Magnetron Sputter-Deposited β-Ga2O3 Films on c-Sapphire Substrate: Effect of Rapid Thermal Annealing Temperature on Crystalline Quality

Gallium oxide (Ga2O3) is a semiconductor with a wide bandgap of ~5.0 eV and large breakdown voltages (>8 MV·cm−1). Among the crystal phases of Ga2O3, the monoclinic β-Ga2O3 is well known to be suitable for many device applications because of its chemical and thermal stability. The crystalline quality of polycrystalline β-Ga2O3 films on c-plane sapphire substrates was studied by rapid thermal annealing (RTA) following magnetron sputtering deposition at room temperature. Polycrystalline β-Ga2O3 films are relatively simple to prepare; however, their crystalline quality needs enhancement. The β-phase was achieved at 900 °C with a crystallite size and d-spacing of 26.02 and 0.2350 nm, respectively, when a mixture of ε- and β-phases was observed at temperatures up to 800 °C. The strain was released in the annealed Ga2O3 films at 900 °C; however, the clear and uniform orientation was not perfect because of the increased oxygen vacancy in the film at that temperature. The improved polycrystalline β-Ga2O3 films with dominant (−402)-oriented crystals were obtained at 900 °C for 45 min under a N2 gas atmosphere.


Introduction
Gallium oxide (Ga 2 O 3 ) is a semiconductive material with a direct bandgap of 4.5-5.0 eV, remarkable thermal and chemical stability, high transparency in ultraviolet (UV) and visible (VIS) regions, and high dielectric constants in the range of 10.2-14.2 [1]. The monoclinic β-Ga 2 O 3 is more stable than the α-, γ-, δ-, and ε-phases with n-type conductivity owing to oxygen vacancy as a donor level [2]. The other four phases are metastable and can transform to β-Ga 2 O 3 at temperatures above 750 • C [1,3]. β-Ga 2 O 3 has attracted attention for future electronic applications, owing to its distinguished properties, such as an ultra-wide bandgap range of 4.7-4.9 eV, a critical electric field (E C ) strength of 8 MV/cm, an excellent electron mobility of 300 cm 2 /V·s, and an exceptional Baliga figure of merit (BFOM) of 3444 [1,4].
Polycrystalline β-Ga 2 O 3 films are relatively simple to prepare; however, their poor crystalline quality does not meet the requirements for certain electronic applications. Conversely, the preparation of single-crystal β-Ga 2 O 3 with high crystalline quality is complicated and costly. β-Ga 2 O 3 films with highly preferred orientation can offer suitable crystalline quality between single crystal and polycrystalline, showing a balance between physical properties and cost [5]. Many efforts have been made by relevant technologies to achieve high crystalline quality polycrystalline β-Ga 2 O 3 films [1,2]; however, suitable substrate is required to prevent a large lattice mismatch and coordination difference at the interface between the films and substrate [6,7].
For the heteroepitaxy of β-Ga 2 O 3 , dislocations are created inside the epitaxy layer to relax the residual stress, degrading the performance of the devices [11]. A buffer layer between the c-plane sapphire substrate and the epitaxy layer can be a solution to reduce the difference in the lattice constants to decrease the residual stress [12][13][14]. In α-Ga 2 O 3 epitaxial growth, the buffer layer using some metal alloys decreased threading dislocation density significantly and eliminated strain accumulation at the α-Ga 2 O 3 -sapphire interface [14][15][16]. Because β-Ga 2 O 3 for power device applications requires significantly fewer defects than for optical device applications, to produce a high-quality monoclinic-phase β-Ga 2 O 3 epitaxial layer for power device applications, high-quality polycrystalline β-Ga 2 O 3 films on a c-plane sapphire substrate are proposed as the buffer layer, which acts as a lattice template in this study. Because the quality of the β-Ga 2 O 3 homoepitaxial layer with fewer defects, such as threading dislocation and twin boundary, was determined according to the dislocation density of the substrate [17], it is necessary to focus on the crystalline quality of the β-Ga 2 O 3 buffer layer.
