Wide band gap (WBG) materials: theory, growth, characterization, and applications

Resume : Oxide-based transparent p−n homojunctions are desirable for the development of transparent electronics. However, most oxide semiconductors are intrinsically n-type and obtaining a reliable p-type wide-gap oxide is still challenging. This can be attributed to the low energy location and the rather flat dispersion of the O 2p orbitals derived valence band (VB) of most transition metal (TM) oxides. Consequently, the development of transparent oxide-based optoelectronic devices is largely hampered by the lack of wide-gap oxides which can achieve both p- and n- (bipolar) conductivity. Here, we showed that by alloying oxides with different electronic band structures, it is possible to achieve bipolar doping in wide gap ternary oxides. Among wide gap oxides, cadmium oxide (CdO) can be doped heavily n-type and has the highest reported electron mobility while nickel oxide (NiO) is one of the very few known p-type TM oxides. Furthermore, NiO has the same rocksalt (RS) structure as CdO, but its valence band maximum (VBM) is ~1 eV above the conduction band minimum (VBM) of CdO. Hence, alloys of these two oxides with a type III band offset can offer a unique opportunity for the development of wide gap ternary oxides which can exhibit both n- and p- conductivity. We found that the conduction type of NixCd1-xO alloy thin films depends on the alloy composition and oxygen stoichiometry. We demonstrated that in the composition range of 0.381 eV above that of the WZ phase, so that alloys with similar composition with the two different structures exhibit a type II band offset. The much higher VBM position of RS alloys favors the formation of native acceptor defects as well as efficient extrinsic p-type doping. Our results offer interesting opportunities for using these alloys for transparent devices that require bipolar conductivity.

Resume : Versatile ZnO semiconducting films with well-defined structural qualities, tunable electrical and optical properties are needed for device applications. However, unintentional structural and point defects and impurities present in ZnO films are main obstructions in achieving controllability, repeatability, and carrier transport in this material [1]. The aim of the present study was to find out relationship between structure and conductivity of thin polycrystalline ZnO films grown under O- or Zn-rich conditions. Two series of ~100 nm thick ZnO films were grown by Atomic Layer Deposition on Si and a-Al2O3 substrates at growth temperature (Tg) range of ~100-300oC. RT-Hall measurements revealed that resistivity increased with Tg and also after rapid thermal annealing (RTP) for both ZnO/Si as well as ZnO/a-Al2O3 films. SIMS experiments found that the content of unintentional H impurity in both as grown and annealed films is higher than the carrier concentrations, showing that not all H atoms play a role of a donor. There exist a certain Tg range of ~160-200oC where preferred orientation showed switching phenomenon influencing conductivity of the films. Strain, dislocation density, and Urbach energy [2] calculations showed that films formed at Tg~160oC had better crystalline quality and higher electrical conductivity, as well as the carrier mobility for both series. Overall, when compared to ZnO/a-Al2O3, the ZnO/Si films showed lower strain and lower dislocation density. Acknowledgement. The work has been performed within the National Science Centre project No. 2018/31/B/ST3/03576. [1] E. Przezdziecka, E. Guziewicz, D. Jarosz, D. Snigurenko, Sulich, P. Sybilski, R. Jakiela and W. Paszkowicz, J. Appl. Phys. 127, 075104 (2020) [2] A. Adhikari, E. Przezdziecka, S. Mishra, P. Sybilski, J.M. Sajkowski and E. Guziewicz, Phys. Stat. Sol. A, 2000669, (2021)

Resume : Among wide-band gap semiconductors, Transparent Conducting Oxides (TCO) are of utmost interest thanks to their optical transparency and low electrical resistivity, that makes them uniquely appealing for applications in optoelectronics, photovoltaics and energy harvesting [1, 2]. One of their peculiarities is the wide tunability of their electrical and optical properties through doping. Typically, TCO films are polycrystalline. However, the growth of epitaxial films would improve conductivity and carrier mobility, especially in thin films, since a good crystalline order is expected to reduce grain boundary scattering as well as trapping of carriers at grain boundaries. Thick epitaxial Al-doped ZnO (AZO) films have been grown via various techniques, exploiting an epitaxial substrate [3, 4]. In our study, we grew AZO films of thickness from 300 nm down to 30 nm on Strontium Titanate (SrTiO3) substrates via Radio Frequency (RF) Magnetron sputtering with optimized conditions to obtain epitaxy and improve carrier density and mobility. The growth was carried out at 400 C. As a reference, we grew polycrystalline AZO films with the same conditions on silicon dioxide (SiO2) amorphous substrates. We investigated surface roughness and mean size of grains of the films with Atomic Force Microscopy (AFM), crystalline order and quality with High Resolution X-Ray Diffraction (HRXRD) and electrical properties by 4W Probe and Hall measurements. Films grow with a clear epitaxial relationship with the substrate, and this happens even for the thinner samples. As a consequence of the improved crystal order the epitaxial films show a resistivity decreased by two orders of magnitude and increased carrier mobility as compared to polycrystalline films on SiO2. The experimental study of the AZO films was completed with optical characterization through ellipsometry measurements. [1] R. A. Afre et al., Rev. Adv. Mater. Sci. 53 (2018) [2] Y. Chen, IOP Conf. Ser.: Mater. Sci. Eng. 423, 012170 (2018) [3] L. M. Trinca et al., Appl. Surf. Sci. 364 (2016) [4] I. Valenti et al., J. Appl. Phys. 118, 165304 (2015)

Resume : Transparent Conductive Oxides (TCO) in solar cells are used as transparent electrodes. They must necessarily have a high optical transmission in order to allow efficient transport of photons to the active layer and also good electrical conductivity which is required to obtain the least losses of transport of photo-generated charges. Aluminum, Magnesium and Tin doped ZnO thin films are some of these. In this contribution, the TCO materials formed of Al, Mg and Sn doped ZnO thin films (AZO, MZO, TZO) were synthesized by spin coating method. The structural, optical and electrical properties of these films are investigated as a function of dopant concentration. X-ray diffraction patterns show that all AZO, MZO and TZO films exhibit the hexagonal würtzite structure with a preferential orientation along [002] direction with no additional peak corresponding to secondary phases. The surface morphologies show that AZO and MZO films are homogeneous and dense while TZO films showed different surface morphologies going from homogeneous to porous structure depending on Sn concentration. An enhancement of optical transmittance of about 95% with a blue shift of the gap energy was found for both Al and Mg doped ZnO layers against 70% of average level transmittance and a red shift of optical window for Sn doped ZnO films. Photoluminescence (PL) measurements at room temperature shows excitonic peaks corresponding to the AZO, MZO and TZO films with a low defects level for 1% Al doped ZnO compared to MZO and TZO films that showing a large intensive band defects centred around 514 nm characterizing the substitution defects of the zinc atoms by oxygen atoms. Electrical measurements show that all the films present an Ohmic comportment with a best electrical conductivity of 2.45 S.cm-1 obtained for 1% AZO in accordance with the photoluminescence analysis. The reduced electrical conductivity shown in our films is attributed to the low mobility of charge carriers due to their diffusion by the grain boundaries.

