Innovative and advanced epitaxy

Resume : The silicon photonic platform is the mainstream technology used currently, due to its scalability up to 12inch wafers and wafer-scale co-integration with high-speed electronics using existing techniques. It is intensively investigated for keeping on track with Moore's Law, by using optical interconnects to provide faster data transfer both between and within microchips. But its interest goes far beyond optical communications at the 1.55µm wavelength used by fiber optic telecommunication systems. In particular, for applications such as 3D imaging or facial recognition, it becomes necessary for the sensor to operate in the near infrared (NIR) wavelength range, particularly at 940nm. CMOS image sensors on silicon substrate absorb the light in the photodiode area but they need to be deep (typically 1 to 10 µm) to absorb the maximum amount of light (especially at long wavelengths) and free of defects, since each defect acts as a center of recombination of the electron-hole pairs formed during the light absorption. By contrast, germanium photodiodes, which are available on the major silicon photonics platforms, typically show a much higher quantum efficiency with an absorption coefficient almost two decades higher than silicon at 940nm. However, the main bottleneck is the fabrication of monocrystalline thick Ge layers on Silicon On Insulator (SOI) substrate, that cannot be fabricated by direct epitaxy. In this presentation, we will review the various fabrication processes developed for the fabrication of GeOI systems. We will also discuss a new fabrication process based on the combination of epitaxy and condensation. We show that different strain relaxation regimes can be distinguished. While before the total oxidation of the underlying SOI layer, the SiGe/SOI samples remain perfectly strained without elastic relaxation, the first stages of relaxation start at concentrations x ? 0.5, where partial and local relaxation were revealed by High Resolution Transmission Electron Microscopy (HRTEM) observations. At this stage, relaxation proceeds through the slow diffusion and motion of matter at the Si1-xGex/SiO2 interface. Large variations of strain distribution on different areas along the interface was quantitatively assessed by Geometric Phase Analysis (GPA). The mechanism results in the morphological evolution and local swelling of the SiGe embedded layer, facilitated by the viscous ?ow of SiO2. GPA confirms that the lateral expansion leads to the relaxation of the strain accumulated during condensation of Ge. This demonstrates that even at low temperature, SiO2 can serve as an efficient compliant substrate for strain engineering of defect-free Ge rich Si1-xGex thin films. At higher Ge concentration, typically for pure Ge, full relaxation is obtained by an original buckling instability phenomenon, which results in a large periodic undulation. To prevent this undulation, periodic patterns are performed by e-beam lithography in the top SiGe epitaxial layer before oxidation. In the areas that do not contain any pattern, TEM cross-section observations show either local undulations of the Ge-rich films or large density of dislocations. However, in the patterned areas, the TEM /GPA analyses show that for optimized patterns periodicity and geometries, a fully relaxed defect-free ultrathin Ge layer can be obtained. The last fabrication step of Ge photodiode which consists of the re-epitaxy of a thick and relaxed Ge layer to maximize the photon absorption is under progress.

Resume : Cu3N is a novel semiconductor in which crystal imperfections such as Cu interstitials and nitrogen vacancies give rise to states that are energetically located inside or very close to the conduction and valence bands respectively but do not give rise to any mid gap states. Consequently, it has been suggested to be a defect tolerant semiconductor that could be attractive as as a solar cell absorber in view of the fact that it has an indirect energy band gap of 1.0 eV but also due to the fact that n- and p-type doping are possible. In addition, Cu3N has been used successfully for energy storage as it can act as a host for Li ions in batteries. Here it is shown that Cu3N can be grown on Al2O3 by halide vapor phase epitaxy in a two-step process using CuCl2 under an excess of 800 ml min-1 NH3 at 600 °C which leads to the deposition of Cu that is subsequently converted into Cu3N under NH3:O2. The reaction of CuCl2 in CH3CH2OH with an excess of NH3 did not lead to the direct growth of Cu3N which is different to the case of halide vapor phase epitaxy of III-V semiconductors. The Cu3N layers obtained in this way have an anti-ReO3 cubic crystal structure and we have clearly observed the M- and R-direct energy band gaps by ultrafast pump-probe spectroscopy in excellent agreement with density functional theory calculations of the electronic structure. We also find that the reaction of CuCl2 under a smaller flow of NH3 will lead to the deposition of cubic Cu2O which is an active topic of investigation due to the observation of giant Rydberg excitons by Kazimierczuk et al. [1] who confirmed the existence of Rydberg excitons with principal quantum numbers as large as n = 25 and giant wavefunction extensions. More recently Rydberg exciton?polaritons were observed in a Cu2O microcavity [2] but all of the above ground-breaking discoveries were observed in naturally occurring Cu2O crystals which has instigated the need for the epitaxial growth of high crystal quality Cu2O. The Cu2O films obtained here have a cubic crystal structure and display a detailed spectral structure by ultrafast pump probe spectroscopy that has not been previously observed. This is currently the main focus of attention along with ongoing investigations into the growth of Cu2O on lattice matched layers other than Al2O3. [1] T. Kazimierczuk, D. Fröhlich, S. Scheel, H. Stolz, M. Bayer, Giant Rydberg Excitons in the Copper Oxide Cu2O, Nature, (2014), 514, 343. [2] Konstantinos Orfanakis, Sai Kiran Rajendran?, Valentin Walther?, Thomas Volz, Thomas Pohl and Hamid Ohadi, Nature Materials, 2022.

Resume : The hexagonal crystal phase of Ge (2H-Ge) is a metastable and extremely promising form, due to the direct bandgap that allows for efficient light emission, particularly in the Ge-rich SiGe alloy. Its synthesis has recently become an important challenge for researchers. Fadaly et al. [1] obtained photoemission from 2H-Ge and 2H-SiGe epitaxial shells, grown around almost lattice-matched wurtzite GaAs nanowire cores. The structural quality of these materials, essential for such applications, has been discussed in Fadaly et al. [2]. There, we identified a specific planar defect, having a triangular shape in the nanowire cross-section, being the I3 basal stacking fault and confined within partial dislocations. The detailed atomistic structure of this complex has been revealed and the harmless electronic nature for emission (no gap states) has been predicted by first principle calculations, and experimentally confirmed. Here, we go further in the defect analysis, modelling possible intrinsic formation mechanisms during the shell growth. In fact, it appears that the defect may originate at any point in the growth front of the shell, independently of the similar ones stemming from the GaAs interface. We propose the key atomistic process leading to the nucleation of the defect triangular apex, considering the growth-front and the dislocation reconstructions, by first principle calculations. An alternative evolution to the original fault is also proposed, possibly leading to a simple point defect. The latter is not easily observed via direct TEM imaging of the nanowires, so we still do not have evidence of its occurrence, in parallel to the extended I3 defects. Therefore, possible initial stages of the two defects just after the fault nucleation have been investigated via first principles calculations, allowing us to argue the stability of the subsequent growth steps. Finally, by considering the structural complex embedding the point defect, we propose a bonding reconstruction and calculate the relative electronic properties, which might constitute an alternative spectroscopic way of assessing whether it appears, or not. [1] E.M.T. Fadaly et al., Direct-Bandgap Emission from Hexagonal Ge and SiGe Alloys, Nature 2020, 580 (7802), 205?209. [2] E.M.T. Fadaly et al., Unveiling Planar Defects in Hexagonal Group IV Materials, NanoLett. 2021, 21, 3619?3625.

Resume : Tuning oxygen mass transport properties at the nanoscale offers a promising approach for developing high performing energy materials. Several strategies for engineering interfaces with enhanced oxygen diffusivity and surface exchange have been proposed. However, the origin and the magnitude of such local effects remain largely undisclosed to date due to the lack of direct measurement tools with sufficient resolution. One potential candidate to retrieve the hidden information is the Transmission Electron Microscope (TEM), a powerful instrument that allows the local observation of structures with sub Å resolution. Furthermore, when talking about TEM, it is important to note that a plethora of techniques based on the same instrument have emerged, both in hardware and data treatment software, beyond the conventional observation methods. Atomic resolution in both structural and analytical images is commonplace, and the ever-increasing size of the latter has been the driving force behind the application of big data algorithms to segment and extract the relevant information from spectra [1,2]. Here we focus on a series of materials with mixed electronic and ionic conduction properties (LaxSr1-xMO3 with X = Mn, Cr and (Co,Fe)) where crystalline defects have been determined as the most efficient conduction pathways. Here we show that the adequate characterization of these defects in terms of local order, strain and stoichiometry is an unavoidable step in the understanding of their functional properties. Supported by Atom Probe Tomography results [3], in this work we present the capabilities of several TEM related techniques, High Resolution TEM and Scanning TEM with Geometric Phase Analysis, Energy-Dispersive X-Ray Spectroscopy and Electron Energy-Loss Spectroscopy (combined with clustering methods), to unveil the local phenomena at the atomic scale that ultimately govern the macroscopic properties of electrochemical devices, thus illuminating the path for charge transport engineering in epitaxially grown thin layers. [1] Blanco-Portals, J., Peiró, F., & Estradé, S. (2022). Microscopy and Microanalysis, 28(1), 109-122. [2] Blanco-Portals, J., et. al. (2022). Ultramicroscopy, 232, 113403. [3] Baiutti, F., et al. . (2021). Advanced Materials, 33(48), 2105622.

Resume : Tantalum arsenide belongs to topological Weyl semimetals (WSM), with unusual electronic and magnetotransport properties. In WSM materials, the combination of distinct features of electronic structure and crystal lattice symmetry (either time reversal or inversion symmetry, but not coincidence of both) leads to occurrence of so called Weyl nodes at the crossing points of two bands without spin degeneracy. The low energy excitations at the Weyl nodes behave as masless Weyl fermions with linear dispersion relations and opposite chiralities. In the band structure of TaAs 12 pairs of Weyl nodes have been identified in the distinct points of the Brillouin zone; but so far TaAs has been obtained only in the form of bulk crystals. We have grown TaAs by molecular beam epitaxy (MBE) on commonly used GaAs(001) substrates, in a III-V MBE system equipped with a valved cracker source for arsenic and e-beam source for Ta. TaAs crystallizes in a body-centred-tetragonal structure with the lattice parameters: a = b = 3.4348 Å; c = 11.641. Using in-situ reflection high energy electron diffraction (RHEED) we observe that in spite of a significant lattice mismatch to GaAs, TaAs grows on GaAs(001) in a fully 2-dimensional layer-by-layer mode, from the very first deposition stage, with no initial 3-dimensional growth typical for epitaxy on the substrates with a high lattice mismatch. The orientation of TaAs is the same as that of the substrate, i.e. TaAs(001) planes are parallel to GaAs(001), but they are rotated by 45 degrees, hence the [110] in-plane direction of the GaAs substrate is parallel to the [010] one of TaAs. Such configuration decreases the lattice mismatch between TaAs and GaAs, but still it is as big as 19%. The twisting of TaAs(001) layers with respect to GaAs(001) substrate is observed in RHEED, and confirmed by X-ray diffraction and transmission electron microscopy (TEM) measurements. Even though no misfit dislocations are identified in TEM cross-sections visualizing the TaAs/GaAs interface, the TaAs surface morphology revealed by atomic force microscopy exhibits stripe-like structure with about 200 nm long and 20 nm thick planar columnar features parallel to GaAs[-110]. This hampers (so far) applicability of angle resolved photoemission spectroscopy (ARPES) to reveal topological properties typical to Weyl semimetals, such as bulk Dirac cones and surface Fermi arcs, due to much larger dimensions of UV-beam spot used in ARPES, but should not be an obstacle in much more local scanning tunneling spectroscopy measurements (to be done). The possibility of obtaining epitaxial layers of TaAs opens a way for integrating this WSM in heterostructures with other materials e.g. ferromagnets or superconductors, which enables investigations of proximity effects, impossible to explore using bulk TaAs crystals accessible so far. This work has been supported by the National Science Centre (Poland), through the project No. 2017/27/B/ST5/02284.

Resume : Transition metal dichalcogenides (TMDs) constitute a class of quantum materials that have gathered a tremendous interest from the solid-state physics community focusing on two-dimensional (2D) materials. They hold promises for numerous applications in photonics and electronics owing to their large exciton binding energies and a transition from an indirect to a direct band gap in the monolayer limit . They also exhibit a strong spin-orbit coupling which is promising for spintronic applications . Until recently, most studies on TMDs have been performed on micrometer-sized flakes mechanically exfoliated from bulk samples . Extensive efforts are beginning to enable the growth of wafer-scale TMD single-crystal films . The absence of lattice matched substrates and the high reactivity of chalcogen atoms prevent the epitaxial growth of monolayers using molecular beam epitaxy (MBE) on usual substrates (Si, Ge, GaAs…). In this presentation, I will discuss our strategy to achieve the growth of single crystalline TMD monolayers by vdW epitaxy . In this regime, the substrate exhibit a van der Waals surface like graphene, mica or Se-passivated GaAs to limit the substrate-epilayer interaction. In a second part, I will present the vdW epitaxy of two TMDs with high spin-orbit coupling on graphene: PtSe2 for the study of spin-charge interconversion phenomena and WSe2 for the study of the valley Nernst effect . Finally, I will demonstrate the clear advantages of MBE and vdW epitaxy to grow well-controlled 2D ferromagnets: Fe5GeTe2 with high Curie temperature close to room and Cr1+dTe2 with tunable magnetic properties by proximity effects and adjusting the stoichiometry d.

Resume : Ferroelectric (FE) binary oxides such as Hafnia and Zirconia are key materials for the development of advanced devices, such as non-volatile memories, neuromorphic computing units and supercapacitors. Very recently, ferroelectricity was observed in ultrathin ZrO2 films grown on Si with a thickness of only 5 Å (Cheema, S. et al., Science, 376(6593), 648-652, 2022). However, ZrO2 ferroelectric films are much less studied than HfO2-based ones. In this work, we will show the impact of the Nb:SrTiO3 (STO) substrate orientation on the stability and ferroelectric properties of ZrO2 films grown by ion-beam sputtering. In particular, we will show how orthorhombic phase pure films with negative piezoelectric coefficient d33 could be grown on (011)-oriented substrates, through domain matching epitaxy, while the films grown on (001)- oriented substrates show a mixture of monoclinic and orthorhombic phases. In addition, the role of oxygen vacancies on the stability and ferroelectric properties of epitaxially grown rhombohedral ZrO2 films on (111)-oriented Nb:STO will be also highlighted.

Resume : Superconductive compound Mo6S6I2 belongs to cluster compounds composed of Mo6X8 units (X=S, Se, Te), where part of chalcogen atoms are replaced by iodine as monovalent substituents [1]. In contrary with many other so-called Chevrel phases with the general formula Mo6S8-xIx, which are of needle-like morphology and differ in the site occupation by sulphur and iodine, like Mo6S2I8 [2] and Mo6S4I6 [3], the Mo6S6I2 compound shows a low degree of morphological anisotropy and crystallizes in three-dimensional semi-cubic shape [4]. It’s structure is describes as the hexagonal rhombohedral structure of the PbMo6S8 type with the lattice parameters: aR=6.563 Ǻ and αR=94.5 Ǻ. The critical temperature (Tc) for the superconducting transition in Mo6S6I2 is 14 K. We report on the first quasi-2-dimensional Mo-S-I compound, which nucleates and grows epitaxially on the surface of Mo6S6I2 crystals in shape of thin and rigid belts. Separated from the Mo6S6I2 surface, they are prone to a cleavage along their length, but they keep their growth as thin lamellas. Building units are similar to constituents of quasi one-dimensional Mo6S2I8. The preliminary structural characterization by high-resolution electron microscopy, Raman spectroscopy and magnetic susceptibility measurements have shown that this completely new compound develops a superconductivity at low temperatures with the Tc≈ 4K. Scanning tunnelling spectroscopy (STS) revealed also a huge energy gap of around 8,5 eV. The compound is stable at normal conditions and might be usable for energy storage material. Results of structural characterization of this new Mo-S-I compound in comparison with the Mo6S6I2 cubes and Mo6S2I8 nanowires will be presented and the growth mechanism will be discussed in a model of epitaxy. [1] A. Perrin, C. Perrin and M. Sergent, “Octahedral clusters in molybdenum(II) and rhenium(III) chalcohalide chemistry”, J.Less-Common Metals 137 (1988) 241. [2] C. Perrin, M. Sergent, “A new family of monodimensional compounds with octa-hedral molybdenum clusters. Mo6X8Y2 (X=halogen, Y=chalcogen)”, J. Chem. Res. (S) 2 (1983) 38. [3] M. Remskar, A. Mrzel, M. Virsek, M. Godec, M., Krause, A. Kolitsch, A. Singh, A. Seabaugh, “The MoS2 Nanotubes with Defect-Controlled Electric Properties”, Nanoscale Res Lett 6:26 (2011). [4] M. Sergent, Ø.Fisher, M. Decroux, C. Perrin, R. Chevrel, “Stabilization of Mo6S8 by halogens; new superconducting compounds: Mo6S6Br2, Mo6S6I2, J. of Solid State Chem. 22 (1977) 87.

