β-Ga2O3 Materials Research Articles

Study on the effects of VO and Hi on the absorption spectrum and conductivity of Ti doped β-Ga2O3

β-Ga2O3 has many excellent properties, but poor electrical conductivity limits its application in optoelectronic devices. At present, some progress has been made in the study of Ti doping on the photoelectric properties of β-Ga2O3. However, few studies on the effects of Ti doping and VO-Hi co-existence on photoelectric properties of β-Ga2O3. Defects (VO) and unintentional impurities (Hi) will inevitably exist during the preparation of β-Ga2O3. Given these problems, the effects of Ti doping and VO-Hi co-existence on photoelectric properties of β-Ga2O3 were studied using first-principles GGA+U method. The electronic structure, dielectric function, absorption spectrum, effective mass, mobility, and conductivity of the system were calculated and analyzed. The results indicate that with the increase of Ti doping concentration, the bandgap of the Ti doped β-Ga2O3 system gradually narrows. Ti doping causes a red shift in the peak of the absorption spectrum and effectively improves the conductivity of the system. The O vacancy reduces the conductivity of the system, while the interstitial H increases the conductivity of the system. Thus, Ti-doped β-Ga2O3 material is helpful for preparing new optoelectronic devices.

Current transport mechanism of lateral Schottky barrier diodes on β-Ga2O3/SiC structure with atomic level interface

Heterogeneous integration of β-Ga2O3 on highly thermal conductive SiC substrate by the ion-cutting technique is an effective solution to break the heat-dissipation bottleneck of β-Ga2O3 power electronics. In order to acquire high-quality β-Ga2O3 materials on SiC substrates, it is essential to understand the influence of the ion-cutting process on the current transport in β-Ga2O3 devices and to further optimize the electrical characteristics of the exfoliated β-Ga2O3 materials. In this work, the high quality of β-Ga2O3/SiC structure was constructed by the ion-cutting process, in which an amorphous layer of only 1.2 nm was formed between β-Ga2O3 and SiC. The current transport characteristics of Au/Pt/Ni/β-Ga2O3 Schottky barrier diodes (SBDs) on SiC were systematically investigated. β-Ga2O3 SBDs with a high rectification ratio of 108 were realized on a heterogeneous β-Ga2O3 on-SiC (GaOSiC) substrate. The net carrier concentration of the β-Ga2O3 thin film for GaOSiC substrate was down to about 8% leading to a significantly higher resistivity, compared to the β-Ga2O3 donor wafer, which is attributed to the increase in acceptor-type implantation defects during the ion-cutting process. Furthermore, temperature-dependent current–voltage characteristics suggested that the reverse leakage current was limited by the thermionic emission at a low electric field, while at a high electric field, it was dominated by the Poole–Frenkel emission from E3 deep donors caused by the implantation-induced GaO antisite defects. These results would advance the development of β-Ga2O3 power devices on high thermal conductivity substrate fabricated by ion-cutting technique.

Radiation effects of high-fluence reactor neutron on Ni/ β -Ga2O3 Schottky barrier diodes

This study investigates the broad-energy-spectrum reactor-neutron irradiation effects on the electrical characteristics of Ni/β-Ga2O3 Schottky barrier diodes (SBDs), where the irradiated neutron fluence was up to 1 × 1016 cm−2. On the one hand, the high neutron fluence of 1016 cm−2 resulted in a reduction in forward current density by two orders of magnitude and an extremely high on-resistance property due to the radiation-generated considerable series resistance in the SBD. On the other hand, the irradiation brought little influence on the Ni/β-Ga2O3 Schottky contact, since the extracted ideality factor and barrier height from temperature-dependent current–voltage (I–V–T) characteristics showed no significant changes after the radiation. Moreover, the capacitance–voltage (C–V) characterization revealed that the net carrier density in the β-Ga2O3 material was only reduced by 25% at the neutron fluence of 1015 cm−2 but a significant reduction by 2–3 orders at 1016 cm−2. Within the neutron fluence range of 2 × 1014 cm−2 up to 1016 cm−2, the carrier removal rates trended to be saturated with the increased fluences, following an exponential regular. In addition, the C–V measurement on the 1016 cm−2 irradiated sample exhibited an obvious frequency dispersion, and the extracted carrier distribution was not uniform.