Resume : Zinc oxide (ZnO) has recently attracted much attention as a sustainable and high potential material for piezoelectric application. It is an abundant and biocompatible semiconducting compound with polar and piezoelectric properties thanks to its wurtzite structure. The piezoelectric properties of ZnO are strongly influenced by its morphology (i.e. shape, dimensions, polarity), but its correlation with the growth processes have not completely been elucidated yet. In this work, we study the morphology transitions of ZnO grown on Si substrate by pulsed-liquid injection metal–organic chemical vapour deposition (PLI-MOCVD) together with the evolution of its structural and piezoelectric properties as a function of the growth temperature as well as the O2 gas and diethylzinc solution flow rates. By only raising the growth temperature, the morphology of ZnO is changed from thin films to nanowires, which is associated with the increase in the piezoelectric amplitude and the improvement of the polarity uniformity as measured by piezo-response force microscopy [1]. Furthermore, the flow rate variation leads to different thin film morphologies with a large change of polarity distribution. These results demonstrate the high ability of the PLI-MOCVD system to tune the ZnO properties as a strong interest for piezoelectric applications. [1] Q. C. Bui et al., ACS Appl. Mater. Interfaces 2020, 12 (26), 29583−29593.

Resume : Wide band gap materials play an important role in many different applications from scintillation to high-performance optoelectronic, ultraviolet, electronic devices and solid-state lighting. In order to develop new materials and improve existing ones advanced spectroscopic methods have to be applied for the characterization using tunable excitation by photons in wide energy range. In recent years’ availability of such short-wavelength synchrotron radiation sources as storage rings and free electron lasers grows and user community consisting of researchers from various disciplines incl. materials science, physics and chemistry [1]. In order to perform such advanced research, several endstations for time-resolved luminescence spectroscopy and measurement techniques (a time correlated multiphoton counting) have been developed by us at the MAX IV Laboratory located in Lund, Sweden. One of these setups is permanently mounted at Finnish-Estonian beamline FinEstBeAMS [2], which provides photons in the energy range of 4.5 – 1000 eV enabling to study valence excitation and inner shell excitation of wide gap materials. In a single bunch operation mode the time resolution as short as 160 ps was achieved [3]. A mobile photoluminescence setup has been implemented to the FemtoMAX beamline, where 100 fs 10 keV X-ray photon pulses are available for investigation of ultrafast processes in scintillators close to real operation conditions and the exceptional time resolution of 28 ps can be exploited [4]. The relaxation processes of electronic excitations in novel wide gap hexafluorogermanate scintillators leading to the ultrafast cross- and intraband luminescence with sub-nanosecond lifetime will be discussed in the basis on the results obtained at these setups by time-resolved luminescence methods [3]. The high quality experimental results are analyzed taking into account electronic band structure calculations from publicly available materials property databases (AFLOW, etc.) Also some examples of other advanced wide gap materials for scintillation and optical applications will be presented and discussed. [1] Y. Hwu and G. Margoritondo, J. Synchr. Radiation 28 (2021) 1014-1029. [2] V. Pankratov, R. Pärna, M. Kirm, et al., Radiation Measurements 121 (2019) 91–98. [3] J. Saaring, A. Vanetsev, K. Chernenko, et al., J. Alloys and Compounds (2021) in press. [4] R.M. Turtos, S. Gundacker, S. Omelkov, et al., npj 2D Materials and Applications 3 (2019) 37.

Resume : Highly accurate material characterization is crucial for development of novel multi-functional materials such as AlScN or HfO2. Here we report on the three-dimensional characterization of the pyroelectric response of selected wide bandgap materials. Our measurement setup consists of high-bandwidth near- (852 nm and 1064 nm) to far- (9.15 µm) infra-red quantum cascade and Fabry-Perot laser sources. By means of the laser intensity modulation method, we reconstructed the in-depth distribution of the spontaneous polarization with sub-micron resolution. In case of AlScN, our findings indicate that the thermal diffusivity and its temperature-dependence differs significantly from what is reported for pure Aluminum Nitride, which we attribute to the dominant role of the Sc dopants.

