α-Ga2O3 Films Research Articles

Stress–strain state in α-Ga2O3 epitaxial films on α-Al2O3 substrates

We consider the stress–strain state in α-Ga2O3/α-Al2O3 heterostructures, in which both constituting phases belong to trigonal crystal system. We utilize the hexagonal geometry (hexagonal cell) to account for structural features of the materials and include in the analysis the complete set of six elastic constants Cij being typical for materials with rhombohedral crystal structure that differs from materials with hexagonal one by the presence of an additional constant C14. This allows us to derive analytical formulas for elastic strains and mechanical stresses in pseudomorphic α-Ga2O3 films on α-Al2O3 substrates with various growth orientations assuming nonzero constant C14.

Heteroepitaxial growth of α-Ga2O3 thin films on a-, c- and r-plane sapphire substrates by low-cost mist-CVD method

Abstract α-Ga2O3 has received wide attention as a wide bandgap semiconductor. Many reports have reported heterogeneous epitaxial growth of α-Ga2O3 thin films, which basically use c-plane (0006) sapphire as a substrate. However, research on the growth of α-Ga2O3 on substrates such as a-, r-plane sapphire is still lacking. This paper systematically analyzes the differences of α-Ga2O3 films grown on different sapphire substrates (a-, c- and r-plane) using the non-vacuum, highly scalable and low-cost mist-CVD method. The in-plane and out-of-plane orientations of all three samples were consistent with their substrates. A series of complementary measurements (XRD, XPS, AFM, TEM, and UV-Vis) were used to characterize the crystallinity, elemental composition, surface morphology, lattice structure, growth rate, and optical bandgap of the α-Ga2O3 grown on substrates of different plane. It is shown that compared to the samples on other two plane substrates, the c-plane α-Ga2O3 films has a smallest FWHM of 0.204°, an ideal band gap width of 5.29 eV, a small Root-Mean-Square (RMS) of 1.64 nm, a growth rate of 11.44 nm/min and a perfect lattice structure among all the three samples. Thus, the c-plane sapphire is more suitable for high-quality α-Ga2O3 growth compared to the other plane substrates.

Свойства резистивных структур на основе полиморфных фаз оксида галлия

The influence of UV radiation and a strong electric field on the I–V characteristics of resistive structures based on polymorphic films of gallium oxide (Ga2O3) is considered. Ga2O3 films were deposited by the method of chloride vapor-phase epitaxy (HVPE) on smooth and structured sapphire substrates with a baseline (0001) orientation. α-Ga2O3 films were produced on smooth substrates, and deposition on patterned sapphire substrates templates produced a mixed structure containing both α-Ga2O3 and ε-Ga2O3. The switching effect was observed in metal / Ga2O3 / metal structures based on two-phase films. Structures pass from a low-resistance (LR) state to a high-resistance (HR) state when exposed to a strong electric field and radiation with λ = 254 nm.

Влияние ультрафиолетового излучения и электрического поля на проводимость структур на основе alpha- и ε-Ga-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-

The influence of ultraviolet radiation and a strong electric field on the current-voltage characteristic of resistive structures based on polymorphic films of gallium oxide is studied. Both types of Ga2O3 films were obtained by the method of chloride vapor-phase epitaxy on smooth and structured sapphire substrates with orientation (0001). In the same process α-Ga2O3 films were deposited on smooth substrates, and gallium oxide films, with regular structures perpendicular to the substrate, containing alternating regions of the α- and ε-phases were deposited on patterned substrate. It’s was observed, that radiation with λ = 254 nm and strong electric transfer structures from a state with low resistance to a state with high resistance. The response time to UV radiation is 5 seconds, and the recovery time less than 1 s.

Influence of Carrier Gases on the Quality of Epitaxial Corundum-Structured α-Ga2O3 Films Grown by Mist Chemical Vapor Deposition Method.