Radio frequency (RF) magnetron sputtering deposition was selected to prepare highquality polycrystalline β-Ga 2 O 3 films on a c-plane sapphire substrate for industrial mass production among the various methods. The RF magnetron sputtering method can produce high-quality films with low cost, high deposition rate, easy control of process parameters, suitable uniformity, high homogeneity, and significant adhesion over a comparatively large area. Polycrystalline β-Ga 2 O 3 films are universally obtained at elevated substrate temperatures during film deposition because β-Ga 2 O 3 crystallization can only be realized at relatively high temperatures [18]. Another method is to fabricate polycrystalline β-Ga 2 O 3 films through a two-step method by post-annealing the as-deposited Ga 2 O 3 films at room temperature. Here, the β-Ga 2 O 3 films were fabricated and characterized by post-annealing with a rapid thermal annealing (RTA) system at various temperatures after deposition onto c-plane sapphire substrates by RF magnetron sputtering at room temperature to enhance the crystalline quality. There have been several previous investigations on the effects of the post-annealing process of sputter-deposited Ga 2 O 3 films [2,19,20], but there have been few studies to apply these films as a lattice template on the c-sapphire substrate for application to power devices, except for optical devices. For the fabrication of the β-Ga 2 O 3 buffer layer at high temperatures, the residual stress of the β-Ga 2 O 3 films should be studied intensively as a function of the temperature.

Experimental Details
Ga 2 O 3 films were deposited on 1 × 1 cm 2 c-plane (0001) sapphire substrates using an RF magnetron sputtering system (IDT Engineering Co., Gyeonggi, Korea) at room temperature [21], with a Ga 2 O 3 (TASCO, Seoul, Korea, 99.999% purity, 5.08 cm diameter) target under a fixed set of process parameters: a pre-sputtering process for 5 min prior to each run, a frequency of 13.56 MHz, an RF sputtering of 100 W, an Ar gas flow rate of 50 sccm, a base pressure of 133.3224 × 10 −6 Pa, a substrate-to-target distance of 5.0 cm, and a vacuum pressure of 999.9178 × 10 −3 Pa during sputtering at room temperature. The deposition time was fixed at 34 min to obtain a constant thickness of approximately 200 nm ( Figure S1, in Supplementary Materials). After the sputtering deposition, the samples were subjected to RTA (GRT-100, GD-Tech Co., Gyeongsangbuk, Korea) from 500 to 900 • C for 45 min under a N 2 gas atmosphere [19,22].
The crystalline structure of the films was analyzed using X-ray diffraction (XRD, PANalytical B.V., Almelo, The Netherlands, X'pert-PRO-MRD, Cu Kα = 0.15405 nm, 40 kV, 30 mA) over a 2θ range of 10-80 • with a step size of 0.026 • and scanning speed of 8.5 • /min. Field emission scanning electron microscopy (FESEM, JEOL, Tokyo, Japan, JSM-7500F) was employed to reveal the morphological characteristics of the Ga 2 O 3 films. X-ray photoelec-Coatings 2022, 12, 140 3 of 12 tron spectroscopy (XPS, Thermo Fisher Scientific Inc., Waltham, MA, USA, K-Alpha + ) was used to analyze the composition and chemical nature of the Ga 2 O 3 films. Field emission transmission electron microscopy (FETEM, JEOL, Tokyo, Japan, JEM-2100F, a field emission gun source at 200 kV) and selected area electron diffraction (SAED) were performed to evaluate the quality of the crystal lattice.

Results and Discussion
The surface morphologies of the as-deposited and annealed Ga 2 O 3 films at different post-annealing temperatures were analyzed using FESEM. Figure 1 shows the top-view FESEM images of the Ga 2 O 3 films on the c-plane (0001) sapphire substrates. None of the asdeposited and annealed Ga 2 O 3 films had extended cracks after the ex situ annealing process using the RTA system [23]. The as-deposited film showed that the surface morphologies comprised fine grains tightly connected with a relatively clear boundary, as shown in Figure 1a. There was no significant difference between the annealed film at 500 • C and the as-deposited films. For the annealed film at 600 • C, the high annealing temperature provided the as-deposited grains with thermal energy to accumulate to form large grains with blurred boundaries. Distorted hexagonal islands occurred on the surfaces of the annealed film at 700 • C, as shown in Figure 1d, similar to the reported island nucleation of ε-Ga 2 O 3 in epitaxial growth on a c-plane sapphire substrate by metalorganic chemical vapor deposition at 650 • C [12,24,25]. It was considered that agglomeration, rather than the merging into large crystals, began to occur at 800 • C in a part of the film, and discontinuities and voids were observed [26]. Uniform, dense, compact, and well-defined grains with clear boundaries were observed in the annealed films at 900 • C; the grain size gradually increased with an increase in the annealing temperature from 500 to 900 • C.