Resume : A recent observation of excitonic Aharonov-Bohm effect in core/shell nanowires has opened an exciting opportunity to study coherently rotating states in these structures [1], which could be, subsequently, applied in the field of quantum information storage. An important task for the observation of these states is the separation of electron-hole wavefunctions within a nanowire. In ref [1], it has been achieved on GaAs crystal phase quantum dots, which are characterized by a type II band alignment. In this work, an alternative approach for fabrication of quantum dots with type II band alignment inside a nanowire built of II-VI semiconductors is presented. The nanowire heterostructures are grown by molecular beam epitaxy by applying the vapor-liquid-solid growth mechanism assisted with gold catalysts. In the first step, ZnTe short axial insertion inside (Zn,Mg)Te nanowire is fabricated by turning off Mg-flux for a few seconds during the (Zn,Mg)Te nanowire growth. Since ZnTe has a smaller bandgap than (Zn,Mg)Te, a sufficiently short insertion can be considered as a quantum dot inside a nanowire. Subsequently, a few monolayers thick ZnSe shell is deposited around the entire nanowire. ZnTe/ZnSe heterostructure is well-known for the type II character, where electrons tend to localize in ZnSe and holes in ZnTe region. In the last step, the nanowires are coated in a (Zn,Mg)Te passivation shell with the thickness of about 20 nm in order to reduce the impact of surface states on the optical emission. Optimization procedure of the above mentioned structure has been performed. In particular, the several parameters, such as: the length of ZnTe axial insertion, the thickness of the ZnSe shell and Mg concentration inside (Zn,Mg)Te are adjusted. Our goal has been to observe an optical emission from ZnTe/ZnSe nanowire quantum dot. In the case of a ZnTe/ZnMgTe reference structure without ZnSe shell, one observes clearly two distinct optical emission lines coming either from the nanowire (Zn,Mg)Te cores or from the ZnTe quantum dot. The addition of only one monolayer thick ZnSe shell significantly changes the optical emission spectrum. The emission energy of the quantum dot emission shifts significantly from 2.38 eV to 2.00 eV. Simultaneously, its intensity drops by about two orders of magnitude and the decays times increase by the factor of about 10. These observations are consistent with the type I to type II band alignment transition resulting from the presence of ZnSe shell. In μ-photoluminescence the broad emission at 2.00 eV splits into several relatively lines coming from individual nanowire quantum dots. This association is confirmed by the appearance of multiexcitonic emission lines when increasing the excitation power. [1] Corfdir P, et al. Adv. Mater. 31 1805645 (2019)

Resume : The application of quantum technologies such as spintronics, solotronics, or quantum computing is highly promising when it comes to miniaturization in modern technology. In order to achieve effective devices, there is a need to investigate the spin properties of im- purity interacting with the semiconductor lattice and conned carriers. Zero-dimensional semiconductor structure such as epitaxial quantum dot (QD) is a model system to probe fundamental interactions in condensed matter. For example, QDs can be used to examine the spin of a single magnetic ion [1,2,3]. Vanadium is a transitional metal with a nuclear spin of 7/2 and 3 electrons on the d shell. It exhibits spin 3/2 in V2+ configuration [4] leading to two possible fundamental states with spin projection +/- 3/2 or +/- 1/2. Particular spin configuration is expected to depend on the strain of the crystal lattice in a QD. In this work we examine self-assembled CdTe QDs doped with V, in ZnTe barrier, fabricated using molecular beam epitaxy. We observed a single QD including a single vanadium dopant and measured the magneto-optical properties of such a system. According to numerical modeling based on experimental data, we conclude that V in this case exhibits spin 1/2, which makes our system a realization of a qubit. [1] L. Besombes et al, Phys. Rev. Lett 93, 207403 (2004). [2] J. Kobak et al, Nature Communications 5, 3191 (2014). [3] T. Smolenski et al, Nature Communications 7, 10484 (2016). [4] M. Herbich et al, Phys. Rev. B 59, 2726 (1999).

Resume : Two-dimensional (2D) magnetic materials and heterostructures garnered significant attention as they are an ideal platform for exploring exotic physical phenomena such as topological spin configurations. Moreover, they are considered to be promising building blocks for the realization of ultra-compact and novel spintronic devices. A crucial aspect in the development of such 2D crystals for applications is their reproducible and large-scale fabrication. In this talk, I will show recent results on van der Waals (vdW) epitaxy of the 2D ferromagnetic metal Fe5-xGeTe2 (FGT, with 0 ≤ x ≤ 2) on graphene and hexagonal boron nitride templates. In this context, the employment of defect engineering to exert control over nucleation during vdW growth will also be discussed. FGT film synthesis was performed via molecular beam epitaxy using elemental Fe, Ge, and Te evaporated from Knudsen cells, and substrate temperatures between 200 and 300 C. Morphological and structural characterization of the heterostructures with various methods show that very good crystalline quality can be achieved. Furthermore, magneto-transport along with magnetometric measurements reveal composition-dependent magnetic anisotropies and Curie temperatures. Importantly, ferromagnetic order with a predominant easy out-of-plane orientation can be stabilized above room temperature by incorporating sufficiently high Fe amounts in the FGT structure. A detailed correlation between the structural evolution and ferromagnetism in the FGT films will be presented. These results are relevant for the further development of wafer-scale fabrication of layered magnetic materials and heterostacks aiming at the realization of multifunctional, atomically thin devices.

Resume : For more than 20 years, III-V semiconductor nanowires (NWs) have been studied to build blocks for future electronic and optoelectronic devices. Nevertheless, making functional NW-based devices is complex, due to the limited capability of current bottom-up and top-down methods to achieve reproducible NWs arrays. VLS growth methods used to obtain high aspect ratio NWs are not a suitable option, since incorporation of catalyst particles in NWs could introduce deep level recombination centers that significantly degrade the electrical and optical properties. Today, the most reliable process to control the growth of well-ordered III-V NWs with high reproducibility is Selective Area Growth (SAG). In this work, we demonstrate the possibility of making III-As NWs arrays by SAG-Hydride Vapour Phase Epitaxy (SAG-HVPE). Perspectives of this work concern multispectral infrared photodetectors for InAs [1] and betavoltaic cells for GaAs [2], which require dense arrays of high aspect ratio NWs. In SAG-HVPE, the chloride precursors provide a suitable environment to implement selective and localized growth since they do not adsorb on the dielectric surface. Moreover, HVPE is a fast growth (up to a few tens of µm/h) cost-effective technique which allows to produce high aspect ratio NWs in limited process time. GaAs NWs growth experiments were carried out on on GaAs (111)B substrates. Systematic studies of the influence of the V/III ratio and growth temperature on the NWs morphology were performed. Doping experiments have been carried out using Si (n-doping) and Zn (p-doping) precursors. Samples were characterized by photoluminescence (PL) and transmission electron microscopy (TEM). GaAs NWs arrays were also grown on Si (111) substrates. The specific conditions required for nucleation and growth of GaAs inside opened Si apertures are described. The temperature is the main factor to control the shape of the NWs: the morphology can be switched from platelets to NWs. TEM analysis showed a perfect zinc blende crystal quality for the platelets. Growth of InAs NWs has also been carried out on GaAs (111)B and Si (111) substrates. Similar to GaAs, the morphology of InAs structures is controlled with different growth parameters. According to COMSOL simulations, the InAs absorptance peak depends on the NWs array design: pitch and diameter of the apertures. Fourier-transform infrared spectroscopy (FTIR) performed at room temperature on InAs NWs grown by SAG-HVPE actually showed a shift of the absorptance peak, from 0.4 eV to 0.8 eV for NWs diameters of 630 nm and 310 nm, respectively [3]. References: [1] R.R. LaPierre et al 2017, J. Phys. D: Appl. Phys. 50 123001. [2] D.L. Wagner, D.R. Novog, and R.R. LaPierre, J. Appl. Phys. 127, 244303 (2020). [3] G. Grégoire, Crystal Growth & Design 2021 21 (9), 5158-5163.

Resume : Quantum confinement in semiconductor heterostructures results in the enhancement of excitonic effects leading to numerous applications related, for instance, to the improvement of the room temperature performance of optical devices. Strikingly, between well-studied two-dimensional and zero-dimensional systems lays an underexplored region of one-dimensional quantum wires (QWRs). In this work, the fabrication of ultra-thin (Cd,Mn)Te nanowires with diameters reaching values below 10 nm is reported. The structures are grown in a system for molecular beam epitaxy by employing the vapor-liquid-solid growth mechanism assisted with gold catalysts. The thinning procedure relies on a thermally induced reverse reaction process, which is applied in-situ on nanowires in the same growth process. Subsequently, the nanowire cores are covered with (Cd,Mg)Te passivation shells to activate the excitonic emission. Importantly, it is found, that the optical emission from ultra-thin nanowires exhibits a distinct blueshift as compared to reference nanowires grown in identical conditions but without the annealing step. This is the first strong indication for the influence of the quantum confinement effects in these structures. While the reference nanowires emit light in the relatively narrow spectral range 1.59 eV – 1.62 eV, the emission from thermally thinned nanowires extends from 1.5 eV to 1.75 eV. Moreover, it is found based on cathodoluminescence study that the emitting objects can be significantly shorter than the nanowires themselves. The emitter region length can be correlated with emission energy, while shorter regions emit at higher energy. This result indicates that excitons can be localized at fluctuations of nanowire core diameter. Therefore, not only the radial but also the axial quantum confinement should be, most likely, taken into account while considering the properties of excitonic emission from ultra-thin (Cd,Mn)Te nanowires. The advantage of the use of Mn-containing II-VI semiconductors, such as (Cd,Mn)Te, is the opportunity to determine the light hole - heavy hole splitting values within individual nanowires by employing the presence of the giant Zeeman effects. A detailed magneto-optical reveals that not only the light hole - heavy hole splitting but also the character of the excitonic emission depends on the emission energy. The nanowires emitting at relatively low energy exhibit a light hole character while the excitonic emission has predominantly heavy hole character in nanowires emitting above 1.70 eV. This finding is explained in terms of competition between strain and radial quantum confinement leading to the light hole character of excitons, and localization of excitons in the axial direction which promotes the heavy hole excitons. This research was partially supported by the National Centre of Science (Poland ) through grant 2017/25/N/ST3/00621, and by the Foundation for Polish Science through the IRA Programme co-financed by EU within SG OP (grant No. MAB/2017/1).

Resume : The AIII-BV-Bi semiconductor, called bismide, is one of the most perspective compounds for the NIR applications because of their unique properties – much faster than of classical semiconductors band gap energy reduction replacing As by Bi atoms and less temperature sensitive Eg. Theoretically it was demonstrated, that 1% of Bi can reduce the band gap energy up to 88 meV. Thus, using GaAs platform and introducing up to 10 % of Bi, the optical features of GaAsBi compound could cover spectral region containing all three telecomunication windows up to1550 nm. Moreover, due to lower band gap energy temperature dependence, bismide alloy based NIR light sources could easily work at room temperature without additional cooling. Despite these advantages their application progress is still hold back by technological challenges: low bismide growth temperature and stoichiometric ratio of Ga and As fluxes used to introduce higher bismuth concentration. The difficulties to insert larger content than 6% Bi due to much larger than As bismuth radius, and tendency to surface segregation, consequently Bi droplets of several hundred nanometers size formation, is one more factor insisting to find the new concepts for GaAsBi and Bi employment in NIR sources. Bulky Bi known as a semimetall with reduction of dimension lower than 60 nm transforms to indirect semiconductor. Further minimization below 16 nm Bi changes from indirect to direct semiconductor, exhibiting 0.7-1.0 eV band gap and extending the application area of GaAsBi and Bi based nanostructures. In this work, bismuth segregation process was explored as an alternative of Bi QDs formation way to usual Stranski-Krastanov method. Bi quantum dots have been formed employing in-situ bismuth segregation process in Molecular Beam Epitaxy (MBE) reactor via annealing of GaAsBi/AlGaAs single or multiple quantum wells (QWs) for a short time at 600-750oC temperatures under arsenic overpressure conditions. The optimization of QWs growth and in-situ annealing conditions was performed to found the crucial technological parameters: quantum barriers composition (which could play Bi blocking layer role as well), GaAsBi well composition (max Bi content) and geometry (well thickness and shape) influencing Bi QD density in the well and its orientation. Featured samples were characterized using high resolution transmission electron microscopy (HR-TEM), meanwhile all grown quantum structures were examined by photoluminescence (PL) measurements.

Resume : Solid-state single and entangled photon emitters linked coherently over long distances with optical fibers enable a new generation of quantum-based communications networks. Currently, epitaxial semiconductor quantum dots (QDs) pave the way as a scalable approach for fabricating deterministic non-classical light sources that can be integrated with other photonic or electronic components in miniaturized form. While multiple epitaxial QD fabrication methods exist, arguably the most successful approaches have been based on local droplet etching (LDE) involving materials based on GaAs and AlGaAs.[1] However, this material system is limited to the 680-800 nm wavelength range owing to their direct band gap range, and thus are not suitable for long-haul transmission over optical fibers. In this work, we present a highly analogous but novel LDE-QD system based on GaSb and AlGaSb [1,2], which emit at telecom wavelengths around 1.5 um with room for expansion towards 2 um and beyond. Here, we first describe how the LDE process works in the GaSb/AlGaSb system. The process is initiated by forming metallic nanodroplets over a flat AlGaSb surface by deposition of Al or Ga [1,2]. These nanodroplets are then annealed under a low Sb-flux which triggers an etching process of the underlying AlGaSb material, thus forming a nanohole on the surface. The nanoholes can then be filled by GaSb and subsequently overgrown with AlGaSb, thus forming GaSb QDs embedded in an AlGaSb matrix. We show how this process of nanohole fabrication can be accurately controlled by fine tuning the epitaxial growth parameters, which allows us to control the morphology and density of the resulting QDs. For example, we demonstrate three-orders-of-magnitude tunability in the nanohole density, as well as the different characteristic morphologies of nanoholes etched by Ga and Al. After understanding the growth process, we demonstrate the different emission properties of GaSb LDE-QDs. In ensemble (large area) low temperature photoluminescence measurements, the QDs show ground state emission at 1.48 µm wavelength with an extremely narrow peak width of around ~8 meV, demonstrating excellent uniformity of the QDs. In single-QD micro-photoluminescence measurements, the QD emission is characteristic of single-exciton dominated spectra with very narrow linewidths and linear power scaling. In addition to these promising emission properties, we show an interesting effect whereby tuning the degree of filling of the nanoholes, the QDs undergo a indirect-to-direct bandgap crossover, which would enable a QD system that could be tuned from highly luminescent to a non-luminescent in a controllable way. 1. M. Gurioli, Z. Wang, A. Rastelli, T. Kuroda, S. Sanguinetti, Nature Materials 18, 799 (2019). 2. J. Hilska, A. Chellu, T. Hakkarainen, Cryst. Growth Des. 21 1917−1923, 2021 3. A. Chellu, J. Hilska, J.-P. Penttinen, T. Hakkarainen, APL Materials 9, pp. 051116, 2021

Resume : Indium antimonide (InSb) has a narrow band gap, high carrier mobility, small effective mass, and is a promising candidate for the implementation of topological superconducting states. Remarkably, the interest in this material has increased in recent years thanks to the possibility to overcome the limitation of its integration with lattice-mismatched substrates in nano-heterostructures. However, the realization of InSb nanostructures with a high crystal quality and a well-controlled morphology is still challenging. In this contribution, we show the growth of free-standing InSb nanostructures on InAs and InP nanowire stems by means of Au-assisted chemical beam epitaxy. We demonstrate that nanowires (1D), nanoflags (2D), and nanocubes (3D) can be obtained by tailoring growth parameters like growth temperature, precursor fluxes, sample rotation, and substrate orientation. Indeed, the InSb shape evolution is a result of the interplay between axial vapor-liquid-solid (VLS) growth, radial vapor-solid (VS) growth, and possibly directional-driven growth. Concerning the nanoflags, through morphological and crystallographic characterization we demonstrate that they are single-crystalline with a defect-free zinc blende structure and stoichiometric composition. The existence of two families of nanoflags, characterized by an aperture angle at the base of 145° and 160°, is observed and modelled [1]. Finally, we have further optimized the size of these free-standing 2D InSb nanoflags. In fact, by employing more robust and tapered NW stems and precisely orienting the substrate with the help of reflection high-energy electron diffraction (RHEED) patterns, we could maximize length and width, and minimize the thickness of the nanoflags [2]. This allowed us to fabricate Hall-bar devices with suitable length-to-width ratio enabling precise electrical characterization of single nanoflags. An electron mobility of ~29,500 cm2/Vs at 4.2 K was measured, which is the highest value reported for free-standing 2D InSb nanostructures in literature. We have also successfully fabricated ballistic Josephson junction devices with 10/150 nm Ti/Nb contacts that show gate-tunable proximity-induced supercurrent (∼ 50 nA at 250 mK) [3]. The devices also show clear signatures of subharmonic gap structures, indicating phase-coherent transport in the junction and a high transparency of the interfaces. Our study provides useful guidelines for the controlled growth of high-quality InSb nanostructures with different shape, and evince the use of 2D InSb nanoflags for fabrication of advanced quantum devices. [1] I. Verma et al., Nanotechnology 31, 384002 (2020). [2] I. Verma et al., ACS Appl. Nano Mater. 4, 5825–5833 (2021). [3] S. Salimian et al., Appl. Phys. Lett. 119, 214004 (2021).