Metal-assisted chemical etching beyond Si: applications to III-V compounds and wide-bandgap semiconductors.

Metal-assisted chemical etching (MacEtch) has emerged as a versatile technique for fabricating a variety of semiconductor nanostructures. Since early investigations in 2000, research in this field has provided a deeper understanding of the underlying mechanisms of catalytic etching processes and enabled high control over etching conditions for diverse applications. In this Review, we present an overview of recent developments in the application of MacEtch to nanomanufacturing and processing of III-V based semiconductor materials and other materials beyond Si. We highlight the key findings and developments in MacEtch as applied to GaAs, GaN, InP, GaP, InGaAs, AlGaAs, InGaN, InGaP, SiC, β-Ga2O3, and Ge material systems. We further review a series of active and passive devices enabled by MacEtch, including light-emitting diodes (LEDs), field-effect transistors (FETs), optical gratings, sensors, capacitors, photodiodes, and solar cells. By reviewing demonstrated control of morphology, optimization of etch conditions, and catalyst-material combinations, we aim to distill the current understanding of beyond-Si MacEtch mechanisms and to provide a bank of reference recipes to stimulate progress in the field.

Photoresponse of large-area atomically thin β-Ga2O3 and GaN materials via liquid metal print synthesized

This study uses liquid gallium metal to synthesize two-dimensional gallium oxide and gallium nitride ultra-thin layers. Addressing the challenges of conventional methods, this approach combines liquid metal properties with Polydimethylsiloxane (PDMS) transfer printing, resulting in large-area, highly clean, and low-residue amorphous Ga-based wide bandgap semiconductors. Transformation of amorphous Ga2O3 into crystallized β-Ga2O3 and GaN is achieved through annealing and ammoniation. Comprehensive characterizations confirm successful changes. Photodetector devices are fabricated and characterized, revealing Ga2O3 photosensitive nature with distinct performance metrics. GaN, while exhibiting lower photoresponse, displays rapid photocurrent recovery, making it promising for photodetector applications. This research offers a scalable method for two-dimensional gallium-group semiconductor synthesis, holding potential for diverse optoelectronic applications.

Role of Native Defects in Fe-Doped β-Ga2O3.

Iron impurities are believed to act as deep acceptors that can compensate for the n-type conductivity in as-grown Ga2O3, but several scientific issues, such as the site occupation of the Fe heteroatom and the complexes of Fe-doped β-Ga2O3 with native defects, are still lacking. In this paper, based on first-principle density functional theory calculations with the generalized gradient approximation approach, the controversy regarding the preferential Fe incorporation on the Ga site in the β-Ga2O3 crystal has been addressed, and our result demonstrates that Fe dopant is energetically favored on the octahedrally coordinated Ga site. The structural stabilities are confirmed by the formation energy calculations, the phonon dispersion relationships, and the strain-dependent analyses. The thermodynamic transition level Fe3+/Fe2+ is located at 0.52 eV below the conduction band minimum, which is consistent with Ingebrigtsen's theoretical conclusion, but slightly smaller than some experimental values between 0.78 eV and 1.2 eV. In order to provide direct guidance for material synthesis and property design in Fe-doped β-Ga2O3, the defect formation energies, charge transitional levels, and optical properties of the defective complexes with different kinds of native defects are investigated. Our results show that VGa and Oi can be easily formed for the Fe-doped β-Ga2O3 crystals under O-rich conditions, where the +3 charge state FeGaGai and -2 charge state FeGaOi are energetically favorable when the Fermi level approaches the valence and conduction band edges, respectively. Optical absorption shows that the complexes of FeGaGai and FeGaVGa can significantly enhance the optical absorption in the visible-infrared region, while the energy-loss function in the β-Ga2O3 material is almost negligible after the extra introduction of various intrinsic defects.

Investigation of a minority carrier trap in a NiO/β-Ga2O3 p–n heterojunction via deep-level transient spectroscopy

The properties of a minority carrier (hole) trap in β-Ga2O3 have been explicitly investigated using a NiO/β-Ga2O3 p–n heterojunction. Via deep-level transient spectroscopy, the activation energy for emission (E emi) and the hole capture cross section (σp ) were derived to be 0.10 eV and 2.48 × 10−15 cm2, respectively. Temperature-enhanced capture and emission kinetics were revealed by the decrease in the capture time constant (τc ) and emission time constant (τe ). Moreover, it was determined that the emission process of the minority carrier trap is independent of the electric field. Taking carrier recombination into account, a corrected trap concentration (N Ta) of 2.73 × 1015 cm−3 was extracted, together with an electron capture cross section (σn ) of 1.42 × 10−18 cm2. This study provides a foundation for the comprehension of trap properties in β-Ga2O3, which is crucial for overcoming self-trapped hole effects when obtaining p-type β-Ga2O3 materials and performance enhancement of β-Ga2O3-based power devices.

