Ga2O3 Phase Research Articles

(100) Plane β-Ga2O3 Flake Based Field Effect Transistor and Its Hydrogen Response

2-dimensional (100) plane β phase Ga2O3 (β-Ga2O3) flake based field effect transistor (FET) was fabricated, and its electrical characteristics was analyzed. The (100) plane β-Ga2O3 flake was mechanically exfoliated from the side wall of plane β-Ga2O3 bulk substrate. The minimum thickness of 57.3 nm was obtained for the very thin (100) plane β-Ga2O3 channel layer of the FET using inductively coupled plasma etching with BCl3/N2 chemistry. The current-voltage characteristics of the FET with various β-Ga2O3 channel thickness was investigated. The dependence of the channel thickness on the drain current density, threshold voltage, transconductance, and field effect mobility was studied. The hydrogen response of the (100) plane Ga2O3 flake based FET with catalytic Pt gate surface was measured in the range of 10–500 ppm at 400 °C, and modeled with a dissociative Langmuir isotherm. The device showed a reliable responsivity to the different concentration of hydrogen exposure, and the responsivity of 25.02% was observed for the 500 ppm hydrogen at 400 °C.

Tracking the phase changes in micelle-based NiGa nanocatalysts for methanol synthesis under activation and working conditions

The hydrogenation of CO2 into high energy density fuels such as methanol, where the required H2 is obtained from renewable sources, is of utmost importance for a sustainable society. In recent years, NiGa alloys have attracted attention as promising catalyst material systems for the hydrogenation of CO2 into methanol at ambient pressures. They thus represent an energy-saving alternative to the Cu-based catalysts employed in today’s catalytic industry that require high pressures for the CO2 hydrogenation. However, the underlying reaction mechanisms for the NiGa system are still under debate. One of the challenges here is to unravel the evolution and coexistence of the different species in the heterogeneous NiGa catalyst system under activation and reaction conditions. To shed light on their evolution under activation in H2 and their catalytic roles under CO2 hydrogenation working conditions on well-defined Ni3Ga1 and Ni5Ga3 nanoparticle (NP) catalysts, we employed a multi-probe approach. It included advanced machine learning-based analysis of operando X-ray absorption spectroscopy data combined with operando powder X-ray diffraction and near ambient pressure X-ray photoelectron spectroscopy measurements, as well as reactivity studies using bed-packed mass flow reactors. In addition, we employed atomic force microscopy and scanning transmission electron microscopy for structural characterization.Under H2 activation at 1 bar total pressure, we concluded the formation of metallic Ni, starting for Ni3Ga1 at 300 °C, and for Ni5Ga3 at 400 °C. At higher temperatures, the formation of NiGa alloys follows. The α’-Ni3Ga1 alloy phase is predominantly formed for the Ni3Ga1 NPs, while the coexistence of α’-Ni3Ga1, δ-Ni5Ga3 and Ga2O3 phases is observed for the Ni5Ga3 NPs after the H2 activation. The formation of the Ga2O3 phase also results in the presence of excess metallic Ni. Under CO2 hydrogenation reaction conditions, Ga partially oxidizes again to form a Ga2O3-rich particle shell for both NP compositions, yet, to a larger extent for the Ni3Ga1 NPs, which, in turn, feature a higher amount of excess Ni. We reveal that metallic Ni is responsible for the high selectivity of the Ni3Ga1 NPs towards the production of methane in our catalytic tests. In contrast, the Ni5Ga3 NPs display a strong selectivity toward methanol production (>92%), more than one order of magnitude higher than that for the Ni3Ga1 NPs, which we ascribe to the presence of the δ-Ni5Ga3 phase.

Ga2O3(Sn) Oxides for High-Temperature Gas Sensors.

Gallium(III) oxide is a promising functional wide-gap semiconductor for high temperature gas sensors of the resistive type. Doping of Ga2O3 with tin improves material conductivity and leads to the complicated influence on phase content, microstructure, adsorption sites, donor centers and, as a result, gas sensor properties. In this work, Ga2O3 and Ga2O3(Sn) samples with tin content of 0–13 at.% prepared by aqueous co-precipitation method were investigated by X-ray diffraction, nitrogen adsorption isotherms, X-ray photoelectron spectroscopy, infrared spectroscopy and probe molecule techniques. The introduction of tin leads to a decrease in the average crystallite size, increase in the temperature of β-Ga2O3 formation. The sensor responses of all Ga2O3(Sn) samples to CO and NH3 have non-monotonous character depending on Sn content due to the following factors: the formation of donor centers and the change of free electron concentration, increase in reactive chemisorbed oxygen ions concentration, formation of metastable Ga2O3 phases and segregation of SnO2 on the surface of Ga2O3(Sn) grains.