Resume : TiO2 thin films were produced by atomic layer deposition (ALD) since the beginning of the nineties, with a much interest starting from 2004 (1). Indeed, anatase TiO2 thin films having long exciton lifetime, semiconductor nature and a wide indirect band gap of 3.2 eV (1) are attractive for many contemporary applications including Li ion batteries, photocatalysis, dielectric layers in capacitors and transparent conductive oxides (TCO)(1–3). We will report on optical and electrical properties of the anatase TiO2 thin films deposited by ALD on single crystalline silicon substrates for microelectronics applications and on transparent glass substrates for TiO2-based TCO layers having high transparency in the visible range. TiO2 thin films were grown by ALD on (100) oriented n-type silicon and Schott glass substrates in a low deposition temperature (TD) range from 100°C to 300°C with titanium tri-isopropoxide (TTIP) precursor and water as oxidizing agent. As-deposited TiO2 films crystallized in stoichiometric anatase phase on both silicon and glass at TD ≥ 250°C. Performed characterizations (TEM, XRD, RBS, AFM) shown that crystallization is promoted with the increase of TD and the film thickness. Moreover crystallization seems to follow a well-known growth mechanism (4): (i) deposited TiO2 is first amorphous, then after reaching a certain critical thickness, (ii) crystalline nuclei form randomly at the surface. (iii) Then the nuclei seem to grow isotropically through the surrounding and the newly deposited matter. Optical properties such as refractive index, absorption coefficient and band gap energy were analyzed by ellipsometric measurement on silicon and by photospectroscopy transmission measurement on glass using a Tauc-Lorentz dispersion model. Results from both methods are coherent, and show a decrease of the indirect band gap energy with the increase of TD confirming the phase transition from amorphous to the anatase phase2. Regarding the dielectric properties, the relative permittivity ε’ and the loss angle tan δ of TiO2 films were studied as a function of the frequency. ε’ increases with TD, due to the higher crystalline quality of the films. Low losses and low relaxation are observed, indicating a low density of charged defects in the films and a good quality of the dielectric properties. All observed structural, optical and electrical properties of TiO2 thin films are consistent, showing the possibility to get high quality stoichiometric crystallized anatase TiO2 with a low thermal budget by ALD on different substrates. This work paves the way for the integration at low temperatures of this versatile oxide for future devices. 1. Niemelä J-P, Marin G, Karppinen M. Titanium dioxide thin films by atomic layer deposition: a review. Semicond Sci Technol. 2017. 2. Elbahri MB, Kahouli A, Mercey B, Prellier W, Lüders U. Effects of oxygen pressure during deposition on the dielectric properties of amorphous titanium dioxide thin films. J Phys Appl Phys. 2019. 3. Mo S-D, Ching WY. Electronic and optical properties of three phases of titanium dioxide: Rutile, anatase, and brookite. Phys Rev B. 1995. 4. Puurunen RL, Sajavaara T, Santala E, Miikkulainen V, Saukkonen T, Laitinen M, et al. Controlling the Crystallinity and Roughness of Atomic Layer Deposited Titanium Dioxide Films. J Nanosci Nanotechnol. 2011.

Resume : Selective absorption and thermal radiation resonating with longitudinal optical (LO) phonon from Au-GaAs mesa-type line and space structures was obtained by our group. This result suggests the application of surface microstructures to THz-mid infrared (IR) optical devices of communication and energy conversion. Previously, radiation resonating with surface or interface phonon polariton (IPhP) or frequencies determined by structural properties has been reported; radiation frequency depends on structural properties or emission direction. In comparison, the LO phonon resonant radiation shows no dependence on these factors, which is an advantage in device application. Here, we have an issue that the dependence on material species, in particular III-nitride has stronger interaction of LO phonon with electric field, meaning the possibility of the emergence of IPhP in the metal-semiconductor structures. Further, we have observed the effects of the modulation of dielectric function on the IPhP mode in a previous report. In this report, we discuss the thermal mid-IR radiation from mesa-type metal-GaN stripe structures from the viewpoints of LO phonon and IPhP resonance. Sample A has mesa structures of GaN on sapphire substrate, where the height was 0.5-1.0 m, and stripe widths of GaN and metal of 2.3-3.8m and 6.2-7.7 m, respectively. Mesa structure was fabricated by RIE. Metal was deposited on the bottom and slope regions of the mesa structure. Another type sample B has no metal deposition at the mesa slope region or no mesa etching. IR reflectance and radiation measurements were conducted using a FTIR spectrometer. Radiation observation was conducted at 580 K. Obtained spectrum was calibrated using emission spectra of a graphite plate. The emission spectrum of sample A showed an eminent peak at approximately 690 cm-1, which is not resonating with the LO phonon at approximately 725 cm-1 at 580 K. When emission polar angle was varied from 0 to 45 degree, the peak wavenumber was nearly constant for the s polarization, while two additional emission peaks at higher and lower energy regions were observed for the p polarization. When metal was not deposited on the slope region, no split for the peak at 690 cm-1 and the LO resonating peak was observed in a region of 725 - 730 cm-1. These results indicate IPhP resonance is dominant in the mesa region, while the LO phonon resonating radiation is mainly yielded by the polarization charges at metal-GaN interface in the bottom region of mesa stripes. The observed wavenumbers of IPhP modes are theoretically obtained only when we take into account the modulation of electric permittivity by the formation of interface polarization charges generated by the thermal occupation of the LO phonon. The mode energies of the IPhPs are dominated by the boundary condition of the GaN-metal in the mesa slope region and the lattice vibration resonating with LO phonon. When the emission polar angle is varied, a pair of mesa slopes yield two different angles between the emission direction and the slopes, which matches to the observation of two additional peaks. The permission of these IPhP resonant emissions are not obtained for the metal-GaAs structure because of the small interaction of optical phonon and electric field. FDTD simulation supports our consideration. These results reveal the particular properties of III-nitride system for the function of IR waveguide or emission based on LO phonon and IPhP resonances and using micro metal-semiconductor stripe structures.

Resume : The Tauc’s method [1] is a much-preferred optical band gap (OBG) evaluate method for amorphous (or glassy) materials. However, regardless its clear and simple purpose, it has been routinely and incorrectly applied to study crystalline semiconductors and dielectrics. In Tauc relation the density of electron states is close to the VB and CB extrema and it is proportional to the square root of the photon energy. Assuming a constant dipole transition matrix element, another possibility to determine OBG is to apply the Cody relation [2]. In spite of the method chosen to determine the OBG of either crystalline or amorphous materials, all of them are influenced by the absorption spectrum α(E) and its following data analysis [3]. Recently very nice revision on OBG was published by A. R. Zanatta proposing a unified methodology to its determination for semiconductors by applying the sigmoid-Boltzmann function to fit α(E) and obtain OBG [3], which works well in case the α(E) is measured at high enough photon energies. In this work we propose an alternative by using the Johs-Herzinger generalized critical point model [4] or Herzinger-Jobs parameterized semiconductor (HJPS) oscillator function [5] to directly fit the α(E) spectra. The results are discussed for amorphous and crystalline ZnO, ZnO2 and YHO thin films by applying Tauc, Cody, sigmoid-Boltzmann function and HJPS. ACKNOWLEDGMENTS This research is supported by the Horizon 2020 Project CAMART² under grant agreement No 739508 and ERAF national project “Smart metal-oxide nanocoating and HIPIMS technologies” No 1.1.1.1/18/A/073. References [1] Tauc, J., Grigorovici, R. & Vancu, A. Optical properties and electronic structure of amorphous germanium. phys. stat. sol. (1966) 15(2), 627–637, https://doi.org/10.1002/pssb.19660150224 [2] Cody, G. D., Brooks, B. G. & Abeles, B. Optical absorption above the optical gap of amorphous silicon hydride. Solar Energy Mater. (1982) 8(1–3), 231–240, https://doi.org/10.1016/0165-1633(82)90065-X [3] A. R. Zanatta, Revisiting the optical bandgap of semiconductors and the proposal of a unified methodology to its determination, Scientific Reports (2019) 9:11225, https://doi.org/10.1038/s41598-019-47670-y [4] P. Petrik, Parameterization of the dielectric function of semiconductor nanocrystals, Physica B: Condensed Matter (2014) V453, 2-7, https://doi.org/10.1016/j.physb.2014.03.065 [5] Craig M. Herzinger; Blaine D. Johs., DIELECTRIC FUNCTION PARAMETRIC MODEL, AND METHOD OF USE, Patent Number: 5796983 [6] M. Zubkins et al., Amorphous ultra-wide bandgap ZnOx thin films deposited at cryogenic temperatures, J. Appl. Phys. 128, 215303 (2020), https://doi.org/10.1063/5.0028901

