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Raman spectroscopy, a versatile and nondestructive technique, was employed to develop a methodology for gallium oxide (Ga2O3) phase detection and identification. This methodology combines experimental results with a comprehensive literature survey. The established Raman approach offers a powerful tool for nondestructively assessing phase purity and detecting secondary phases in Ga2O3 thin films. X-ray diffraction was used for comparison, highlighting the complementary information that these techniques may provide for Ga2O3 characterization. Few case studies are included to demonstrate the usefulness of the proposed spectroscopic approach, namely the impact of deposition conditions such as metal–organic vapor-phase epitaxy and pulsed electron deposition (PED), and extrinsic elements provided during growth (Sn in the case of PED) on Ga2O3 polymorphism. In conclusion, it is shown that Raman spectroscopy offers a quick, reliable, and nondestructive high-resolution approach for Ga2O3 thin film characterization, especially concerning phase detection and crystalline quality. [ABSTRACT FROM AUTHOR]

• Inexpensive technological process for high-yield production of semi-insulating InP wafers. • The obtained semi-insulating wafers have properties that outperform those of standard Fe-doped InP. • The procedure is simple and reproducible, ready for industrial application. A novel technique for preparation of high-resistivity indium phosphide (InP) via post-growth treatment of undoped n-type wafers is presented. The method includes the deposition of a controlled quantity of iron on both faces of as-cut wafers by a simple chemical bath, and the subsequent Fe diffusion by thermal annealing. The reproducible low - to – high resistivity conversion is explained considering two simultaneous phenomena: the annealing-controlled in-diffusion of Fe deep acceptors and the out-diffusion of hydrogen-related shallow donors. Differently from standard Fe-doped melt-grown InP single crystals, this process does not suffer from segregation, thus the Fe-concentration is constant from wafer to wafer, with no striations and radial gradients deriving from the convex solid–liquid interface during growth. Main advantages of the developed process are: i) an entire undoped InP boule may be sliced and converted to semi-insulating, which makes the process cost-effective; ii) reproducible and uniform semi-insulating properties from batch to batch of wafers; iii) the Fe incorporation is precisely controlled, and minimized, so that electrical characteristics of Fe-diffused wafers are superior to those of traditional semi-insulating melt-grown InP crystals doped via addition of elemental Fe to the melt. [ABSTRACT FROM AUTHOR]

InGaN/GaN heterostructures have been deposited by MOVPE onto (0 0 0 1) sapphire substrates. It has been noted that the Indium incorporation efficiency depends on different growth parameters, namely the growth rate, reaction kinetics and partial pressure of H2 in the reaction cell. In this work the InGaN composition has been investigated by different techniques and the incorporation efficiency of indium is then correlated with substrate temperature, substrate rotation, H2 partial pressure and input flows of TMG and TMI. [Copyright &y& Elsevier]

Atomically thin molybdenum disulfide (MoS2) is a two-dimensional semiconductor with versatile applications. The recent adoption of liquid molybdenum precursors in chemical vapor deposition has contributed significantly to the reproducible wafer-scale synthesis of MoS2 monolayer and few-layer films. In this work, we study the effects of the carrier gas flow rate on the properties of two-dimensional molybdenum disulfide grown by liquid-precursor-intermediate chemical vapor deposition on SiO2/Si substrates. We characterized the samples using Optical Microscopy, Scanning Electron Microscopy, Raman spectroscopy, and Photoluminescence spectroscopy. We analyzed samples grown with different nitrogen carrier flows, ranging from 150 to 300 sccm, and discussed the effect of carrier gas flows on their properties. We found a correlation between MoS2 flake lateral size, shape, and number of layers, and we present a qualitative growth model based on changes in sulfur provision caused by different carrier flows. We show how the use of liquid precursors can allow for the synthesis of homogeneous, single-layer flakes up to 100 µm in lateral size by optimizing the gas flow rate. These results are essential for gaining a deeper understanding of the growth process of MoS2. [ABSTRACT FROM AUTHOR]