Innovative Infrared-pulsed laser assisted RF sputtered β-Ga2O3 thin film at low temperature process

Innovative new method for the pulsed laser assisted radio-frequency sputtering at low temperature process was successfully applied for the β phase Ga2O3 thin film as wide band gap materials. Thin film of wide bandgap material, β-Ga2O3, was successfully deposited on SiO2/Si and quartz substrates by employing radio-frequency sputtering system with assisting process of ytterbium-laser irradiation. Both sputtering system and infrared pulsed laser irradiation process were employed at the same time to increase the acquired energy densities for the adatoms on the thin film growth process. X-ray diffraction patterns (XRD) showed the prominent peaks of unique crystalline properties of Ga2O3 thin films in β phase. The optical energy bandgap of Ga2O3 thin films derived from spectral transmittance was measured in the range of 4.90–5.06 eV for a variation of laser power. High resolution X-ray photon spectroscopy (XPS) analysis showed that O 1s and Ga 3d position in core level. We confirm that infrared optical energy was effectively added to fabricate β-Ga2O3 thin film by employing the Yb-laser-assisted RF sputtering process.

Growth and characteristics of β-Ga2O3 thin films on sapphire (0001) by low pressure chemical vapour deposition

Monoclinic β phase Ga2O3 has been considered as a promising candidate for next generation radio frequency and high-power devices because of its ultrawide bandgap and high breakdown field. The development of epitaxial growth techniques and the synthesis of high quality β-Ga2O3 thin films are crucial for the device applications. In this work, the heteroepitaxial β-Ga2O3 thin films were grown on sapphire (0001) substrates by low pressure chemical vapor deposition. The influence of growth parameters such as the source/substrate temperatures, the oxygen/Ar gas flow rates, and the surface morphology of substrate on the resultant crystallinity and surface roughness of the β-Ga2O3 films were investigated. The β-Ga2O3 heteroepitaxial layer on sapphire exhibits an RMS roughness of 1.82 nm, an XRD rocking curve of 1.18°, and a growth rate of 0.72 μm/h. The β-Ga2O3 film grown on the buffer layer exhibit a smoother surface, whereas the chemical etching and annealing lead to an improved crystallinity and a rough surface. However, there is a trade-off between the crystallization and the surface roughness.

The role of Ga and Bi doping on the local structure of transparent zinc oxide thin films

Transparent undoped ZnO and additionally doped with Ga and Bi thin films were produced by magnetron sputtering. The thin films were comprehensively characterized by X-ray absorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission and scanning transmission electron (TEM, STEM) microscopy and Raman spectroscopy. All undoped and doped films crystallise in a ZnO phase with the hexagonal wurtzite crystal structure. The local structure of the thin films was investigated by temperature-dependent X-ray absorption spectroscopy at the Zn and Ga K-edges, as well as at the Bi L3-edge. It was found that the doping of Ga3+ and Bi3+ ions in the ZnO wurtzite structure produces distinct effects on the thin film microstructure. The substitution of Zn2+ ions by smaller Ga3+ ions introduces a static disorder to the thin film structure, which is evidenced by an increase in the mean-square relative displacements σ2(Zn‒O) and σ2(Zn‒Zn). At the same time, large Bi3+ ions do not substitute zinc ions, but are likely located in the disordered environment at the ZnO grain boundaries. This conclusion was directly supported by energy-dispersive X-ray spectroscopy combined with TEM and STEM observations as well as by resonant and non-resonant μ-Raman experiments at room temperature, where the ZnO and ZnO:Bi spectra are similar, suggesting a lack of structural disorder in the wurtzite cell. On the other hand, the Raman disorder-activated phonon is pronounced for Ga-doping of the ZnO lattice, confirming the compositional disorder. Both XRD and XPS ruled out Ga2O3 phase in Ga-doped ZnO; conversely, Bi2O3 and a small amount of Bi‒metal phases are clearly discerned by XPS experiments, further suggesting that Bi is not incorporated in the ZnO wurtzite cell, but segregated to grain boundaries.