Coatings 2022, 11, x FOR PEER REVIEW 3 of 12 8.5°/min. Field emission scanning electron microscopy (FESEM, JEOL, Tokyo, Japan, JSM-7500F) was employed to reveal the morphological characteristics of the Ga2O3 films. X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Inc., Waltham, MA, USA, K-Alpha + ) was used to analyze the composition and chemical nature of the Ga2O3 films. Field emission transmission electron microscopy (FETEM, JEOL, Tokyo, Japan, JEM-2100F, a field emission gun source at 200 kV) and selected area electron diffraction (SAED) were performed to evaluate the quality of the crystal lattice.

Results and Discussion
The surface morphologies of the as-deposited and annealed Ga2O3 films at different post-annealing temperatures were analyzed using FESEM. Figure 1 shows the top-view FESEM images of the Ga2O3 films on the c-plane (0001) sapphire substrates. None of the as-deposited and annealed Ga2O3 films had extended cracks after the ex situ annealing process using the RTA system [23]. The as-deposited film showed that the surface morphologies comprised fine grains tightly connected with a relatively clear boundary, as shown in Figure 1a. There was no significant difference between the annealed film at 500 °C and the as-deposited films. For the annealed film at 600 °C, the high annealing temperature provided the as-deposited grains with thermal energy to accumulate to form large grains with blurred boundaries. Distorted hexagonal islands occurred on the surfaces of the annealed film at 700 °C, as shown in Figure 1d, similar to the reported island nucleation of ε-Ga2O3 in epitaxial growth on a c-plane sapphire substrate by metalorganic chemical vapor deposition at 650 °C [12,24,25]. It was considered that agglomeration, rather than the merging into large crystals, began to occur at 800 °C in a part of the film, and discontinuities and voids were observed [26]. Uniform, dense, compact, and well-defined grains with clear boundaries were observed in the annealed films at 900 °C; the grain size gradually increased with an increase in the annealing temperature from 500 to 900 °C.  The as-deposited Ga 2 O 3 films exhibited amorphous or microcrystalline structures. A post-annealing process was performed by RTA to improve their crystallinity. Figure 2a shows the XRD patterns of the as-deposited and annealed Ga 2 O 3 films at various annealing temperatures in the 2θ range of 10-80 • . The XRD patterns of the annealed film at 500 • C show broad amorphous features and weak peaks along with (006) and (003) diffraction peaks from the sapphire substrate, which indicates that the β-Ga 2 O 3 crystallization was not easily achieved under the low annealing temperature of 500 • C. It was observed that three diffraction peaks along (-201), (-402), and (-603) at 2θ = 18.38 • , 38.21 • , and 58.84 • , respectively, corresponded to β-Ga 2 O 3 at the annealing temperature of 600-900 • C (ICDD/JCPDS PDF card No. . This reveals that the arrangement of oxygen atoms in the β-Ga 2 O 3 (-201) plane was equivalent to that in the c-plane sapphire [27,28]. In contrast, the annealed Ga 2 O 3 films at 600 and 700 • C comprised three diffraction peaks along (0002), (0004), and (0006) at 2θ = 19.09 • , 37.62 • , and 59.12 • , respectively, corresponding to ε-Ga 2 O 3 . The shoulder of ε-Ga 2 O 3 at the diffraction peak along (-402), corresponding to β-Ga 2 O 3 at 2θ = 38.12 • , disappeared in the annealed Ga 2 O 3 film at 800 • C, while a diffraction peak along (0002) remained at 2θ = 19.10 • , corresponding to ε-Ga 2 O 3 . The metastable ε-phase transformed into the thermodynamically stable β-phase within the range of 700-800 • C [3,12,29,30]. The intensities of the diffraction peaks along (-201), (-402), and (-603) increased with an increase in temperature, as shown in Figure 2a. The as-deposited Ga2O3 films exhibited amorphous or microcrystalline structures. A post-annealing process was performed by RTA to improve their crystallinity. Figure 2a shows the XRD patterns of the as-deposited