• Heterostructures of ε-Ga 2 O 3 /GaN have been grown by MOCVD. • Growth of GaN on ε-Ga 2 O 3 at 1050 °C results in strong diffusion of O into GaN. • Growth of ε-Ga 2 O 3 on GaN occurs through initial 3D-island formation. • A 2DEG is formed at the ε-Ga 2 O 3 -on-GaN interface. • Novel HEMTs were obtained from ε-Ga 2 O 3 /GaN heterostructures. Heterostructures made of GaN and ε-Ga 2 O 3 epitaxial layers may be very interesting because they could exploit the high electron mobility of GaN combined with the ferroelectric character of ε-Ga 2 O 3. We have explored the possibility of using ε-Ga 2 O 3 templates, deposited by metalorganic chemical vapor deposition on sapphire substrates, in order to reduce the lattice mismatch of GaN with sapphire. Considering that ε-Ga 2 O 3 is metastable and undergoes a first phase transition at around 700 °C, the GaN layers were deposited at two different temperatures (690 °C, 1050 °C). Preliminary electrical and SIMS investigations have evidenced the diffusion of oxygen from the ε-Ga 2 O 3 to the GaN epitaxial layer, which results in an n-type conductivity and a sheet resistance as low as 70 Ohm/sq in a 1 μm thick GaN layer. The rocking curve of the GaN layers grown ε-Ga 2 O 3 /sapphire at standard high temperature (1050 °C) indicates a crystal quality worse than for GaN deposited directly on sapphire. In parallel, we studied the nucleation of ε-Ga 2 O 3 on GaN templates. We evidenced that ε-Ga 2 O 3 nucleates in 3D islands on the surface of GaN grown on on-axis sapphire, with coalescence taking place as the layer grows thicker. The use of off-cut sapphire substrates, instead, permits to inhibit islands formation, resulting in a smoother layer. The possibility of obtaining uniform and very thin ε-Ga 2 O 3 layers on GaN layers opens interesting possibilities for the development of novel high electron mobility transistors (HEMT). [ABSTRACT FROM AUTHOR]

Semiconductor photodetectors can work only in specific material-dependent light wavelength ranges, connected with the bandgaps and absorption capabilities of the utilized semiconductors. This limitation has driven the development of hybrid devices that exceed the capabilities of individual materials. In this study, for the first time, a hybrid heterojunction photodetector based on methylammonium lead bromide (MAPbBr3) polycrystalline film deposited on gallium arsenide (GaAs) was presented, along with comprehensive morphological, structural, optical, and photoelectrical investigations. The MAPbBr3/GaAs heterojunction photodetector exhibited wide spectral responsivity, from 540 to 900 nm. The fabrication steps of the prototype device, including a new preparation recipe for the MAPbBr3 solution and spinning, will be disclosed and discussed. It will be shown that extending the soaking time and refining the precursor solution's stoichiometry may enhance surface coverage, adhesion to the GaAs, and film uniformity, as well as provide a new way to integrate MAPbBr3 on GaAs. Compared to the pristine MAPbBr3, the enhanced structural purity of the perovskite on GaAs was confirmed by X-ray Diffraction (XRD) upon optimization compared to the conventional glass substrate. Scanning Electron Microscopy (SEM) revealed the formation of microcube-like structures on the top of an otherwise continuous MAPbBr3 polycrystalline film, with increased grain size and reduced grain boundary effects pointed by Energy-Dispersive Spectroscopy (EDS) and cathodoluminescence (CL). Enhanced absorption was demonstrated in the visible range and broadened photoluminescence (PL) emission at room temperature, with traces of reduction in the orthorhombic tilting revealed by temperature-dependent PL. A reduced average carrier lifetime was reduced to 13.8 ns, revealed by time-resolved PL (TRPL). The dark current was typically around 8.8 × 10−8 A. Broad photoresponsivity between 540 and 875 nm reached a maximum of 3 mA/W and 16 mA/W, corresponding to a detectivity of 6 × 1010 and 1 × 1011 Jones at −1 V and 50 V, respectively. In case of on/off measurements, the rise and fall times were 0.40 s and 0.61 s or 0.62 s and 0.89 s for illumination, with 500 nm or 875 nm photons, respectively. A long-term stability test at room temperature in air confirmed the optical and structural stability of the proposed hybrid structure. This work provides insights into the physical mechanisms of new hybrid junctions for high-performance photodetectors. [ABSTRACT FROM AUTHOR]

Orthorhombic κ-Ga2O3 thin films were grown for the first time on polycrystalline diamond free-standing substrates by metal-organic vapor phase epitaxy at a temperature of 650 °C. Structural, morphological, electrical, and photoelectronic properties of the obtained heterostructures were evaluated by optical microscopy, X-ray diffraction, current-voltage measurements, and spectral photoconductivity, respectively. Results show that a very slow cooling, performed at low pressure (100 mbar) under a controlled He flow soon after the growth process, is mandatory to improve the quality of the κ-Ga2O3 epitaxial thin film, ensuring a good adhesion to the diamond substrate, an optimal morphology, and a lower density of electrically active defects. This paves the way for the future development of novel hybrid architectures for UV and ionizing radiation detection, exploiting the unique features of gallium oxide and diamond as wide-bandgap semiconductors. [ABSTRACT FROM AUTHOR]