Compensation of Zn substitution and secondary phase controls effective mass and weighted mobility in In and Ga co-doped ZnO material

Conductivity σ and thermal conductivity κ are directly related to carrier concentration while Seebeck coefficient S is inversely proportional to carrier concentration. Therefore, improving thermoelectric (TE) performance is challenging. Here, the first-time analysis of secondary phase-controlled TE performance in terms of density-of-state effective mass md∗, weighted mobility μw and quality factor B is discussed in ZnO system. The results show that the secondary spinel phase Ga2O3(ZnO)9 not only impacts on κ but also on σ and S at high temperature, while the effect of carrier concentration seem to be dominant at low temperature. For the high-spinel-segregation sample, a compensation of dopant atoms from the spinel to substitutional sites in the ZnO matrix at high temperature leads to a low decreased rate of temperature-dependent md∗. The compensation process also induces a band sharpening, a small μw reduction, and a large B enhancement. As a result, In and Ga co-doped ZnO bulk with the highest spinel segregation achieves the greatest PF improvement by 112.8%, owing to enhanced Seebeck coefficient by 110% as compared to the good Zn-substitution sample.

Controlled thin-film deposition of α or β Ga2O3 by ion-beam sputtering

The wide bandgap of oxide semiconductors enables solid-state devices with functionalities beyond the reach of technologically more well-established materials such as silicon, germanium, or gallium nitride. These functionalities, among others, include optical transparency paving the way for transparent (opto-)electronics like ultraviolet light emitters or solar-blind detectors as well as large breakdown fields enabling high-power electronic applications. In particular, Ga2O3 is considered as oxide compatible with the nitride III-V materials. As a particular challenge, Ga2O3 may crystallize in different polymorphs. All possess large optical bandgaps beyond 4.8 eV and, thus, are of great interest for such devices. Nowadays, Ga2O3 thin films are prepared by a plethora of different synthesis methods including physical as well as chemical variants, however, reports on a controlled manufacturing process of individual Ga2O3 phases with sputtering techniques are currently scarce. Here, we grow Ga2O3 thin films by ion-beam sputtering on sapphire substrates of different orientation and demonstrate that selective controlled synthesis of different Ga2O3 polymorphs is feasible. The monoclinic β-phase and the corundum-type α-phase are stabilized when deposited on (0001) and (10–10) sapphire substrates, respectively. A stacking rather than a mixture of both phases is observed when thin films of α-phase exceed a certain thickness. Thus, thickness control enables the transition between the two phases. The proposed technique holds promise for ion-beam sputter methods to be employed in Ga2O3-related device build-up.

Two-dimensional gallium and indium oxides from global structure searching: Ferromagnetism and half metallicity via hole doping

There has been tremendous research effort in hunting for novel two-dimensional (2D) materials with exotic properties, showing great promise for various potential applications. Here, we report the findings about a new hexagonal phase of 2D Ga2O3 and In2O3, with high energetic stability, using a global searching method based on an evolutionary algorithm, combined with density functional theory calculations. Their structural and thermal stabilities are investigated by the calculations of their phonon spectra and by ab initio molecular dynamics simulations. They are predicted to be intrinsically non-magnetic stable semiconductors, with a flatband edge around the valence band top, leading to itinerant ferromagnetism and half-metallicity upon hole doping. Bilayer Ga2O3 is also studied and found to exhibit ferromagnetism without extra hole doping. The Curie temperature of these materials, estimated using Monte Carlo simulations based on the Heisenberg model, is around 40–60 K upon a moderate hole doping density.

Preparation and properties of heteroepitaxial β-Ga2O3 films on KTaO3 (100) substrates by MOCVD

Ga2O3 films were deposited on (100) oriented KTaO3 substrates by the metal-organic chemical vapor deposition technology. Structural and chemical composition analyses confirmed that the as-deposited film at 850 °C was β phase Ga2O3 with high crystalline quality. The epitaxial relationships between the film and substrate were determined as β-Ga2O3 (100) || KTaO3 (100) with β-Ga2O3 [001] || KTaO3 〈011〉 by X-ray diffraction, high resolution transmission electron microscopy and selected area electron diffraction results. The average transmittance in the visible range of the sample prepared at 850 °C was 73.6%, and the optical band gap of the β-Ga2O3 film was approximately 4.79 eV with a resistivity of 5.01 × 103 Ω·cm.

