Epitaxial integration of dissimilar materials: challenges and fundamentals

Resume : Atomically thin transition metal dichalcogenides (TMDC) are potential materials for next generation applications for optoelectronics or memory devices. Especially, electrical properties of TMDC can be varied depending on certain oxide substrate. Among them, semiconducting TMDC can show photovoltaic performance in the heterostructure. Here, we utilized n-type MoS2 and p-type WSe2 as semiconducting materials. We used epitaxial PbTiO3 thin films grown by a pulsed laser deposition method. Here, we used conductive-atomic force microscopy (C-AFM) to obtain charge transport in the atomic sheets and photocurrent was obtained by using C-AFM with light illumination. In addition, change of conducting states was observed with different polarization states of PbTiO3 thin films. Especially, resistive switching was obtained at the atomic sheets assisted by ferroelectric thin films. Bound charges from ferroelectric polarization gives variation of free carriers in the atomic sheets resulted from Coulomb interaction. Therefore, contribution of each carrier from external stimulation at the atomic sheets can be analyzed. To get exact contribution of photo-induced carriers, we used Kelvin probe force microscopy to investigate surface potential and surface photovoltage measurement was also conducted. From this point of view, photovoltaic non-volatile memory device can be suggested as a new multi-functional electronic device and we suggest the vertical device consisting of 2-dimensional materials and ferroelectrics.

Resume : The properties of layered material heterostructures are crucial to the success of devices based upon the novel capabilities of 2D materials. Theoretical studies of such heterostructures have been limited by the large system sizes required to study incommensurate or rotated interfaces. We have employed high-accuracy linear-scaling DFT calculations utilising non-local van-der-Waals functionals to explore large-scale models of a range of heterostructures of interest to device applications. I will present results for heterostructures including MoS2/MoSe2, MoSe2/WSe2, TMDC/Graphene, and hBN/Phosphorene in a range of geometries. Band-alignments and modifications of the electronic structure upon stacking and rotation of different monolayers can be obtained by unfolding the supercell spectral function into the primitive cells. Changes in spectral weight and band-structure between the monolayers and heterostructured interfaces show how lattice mismatch (MoS2/MoSe2) or spacer layers (Phosphorene/hBN/Phosphorene) allows the component monolayers to retain more independence in heterostructures than in homo-stacks. Finally, applying electric fields allows the behaviour of gated structures to be predicted and explained.

Resume : The formation of low dimensional systems is a powerful tool for tuning the physical properties of materials. The unique characteristics of two-dimensional (2D) nanomaterials make them highly attractive for a wide range of applications. A variety of approaches have been explored to synthesize and control their size, morphology and crystal structure. Template-directed synthesis represents a convenient and versatile route and both organic and inorganic-based materials are usually employed for this purpose. Since carbon nanotubes (CNTs) can have a wide range of diameters and lengths, their inner hollow cavity can be filled with a large variety of compounds. Furthermore, the presence of CNTs might create synergistic effects with the filling material, resulting in unique properties and superior performance. Despite their interest, the number of reports on single-walled inorganic nanotubes is limited because the formation of their multi-walled counter parts is favored during the synthesis. Here we report on the synthesis of individual layers of several 2D van der Waals solids, namely CeI3, CeCl3, TbCl3 and ZnI2[1], and PbI2[2] using CNTs as directing agents. Recent theoretical studies reveal that these heterostructures, consisting of single-layers and a carbon host can substantially enhance the visible light response, suggesting potential applications in novel 2D optoelectronics and photovoltaics[3]. Once confined, the metal halides can adopt different structures including single-layered metal halide nanotubes, which formation is greatly enhanced by increasing the temperature of synthesis[1]. This opens up a new strategy for the isolation of individual layers of a wide variety of metal halides, a family of 2D materials that has been barely explored. [1] Sandoval S., Pach E., Ballesteros B., Tobias, G. Carbon 123, 129 (2017). [2] Cabana L., Ballesteros B., Batista E., et al. Adv. Mater. 26, 2016 (2014) [3] Zhou M., Duan W., Chen Y., Du A. Nanoscale 7, 168 (2015).

Resume : Doping graphene layers presents a difficult practical and fundamental problem. We consider theoretically, the possibility of electrostatic doping of graphene by the intrinsic field of a polar substrate. By way of example, we perform density functional theory calculations for a graphene sheet placed on the (111)-oriented perovskite SrTiO3 surface. We find that the Fermi surface moves well below the Dirac point of graphene, resulting simultaneously in a fast conducting channel in graphene, and a slow (large-effective-mass) channel at the oxide surface. Additionally, electrostatic gating may open a way to explore peculiar states that, through the “no-crossing”, represent a hybrid carrier that exists simultaneously in both materials. . Moreover, further theoretical analysis suggests that coupling of electrons in graphene to interfacial hybrid plasmon/optical modes may result in effective attractive electron-electron interaction that, in turn, could result in electron pairing and lead to superconductivity.

Resume : Stacking few-monolayer two-dimensional (2D) semiconductor with an insulator is critical to enable electrostatic control of electron transport in 2D film. However, difference in effective work-function (EWF) between the 2D “channel” and the control gate may give rise to built-in potentials which disturb electron motion. This mandates thorough examination of the electrostatics at the interfaces of 2D semiconductors as affected by the insulating material. In this work we used internal photoemission of electrons (IPE) from 1 monolayer MoS2 films synthesized on top of SiO2 or Al2O3 to determine energy of the semiconductor valence band (VB) relative to the reference level of the insulator conduction band (CB). This allows us to compare the VB top energy in MoS2 to that of the (100)Si substrate crystal at the interface with the same insulator. Despite the CB in Al2O3 found to be ≈1 eV below that in SiO2 as measured relative to the Si VB edge, we observe nearly no shift of the spectral threshold of IPE from the MoS2 VB. This observation indicates violation of electroneutrality at the MoS2/Al2O3 interface causing increase of EWF by ≈1 eV. This conclusion is supported by much weaker field dependence of the IPE threshold at the MoS2/Al2O3 interface as compared to the MoS2/SiO2 one suggesting presence of negative charges and/or interface dipole. Therefore, the commonly accepted Schottky-Mott picture appears to be not appropriate to describe the band alignment at 2D/insulator interfaces.

Resume : Two-dimensional materials, such as graphene, epitaxially grown on surfaces can form a variety of moire superstructures due to lattice parameter and/or symmetry mismatch. We give a very general formulation of the problem in terms of two generic 2D Bravais lattices and propose a systematic procedure to determine their coincidence network for a given misorientation. The procedure allows to identify the minimum size simulation cell to be used, for instance, in atomistic simulations. We discuss the application to the specific case of a hexagonal lattice over a square lattice and compare our findings with the experimental evidence of graphene grown on a Ni(100) surface [1]. [1] Z. Zou, V. Carnevali, M. Jugovac, L.L. Patera, A. Sala, M. Panighel, C. Cepek, G. Soldano, M.M. Mariscal, M. Peressi, G. Comelli, C. Africh, Graphene on nickel (100) micrograins: Modulating the interface interaction by extended moiré superstructures, Carbon (2018), doi: 10.1016/j.carbon.2018.01.010.

Resume : The transfer process of graphene onto the surface of ferroelectric thin films is well known. However, for many devices, is required high quality oxide thin films to be grown on the surface of graphene. This step is not understood. It is not clear why the oxide should adopt the epitaxy of the underlying oxide layer when it is deposited on graphene where there is no lattice match. To date there has been no explanation or suggestion of mechanisms which clarify this step. Here we present a mechanism, supported by first principles simulation and structural character-isation results, for the growth of oxide ferroelectric thin films on graphene. We describe the growth of epitaxial SrTiO3 (STO) thin films on a graphene and show that local defects in the graphene layer (e.g. grain boundaries) act as bridge-pillar spots that enable the epitaxial growth of STO thin films on the surface of the graphene layer. In this study, SrTiO3 layers with thicknesses varying from 10nm to 100 nm were deposited at a temperature of 850 oC, and oxygen pressure ranging from 0.01 mTorr to 300 mTorr using pulsed laser deposition (PLD). The two-dimensional growth of the STO layer was monitored in-situ by RHEED. To measure its electrical properties, the STO film was covered with a 50 nm Au layer using dc magnetron sputtering; and MIM capacitor structures were formed with pho-to-lithography followed by ion-milling. The surface of the STO film was analyzed using AFM, while its crystal structure was examined by x-ray, SEM and TEM. Results of the electrical measurements carried out within a tempera-ture range of 77K up to 100oC will be presented. The suggested mechanism for epitaxial growth of oxides on graphene, offers new directions to exploit the development of ferroelectric/graphene multilayer structures and devices.