• S b 2 S e 3 - based polycrystalline thin film solar cells exhibiting 5 % efficiency. • Innovative back contact material, based on low costs, low toxicity and abundant elements such as Fe, S and O. • High scalability back contact deposition technique: R.F. magnetron sputtering at RT. • Ohmic behaviour and low contact resistivity between S b 2 S e 3 and back contact material. Among the most promising material for thin-film photovoltaics, S b 2 S e 3 (ASe) plays a pivotal role, being constituted of Earth-abundant elements and owing to its optical and electrical properties. Recently, ASe solar cells exhibited relatively good photovoltaic performances, reaching a considerable efficiency close to 9 %. Despite these remarkable results, much is still to be investigated, particularly regarding the active materials, interfaces, and electrical contacts. In this study we propose an alternative back contact for ASe solar cells. It is based on Fe, S and O elements, and was deposited at room temperature by radiofrequency magnetron sputtering. XRD and Raman measurements revealed that it is actually made of two materials: F e 3 O 4 , in orthorhombic and cubic phases and F e S in troilite phase. This material provided an ohmic contact on top of the ASe film; its contact resistivity, extrapolated from current vs voltage characteristics of complete ASe-based solar cells, was estimated to be 0.8 Ω · c m 2. Monitoring the photovoltaic parameters, we observed a negligible average change after three months. [ABSTRACT FROM AUTHOR]

In this study, a dual-frequency method for the impedance/admittance measurements on illuminated junctions has been critically analyzed as an appropriate methodological approach for the measure of the depletion capacitance, both under steady-state light and during on-off illumination cycles. A planar Pt/κ-Ga 2 O 3 Schottky diode was used as a test structure. Then capacitance-voltage (C-V) measurements were carried out under various applied AC frequencies and monochromatic UV-C light (λ = 254 nm) and compared to dark values. Transient photo-capacitance (TPC) measurements were performed at fixed applied voltages and the same AC frequencies and illumination conditions as the C-V measurements. Thanks to capacitance-frequency (C-f) measurements, the device under test was preliminary demonstrated to be well represented, both in dark and under light, by the equivalent circuit required for the applicability of the dual-frequency approach. It must include a depletion capacitance, parallel leakage resistance, and a series resistance. Dual-frequency capacitance resulted comparable to the single frequency C-V and TPC data obtained in the high-frequency limit for series configuration measurements and those obtained at low frequency in measurements carried out in parallel configuration. The experimental conditions of reliable data acquisition were discussed leading to the interpretation of the results in terms of the presence of slow-response traps. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: A novel melt crystal growth technique was developed and applied for growing bulk In2O3 single crystals. In this new method the In2O3 starting material inside an inductively heated metal crucible is subjected to a controlled decomposition in such a way that a certain amount of free metallic indium forms. As a result, the electromagnetic field from an RF coil couples also to the In2O3 starting material, in addition to the metal crucible, which facilitates the melting. Liquid In2O3 has good electrical conductivity so that eddy currents are induced in the melt close to the crucible wall. This in turn induces a counter magnetic field that ultimately leads to levitation of a portion of the molten In2O3. The amount of the levitated material depends on the mutual RF coil–crucible position, their configurations as well as other components of a growth furnace. A consequence of the partial levitation of In2O3 melt is the formation of a liquid neck between the levitated and the quiescent melt portions. This neck is crucial as it acts as a seed during the crystallization process. The neck can be further shaped by controlled overheating or soaking. By cooling down, two single crystals are formed on the opposite sides of the seed. We named this new crystal growth technique “Levitation-Assisted Self-Seeding Crystal Growth Method”. It is in principle applicable to any oxides whose melts are electrically conductive. Thanks to this method we have successfully grown truly bulk In2O3 single crystals from the melt for the first time worldwide. [Copyright &y& Elsevier]

Indium oxide as thin film and powder was prepared by the sol-gel method. The structural characterization of the film and powder showed that similar crystallite sizes are obtained in samples heat-treated at the same temperature. Thermo-gravimetric analysis and differential scanning calorimetry were performed in order to determine the activation energies of In 2 O 3 formation from precursors and subsequent crystal growth of both kinds of samples. Absorption measurements showed that the bandgap of the sol-gel polycrystalline films deposited on c-oriented sapphire is in excellent agreement with the one found in bulk and epitaxial indium oxide. The annealed films were found to be highly conductive with carrier concentration generally in excess of 10 19 cm − 3 . It was observed that the electrical characteristics of films annealed at 700 °C remained stable while those of films annealed at lower temperatures exhibited remarkable changes when measurements were repeated after seven months. [ABSTRACT FROM AUTHOR]