Thickness-dependent thermal conductivity of mechanically exfoliated β -Ga2O3 thin films

Beta-phase gallium oxide (β-Ga2O3), the most thermally stable phase of Ga2O3, has stimulated great interest in power electronics due to its ultra-wide bandgap (∼4.9 eV) and high breakdown electric field. The relatively low thermal conductivity of β-Ga2O3, however, limits the device performance due to excessive temperature driven by self-heating. Recently, integrating β-Ga2O3 thin films on substrates with high thermal conductivities has been proposed to improve heat rejection and device reliability. In this work, we prepare high-quality single-crystal β-Ga2O3 thin films by mechanical exfoliation of bulk crystals and study their thermal transport properties. Both the anisotropic thermal conductivity of β-Ga2O3 bulk crystals and the thickness-dependent thermal conductivity of β-Ga2O3 thin films are measured using the time-domain thermoreflectance technique. The reduction in the thin-film thermal conductivity, compared to the bulk value, can be well explained by the size effect resulting from the enhanced phonon-boundary scattering when the film thickness decreases. This work not only provides fundamental insight into the thermal transport mechanisms for high-quality β-Ga2O3 thin films but also facilitates the design and optimization of β-Ga2O3-based electronic devices.

Photocatalytic Activity of Metal Oxide Supported Gallium Oxide for CO2 Reduction with Water

Abstract We have examined photocatalytic activities of Ga2O3 supported by various metal oxides for water splitting and CO2 reduction with water under UV light irradiation. The Ga2O3 photocatalyst supported by metal oxides was prepared by an impregnation method. We have found that some oxides used as a supporting oxide of Ga2O3 photocatalyst improve activities of water splitting and CO2 reduction. In particular, the CO production rates are well correlated to various properties of the supporting oxide. These oxides satisfy the following three criteria significantly enhancing the CO production rate. (1) Their band gap is wider than that of Ga2O3 and the position of its conduction band is at a negative potential with respect to that of Ga2O3. (2) They are not miscible with Ga2O3 and (3) mixed phases of α and γ or defective γ phase of Ga2O3 are well dispersed on them exhibiting columnar shape. These criteria would be useful for further improvement of the activity for photocatalytic CO2 reduction with water by Ga2O3.

Growth, structural and optical properties of coherent κ-(AlxGa1−x)2O3/κ-Ga2O3 quantum well superlattice heterostructures

High quality heteroepitaxial (001)-oriented κ-(AlxGa1−x)2O3/κ-Ga2O3 quantum well superlattice heterostructures were deposited by tin-assisted pulsed laser deposition on c-sapphire substrates. Sharp superlattice fringes up to the ninth order in XRD patterns for Al-contents up to about 50 at. % confirm excellent structural quality and smooth interfaces in the multilayers on par with reports on homoepitaxial superlattices in the monoclinic modification. By employing elliptically segmented targets, the Al-content in the barrier layers of the superlattices was systematically varied in a range of 0.1 ≤ x ≤ 0.5 in a controlled and quasi-continuous manner. An in-depth investigation employing XRD 2θ-ω scans and reciprocal space map measurements on superlattices with different periods as well as single quantum well samples suggests coherent growth of the superlattices for application-relevant quantum well widths. The critical thickness for coherent growth of κ-Ga2O3 on κ-(AlxGa1−x)2O3 was further estimated to be at least 50 nm and 3 nm for x = 0.2 and x = 0.3, respectively. We determined absorption energies in optical transmission spectra for superlattices with x = 0.3 well below the bandgap of the barrier layers that decrease with increasing quantum well width suggesting transitions between localized states in the quantum wells as their origin. These results render superlattices in the metastable orthorhombic phase of Ga2O3 as a promising active layer for quantum well infrared photodetector applications.

Liquid Metal-Based Route for Synthesizing and Tuning Gas-Sensing Elements.