This report systematically investigates the influence of different carrier gases (O2, N2, and air) on the growth of gallium oxide (Ga2O3) thin films on c-plane sapphire substrates by using the mist-CVD method. Although XRD and Raman measurements show that the pure corundum-structured α-Ga2O3 with single (0006) plane orientation was successfully obtained for all three different carrier gases, the crystal quality could be greatly affected by the carrier gas. When O2 is used as the carrier gas, the smallest full-width at half maximum (FWHM), the very sharp absorption cutoff edge, the perfect lattice structure, the highest growth rate, and the smooth surface can be obtained for the epitaxial α-Ga2O3 film as demonstrated by XRD, UV-VIS, TEM, AFM (Atomic Force Microscope), and SEM measurements. It is proposed that the oxygen content in carrier gas should be responsible for all of these results. XPS (X-ray photoelectron spectroscopy) analysis also confirms that more oxygen elements can be included in epitaxial film when O2 is used as the carrier gas and thus help improve the crystal quality. The proper carrier gas is essential for the high quality α-Ga2O3 growth.

On the origin of dislocation generation and annihilation in α -Ga2O3 epilayers on sapphire

Epitaxial film quality is critical to the success of high-performance α-Ga2O3 vertical power devices. In this work, the origins of threading dislocation generation and annihilation in thick α-Ga2O3 films heteroepitaxially grown on sapphire by the mist-CVD technique have been examined by means of high-resolution X-ray diffraction and transmission electron microscopies. By increasing the nominal thickness, screw dislocations exhibit an independent characteristic with a low density of about 1.8 × 106 cm−2, while edge dislocations propagating along the c-axis are dominant, which decrease down to 2.1 × 109 cm−2 in density for an 8 μm-thick α-Ga2O3 layer and exhibit an inverse dependence on the thickness. In the framework of the glide analytical model, parallel edge dislocations are generated at the interface due to the misfit-induced strain relaxation, while the dislocation glide and coalescence result in the annihilation and fusion behaviors. The optimal thick α-Ga2O3 with low dislocation densities may provide a prospective alternative to fully realize α-Ga2O3 power devices.

Growth and Photocatalytic Properties of Gallium Oxide Films Using Chemical Bath Deposition

Gallium oxide (Ga2O3) thin films were fabricated on glass substrates using a combination of chemical bath deposition and post-annealing process. From the field-emission scanning electron microscopy and x-ray diffraction results, the GaOOH nanorods precursors with better crystallinity can be achieved under higher concentrations (≥0.05 M) of gallium nitrate (Ga(NO3)3). It was found that the GaOOH synthesized from lower Ga(NO3)3 concentration did not transform into α-Ga2O3 among the annealing temperatures used (400–600 °C). Under higher Ga(NO3)3 concentrations (≥0.05 M) with higher annealing temperatures (≥500 °C), the GaOOH can be transformed into the Ga2O3 film successfully. An α-Ga2O3 sample synthesized in a mixed solution of 0.075 M Ga(NO3)3 and 0.5 M hexamethylenetetramine exhibited optimum crystallinity after annealing at 500 °C, where the α-Ga2O3 nanostructure film showed the highest aspect ratio of 5.23. As a result, the photodegeneration efficiencies of the α-Ga2O3 film for the methylene blue aqueous solution can reach 90%.

Atomic layer deposited α-Ga2O3 solar-blind photodetectors

Low temperature atomic layer deposition was used to deposit α-Ga2O3 films, which were subsequently annealed at various temperatures and atmospheres. The α-Ga2O3 phase is stable up to 400 °C, which is also the temperature that yields the most intense and sharpest reflection by x-ray diffraction. Upon annealing at 450 °C and above, the material gradually turns into the more thermodynamically stable ε or β phase. The suitability of the materials for solar-blind photodetector applications has been demonstrated with the best responsivity achieved being 1.2 A W−1 under 240 nm illumination and 10 V bias, for the sample annealed at 400 °C in argon. It is worth noting however that the device performance strongly depends on the annealing conditions, with the device annealed in forming gas behaving poorly. Given that the tested devices have similar microstructure, the discrepancies in device performance are attributed to hydrogen impurities.