Resume : A new generation of piezoelectric generators based on 1D-nanostructures is appeared, these last years, to develop autonomous power micro-systems. Thanks to their high crystalline quality, their high mechanical properties and their large surface-to-volume, nanowires (NWs) present advantages to significantly enhance the generator conversion efficiency. However, due to their nanoscale dimensions, the nanostructures are also characterized by “new” properties, non-significant at micrometric scales, that can lead to a strong modulation of their characteristics. Among these “new” properties, we can cite the modulation of the free carrier concentration in the NW volume due to the presence of the surface charge (SC) effects, strongly pronounced in sub-100 nm wide GaN NWs. Although these effects are under debate, their potential detrimental (in nanoscale electronic and optoelectronic devices) or advantageous (in nanoscale piezoelectric devices) roles have already been pointed out. In-depth understanding and control of the influence of SC on the NW properties is thus crucial to fully take advantage of these 1D-nanostructures and thus develop high-efficient NW-based devices. Characterization at the nanoscale is challenging, and the development of new nano-characterization techniques is required. Here we investigate the effects of the SC through the characterization of the piezo-conversion properties of GaN NWs, by using a new advanced nano-characterization tool. It is well known that the free carriers degrade the piezoelectric response, due to their screening of the piezoelectric charges. The modulation of their concentration by varying the SC effects thus results in the variation of the piezo-response of NWs. Thanks to two different experimental configurations, we demonstrate the influence of the NW environment as well as of the NW dimensions on the expression of the SC and thus on the piezoelectric conversion properties of GaN NWs. Especially, we establish that the SC effects can be extremely favorable for strongly improving the electromechanical conversion efficiency of GaN NWs.

Resume : Boron nitride (BN) in the sp2 – hybridized structure is a material with extremal resistance to external conditions, characterized at the same time by a wide band gap (approximately 6 eV) and two-dimensional nature [1]. Such a combination of properties provides a wide range of possible applications, from deep UV light sources, through protective layers and insulating barriers to one of the building blocks in van der Waals heterostructures [2]. Despite the great progress in the growth technologies of the BN layers [3,4], there is no suitable method that would provide high-quality and large area layers, which would scale with the size of the substrate used. In this communication, we present our achievements in the growth of boron nitride by Metal Organic Vapour Phase Epitaxy (MOVPE) – a method that could fulfill the above-mentioned requirements. In this technique, BN is grown on sapphire substrates as a product of the chemical reaction between ammonia (nitrogen precursor) and triethylborane (TEB, boron precursor). The traditional growth modes used in MOVPE are still very limited in obtaining high-quality layers. Our new approach – two-stage epitaxy [5] allows overcoming some of the limitations of previous methods. In this regime, a thin (a few nanometers), preordered layer grown in Continuous Flow Growth mode (CFG) [6] is treated as a buffer for a more effective growth by Flow-rate Modulation Epitaxy [7]. Such a combination allows to avoid chaotic nucleation on the substrate and leads to the formation of boron nitride with a lattice constant very close to those of the best bulk h-BN crystals. Our studies show that achieving boron nitride with properties comparable to those of bulk material but on a large scale by MOVPE may be within reach in the near future [8]. Further tuning of the method and searching for ideal parameters and substrates for growth may allow to transfer the material from research laboratories to practical applications. The advantages and prospects for further development of this novel approach (i.e. influence of the buffer layer, the substrate used, and pregrowth procedures) will be discussed. [1] K.S. Novoselov et al., Science 353, 6298, aac9439 (2016) [2] A.F. Rigosi, et al., J. Phys. Mater. 4, 032003 (2021) [3] K. Zhang et al., J. Mater. Chem. C 5 11992–12022 (2017) [4] A. Pakdel et al., Chem. Soc. Rev. 43 934–59 (2014) [5] A.K. Dabrowska et al., 2D Mater., 8, 015017 (2021) [6] K. Pakuła et al., arXiv 1906.05319 (2019) [7] Y. Kobayashi et al., Journal of Crystal Growth 310, 5048–5052 (2008) [8] K. Ludwiczak et al., ACS Appl. Mater. Interfaces 13, 40, 47904–47911 (2021) Acknowledgment: This work was partially supported by the National Science Centre under grant No. 2019/33/B/ST5/02766

Resume : Silicene is an attractive candidate for the realization of 2D field effect transistors (2D-FET), owing to its predicted high carrier mobility and bandgap tunability. Its electronic properties can be controlled through an external electric field, perpendicular to the 2D lattice: this requires the deposition of an insulating layer that directly interfaces silicene without perturbing its two-dimensional nature. A promising material is calcium fluoride (CaF2), a crystalline insulator with a high dielectric constant (e=8.43) and wide bandgap (Eg=12.1 eV), known to form a quasi van der Waals interface with 2D materials and with excellent insulating properties even at ultra-thin scales. Its fluorine-terminated surface is chemically inert and free of dangling bonds, strongly reducing the density of traps at the interface. The integration of crystalline CaF2 in MoS2-based FETs has already been successfully shown, yielding high on/off current ratios up to 107 and small hysteresis. A high quality, pinhole-free, single crystal CaF2(111) layer can be grown at relatively low temperatures (250°C) on Si(111) by molecular beam epitaxy (MBE), thanks to the small lattice mismatch (<1%). The obtained material is characterized by an extremely low concentration of defects, allowing layers with a thickness of just 2 nm to be able to withstand high electric fields - up to 27.8 MV/cm - with negligible leakage currents. We demonstrate the epitaxial growth of a thin layer of crystalline CaF2 on 1 monolayer of silicene by MBE. Low energy electron diffraction analysis proves that CaF2 grows epitaxially on all phases of silicene, strictly following the orientation of the 2D lattice underneath. In-situ X-ray photoemission spectroscopy data evidence that, upon CaF2 deposition, no changes in the chemical state of the silicon atoms can be detected, proving that no Si-Ca or Si-F bonds are formed: this clearly shows that the 2D layer is pristinely preserved underneath the insulating layer. The vibrational properties of silicene are analyzed by polarized Raman spectroscopy, evidencing a structural modification of the 2D layer caused by the weak interaction with CaF2. The encapsulated material retains a bidimensional geometry but a clear shift of the vibrational modes hints at an increased buckling and, consequently, at a greater Si-Si bond length. This structural modification could be exploited to open a larger bandgap in silicene. Our work shows that CaF2 and silicene can be successfully interfaced, paving the way for the integration of silicon-based 2D materials in functional devices.

Resume : Monolayer transition metal dichalcogenides (TMDs) are two-dimensional materials with exceptional optical properties such as high oscillator strength, valley-related excitonic physics, efficient photoluminescence, and several narrow excitonic resonances. However, the above effects have been explored so far only for structures produced by techniques involving mechanical exfoliation and encapsulation in hBN, inevitably inducing considerable large-scale inhomogeneity. On the other hand, techniques which are essentially free from this disadvantage, such as molecular beam epitaxy (MBE), have to date yielded only structures characterized by considerable spectral broadening, which hinders most of the interesting optical effects. In this talk, I will present the MBE-grown TMD (MoSe2) exhibiting narrow and fully resolved spectral lines of neutral and charged exciton [1]. Moreover, our monolayers exhibit unprecedented high spatial homogeneity of optical properties, with variation of the exciton energy as small as 0.16 meV over a distance of tens of micrometers. Importantly, good optical properties are achieved for as-grown samples, without any post-growth exfoliation and encapsulation in hBN. Our best recipe for MBE growth includes an extremely slow growth rate, the annealing at very high temperatures, and the use of atomically flat hBN substrate in the form of flakes exfoliated from bulk. Moreover, comparable results are also obtained using an hBN substrate that we grow by MOCVD on 2” Al2O3 wafers [2]. Our optical characterization includes low-temperature PL, PLE, reflectivity, magneto-spectroscopy, time resolved spectroscopy and room-temperature Raman scattering and SHG [1,3]. We compare structural and optical properties of MoSe2 grown on exfoliated hBN to properties of various TMDs (MoTe2 [4,5], NiTe2 [6], WSe2, VSe2) grown on various substrates (2D, 3D, polycrystalline). This reveals particularly high diffusion parameters of transition metals on hBN [5], the role of distribution of orientation of TMD grain domains, the tendency to merge grains or form bilayers and 3D structures. [1] W. Pacuski et al., Nano Letters 20, 3058 (2020). [2] K. Ludwiczak et al., ACS Appl. Mater. Interfaces 13, 47904 (2021). [3] K. Oreszczuk et al., in preparation (2022) . [4] Z. Ogorzałek et al., Nanoscale 12, 16535 (2020). [5] B. Seredyński et al., ArXiv:2111.12433 (2021). [6] B. Seredyński et al., Cryst. Growth Des. 21, 5773 (2021).

Resume : The path to the implementation of quantum computers relies on the availability of suitable material platforms. Semiconductor-superconductor hybrid systems resulting in Andreev quantum bits (qubits) are among the promising candidates, as high-quality superconducting thin films with transparent interfaces to a low-D semiconductor will make it possible to improve the coherence time and strong qubit-qubit coupling [1]. In this work we have demonstrated that in-situ growth of Al films on near-surface InAs 2D electron gases (2DEGs) can be grown by Molecular Beam Epitaxy on GaAs substrates with quality comparable to state-of-the art growth on InP [2]. Adaptation of the metamorphic growth protocol previously adopted by us for In0.75Ga0.25As on GaAs (001) [3] allowed us to reach low-T electron mobilities up to 8X10^5cm^2/Vs in undoped deep InAs/In0.81Ga0.19As 2DEGs, while preserving values around 3.6X10^4cm^2/Vs for 2DEGs at 10nm from the surface, with a charge around 3.5X10^11/cm^2. Si doping allows tuning the charge density up to 1X10^12/cm^2, with mobilities up to 5.7 and 8.6X10^4cm2/Vs on VdP and Hall bar structures, respectively. Shubnikov-de Haas oscillations on Hall bar structures show well-developed quantum Hall plateaus with a single occupied subband, including Zeeman split-features at ν=3 and 5. In-situ growth of Al films was done at about -60C with a growth rate of about 0.5A/sec. AFM and X-ray reflectivity showed that Al deposition is conformal to the underlying semiconductors, preserving the cross-hatched pattern typical for metamorphic growth and a RMS roughness of about 1nm. Synchrotron radiation XRD rocking curves showed the existence of (111) Bragg peaks, though with intensity much weaker than Al grown on GaAs (001). XRD results will be complemented by cross-sectional TEM data to better assess the Al film crystalline properties. Resistivity measurements as a function of temperature on 10nm-thick films gave values of a few X10^-9Ωm at 4K and 2.5X10^-8Ωm at RT, comparable to the best Al layers on GaAs [4]. Superconducting proximity effect was observed in a Josephson junction. The observed phenomenology opens the way to the exploitation of Andreev physics on GaAs-based technology. 1. Lee, J. S. et al. Transport Studies of Epi-Al/InAs Two-Dimensional Electron Gas Systems for Required Building-Blocks in Topological Superconductor Networks. Nano Lett. 19, 3083–3090 (2019). 2. Wickramasinghe, K. S. et al. Transport properties of near surface InAs two-dimensional heterostructures. Appl. Phys. Lett. 113, 262104 (2018). 3. Capotondi, F. et al. Strain induced effects on the transport properties of metamorphic InAlAs/InGaAs quantum wells. Thin Solid Films 484, (2005). 4. Fan, Y.-T. et al. Atomic-scale epitaxial aluminum film on GaAs substrate. AIP Advances 7, 075213 (2017).

Resume : Avalanche photodiodes (APDs) detectors for synchrotron radiation have been traditionally based on silicon. GaAs-based APDs are however a promising alternative for particularly demanding applications for high energy, ultra-short pulsed light sources [1]. We developed GaAs/AlGaAs APDs with separated absorption and multiplication regions (SAM-APDs) [2]. The multiplication region is made of a staircase-like periodic bandgap modulation, which favors well defined multiplication of electrons and suppresses multiplication of holes (lower noise of device) [3]. The amplitude of this modulation, which determines multiplication efficiency and excess noise, is limited by the indirect AlxGa1-xAs bandgap at x>0.45. In previously fabricated devices, the staircase period consisted of GaAs and Al0.45Ga0.55As layers and a linearly graded AlxGa1-xAs region (0.45>x>0), grown by Molecular Beam Epitaxy (MBE). Numerical simulations showed that, for a given multiplication, a larger conduction band discontinuity in the first steps is beneficial to reduce excess noise [4]. In this work we have realized this concept by replacing GaAs with In0.2Ga0.8As in the first two steps. Due to the 1.5% lattice mismatch between GaAs and In0.2Ga0.8As, preliminary investigations were done to exclude lattice relaxation through dislocations. A set of samples was grown by MBE on GaAs (001) consisting of a 8.6nm InyGa1-yAs grading (0>y>0.2) followed by In0.2Ga0.8As of thicknesses ranging from 5 nm to 1 um. XRD, Raman spectroscopy and AFM experiments showed that the structures are strained and dislocation-free up to a critical In0.2Ga0.8As thickness of at least 60nm. Based on this, we fabricated a SAM-APD as in [3] where in the 2 initial steps the 35nm-thick GaAs layer was replaced by In0.2Ga0.8As and the graded region extended accordingly. We will show preliminary tests on multiplication efficiency and excess noise under illumination with a green laser. [1] G. Lioliou et al., Nucl. Instrum. Meth. A 813 (2016) 1. [2] F. Capasso et al., IEEE T Electron Dev. 30 (1983) 381. [3] T. Steinhartova et al., 2017 JINST 12 C11017. [4] A. Pilotto et al., IEEE T Electron Dev. 66 (2019) 1810.

Resume : ZnCdO ternary alloys are being actively studied due to the possibility of energy band gap modification. The band gap shrinkage for Zn1–xCdxO from 3.1 eV to 1.8 eV is possible due to the presence of the CdO, in addition to the direct band gap (2.35 eV), two indirect band gaps 1.28 and 0.8 eV at room temperature. Also, doping with rare earth elements (Sm3 , Ce3 , Y3 , La3 , Nd3 and Eu3 ) allows manipulation of the optical characteristics of ZnO and CdO materials. Properties ZnCdO solid alloys doped with Eu are practically not studied. In this work we present the results of investigation of {ZnCdO/ZnO}30 multiple quantum wells (MQWs) doped with Eu deposited on quartz and on Si substrates. The {ZnCdO/ZnO}30 quantum structures were doped with Eu using the plasma-assisted molecular beam epitaxy (PA-MBE) method. The difference in the Eu concentrations in the samples was obtained by different temperatures of the europium effusion cell, as well as in the localization of Eu (in barriers or in the whole quantum structure). The obtained MQWs were annealing at various temperatures (700 and 900°C) in an O2 atmosphere for 5 minutes. Analysis of the depth composition and profile of elements (Cd, Eu, Zn and O) in MQWs was carried out by secondary ion mass spectrometry (SIMS) and the concentration of Eu before and after annealing was estimated. The distribution of Eu after annealing is uniform for all {ZnCdO/ZnO}30 MQWs, but its concentration is reduce. To study the effect of temperature on the band gap, the transmission spectra of as grown and annealed at 900 oC MQWs doped with Eu deposited on quartz substrates were measured in the range of 10-290 K. The post-annealing effect has a great influence on the MQWs transmission spectra was noted. For as grown MQW with a higher Eu concentration (9,38*1018 atoms/cm3), the band gap is 3.322 eV at low temperatures, while for as grown sample with a lower Eu concentration (3.59*1016 atoms/cm3), the band gap is about 3.268 eV. Due to a reduction in the Cd and Eu concentration in the annealed samples in both MQWs energy band gap shift (to 3.314 eV and 3.298 eV, respectively) were observed. For all structures, a blue shift of the band gap with increasing temperature was observed. The temperature dependence of the optical band gap of as grown and annealed {ZnCdO/ZnO}30 MQWs doped with Eu was investigated using the empirical Varshni and the Bose-Einstein (B-E) equations. The obtained fitting parameters from the two models are quite close to each other, however, it was noted that the B-E model is better represented the entire temperature range. This work was supported in part by the Polish National Science Center, Grants No. 2019/35/B/ST8/01937, and 2021/41/B/ST5/00216.