and annealed Ga2O3 films at various annealing temperatures in the 2θ range of 10-80°. The XRD patterns of the annealed film at 500 °C show broad amorphous features and weak peaks along with (006) and (003) diffraction peaks from the sapphire substrate, which indicates that the β-Ga2O3 crystallization was not easily achieved under the low annealing temperature of 500 °C. It was observed that three diffraction peaks along (-201), (-402), and (-603) at 2θ = 18.38°, 38.21°, and 58.84°, respectively, corresponded to β-Ga2O3 at the annealing temperature of 600-900 °C (ICDD/JCPDS PDF card No. . This reveals that the arrangement of oxygen atoms in the β-Ga2O3 (-201) plane was equivalent to that in the c-plane sapphire [27,28]. In contrast, the annealed Ga2O3 films at 600 and 700 °C comprised three diffraction peaks along (0002), (0004), and (0006) at 2θ = 19.09°, 37.62°, and 59.12°, respectively, corresponding to ε-Ga2O3. The shoulder of ε-Ga2O3 at the diffraction peak along (-402), corresponding to β-Ga2O3 at 2θ = 38.12°, disappeared in the annealed Ga2O3 film at 800 °C, while a diffraction peak along (0002) remained at 2θ = 19.10°, corresponding to ε-Ga2O3. The metastable εphase transformed into the thermodynamically stable β-phase within the range of 700-800 °C [3,12,29,30]. The intensities of the diffraction peaks along (-201), (-402), and (-603) increased with an increase in temperature, as shown in Figure 2a. The full width at half maximum (FWHM) of the (-402) diffraction peak, as a function of the annealing temperature, is shown in Figure 2b. This FWHM decreased to 0.428° with an increase in temperature from 600 to 900 °C, which was attributed to the large driving energy to migrate atoms to suitable lattice sites to achieve high crystalline quality at 900 °C [32]. However, it was considered that certain stresses caused by internal/external factors affected the structure of the films at this temperature [33,34]. The crystallite size of the films was estimated from the (-402) diffraction peak using both the Debye-Scherrer for- The full width at half maximum (FWHM) of the (-402) diffraction peak, as a function of the annealing temperature, is shown in Figure 2b. This FWHM decreased to 0.428 • with an increase in temperature from 600 to 900 • C, which was attributed to the large driving energy to migrate atoms to suitable lattice sites to achieve high crystalline quality at 900 • C [32]. However, it was considered that certain stresses caused by internal/external factors affected the structure of the films at this temperature [33,34]. The crystallite size of the films was estimated from the (-402) diffraction peak using both the Debye-Scherrer formula D D-S = 0.94 λ/ω·cosθ and Williamson-Hall equation D W-H = 0.94 λ/(ω·cosθ−ε·sinθ) [35][36][37][38], where λ is the Kα radiation wavelength of Cu (λ = 0.15406 nm), ω is the FWHM of the (-402) diffraction peak, ε is the lattice strain, and θ is the Bragg angle corresponding to the (-402) diffraction peak. The crystallite size gradually increased from 15. increase, while the slopes of the increase in D W-H were relatively lower than those in D D-S at 700 and 900 • C. In the Williamson-Hall equation, the Bragg angle of the diffraction peak along (-402) shifted toward lower values from 2θ = 38.21 • to 37.98 • with an increase in temperature from 600 to 900 • C. This shift was revealed as one of the main reasons for the larger values of D W-H compared to those of D D-S , although the effect of strain remains to be further investigated. According to Bragg's law, the shift in the Bragg angle was affected by a change in the spacing of the crystallographic planes, where the tensile strain increases the d-spacing, causing a shift in the Bragg angle of the diffraction peak towards lower 2θ values, whereas compressive strain decreases the d-spacing, resulting the shift towards higher 2θ values in the XRD pattern [39]. Average grain size, which was estimated using SEM images and ImageJ software [40], is also shown in Figure 2c.