Resume : The high-tech impact of combining silicon substrate with the properties of functional oxides is enormous given its relevant role in the development of novel efficient devices. However, the precise control of interfaces and crystallization mechanisms at the nanoscale of dissimilar materials need to be further developed. As an example, the integration of high quality epitaxial oxide films and nanostructures on silicon as hybrid structures rest extremely challenging because these materials are structurally and thermally different. This work describes promising strategies used to accommodate advanced oxide nanostructures and thin films on silicon substrate via chemical solution deposition approach. Divers examples will be presented separated in two different approaches i.e: (i) perovskite oxides with enhanced physical properties performed by combining chemical and physical techniques, such as epitaxial BiFeO3, BaTiO3 or La0.7Sr0.3MnO3 nanostructured thin films on silicon using the combination of soft chemistry and molecular beam epitaxy [1,2], and (ii) oxide materials entirely performed by soft chemistry, such as nanostructured piezoelectric quartz thin films on silicon [3] or ferroelectric oxide nanowires thin films epitaxially grown on silicon [4] As a conclusion, this presentation will cover cutting-edge strategies based on the potential of combining epitaxial growth and chemical solution deposition to develop functional oxide nanomaterials on silicon with novel structures and improved physical properties. [1] Electric and Mechanical Switching of Ferroelectric and Resistive States in Semiconducting BaTiO3–δ Films on Silicon. A. Gómez, J. M. Vila‐Fungueiriño, R. Moalla, G. Saint‐Girons, J. Gázquez, M. Varela, R. Bachelet, M. Gich, F. Rivadulla and A. Carretero‐Genevrier - Small, (2017) [2] Integration of functional complex oxide nanomaterials on silicon J. Vila-Fungueirio, R. Bachelet, G. Saint-Girons, M. Gendry, M. Gich, J. Gazquez, E. Ferain, F. Rivadulla, J. Rodriguez-Carvajal, N. Mestres, A. Carretero-Genevrier Frontiers in Physics 3, 38 (2015) [3] Soft chemistry based routes to epitaxial A-quartz thin films with tunables textures A.Carretero-Genevrier, M.Gich, L.Picas, J.Gazquez, , D.Grosso, C.Boissiere, J.Rodriguez-Carvajal, and Clement Sanchez. Science. Vol 340. Pp 827-831 (2013) [4] Direct Monolithic Integration of Vertical Single Crystalline Octahedral Molecular Sieve Nanowires on Silicon. A. Carretero-Genevrier, J. Oro-Sole, J. Gazquez, C. Magn, L. Miranda, T. Puig, X. Obradors, E. Ferain, C. Sanchez, J. Rodriguez-Carvajal, N. Mestres. Chemistry of Materials 26 1019 (2014)

Resume : Inorganic Perovskites based on Ferroelectric oxide (FEPO) materials can be of great interest to several energy applications including photovoltaics. When compared to hybrid perovskites, FEPO cells can benefit of strong reproducibility and high stability. Furthermore, there is no need for a p-n junction as in conventional photovoltaic devices since the electric polarization from ferroelectricity is responsible for electron-hole separation. Yet, few work has concerned FEPO based solar cells. The main challenges are developing FEPO materials with high absorption with the solar spectrum, an appropriate bandgap and a well ordered structure. In this work, we report on the structural, optical and electrical properties of Bi2FeCrO6 (BFCO) oxide layers by the pulsed laser deposition (PLD) technique. High quality epitaxial growth and phase-pure films are demonstrated by X-ray diffraction. We have investigated the variation of parameters such as the bandgap versus the growth conditions, and demonstrated that it can be tuned from 1.9 to 2.6 eV. Modeling of the structure has confirmed our experimental results [1]. The ferroelectric properties were studied using the piezoresponse force microscopy (PFM) technique. Finally, test devices based of BFCO were fabricated and their photovoltaic properties will be presented. References: [1] A.Quattropani , D.Stoeffler, T.Fix, G.Schmerber, M.Lenertz,, G.Versini ,J.L.Rehspringer, A Slaoui, A.Dinia and S.Colis, DOI: 10.1021/acs.jpcc.7b10622

Resume : The substituted manganite perovskite La2/3Sr1/3MnO3 (LMSO) is an interesting material for spintronic devices due to its ferromagnetic properties at room temperature and its half-metallic character suggesting a strong spin polarization at the Fermi level. It is characterized by a small magnetic anisotropy, a low coercive field and a low resistivity. All these properties recommend this system to be used as an electrode in magnetic tunnel junctions that can be further integrated in spintronic devices. However, as already pointed out by previous studies, the transport and magnetic properties are strongly correlated to the quality of the magnetic/non-magnetic interfaces. It was suggested that the interfaces should be controlled at the atomic scale as this was shown in the case of the Fe/MgO system. The present work is focused on the growth, structural, morphological and magnetic properties of LSMO/SrTiO3/LSMO heterostructures. The samples are deposited by pulsed laser deposition on atomically flat TiO2-terminated SrTiO3(100) substrates and the growth is monitored by RHEED over the whole deposition process. The intensity oscillation of the RHEED specular spot show a layer-by-layer growth for the whole stack. X-ray diffraction and scanning transmission electron microscopy observations indicate clearly that the samples are single crystalline and that the interfaces are flat at atomic level and exempt of interdiffusion. Room temperature magnetic measurements show a sharp two step magnetization reversal corresponding to the two LSMO layers. Along with the structural characterization, these measurements let us think that magnetic tunnel junctions with large magnetoresistance ratios are expected paving thus the way for stable full oxide sprintronic devices.

Resume : A recent discovery of growth methods that allow for growing transition metal oxides or TMOs directly on Si, created revolutionary opportunities in silicon photonics, a hybrid technology combining semiconductor logic with optical information technologies. I will discuss the recent progress in development and integration of nonlinear optical materials that will enable combining semiconductor logic with fast broadband optical. This combination advances the nonlinear optical materials for defense applications and telecom industries, where the benefits of reliability, smaller size and weight, and lower cost of ownership are compelling. In particular, neurocomputing may benefit from this technology. A typical neurocomputer is a network of simple artificial neurons that process information in parallel. The salient features of this paradigm, its massive connectivity and parallelism, are most naturally realized in optics, where analog inner products and simple point nonlinearities are precisely the computations needed. Optics and neural networks present an ideal match of requirements and capabilities. I will briefly discuss possible opportunities in neuromorphic computing created by integration of TMOs with silicon.

Resume : The crystal structure of perovskite oxides (general formula ABO3) can host an impressive variety of cations without being significantly modified. In addition, this crystal structure is very flexible against insertion of native or foreign atomic planes, as typically exemplified by the stability of oxide based Ruddlesden-Popper homologous series. These specificities confer on this material class unparalleled flexibility for functional properties engineering, and molecular beam epitaxy offers unrivalled growth control capability to take the best advantage of this flexibility. In this context, we will first give a global overview of the different levers to engineer the functional properties of perovskite oxides. We will then show how molecular beam epitaxy enables implementing specific growth strategies to improve the structural quality of SrTiO3 epitaxial layers grown on Si and GaAs, enabling the integration of perovskite oxides on semiconductor platforms. In the end, we will show how epitaxial structuration down to the atomic layer level, allowed by molecular beam epitaxy, enables for tuning the optical response of perovskite oxides, leading to the emergence of promising properties such as hyperbolicity in the optical domain.