Low frequency noise in β -Ga2O3 based nanoelectronic devices

In this work, low frequency noise in β-Ga2O3 nanowire-based (NW) electronic devices is analyzed, which exhibits different behaviors as the device size scales down. The noise spectrum for the narrower NW (∼80 nm) is closer to 1/f characteristics, whereas it starts to show evident 1/f2 components as the NW size gets thicker (∼200 nm), giving clear signs of distinctive features for the bunch of traps at the NW interface or in the bulk. Our results show that 1/f noise in these NW electronic devices seems predominantly originated from an aggregated effect of the intricate trap states close to the β-Ga2O3 NW surface or interface with a wide range distribution, while finite groups of active deep traps play a critical role in contributing 1/f2components via generation-recombination or random telegraph signal processes. Notably, as the bias voltage increases, the 1/f2 components in the noise spectra get more overwhelming and would shift toward lower frequencies, suggesting that electric ionization effects would screen the shallow traps close to the surface or interface based on the Poole–Frenkel model. The Hooge's constants extracted from the 1/f noise component for these β-Ga2O3 NW-based devices fall in the range of 0.008–0.019, which are comparable to those of the best reported devices based on other wide bandgap semiconductor with nanoscale structures, including GaN, ZnO, and SnO2. This work may give hints of revealing the sophisticated dynamic behaviors of traps in the surface/volume β-Ga2O3 materials and electronic devices in the nanoscale by low frequency noises.

Assessment of bonding characteristic of β-(AlxGa1−x)2O3 alloys from photoluminescence and x-ray absorption near edge spectroscopy

Bonding characteristics of the β-(AlxGa1−x)2O3 alloy have been assessed from red luminescence and x-ray absorption near edge spectroscopy (XANES). Red luminescence, known as R1 and R2 lines, originates from crystal field split d-levels of unintentionally present Cr3+ ions in the β-(AlxGa1−x)2O3 alloy, is found to blueshift with Al composition. This indicates that the nephelauxetic effect in β-Ga2O3 materials decreases with Al substitution, which results to a decrease in the covalency of the β-(AlxGa1−x)2O3 alloy. The XANES experiments at the oxygen K-absorption edge provide clear evidence of the decrease in the covalency of the alloy, where an absorption peak representing the covalency in the β-Ga2O3 material is found to decrease with an increase in the Al composition. Furthermore, the nature of the observed red luminescence for the β-(AlxGa1−x)2O3 alloy is determined to be excitonic recombination. Our results further confirm the fact that the red luminescence from Cr3+ ions is an excellent local probe to assess the bonding characteristic of the host material.

A solar-blind photodetector with ultrahigh rectification ratio and photoresponsivity based on the MoTe2/Ta:β-Ga2O3 pn junction

High-performance solar-blind photodetectors have attracted increasing attention due to their important implications in military, medical, and industrial applications. Among a large number of wide-bandgap semiconductors, the quasi-two-dimensional β-Ga2O3 material is considered as a novel candidate for the solar-blind region (200–280 nm) owing to its suitable bandgap and superior optoelectronic properties. Herein, we report an outstanding solar-blind photodetector based on the MoTe2/Ta:β-Ga2O3 pn junction, which exhibits excellent optoelectrical properties with a superhigh rectification ratio ∼1.0 × 107, an ultrahigh responsivity (R ∼358.9 A/W), a large detectivity (D* ∼3.1 × 1012 Jones), an excellent external quantum efficiency (EQE ∼1.76 × 105%), a fast photoresponse (21.1/84.5 ms), and a high rejection ratio (∼1.1 × 103) under forward bias (1.5 V) and deep-ultraviolet (UV) illumination. These superior photoresponses result from the rapid separation of photoexcited electron-hole pairs driven by the synergistic effect of the forward bias and built-in electric field of the pn junction. Moreover, the MoTe2/Ta:β-Ga2O3 pn junction photodetector also demonstrates competitive optoelectronic characteristics in self-powered mode with an ultralow dark current of 8.5 fA and a fairly high R of 9.2 mA/W. As a result, this work provides an alternative solution for designing and fabricating high-performance solar-blind photodetectors with potential applications in UV imaging, environmental monitoring, and insecure communication.