The diffraction data analysis with lattice constants (a, b, c, and β) was refined using X'Pert HighScore Plus software (Panalytical B.V., Almelo, The Netherlands) [41,42].  [44]. The wave function of the conduction band bottom generally comprises 4s orbitals of Ga 3+ ions in octahedral sites [29]. A compressive strain may lead to an increased octahedral occupancy by Ga 3+ ions, forming a compact structure in the unit cell, whereas a tensile strain in the film may lead to an influx of Ga 3+ ions in the tetrahedral sites, forming a relatively loose structure [29]. Figure 3f shows the strain (ε) due to crystal imperfections and distortions of the Ga 2 O 3 films that were annealed at 600, 700, 800, and 900 • C, as calculated using the equation ε = ω/4tanθ, where ω is the FWHM of the predominant diffraction peak and θ is the Bragg angle corresponding to the diffraction peak obtained from the XRD data [45][46][47]. The strains along the (-402) orientation decreased from a maximum of 8.562 × 10 −3 at 600 • C to a minimum of 6.187 × 10 −3 at 900 • C. With an increase in the annealing temperature from 600 to 900 • C and a relaxation in the compressive strain, an increasing number of Ga 3+ ions occupied the oxygen tetrahedral sites from the oxygen octahedral sites; therefore, the volume of the unit cell showed the slightly volumetric contraction at 700 • C although the compressive strain released rapidly, while volumetric expansion occurred for compact structures at 800-900 • C as the compressive strain released, as shown in Figure 3e,f. The volumetric expansion and transformation of the ε-phase into the thermodynamically stable β-phase occurred at 800 • C [24,25,29]. The proportion of octahedrally and tetrahedrally coordinated Ga is 1:1 in the monoclinic β-phase when the disordered Ga atoms occupy octahedral and tetrahedral sites to give the 2:3 stoichiometry in the ε-phase [48]. It is suspected that volume expansion was limited at 900 • C as a relatively increased octahedral occupancy by Ga 3+ ions due to the transformation from εto β-phase. The dislocation density (δ) of the Ga 2 O 3 films was calculated at the same annealing temperatures using the equation δ = 1/D 2 , where D is the crystallite size obtained from the XRD data. A similar trend in the dislocation density was obtained for the strain along the (-402) orientation (not shown). The lowest value of 2.3790 × 10 15 line/m 2 along the (-402) orientation was obtained at 900 • C when the dislocation density decreased from the highest value of 4.2730 × 10 15 line/m 2 at 600 • C. This indicates that the largest crystallite size (Figure 2b) was observed at 900 • C as a result of the released compressive strain and dislocation density.
volumetric expansion and transformation of the ε-phase into the thermodynamically stable β-phase occurred at 800 °C [24,25,29]. The proportion of octahedrally and tetrahedrally coordinated Ga is 1:1 in the monoclinic β-phase when the disordered Ga atoms occupy octahedral and tetrahedral sites to give the 2:3 stoichiometry in the ε-phase [48]. It is suspected that volume expansion was limited at 900 °C as a relatively increased octahedral occupancy by Ga 3+ ions due to the transformation from ε-to β-phase. The dislocation density (δ) of the Ga2O3 films was calculated at the same annealing temperatures using the equation δ = 1/D 2 , where D is the crystallite size obtained from the XRD data. A similar trend in the dislocation density was obtained for the strain along the (-402) orientation (not shown). The lowest value of 2.3790 × 10 15 line/m 2 along the (-402) orientation was obtained at 900 °C when the dislocation density decreased from the highest value of 4.2730 × 10 15 line/m 2 at 600 °C. This indicates that the largest crystallite size (Figure 2b) was observed at 900 °C as a result of the released compressive strain and dislocation density.  Figure 3f shows the interplanar distances corresponding to the d-spacings for the monoclinic Ga2O3 orientation along the (-402) plane with a change in temperature from 600 to 900 °C. The d-spacing value corresponding to the (-402) plane was calculated using the equation d-402 = λ/2sinθ, where λ is the Kα radiation wavelength of Cu (λ = 0.15406 nm) and θ is the Bragg angle corresponding to the diffraction peak obtained from the XRD data [49]. The d-402 value of the annealed Ga2O3 films was lower than the bulk value (0.249 nm) because the films were compressively strained within the range of 600-900 °C [50], which was generally attributed to the different equilibrium lattice spacing of the