Heteroepitaxial growth of thick α-Ga2O3 film on sapphire (0001) by MIST-CVD technique

The 8 μm thick single-crystalline α-Ga2O3 epilayers have been heteroepitaxially grown on sapphire (0001) substrates via mist chemical vapor deposition technique. High resolution X-ray diffraction measurements show that the full-widths-at-half-maximum (FWHM) of rocking curves for the (0006) and (10-14) planes are 0.024° and 0.24°, and the corresponding densities of screw and edge dislocations are 2.24 × 106 and 1.63 × 109 cm−2, respectively, indicative of high single crystallinity. The out-of-plane and in-plane epitaxial relationships are [0001] α-Ga2O3//[0001] α-Al2O3 and [11-20] α-Ga2O3//[11-20] α-Al2O3, respectively. The lateral domain size is in micron scale and the indirect bandgap is determined as 5.03 eV by transmittance spectra. Raman measurement indicates that the lattice-mismatch induced compressive residual strain cannot be ruled out despite the large thickness of the α-Ga2O3 epilayer. The achieved high quality α-Ga2O3 may provide an alternative material platform for developing high performance power devices and solar-blind photodetectors.

Halide vapor phase epitaxial growth of β-Ga2O3 and α-Ga2O3 films

Halide vapor phase epitaxy was used to grow homoepitaxial films of β-Ga2O3 on bulk (010) crystals and heteroepitaxial films of α-Ga2O3 on c-plane sapphire substrates. The β-Ga2O3 substrates were prepared prior to growth to remove sub-surface damage and to apply various miscuts to their surfaces. Structural and electrical properties were found to be most impacted by the crystallinity of the β-Ga2O3 substrate itself, while the surface morphology was found to be most impacted by the miscut of the substrate. The appropriate choice of growth conditions and the miscut appear to be critical to realizing smooth, thick (>20 µm) homoepitaxial films of β-Ga2O3. The α-Ga2O3 films were grown on commercially available c-plane sapphire substrates, and the film morphology was found to be strongly impacted by the surface finish of the sapphire substrates. The α-Ga2O3 films were found to be smooth and free of additional phases or crystal twinning when the sapphire was sufficiently polished prior to growth.

Electrical properties, structural properties, and deep trap spectra of thin α-Ga2O3 films grown by halide vapor phase epitaxy on basal plane sapphire substrates

Undoped epitaxial films of α-Ga2O3 were grown on basal plane sapphire substrates by halide vapor phase epitaxy (HVPE) in three different modes: standard HVPE, HVPE with constant flow of Ga and pulsed supply of O2 (O2-control growth regime), and with constant flow of O2 and pulsed delivery of Ga (Ga-control growth fashion). The best crystalline quality as judged by x-ray symmetric and asymmetric reflection half-widths and by atomic force microscopy morphology profiling was obtained with the O2-control deposition, and these results appear to be the best so far reported for α-Ga2O3 films. All grown α-Ga2O3 epilayers were high-resistivity n-type, with the Fermi level pinned near Ec − 1 eV deep traps. Photoinduced current transient spectra also showed the existence in standard HVPE samples and samples grown under the O2-control pulsed growth conditions of deep hole traps with levels near Ev + 1.4 eV whose density was suppressed in the Ga-control pulsed HVPE samples. The levels of the dominant deep traps in these α-Ga2O3 samples are close to the position of dominant electron and hole traps in well documented β-Ga2O3 crystals and films.

Sub-microsecond response time deep-ultraviolet photodetectors using α-Ga2O3 thin films grown via low-temperature atomic layer deposition