Electrical performance study of Schottky barrier diodes using ion implanted β-Ga2O3 epilayers grown on sapphire substrates

β-Ga2O3 epilayers with a 210 nm thickness were successfully grown on c-plane sapphire substrate by metalorganic chemical vapor deposition (MOCVD). Highly resistive unintentionally doped (UID) epilayers are studied, as are Si ion-implantated samples with doses of 1✕1014, 6✕1014 and 1✕1015 cm−2 at 30 keV to improve β-Ga2O3 epilayer conductivity. Schottky barrier diodes (SBDs) were fabricated using optimized growth parameters and electrical measurements performed for both UID and Si-implanted β-Ga2O3 material. The forward current density (J) of an UID SBD at 2 V was 62 nA/cm2 with breakdown voltage (Vbr) of 1.03 kV and leakage J of 10 μA/cm2. The forward J improved by eight orders of magnitude after Si implantation. In this case, Vbr reduced to 680 V combined with a high leakage J of ∼A/cm2 for doses of 1✕1015cm−2. A hard breakdown of 553 V was achieved with a Si-ion dose of 1✕1014 cm−2 with comparatively less leakage than samples with higher dose amounts.

Effect of H, He and Ar irradiation on β-Ga2O3 material

In this work, the projected range, the nuclear stopping power(Sn), electronic stopping power (Se), vacancies, replacement collisions, and total target damage of β-Ga2O3 by H, He and Ar ion bombardment are investigated by the Monte Carlo SRIM software. The ions were vertically incident on a 1-um β-Ga2O3 material, and their energies varied from 100 keV to 1 MeV. The results showed that the projected range increased for all three ions with increasing incident energies, with H ions having the largest projected range. The Sn for Se with incident energy of 0-1MeV in β-Ga2O3 is much greater than the Sn for H and He, while the Sn for Ar decreases gradually with the increase of incident ion energy, and the Se shows a gradual increasing trend. Compared with H and He, the number of vacancies induced by Ar were the largest at the same incident energy and thickness of β-Ga2O3. In addition, it is obvious that the total target damage of Ar on β-Ga2O3 material were much more serious than H and He.

Giant Reduction in the Thermal Conductivity of Porous β-Ga2O3 with Multiple Heat-Blocking Mechanisms

In this work, multiple strategies have been adopted to suppress the lattice thermal conductivity in β-Ga2O3 by point defects, grain boundaries, secondary phases, and pores. Porous β-Ga2O3 was prepared by a convenient calcination method together with spark plasma sintering technology. Positron annihilation lifetime and cathodoluminescence measurements reveal the existence of small micropores and various vacancies in the as-sintered β-Ga2O3. Macropores with sizes on the order of hundreds of nanometers are also observed by scanning electron microscopy. Due to these pores, the as-sintered β-Ga2O3 shows a high porosity of 63.4% and possesses abundant boundaries between the solid matrix and the air. In addition, the secondary phase of ε-Ga2O3 is also revealed by X-ray diffraction. The above microstructures cause strong enhancement in phonon scattering. As a result, an extremely low thermal conductivity of 0.30 W m–1 K–1 is obtained at room temperature in the as-sintered β-Ga2O3, which is far below the amorphous limit (1.15 W m–1 K–1) and is the lowest value reported so far for β-Ga2O3. Our work provides an avenue to reduce the thermal conductivity for the β-Ga2O3 materials and is believed to be more widely applicable for many other materials.