films with the substrate in the much thinner films than the substrate [51]. The d-402 values of the  Figure 3f shows the interplanar distances corresponding to the d-spacings for the monoclinic Ga 2 O 3 orientation along the (-402) plane with a change in temperature from 600 to 900 • C. The d-spacing value corresponding to the (-402) plane was calculated using the equation d -402 = λ/2sinθ, where λ is the Kα radiation wavelength of Cu (λ = 0.15406 nm) and θ is the Bragg angle corresponding to the diffraction peak obtained from the XRD data [49]. The d -402 value of the annealed Ga 2 O 3 films was lower than the bulk value (0.249 nm) because the films were compressively strained within the range of 600-900 • C [50], which was generally attributed to the different equilibrium lattice spacing of the films with the substrate in the much thinner films than the substrate [51]. The d -402 values of the annealed Ga 2 O 3 films increased from 0.23520 to 0.23596 nm with an increase in temperature from 600 to 900 • C. The release of the compressive strain causes a shifting of the diffraction peak along (-402) towards a lower Bragg angle [39], as shown in Figure 2, which increases the d-spacing in the above equation. The slope of the increase in d -402 gradually decreased with the gentle release in the compressive strain at 900 • C. The d-spacing finally approached the more standard value due to the gradual shifting of the diffraction peak along (-402) towards a lower Bragg angle under the same condition, which was possibly due to the Coatings 2022, 12, 140 7 of 12 existence of considerable point defects, including oxygen vacancy in the lattice [8,33,52]. It became necessary to examine the XPS analysis results to identify the cause of the rapid decrease in temperature.
To further study the quality of the Ga 2 O 3 films, particularly the oxygen vacancies inside the films, the chemical compositions and bonding energies of the films were investigated by XPS at the annealing temperatures of 800 and 900 • C, showing a sharp difference in the lattice characteristics (Figure 2), as shown in Figure 4. All the XPS spectra were deconvoluted using XPSPEAK4.1 software (Washington State University, Pullman, WA, USA). The spectrum of the C 1s peak with a binding energy of 285 eV was used as a reference for data calibration. High-resolution narrow scans were employed to examine the core-level elements, such as Ga 2p, Ga 3d, and O 1s, in the Ga 2 O 3 films. The core-level XPS spectra of the annealed Ga 2 O 3 film at 800 • C shifted by 0.677-0.870 eV toward a higher binding energy than that at 900 • C. The chemical shift towards higher binding energy without any obvious change in the spectral shape is due to the large electronegativity difference between the coordinating groups. This difference can be attributed to the formation of a large number of crystal defects in the annealed Ga 2 O 3 film at 800 • C, which shows the presence of both β-Ga 2 O 3 and ε-Ga 2 O 3 phases, as observed from the XRD results. The Ga 2p doublet was symmetric and narrow at the binding energies of 1118.49 and 1117.76 eV (Ga 2p 3/2 ) and 1145.36 and 1144.64 eV (Ga 2p 1/2 ) in the annealed Ga 2 O 3 films at 800 and 900 • C, respectively, with a spin-orbit splitting (SOS) energy of 26.87 eV, as shown in Figure 4a,d. The small peaks at 1138.37 and 1136.77 eV were loss features between two spinorbit coupled peaks originating from inelastic interactions between the emerging electrons, which reduced their energy. Because the Ga 2p peaks have no appreciable shoulder-like feature, as direct evidence for the gallium interstitials, which also have very high formation energy [53], the origin is deduced to be rather due to oxygen vacancies or others.
annealed Ga2O3 films increased from 0.23520 to 0.23596 nm with an increase in temperature from 600 to 900 °C. The release of the compressive strain causes a shifting of the diffraction peak along (-402) towards a lower Bragg angle [39], as shown in Figure 2, which increases the d-spacing in the above equation. The slope of the increase in d-402 gradually decreased with the gentle release in the compressive strain at 900 °C. The d-spacing finally approached the more standard value due to the gradual shifting of the diffraction peak along (-402) towards a lower Bragg angle under the same condition, which was possibly due to the existence of considerable point defects, including oxygen vacancy in the lattice [8,33,52]. It became necessary to examine the XPS analysis results to identify the cause of the rapid decrease in temperature.