We report on the fabrication and characterization of Cu(In,Ga)Se 2 (CIGS)-based thin film bifacial solar cells using Al-doped ZnO (AZO) as cost-effective and non-toxic transparent back contact. We show that, by depositing both CIGS and AZO by Low Temperature Pulsed Electron Deposition at a maximum temperature of 250 °C, a good ohmic contact is formed between the two layers and good quality solar cells can be fabricated as a result. Photovoltaic efficiencies as high as 9.3% (front illumination), 5.1% (backside illumination) and 11.6% (bifacial illumination) have been obtained so far. These values are remarkably higher than those previously reported in the literature. We demonstrate that this improvement is ascribed to the low-temperature deposition process that avoids the formation of Ga 2 O 3 at the CIGS/AZO interface and favours the formation of a low-resistivity contact in agreement with device simulations. [ABSTRACT FROM AUTHOR]

A comprehensive study by high-resolution transmission electron microscopy (TEM) and X-ray diffraction (XRD) was carried out on Ga2O3 epilayers grown at low temperature (650 °C) by vapor phase epitaxy in order to investigate the real structure at the nanoscale. Initial XRD measurements showed that the films were of the so-called ε phase; i.e. they exhibited hexagonal P63mc space group symmetry, characterized by disordered and partial occupation of the Ga sites. This work clarifies the crystal structure of Ga2O3 layers deposited at low temperature at the nanoscale: TEM investigation demonstrates that the Ga atoms and vacancies are not randomly distributed, but actually possess ordering, with (110)-twinned domains of 5–10 nm size. Each domain has orthorhombic structure with Pna21 space group symmetry, referred to as κ-Ga2O3. Further XRD analysis carried out on thicker samples (9–10 μm) confirmed this finding and provided refined structural parameters. The six (110)-type twinned ordered domains together – if the domain size falls below the actual resolution of the probing techniques – can be misinterpreted as the disordered structure with its P63mc space group symmetry usually referred to as ε-Ga2O3 in the current literature. The crystal structure of these Ga2O3 layers consists of an ABAC oxygen close-packed stacking, where Ga atoms occupy octahedral and tetrahedral sites in between, forming two types of polyhedral layers parallel to (001). The edge-sharing octahedra and the corner-sharing tetrahedra form zig-zag ribbons along the [100] direction. Anti-phase boundaries are common inside the domains. The polar character of the structure is confirmed, in agreement with the characteristics of the Pna21 space group and previous observations. [ABSTRACT FROM AUTHOR]

• The origin of a photogain in resistive UV-C photodetector is discussed in the light of trapping phenomena. • UVC-to-visible rejection ratio higher than 104 and photogain higher than 103 are obtained. • The solar-blind performances are investigated under the spectral irradiation of a solar simulator. • From the gain saturation for high applied voltages the minority carrier diffusion length is estimated. • On-off switching times under of band-to-band energy radiation are comparable/better than in other Ga2O3 polymorphs. Electrical and optical properties of nominally-undoped high-resistivity κ -Ga 2 O 3 epitaxial films are presented, along with the ultraviolet detection performance of solar-blind photodetectors fabricated from these epilayers. The photodetectors exhibited a solar-blind rejection ratio higher than 104, reproducible on–off switching times and a remarkable photo-gain (up to 103). The latter can be ascribed to either a strong trapping effect of free holes by deep levels inducing a majority carrier excess Δ n with respect to the minority carriers Δ p , or to a hole mobility several orders of magnitude lower than the electron mobility, both the hypothesis being consistent with literature data. A saturation of the gain, dependent on detector size, was observed beyond a certain applied voltage, that is consistent with a minority carrier diffusion length of a few 10-5 cm. Non-critical growth conditions, reproducibility and intrinsic spectral selectivity together with good UV-detection performance make κ-Ga 2 O 3 a suitable material for solar-blind ultraviolet photodetectors. [ABSTRACT FROM AUTHOR]

The article discusses a study related to the high-voltage field effect on transistors by Wan Sik Hwang and colleagues that was published in the 2014 issue is presented. Topics discussed include nanoscale semiconductor materials are used as materials for transistors to produce logic switches, miniaturization of power switches would provide integration with various electronic systems, and nanomembranes of semiconductor gallium oxide.

Epitaxial β-Ga2O3 layers have been grown on β-Ga2O3 (100) substrates using metal-organic vapor phase epitaxy. Trimethylgallium and pure oxygen or water were used as precursors for gallium and oxygen, respectively. By using pure oxygen as oxidant, we obtained nano-crystals in form of wires or agglomerates although the growth parameters were varied in wide range. With water as an oxidant, smooth homoepitaxial β-Ga2O3 layers were obtained under suitable conditions. Based on thermodynamical considerations of the gas phase and published ab initio data on the catalytic action of the (100) surface of β-Ga2O3 we discuss the adsorption and incorporation processes that promote epitaxial layer growth. The structural properties of the β-Ga2O3 epitaxial layers were characterized by X-ray diffraction pattern and high resolution transmission electron microscopy. As-grown layers exhibited sharp peaks that were assigned to the monocline gallium oxide phase and odd reflections that could be assigned to stacking faults and twin boundaries, also confirmed by TEM. Shifts of the layer peak towards smaller 2 θ values with respect to the Bragg reflection for the bulk peaks have been observed. After post growth thermal treatment in oxygen-containing atmosphere the reflections of the layers do shift back to the position of the bulk β-Ga2O3 peaks, which was attributed to significant reduction of lattice defects in the grown layers after thermal treatment. [ABSTRACT FROM AUTHOR]