Resume : Charge trapping observed at 300 and 77 K in ferroelectric (FE, annealed Al- or Si-doped) and non-ferroelectric (unannealed and/or undoped) HfO2 films grown by atomic layer deposition revealed the presence of “deep” and “shallow” electron traps with volume concentrations in the 10^19-cm^3 range [1]. These results demonstrate a broad spectrum of electron trapping sites present in undoped HfO2 layers. These include deep states with optical depopulation energies in the 2.0–3.5-eV range, but also traps which are thermally depopulated with activation energies below 0.7 eV and around 0.2 eV. The samples typically contain crystalline regions of non-FE (monoclinic, tetragonal, cubic) and orthorhombic FE phases intermixed with amorphous regions. The trap energy depth and marginal sensitivity of their concentration to crystallization annealing or film doping with Si or Al suggests that these traps are associated with boundaries between crystalline grains and interfaces between crystalline and amorphous regions in HfO2 films. To understand these results, we performed density functional theory (DFT) calculations of electron trapping at surfaces of monoclinic, tetragonal, and orthorhombic phases of HfO2 and at interfaces of these grains with amorphous regions. Some of the trap energies are consistent with those predicted for electron polarons in m-HfO2 (Etr ∼ 0.2 eV) [2] and polaron and bi-polaron photo-depopulation energies in the bulk of amorphous HfO2 (2.0–3.5 eV) [3]. The trap states calculated at the interfaces between the crystalline and amorphous regions are consistent with the observed thermal ionization energies (0.7–1.0 eV below the HfO2 CB). This behaviour suggests the trapping sites are related to morphology and disorder in HfO2 films rather than to the oxygen deficiency and support their intrinsic polaronic nature. Their depth is correlated to the extent of oxygen coordination of Hf sites trapping the extra electrons. We suggest a model that at 77 K electrons are initially trapped in shallow polaron states in monoclinic and orthorhombic HfO2. Some of these electrons escape to an electrode while others hop to grain boundaries and eventually end up in amorphous regions, where they are getting trapped at deeper states. [1] R. A. Izmailov et al., Phys. Rev. Mater. 5, 034415 (2021) [2] D. Muñoz Ramo et al., Phys. Rev. Lett. 99, 155504 (2007) [3] J. Strand et al. Nanotechnology 29, 125703 (2018).

Resume : By use of scanning probe microscopy and spectroscopy (STM and STS), X-ray photoelectron spectroscopy (XPS), and theoretical calculations we investigated the topography and electronic properties of sub-monolayer HfO2 films on highly ordered pyrolytic graphite (HOPG). Crystalline HfO2 nanoislands were grown fully oxidized via pulsed laser deposition at elevated temperatures (> 650°C) by van der Waals epitaxy. STS on homogenously formed and well separated nanoislands revealed a bulk-like size of the apparent bandgap. At the same time, in-gap states close to the Fermi level were found. By use of STM with atomic resolution the first nucleation states of HfO2 nanocrystallites (2.7-4.5 Å in diameter) located next to carbon vacancies in the topmost HOPG layer were identified. In addition, carbon incorporation into HfO2 nanocrystallites was indicated by XPS measurements. Using evolutionary algorithm (USPEX) in combination with Vienna Ab Initio Simulation Package (VASP) a series of possible HfmOn(:C) clusters (with m in range from 3 to 10, and n in range from 3 to 22) with and without incorporation of carbon atoms were simulated. By comparison of the lowest-energy structure and the LDOS of the theoretically deduced nanocrystallites with the experimental results the most relevant nuclei formed during the first growth steps of HfO2 were identified. This finding will be of major significance for the practical growth of the dielectric HfO2 on graphene-based devices.

Resume : Intense recent research efforts are directed toward the development of novel materials morphologies, such as nanoparticles, able to efficiently convert ionizing radiation into light. Fluorescent or scintillating materials attract great interest in lighting technology, solar applications, theranostics and self-lighting photodynamic therapy. Moreover, additive manufacturing advancements suggest the possibility to fabricate nanoparticle-based materials applicable in several fields. Therefore, novel luminescent nanomaterials free from toxic or expensive elements, such as rare earths, are desirable building blocks for promoting future developments in all these fields. In this work, the structural and morphological properties of pure and Ti-doped HfO2 nanoparticles are studied in dependence of calcination temperatures leading to a fine tuning of either morphology and functional defect configuration. Indeed, tuneable optical features are observed in undoped monoclinic HfO2 nanocrystals and their dependence on the structural properties of the material at the nanoscale are disclosed. TEM, XRD, XPS, and surface area data combined with the analysis of the luminescence allowed us to identify the dual nature of the broad emission at 2.5 eV, where an ultrafast defect-related intrinsic luminescence overlaps with a slower emission ascribed to extrinsic Ti impurities.[1] When titanium is employed as luminescence activator, a bright emission is observed under excitation with both UVC radiation and X-rays. [2] These results represent useful insights toward the design of novel promising nanomaterials suitable for rare-earth-free UV pumped white LEDs based on intrinsic defect engineering, and for X-ray triggered oncological therapies by using the Ti (IV)-related bright radioluminescence to excite photosensitizer molecules for singlet oxygen generation. [1] Villa, I.; Vedda, A.; Fasoli, M.; Lorenzi, R.; Kränzlin, N.; Rechberger, F.; Ilari, G.; Primc, D.; Hattendorf, B.; Heiligtag, F. J.; Niederberger, M.; Lauria, A., Size-Dependent Luminescence in HfO2 Nanocrystals: Toward White Emission from Intrinsic Surface Defects. Chem. Mater. 2016, 28 (10), 3245-3253. [2] Villa, I.; Moretti, F.; Fasoli, M.; Rossi, A.; Hattendorf, B.; Dujardin, C.; Niederberger, M.; Vedda, A.; Lauria, A., The Bright X-Ray Stimulated Luminescence of HfO2 Nanocrystals Activated by Ti Ions. Advanced Optical Materials 2020, 8 (1), 1901348.