There is a strong demand for developing tunable and facile routes for synthesizing gas-sensitive semiconducting compounds. The concept of synthesizing micro- and nanoparticles of metallic compounds in a tunable process, which relies on liquid metals, is presented here. This is a liquid-based ultrasonication procedure within which additional metallic elements (In, Sn, and Zn) are incorporated into liquid Ga that is sonicated in a secondary solvent. We investigate liquid metal sonication in dimethyl sulfoxide (DMSO) and water to show their impact on the size, morphology, and crystal structure of the particulated products. The synthesized materials are annealed to investigate their responses to model reducing (H2) and oxidizing (NO2) gas species. The preparation process in DMSO gives rise to predominantly monoclinic Ga2O3 crystals which are favorable for gas sensing, while the emergence of rhombohedral Ga2O3 phases from the water sonication process led to inactive samples. The ease of tunability without hazardous precursors during the synthesis procedure is demonstrated. The route presented here can be uniquely employed for designing and engineering on-demand functional materials for sensing applications.

Ellipsometric determination of anisotropic optical constants of single phase Ga2O3 thin films in its orthorhombic and monoclinic phases

The anisotropic optical constants of high-quality, single crystalline Ga2O3 films in both orthorhombic (ε) and monoclinic (β) phases were obtained from ellipsometry study. The (0001)-oriented ε-Ga2O3 and a (−201) β-Ga2O3 epilayers were grown on (0001) α-Al2O3 substrates by mist-chemical vapor deposition and pulsed laser deposition, respectively. The experimental bandgaps derived from optical constants were found to be related with the allowed optical transitions from valence sub-bands to conduction band minimum in terms of the polarization-selection rules in such asymmetric structures. The pseudo-ordinary (a-b plane) and extraordinary (c axis) optical bandgaps of the ε-Ga2O3 film were estimated to be 4.85 ± 0.02 and 4.76 ± 0.02 eV, respectively. The pseudo ordinary (b axis) and extraordinary (a-c plane) optical bandgaps of the β-Ga2O3 film were estimated to be 4.92 ± 0.02 and 4.57 ± 0.02 eV, respectively. The energy difference between the two bandgaps of the ε-Ga2O3 is about 0.09 eV, which matches the theoretically calculated energy difference (about 0.06 eV) of the highest two valence bands.

Formation of Tl-1212 phase in Ga-substituted Tl(BaSr)Ca1−xGaxCu2O7 superconductor

The effect of Ga substitution on the superconductivity of Tl(BaSr)Ca1-xGaxCu2O7 (x = 0–0.5) is reported. The samples were prepared by solid-state reaction method. XRD patterns showed a single Tl-1212 phase for the x = 0 and 0.1 samples. Unreacted Ga2O3 and other impurity phases were observed in the higher substituted (x = 0.2 to 0.5) samples. The temperature-dependent resistance measurements showed metallic normal state behavior for all samples with onset transition temperature Tc-onset between 94 and 66 K and zero transition temperature Tc-zero between 86 and 55 K. AC susceptibility measurement showed the highest superconducting transition (95 K) and peak temperature (89 K) for non-substituted sample. The intergranular critical current density Jc at Tp for all samples was between 16 and 22 A cm−2. This work showed that the intergranular coupling and flux pinning energy were suppressed with Ga substitution. Some Ga+2 were most likely incorporated at Ca+2 for x = 0.1 and some resided at the grains for x ≥ 0.2 samples. This work showed that a single superconducting Tl-1212 phase was formed for x ≤ 0.1 samples. This work also showed that the internal lattice constrain which is the ratio of the lattice parameter (a/c) is important in improving the transition temperature of the Tl-1212 phase.

Influence of Ga2O3, CuGa2O4 and Cu4O3 phases on the sodium-ion storage behaviour of CuO and its gallium composites.