Review of Radiation-Induced Effects on β-Ga2O3 Materials and Devices

β-Ga2O3 has become an ultimate choice of emerging new-generation material for its wide range of compelling applications in power electronics. In this review, we have explored the available radiations in the atmosphere and the effects of radiation on the β-Ga2O3 material and devices. The focus in this review summarizes various studies covering different radiation such as swift heavy ions, protons, neutrons, electrons, Gamma, and X-rays to understand the radiation-induced effects on the structure and their reliable performance in harsh environments. In addition, we focused on the various pre-existing defects in β-Ga2O3 and the emergence of radiation-induced defects that provoke a severe concern, especially from the device performance point of view. This review presents the irradiation-induced effects on the devices such as high-power devices such as Schottky barrier diodes (SBDs), field-effect transistors (FETs), metal-oxide-semiconductor (MOS) devices, and photodetectors. Some key studies including the changes in carrier concentration with a removal rate, Schottky barrier height (SBH), ideality factor, defect dynamics dielectric damage, interface charge trapping, a thermally activated recovery mechanism for charge carriers at elevated temperature, and diffusion length of minority charge carriers. These reports show that β-Ga2O3-based devices could be deployable for space or high-radiation terrestrial applications. These results provide/suggest a better device design based on the radiation degradation studies in the state-of-the-art β-Ga2O3 devices.

Patterning the surface structure of transparent hard-brittle material β-Ga2O3 by ultrashort pulse laser

Femtosecond laser direct writing technology offers a powerful and advantageous tool for precise three-dimensional (3D) micro/nanofabrication of transparent hard and brittle materials. In this paper, micro-craters, micro/nano-rectangular columns, and microchannels structures with different spatial characteristics are achieved after irradiation of crystalline (100) β-Ga2O3 via an optical-fiber femtosecond laser (515/1030 nm, 285 fs, 100 kHz) with different processing parameters in air ambient. Laser scanning confocal microscope, atomic force microscopy, and Raman spectroscopy are employed to characterize the roughness and constitutive property of the surface structure under tightly focused laser conditions. The influences of different pulse numbers and different pulse energies on the morphology of the pits were investigated. Through precise control and optimization of processing parameters, the formation of cracks and chipping can be effectively avoided. Crack-free ablation holes with smooth edges were fabricated under low-energy (E = 22 nJ) single-pulse conditions. Furthermore, the effects of different laser processing parameters (single-pulse scanning interval, pulse number and scanning repetition times) on the microchannel morphology were also explored. From this work, high-precision micro-hole arrays, micro-nano rectangular column arrays, and microchannels were successfully fabricated on (100) β-Ga2O3 substrates. Due to the interaction of single-pulse femtosecond laser with β-Ga2O3 materials, this fast fabrication method of 3D submicron structures has potential applications in sensing, photonics, or electronics.

Perspective on atomic scale investigation of point and extended defects in gallium oxide

Beta-gallium oxide (β-Ga2O3) has recently attracted significant attention as an outstanding candidate for ultra-wide bandgap applications due to its unique advantages. Point and extended defects in β-Ga2O3 can significantly reduce the net doping and play an essential role with their functionality in advancing β-Ga2O3 device performance. It is, therefore, critical to gain an atomic level understanding of the structure of the defects and how they correlate to important properties of defects in β-Ga2O3. In this Perspective, we provide an overview of the recent characterization works involving scanning transmission electron microscopy and related techniques revealing the detailed structure of various point and extended defects in β-Ga2O3 and β-(AlxGa1−x)2O3 heterostructures. This article aims to offer insight into how defects determine important aspects of the material, such as in crystal growth, dopant incorporation and activation, and phase stability. The new information that we summarize here is expected to help achieve atomic scale control of defects in β-Ga2O3 materials and devices for development of the next generation power electronics applications.

Elastic strain modulation of energy bandgap in β-Ga2O3 sheet: Experimental and computational investigations