Resume : Integration of the semiconductors on the nanoscale level is a modern challenge for the present materials science. This became crucial after invasion of two-dimensional (2D) materials and need for design of artificial 3D crystals or vdW heterostructures (2D HEX). The presently dominating approach for 2D HEX fabrication is assembling of separate flakes into a stack. While the highly desirable direct growth of 2D HEX can be realized via the van der Waals epitaxy through accurate growth parameters tuning and proper substrate choice. Graphene (Gr), being a dangling bonds free surface can serve as appropriate substrates for vdW epitaxy. We have studied the conditions for epitaxy of thin metal oxide films - ZnO, Ga2O3 - on Gr/SiC substrate templates by MOCVD and spray pyrolysis. The samples were studied by HRTEM, HRXRD, SEM and AFM methods; it is revealed that despite the low growth temperature and large lattice mismatch, the films deposited possess advanced crystal properties (i. e. are strain-free, with no in-plane epitaxial relationship and are weakly attached to the substrate). An unusually high ultraviolet and visible photoluminescence were observed from the ZnO/Gr/SiC heterostructures in contrast to identical samples on conventional substrates. The phenomena observed are explained with the graphene role as a dangling bond free substrate, providing conditions for vdW epitaxy of metal oxides and eventually leading to novel heterostructures with added functionality.

Resume : The epitaxial layer transfer process was introduced to integrate high-quality and ultrathin III-V compound semiconductor layers on any substrate. However, this technique has been limited to fabricate sub-micron nanoribbons due to the diffraction limit of photolithography. In order to overcome this limitation and scale down its width to sub-50nm, we need either costly short wavelength lithography system or non-optical patterning method. In this work, high-quality III-V compound semiconductor nanowires were fabricated and integrate on a Si/SiO2 substrate by a soft-lithography top-down approach and an epitaxial layer transfer process, using MBE-grown ultrathin InAs as a source wafer. The width of the InAs nanowires is controlled by using solvent-assisted nanoscale embossing (SANE), descumming, and etching processes. By optimizing these processes, the NWs with their width below 50 nm were readily obtained. The InAs NWFETs prepared by our method demonstrate peak electron mobility of ~1600 cm2/Vs, indicating negligible material degradation during SANE process.

Resume : Understanding the fundamental properties of graphene when its topography is patterned with the use of a compliant substrate is essential to improve the performances of graphene sensors. In this work, we suspend a graphene monolayer on SiO2 nanopillar arrays to form a puckered graphene-on-lattice and investigate the strain and electrical transport by combining Raman spectroscopy with nanometre-scale characterization tools such as tip enhanced-Raman spectroscopy (TERS), infrared near-field microscopy (s-SNOM) and four-point probe transport measurement with scanning tunnelling microscopy (STM). Despite a nonuniform strain in the graphene-on-lattice, the nanoscale investigation reveals that the resistivity is governed by a thermally activated transport and not the strain. We show that the high thermal activation energy results from a low charge carrier density and a periodic change of the chemical potential induced by the interaction of the graphene monolayer with the nanopillars, making the use of graphene-on-lattice attractive to further enhance the electronic response of future graphene sensors.

Resume : Indium Arsenide nanowires (NWs) were grown on Silicon (100) substrate using Silver (Ag) nano particles as catalyst in atmospheric pressure Metal-Organic Chemical Vapor Deposition (MOCVD) technique. Ag nanoparticles were prepared by reduction of AgNO3 with monoethanolamine in aqueous solution. NWs were grown at three different temperatures for a fixed value of III/V ratio to observe the temperature dependence. At low temperature agglomeration of species occurs. At high temperature both density and length of NWs decrease. Substrate temperature has a direct effect on effective adatom’s diffusion length (λa) and mobility on the substrate surface. At the higher temperature end, λa is desorption limited and it increases with lowering the temperature as the conventional diffusion length of a single adatom. At low temperature more nucleation occurs at substrate surface and eventually surface density of islands increases. When λa is surface nucleation limited, adatoms are trapped by surface nucleuses and cannot reach the wire base which results a reduction in adatom’s diffusion length. So, λa has a maximum value at a particular temperature which is near about 550 ˚C in our case. Morphology and crystal structure has been investigated by Scanning Electron Microscopy (SEM) and High Resolution Transmission Electron Microscopy (HR-TEM). Grown NWs have pure zinc blend crystal phase with stacking faults. Further Raman Spectroscopy has been done to exactly determine the existence of InAs nano crystal on Si substrate. Down shifting of Transverse and Longitudinal optical (TO & LO) phonon peaks along width broadening w.r.t. InAs bulk crystal’s phonon peak, were observed. Inconclusively those non idealities of Raman Peaks are mainly caused by crystal defects.

Resume : We have investigated the properties of GaAs single solar cell grown on Si substrates with directly grown GaAs buffers which has been improved by the thermal cycling annealing method. In this study, the GaAs buffer was directly grown on Si without the growth of Ge and graded SiGe layers. In case of directly grown GaAs buffer, the formation of GaAs seed grown at low temperature was important because it related on lateral direction growth to alleviate the lattice mismatch between Si substrates and GaAs. As the V/III ratio and growth rates of GaAs seed were low and increased, the quality of GaAs buffer was well improved. After the growth of GaAs seed, the GaAs buffer was grown with TCA method. As the period of TCA was increased, the quality of GaAs buffer was improved. The full width half maximum (FWHM) of double X-ray diffraction (DXRD) and dislocation of GaAs buffer on the optimal condition were 102 arcsec and about 1 x 15 cm2, respectively. Finally, the efficiency of the GaAs single solar cell was grown on the optimal GaAs buffer was 15.7 %. This result was 69 % compared to it of the same solar cell structure grown on GaAs substrate.

Resume : Electric field control of magnetization allows further miniaturization of integrated circuits relying on functional layers for binary bit processing and data storage as it eliminates the need for bulky systems such as solenoids to induce magnetic fields. Tailoring magnetoelectric coupling in bulk of multiferroic thin films and piezoelectric strain control of spin orientation in layers with magnetic ordering have been two approaches that were investigated extensively. One other phenomenon, namely spin-dependent screening has been studied mostly from the point of spin-selective tunnel junctions. Here, we analyze the spin-dependent screening of ferroelectric polarization in a ferroelectric film interfacing ferromagnetic electrodes. We develop a formalism using a thermodynamic approach combined with electrostatics that takes into account the spin-degree of freedom of carriers in the magnetic electrode. The competition between the electrostatic effects and the mean-field exchange between spins in the magnetic electrodes gives rise to various degrees of screening of the ferroelectric polarization. We show that the density of states for spin subbands can vary strongly near the interfaces due to the competition between the magnetic and electrostatic energies. The carrier and spin population near the ferroelectric/ferromagnetic interface is also a strong function of the polarization profile across the ferroelectric thin film, a point overlooked in previous reports.

Resume : VO2 is an archetypal correlated oxide system and undergoes a temperature driven metal-insulator transition (MIT) near 340 K with a concomitant change in crystal symmetry. Resistivity of the system changes 3 to 5 orders of magnitude at the phase transition and transmittance of far infrared region decreases significantly at the metallic phase, which can be used in various applications such as metal-insulator-transition devices and smart window. A fundamental understanding of the metallic phase and its role in triggering the MIT is critical to engineering devices based on the MIT in VO2 and related materials. In this work, we deposit epitaxial VO2 films by aerosol-assisted chemical vapor deposition on sapphire substrates of different surface orientations, C (0001), M (101 ̅0) and A (112 ̅0) planes. X-ray diffraction and transmission electron microscopy results show high structural quality of the films. Depending on the substrate orientation, VO2 films are grown with (020)/(002), (2 ̅31)/(202), and (200)/(2 ̅11) planes of VO2 being parallel to the substrate surfaces of C-, M-, and A-sapphires, respectively. We expect that the films are subject to either tensile or compressive strain induced by substrate due to the structural difference between VO2 and the substrates. The temperature dependencies of the resistivity of the films are also different depending on the film orientations. We discuss the effect of strain on MIT of VO2 film grown on the different orientations of sapphire substrates. Also, Ruthenium doping effects on the structural and electrical properties of VO2 will be discussed in conjunction with MIT of the system.