Resume : The determination of absolute surface and interface energies of materials is one challenging issue in today’s epitaxial developments, as it fundamentally governs the nucleation and physical properties of heterogeneously integrated materials and devices for photonics, electronics or energy harvesting applications.[1] Especially, determining the hetero-interface absolute energies remain tricky, because of the difference of chemistry between the two materials considered. In this work, we use Density Functional Theory, with a fictitious hydrogen atoms charge compensation strategy, to determine the surface and interface absolute energies in the III-V/Si heterogeneous materials association case [1,2]. This method is expected to be applicable for all other heterogeneous materials association. We finally show how, in the specific case of III-V/Si heterogeneous epitaxy, it allows understanding structural properties of samples, and consider their use for applications in photonics, solar cells, or solar hydrogen production through water splitting [3,4]. This research was supported by the French National Research Agency PIANIST Project (Grant no. ANR-21-CE09-0020), and NUAGES Project (Grant no. ANR-21-CE24-0006). References: [1] I. Lucci et al., “Universal description of III-V/Si epitaxial growth processes” Physical Review Materials 2 (6), 060401, 2018 [2] I. Lucci et al., “A Stress-Free and Textured GaP Template on Silicon for Solar Water Splitting”, Advanced Functional Materials, 28(30):1801585, 2018. [3] M. Alqahtani et al., “Photoelectrochemical water oxidation of GaP1− xSbx with a direct band gap of 1.65 eV for full spectrum solar energy harvesting”, Sustainable Energy & Fuels 3, 2019. [4] L. Chen et al., Advanced Science 2022

Resume : Among ferroelectrics a new class of materials with high potentialities for spintronic applications has recently been introduced as ferroelectric Rashba semiconductors[1,2]. Main results, obtained on GeTe thin films, have demonstrated that the reversal of the ferroelectric polarization under an electric field leads to a consistent change of the spin chirality of the band structure[3,4]. An effective spin-to-charge conversion has also been demonstrated in a ferromagnetic-GeTe heterostructure[5-7] and a nonreciprocal charge transport up to room temperature has been detected[8]. However the influence of the domain structure on these phenomena still remains unclear. Given the rhombohedral structure of GeTe (R3m space group) and the existence of an electric dipole in the <111> direction, eight possible polar domain orientations are anticipated in this system. In spite of the growing interest in such ferroelectric Rashba semiconductors[9], the detailed polar domain structure and spatial organization has not been studied so far. These studies are a prerequisite for the controlled switching of ferroelectric domains and the understanding of aging properties. In this presentation we will address the ferroelectric nanodomains organization of GeTe thin films grown on Si(111)[10], the domain wall type, and the structure of the interface with the substrate. As reported by Wang et al.[11], quasi-single crystalline GeTe thin films can be grown on Si(111) by molecular beam epitaxy using a pre-deposition of 1 monolayer (ML) of Sb onto the substrate. It is an ideal platform to study and control ferroelectric domains as they are no more limited by grain boundaries. We have determined by X-ray diffraction (3D reciprocal space maps) in combination with low energy electron microscopy (LEEM) the volume fraction of the ferroelectric domains and the domains size in a large range of film thickness (10-1800nm). Second harmonic generation (SHG) microscopy combined to polarimetry analysis reveal the local symmetry of these domains. Using high resolution transmission electron microscopy (HR-TEM) we show that domain walls are only of 71° and that the GeTe/Si interface is stabilized by misfit dislocations that relax the large lattice parameter mismatch between both lattices. The reversible decay/growth of the ferroelectric nanodomains under annealing/cooling, as demonstrated by in situ LEEM, is attributed to the thermal stress induced by the large difference of linear thermal expansion coefficients of both materials. [1] D. Di Sante et al., Adv. Mater. 2013, 25, 509. [2] M. Liebmann et al., Adv. Mater., 2016, 28, 560. [3] C. Rinaldi et al., Nano Lett., 2018, 18, 2751. [4] J. Krempasky et al., Phys. Rev. X, 2018 8, 021067. [5] C. Rinaldi et al., APL Mater., 2016, 4, 032501. [6] J. Slawinska et al., Phys. Rev. B., 2019, 99, 075306. [7] S. Varotto et al., Nature Electron. 2021, 4, 740 [8] Y. Li et al., Nat. Commun., 2021, 12, 540 [9] S. Picozzi, Frontiers in Physics, 2014, 2, 10. [10] B. Croes et al., Phys. Rev. Mater., 2021, 5, 124415. [11] R. Wang et al., J. Phys. Chem. C, 2014 118, 29724. Acknowledgments: The authors thank funding from Excellence Initiative of Aix-Marseille University A*MIDEX, a french "Investissements d'Avenir" program through the AMUtech institute. This work has also been supported by the ANR grants HOLOLEEM (ANR-15-CE09-0012) and TOPELEC (ANR-18-CE92-0052)

Resume : Oxide semiconductors are promising candidates for optoelectronic applications because of advantages like large bandgap energy, high mechanical and chemical stability. Among the oxide semiconductors, group-II oxides have been increasing interest as they share similar features with group-III nitrides. CdO is one of the oldest known semiconductor oxides that have been studied widely because of its high electron concentration, high transparency, high electron mobility, low resistivity, and high exciton binding energy. However, a relatively low bandgap of CdO, which is 2.3 eV (at the ambient condition), along with two indirect band gaps of 1.2 eV (Σ3 and Γ1) and 0.8 eV (L3 and Γ1), restricts its use only in the visible wavelength range of applications. Development of advanced growth techniques and in situ characterization methods allowed not only to grow high quality CdO layers but also related ternary alloys (such as ZnCdO and CdMgO) which in turns can be used in many wide spectral range applications. Among many growth techniques used to grow CdO layers, plasma-assisted molecular beam epitaxy (PA-MBE) allows most precise control of growth parameters, such as growth rate, flux of Cd, which could be monitored by a variety of in situ characterization techniques such as flux monitor, laser reflectometry, and desorption mass spectroscopy. In this work we have investigated a series of CdO layers grown on m-plane sapphire substrate using PA-MBE technique. CdO layer on m-plane substrate have a strong preferential <110> orientation. During the growth process, the oxygen flow and power of oxygen plasma was fixed and Cd flux was changes and controlled by varying the temperature of Cd effusion cell. The growth rate was estimated in situ using laser reflectometry. The relation between Cd and O2 parameters during growth influences on the stoichiometry of the CdO layers. As a results we have a chance to analyze samples grown in O-rich and in Cd-rich condition as the Cd effusion cell temperature increases from 370ºC to 430ºC. The change in stoichiometry of CdO layers from O-rich to Cd-rich condition affects morphological, optical and electrical properties which is manifested in roughness parameter, optical bandgap and electrical conductivity of corresponding layers. Surface morphology of CdO layers were studied using scanning electron microscope (SEM) and atomic force microscope (AFM) techniques. The optical transmittance and bandgap of CdO layers were investigated using UV-Vis spectroscopy at room temperature. These results will provide importance of growth parameters of CdO layers during the growth process, which subsequently leads to more accurate control of quantum structures and CdMgO and ZnCdO ternary alloys towards new optoelectronic devices. The work was supported by the Polish NCN project DEC-2021/41/N/ST5/00812, and DEC-2021/41/B/ST5/00216.

Resume : High-quality crystalline semiconductor free-standing membranes (SFSM) of functional materials are of paramount importance for fundamental research and device applications. Compared to classical substrate-bound structures, SFSM form offers extra degrees of freedom for implementations that cannot be provided by traditional hetero-integration techniques. Furthermore, SFSM of various materials can be stacked, enabling easy coupling of physical properties between dissimilar materials. Additionally, the freestanding form makes these structures lightweight and flexible. Using SFSM for the device production instead of bulky wafers provides significant cost saving through multiple reuses of the same substrate, especially for non-silicon-based materials with orders of magnitude higher prices. Accordingly, new emerging techniques such as Van der Waals epitaxy [1] and remote epitaxy [2], using 2D materials (graphene, hexagonal boron nitride, etc.) as weak bonding interface, show huge potential for the fabrication of various SFSM. Nevertheless, those methods are still in the early stages of development, especially those targeting large-scale production, compared to other techniques such as nanostructured substrates [3,4] or epitaxial lift-off [5]. In this work, we demonstrate the formation of homogenous edge-to-edge porous Ge (PGe) layers on industry-standard 100 mm wafer-scale by bipolar electrochemical etching with the ability to modulate the corresponding thickness and porosity. These PGe nanostructures maintain high material purity and substrate crystal orientation with excellent surface conditions suitable for epitaxial growth. Moreover, we study the initial stages of epitaxial growth on porous substrate. Morphological evolution shows the transition from 3D nucleation on nanostructured substrate towards the coalescence of the layer and continuous 2D growth. The growth of ultra-thin Ge layers on top of PGe substrate is demonstrated. This Ge epilayer is easily detachable due to the nanostructured PGe interface. Our findings lay the groundwork for the wafer scale production of free-standing monocrystalline Ge membranes for applications in light and flexible optoelectronics. [1] Y. Kobayashi et al., Nature 484, 223?227 (2012) [2] Kim, Y., et al., Nature 544, 340?343 (2017) [3] Park et al., Joule, vol. 3, p. 1782?1793 (2019) [4] A. Boucherif et al., Appl. Phys. Lett., vol. 102, p. 011915, no. 1 (2013) [5] A. van Geelen et al., Mater. Sci. Eng. B, vol. 45, (1997)

Resume : To deposit Mo2C and W2C coatings onto different non-oriented metal substrates, electrolysis of Na2WO4-based oxide melts was applied. In such a case, it is necessary to obtain continuous cathode deposits with preset properties (structure, orientation, and crystallite size). During electrolysis, an important role is played by the initial stages of crystal nucleation. The initial stages of crystal nucleation were studied by the electrochemical methods used in the study of phase formation with simultaneous investigation of the microstructure of the substrate surface and electrolysis products. The initial stages of the nucleation of Mo2C and W2C crystals in tungstate-molybdate-carbonate melts of a certain composition on Ag, Au, Cu, Pt, and Ni substrates were studied by electrodeposition method depending on the electro-crystallization conditions: temperature, deposition duration, initial current pulse, and current density. It was found that the first crystals start growing immediately after the appearance of the peak on the switching curves. The repeated switching-on the current within a short period of time (5–10 s) does not give rise to peaks formation. These facts indicate that the crystallization overvoltage is associated with three-dimensional nucleation. From the experimental results, the crystallization overvoltage values were evaluated appearing due to a considerable energy loss for synthesis components nucleation at the first moments of synthesis. These values at the Ag electrodes within the temperature range 973–1023 K are 8–40 mV. During the metal carbides deposition onto the same metals substrates was not accompanied by crystallization overvoltage. This overvoltage took place only at low values for metals with high exchange current. Under these conditions, the surface diffusion stage does indeed limit the electrode process rate. With an increase of the carbide deposition rate, the number of crystallization centers increases, reducing decelerations caused by surface diffusion. As a result, with higher overvoltages, the limiting stage is seemingly changed so that the process rate is determined either by the electron transfer rate or by the rate of diffusion from the melt volume. It is characteristic that the height of the overvoltage maximum peaks for the abovementioned metals is proportional to the reciprocal time of their formation. This seems to be associated with the penetration of the part of the deposited components into the substrate bulk due to solid-phase diffusion and allows to qualitatively characterize the degree of inertness of the substrate material. The experimental study of the initial stages of Mo2C electrocrystallization from the tungstate–molybdate–carbonate melts with the electrodes made from various materials over a wide temperature range allows to put forward the following concepts of nucleation. Thus, for example, with the inert substrates at T < 1073–1173 K, considerable crystallization hindrances were observed associated with the formation of three-dimensional nuclei. An increase in the electrolysis temperature facilitates diffusion of the components atoms into the substrate resulting in a decrease of the crystallization overvoltage. Simultaneously, a transition from the three- to two-dimensional nucleation is observed and, in some cases, to depolarization phenomena due to solid-phase saturation of the electrode boundary layers with the synthesis components (molybdenum and carbon) and to the formation of an alloy with the electrode material.

Resume : QCLs (quantum cascade lasers), InGaAs 1.7 and ext-PDs (extended InGaAs photodetectors) are very attractive materials for use in sophisticated photonics and microelectronics such as optical recording, scanners, laser spectroscopy, thermal or gas sensing. Once a novel laboratory curiosity, QCLs and ex-PDs are now becoming commercially available products, shaping new and exciting applications. Optical wireless power transmission (OWPT) is another exciting field. Photovoltaic devices able to convert not only sunlight but also laser power offer new promising technological capabilities. These Optical Power Converters (OPCs), also called Laser Power Converters (LPCs), have been used for power links and radio-over-fiber, long-distance and safe laser power beaming, power electronics, and other applications. In this work we present the intricacies and challenges of the development and industrial production of MOCVD-grown QCLs, VCSELs, LW-VCSELs, PVCs (solar cells and lasers) EELs (edge emitting lasers for use in telecommunication) and ext-PD structures, fabricated in Vigo Photonics. Different types of epitaxial structures were grown using AIX 2800 G4 production system, in 12 x 3” or 4” wafer configuration per growth run. Structural and electrical properties of epitaxial layers were examined using X-ray diffraction (XRD), high resolution secondary ion mass spectroscopy (HRSIMS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), electrochemical capacitance-voltage technique (ECV), mapping of photoluminescence (PL) and others. AIX 2800 G4 production reactor, whose processes are characterized by highly complicated chemical reactions, enables development of sophisticated epitaxial structures. In multi-wafer reactor chambers, gas residence is much longer when compared with small R&D systems and the growth zone is much larger. As a consequence, in case of QCLs, the hetero-interfaces of InGaAs/InAlAs are inevitably graded, which negatively impacts optical and electrical parameters of QCL devices. In addition, QCL structure requires the exact layer thickness as well as absolute precision and repeatability in the development of strained InAlAs and InGaAs epi-layers. In case of ext-PDs fabrication, the crucial thing is to develop a specially designed and graded buffer layer and try to avoid numerous misfit defects and anomalies in dark current. The challenge in OWPTs epitaxial layers development is to achieve not only higher conversion efficiency, but also higher output powers. We consistently improve MOCVD technology by enhancing capability and flexibility in terms of alloy composition, dopant, and thickness control, within-wafer uniformities and reproducibility, and by optimising thermal budget, which results in higher interface quality. We succeeded in obtaining high crystallographic quality and repeatability of the epi-layers, leading to QCL high optical power (>2W), ext-PDs high-performance operation with low dark current density (less than 1E-4 A/cm2 at -100mV/RT) and high quantum efficiency. VCSEL, LW-VCSEL and EEL lasers displayed high optical parameters due to nanometer-scale thickness control and optimization of strain and efficiency of quantum wells. Acknowledgements This research was supported by The Polish National Centre for Research and Development grant MAZOWSZE/0032/19-00.

Resume : High-quality monocrystalline materials and nanostructures are key to high-efficiency optoelectronic devices (Polman et al., Science, 2016), plasmonic materials (Wu et al., Adv Mater, 2014) and metasurfaces (Koenderink et al., Science 2015). A large variety of materials can be synthesized at low temperature in solution as colloidal single crystals, combining the advantages of high-quality material, low-cost fabrication, and potential for large area integration into nanophotonic and plasmonic devices, but are typically limited to Platonic solid shapes. We recently demonstrated that colloidal single-crystal gold nanocubes, could be used as building blocks for the fabrication of continuous monocrystalline nanostructures via liquid-phase epitaxy of adjacent nanocubes at near ambient temperature (Capitaine et al, Adv Mater., 2022). This consitutes a remarkable example that bridges the gap between bottom-up and top-down approaches. We demonstrate here that the strategy developed for gold nanocube epitaxy, can serve as a model for other noble metals, such as silver. We investigate similarities and differences in the epitaxy mechanism of single-crystal silver nanocubes by performing a set of ex-situ and operando optical experiments on a nanocube dimer model system and comparing the results to FDTD simulations. Monodisperse 40 and 50-nm Ag single-crystal nanocubes are first synthesized in solution by polyol synthesis. Next, using capillary forces they are self-assembled in a polydimethylsiloxane (PDMS) mould containing pre-defined traps, with individual nanocube subunits lying face-to-face in a dimer configuration. Then nanocubes are epitaxially connected and transformed into continuous monocrystalline at low temperature by inducing an equilibrium between nanocube etching and growth in an aqueous acidic medium in the presence of a silver salt. The insights gained via this model system about the chemical framework for nanocube epitaxy is then exploited to control the epitaxial welding of larger complex structures such as curved lines assembled by capillary forces and closely-packed Langmuir-Blodgett films on PDMS for the first time, showing that this unique approach is highly versatile for the fabrication of monocrystalline silver nanostructures. We use high-resolution transmission electron microscopy and electron backscatter diffraction to provide evidence for epitaxy and study the impact of nanocube misalignment. Finally, we show that such nanostructures and thin films can be transferred from PDMS to various substrate by contact printing. This paves the way for swift integration into optoelectronic devices of high-quality monocrystalline silver mirrors, contacts and/or large-scale nanophotonic surfaces. This is particularly relevant for devices that are incompatible with classic nanopatterning strategies such as perovskite layers.