To further study the quality of the Ga2O3 films, particularly the oxygen vacancies inside the films, the chemical compositions and bonding energies of the films were investigated by XPS at the annealing temperatures of 800 and 900 °C, showing a sharp difference in the lattice characteristics (Figure 2), as shown in Figure 4. All the XPS spectra were deconvoluted using XPSPEAK4.1 software (Washington State University, Pullman, WA, USA). The spectrum of the C 1s peak with a binding energy of 285 eV was used as a reference for data calibration. High-resolution narrow scans were employed to examine the core-level elements, such as Ga 2p, Ga 3d, and O 1s, in the Ga2O3 films. The core-level XPS spectra of the annealed Ga2O3 film at 800 °C shifted by 0.677-0.870 eV toward a higher binding energy than that at 900 °C. The chemical shift towards higher binding energy without any obvious change in the spectral shape is due to the large electronegativity difference between the coordinating groups. This difference can be attributed to the formation of a large number of crystal defects in the annealed Ga2O3 film at 800 °C, which shows the presence of both β-Ga2O3 and ε-Ga2O3 phases, as observed from the XRD results. The Ga 2p doublet was symmetric and narrow at the binding energies of 1118.49 and 1117.76 eV (Ga 2p3/2) and 1145.36 and 1144.64 eV (Ga 2p1/2) in the annealed Ga2O3 films at 800 and 900 °C, respectively, with a spin-orbit splitting (SOS) energy of 26.87 eV, as shown in Figure 4a,d. The small peaks at 1138.37 and 1136.77 eV were loss features between two spin-orbit coupled peaks originating from inelastic interactions between the emerging electrons, which reduced their energy. Because the Ga 2p peaks have no appreciable shoulder-like feature, as direct evidence for the gallium interstitials, which also have very high formation energy [53], the origin is deduced to be rather due to oxygen vacancies or others.    The Ga 3d peaks in Figure 4b,e were deconvoluted into two peaks situated at 21.06 and 20.39 eV (Ga 3d 5/2 ) and 20.31 and 19.92 eV (Ga 3d 3/2 ) for the annealed Ga 2 O 3 films at 800 and 900 • C, respectively. This was in addition to the overlapped O 2s peaks at 23.70 and 23.12 eV. The Ga 3d 5/2 peaks centered at the high binding energies of 21.06 and 20.39 eV represented the Ga 3+ oxidation state associated with the Ga-O bond expected in the Ga 2 O 3 [54]. The Ga 3d 3/2 peaks at the low binding energies of 20.31 and 19.92 eV may be related to the Ga + or Ga 2+ oxidation state in the GaO x bond; this observation suggests either a Ga-rich growth or a presence of oxygen vacancies near the surface.
In Figure 4c,f, each O 1s peak in the annealed films at 800 and 900 • C comprised two peaks, O(I) at 531.62 and 530.81 eV, corresponding to the lattice oxygen Ga-O bonds of Ga 2 O 3 and O(II) at 533.11 and 532.24 eV, respectively, associated with defected oxygen sub-lattice such as oxygen-related vacancies in the Ga 2 O 3 films [55]. However, the area ratio of the O(II) to O 1s peaks in the annealed films increased from 11.73% at 800 • C to 17.69% at 900 • C, indicating that the annealed film at 900 • C had a slight increase in the oxygen vacancies. Therefore, it was inferred that prominent defects which resulted in shifting of binding energy at 800 • C are other defects including dislocation rather than the oxygen vacancy, which needs to be investigated more closely in a follow-up study.