Resume : BiFeO3 (BFO) is a well-known room temperature multiferroic material and shows large polarization. Recent reports on strain-induced tetragonal-like (T-like) structure of BFO film on LaAlO3 (LAO) substrate attracted much attention due to its extraordinary properties of giant polarization and domain structures. However, the origin of the structural change to T-like phase is not understood clearly because of the observation of T-like phase in small-strained films grown on SrTiO3 (STO). In this work, T-like phase of BFO films are grown on c-sapphire substrate at low temperature of 440 C by aerosol-assisted chemical deposition. Cocktailed precursor solutions of Bi and Fe source materials are controlled to grow stoichiomatric BFO films by changing their molar ratios of the sources. The structure of BFO films grown on LAO and STO by the same deposition conditions result in T-like and usual rhombohedra structure, respectively, confirming that substrates can affect the structure of BFO films. We report effects of deposition condition of vapor supply on the structure of Bi-Fe-O complex films. We also report the orientation of sapphire substrates which affect the growth direction of BFO films as well as their structures.

Resume : Black phosphorus (BP) is a promising 2D semiconductor with an intrinsic direct bandgap and considerable carrier mobility attracting lots of interests.[1-4] The thickness dependent intrinsic bandgap of BP ranges from 0.3 eV in bulk to 2.0 eV in monolayer analogous, which is a good candidate for photoelectric devices.[5, 6] Moreover, it is reported that the carrier mobility of BP transistor reaches up to 1000 cm2/V.s at room temperature, which is much higher than the other 2D counterparts suggesting that BP is competent for the future integrated circuits.[1, 7, 8] More importantly, the puckered honeycomb atomic structure leads to more profound properties in BP, such as the zigzag direction is more preferential for electric conductance and strain engineering.[8, 9] It is predicted that the giant anisotropy of BP can be tuned by external voltage and strain.[10, 11] However, the weak ambient stability is the Achilles’ Heel, naked BP suffers from the strong hydrophile and degrades quickly in ambient environment for several hours due to the adsorption of oxygen and moisture. This feature irritates lots of physical or chemical methods to isolate the BP from the oxygen or moisture, such as Al2O3 capping[12], PxOy capping[13] or chemicals passivation[14]. Among these methods, poly(methyl methacrylate) (PMMA) is a kind of common organic material and convenient to encapsulate the BP for protection. Our experiments prove that PMMA encapsulation can not only slow the degradation but also modify the electrical transport property of BP devices effectively. Herein, we investigated the transport property of BP field effect transistor modified by the PMMA encapsulation comprehensively. It was found that PMMA encapsulation resulted in electron doping in BP. The charge neutral point moved to the negative after PMMA encapsulation. Moreover, the on/off ratio had been increased by about six times. These significant changes are attributed to the surface charge transfer induced by fixed positive charge in the interface between the PMMA layer and BP. Since cross-linking the PMMA film by electron beam irradiation with high dose exposure (over 20000 μC/cm2) can create more free radicals, it was found that more obvious charge neutral point negative motion was observed. The ambipolar BP field effect transistor was dominated by the electron conductance in the absent of gate voltage. Through analyzing the I-V characteristics of BP field effect transistors encapsulated by the cross-linked PMMA, we discovered that the PMMA encapsulation can effectively reduce the Schottky barrier height between the BP and electrodes. Based on the heavy n-type doping by cross-linked PMMA encapsulation, BP heterostructure with half channel covered with PMMA was fabricated. PN junction based on BP shows obvious bipolar switching behavior under various applied gate voltages suggesting a promising application in future logic circuit. Reference [1] L. Li, Y. Yu, G. J. Ye, Q. Ge, X. Ou, H. Wu, D. Feng, X. H. Chen, Y. Zhang, Nat. Nanotechnol. 2014, 9, 372. [2] S. Zhang, J. Yang, R. Xu, FanWang, W. Li, M. Ghufran, Y.-W. Zhang, Z. Yu, G. Zhang, Q. Qin, Y. Lu, ACS Nano 2015, 8, 9590. [3] A. Castellanos-Gomez, L. Vicarelli, E. Prada, J. O. Island, K. L. Narasimha-Acharya, S. I. Blanter, D. J. Groenendijk, M. Buscema, G. A. Steele, J. V. Alvarez, H. W. Zandbergen, J. J. Palacios, H. S. J. van der Zant, 2D Mater. 2014, 1, 025001. [4] H. O. Churchill, P. Jarillo Herrero, Nat. Nanotechnol. 2014, 9, 330. [5] S. Das, W. Zhang, M. Demarteau, A. Hoffmann, M. Dubey, A. Roelofs, Nano Lett. 2014, 14, 5733. [6] H. Liu, A. T. Neal, Z. Zhu, Z. Luo, X. Xu, D. Tomanek, P. D. Ye, ACS Nano 2014, 8, 4033. [7] S. P. Koenig, R. A. Doganov, H. Schmidt, A. H. Castro Neto, B. Özyilmaz, Appl. Phys. Lett. 2014, 104, 103106. [8] F. Xia, H. Wang, Y. Jia, Nat. Commun. 2014, 5, 4458. [9] Y. Y. Li, Z. X. Hu, S. H. Lin, S. K. Lai, W. Ji, S. P. Lau, Adv. Func. Mater. 2017, 27, 1600986. [10] R. Fei, L. Yang, Nano Lett. 2014, 14, 2884. [11] X. Peng, Q. Wei, A. Copple, Phys. Rev. B 2014, 90, 085402. [12] J.-S. Kim, Y. Liu, W. Zhu, S. Kim, D. Wu, L. Tao, A. Dodabalapur, K. Lai, D. Akinwande, Sci. Rep. 2015, 5, 8989. [13] J. Pei, X. Gai, J. Yang, X. Wang, Z. Yu, D. Y. Choi, B. Luther Davies, Y. Lu, Nat. Commun. 2016, 7, 10450. [14] C. R. Ryder, J. D. Wood, S. A. Wells, Y. Yang, D. Jariwala, T. J. Marks, G. C. Schatz, M. C. Hersam, Nat Chem 2016, 8, 597.

Resume : Ferroelectric tunnel junctions (FTJs) have drawn considerable attention for non-volatile memory applications. FTJs consist of two metal electrodes separated with a thin ferroelectric barrier. It is widely believed that the FTJ performance is sensitive to the electrode material used or the interface between the electrode and the ferroelectric barrier. Recently, graphene has been used as electrodes to change the interface behavior to enhance the device performance in FTJs. Also, using semiconductor electrodes has been shown to greatly increase the switching characteristics. Here, we report the tunneling electroresistance effect (TER) of BaTiO3/Nb:SrTiO3 heterostructure with graphene electrodes. We find that a large TER ratio above 10000% is achieved at room temperature, which is important to non-volatile memory applications. The result suggests a new route to enhance the non-volatile memory performance and design the memory device architecture.

Resume : Scandium nitride ScN is a rocksalt semiconductor with a high hardness and a high melting point and a very interesting optical and electrical properties. Unfortunately, due to the high oxygen affinity of the Scandium, ScN has to be synthesized under an ultrahigh-vacuum environment to minimise the oxygen and hydrocarbon impurities. In this study, ScN thin films with thicknesses varying from 20 nm to 200 nm were deposited by DC magnetron sputtering in an ultrahigh vacuum system. Different deposition temperatures were tested to observe the evolution of ScN properties as a function of the crystalline quality and the chemical compound of films. It has been observed that defects induced by a weak crystallization tend not to ensure the chemical stability of the ScN, and can cause oxidation under atmospheric conditions. Nevertheless, the formed scandium oxide layer plays the role of a passive layer at the surface of ScN samples to protect them from oxidation. A high dependence of nitrogen incorporation is observed from the deposition energy used during the growth of layers. A clear evolution of direct bandgap energies leads to the crystalline quality of films varying from ~ 3 eV to ~ 2.1 eV for the direct bandgap. The electrical resistivity, as well as the electron mobility of ScN films, are strongly linked to the oxygen impurities and grain sizes in these films. The results indicate that the expected ScN properties are observed for films thicker than 100 nm.