Resume : Ferroelectric materials can be used as a negative capacitance element at the gate of a field effect transistor with the purpose of decreasing the fundamental limit of subthreshold swing of 60mV/decade. Many investigations have been performed in the last years for understanding both NC dynamic regime during polarization switching but also the possibility of stabilization of a steady-state NC in FE multilayers and superlattices. The present study evaluates switching characteristics of the FE/isolator/FE multilayered thin films structures exhibiting multiple polarization states. Ferroelectric PZT thin films are used intercalated by SrTiO3 or BaTiO3 isolator layers. All structures are epitaxial grown by pulsed laser deposition and the structural quality is investigated by X-ray diffraction and TEM microscopy. The negative capacitance dynamic regime is investigated during atypical step-like switching and we also evaluate the possibility to stabilize a negative capacitance steady state in the intermediate polarization states. The experimental investigations were doubled by numerical calculations using density functional theory (DFT). In order to simplify the problem, the PbTiO3 ferroelectric material was chosen and the PTO/STO/PTO multilayer structure was constructed. In this configuration the individual polarization of each layer was prepared in several arrangements: both polarizations having the same direction (representing the polarization saturation case) or both polarizations pointing towards each other or away from each other (representing the intermediate polarization state). The structures were structurally relaxed and it was evidenced that for the intermediate state one of the layers evolved towards an almost centrosymmetric state with a near zero polarization. This is a finger print of a steady-state negative capacitance state.

Resume : Surface X-ray diffraction (SXRD) is a powerful structural probe for epitaxial thin films. Sb(0001) ultrathin films in tensile strain have been grown by molecular beam epitaxy on InAs(111)B substrates and their detailed atomic structures derived using SXRD. Features considered in the structural modelling include interfacial intermixing, surface roughness, individual layer relaxations, and rotational twin domains. A four-bilayer film showed significant structural relaxation in every layer, while a 19-bilayer films showed consistent "bulk" relaxation with strongly modified relaxations in layers near both the surface and interface. Both films included rotational twin domains and interfacial mixing over <3 atomic layers depended on the growth history.

Resume : Molecular beam epitaxy (MBE) has been used to grow several topologically nontrivial materials involving In, Bi, Sb, Sr and Mn. Analysis of the films using X-ray photoelectron spectroscopy (XPS) provides valuable information towards growth optimisation. The results of in situ XPS (in a chamber connected to the MBE system), XPS via vacuum suitcase transfer, and XPS via air transfer will be discussed. For Mn- and Sr-containing materials, their very high sensitivity to surface contamination strongly favours in situ or vacuum suitcase transfer.

Resume : The geometrical strain engineering of magnetic perovskite thin films enables rich magnetic properties that are yet to be fully explored. One route to obtain these complex magnetic thin films is to use substrates with different surface geometries and anisotropic strain states. (111)-orientated SrTiO3 used in this work has hexagonal surface geometry that provides magnetic thin films with unique anisotropic strain and high oxygen octahedral connectivity compared to STO (001), in which the magnetic states are highly tuneable by external influences such as temperature and magnetic fields. The magnetic domain configurations in addition to structural information can be imaged by Scanning transmission electron microscopy-differential phase contrast (STEM-DPC) technique directly and can be coupled with in-situ temperature and field control. A prerequisite for this technique is the need for high quality plan-view magnetic samples over a large area. La0.7Sr0.3MnO3 (LSMO) is a widely reported room temperature ferromagnet, making the material ideal for in-situ STEM-DPC study. In this work we use LSMO epitaxial grown by pulsed laser deposition on STO (111) substrate as a model system for in-situ STEM-DPC study. Thin film characterisation reveals by layer growth by RHEED, and fully single crystalline epitaxy confirm with XRD. The sample was also characterised by vibrating sample magnetometer confirming the presence of room-temperature ferromagnetism. The TEM plan-view sample was prepared by both conventional tripod polish in combination with focused ion beam (FIB) and solely FIB. A comparison is made on different methods in term of preparation difficulties and sample quality. The conventional tripod polish utilises a wedge polish method with final thinning in FIB. Meanwhile, for solely FIB preparation the bulk sample uses a conventional lift-out method, but optimised for a larger surface area. The bulk is then mounted to a TEM grid holder on pin, where the lift-out grid is lying flat instead of upright on the holder and thinned down to electron transparent. The use of alignment marks enables one to perform precision mounting to the holder and corrections in the tilt during thinning. Using STEM-DPC we correlate structure and magnetic properties, and the effect of the different sample preparation routines.

Resume : GaAsBi/GaAs multiple quantum wells (MQW) are promising candidate to be used as an active area in near infrared GaAs-based lasers or LEDs due to unique properties, such as, fast bismuth incorporation induced band gap reduction, increase in spin-orbit splitting and less temperature sensitive band gap. However, the growth of high optical quality GaAsBi quantum well (QW) layers is still a challenge. One of the methods to grow high quality GaAsBi MQWs is two-substrate-temperature molecular beam epitaxy (MBE) technique. In this work five periods of GaAsBi/GaAs MQW grown by molecular beam epitaxy using conventional single-substrate temperature (SST) and two-substrate-temperatures (TST) techniques are compared. In the SST mode, the substrate temperature was set to 370 °C for growth of both GaAsBi QW and GaAs barrier layers. In the TST growth mode, the substrate temperature was set to 370 °C for GaAsBi layers and higher temperature of about 450 °C - for the growth of GaAs layers. The structural and optical characterisation of GaAsBi/GaAs MQW was carried out. X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) indicated higher structural quality of GaAsBi/GaAs MQW structures grown using SST technique. The intensities of photoluminescence (PL) at room temperature were comparable for both MQW structures. However, the temperature-dependent PL measurements revealed that optical quality of MQW structures grown using TST technique is higher than MQW structures grown using conventional one-temperature MBE growth. PL peak position and full-width-at-half-maximum (FWHM) variation with temperature showed high carrier localisation of GaAsBi/GaAs MQW structure grown using SST technique. Moreover, the much faster PL intensity quenching, especially at low temperatures, of MQW grown using SST mode in comparison to TST-grown MQW was observed. High room temperature PL intensity of SST-grown GaAsBi/GaAs MQW was explained by decreased photoexcited carrier mobility due to localisation effect. The estimated scale of randomly distributed field fluctuations for SST grown MQWs was around 25 meV, comparable with thermal energy of carriers at room temperature.

Resume : Mn5Ge3(001)/Ge(111) epitaxial films (hexagonal D88 structure, space group P63/mcm) attracted a lot of interest as a potential source of polarized carriers into Ge, making them directly compatible with the mainstream silicon technology. This material reveals also a strong uniaxial magnetocrystalline anisotropy creating the opportunity to combine spintronics with the data storage. For practical applications it is interesting to add some carbon admixture, which boosts the Curie temperature of the Mn5Ge3Cx films from 296 K for x=0 up to 445 K for x=0.5. Unfortunately, doping with carbon leads to a significant drop of magnetocrystalline anisotropy constant Ku. The ferromagnetic resonance study performed at 15 K reveals the change of Ku from 5,97 ∙ 10^5 J/m3 in Mn5Ge3 down to 8,06 ∙ 10^4 J/m3 in the Mn5Ge3C0.5 film. To investigate the microscopic origin of this spectacular drop of magnetocrystalline anisotropy we performed the 55Mn NMR study in the entire carbon concentration range. Our previous NMR study performed on the 300 nm film with x=0.2 [1] has shown that carbon enters interstitially in the vicinity of the (6g) crystallographic positions, occupying the (2b) octahedral voids and reducing the magnetic moment of Mn atoms in the corners of a host octahedron by 0.7μB. The orbital moment anisotropy of (6g) Mn - measured as a difference between its value along the hexagonal c-direction and in the c-plane - dropped from 0.151μB for x=0 down to 0.058μB for x= 0.2. This change was postulated to be at the origin of the reduced magnetocrystalline anisotropy of the Mn5Ge3C0.2 film [1]. In this study we extend the 55Mn NMR study to a full carbon concentration range. We find that the hyperfine field anisotropy in the (6g) Mn sites continues to drop as a linear function of the carbon content. These results show that the main mechanism responsible for the observed drop of the uniaxial magnetocrystalline anisotropy upon carbon doping consists in a rapid increase of the carbon-affected Mn sites, prompted by the highly ordered carbon incorporation into the Mn5Ge3 crystal lattice [2]. They also confirm the scenario of a single ion origin of magnetocrystalline anisotropy in these compounds. References: [1] R. Kalvig, E. Jędryka, M. Wójcik, G. Allodi, R. De Renzi, M. Petit and L. Michez, Phys. Rev. B 97, 174428 (2018) [2] R.Kalvig, E. Jędryka, M. Wójcik, M. Petit and L. Michez, Phys. Rev. B 105, 094405 (2022)

Resume : Sybilla equipment is addressing many of the challenges faced by the very promising and expanding field of oxide thin film deposition. Chemical Beam Epitaxy (CBE) enables deposition of multi-element oxides (up to 5 possible), either homogenously or in combinatorial mode (i.e with controlled precursor flow gradient emitted onto the substrate, in good agreement with theoretical model predictions). High homogeneity films +/-1% can be achieved, even on large substrates (450 mm wafer will be shown). Precursor decomposition can be initiated either thermally (substrate heating) or by irradiating with energetic beams (laser and electron activations were studied). Additive growth can be obtained by such localized irradiation, or alternatively depositing through shadow masks and benefiting from the line-of-sight nature of the technique (and exploiting the precursor decomposition kinetics not to damage masks). The multi-parameter nature of the deposition technology (precursor nature, different flows, temperature) allows to tune growth rate (from few nm/h to several um/h) as well as thin film physico-chemical properties (chemical composition, film morphology, crystallinity, etc.) and functional properties. Combinatorial growth reveals a very efficient facility to optimize processes (in one shot, saving time and resources) and address new thin film architectures. Some epitaxially grown materials such as TiO2 and LiNbO3 will be presented.

Resume : Metal Organic Vapor Phase Epitaxy (MOVPE) is a well-established tool for producing high quality epitaxial films for many optoelectronic device components. Despite the extensive technological use of MOVPE, there are still a large number of unresolved issues. One of the major outstanding technological issues, linked to fundamental aspects of epitaxial growth processes, is the ?leakage? of p-type dopant Zn into surrounding III-V regions during the growth process. The lack of controllability of Zn incorporation and diffusion processes in III-V layers hinders the possibility of developing all III-V integration platforms, with multi-device stacking, or just even reliable simple inverted lasers with a p-i-n structure instead of the usual n-i-p design. Here, we report a finding on Zn diffusion, which remained hidden despite around 40 years of research on the topic. Zn is reported to have a relatively high diffusivity in InP at temperatures greater than 500°C, i.e. typical process temperatures of MOVPE. The diffusion of Zn has been heavily studied, especially looking at back-diffusion (opposite to growth direction), but remarkably studies in the growth direction are either lacking/few or the issue is addressed as part of more complex interplayed structures. To examine the Zn dopant behaviour in the growth direction, we grew a very simple structure: 200 nm Zn doped InP layer sandwiched between two undoped layers. Secondary Ion Mass Spectroscopy (SIMS) results show that Zn exhibits an asymmetric dopant concentration profile and appears for an unexpected ~1500 nm into the undoped layer in the growth direction. The diffusion in the opposite of the growth direction is more modest and can be accurately explained by well-known diffusion laws. However, in the growth direction the concentration profile is significantly different from what would be expected from diffusion processes alone. A comparative experiment with GaAs-based materials shows similar, but less extensive post growth tail, demonstrating also that the carry-over of Zn in InP is not a reactor memory effect, but is a material specific phenomenon. We have developed a model combining Fick?s second law of diffusion and material incorporation mechanisms together to understand Zn concentration profiles in InP layers. The tailing in the growth direction is modelled based on the assumption that either Zn (or the precursor DEZn) acts as a surfactant accumulating on the surface layer and gradually continues to incorporate into InP after the Zn source has been shut off. We used the Crank-Nicholson algorithm to numerically calculate this behavior with a finite difference approach which requires a set of boundary conditions which dynamically evolve to mimic the accumulation of the materials on the sample surface. The dopant concentration depth profiles that we have simulated with this method agree very well with the SIMS results. We will discuss in detail our experimental results and simulations, and our unique solution to the Zn diffusion problem. This will include Zn diffusion and accumulation behavior in a variety of other III-V materials, and a discussion of major consequences for III-V device design. The results of this work can enable growth designs for novel inverted laser structures and multicomponent devices, and broadly eliminate existing constraints while opening for new technological solutions.

Resume : Ferroelectric materials with perovskite structure, with ABO3 formula, are used in a wide variety of applications due to their multiple functional properties including the spontaneous polarization that can be manipulated by various stimuli: electric fields, temperature, mechanical stress and light. Ferroelectric layers are used in many electronic and sensing applications, such as ferroelectric field effect transistors, negative capacitance field effect transistors or pyroelectric infrared detectors. Here we present effects induced by doping on A or B positions of epitaxial Pb(Zr,Ti)O3 (PZT) thin films with Zr/Ti ratio of 20/80. This material is widely used due to the large value of polarization, reaching about 1C/m2, and to the fact that high quality epitaxial films can be obtained on single crystal SrTiO3 substrates by using the pulsed laser deposition (PLD) method.Th e PZT films were deposited by PLD from in-house produced targets using precursor metal oxides of at least 99.99 % purity. In order to obtain n doping, the target was doped with about 1 % Nb, Bi or La and for p doping 1 % Fe or K was introduced. The choice of doping species was not random but was guided by DFT calculations. We studied the effect of doping on the PbTiO3 structure (we chose PTO in order to simplify the numerical problem) using a large super cell obtained from 3x3x3 PTO unit-cells. A single Ti or Pb atom was replaced by either Nb, Bi, La, Fe or K, to obtain approximately 3.7% doping concentration. First we deposit thin films from doped targets to compare to that obtained from un-doped target. Then to produce bi-layer structures by successive deposition of one n-type PZT layer and one p-type PZT layer, with the aim to obtain a p-n ferroelectric homojunction. It was found that these p-n structures present polarization hysteresis and butterfly-shaped capacitance-voltage (C-V) characteristics, as for a ferroelectric capacitor, although the current-voltage (I-V) characteristics of the bi-layer structures are symmetric and linear as for a resistor. Moreover, it was found that the capacitance of the bi-layer structure is larger than that of the component layers. This is the signature of a stabilized negative capacitance effect. In summary, a relatively low doping concentrations can trigger significant differences in the macroscopic properties of epitaxial PZT films, although the structural properties are similar. The differences can be explained by different electronic properties induced by doping that acting as acceptor, and or doping that acting as donors. The p-type doping or n-type doping are evidenced by the special poling procedure used in PFM investigations. Results open new perspective in obtaining genuinely ferroelectric p-n homojunctions.

Resume : A number of previous publications reveal that many composite semiconductor oxides have better gas sensitivity than a single semiconductor metal oxide. Both titanium oxide (TiO2) and zinc oxide (ZnO:Al) are wide band gap n-type semiconductors, their band gap energies are similar to each other (about 3.2-3.4 eV), and they both possess good gas sensing properties. In this paper, the TiO2:Nb thin films were reactively sputtered on glass and ZnO:Al/glass substrates, with the resistivity of 1.7 × 10− 4 Ω⋅cm, an transmittance of about 90%, mobility of 25 cm2/V∙s and a thickness of 400 nm. TiO2:Nb thin films were prepared at different temperature by RF magnetron sputtering at 100 W RF power. The distance from sample to the target was 6–7 cm. The working pressure was maintained at 4 × 103 Pa. The crystallographic structures of the TiO2:Nb thin films were studied via X-ray diffraction) method. Before annealing in vacuum the TiO2:Nb thin films show both polycrystalline and amorphous phases. Annealing in vacuum atmosphere showed substantial improvement of structural properties of TiO2:Nb thin films. The EDX spectra indicate the presence of Ti, Zn, and O elements. The Ti:Zn atomic contents in the thin film depends of the substrate temperature. The gas sensing properties are characterized in term of resistance and gas-sensing response. The gas sensing properties of ZnO/TiO2 RF composite thin films toward acetone and ethanol will be discussed in function of the substrate temperature.