The influence of intrinsic point defects on the electronic structure of n-type In2O3 single crystals grown by two methods, namely chemical vapor transport and from the melt, was examined by scanning tunneling spectroscopy. This method is a very surface sensitive technique for measuring the local density of states. So far not resolved states within the fundamental band gap have been observed. The gap states have been studied at different crystals and after several annealing treatments in an oxidizing atmosphere. The spectroscopic results will be compared to state of the art DFT-calculations revealing hints on the origin of the gap states of In2O3 to be due to oxygen vacancies as well as indium and oxygen interstitials. [ABSTRACT FROM AUTHOR]

SnO2 is a semiconductor with a wide optical bandgap (3.5 eV), which makes it an attractive transparent semiconducting oxide (TSO) for electronic and opto-electronic applications. At elevated temperatures it is, however, much more unstable than other TSOs (such as ZnO, Ga2O3, or In2O3). This leads to a rapid decomposition even under very high oxygen pressures. Our experiments showed that stoichiometric SnO2 does not melt up to 2100 °C, in contradiction to earlier published data. Bulk SnO2 single crystals, that could provide substrates for epitaxial growth, have not been reported so far. Hereby we report on truly bulk SnO2 single crystals of 1 inch diameter grown by physical vapor transport (PVT). The most volatile species during SnO2 decomposition is, in addition to oxygen, SnO, which is stable in the gas phase at high temperature and reacts again with oxygen at lower temperatures to form SnO2. We identified a relatively narrow temperature window, temperature gradients and a ratio of SnO/O2 for providing the best conditions for SnO2 single crystal growth. X-ray powder diffraction (XRD) proved the single SnO2 phase. Moreover, by selecting a suitable SnO/O2 ratio it was possible to obtain either n-type conductivity with electron concentrations up to 2 × 1018 cm−3 and electron mobilities up to 200 cm2 V−1 s−1, or insulating behavior. The crystals exhibited an optical absorption edge located at 330-355 nm, depending on the crystal orientation, and a good transparency over visible and near infrared (NIR) spectra. [ABSTRACT FROM AUTHOR]

A single phase InN epitaxial film is grown on a bulk In2O3(111) wafer by plasma-assisted molecular beam epitaxy. The InN/In2O3 orientation relationship is found to be (0001) II (111) and [100] II [11]. High quality of the layer is confirmed by the small widths of the x-ray rocking curves, the sharp interfaces revealed by transmission electron microscopy, the narrow spectral width of the Raman E2h vibrational mode, and the position of the photoluminescence band close to the fundamental band gap of InN. [ABSTRACT FROM AUTHOR]

Epitaxially strained NaNbO3 films were grown by liquid-delivery spin metal-organic chemical vapour deposition on several oxide substrates, inducing tensile and compressive lattice strain. High-resolution X-ray diffraction measurements reveal that coherently grown compressively strained NaNbO3 films on NdGaO3 exhibit the orthorhombic c phase. With increasing in-plane strain a first structural phase transition to the monoclinic r phase and, further on, for films grown under tensile strain on rare earth scandates, a second phase transition to the aa phase, are observed. Our results are in good agreement with the pathway of phase transitions predicted by Diéguez, Rabe & Vanderbilt [ Phys. Rev. B, (2005), 72, 144101] for NaNbO3 films. [ABSTRACT FROM AUTHOR]

Abstract: Thin films of different phases of lead-free sodium–bismuth–titanate were epitaxially grown on SrTiO3 and NdGaO3 substrates by metal–organic chemical vapor deposition. Aurivillius phases with m =3, m =3.5 and m =4 and the Na0.5Bi0.5TiO3 perovskite phase were obtained by varying the substrate temperature and the Na/Bi ratio in the gas phase. Investigations of the impact of crystallographic structure and incorporated film lattice strain on local piezo- and ferroelectric properties were carried out by piezoresponse force microscopy experiments using dual AC resonance tracking combined with a tip-sample contact modeling procedure to determine an effective value for the piezoelectric coefficient for each sample. Comparative piezoresponse force microscopy measurements revealed a significant increase of the average effective piezoelectric coefficient d zz when the film structure changed from an Aurivillius phase to the perovskite phase. Films of perovskite phase have also shown the possibility of local tip-induced polarization switching. [Copyright &y& Elsevier]