Resume : We report on a high-performance vertically-injected broadband UV-to-IR photodetector based on Gd-ZnO nanorods (NRs)/CH3NH3PbI3 perovskite heterojunction fabricated on metal substrates. For the first time, the perovskite-based photodetector is demonstrated to extend its detection capability from deep UV to infrared (IR) (> 1000 nm) wavelength range with high room temperature responsivity. The Gd-ZnO NRs were grown by pulsed laser deposition (PLD) on metal substrate, then perovskite was synthesized on NRs. The PLD optimized parameters are 100 mTorr oxygen partially pressure, 350 mJ laser power, 9 cm target-substrate distance, and 650 C growth temperature. X-ray diffraction and scanning electron microscopy measurements confirmed that high-quality single crystal ZnO NRs were obtained. Photoluminescence measurements revealed high optical properties of Gd-doped ZnO NRs/perovskite. The electrical characterizations were carried out by a halogen light source and a tunable infrared laser (1260–1630 nm) that showed our perovskite-based photodetector is sensitive to a broad spectral range (250−1357 nm). Our device structure leads to a high photo-responsivity of 28 and 0.22 A/W with high detectivity values of 1.1×10^12 and 9.3×10^9 Jones, for white light and IR illumination, respectively. Absorption spectra demonstrate that the IR detection is due to intraband transition in the perovskite material. Metal substrate boosts carrier injection, resulting in higher responsivity compared to that of the conventional devices grown on glass, whereas the presence of Gd increases the ZnO NR performance. This work can lead to a high-performance ingenious cost-effective broadband photodetector design for large-scale applications.

Resume : The monolithic integration of III-V based laser sources on a silicon chip requires avoiding/limiting the formation of antiphase disorder at the polar-on-nonpolar III-V/Si interface [1]. We present here a detailed scanning tunneling microscopy (STM) study of epitaxial GaP layers grown by molecular beam epitaxy (MBE) [2] on nominal and vicinal Si(001) substrates for photonic applications. The surface morphology of GaP films with thickness varying between 3 and 220 nm is investigated from large scale down to the atomic level. We show that the use of vicinal Si(001) substrates presenting bi-atomic steps only promotes growth of a dominant polarity at the early deposition stages, allowing the complete annihilation of the antiphase boundaries (APBs) for thicker GaP films. From atomically resolved STM images, we also observe that the terminal GaP facets at the early growth stages are strongly modified by the use of a vicinal Si(001) substrate. Based on these STM observations, the three-dimensional GaP/Si(001) MBE growth mode is discussed in terms of surface/interface energies computed by ab-initio calculations. This study gives new ideas on the effect of a controlled substrate miscut on APBs formation and annihilation mechanisms. This work is supported by the French National Research Agency project ANTIPODE (Grant no. 14-CE26-0014-01), Région Bretagne and Rennes Métropole. [1] H. Kroemer, J. Cryst. Growth 81, 193 (1987). [2] Y. Ping Wang et al., Appl. Phys. Lett. 107, 191603 (2015).

Resume : Recently, in the water splitting context[1], [2], it has been demonstrated that the texturation of surfaces at the electrode level greatly enhances the efficiency of BiVO4 photoanodes in PEC devices[3]. Moreover, in the study of the semiconductor photocatalyst materials[4], many researches proposed to use the GaP semiconductor as a photoelectrode in PEC devices[5] especially because its bandgap (2.26 eV) is larger than the 1.73 eV photopotential needed for water splitting. In this work, we developed by surface energy engineering a large scale textured GaP template monolithically integrated on Si, for water splitting applications. We first discuss the stability of the {114} facets, based on scanning tunneling microscopy images and density functional theory calculations. We then show that change of the surface energy for experimentally promoting the GaP(114) surface texturation can be achieved through (i) destabilizing the GaP(001) surface by using a vicinal Si substrate or through (ii) favoring the {114} facets formation by changing the group-V atmosphere above the surface on a miscut-free GaP substrate. This work is supported by the French National Research Agency project ANTIPODE (Grant no. 14-CE26-0014-01) and Région Bretagne. The ab initio simulations have been performed on HPC resources of CINES under the allocation 2016-[x2016096724] made by GENCI (Grand Equipement National de Calcul Intensif). [1] M. G. Walter et al., ‘Solar Water Splitting Cells’, Chem. Rev., vol. 110, no. 11, pp. 6446–6473, Nov. 2010. [2] A. Fujishima and K. Honda, ‘Electrochemical Photolysis of Water at a Semiconductor Electrode’, Nature, vol. 238, no. 5358, p. 37, Jul. 1972. [3] J. Zhao et al., ‘High-Performance Ultrathin BiVO4 Photoanode on Textured Polydimethylsiloxane Substrates for Solar Water Splitting’, ACS Energy Lett., vol. 1, no. 1, pp. 68–75, Jul. 2016. [4] A. Kudo and Y. Miseki, ‘Heterogeneous photocatalyst materials for water splitting’, Chem. Soc. Rev., vol. 38, no. 1, pp. 253–278, 2009. [5] E. E. Barton, D. M. Rampulla, and A. B. Bocarsly, ‘Selective Solar-Driven Reduction of CO2 to Methanol Using a Catalyzed p-GaP Based Photoelectrochemical Cell’, J. Am. Chem. Soc., vol. 130, no. 20, pp. 6342–6344, May 2008.

Resume : The monolithic integration of III-V semiconductors on Si is of great interest for photonics due to its potential to provide very large scale integration[1]. So far, driving forces for III-V/Si crystal growth, heterointerfaces formation and antiphase boundaries formation have not been fully understood. In this work, we explain in terms of thermodynamics the GaP/Si 3D-growth mode extending this concept to other systems which show the same growth mode, such as AlSb/Si(001) or AlN/Si(111). Surface and interface energies have been computed by ab-initio calculations (DFT) [2] to determine the free energy changes for describing the thermodynamic process of islands formation [3]. We show that the 3D growth is thermodynamically favorable for most common III-V semiconductors hetero-integration on Si. We finally propose a general III-V/Si growth mechanism where respective contributions of thermodynamics and kinetics are discussed, and the antiphase boundaries formation is clarified. This work is supported by the French National Research Agency project ANTIPODE (Grant no. 14-CE26-0014-01) and Région Bretagne. The ab initio simulations have been performed on HPC resources of CINES under the allocation 2016-[x2016096724] made by GENCI (Grand Equipement National de Calcul Intensif). [1] M. Liao et al., ‘Monolithically Integrated Electrically Pumped Continuous-Wave III-V Quantum Dot Light Sources on Silicon’, IEEE J. Sel. Top. Quantum Electron., vol. 23, no. 6, pp. 1–10, Nov. 2017. [2] O. Romanyuk, T. Hannappel, and F. Grosse, ‘Atomic and electronic structure of GaP/Si(111), GaP/Si(110), and GaP/Si(113) interfaces and superlattices studied by density functional theory’, Phys. Rev. B, vol. 88, no. 11, p. 115312, Sep. 2013. [3] P. Müller and R. Kern, ‘Equilibrium nano-shape changes induced by epitaxial stress (generalised Wulf–Kaishew theorem)’, Surf. Sci., vol. 457, no. 1, pp. 229–253, Jun. 2000.

Resume : Hybrid heterostructures based on two dimensional (2D) and other semiconductors provide the opportunity to build multi-functional electronic devices for applications in broadband photodetectors, junction field-effect-transistors and memory devices. In this study, we demonstrate a multi-functional heterojunction diodes based on black phosphorus and n-InGaAs nanomembrane semiconductors, which exhibit gate tunable, photoresponsive, and programmable diode characteristics. This gate-tunable diode properties enable the device to offer both rectifying and switching behaviors with a maximum rectifying ratio of 2,500 and on/off ratio exceeding 105 for diode and transistor devices, respectively. The device also exhibits non-volatile memory properties including large hysteresis and stable retention of storage charges due to the native phosphorus oxide (POx). By combining the memory and gate tunable rectifying behaviors, the rectifying ratio of the device can be controlled and memorized from 0.058 to 400 by the application of gate pulse. Moreover, the device could generate three different electrical signals by combination with photoresponsivity of 46 mA/W and gate tunable electrical property, which has a potential application in multiple logic operator. This work presents further understanding of multi-functional heterostructure device made of 2D and compound semiconductor with opportunities for exploring novel design of heterostructure.