Resume : Recent research regarding Boron Nitride (BN) layer growth focuses on the fabrication of the BN films with sp2 orbital hybridization, in particular hexagonal (h-BN) or rhombohedral (r-BN) polytypes. This is because they can be easily used as substrates for other 2D materials [1] or in flexible electronics [2]. Fabrication of such films using Metal-Organic Chemical Vapor Deposition (MOCVD) requires growth temperatures as high as possible in order to reach the transport-limited growth regime [3]. However, layers grown in the kinetic-limited regime also have interesting properties. BN layers grown at low temperatures exhibit a wide range of refractive index values [4]. Moreover, low growth temperatures allow for incorporating carbon atoms into BN films. This allows tuning the bandgap of the fabricated films in the range of 0.5-4.0 eV [5]. However, fabricating BN layers with very high carbon concentration remains a challenge. One of the key parameters in the growth of BN-based layers via MOCVD is the flow ratio of the nitrogen source (typically ammonia) to the boron source (typically triethylborane or trimethylborane) [4]. In this communication, we show that for layers grown at temperatures below 1200 ºC, we are able to switch between the growth of two kinds of layers with radically different optical and electrical properties by altering only the ammonia flow rate through the reactor chamber. For ammonia flow rates comparable to the triethylborane (TEB) flow, we were able to fabricate porous and transparent BN layers with low carbon concentrations and therefore high resistance values of MΩ or greater on a 1×1 cm2 sample. For ammonia flow rates in high excess (up to 50 times) of the TEB flow rates, we were able to fabricate nontransparent, highly absorptive layers with very high carbon concentration and resistance values in the range of single kΩ or less on a 1×1 cm2 sample. Moreover, these two types of layers exhibit radically different refractive index values. In the case of the transparent BN films, the effective refractive index values are lower than the refractive index of the bulk BN [6], while for the non-transparent films, the refractive index values are much higher. The causes of these differences as well as possible applications will also be discussed. This research was funded by the National Science Centre, grants no. 2019/33/B/ST5/02766 and 2020/39/D/ST7/02811. [1] K. Ludwiczak et al., ACS Appl. Mater. Interfaces 13, 47904 (2021) [2] J. Iwański et al., Acta Phys. Pol. 4, 139 (2021) [3] A. K. Dąbrowska et al., 2D Mater. 8, 015017 (2020) [4] K. Pakuła, et al., arXiv:1906.05319 (2019) [5] W. Xie et al., Journal of Carbon Research 2, 2 (2016) [6] A. Segura et al., Phys. Rev. Mater. 2, 024001 (2018)

Resume : INTRODUCTION Modern motion sensing solutions require encoders, devices that can ensure detection of small position variations and speed changes in different metrology instruments and other high precision machinery. Today, optical scales are considered to be one of the most accurate encoders, however it lacks versatility when operating near dew point, where fogging mechanism can hinder its applications due to signal disruption. To overcome such obstacle, optical transparency could be spared by developing an anti-fogging, scratch-resistant hydrophilic diamond-like carbon (DLC) films and nanocomposites. Therefore, in the present research, diamond-like nanocomposites containing SiOx and DLC carbon films doped by nitrogen were grown and studied. EXPERIMENTAL/THEORETICAL STUDY In this study, diamond-like nanocomposite film containing SiOx deposition was carried out using an anode layer ion source from the hexamethyldisiloxane vapor and hydrogen gas mixture. DLC films were doped by nitrogen and grown using two different methods. Those methods were graphite target reactive magnetron sputtering and inductively coupled plasma (ICP) plasma beam-assisted magnetron sputtering. For structure and morphology, investigations were carried out using Raman scattering spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). In addition, water contact angle of the samples was studied. RESULTS AND DISCUSSION In all cases, DLC films containing SiOx exhibited hydrophobic behavior, though post-treatment using oxygen plasma, allowed to reduce the wetting angle to 1o. This can be related to OH group formation on the surface of our samples. The growth dynamics of non-hydrogenated DLC films doped with nitrogen (DTAD: N) were studied in terms of nitrogen flow and time. When grown on the glass using 25 sccm nitrogen flow, the wetting angle of the DTAD:N films have decreased to 18o over the course of 10 min. duration of the measurement. These results were further explained from the chemical composition observations. No correlation between the contact angle, structure and roughness of our films was found. CONCLUSION In conclusion, hydrophobic behavior of fabricated DLC:SiOx films was changed to superhydrophilic, using oxygen plasma post-treatment. This was explained by the formation of -OH functional groups. Decreased contact angle was observed in hydrogen-free diamond-like carbon (DLC) films doped with nitrogen. Changes in chemical composition explain these observed results. ACKNOWLEDGMENTS This research was supported by the European Social Fund under the Measure 01.2.2-LMT-K-718 ‘Targeted Research in Smart Specialisation Areas’ (project No 01.2.2-LMT-K-718-03-0058).

Resume : Recently, graphene and two-dimensional (2D) transition metal dichalcogenides (TMDCs with general formula MX2, where M represents transition metal and X represents chalcogen) open a great potential for direct integration with silicon (Si) technology, due to their outstanding chemical, physical, electronic, and optical properties. Here, we report the synthesis details of graphene and 2D layers using bottom-up techniques, their characterizations, and potential applications. In term of graphene, single and multilayers were synthesized using Chemical Vapour Deposition (CVD) method and transferred on varieties of rigid and flexible substrates, analyzed by Raman spectroscopy, optical and electrical measurements. For TMDCs layers (as PtSe2, WSe2) we used an atmospheric pressure thermally assisted conversion (TAC) method of pre-deposited by Magnetron Sputtering technique metal layers. The existence of 2D PtSe2 and WSe2 layers were confirmed using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy analysis. In addition, Optical microscopy (OM) and atomic force microscopy (AFM) mapping of WSe2 revealed the formation of isolated flakes with different shapes, mainly concentrated near the substrate’s edges, which tended to form clusters and to further overlap to continuous layers. Performed photoluminescence measurements confirmed the existence of atomically thin flakes and 2H-WSe2 continuous layers. Spectroscopic ellipsometry were used to determine the thicknesses. Based on optical and electrical measurements, graphene and selected TMDCs nanolayers has been proposed as supporting conducive electrodes for tunable liquid crystal (LC) phase retarders. The functionality of the assembled LC devices was proved by electro-optical measurements such as threshold and saturation voltage and the phase retardation. Moreover, resistor-like behavior of PtSe2 and WSe2 layers suggested their unlimited prospects for integration into a variety of heterostructures. Acknowledgments The authors acknowledge the COST Action CA20116 “European Network for Innovative and Advanced Epitaxy”, OPERA. V. M. acknowledges the financial support by the European Regional Development Fund within the Operational Programme ‘Science and Education for Smart Growth 2014–2020’ under the Project CoE ‘National Center of Mechatronics and Clean Technologies’ BG05M2OP001-1.001-0008-C01 and Bulgarian Science Fund under the project number КП-06-Н-58/12.

Resume : Self-assembled Network of Nanostructures in Functional Oxide Thin Films Z. Konstantinovic1, V. Fuentes2, Ll. Balcells2, A. Pomar2 and B. Martinez2 1 Center for Solid State Physics and New Materials, Institute of Physics Belgrade, University of Belgrade, Serbia 2 Institut de Ciencia de Materials de Barcelona, ICMAB-CSIC, Campus UAB, 08193 Bellaterra, Spain. Complex oxide thin films are often elastically strained, due to film-substrate lattice mismatch, and this lattice strain can, in some cases, select preferential growth modes leading to the appearance of different self-organized nanostructured morphologies. This behaviour offers enormous potential for the implementation of new nanodevices while, at the same time, attracts great attention due to their rich physics. Among other complex oxides of interest, strongly correlated systems, such as manganese perovskites with colossal magnetoresistance and half metallic characteristics, environment friendly multiferroics and/or spin-orbit coupling perovskite, have emerged as promising candidates for miniature spintronic devices. In this work we report on the controlled fabrication of self-assembled networks of nanostructures in various functional oxide thin films. In highly epitaxial La2/3Sr1/3MnO3 thin films, all nano-objects form long-range ordered arrays running in the steps’ direction defined by the miscut angle of underlying substrate [1]. A very similar behavior is observed in multifferoic BiFeO3 thin films [2], as well as in SrIrO3 system that exhibits a strong spin-orbit coupling [3]. It is shown that when no structural relaxation exits, self-organization process is directly promoted by the topological features of the substrate. Common features of the spontaneous formation of self-organized nanostructure formation in various systems are summarized considering the impact of kinetic growth effects and the presence of the lattice strain. The influence of the presence of such regular ordered nanostructured arrays on the functional properties is also discussed. Finally, the growth of self-assembled nanoparticles on top of nanostructured antidots arrays is explored [4]. [1] Z. Konstantinović, J. Santiso, Ll. Balcells and B. Martínez, Small 5, 265 (2009) [2] B. Colson, V. Fuentes, Z. Konstantinović, D. Colson, A. Forget, N. Lazarević, M. Šćepanović, Z.V. Popović, C. Frontera, Ll. Balcells, B. Martínez, A. Pomar, Journal of Magn. Magn. Mat 509, 166898 (2020). [3] V. Fuentes, Z. Konstantinović, Ll. Balcells, B. Martínez and A. Pomar under preparation [4] Z. Konstantinović, F. Sandiumenge, J. Santiso, Ll. Balcells and B. Martínez, Nanoscale 5, 1001 (2013).

Resume : The rapid improvement in materials growth and processing techniques has produced novel structures with unique functionalities. Electron microscopy plays a key role in understanding the structure of these novel materials and controlling them at an atomic level. The development of spherical aberration correctors for electromagnetic lenses established a major improvement in the new generation of electron microscopes. Using aberration-corrected scanning transmission electron microscopy (STEM), we analysed in detail the domain structure of PbTiO3/(La,Sr)MnO3 ferroelectric capacitors with ultra-thin active ferroelectric layers. Annular Bright Field (ABF) imaging was used to directly visualise both the heavy and light elements, measuring their relative displacement and dipole distribution, unit cell by unit cell. For a given system with a set lattice strain, the depolarization field becomes relatively large at smaller film thicknesses. The polarisation maps of different PbTiO3 thin films revealed a clear influence of the asymmetric screening of the depolarizing field on the equilibrium domain pattern. The dipole distribution reveals an evolution from conventional 180° Kittel-type domains to flux closure and vortex-type domain configurations with reduced film thickness. Additionally, effects such as polarisation and octahedral tilt suppression can be observed local to the interface. Supported by DFT calculations, we show how the different polarisation orientations interact with the LSMO at the interface. This provides insight into how such devices may be designed and tuned to achieve the desired performance. Additionally, ferroics can form vortices and skyrmions under particular boundary conditions. Here, we focus on the formation of modulated ferroelectric vortices in PbTiO3 sandwiched between two SrRuO3 electrodes on SrTiO3-buffered DyScO3 (110) substrate. The local structure is resolved using a combination of STEM and conventional TEM. Two orthogonal modulations were revealed. This incommensurate polar crystal may be an equivalent to the incommensurate spin crystals found in ferromagnetic materials. 1. Pantel, D., Goetze, S., Hesse, D. & Alexe, M. Reversible electrical switching of spin polarization in multiferroic tunnel junctions. Nat. Mater. 11, 289–293 (2012). 2. Stengel, M., Vanderbilt, D. & Spaldin, N. A. Enhancement of ferroelectricity at metal–oxide interfaces. Nat. Mater. 8, 392–397 (2009). 3. Peters, J. J. P., Apachitei, G., Beanland, R., Alexe, M. & Sanchez, A. M. Polarization curling and flux closures in multiferroic tunnel junctions. Nat. Commun. 7, 13484 (2016). 4. Rusu, D., Peters, J. J. P., Hase T. P. A., Gott J. A., Nisbet G.A.A., Strempfer J., Haskel D., Seddon S.D., Beanland R., Sanchez, A. M & Alexe M. Ferroelectric incommensurate spin crystals. Nature 602, 240–244 (2022)

Resume : Recently, transparent conducting oxide (TCO) thin films have attracted considerable attention because of their simultaneously high transparency in the visible spectrum and low resistivity. In the TCO family II−VI oxides ternary alloys have attracted considerable interest of the scientific community due to the possibility of modulating their interesting optoelectronic properties. Direct bandgap of 2.5 eV in CdO is reported, whereas in case of MgO the energy gap of 7.8 eV is observed. Thus in (CdO-ZnO-MgO) oxide system we will able to control energy gap in a wide range from yellow to deep UV (UVC) region. Alternate growth of MgO, CdO and ZnO thin layers (quasi ternary alloys) can enhance advantages and reduce shortcomings of the individual oxide films (CdO and ZnO and MgO) and random alloys for particular new applications. Short period superlattices {CdO/MgO}n and {CdO/ZnO}n was grown by PA-MBE on different substrates. The good quality of crystals was confirmed by High Resolution X-ray Diffraction and Transmission Electron Microscopy (TEM) techniques. X-ray diffraction confirmed cubic structure of the samples in case of CdO/MgO SLs and cubic-hexagonal structures in case of CdO/ZnO SLs. Reciprocal space maps were collected to analyze the strain in the SLs layers with the very high precision. Local lattice parameter were measured by digital processing of experimental HRTEM images. From TEM and small angle X-ray measurements thicknesses the growth rates of individual layers of MgO and CdO and ZnO have been extracted. The thicknesses of the CdO, ZnO and MgO sublayers influence the measured band gap. The direct band gap of {CdO/MgO} superlattice were tuned from 2.6 eV to 6 eV by varying the thickness of CdO from 1 to 12 monolayers while maintaining the same MgO layer thickness. Obtained values of direct and indirect band gaps are higher than those theoretically calculated by ab initio method still obtained data have the same trend. The presented CdO/ZnO short period superlattices have a good transparency and we are able to change the energy gap for this structures in the range from 2,68 to ~3,1 eV. This work shows that {CdO/MgO} and {CdO/ZnO} quasi alloys can be useful in developing new optoelectronic devices such as detectors for visible, and UV A, UV B and UV C region as well. This work was supported in part by the Polish National Science Center, Grants No. 2019/35/B/ST8/01937, and 2021/41/B/ST5/00216.

Resume : In this work, we report the influence of Mg and Oxygen contents on the structural and luminescence properties of Eu doped ZnMgO layers. ZnMgO:Eu thin layers are deposited on a-Al2O3 substrates by oxygen plasma assisted molecular beam epitaxy. The samples are grown at 550°C under two different experimental conditions; i) varying Mg fluxes from sample to sample while keeping the oxygen flow constant; ii) varying the oxygen flow while keeping all other parameters constant. EDX measurements are carried out to determine specific elemental composition. Compositional depth profiling is performed using Secondary Ion Mass spectrometry. Structural investigations are carried out using XRD and SEM techniques. Both magnesium and oxygen is found to have crucial impact on the luminescence properties of Europium as seen from the photoluminescence and cathodoluminescence analysis.