CuO and its gallium composites with various compositions are successfully fabricated by using a hydrothermal technique followed by calcination at 900 °C. The added Ga precursors formed oxides in the composites, such as Ga2O3, CuGa2O4 and Cu4O3, as confirmed through the X-ray diffraction patterns as well as the HRTEM and SAED patterns. Further HRTEM analysis also confirmed that Cu4O3 and CuGa2O4 phases reside on the surface of CuO in the composites with a CuO : Ga ratio of 90 : 10. The contents of various oxide phases varied when we increased the amount of Ga in the CuO composites. Changing the ratios of CuO and Ga precursors in the composites is quite effective in tailoring the sodium-ion storage behaviour of CuO. The resultant CuO/Ga composites exhibit remarkable electrochemical performance for sodium-ion batteries in terms of capacity, rate capability and cycling performance. The composite containing 90% CuO and 10% Cu/Ga oxides delivers the highest charge capacity of 661 mA h g−1 at a current density of 0.07 A g−1 with a capacity retention of 73.1% even after 500 cycles. The structure and morphology of the composite (90% CuO and 10% Cu/Ga oxides) was successfully retained after 500 cycles, which was confirmed through ex situ XRD, SEM and HRTEM analyses. The composite also exhibited remarkable rate capability in which it delivered 96 mA h g−1 even at a high current density of 6.6 A g−1. The enhanced electrochemical performances of CuO and its gallium composites are attributed to the presence of Cu4O3 and CuGa2O4 phases. The Cu4O3 phase is actively involved in the redox reaction and the CuGa2O4 phase stabilizes the CuO phase and buffers the volume expansion of CuO during cycling. The present approach eplores great opportunities for improving the electrochemical performance of oxide based anode materials for sodium-ion batteries.

Influence of Annealing Temperature on the Properties of ZnGa2O4 Thin Films by Magnetron Sputtering

Zinc gallate (ZnGa2O4) thin films were grown on sapphire (0001) substrate using radio frequency (RF) magnetron sputtering. After the thin film deposition process, the grown ZnGa2O4 was annealed at a temperature ranging from 500 to 900 °C at atmospheric conditions. The average crystallite size of the grown ZnGa2O4 thin films increased from 11.94 to 27.05 nm as the annealing temperature rose from 500 to 900 °C. Excess Ga released from ZnGa2O4 during thermal annealing treatment resulted in the appearance of a Ga2O3 phase. High-resolution transmission electron microscope image analysis revealed that the preferential crystallographic orientation of the well-arranged, quasi-single-crystalline ZnGa2O4 (111) plane lattice fringes were formed after the thermal annealing process. The effect of crystallite sizes and lattice strain on the width of the X-ray diffraction peak of the annealed ZnGa2O4 thin films were investigated using Williamson-Hall analysis. The results indicate that the crystalline quality of the deposited ZnGa2O4 thin film improved at higher annealing temperatures.

One-Step Growth of Amorphous/Crystalline Ga2O3 Phase Junctions for High-Performance Solar-Blind Photodetection.

The pursuit of high-performance photodetectors functioning in the solar-blind spectrum is motivated by both scientific and practical applications ranging from secure communication, monitoring, sensing, etc. In particular, the fabrication of heterojunctions based on the wide band gap semiconductors has emerged as an attractive strategy to promote the high-efficient photogenerated electron/hole pair separation. However, the precisely controlled growth of heterojunctions remains a huge challenge. The lattice mismatch leads to the formation of defects and/or dislocations at the interface, deteriorating the performance of devices and limiting their envisioned applications. Here, we demonstrate a simple one-step growth of amorphous/crystalline Ga2O3 phase junctions by using sputtering technique, yielding a large responsivity of 0.81 A/W, a superior photo-to-dark current ratio over 107, and an ultrahigh response speed of ∼12 ns. Compared to the previous reported solar-blind photodetectors, the obtained detectivity ≈ 5.67 × 1014 Jones is increased by 2 orders of magnitude. Such excellent photoresponse characteristics can be understood by the interfacial built-in field-promoted electron/hole pair separation for the amorphous/crystalline Ga2O3 phase junctions. Our results provide a novel path toward realizing high-performance optoelectronics functioning in the solar-blind spectrum.

Characterization of single crystal β-Ga2O3 films grown on SrTiO3 (100) substrates by MOCVD

Gallium oxide (Ga2O3) films have been deposited on SrTiO3 (100) substrates by using the metal-organic chemical vapor deposition (MOCVD) method. Post-deposition annealing was performed at different temperatures. XRD θ-2θ scans displayed that the annealing at 1000 °C leaded to β phase Ga2O3 film with the best crystalline quality. Microstructural and chemical composition analyses revealed that this film was single crystal β-Ga2O3 with stoichiometric ratio. The epitaxial relationships were clearly determined as β-Ga2O3 (100) || SrTiO3 (100) with β-Ga2O3 [001] || SrTiO3 <011>. All of the prepared Ga2O3 samples have average transmittances in the visible range of more than 70%.