Resume : Recent research has explored the promising future of BiFeO3 180° ferroelectric domain walls as they have improved piezoelectric response and enhanced dielectric properties. However, the detailed atomic structure at the domain walls has not been fully investigated. In this work, we examined the polarization configuration and the atomic scale structure by using aberration-corrected (STEM) Z-contrast imaging with sub-Angstrom resolution. Results indicated that there are two types of polarization configurations. Moreover, the customized Matlab image analysis demonstrated that the lattice parameters depend on the polarization configurations. The observed variations in lattice parameters across the domain walls may be due to the oxygen octahedral tilting of the unit cells. According to the first principle density functional theory calculation we conducted, the oxygen octahedral tilting angle increased at the domain walls which lead to an enlargement of lattice spacing between the unit cells. These results furnish a preliminary understanding about the relationship between polarization configuration and atomic scale structure. Hence, it could provide a critical insight into the application of BiFeO3 ferroelectric material in different electronic devices.

Resume : Integration of III-V photonics on Si substrate remains a very hot topic in recent years[1,2]. GaP shows a strong advantage of low-lattice mismatch to Si and has been widely studied. However, the GaP material has an indirect bandgap and relatively low carrier mobility, which intrinsically limit its optical performances. Therefore, the pseudomorphic integration of InGaP on a relaxed SiGe/Si pseudo-substrate is expected to overcome these issues. In this work, the optical properties of an InGaP/SiGe/Si sample were studied by temperature- and power- dependent photoluminescence (PL). With the additional help of transmission electron microscopy images and chemical mechanical polishing technique, one PL emission peak is found to come from surface InP quantum dots. Other PL emission peak with a superlinear power dependence is tentatively considered to be co-contributed by InGaP host material[3] and In-rich antiphase boundaries, in good correlation with theoretical predictions[4]. This work is supported by the French National Research Agency project ANTIPODE (Grant no. 14-CE26-0014-01) and Région Bretagne. [1] M. Liao et al., “Monolithically Integrated Electrically Pumped Continuous-Wave III-V Quantum Dot Light Sources on Silicon”, IEEE Journal of Selected Topics in Quantum Electronics, 23(6), 2017. [2] C. Cornet, Y. Léger, and C. Robert, “Integrated Lasers on Silicon”, ISTE-Elsevier, 2016. [3] U. Kops et al., “Intrinsic Quantum Dots in Partially Ordered Bulk (GaIn)P”, Physical Review B, 61(3), 2000. [4] E. Tea et al., “Theoretical Study of Optical Properties of Anti Phase Domains in GaP”, Journal of Applied Physics, 115(6), 2014.

Resume : Semiconductors with ultra-high doping level are attractive for the near- and mid-infrared plasmonics. In III-V compound semiconductors characterized by high electron mobility and low effective mass the plasma edge can be tuned by changing the doping level. In this paper we present the formation of ultra-high doped p-type and n-type GaAs utilizing ion implantation of Te, S and Zn , followed by millisecond-range flash lamp annealing (FLA). The depth distribution of implanted elements obtained by SIMS shows that the ms-range FLA suppresses the diffusion of dopants, while simultaneously the ms-range annealing is sufficient to recrystallize the implanted layer as shown by XRD. The effective carrier concentration is in the range of ~1020 cm-3 suggesting full activation of implanted elements. The structural and optical properties of ion implanted and annealed GaAs were investigated using XRD and micro-Raman spectroscopy. The plasmonic properties are confirmed by Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy.

Resume : We have theoretically investigated the 1.55 µm p-i-n GaNAsBi-based multiple quantum wells (MQWs) using a self-consistent calculation combined with 16x16 BAC model. Their performances are evaluated in terms of optical gain and radiative current density J_rad. We have found that J_rad reduces by increasing the well thickness〖 L〗_ω. The quantum confined Stark effect as well as the doping effect on spontaneous emission and radiative current density in ideal lasers are also discussed. The optical properties of the heterostructure are improved when the MQWs are doped. The optimization of well parameters can be used as a basis for GaNAsBi-based lasers intended for optical fiber telecommunication wavelength.

Resume : Successful integration of 2D materials in large scale production requires metrology to monitor the material quality and uniformity. The standard methods for electrical characterization of 2D materials are Hall Effect and van der Pauw type measurements or characterization of device performance in which a 2D material is integrated. Several fabrication steps are involved in fabricating these structures. Each of these steps risks altering the properties of the 2D material. Due to the permanent nature of these Hall and van der Pauw structures, only a single area of the 2D material is probed, making it difficult to monitor the overall material uniformity. Together these effects make it hard to generate adequate statistics. In this study, we will present the use of micro four-point probes (M4PP) as a possible tool for the characterization of field effect in 2D materials. By using M4PP to characterize the 2D materials, they do not need to be patterned, thus reducing processing steps, and adds the possibility of mapping material uniformity in an automated fashion. The uniformity both locally and globally can then be compared across samples. We will present data concerning the uniformity of MoS2 and reproducibility of the measurements and compare it to van der Pauw measurements on similar samples. The measured samples consist of MoS2 transferred to either SiO2 or PMMA coated highly doped silicon.

Resume : Epitaxial Gd-doped SrxBa1-xNb2O6 films were fabricated on Pt coated MgO single crystals substrates using pulsed laser deposition (PLD). Their structural properties were investigated by X-ray diffraction. The ?-2? scan showed single crystalline Gd-doped SBN layer with < 211> orientation perpendicular to the substrate plane. Electrical measurements showed that the Gd-SBN films had good ferroelectric properties. Ramanent polarization of 0.7 µC/cm2 and coercive field of 0.4 kV/cm2 were obtained. The pyroelectric properties of Gd-doped SBN thin films will also be investigated to show the potential of Gd-SBN film applied in pyroelectric device.

Resume : Photoluminescence (PL) and photoreflectance (PR) measurements have been used to determine the band gap energy of InGaAs and GaAsBi alloys. The temperature dependence of optical transitions in the temperature ranging from 12 to 300 K were investigated. Three theoretical models (Varshni, Viňa, and Pässler) were used to fit the experimental points. The results show that the temperature dependence of the fundamental gap energy is attributed to the sum of two contributions: the thermal expansion (TE) and the electron-phonon interaction (EPI). For InGaAs with different In concentration (x=0, 0.08, 0.16 and 0.37), the contribution to the EPI of the longitudinal optical phonon increases, relatively to the longitudinal acoustical phonon, with increasing In concentration. In the low temperature range, the small decrease in the optical transition energy is explicated by a competition between the two contributions (TE and EPI). At high temperature, the linear decrease of gap energy is explained essentially by the EPI. Additionally, we have estimated the thermal expansion coefficient of GaAsBi with different Bi concentration (x=0.013, 0.037 and 0.048). The results show a slowly decrease in the TE with temperature and Bi composition. This effect is considered to play an important role in the reduction of the temperature dependence of the band gap energy of GaAsBi alloys. GaAsBi alloys are important materials for the fabrication of temperature insensitive optical devices.

Resume : Because of GaAs properties such as high electronic mobility and direct band gap, its integration on Si substrate has been studied for fields such as high mobility electronics and integration of novel functionalities in photonics. GaAs integration on Si substrate is a major challenge due to the formation of dislocation (4% lattice mismatch) and other defects like anti-phase domains due to their polar/non polar interface nature. In this study, we will show the possibility to integrate GaAs µ-crystal on Si (001) and (111) by epitaxial lateral overgrowth from randomly dispersed or localized nano-seeds. Those ones are made of Si or Ge and aim to improve the GaAs crystal nucleation. Indeed, lattice parameters accommodation will occur within a limited surface allowing GaAs to relax laterally. Thus we will describe the GaAs crystallinity, in function of the substrate and nano-seed types. In particular, we will show how the morphology of Ge can be detrimental to the GaAs cristallinity. One of the most interesting results had been obtained on patterned Si (111) type substrate; with a 100% nano-opening fill rate and around 75% of high epitaxial quality GaAs µ-crystal. Scanning Electron Microscopy and Transmission Electron Microscopy had been used to observe the crystalline structure; Energy Dispersive X-rays spectrometry analysis had been performed in order to localize the Ge nano-seed. These different results will be presented and discussed.