Abstract: The structural properties of the heteroepitaxially grown AlN on off-axis 4H-/6H-SiC by PVT are presented in dependence of the seed polarity. In addition to long-term growth runs (48–72h) short-term experiments were carried out in order to investigate the initial growth stage. SEM, EDX, HR-TEM and HR-XRD studies show different growth modes for C-polar and Si-polar SiC seeds. The main cause is the anisotropic oxidation of SiC by Al2O(g) which is formed during heating up in the AlN source. The type of the growth mode is essential for the defect density in the grown bulk AlN crystal. The growth on C-polar SiC leads to a higher crystal quality of the bulk AlN with up to two orders of magnitude lower etch pit densities and smaller rocking curve widths and hence mosaicity compared to the growth on Si-polar SiC. New initial 3D island growth models for both seed polarities are introduced. [Copyright &y& Elsevier]

Abstract: An approach to deposit polycrystalline silicon layers on amorphous substrates is presented. It is shown that metastable amorphous silicon can be transformed into its more stable microcrystalline structure at a temperature below 330°C via an intermediate liquid solution stage. In particular, the interaction of liquid indium nanodroplets in contact with amorphous silicon is shown to lead to the formation of circular polycrystalline domains. Crystallinity of these domains is analyzed by micro-Raman spectroscopy. The droplet size necessary for the onset of crystallization is related to the temperature of the film. Full coverage of the substrate with microcrystalline silicon has been obtained at 320°C within less than one hour. These films might act as seeding layers for further enlargement by steady-state liquid phase epitaxy. [Copyright &y& Elsevier]

Transparent conducting oxides (TCOs) pose a number of serious challenges. In addition to the pursuit of high-quality single crystals and thin films, their application has to be preceded by a thorough understanding of their peculiar electronic structure. It is of fundamental interest to understand why these materials, transparent up to the UV spectral regime, behave also as conductors. Here we investigate In2O3 and Ga2O3, two binary oxides, which show the smallest and largest optical gaps among conventional n-type TCOs. The investigations on the electronic structure were performed on high-quality n-type single crystals showing carrier densities of ~1019 cm-3 (In2O3) and ~1017 cm-3 (Ga2O3). The subjects addressed for both materials are: the determination of the band structure along high-symmetry directions and fundamental gaps by angular resolved photoemission (ARPES). We also address the orbital character of the valence- and conduction-band regions by exploiting photoemission cross sections in x-ray photoemission (XPS) and by x-ray absorption (XAS). The observations are discussed with reference to calculations of the electronic structure and the experimental results on thin films. [ABSTRACT FROM AUTHOR]

Abstract: Progress in low-temperature growth of crystalline silicon from a metallic solution is presented. Growth has been performed on glass that is coated with an electrically conductive layer. Thermodynamic stability of this interlayer in contact with the saturated growth solution is an essential precondition for subsequent process steps. Therefore, materials involved in the process must be properly chosen. Molybdenum disilicide thin films are shown to withstand the solution contact. In this way, further processing, that includes the formation of seed crystals by use of the vapor–liquid–solid mechanism and subsequent outgrowth from a low-temperature metallic solution, is successful. Growth of {111} faceted silicon crystallites on glass with a size of up to in diameter has been shown. [Copyright &y& Elsevier]

Abstract: Self-assembled and ordered silicon and germanium nanowires grown by physical vapor deposition (PVD) via vapor–liquid–solid (VLS) mechanism are presented. The morphology of the nanowires has been investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Differences in the orientation of the homoepitaxially grown nanowires between silicon and germanium were observed. Most silicon nanowires grew in [111] direction on the (111) Si substrate whereas germanium nanowires grew in direction on the (111) Ge substrate. Nucleation energies as a function of supersaturation were considered. The results of these calculations could explain the behavior of Si and Ge wires in terms of growth direction. A method to position nanodroplets and thus to obtain a regular arrangement of nanowires is also presented. For this purpose, substrates were structured with nanopores by focused ion beams (FIB) before inserting them into the growth chamber. Gold droplets have been successfully ordered both on silicon and on germanium substrates. A regular array of epitaxial silicon nanowires has been obtained as well. [Copyright &y& Elsevier]