Resume : Atomic layer deposited (ALD) hafnium oxide (HfO2) based materials are extensively used as insulating or active layers in a multitude of electronic devices. The addition of aluminum to the binary HfO2 has proved to enhance its electrical properties increasing the dielectric permittivity, reducing the leakage current, and improving the reliability. It is known that the electrical properties are strongly correlated to atomic defects, which generate localized electronic states inside the bandgap that act as charge traps. Nevertheless, the effect of the presence of aluminum on the trap properties is still under debate. Different ALD precursors can also strongly affect the material electrical behavior. Here we present the traps characterization of HfO2 and Al-doped HfO2 (AlHfO) -based TaN/oxide/TaN capacitors with four different Al concentrations (from 0% to 20%) and deposited using various precursor combinations. The films are grown in a Savannah reactor (Cambridge Nanotech) at 300°C using trimethylaluminum and (MeCp)2Hf(Me)(OMe) as metal precursors for aluminum and hafnium, respectively, and using either water or ozone as oxygen precursor. The AlHfO films are grown by alternating HfO2 and Al2O3 ALD cycles with different ratios to control the Al content. Defect properties are extracted from current-voltage characteristics using the Ginestra(TM) simulation software, which self-consistently solves the Poisson’s and charge continuity equations considering all the different conduction mechanisms relevant in dielectrics (i.e. direct and Fowler-Nordheim tunneling, conduction/valence band carrier drift, and multi-phonon trap-assisted-tunneling).[1] In every film two kinds of electron traps are detected. In ozone-based films, 2·10^19 traps/cm3 are normally distributed in energy around 1.9eV below the oxide conduction band minimum (CBM), and 1.4·10^19 traps/cm3 are uniformly distributed from 2.4eV to 4eV below the CBM. The presence of aluminum does not affect the traps properties but increases the oxide band gap (from 5.5eV to 6.2eV) and reduces the electron affinity accordingly, leading to the increase of the injection barrier height. The use of water as oxygen source during the oxide deposition introduces 1.4·10^19 cm-3 fixed positive charges uniformly distributed along the dielectric thickness and reduces traps ionization energies by approximately 0.2eV. On the other hand, the different aluminum content, or the use of different oxygen sources, do not impact significantly on the density of the electron traps. [1] P. La Torraca, F. Caruso, A. Padovani, S. Spiga, G. Tallarida, L. Larcher, “Extraction of Defects Properties in Dielectric Materials From I-V Curve Hysteresis”, IEEE Electron Device Lett. 42 (2021) 220-223.

Resume : Quantum efficiency of direct wide band gap semiconducting optoelectronic devices is determined by the electron and hole recombination time. However, polarization induced electric field inside piezoelectric materials (for example, along the ZnO c-axis) separates the carriers, thus reducing the optical performances [1]. To study in detail the polarity effects on the electrical, structural and optical properties a series of ~100 nm thick ALD-ZnO films were deposited on a-and c-Al2O3 substrates at 6 different growth temperatures ranging from 100 to 300oC. The films are found to be polycrystalline with average crystallites sizes of ~10-80 nm which increased slightly after Rapid thermal process (RTP) to ~40-120 nm. An increase in the semipolar orientation [101] is observed for ZnO films grown on a- respect to c-Al2O3 with an exception for the ~160-250degC growth window where both polarities compete regardless of the substrate chosen. The RTP was found beneficial for reducing the unwanted polarity as well as for improving the overall structural characteristics (strain, micro-strain, and dislocation density). SIMS and RT-Hall measurements performed on the as grown and annealed films indicate no polarity effects on the inclusion of unintended impurities (H, C, and N) and a similar trend in the resistivity changes, respectively. Acknowledgements. The work has been performed within the National Science Centre projects No. 2018/31/B/ST3/03576 and UMO-2016/22/E/ST3/00553. [1] Waltereit, P., et al. Nature 406.6798 (2000): 865-868.

Resume : GaN is popularly known to be one of the wide-bandgap semiconductor materials and has been strongly expected to be used in high-breakdown-voltage and high-power electronic devices. In particular, AlGaN/GaN-based high-electron-mobility transistor (HEMT) structures can fully benefit from the wide-bandgap advantages of GaN, so they are attracting attention as next-generation power devices that can help realize mobile communications and an energy-saving society using high-output, high-speed field-effect transistors. However, large quantities of electron traps that form defect levels exist in the AlGaN and GaN layers in the GaN crystal system. The AlGaN/GaN-based HEMTs are prone to the “current collapse phenomenon” in which the electrons in the channel are captured by the traps to increase the ON resistance. In addition, the basic properties of such crystal defects need to be studied in depth. In this study, the drain electrode side was examined for a 10 MHz to 10 GHz frequency sweep using a vector network analyzer in the operating state of the AlGaN/GaN-based HEMT, with simultaneous measurement of the fluctuation of the drain current with respect to frequency variations. Thus, it was confirmed that the current collapse phenomenon occurs below a specific frequency while being suppressed above this value. When a radio frequency is used to sweep the drain side of the AlGaN/GaN-based HEMT, the electric field near the drain electrode gradually accelerates the electrons arriving at the drain with increasing frequency. During this time, some of the channel electrons in the low-frequency region are attracted to the level energies of the crystal defects before their speed increases by the sweep frequency effect; therefore, they are trapped by the crystal defects to cause the current collapse phenomenon. However, the electron velocity increases above a specific frequency, and the channel movement time is reduced compared with the time constant required for the electrons to be trapped in the crystal defect levels. Therefore, the channel electrons can reach the drain region without being trapped by the crystal defects. This indicates that the crystal defects determine the channel carrier trapping based on the frequency. Previous studies have only qualitatively shown that the channel electrons are not trapped by high frequencies and that the current collapse phenomenon can be thus suppressed; however, this study clarifies the frequency response characteristics of the crystal defects.