Resume : III-V nitride materials combine unique properties such as a direct and tunable band gap. InAlN/GaN device structures, developed for HEMT, ensure a spontaneous high polarization with high 2DEG (3.5 x 1013 cm-2). Moreover, MOCVD deposition technique gives excellent crystalline quality for ultra-thin layers (below 8 nm thickness). Such materials are also known to be chemically stable at working temperature and poorly reactive to air exposure. Nevertheless, thermal annealing treatment during ohmic contact engineering can harm the quality of 2DEG. We currently studied by XPS the chemical surface evolution of InAlN, from 850°C to 950°C, under O2 rich and Ar atmosphere, in order to understand the degradation mechanisms usually running through the HEMT elaboration process. We firstly found that O2 thermal treatment increases the oxygen content forming an ultra-thin and covering oxide top layer. Important chemical change is observed with the appearance of a new N1s band at 404 eV attributed to interstitial N2 molecules trapped in the structure. Enlargement of the usual N1s band at 397.3 eV (accredited to InAlN) can also be seen and was attributed to N-N bonded defects. Ageing of the samples was finally performed showing that the N2 interstitial molecules are slowly decreased and almost totally gone after 1 year. Unlike this evolution, N-N defects are still present justifying a deeper incorporation inside the layer. To our knowledge, this unusual behaviour was not reported before.

Resume : Recently, additive manufacturing (AM) technology, also known as three-dimensional printing (3D printing), has been well developed. Numerous new designs had been developed and realized by additive manufacturing technology. However, some of the 3D printable commercial materials should be modified or further improved to meet the environmental requirement for the unique applications. For example, the wear resistance and corrosion resistance of the Titanium impeller should be improved/modified depending on the application environment. In this study, the high wear and corrosion resistance amorphous alloy is introduced to be sputtered on the surface of the 3D printed Titanium alloy to modify the physical and chemical properties on the surface. Meanwhile, thanks to the lacking of the grain boundary of amorphous alloy, the poor surface roughness of the as printed Titanium alloy could be improved by appropriately controlling of the sputtering parameter at the same time. The amorphous nature and morphology of the samples were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Meanwhile, all of the results were discussed in details.

Resume : 2D nanomaterial graphene is at the top of the significant interest due to the giant electron and hole mobility, charge carrier multiplication, flexibility, optical transparency, chemical inertness. One of the hindrances stopping the wider application of the graphene in semiconductor device technology is complex procedure used for fabrication of the graphene layers by using graphene transfer. Cu or Ni catalytic foils. Afterward, follows the long process of the graphene transfer onto the targeted semiconductor or dielectric substrates. During that process, graphene can be contaminated by different adsorbents. Transfer cause wrinkles or ripples to form on graphene. In such a case control of the graphene layer or graphene-semiconductor contact properties is complicated. Recently there were shown that direct synthesis of the graphene on semiconducting or dielectric substrates is possible. However, this technology and related studies are at the very beginning. In present research graphene layer were directly synthesized by microwave plasma enhanced chemical vapor deposition on monocrystalline Si substrates and SiO2 films. Nitrogen doping of the directly synthesized graphene layers was considered. Structure, composition and morphology of the films was investigated by Raman scattering spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy. Deposition temperature, synthesis time and pressure effects on the structure of the directly synthesized graphene layers was studied. Research on effects of the hydrogen and methane gas flow ratio on the structure of the directly synthesized graphene layers was performed. Composition and structure of the nitrogen doped directly synthesized graphene were investigated.

Resume : Recent advances in MOVPE growth enabled realization of high quality wafer-scale h-BN1, subsequently III-nitride device structures were grown on it. This new architecture added much needed functionalities and are very attractive for flexible electronic and optoelectronic applications. Previously, we have demonstrated growth of III-nitride LED structures on h-BN buffered sapphire substrates and simple wafer-scale mechanical liftoff using metallic tapes2. AlGaN/GaN high electron mobility transistors (HEMT) grown on h-BN templates is very interesting for cost reduction, higher performance and flexible applications. Further h-BN acts as a good heat sink solving the self heating issues. In the present investigation AlGaN/GaN HEMTs were grown on 2” and 4” h-BN buffered sapphire substrates. We overcame growth issues like delamination during growth, cracking and other strain related degradations by optimizing the growth conditions and thicknesses of different constituent layers. Detailed cross sectional transmission electron microscope analysis on the interfaces between 3D bonded III-nitride and 2D layered h-BN buffered sapphire substrates shows that the AlGaN/GaN heterostructure are single crystalline and has abrupt interfaces. Front-end HEMT process was adapted to suit the fragile as-grown device structures. The modified process yielded more than 1000 devices that show typical I-V characteristics of an AlGaN/GaN HEMTs device results on a 2” wafer. Subsequently the processed HEMTs were lifted off and transferred to metallic platforms with good performances. This transfer of III-N based devices using 2D h-BN opens new avenues to use suitable substrates, and is promising for next-generation wearable and portable opto-electronic devices. Ref: 1. Li, X., et al., Crystal Growth & Design, 16, 3409−3415 (2016) 2.Ayari, T., et al., Appl. Phys. Lett. 108, 171106 (2016)

Resume : The growth of cubic GaN layers on (113) and (001) oriented GaAs substrates were investigated by metal organic vapor phase epitaxy (MOVPE). The growth of low temperature GaN nucleation layers (500-550 °C) was in-situ monitored by laser reflectometry (LR). The simulation of experimental reflectivity curves, by using an optical model which includes both time-dependent growth rate and surface roughness profiles, shows differences between growth kinetics depending on substrate orientation. Thus, growth anisotropy was observed. The effect of high temperature (750-900 °C) GaN sublayers grown on 130 nm thick GaN buffer layer deposited on (113) GaAs substrate at low temperature (550 °C) was analyzed. The GaN sublayers were ex situ characterized by scanning electron microscope (SEM), high resolution X-ray diffraction (HRXRD) and room temperature cathodoluminescence (RT-CL). Based on comparative study between structural and optical qualities of GaN layers grown on the two substrate orientations, we conclude that the crystalline structure strongly depends on both temperature and substrate orientation. The SEM micrographs and 2θ/ω spectra showed that the (113) oriented GaAs promotes the alignment of cubic GaN (113) columns at a growth temperature close to 850°C. For this temperature, the RT-CL spectra exhibited only cubic GaN emission (3.23eV) for the two orientations (001) and (113) of GaAs substrate. Keywords: MOVPE, c-GaN(113), GaAs(113) substrate, HRXRD, Cathodoluminescence.

Resume : Quantum Dots (QDs) are considered as an efficient active layer for semiconductor lasers whereby their physical dimension is the key parameter to be controlled. In this work, we have studied experimentally the growth of InAs QDs on InP(001) substrate by MOVPE and established a theoretical model explaining our observations. In variance with the classical Stranski-Krastanov growth, we show that during the growth there is an intermediate stage whereby although first large 3D islands are formed, an increased density of small QDs if formed concurrent with the shrinkage of the large 3D islands. As a result, the growth can be divided into three regimes: 2D layer growth, large 3D islands growth and QDs growth. To explain this evolution, a thermodynamic model has been developed accounting for the process driven by surface energy, elastic relaxation energy and inter-island interaction energy. It will be shown that the balance between the surface energy and the elastic relaxation energy provides the 2D layer to large 3D islands transition (roughly at 3 ML of InAs deposition). However, we have found that a competing process is the inter-island interaction which explains the volume shrinkage of the early large 3D islands with the increase of density and small QDs formation.

Resume : We present two approaches to understanding graphene growth on copper which are applicable to a wide range of "weak epitaxy" systems. In both models, one based on lattice Monte Carlo and the other on molecular dynamics, the substrate is treated in a purely abstract way. This allows aspects of so-called Van der Waals epitaxy to be studied in a computationally efficient way. The models address two important aspects of graphene growth: how high quality single crystal material can be grown on rough, highly dynamic (sublimating) copper surfaces, and how mismatch epitaxy with small-angle offsets occurs.The abstract substrate approach will be discussed in the context of other epitaxial systems with weak interactions and symmetry mismatch.