A photodetector responding to deep-ultraviolet (DUV) wavelengths (λ = 200–300 nm) is developed via the growth of crystalline α-Ga2O3 thin films with a bandgap of 5.2 eV on sapphire (0006) substrates using atomic layer deposition (ALD), at a low temperature of approximately 250 °C. The achievement of crystalline α-Ga2O3 films at such a low temperature is viable owing to the lattice match between α-Ga2O3 and sapphire crystals, which is confirmed by X-ray diffraction measurements and high-resolution transmission electron microscopy analysis. Metal–semiconductor–metal photodetectors (active area of 30 × 30 μm2) using 10-nm-thick α-Ga2O3 films exhibit a rise time (time required for the photocurrent to increase from 10% to 90% of its final value under illumination) of 539 ns at λ = 266 nm. Such an ultrafast response to DUV with λ = 266 nm is maintained for 3-nm-thick α-Ga2O3 photodetectors, suggesting that our ALD process is adequate for obtaining high-quality ultrathin α-Ga2O3 films. Measurements of the wavelength-resolved photocurrents reveal that the α-Ga2O3 photodetectors respond selectively to DUV wavelengths (λ = 200–300 nm), without responding to other longer wavelengths (λ > 300 nm). The responsivity is maximized to 0.76 A/W at λ = 253 nm, and drops off at λ ≈ 300 nm (i.e., a cutoff wavelength). The dark current measured at 10 V is as low as 0.5 pA, and the signal-to-noise ratio reaches 104, both of which underpin the pristine material quality of the ALD-grown α-Ga2O3 films. We believe that the fabrication of photodetectors using α-Ga2O3 thin films at such a low temperature will provide an economically feasible solution for high-performance DUV detection and sensing applications.

α-Ga2O3 grown by low temperature atomic layer deposition on sapphire

α-Ga2O3 is a metastable phase of Ga2O3 of interest for wide bandgap engineering since it is isostructural with α-In2O3 and α-Al2O3. α-Ga2O3 is generally synthesised under high pressure (several GPa) or relatively high temperature (∼500 °C). In this study, we report the growth of α-Ga2O3 by low temperature atomic layer deposition (ALD) on sapphire substrate. The film was grown at a rate of 0.48 Å/cycle, and predominantly consists of α-Ga2O3 in the form of (0001)-oriented columns originating from the interface with the substrate. Some inclusions were also present, typically at the tips of the α phase columns and most likely comprising ε-Ga2O3. The remainder of the Ga2O3 film – i.e. nearer the surface and between the α-Ga2O3 columns, was amorphous. The film was found to be highly resistive, as is expected for undoped material. This study demonstrates that α-Ga2O3 films can be grown by low temperature ALD and suggests the possibility of a new range of ultraviolet optoelectronic and power devices grown by ALD. The study also shows that scanning electron diffraction is a powerful technique to identify the different polymorphs of Ga2O3 present in multiphase samples.

Electrical characterization of Si-doped n-type α-Ga2O3 on sapphire substrates

ABSTRACTIssues on tin (Sn) doping in the mist chemical vapor deposition of α-Ga2O3 was attributed to conversion of ionization states of Sn from Sn4+ to Sn2+ and/or out-diffusion of Sn after thermal annealing, resulting in highly resistive films. Silicon (Si) doping, instead, was succeeded by the use of a novel doping source of chloro-(3-cyanopropyl)-dimethylsilane [ClSi(CH3)2((CH2)2CN)]. Si was uniformly incorporated in the α-Ga2O3 films and the electrical properties were stable even after the thermal annealing. The activation ratio as donors was nearly 100%. The carrier (electron) concentration was controlled to the level of 1018 cm-3 by the [Si]/[Ga] ratio in the source solution. The maximum mobility was 31.5 cm2/V⋅s, in contrast to 24 cm2/V⋅s with Sn doping, suggesting advantage of Si doping . However, the reduction in dislocation defects in the α-Ga2O3 films formed owing to heteroepitaxy on sapphire was suggested to be the important subject to be considered in the next stage.

Conductivity control of Sn-doped α-Ga2O3 thin films grown on sapphire substrates

We achieved the successful fabrication of Sn-doped α-Ga2O3 thin films with higher electron mobility and wider conductivity controls by improving the crystal quality. α-Ga2O3 films showed n-type conductivity with a maximum electron mobility of 24 cm2 V−1 s−1. The carrier concentration was successfully controlled in the range of 1017–1019 cm−3. Crystal defects such as dislocations severely compensate the free carriers in α-Ga2O3 films and restrict the mobility at low carrier concentrations. Therefore, to achieve further conductivity control and higher mobility, improving the crystallinity of α-Ga2O3 films is necessary.