Resume : Ultraviolet photodetectors are widely used in the biomedical field, as flame sensors, and for ultraviolet photolithography. Ultraviolet rays have extremely high energies owing to their wavelengths, and their effects on the human body cannot be denied. Further, their usage is limited; hence, ultraviolet-light-receiving elements are indispensable when using ultraviolet rays. However, the light-receiving elements are easily damaged by the influence of the ultraviolet radiation. To ensure sufficient strength and durability against ultraviolet rays, photodetectors made of wide-bandgap semiconductors, such as GaN and AlGaN, have been studied. Most of the currently utilized ultraviolet-light-receiving devices are composed of a combination of a pin diode and an amplifier. The pin diode receives the ultraviolet rays, and the amplification mechanism magnifies the output signal. However, the amplification mechanism and light-receiving element are manufactured by separate processes, and each device forming step requires subsequent assembly steps. Therefore, we propose an optical receiver fabricated from AlGaN/GaN-based high-electron-mobility transistors (HEMTs) as both photodetectors and amplifiers. Such photodetectors and amplifiers made of the same material can be fabricated on a single substrate simultaneously. Moreover, even if the received signals are weak, amplification and speeding up can be easily realized by the two-dimensional electron gas of the HEMTs. To confirm the performance of the AlGaN/GaN-based HEMT device as a photodetector, it was irradiated with ultraviolet light of wavelength 370 nm corresponding to the bandgap of GaN, and the light-receiving characteristics were measured. The expected results were confirmed in the form of an increase in the drain current in the linear region of the field-effect transistor compared with the measurements without ultraviolet irradiation. From this increase in drain current, it is considered that light absorption occurs in the GaN layer to generate carriers, which is detected as current amplification. Thus, it is considered possible to realize an optical receiver using the AlGaN/GaN-based HEMT as a photodetector that reacts to ultraviolet wavelengths and have a HEMT structure.

Resume : Compound materials formed from group II-VI semiconductor oxides are promising candidates for electronics applications. A unique combination of high exciton binding energy and a wide range of bandgap tunability is a great technological advantage to make use of these materials in optoelectronics. From these group-II-VI oxides, CdO is one of the oldest transparent conducting oxides (TCOs) used for the fabrication of many photovoltaic applications. However, a low bandgap of CdO, which is 2.3 eV (at the ambient condition), restricts its use only in the visible range of application. On the other hand, MgO, which is a direct bandgap semiconductor oxide having a bandgap of 7.8 eV can be alloyed with CdO to make the device used for light emission over UV to Visible regime. A recent report by Przezdziecka et. al. suggests that the bandgap of CdO/MgO superlattice structures(SLs) can be tuned from 2.6 eV to 6 eV by varying the thickness of monolayer in SLs. In this work we have studied Cd1-xMgxO alloy deposited on c- and m- oriented sapphire substrates using plasma-assisted MBE technique. MBE is an epitaxial growth technique in which, the thermal equilibrium has made it possible to overcome the natural solubility limit of these two ternary oxides. Introducing Mg into CdO results in interesting properties to observe in compositional graded Cd1-xMgxO alloys. The concentration of Mg dopant was varied from 0 to 96%. Structural investigation of these alloys was carried out using X-ray diffraction technique. It is observed that with an increase in Mg2 dopant in CdMgO alloys, the lattice constant decreases. The content of Cd and Mg in the alloys was determined using the Energy-dispersive X-ray spectroscopy (EDX) technique. The optical properties of thin films were investigated by UV-Vis spectroscopy at room temperature. From the absorption curve, the optical bandgaps were determined and it was found that the bandgap of films changes from 2.6 eV to 5.6 eV with varying Mg concentration. Using the Modified Vegard’s law, the calculated lattice parameters were fitted with Cd concentration, and the bowing constants were found to be -0.1003 and -0.0826 nm for CdMgO alloys on c- and m-plane sapphire substrate respectively. From the negative bowing value, we can say that the interaction between O-2p and Cd-3d is repulsive. We have studied the influence of Cd-rich to Mg-rich conditions. These results suggest that CdMgO alloys can be a promising material for deep UV light emission and /or detection applications.

Resume : The electronic structure of ZnO:Yb epitaxial layers grown on GaN/sapphire substrates was studied using X-ray absorption spectroscopy at the Zn L3,O K and Yb M5 edges at Solaris light source. Knowing that the ZnO crystallizes in an anisotropic wurtzite structure, the linear polarization of synchrotron radiation was exploited to estimate the influence of the crystal structure anisotropy on the distribution of the local density of states at the site of Zn and O. The calculated partial density of states describes the observed anisotropy in the measured spectra. Influence of the core–hole effect on the analyzed absorption spectra was verified. Acknowledgements: These research took place at the National Synchrotron Radiation Centre SOLARIS, at the PEEM/XAS beamline. The experiment was performed thanks to collaboration of the SOLARIS Team. The work was partially supported by the Interdisciplinary Centre for Mathematical and Computational Modelling (ICM) at University of Warsaw, Poland, grant ID G59-20. The samples growth was co-financed by international project supported bythe Polish Ministry of Science and Higher Education (3846/HZDR/2018/0) and Helmholtz-Zentrum Dresden-Rossendorf (17000941-ST).