Resume : Nanomultiferroic composites are promising systems where the interfacial coupling between different ferroics can be driven by magnetic, voltage and strain mediation approaches to explore new technological applications [1]. Bulk FeRh alloy chemically ordered in the CsCl-type B2 phase, is characterized by a remarkable magneto-structural transition close to room temperature (~350 K) from an anti-ferromagnetic phase to a ferromagnetic one. Remarkably, we previously showed that in nanocrystallites (NC) as small as 5 nm, B2 phase is attainable through UHV high-temperature annealing, but depicts high magnetization down to 3 K [2], confirming the stabilization of intrinsic ferromagnetism in B2 FeRh NCs. Moreover as in nanoscale systems, the magnetic order of FeRh is very sensitive to interfaces, strain and surface terminations [2, 3], we have deposited such NCs on perovskite oxides substrates. Here, we focus on studying ultimately small nanomultiferroic systems (3 to 7nm FeRh NCs) on perovskite oxides, SrTiO3 (001) crystals (STO) and BaTiO3 (BTO) thin films. Material choices are based on structural compatibility, in a goal of using the coupling of strain and magnetic order to obtain voltage control over metamagnetic FeRh transition, as this was previously observed in hybrid multiferroic: 22 nm-thick FeRh film on BTO crystal [4]. We show that randomly deposited particles on BTO or STO adopt specific orientations after annealing. In addition to the usual epitaxy relationship [100]FeRh//[110]STO & [001]FeRh//[001]STO as expected for thin films, another orientation (45° rotation) has been observed, together with more complex geometries (particles lying on a {110} facet). For the later, a coincidence between {211}FeRh and {130}STO seems to be the driving force, and results in “satellite peaks” in some FeRh rings, around the main orientations. Nevertheless, important questions need to be discussed, such as the effects of reduction on the nanostructure. Indeed, we have observed that unprotected FeRh particles, which are then oxidized, can be reduced by UHV annealing, which can explain why epitaxy had been observed for annealed particles that subsequently appeared to be (partially) oxidized with XAS measurements at room or low temperature. Ref: [1] W. Eerenstein, N.D. Mathur, J.F. Scott, Nature, 442, pp. 759-765 (2006); N. A. Spaldin and R. Ramesh, Nature Mater.18, 203 (2019) [2] A. Hillion et al., Phys. Rev. Lett. 110, 087207 (2013) ; V. Dupuis et al. Beilstein J. Nanotechnol. 7, 1850 (2016) [3] Liu et al., Euro. Phys. Lett. 116, 27006 (2016) ; Lewis et al., J. Phys. D 49, 323002 (2016) [4] Cherifi et al., Nature Mater. 13 (2014) 345 ; Liu et al., Nature Comm. 7 (2016)

Resume : Ferroic materials may find application in several fields including ultrafast data reading, enhanced storage capacity, laser-assisted electronic data recording in ferroelectric memories, catalytic materials, and sensor devices. Nanopatterning metal oxides using sustainable methodologies may be a way to modify the properties of metal oxide thin films. In this work, it will be presented several examples in which the ordered thin layers were prepared with fine control of the porous structure and great homogeneity. Nanoporous thin films of lead titanate [1,2] and bismuth ferrite [3], as examples of oxides with perovskite and ferrite structures, were prepared by block-copolymer self-assembly (Figure 1). Films with different thicknesses and pore organizations were also prepared. The films were characterized in relation to the structure and morphology by X-rays diffraction, scanning electron microscopy and atomic force microscopy. The electric properties were measured at nanoscale by piezoelectric-response force microscopy and are compared with dense counterparts with similar thickness. Nanopatterned PbTiO3 films present higher switchable polarization and (d33)eff piezoelectric coefficient than dense thin films (137.6 ± 6.7 pm/V and 49.6 ± 0.7 pm/V versus 85.1±2.0 pm/V and 35.7±0.8 pm/V, respectively), demonstrating the positive effect of porosity on the enhancement of the ferroelectric properties. Nanopatterned BiFeO3 layers with 66 nm of thickness and average pore diameter of 100 nm at 600 °C were prepared. The large vertical porosity markedly enhances the local electric and macroscopic magnetic properties when compared with the dense counterparts. The vertical porosity orients the piezoelectric domains and reduces the energy necessary to reorient the dipoles. The induced instability in the dipole-dipole interactions results in the increase of the effective piezoelectric coefficient. [1] A. Castro, P. Ferreira, B. J. Rodriguez; P.M. Vilarinho, J. Mater. Chem. C (2015) 3, 1035–1043. [2] A. Castro, P. Ferreira, P.M. Vilarinho, J. Phys. Chem. C (2016) 120, 10961−10967. [3] A. Castro, M.A. Martins, L.P. Ferreira, M. Godinho, P.M. Vilarinho, P. Ferreira, J. Mater. Chem. C, (2019) 7, 7788-7797. Acknowledgement This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020, financed by national funds through the FCT/MEC (PIDDAC). NANOTRONICS (IF/300/2015), FLEXIDEVICE (PTDC/CTM-CTM/29671/2017) and Cost action OPERA CA 20116 are also thanked.

Resume : Optimally doped manganites (La0.7A0.3MnO3 (A=Ca, Sr, Ba)) is one of the most investigated correlated oxides due to its interesting properties: ferromagnetic metal with high spin polarization, ordering above room temperature (in bulk), colossal magnetoresistance [1,2]. However, the finding of a magnetic dead layer [3] has partially hindered its further exploration in devices, since La0.7A0.3MnO3 becomes non-ferromagnetic and insulating typically below thicknesses of 5-unit cells (uc) [4]. Here, we present our results using x-ray magnetic circular dichroism (XMCD) and x-ray linear dichroism (XLD) on optimally doped manganites interfaced with SrRuO3 (SRO) where we show that the magnetic dead layer can be hindered by the addition of this interfacial layer [5,6]. We show that 1 uc La0.7Ba0.3MnO3 (LBMO) inserted between two layers of SRO, 3uc-thick each, is ferromagnetic up to 130K. The XMCD shows clear Mn remanence at this temperature. Element-specific magnetization curves evidence the antiferromagnetic coupling between Mn and Ru. DFT calculations show that due to the SRO buffer layer, LSMO preserves the octahedral rotations from bulk. [5] Further, we investigated different thicknesses of La0.7Sr0.3MnO3 (LSMO) grown on SrTiO3 (STO) compared to LSMO/SRO bilayers on STO [6]. The orbital occupation of Mn in these two interfaces has been quantified by XLD. We observe that the Mn orbital occupation in the interface with SRO or STO is different, as previously predicted by DFT [7]. However, in the ultra-thin limit the interfaces have similar orbital occupation. We also observe that the Mn valence is different in these two interfaces. Our work shows that the magnetically stability of 1-2uc-thick LSMO when interfaced with SRO can be attributed to multiple factors. From one point of view, the overlap of Mn and Sr bands at LSMO/SRO interface, shown by DFT, leads to a 3D electronic structure, hindering quantum confinement. Further the SRO interlayer provides a buffer allowing the bulk MnO6 octahedral rotations and the preservation of the bulk Mn valence. [1] J.-H. Park, E. Vescovo, H.-J. Kim, C. Kwon, R. Ramesh and T. Venkatesan. Nature 392, 794–796 (1998). [2] Y. Tokura, Critical features of colossal magnetoresistive manganites. Rep. Prog. Phys. 69, 797 (2006). [3] M. Bibes, S. Valencia, Ll. Balcells, B. Martınez, J. Fontcuberta, M. Wojcik, S. Nadolski, and E. Jedryka. Phys. Rev. B 66, 167 (2002). [4] M. Huijben, L. W. Martin, Y.-H. Chu, M. B. Holcomb, P. Yu, G. Rijnders, D. H. A. Blank, and R. Ramesh. Phys. Rev. B 78, 094413 (2008). [5] C. Piamonteze, F. Bern, S. R. V. Avula, M. Studniarek, C. Autieri, M. Ziese, and I. Lindfors-Vrejoiu. Appl. Phys. Lett. 118, 152408 (2021). [6] A. Zakharova , M. Caputo, E. B. Guedes , M. Radovic, F. Nolting , and C. Piamonteze. Phys. Rev. Mater. 5, 124404 (2021). [7] Lv, K., Zhu, H. P., Zou, W. Q., Zhang, F. M. & Wu, X. S. J. Appl. Phys. 117, 185305 (2015).

Resume : Recent, the II-VI wide band gap semiconductors have application in a variety of solid-state electronic devices, such as light emitting diodes (LED), photosensors, thin-film transistors and solar cells. Several of the wide band gap II-VI compounds have been prepared with good n-type conductivity, the only compound which has shown low enough resistivity in p-type form to be useful in p-n junction devices is ZnTe. From the standpoint of the crystal lattice mismatch (0.216 Å), the n-type CdS and p-type ZnTe thin films present a suitable combination to create a heterojunction (HJ). ZnTe-CdS epitaxial HJs have been prepared by close space sublimation (CSS) method by consequently deposition of CdS and ZnTe films on ZnO/glass substrates. ZnO thin films were deposited by DC magnetron sputtering using ZnO:Ga:Cl ceramic targets. The effect of deposition temperature, film thickness and Ga and Cl concentrations on the electrical properties of ZnO thin films will be discussed herein. From the Hall measurements using the van der Pauw method, the resistivity value of 1.5 × 10− 4 Ω⋅cm, an average transparency of about 90%, mobility of 25 cm2/Vs were estimated for optimized ZnO:Ga thin films with a thickness of 400 nm. Also, the crystalline structure of each component of the ZnO/CdS/ZnTe heterostructure studied by x-ray diffractometer with CuKα radiation (λ = 0.15406 nm will be discussed. The optical properties measured with the Cary 60 UV-Vis spectrophotometer, such as film absorption coefficient and optical band gap in the range 300–1000 nm will be analyzed. The current–voltage (I–V) and capacitance–voltage (C–V) measurements were conducted using a Keithley 2400, a solar simulator (AM1.5, 100 mW/cm2) and Labtracer software. We optimized the photoelectrical parameters of ZnO/CdS/ZnTe HJs in function of ZnTe films substrate and source temperatures. Next, for optimized ZnO/CdS/ZnTe HJ with open circuit voltage 0.65 V, short circuit current densities of about 12 mA/cm2, electrical data were recorded in the (270-400) K temperature interval, dark. The forward current transport mechanism is discussed. Field for improving the photovoltaic performance of the ZnO/CdS/ZnTe HJs could be intermediate band solar cells.

Resume : Enhancement of the Raman signal intensity is currently among the most researched directions in developing Raman-based characterization techniques of all 2D materials as it elevates detection limits of their fine structural properties. The interest in Raman signal intensification is also triggered by the wide range of applications it can benefit, such as biochemistry and biosensing, polymer and materials science, catalysis, electrochemistry, the study of high-temperature processes, and the detection of hazardous gases [1]. In this work, we demonstrate a method for the enhancement of Raman active modes of hydrogen-intercalated [2] quasi-free-standing epitaxial chemical vapor deposition graphene and the underlying semi-insulating 6H–SiC(0001) substrate through constructive signal interference within atomic-layer-deposited amorphous Al2O3 passivation. We find that an optimum Al2O3 thickness of 85 nm for the graphene 2D mode and one of 82 nm for the SiC longitudinal optical A1 mode at 964 cm–1 enable a 60% increase in their spectra intensities. We demonstrate the method’s efficiency in Raman-based determination of the dielectric thickness and high-resolution topographic imaging of a graphene surface [1,3]. The research leading to these results has received funding from the Research Foundation Flanders (FWO) under Grant No. EOS 30467715, the National Science Centre under Grant Agreement No. OPUS 2019/33/B/ST3/02677 for project “Influence of the silicon carbide and the dielectric passivation defect structure on high-temperature electrical properties of epitaxial graphene,” and the National Centre for Research and Development under Grant Agreement No. LIDER 0168/L-8/2016 for the project “Graphene on silicon carbide devices for magnetic field detection in extreme temperature conditions.” Karolina Piętak acknowledges financial support from the IDUB project (Scholarship Plus programme). [1] K. Piętak, J. Jagiełło, A. Dobrowolski, R. Budzich, A. Wysmołek, T. Ciuk, Enhancement of graphene-related and substrate-related Raman modes through dielectric layer deposition, Applied Physics Letters, 120, 063105 (2022). [2] M. Szary, S. El-Ahmar, T. Ciuk, The impact of partial H intercalation on the quasi-free-standing properties of graphene on SiC(0001), Applied Surface Science, 541, 158668 (2020). [3] A. Dobrowolski, J. Jagiełło, D. Czolak, T. Ciuk, Determining the number of graphene layers based on Raman response of the SiC substrate, Physica E: Low-dimensional Systems and Nanostructures, 134, 114853 (2021).

Resume : Properties of undoped and doped epitaxial PZT films will be reviewed. It will be shown that trace impurities from the target can have a significant impact on the macroscopic electrical properties. It will be also shown that low level doping with Nb and Fe can induce changes in the direction of the ferroelectric polarization in the as-grown films, upward in Nb doped films (assumed as n-type doping), and downward in Fe doped films (assumed as p-type doping). p-n homojonction were also grown, with current-voltage characteristics suggesting a resistive behavior. At the end, considerations on the polarization switching in epitaxial films will be made, supporting homogeneous switching triggered by charge injection at the electrode interfaces.

Resume : In spite of a continuous interest in AlGaN based deep-UV emitters in view of various applications, these devices are still exhibiting an insufficient efficiency, which prevents them from being widely used. Discarding the dislocation density, which is not a drastic limitation if below 108 cm-2, the limitation of layer-based deep-UV LEDs primarily relies on low light extraction efficiency and difficult electrical injection. Due to their unique geometry, AlGaN nanowires (NWs) are promising to address those issues: the absence of extended defects, as well as the morphology of nitride NWs, are favorable to improve the dopant incorporation and light extraction. As a matter of fact, some of us recently reported axial pn-junction realization with Mg acceptors in the p-region and Si donors in the n-region [1]. Furthermore, the stabilization of Si in shallow state in AlN NWs was recently demonstrated [2]. In this work, we review the realization of AlN NW based light emitting diodes grown by plasma-assisted molecular beam epitaxy. For both n-type and p-type NW sections as well as for active region, the use of binary AlN allows one to overcome the spontaneous formation of AlN/GaN superlattice-like structure of AlGaN and concomitant alloy inhomogeneities making difficult the control of active region composition and emission wavelength [3]. By contrast, the use of short period AlN/GaN superlattices in the active region is one key to reach deeper UV, between 210 and 240 nm. Current voltage (I-V) measurements of the device show a rectifying behavior typical of pn junction assessing the effective n-type and p-type doping of the AlN sections. Electron beam induced current (EBIC) was used to identify the position of the space-charge region induced by the pn-junction within the NW, which was compared to the location of the cathodoluminescence (CL) signal. The correlation between EBIC and CL assesses the spatial superposition of both electric field and emitting area, a prerequisite to ensure light emission from electrical injection. Finally, electroluminescence (EL) spectra were acquired, exhibiting a peak at 285 nm, in agreement with the CL signal. The peak intensity follows the standard behavior of a diode, i.e. a linear increase of the optical power with current, before dropping. Various active area designs for wideband emission will be discussed in order to match DNA absorption band in view of sanitization applications. [1] A.-M. Siladie et al, Nano Lett. 19, 8357-8364 (2019) [2] R. Vermeersch et al., Appl. Phys. Lett. 119, 262105 (2021) [3] A. Pierret et al, M. Belloeil et al, Nano Lett. 16, 960−966 (2016)

Resume : Several types of lasers, such as, solid-state, semiconductor, gas, excimer, and dye lasers, have been developed. Today lasers are used in many fields, particularly in optical fiber communication, optical digital recording, material processing, biology and medicine, spectroscopy, imaging, entertainment, and many others. Due to exceptional material properties and/or investigation conditions various application require unique set of laser parameters - emission wavelength, its tunability, beam quality, operation temperature, optical output power, as well as convenient method of laser excitation, power consumption, high-speed modulation and device size is very important. Vertical-external-cavity surface-emitting lasers (VECSEL) also called optically pumped semiconductor lasers (OPSL) or semiconductor disk laser (SDL) belong to relatively new laser family that combines many of the desirable properties. VECSELs were developed to overcome key problems typical to conventional semiconductor lasers. In comparison to both types of electrically pumped Vertical-cavity surface-emitting lasers (VCSELs), which emit circular fundamental transverse mode beam but exhibit low power and edge emitting lasers (Fabry-Perot and DFB) that can reach high output power but an asymmetric beam with strong angular divergence, VECSELS are capable to generate high optical power with circular beam quality. In this work the main activity was focused to investigation of the technological parameters required to grow high quality GaAsBi quantum structures, balance the strain during the growth procedure, obtain sharp interfaces and investigate photoluminescence properties. Multiple quantum well structures were grown using solid-source MBE system (Veeco GENxplor R&D) equipped with standard cells for metallic Al, Ga, Bi and unique As design source generating arsenic dimers flux. Semi-insulating GaAs substrate oriented in (001) crystalline plane was selected as substrate. During this work the thickness of the quantum wells has been kept constant. Two different types of design were investigated, one consisting of two GaAsBi quantum wells with AlGaAs parabolic barriers and one with three rectangular GaAsBi/GaAs quantum wells encapsulated into AlGaAs parabolic barriers. To obtain high crystalline quality and enhanced optical properties of MQW structures the temperature of the substrate was varied from 420°C to 425°C, the beam equivalent pressure ratios of Bi/Ga and As/Ga were changed in the ranges of 1.05-1.10 and 0.4-3.0, respectively. The HR-TEM and STEM investigation evidenced that GaAsBi QWs with AlGaAs PQBs were formed successfully. Photoluminescence measurements determined the relationship between growth conditions and emission energy and intensity and highlighted the crucial growth parameters - As/Ga beam equivalent pressure ratio, Bi flux, growth rate and substrate temperature.