[Display omitted] • Growth parameters limits of BGaAs alloys for dual application of BInGaAs QW. • Coupling effect of growth temperature and diborane flux for the first time. • Morphological, structural, optical, and electrical properties. • Engineering of carrier localization in B-III-V materials. • Experimental and theoretical investigations on ternary and quaternary B-III-V. BGaAs/GaAs epilayers and BInGaAs/GaAs quantum well (QW) have been prepared using metal–organic chemical vapor deposition under different growth conditions, and their physical and structural properties have been examined. SEM-EDS investigation showed a dependence of surface properties of the ternary compound on the growth conditions. High-resolution X-ray diffraction evidenced a tensile strain for the ternary alloys whatever the growth condition, while the quaternary QW always shows a compressive strain state. Room temperature optical absorption allowed to follow the variation of the bandgap with boron incorporation. Photoluminescence measurements confirmed the carrier-localization phenomenon and its dependence with the growth conditions. Deposition temperature and diborane (B 2 H 6) flow rate are with particularly significant effects on the optical properties: lower diborane flow rate and high growth temperature enhance the radiative emission. Computer simulation using localized state ensemble model quantitatively relates the lattice inhomogeneity to the optical properties and suggests a way to engineer the localization phenomenon and avoid clustering effects. Electrical investigations by current–voltage, capacitance and conductance methods have been performed for the first time on selected BGaAs samples. The ideality factor of Schottky barriers has been determined, while their height and film doping level could only be approximately estimated. Such physical properties make boron-based alloys very promising for applications in multijunction solar cells. [ABSTRACT FROM AUTHOR]

Deep and shallow electronic states in undoped and Si-doped ε-Ga 2 O 3 epilayers grown by MOVPE on c-oriented Al 2 O 3 were investigated by cathodoluminescence, optical absorption, photocurrent spectroscopy, transport measurements, and electron-paramagnetic-resonance. Nominally undoped films were highly resistive, with a room temperature resistivity varying in the range 107- 1013 Ωcm depending on the carrier gas used during growth. Films grown with He carrier were generally more resistive than those grown with H 2 carrier and exhibited a Fermi level located at about 0.8 eV below the conduction band edge, which tends to shift deeper with temperature. This can tentatively be attributed to the combined action of deep donors (probably carbon impurities and oxygen vacancies) and deep acceptors (Ga vacancies and related complexes), which compensate residual shallow donors. There are strong experimental hints that nitrogen also behaves as deep acceptor. Room temperature resistivity as low as 0.42 Ωcm and electron concentrations around 1018 cm−3 were obtained by silicon doping. Si was confirmed to act as shallow donor with sufficiently high solubility. A variable range hopping conduction was observed in a wide temperature interval in the n -type layers, and compensation by native acceptors also plays a major role on conduction mechanisms. Previous evaluations of curvature and anisotropy of the conduction band are confirmed, which allows for the estimation of the electron effective mass. The present experimental data are discussed considering the theoretical predictions for point defect formation in the ε-polymorph as well as literature data on extrinsic and intrinsic defects in β-Ga 2 O 3. One Sentence Summary. Shallow and deep electronic defects detected in highly resistive and highly conductive ε-Ga 2 O 3 thin films and survey of literature data. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports on the bahavior of Au and In as solvents for the growth of silicon nanowires on a Si(111) substrate via vapor–liquid–solid (VLS) mechanism. Gold is the mostly used solvent for growing silicon nanowires but in the present work indium was also applied, as it may bring some advantages for later electronic application of the wires. The main focus of this work is the behavior of gold and indium on a silicon substrate but also the different morphologies and distributions of the grown wires are compared. Individual metal droplets have been located in pre-structured nanopores to serve as starting points for wire growth. The method used to exactly position the metal droplets and thus obtain a regular arrangement of nanowires is also presented. [Copyright &y& Elsevier]

Abstract: AlN boules, 35mm in diameter and up to 25-mm long, were grown on TaC crucible lid in an inductively heated reactor. The growth rates range between 100 and 300μm/h. The boules grown on TaC show a columnar structure mostly composed of 〈0001〉 grains. The largest grains (4–5mm in diameter) were sliced and used for subsequent growth runs. Successful epitaxial seeding and growth on the starting AlN wafer was demonstrated and confirmed by electron back-scatter diffraction (EBSD) measurements. Crystals were grown on both Al and N surfaces of the seeds up to a maximum diameter of about 9mm so far. Formation of oxy-nitride layers, very detrimental to the further AlN deposition, could be avoided when starting from pre-sintered source powder. Secondary ion mass spectroscopy (SIMS) measurements on axial cuts revealed a relatively low oxygen content, with variable distribution along the growth direction (290ppm near seed, 100ppm near external surface). [Copyright &y& Elsevier]