Resume : Zinc oxide (ZnO) due to its direct wide bandgap and high exciton binding energy has significant potential in many applications. To expand the functionality of ZnO-based devices, solid alloys can be used. Therefore, the study of the physical properties of up to now poorly tested Zn1-xCdxO solid solutions is justified. Synthesis of Zn1−xCdxO alloys is possible by various methods and we propose to grow quasi ternary alloys based on short period superlattices. In this work we present the results of investigation of quasi ternary alloys in the form of {ZnO/CdO}m short-period superlattices (SLs) with varying layers thicknesses. The structures were grown on (10-10) m-plane sapphire substrate (Al2O3) by molecular beam epitaxy method (MBE). To study the thermal stability of the structures the SLs were annealed for 5 minutes in an O2 environment at a temperature of 900 °C. The chemical composition and interface morphology of as-grown and annealed SLs were examined by Secondary-Ion Mass Spectrometry (SIMS), Scanning Electron Microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX) and Atomic Force Microscope (AFM). The Cd concentration in as-grown SLs varies from 46% to 7.3%, depending on the structure design. The high quality of the as grown SLs is evidenced by the XRD study and low RMS value within 0.238 - 2.29 nm. Upon annealing in oxygen, the properties of {CdO/ZnO}m SLs were studied again. SIMS measurements showed that structural deformation of the SLs takes place. In particular, the SL structure is entirely destroyed, because of diffusion of Cd atoms. It results in a significant decrease of average Cd content in the samples. Also, the SIMS plots revealed an inhomogeneous depth distribution of cadmium. For a more detailed analysis of the Cd distribution with depth, the low temperature (~ 5 K) cathodoluminescence spectra (CL) were collected from different sample depths (~20, 70, 200 and 400 nm). The CL reveals a redshif of about 4 - 60 meV with depth and strongly depends on the structure, namely on Cd contents. The CL spectra reflect the Cd profiles measured with SIMS. Based on the analysis of SIMS and CL data, it was found that the lower the Cd content in as-grown {ZnO/CdO}m SLs, the more uniform the distribution of cadmium over the depth profile after annealing. This work was supported by the Polish National Science Center under Grant No. 2019/35/B/ST8/01937

Resume : Thermally nonequilibrium phonons generated by the operation of light-emitting devices and transistors deteriorate their operation properties. For example, the population distribution in excitonic states shifts to higher energy levels with low radiation rate, and electrons in the channel of a transistor are scattered leading to the decrease in electron velocity and a further increase in the resistivity. In addition to the device design by electronic viewpoint, thermal energy control is desired, where phonon transport analysis in bulk and heterostructures are required. Previously, thermoreflectance and Raman thermography are investigated. However, separation of heating position and temperature probing position is required to analyze the phonon transport through heterointerfaces. The effects of crystal defects on thermal energy transport have been extensively discussed in many articles. However, microscopic local imaging of heat transport at a specific defect is still a challenge. In Raman measurements, mode-separated information of phonons can be obtained, which is an advantage compared with the pumpprobe method of photo reflectivity. In this paper, we will show the phonon transport analysis by simultaneous irradiation of two laser beams, where the phonon generation and temperature probing are conducted by two lasers. GaxIn1-xN(100nm)/GaN(3m) heterostructures were grown on sapphire substrates by metalorganic vapor phase epitaxy. Two samples with InN mole fraction x of 0.84 and 0.05 were investigated. Microscopic Raman scattering spectroscopy was conducted by selecting a pair of lasers from the wavelength values of 532 nm, 325 nm, and 266 nm. A pair of lasers were incident on samples through an objective lens with NA=0.46 or a reflective objective with NA=0.5. Measurements are performed at room temperature. Raman signals for E2(high) and A1(LO) were obtained. The energy relaxation of generated carriers to the band edges yielded a local heating in a GaInN layer. When the Raman signal of the GaN layer was probed by the 532-nm laser, and the GaInN layer was heated by a UV laser. For the sample of x=0.84, reciprocal space mapping of x-ray diffraction (XRD) analysis showed the partial lattice relaxation. In microscopic PL images, triangle structures originating from dark lines in the regions of high misfit dislocation (MD) density were observed. Raman images of E2(high) mode of the GaN with the heating in the GaInN layer by the 325 nm laser showed the same triangle structure observed in the PL image. The temperature increase in the region with lower PL intensity was lower, which indicates the phonon transport blocking in the vicinity of misfit dislocations. The average temperature increase was estimated to be approximately 50-60 K. On the other hand, for the sample with x=0.05, which shows pseudo morphological growth in the XRD analysis, no triangle structures were observed in PL or Raman imaging. The temperature increase was less than 20 K. This result indicates the higher dissipation of heat energy or phonons in crystals with higher quality. It was found that the lateral phono transport is decayed to a half of the maximum at the position of 10 m from the heating position. These results indicate the Raman imaging using double lasers is effective to analyze phonon transport 　　　　　　　through heterointerfaces and to lateral direction inside layers.

Resume : Sesquioxides of rare earth elements with +3 ion as Y2O3, Eu2O3, and Lu2O3 are well known for their high optical excitation threshold. An experimental investigation was conducted to explore divalent (+2) yttrium in solid phase in the scope of this research. We produced nanocrystalline-amorphous semiconducting YO thin film at 623 K (± 5) utilizing reactive DC magnetron sputtering. XRD and TEM measurements reveal that the formation of yttrium monoxide is accompanied by the crystalline Y2O3 and Y. In addition to this, spectroscopic investigations such as ellipsometry and XPS and temperature-dependent resistivity measurement denote the semiconducting yttrium oxide has the dominant formation character over Y and Y2O3. The results presented here may facilitate improvements in understanding the oxidation dynamics of rare earth elements, which has a key role in todays and future technology.

Resume : The present study compares two types of TiO2 and magnetic particle (TiO2/MP) composites, based on the source of the MP component and how that influences the structural proprieties of the composites and their applicability on the field of heteorgeneous photocatalysis. As the base photocatalyst of the composite in both cases TiO2 anatase nanoparticles were chosen, TiO2 being a widely researched semiconductor with good photocatalytic properties and easy controllability of size and shape during synthesis. Starting from titanium isopropoxide (TIP) precursor, ethanol and acetic acid, the TiO2 nanoparticles were synthesized using a modified sol-gel hydrothermal method, in a stainless steel autoclave, on 180 °C. The magnetic component of the composites had two fundamentally different sources: one originated from natural magnetic sand, the other was synthetic. In order to prepare the TiO2/natural magnetic particle (TiO2/N-MP) composites, natural sand with magnetic properties was collected from the streambed of a tributary of river Olt (Brașov county, Romania), and processed by micro grinding in a planetary ball mill to an optimal particle size. The as obtained N-MP was introduced in the TiO2 synthesis at the very first step, to obtain composites of N-MP with TiO2 anatase hydrolyzed on their surface. In the case of TiO2/synthetic magnetic particle (TiO2/S-MP) composites a mixture of magnetite (Fe3O4) and maghemite (γ–Fe2O3) iron oxide nanoparticles were synthesized via a simple precipitation method, using FeCl2.4H2O as precursor. The TiO2/S-MP composites were prepared with the same method, as the TiO2/N-MP, except using the synthetic iron oxide nanoparticles. The composition, the crystal structure, the size, the morphology and the optical properties of all the components and both TiO2/N-MP and TiO2/S-MP composites were characterized employing X-ray diffraction, Scanning Electron Microscopy, Raman spectroscopy and Diffuse reflectance spectroscopy. Beside the structural properties and the stability of the composites, their photocatalytic properties were also studied, under UV-A irradiation for the removal of methyl orange (MO) dye and paracetamol from aqueous solution, with promising results. In order to investigate the influence of the MP source on the photocatalytic activity of the composites, the results were also compared with the photocatalytic effieciency of pure anatase TiO2. Acknowledgements.This work was supported by the PN-III-P1-1.1-TE-2019-1318-project. K. Saszet acknowledges the financial support of the Collegium Talentum scholarship provided by the Sapientia Hungariae Foundation.