Elastic strain engineering has proven to be an efficient tool to substantially modify the physical properties of semiconductors. Likewise, modification of properties by elastic strain based on 2D materials has drawn a considerable attention recently. While on ultrawide-bandgap (UWBG) materials, emergent alternatives for next-generation high-power and high-efficient electronic devices, there exist only a few experimental reports related to the strain modification of fundamental physical properties. According to the theoretical prediction, metallization and indirect-to-direct transition can be achieved in diamonds through elastic strain. In previous work, the modulation of bandgap and indirect-to-direct bandgap transition in 2D β-Ga2O3 under uniaxial strains were demonstrated. The present work reports a modified two-point bend method to apply the elastic strain to β-Ga2O3 material continuously without any damage in the crystal quality, and investigates the changes in bandgap caused by the strain-induced deformation. Free-standing β-Ga2O3 sheets exhibit a sizeable reduction in the bandgap of up to 30% (from 4.9 eV to 3.4 eV) under the bending state, as measured by X-ray photoelectron spectroscopy (XPS). Based on the predictions obtained from first-principles calculations, a strong anisotropic character in the Young's modulus of β-Ga2O3 indicates the existence of strain gradient in the free-standing β-Ga2O3 sheets under bending states, which was evinced by the confocal Raman spectroscopic measurements. Moreover, the lattice distortion caused by the strain gradient and elastic strain can be attributed to the dramatic changes in the bandgap. More importantly, in as-bent samples, the variation of bandgap was continuous and recoverable. These findings provide a direct experimental evidence of bandgap modification of β-Ga2O3 by elastic strain, and indicate a much larger and more versatile platform for the application of flexible Ga2O3 based devices to a wide range of application areas.

Thin-film temperature sensors based on LPD-fabricated β-Ga2O3 Schottky diodes

We report the synthesis of wide-bandgap β-Ga2O3 nanocrystalline thin films via the low-cost and non-vacuum-based liquid phase deposition (LPD) method. The morphological evolution of the nanocrystalline β-Ga2O3 grains was investigated as a function of the growth temperature, processing time, and pH value of the precursor solution. We successfully calcined gallium oxide hydroxide GaO(OH) through a 3-h annealing process at 800 °C to convert it into β-Ga2O3. We fabricated horizontal-structured Ni/β-Ga2O3 Schottky diodes and investigated the electrical characteristics pertinent to sensing temperature in the range of 100−800 K. The temperature sensitivity of the Ni/β-Ga2O3 Schottky-junction temperature sensors, defined as the temperature dependence of junction voltage at a fixed bias current of 10 µA, peaked at −2.924 mV/K in the range between 300 and 500 K. At room temperature, we measured a barrier height of 0.915 eV and a Richardson constant of 43.04 ± 0.01 A/cm2 K2 from the Ni/β-Ga2O3 Schottky junctions. These results indicate that the LPD-synthesized β-Ga2O3 material and devices hold promising potential for sensing applications especially at high temperatures.

Mis-cut direction of substrate effect on the photoresponse characteristic of β-Ga2O3 film

β-Ga2O3 epitaxial film was grown on 6° mis-cut angle (toward <112‾0> and <101‾0> directions) c-plane sapphire substrate by metalorganic chemical vapor deposition (MOCVD). The influence of mis-cut sapphire substrate on β-Ga2O3 film and the photoresponse properties of β-Ga2O3 photodetectors were studied. Compared with non-miscut substrates, both mis-cut sapphire substrates can obtain high crystal quality β-Ga2O3 films by reducing the in-plane rotational symmetry and promoting the step flow growth mode. Compared with the mis-cut toward <112‾0> direction, the substrate of mis-cut toward <101‾0> direction showed the highest crystal quality and largest growth rate due to smaller edge step terrace width and large step density. Three β-Ga2O3 photodetectors were fabricated on films grown on the non-miscut and the two mis-cut substrates. The photoresponse characteristics of β-Ga2O3 photodetectors based on sapphire substrate of mis-cut toward <101‾0> direction can reach 65 μA (photocurrent), 3.5 × 103 (Ip-Id/Id) and 3.21 A/W (responsivity, under 254 nm light). These results demonstrate an important guidance to develop the industrialization of β-Ga2O3 materials and devices.

Anisotropic complex refractive index of β-Ga2O3 bulk and epilayer evaluated by terahertz time-domain spectroscopy

Homoepitaxial film and semi-insulating bulk β-Ga2O3 with (001) orientation were studied using terahertz time-domain spectroscopy (THz-TDS) in the frequency region from 0.2 to 3.0 THz parallel to the [100] and [010] directions. The static permittivity of the bulk was determined to be 10.0 and 10.4 along the a-axis and b-axis, respectively, and the refractive index values at 0.2 THz are 3.17 and 3.23 for each axis. The electrical resistivity of the epilayer was extracted with good accuracy by employing the Drude–Lorentz model and without the use of electrical contacts. This noninvasive and contact-free material evaluation through THz-TDS proves to be a powerful tool for probing and obtaining various types of information about β-Ga2O3 materials such as bulk and thin films for the development of β-Ga2O3-based device applications.