Abstract: Two-inch-diameter γ-LiAlO2 single crystals were grown from the melt by Czochralski method. The crystals were examined by optical methods, high-resolution X-ray diffraction and transmission electron microscopy (TEM). Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to determine the Li/Al ratio in the residual melts. The Li-evaporation from both melt and grown crystal is the main problem in the γ-LiAlO2 growth and has to be controlled by acting on the vertical temperature gradient. Shallow gradients increase the Li-evaporation from the crystal surface resulting in boules with a milky rim. On the other hand, steep gradients may induce cracks in the boule and enhance the Li2O escape from melt with consequent variation of the composition. ICP-OES investigations reveal that melt compositions can vary in the range from 46.5 to 50mol% Li2O to obtain transparent LiAlO2 crystals. Beyond this value, the formation of inclusions inside the crystals is probable. We have established an optimized growth assembly, which allows remaining the melt composition stoichiometric. The as-grown crystals exhibit defects like subgrains, twins and a core of voids and fine-grained inclusions. The latter could be characterized by TEM as submicron LiAl5O8 crystallites. [Copyright &y& Elsevier]

Abstract: (Cr, Al)2O3 (k 0 =2), (Ga, Al)2O3 (k 0 =0.3), Gd3(Cr, Ga)5O12 (k 0 >1), (Gd, Yb)3Ga5O12 (k 0 >1) and (Yb, Y)3Al5O12 (k 0 >1) fibers have been grown by the micro-pulling-down method. k 0 is the equilibrium distribution coefficient of the dopant with respect to the given host phase. The axial and radial dopant distribution was measured by electron probe microanalysis. The growth interface was frozen by pulling-down the fiber rapidly and quenching the molten zone. In the case of growth with a thin melt zone of about 30–70μm between growth front and crucible nozzle radial dopant distributions in form of a homogeneous core and a peripheral rim of different composition were found in all fibers. The growth interface has a bump into the crucible nozzle. For (Yb, Y)3Al5O12, Gd3(Cr, Ga)5O12, (Gd, Yb)3Ga5O12 and (Ga, Al)2O3 mainly diffusional transport in the crucible nozzle was found. The dopant concentration is higher or lower at the rim than in the core for k 0 <1 or k 0 >1, respectively. This axial segregation corresponds to the bend interface. In the case of (Cr, Al)2O3 convective transport inside the crucible nozzle was found. Also, different to the diffusive case, the Cr2O3 concentration in the rim is higher than in the core region although k 0 >1. The growth with a higher melt zone of about 200μm has the same axial segregation behaviour but the radial segregation is strongly suppressed. The interface is spherical. The experiments show, that the radial and axial concentration profiles that are the result from the micro-pulling-down growth of solid solutions are influenced by the distribution coefficient, the geometry of the crucible, the height of the molten zone and melt properties. [Copyright &y& Elsevier]

Abstract: Zinc oxide single crystals were obtained by solidification of a melt contained in an iridium crucible. To the authors knowledge this is the first report on crystal growth of zinc oxide from the melt using an inductively heated metal crucible. This proves that under particular conditions metals are capable of withstanding liquid zinc oxide at high temperatures. Optical microscopy as well as X-ray rocking curve measurements reveal the high crystalline quality of the wafers with full-width at half-maximum values of 22arcsec. Annealing of as-grown samples was observed to have a big influence on optical properties. [Copyright &y& Elsevier]

The good control of the n -type doping is a key issue for the fabrication of efficient devices based on ε-Ga 2 O 3 epilayers. In this work we studied the possibility of doping the ε-Ga 2 O 3 thin films, epitaxially grown on c -oriented sapphire by metal-organic chemical vapor deposition, by means of a post-deposition treatment. For the first time, the n -type doping was achieved by depositing a tin-rich SnO 2 film on top of the ε-Ga 2 O 3 layer and keeping this bi-layer system for 4 h at a temperature of 600 °C in an evacuated furnace. The diffusion of Sn atoms into the ε-Ga 2 O 3 film is evidenced by time-of-flight secondary-ion mass spectrometry depth profiles. Room-temperature resistivity of the order of 1 Ω•cm is obtained and the electrical characterization revealed a conduction mechanism based on variable range hopping, according to the Mott's model. [Display omitted] [ABSTRACT FROM AUTHOR]

A correction to the article "High-voltage field effect transistors with wide-bandgap β-Ga2O3 nanomembranes" by Wan Sik Hwang and others that was published in the May 2014 issue of the journal is presented.

Germanium nanowires (NWs) were grown onto Ge(111) substrates by the vapor-liquid-solid process using gold droplets. The growth was carried out in a molecular beam epitaxy chamber at substrate temperatures between 370 °C and 510 °C. The resulting nanowire growth rate turns out to be highly dependent on the substrate temperature exhibiting the maximum at T = 430 °C. The temperature dependence of growth rate can be attributed to surface diffusion both along the substrate and nanowire sidewalls. Analyzing the diffusive material transport yields a diffusion length of 126 nm at a substrate temperature of 430 °C. [ABSTRACT FROM AUTHOR]