Resume : The Inverse Faraday Effect (IFE) - the appearance of stationary magnetic moments magnetization caused by circulary polarized light - has been mostly studied in magnetic materials. Recently, the IFE was predicted in the periodic lattice of metallic disks or spheres placed the vicinity of two dimensional electron liquid and subjected to the external circularly polarized radiation. The radiation causes the DC current loops in the electron liquids, thus leading to appearance of static magnetic moments. Physically, the interaction between metal disks and two dimensional electron liquid, the "twisted" plasmonic modes are excited which lead (due to rectification) to appearance of DC circulating current In this work, we present the basic idea of IFE enhanced by twisted plasmons. The mechanism of the effect is described and the theoretical predictions are presented. The GaN/AlGaN is proposed as a basic system for the experimental realization of the IFE. In order to observe IFE experimentally, the GaN/AlGaN HEMT like structure was prepared with 2 dimensional electron gas as a channel. On top of the structure, the periodic lattice of metal disks were fabricated with use of electron beam lithography. In addition, the frequency of twisted plasmonic modes can be slightly tuned with use of conducting back - gate layer that is present in epitaxial structure. We present the theoretical predictions and the technological realization of the structure as well as its basic characterization.

Resume : Zirconium dioxide of various phase composition, widely used in modern technology, should have high stability of characteristics under various radiation effects. This is especially important when used in military and space technology, as well as in the nuclear industry. The determining factor influencing the stability of the luminescence properties of oxide dielectrics under irradiation is the formation of radiation-induced defects in the anion sublattice (oxygen vacancies). Investigation of the effect of the size of nanoparticles, doping in the synthesis of micro- and nanostructured compacts and ceramics based on ZrO2 is necessary to predict and increase the radiation resistance, as well as to elucidate the nature of the intrinsic luminescence of ZrO2, which has not yet been finally established [1-3]. Micro- and nanostructured compacts were prepared by uniaxial cold pressing of monoclinic ZrO2 nanopowder and tested using SEM (Carl Zeiss Sigma VP), and X-ray fluorescent analysis. The compacts were irradiated by 130 keV and 10 MeV electrons (Linear Electron RADAN Accelerator, LERA-10-10C, Yekaterinburg, Russia) and 200 MeV Xe ions (cyclotron DC-60, Nur-Sultan, Kazakhstan). Thermostimulated luminescence (TSL) of irradiated samples showed the presence of different peaks. A number of TSL peaks have been identified. The kinetic parameters of the TSL were also calculated. References 1. C. Barry Carter, M. Grant Norton. Ceramic Materials. Science and Engineering. Springer, 2007. 716 p. 2. Salari, S., and F.E. Ghodsi. Journal of Luminescence 182 (2017): 289-299. 3. Aleksanyan E., Kirm, M., Feldbach, E., & Harutyunyan, V. Radiation Measurements 90 (2016): 84-89. Keywords: zirconium dioxide, electron beam, swift heavy ions, thermostimulated luminescence, kinetic parameters.

Resume : Among the many oxides, aluminum-magnesium spinel MgAl2O4 is one of the most radiation-resistant dielectric materials that are promising for use in nuclear power devices. [1]. At present, most of experimental studies about radiation defects in spinel were received using high-energy electrons, neutrons, and low energy ions [2, 3]. The aim of this work is to study radiation defects in MgAl2O4 irradiated with swift heavy ions using picosecond pulsed photoluminescence. Spinel single crystals were irradiated with 710 MeV Bi, 167 MeV Xe, 107 MeV Kr and 46 MeV Ar ions up to the fluence 2·1013 сm-2 at IC-100 and U-400 cyclotrons (Dubna, Russia). The photoluminescence spectra excited by picosecond laser source (wavelength 445 nm, pulse duration FWHM < 80 ps) have been measured at room temperature using the confocal microscope Integra Spectra, NT-MDT. It was shown that the PPL spectra from unirradiated MgAl2O4 contain sharp lines with a maximum at 1.8 eV related to emission of Cr3+ ions and a band at 2.35 eV, which is assigned to Mn2+ ions. As a result of irradiation with high energy Kr ions, a broad emission band appears at 1.7 – 2.5 eV (500 – 750 nm), which indicates the radiation origin of the corresponding luminescence centers. Similar bands were observed in the PPL spectra of MgAl2O4 irradiated with Bi, Ar and Xe ions. It was found that radiative lifetimes equal 8 ns, 8.9 ns, 12.3 ns and 13.7 ns for the emission wavelength 500 nm, 520 nm, 620 nm and 650 nm, respectively. References: 1. S.S. Raj, S.K. Gupta, V. Grover, K.P. Muthe, V. Natarajan, A.K. Tyagi. Journal of Molecular Structure 2015; 1089: 81-85. 2. A. Lushchik, E. Feldbach, E.A. Kotomin, I. Kudryavtseva, V.N. Kuzovkov, A.I. Popov, V. Seeman, E. Shablonin. Scientific reports, nature search 2020; 10: 1-9. 3. E. Feldbach, I. Kudryavtseva, K. Mizohata, G. Preditis, J. Raisanen, E. Shablonin, A. Lushchik Optical Materials 2019; 96: 1-7. Keywords: aluminum-magnesium spinel MgAl2O4, swift heavy ions, pulsed photoluminescence, radiation defects.