Crack-free thick (∼5 µm) α-Ga2O3 films on sapphire substrates with α-(Al,Ga)2O3 buffer layers

To obtain crack-free thick α-Ga2O3 films on sapphire substrates, effects and behaviors of buffer layers have been investigated. With the growth of an α-Ga2O3 layer, there appeared an unintentionally formed layer in the sample, which was associated with stress accumulation and could be the seed for crack generation. We obtained a thick (∼5 µm) α-Ga2O3 layer on a sapphire substrate with the insertion of α-(Al0.12Ga0.88)2O3/α-(Al0.02Ga0.98)2O3 buffer layers, and for this sample, we did not observe the intermediate layer, suggesting that the buffer layers were effective for eliminating the stress accumulation at the α-Ga2O3/sapphire interface region.

Surface termination structure of α-Ga2O3 film grown by mist chemical vapor deposition

The surface structure of α-Ga2O3(0001) grown on an α-Al2O3(0001) substrate by mist chemical vapor deposition was studied by coaxial impact-collision ion scattering spectroscopy (CAICISS) and atomic force microscopy (AFM). The minimum step height observed in the AFM image was 0.21 ± 0.01 nm, coinciding with the height of three atomic layers of α-Ga2O3(0001). It was revealed by CAICISS analysis that the surface of α-Ga2O3(0001) is terminated by a Ga layer followed by an O layer, which is consistent with the surface termination of α-Al2O3(0001). A structural model taking surface relaxation into account was also constructed by fitting the simulated curve for the azimuth angle dependence of the Ga intensity to the experimental dependence. The resultant structural model is similar to the model of an α-Al2O3(0001) surface, which indicates analogous behavior in corundum crystals.

Structural and Optical Properties of α-Ga2O3 Films Deposited on Sapphire (10‾10) and (01‾12) Substrate by MOCVD

Ga2O3 thin films were grown on sapphire m-cut () and r-cut () orientations substrates at different temperatures by metal-organic chemical vapor deposition. Structural and optical properties of the Ga2O3 films were investigated including the influence by annealing for the obtained films. The Ga2O3 films on sapphire () and () substrate are α-Ga2O3. The crystallization of the films decreases after annealed at 900 °C. The average transmittance of the samples in the visible wavelength range was over 86% and the optical band gap Eg was about 4.755.15 eV. The Eg of the samples increases after annealing at 900 °C.

Evaluation of Misfit Relaxation in α-Ga2O3 Epitaxial Growth on α-Al2O3 Substrate

Corundum-structured α-Ga2O3 epitaxial thin films were grown on c-plane α-Al2O3 (sapphire) substrates by a mist chemical vapor deposition method. To reveal the defect structures, the α-Ga2O3 film was observed by high-resolution transmission electron microscopy (TEM). We found that the α-Ga2O3 thin film was in-plane compressive stressed from the α-Al2O3 substrate. Although misfit dislocations were periodically generated at the α-Ga2O3/α-Al2O3 interface owing to the large lattice mismatches between α-Ga2O3 and α-Al2O3, 3.54% (c-axis) and 4.81% (a-axis), most of the misfit dislocations did not thread through the layer. An extra-half plane was {2̄110} consisting only of Ga. Screw dislocations were not confirmed, i.e., the density was under 107 cm-2. The threading dislocation density was 7 ×1010 cm-2.

Heteroepitaxy of Corundum-Structured α-Ga2O3 Thin Films on α-Al2O3 Substrates by Ultrasonic Mist Chemical Vapor Deposition

Ga2O3 thin films of the α-phase, that is, the corundum structure (in the trigonal system), have been epitaxially obtained on sapphire (α-Al2O3) substrates, in contrast to the strong tendency of Ga2O3 to assume a heterogeneous crystal structure, that is, the β-gallia structure (in the monoclinic system) on sapphire. This result is advantageous for high-quality films and is due to the growth by mist chemical vapor deposition (CVD) at low temperatures of 430–470 °C. The α-Ga2O3 films have narrow full-widths at half maximum (FWHMs) in their X-ray diffraction rocking curves, for example, about 60 arcsec. The root mean square (RMS) roughness of the surface was as small as 1 nm. The optical band gap energy obtained was 5.3 eV, and the films were almost completely transparent in the near-ultraviolet and visible regions. The epitaxial growth of α-Ga2O3 films on sapphire is beneficial for the fabrication of oxide optical and electronic devices.