Resume : Growth of Ge on Si is difficult due to lattice mismatch of 4% .This lattice mismatch leads to misfit dislocations which reach surface as threading dislocations resulting in defective films and ultimately are detrimental to device performance. Despite, several methods for growing epitaxial germanium films on Si (100) have been reported such as MBE[1] and also using vapor phase epitaxy such as two-step process using RPCVD[2] and three step process using MOCVD[3]. However the growth of epitaxial Ge on Si (110) is even more difficult. It is reported that one of the primary factors limiting crystalline quality on Si(111), (110), and (112) is the tendency towards stacking fault and micro-twin formation. These micro-twins are believed to arrest the propagation of strain-relieving misfit dislocations necessitating further nucleation events and an overall rise in threading dislocation density(TDD),[4]. Still several methods like surfactant mediated MBE [5], selective growth techniques such as aspect ratio trapping (ART) and epitaxial lateral overgrowth (ELO) [6]. Apart from these methods two step process using LT and HT layers to reduce formation of stacking faults have also been reported [7]. In this work, we report relatively unexplored liquid phase epitaxial growth method to grow epitaxial Ge films on Si(110). Liquid phase epitaxial growth has inherent challenges due to its sensitivity to substrate cleanliness as well as deposition and growth chamber conditions. We present study of liquid phase epitaxial growth of germanium thin films on Si(110) after annealing of amorphous Ge films in hydrogen ambient. SEM and XRD measurements were done to observe film morphology and quality. Finally, AFM roughness measurements were done on films with lowest threading dislocation densities. 1 μm thick amorphous germanium films were deposited Si(110) wafers using OXFORD Plasma lab PECVD deposition system 3500C, with GeH4 10 sccm, N2 100 sccm and H2100 sccm, and RF power density of 0.20 W/cm2. RCA cleaning followed by HF dip was done for Si wafers prior to deposition. RCA cleaning of silicon wafers was done to ensure removal of organic as well as metal contaminants from the substrate which might contaminate the deposited film and subsequently hinder the re-crystallization process .The depositions were preceded by a plasma cleaning of the PECVD chamber to avoid any contamination on the deposited film. The PECVD grown Ge layers were annealed above melting temperature of Ge (9370C) in a clean tube furnace in hydrogen ambient devoid of chamber contamination above melting temperature. The films annealed in tube furnace were oriented in (110) following orientation of Si (110) beneath. Both amorphous and re-crystallized films were observed under Zeiss ultra 55® Scanning Electron microscope. X-ray diffraction measurements were done using Rigaku Smartlab® X-ray diffractometer for films crystallized in tube furnace. High resolution rocking curve XRD measurements were done to estimate threading dislocation density on the basis of rocking curve FWHM. AFM measurements were done to find surface roughness of films. XRD θ-2θ measurements for different film thickness show highly oriented Ge films in (220) direction following orientation of underlying silicon.Fig.2 shows XRD curve for films and it is evident that film is highly oriented in (220) . Clearly the film has re-crystallized epitaxially upon melting and solidification on Si(110) surface. Epitaxial quality is the measure of spread in the rocking curve of specular and non-specular reflection XRD peaks. Full with at half maximum of the peaks are used to infer the mosaicity and defects such as dislocation. The full-width half-maximum (FWHM) of all are rocking scan peaks are <= 0.15 degrees for 1 μm thick films From rocking curve scans it was seen that FWHM do not vary for different planes indicating similar dislocation densities. The total dislocation density calculated from rocking curve scans is 4.3 x 10^9 cm-2 . There is a thickness dependence on the epitaxial quality of films grown on Si. This is already observed in our previous work for Ge films grown on Si (100) [8] where 1 micron thick films had lowest defect density. Similar trends are expected for films grown on Si(110) substrates and hence only results corresponding to 1 micron films with lowest defect density are discussed here. The defect density in Ge(110) films is found to comparable with Ge(100) films[8] as contrary to observed by [4]. This indicated that high temperature of growth results in annihilation of dislocations and stacking faults resulting in similar quality of films as observed for Ge (100). XRD phi scan was done to confirm in plane epitaxial relationship between Si substrate and germanium films crystallized using LPE . Phi scan is done corresponding to (111) and (400) planes of germanium for hydrogen ambient annealed films. The films showed two-fold rotational symmetry confirming epitaxial nature . Roughness measurement is a requisite for device application and hence were measured for the films. The AFM scan was done on an area of 5x5 um for 1 micron films annealed in hydrogen ambient and found to be ~ 80 nm. In conclusion , we have presented a method to grow epitaxial thin-film of germanium on silicon (110) using a relatively simple one-step process. The films were crystallized by annealing at a temperature slightly higher than the melting point (9370 C) of germanium in hydrogen ambient and cooling slowly .The Si(100) wafer underneath oriented the Ge melt leading to high-quality Ge epitaxial films. SEM and XRD show that the resulting films were highly oriented, continuous and crack free respectively. The rocking curve (Omega scan) confirmed that the1 micron films are highly-oriented with a FWHM of only ~0.15 degree resulting in dislocation density of ~10^9 cm-2. Also, phi scan for LPE films confirms high quality epitaxial growth. References [1] J. Liu, H. J. Kim, O. Hul’ko, and Y. H. Xie,S. Sahni, P. Bandaru, and E. Yablonovitch, “Ge films grown on Si substrates by molecular-beam epitaxy below 450°C,” J. Appl. Phys. ,vol.96, 2004 . [2] V.A. Shah, A. Dobbie, M. Myronov, D.R. Leadley, “High quality relaxed Ge layers grown directly on a Si(0 0 1) substrate” Solid-State Electronics,vol. 62pp.189–194, 2011. [3] Kwang Hong Lee, Adam Jandl, Yew Heng Tan, Eugene A. Fitzgerald, and Chuan Seng Tan, “Growth and characterization of germanium epitaxial film on silicon (001) with germane precursor in metal organic chemical vapour deposition (MOCVD) chamber”,AIP Advances vol.3, p.092123,2013. [4] Minjoo L. Lee a,b,*, Dimitri A. Antoniadis a, Eugene A. Fitzgerald, “Challenges in epitaxial growth of SiGe buffers on Si (111), (110), and (112),” Thin Solid Films 508 ,pp.136 – 139, 2006. [5] T. F. Wietler, E. Bugiel K. R. Hofmann, “Surfactant-Mediated Epitaxy Of Relaxed Low-Doped Ge Films On Si(001) With Low Defect Densities,” Appl. Phys. Lett. 87, 182102 ,2005. [6] J.-S. Parkz et.al, “ Low-Defect-Density Ge Epitaxy on Si(001) Using Aspect Ratio Trapping and Epitaxial Lateral Overgrowth” ,Electrochem. Solid-State Lett. volume 12, issue 4, pp.142-144, 2009. [7] Van Huy Nguyen, A. Dobbie, M. Myronov, and D. R. Leadley, “High quality relaxed germanium layers grown on (110) and (111) silicon substrates with reduced stacking fault formation ,” Journal of Applied Physics 114, 154306 ,2013. [8] Chaurasia, Saloni, Srinivasan Raghavan, and Sushobhan Avasthi. “High Quality Epitaxial Germanium on Si ( 100 ) for low -cost III-V”. In: 44th IEEE Photovoltaic Specialist Conference(IEEE-PVSC 2017).

Resume : Highly mismatched heteroepitaxial growth is usually assumed to occur either in a layer-by-layer mode, where elastic strain energy is reduced by misfit dislocation formation, or in one of several 3D modes where lateral strain relaxation contributes to elastic energy reduction. MnSb is a double-hexagonal close-packed ferromagnetic weak metal with technological potential in semiconducor spintronics. It can be grown on GaAs(111) in the (0001) orientation, with 3.2% lattice mismatch. We use synchrotron X-ray diffraction, scanning tunnelling microscopy and first-principles calculations to investigate its unusual growth mode and strain relaxation. Broad flat-topped islands in both coherently strained and relaxed states co-exist in the first few nanometres of film growth.