Resume : The direct epitaxial growth of silicon and germanium on silicon (Ge-on-Si) has fostered the development of visible - near-infrared detectors. A viable route to enhance the responsivity of these photodetectors might be exploiting the micro-structuring of the absorbing layer to increase the effective volume of interaction between light and matter. In this work we report on a new type of detectors, obtained from Si and Ge micro-crystals epitaxially grown on a patterned Si substrate [1,2]. The faceted morphology and relatively high aspect ratio of the microcrystals is seen to enhance the fraction of absorbed light and the detector responsivity in the wavelength region of the indirect regime of absorption, as compared to conventional planar devices. Modeling of the visible - near-IR absorption properties of the Si and GeonSi micro-crystals has been performed by finite difference time (FDTD) simulations [3,4]. This confirms that crystal faceting and pattern periodicity lead to enhanced light absorption as compared to conventional epitaxial layers and makes Si-Ge micro-crystals promising building blocks for optoelectronic devices operating in the VIS- NIR spectral region. A first step of electrical and optical characterization was made by means of a manipulator with a tip of 100 nm, enableing the electrical contact of a single microcrystal. The IV curves confirmed the pin doping profile inside the microcrystals, and its optimization by the implantation of the final p-doped top layer. The responsivity of the single microcrystal proved the VIS -NIR photoresponse. The main challenge in realizing vertically illu-minated photodiodes based on microcrystals is the formation of a top transparent contact that can adapt to the surface morphology and bridge the 100-200 nm gap between adjacent microcrystals. To this purpose, graphene can be used as a suspended continuous top contact. The Ge microcrystals fabricated devices have been characterized by electrical and optical measurements that confirm the near-IR photoresponse [4]. Responsivity measurements experimentally confirm the enhanced absorption close to the germanium indirect gap. This responsivity enhancement is linked to light-trapping effects taking place within the micro-crystal array. 1. C. V. Falub et al., Scaling Hetero-Epitaxy from Layers to Three-Dimensional Crystals, Science, vol. 335, no. 6074, doi: 10.1126/science.1217666. 2. R. Bergamaschini et al., Self-aligned Ge and SiGe three-dimensional epitaxy on dense Si pillar arrays, Surf. Sci. Rep., vol. 68, no. 3–4, 2013, doi: 10.1016/j.surfrep.2013.10.002. 3. J. Pedrini et al., Broadband control of the optical properties of semiconductors through site-controlled self-assembly of mi-crocrystals, Opt. Express, vol. 28, no. 17, doi: 10.1364/OE.398098. 4. V. Falcone, et al., Graphene/Ge microcrystal photodetectors with enhanced infrared responsivity, APL Photonics, https://doi.org/10.1063/5.0082421.

Resume : Distributed Bragg Reflectors (DBRs) are commonly used in optoelectronic devices which require an optical cavity, like lasers or diodes. In DBRs, layers with high and low values of refractive index are deposited alternately on top of each other. Such a structure allows for achieving almost 100% reflectance value for a wavelength range dependent on the refractive index contrast between the two types of layers as well as their thicknesses [1]. Although epitaxial techniques such as Metal-Organic Chemical Vapor Deposition (MOCVD) have been used to fabricate DBR structures for more than three decades, utilizing Boron Nitride (BN) to achieve this is a relatively new idea [2,3]. Commonly, the difference in the refractive index values between the two component layers of the DBR is a result of the use of two different materials or at least different amounts of specific elements in ternary or quaternary compounds. Additionally, the refractive index contrast may also be achieved by introducing porosity to at least one component layer [4,5]. In this communication, we show the fabrication and optical performance of DBRs in which both component layers are made of BN, but exhibit varying levels of porosity and therefore have different values of the refractive index. We are able to achieve this by manipulating only the temperature during the growth of the sample [6]. In contrast to the case of DBRs made of GaN or AlGaN [4,5], the fabrication of the whole structure is possible in a single MOCVD process, without the need for any post-process etching. For DBRs consisting of 15.5 layer pairs, we were able to achieve a peak reflectance reaching 90% [6], the position of which can be easily tuned in the visible and infrared parts of the electromagnetic spectrum. We also show that the fabricated DBRs can be easily utilized in the construction of optical microcavities. This research was funded by the National Science Centre, grants no. 2019/33/B/ST5/02766 and 2020/39/D/ST7/02811. [1] P. Yeh, Optical Waves in Layered Media, John Wiley & Sons: New York, 2005. [2] Q. Li et al., Opt. Mater. Express 11, 180-188 (2021) [3] F. AlQatari et al., Mater. Res. Express 8, 086202 (2021) [4] F. H. Fan et al., Sci. Rep. 7, 4968 (2017) [5] Y. Tian et al., Materials 15, 3536 (2022) [6] A. Ciesielski et al., arXiv:2206.02168 (2022)

Resume : Electrochemical etching (ECE) has recently emerged as a very useful method to fabricate porous GaN of a decreased refractive index (1) or for removal of sacrificial layers and structure lift-off (2). Due to high selectivity against n-type doping, ECE technique can also serve as a tool to detect inhomogeneity in Si doping in GaN grown by molecular beam epitaxy (3). We demonstrate that the lateral growth rate of atomic steps during epitaxy of GaN:Si in metal rich conditions impacts local Si incorporation and we can resolve morphology-induced differences of Si doping within the layers using ECE. Most of the reports focus on ECE of n-type GaN for which the process is well controlled and wide range of porosities can be easily obtained. In this report we show the first controlled ECE of p-type GaN with a tunnel junction used for effective carrier injection (4). As a consequence of different band-bending at the at the semiconductor-electrolyte interface in case of n-type and p-type, the etching conditions are different. Threshold etching bias is lower for p-type and complete material removal could be obtained easier than for n-type of similar doping. In the last part of the talk, applications of porous GaN in device structures will be shown with particular focus on edge emitting laser diodes (LDs) (5). Porous GaN is used here as a material of lower refractive index in the bottom cladding. We disclose the process-flow we used to implement porous layer in a laser structure. We show optical and electrical characteristics of LDs with porous claddings. Porous LDs show surprisingly low slope efficiencies (0.2 W/A), relatively high threshold current densities (9.2kA/cm2) and blue-shifted emission (448.7 nm) when compared to standard LDs. Finally, we list the reasons of high losses and discuss possible routes for improvements. Acknowledgements: This work received funding from the Foundation for Polish Science (POIR.04.04.00-00-4463/17-00 and POIR.04.04.00-00-210C/16-00) and from National Science Centre Poland (2019/35/D/ST5/02950, 2019/35/D/ST3/03008 and 2019/35/N/ST7/02968). The research leading to these results has also received funding from the Norway Grants 2014-2021 via the National Centre for Research and Development grant no. NOR/SGS/BANANO/0164/2020. 1. C. Zhang et al., New Directions in GaN Photonics Enabled by Electrochemical Processes. ECS Transactions 72, 47-56 (2016). 2. S. H. Park et al., Wide bandgap III-nitride nanomembranes for optoelectronic applications. Nano Lett 14, 4293-4298 (2014). 3. M. Sawicka et al., Revealing inhomogeneous Si incorporation into GaN at the nanometer scale by electrochemical etching. Nanoscale 12, 6137-6143 (2020). 4. N. Fiuczek et al., Electrochemical etching of p-type GaN using a tunnel junction for efficient hole injection. Acta Materialia 234, 118018 (2022). 5. M. Sawicka et al., Electrically pumped blue laser diodes with nanoporous bottom cladding. Optics Express 30, 10709-10722 (2022).

Resume : The use of hydrogen (H2) as a clean, efficient, and sustainable energy source has been expanding into various fields since it will permit the reduction of the CO2 gas generated by the combustion of fossil fuels. H2 is the smallest molecule, colorless, odourless, tasteless, and is a flammable gas, and it cannot be detected by human senses, unless it is present in high concentrations. In this regard, rapid and accurate H2 gas measurement is essential to alert to the formation of potentially explosive mixtures with air, to help prevent the risk of an unexpected explosion. In the present work, ultrathin platinum-gadolinium PtxGd1-x (x=1, 0.75, 0.5 and 0.25) alloy films are fabricated using co-sputtering with approximately 2nm in thickness onto SiO2/Si substrate. The stoichiometry, structural and electrical resistivity characterization of the films are obtained from EDX, RBS and XPS measurements. The structural characterization of ultrathin PtGd films showed smooth coverage of the surface, in a fcc crystalline plane (111). The resistive properties of the films are tested towards a hydrogen gas sensor between 25 °C ? 150 °C in the gas concentration ranging from 0.5% to 5%. The surface scattering phenomenon could explain the hydrogen gas sensing mechanism of ultrathin PtGd alloy thin films. It will discuss the effect of alloy composition, temperature and hydrogen concentration on the gas sensing properties of PtGd thin films. Acknowledgment: This study was supported by The Scientific and Technological Research Council of Turkey (TUBITAK) with a project number of 121M681.

Resume : The detectivity of a magnetic sensor is defined as the ratio of its intrinsic noise to its sensitivity. We previously demonstrated that very low levels of low frequency noise can be achieved in high epitaxial quality La2/3Sr1/3MnO3 (LSMO) thin films [1]. Since LSMO is a room temperature ferromagnetic metal with Curie temperature around 360K, it is a good candidate for the realization of anisotropic magnetoresistive sensors operated at room temperature, even if they do not reach same sensitivity values as other spintronic sensors such as Giant Magnetoresistances (GMR) or Tunnel Magnetoresistances (TMR). Anisotropic Magnetoresistance (AMR) depends on the angle between the direction of the electrical current and that of the magnetization. It is therefore important to control the magnetic anisotropy in LSMO thin films, and in particular to achieve an uniaxial magnetic anisotropy, with a relatively low anisotropy field, named Ha, while keeping a very low electrical noise. We will present results obtained with AMR sensors patterned in 30 to 60 nm thick epitaxial La2/3Sr1/3MnO3 (LSMO) thin films, deposited on 4°, 6° or 8° vicinal SrTiO3 (STO) substrates by pulsed laser deposition. The use of a vicinal substrate, which presents a surface miscut angle regarding to the crystallographic plane, was used to induce uniaxial anisotropy with an easy magnetic axis along the step edge directions [2]. A Wheatstone bridge design was used in order to get rid of common perturbations, such as temperature drift. The measured AMR curves could be compared to the expected behaviour considering the Stoner-Wohlfarth model for coherent magnetization reversal. In order to achieve a higher sensitivity, thus the targeted lower detectivity, several parameters were considered, such as LSMO film thickness, LSMO deposition temperature, vicinal angle of the substrates and design of the electrical contacts. Dedicated electronics readout and gradiometric configuration were considered. The lowest detectivity at 310K was 1 nT∙Hz-1/2 at 1 Hz and 300 pT∙Hz-1/2 at 1 kHz in a single-layer sensor without using any performance enhancing techniques such as magnetic flux focusers or modulation techniques [3]. Such AMR sensors are promising for biomedical applications where small size devices are needed at low frequency. ---------------------------- [1] L. Méchin, S. Wu, B. Guillet, P. Perna, C. Fur, S. Lebargy, C. Adamo, D.G. Schlom, J.M. Routoure, Experimental evidence of correlation between 1/f noise level and metal-to-insulator transition temperature in epitaxial La0.7Sr0.3MnO3 thin films, J. Phys. D: Appl. Phys. - Fast Track Communication 46 202001 (2013) [2] P. Perna et al., Engineering Large Anisotropic Magnetoresistance in Half-Metallic Manganite Films at Room Temperature, Adv. Funct. Mater., 27, 1700664 (2017) [3] L.G. Enger et al., Sub-nT resolution of Single Layer Sensor Based on the AMR Effect in La2/3Sr1/3MnO3 Thin Films, IEEE Transactions on Magnetics 58 (2) 4001204 (2022)

Resume : Quantum cascade lasers (QCL) are laser devices based on inter-subband transitions in semiconductor heterostructures. QCLs are ideal candidates for the mid to far-infrared spectral region, as the emission wavelength is determined by the layer width and array and not by the bandgap. QCLs have been of major interest for a range of industrial applications, including infrared imaging and spectroscopy, as well as various defense applications such as range detection and free-space optical communications. The superlattices based on the InGaAs/InAlAs heterostructure are mostly used in the construction of QCLs. The emission wavelength of lattice-matched InGaAs/InAlAs QCLs with InP substrate covers the infrared region from 3.5 to 24 μm and this gives them the opportunity to use them in applications in chemical and biological sensing or spectroscopy. A particularly promising area of application for QCLs is gas detection in the first atmospheric window (2.9-5.3 µm). Besides line-of-sight telecommunications, military countermeasures based on emissions in the first atmospheric window are also of interest. Laser realization within this region is particularly difficult due to the large conduction band discontinuity (∆Ec) required. The most mature material system used for short wavelength QCLs is the strain-compensated InP-based InGaAs/InAlAs. The QCL core contains hundreds or even thousands of thin layer repeats with a thickness in the 0.5–10 nm range. The growth of QCLs is crucial in terms of alloy composition, layer thickness, and hetero-interface quality of hundreds of ultrathin epitaxial layers. In the case of such complex structures, molecular beam epitaxy (MBE) is often used because it provides a specific composition in individual layers and very high-quality interfaces between them. Therefore, epitaxial growth of such layers with the metal-organic vapor phase epitaxy (MOVPE) technique is very difficult and it is important to find the optimal technological parameters. Growth kinetics depend on various factors such as flow rate of precursors, process pressure, and temperature, and they strongly affect the growth of individual layers. However, due to the high vacuum working processes, the MBE technique has the low QCL production capacity and high QCL cost. Therefore, another competing technique MOCVD was used to grow high quality QCL wafers with high yield. There are few studies in the literature about the effect of inter-layer transition time on quantum well, multilayer structures and lattice-match InGaAs/InAlAs QCL. However, transition time studies have not been performed for strain-compensated QCL structures to the best of our knowledge. In this study, we will examine the optimization studies of the combination of Si-doped and undoped inter-layer transition time in the strain compensated In0.67Ga0.33As/In0.36Al0.64As QCL structure grown on InP substrate by MOVPE. After growth, the crystal quality and thickness sensitivity of the samples will be investigated with a high resolution X-ray diffractometer (HRXRD). Optical transitions in QCL active region structures will be determined by photoluminescence measurements. In addition, optical interband transitions will be calculated with the Nextnano simulation program and comparison between experiment and theory will be investigated.

Resume : In order to meet the requirements for future optical communication systems, optical circuits have to be cheaper, smaller, less power hungry and deliver ever-growing data rates. It has long been proposed that silicon photonics, due to compatibility with large volume CMOS fabrication processes, has a unique potential to deliver ultra-compact optoelectronic chips meeting these requirements. Although there has been significant progress in the development of silicon photonics over recent years, handling the multiple wavelengths that arrive simultaneously through optical fibers remains a significant challenge. A potential solution is to integrate the superior nonlinear and amplification performance of Yttria-Stabilised Zirconia (YSZ) with Si photonics. This offers the perspective of simultaneously combining frequency comb generation and light amplification in the same chip. In order to achieve high performance, however, excellent crystalline quality is required for YSZ films on silicon. Unfortunately, this is difficult because of the amorphous native oxide layer which forms spontaneously at the surface of the silicon and thus prevents epitaxial growth. In this work we explore possibility of integrating epitaxial YSZ on silicon through Epitaxial Lift-Off (ELO). ELO is based on the use of a sacrificial epitaxial buffer layer between the substrate and the layer to be transferred. This underlayer acts as both a crystallographic template and a sacrificial release layer. There is an epitaxial relationship between this underlayer and both the single crystal substrate and the layer to be transferred. The template layer is chosen so as to be more susceptible to chemical etching than the layer to be transferred such that preferential dissolution can be achieved. ELO offers several advantages over other lift-off techniques. In particular, it reveals the pristine epitaxial interface of the overlayer, which is very smooth and thus amenable to Van der Waals bonding on a new host substrate. This kind of direct fusion bonding requires no interfacial bonding layer and thus can offer excellent electrical, thermal and optical coupling to the new host. In this study we chose to adopt zinc oxide (ZnO) as the sacrificial template layer because it is readily chemically etched and because it is a compliant material which can readily form an epitaxial relationship with significantly mismatched substrate. In previous work ELO of YSZ was performed using ZnO coated c-sapphire substrates [1]. In this work we employ ZnO-buffered YSZ(111) substrates in order to profit from the superior epitaxial quality on this substrate. [1] Use of Sacrificial Zinc Oxide Template Layers for Epitaxial Lift-Off of Yttria-Stabilised Zirconia Thin Films D. J. Rogers et al. Proc. of SPIE 11687 (2021) 116872C-1

Resume : Thin flms of oxides were grown on glass substrates using pulsed laser deposition (PLD) technique and characterized by atomic force microscopy (AFM), X-ray diffraction ray (XRD), scanning electron microscopy (SEM), and Water Contact Angle (WCA) measurements. The growth of human bone marrow mesenchymal stem cells (hBMMSCs) was studied on these films for different periods. Their adhesion, proliferation, and di􏰁erentiation were assessed by di􏰁erent techniques, including SEM, differentiation staining, and reverse transcription polymerase chain reaction (RT-PCR). The data obtained suggest that vanadium oxide impairs normal healthy cell development and inhibits the proliferation of hBMMSCs and their osteogenic and chondrogenic differentiation. These results provide new information on the cytotoxicity of vanadium oxides to human stem cells and offer opportunities for a deeper understanding of the use of oxide thin films for medical applications.