TEM was conducted to examine the crystalline quality of the annealed Ga 2 O 3 films on the c-plane (0001) sapphire substrates at 800 and 900 • C, which were the changed conditions in the crystalline state at all annealing temperatures. Figure 5 shows the high-resolution TEM images, SAED patterns, and inverse fast Fourier transform (IFFT) images of the films. Figure 5a-c show the TEM image, SAED pattern, and inverse Fourier transform (IFFT) image, respectively, of the annealed Ga 2 O 3 film at 800 • C. Three nanocrystalline planes (systems of parallel equidistant lines) with interplanar distances of 0.23588, 0.14820, and 0.46930 nm corresponding to β-Ga 2 O 3 were observed [56]. The TEM image of the films at 800 • C shows Moiré fringes due to several minute β-Ga 2 O 3 (-402) nanocrystals with a pseudomorphic coherence for (-603) and (-201) orientations, which may have originated from the partial symmetry mismatch between the monoclinic β-Ga 2 O 3 and hexagonal ε-Ga 2 O 3 [57][58][59]. Notably, the Moiré fringes did not necessarily appear at high dislocation densities in the films [60]. Detailed information on the lattice dislocation structure of the nanocrystals in the film at 800 • C was obtained by the IFFT of the TEM image. The IFFT image, obtained from the center of the TEM image in Figure 5a, shows linear defects, including an edge dislocation (T-shaped symbol) and screw dislocations (red-dashed rectangular frames) [61].
The TEM image in Figure 5d shows that the orientation of the annealed Ga 2 O 3 film at 900 • C improved despite the remaining dislocations, which comprised regions with a better atomic arrangement, showing an aligned arrangement sloping with distinct angles and interplanar distances of upward to the left with 0.23529 nm. These correspond to the d-spacing of the monoclinic β-Ga 2 O 3 (-402) plane with similar XRD results in Figure 3f. Figure 5d shows that the annealed Ga 2 O 3 films at 900 • C exhibit a polycrystalline nature with β-Ga 2 O 3 (-402) texture-dominated crystal films, correlating with the XRD results. As shown in the SAED pattern, diffraction spots and dispersive diffraction rings were observed, owing to the presence of polycrystalline components in the films. The IFFT image obtained from the center of the TEM image in Figure 5d shows ordered lattice fringes with the same d-spacings, as shown in Figure 5f. This confirms that linear defects, such as dislocations and boundaries, as shown in Figure 5c, decreased. As expected for the annealed Ga 2 O 3 film at 900 • C with the relaxed strains, it can achieve significantly reduced defects and relatively clear and uniform β-Ga 2 O 3 , although they were not perfect because of the unintended occurrence of point defects at 900 • C.
served, owing to the presence of polycrystalline components in the films. The IFFT image obtained from the center of the TEM image in Figure 5d shows ordered lattice fringes with the same d-spacings, as shown in Figure 5f. This confirms that linear defects, such as dislocations and boundaries, as shown in Figure 5c, decreased. As expected for the annealed Ga2O3 film at 900 °C with the relaxed strains, it can achieve significantly reduced defects and relatively clear and uniform β-Ga2O3, although they were not perfect because of the unintended occurrence of point defects at 900 °C.

Conclusions
Improved crystalline quality β-Ga2O3 films on c-plane sapphire substrates were fabricated by RF magnetron sputtering deposition followed by RTA at 900 °C for 45 min. The amorphous nature of the Ga2O3 films was observed in the as-deposited and annealed films at a low temperature of 500 °C; a mixture of ε-and β-phases was observed within the range of 600-800 °C, whereas only the β-phase appeared with a crystallite size of 26.02 nm at 900 °C. The d-spacing decreased and approached the standard value when the strain

Conclusions
Improved crystalline quality β-Ga 2 O 3 films on c-plane sapphire substrates were fabricated by RF magnetron sputtering deposition followed by RTA at 900 • C for 45 min. The amorphous nature of the Ga 2 O 3 films was observed in the as-deposited and annealed films at a low temperature of 500 • C; a mixture of εand β-phases was observed within the range of 600-800 • C, whereas only the β-phase appeared with a crystallite size of 26.02 nm at 900 • C. The d-spacing decreased and approached the standard value when the strain was consistently relaxed with an increase in the annealing temperature to 900 • C. Although the dislocation density in the annealed Ga 2 O 3 films was reduced at 900 • C, a clear and uniform orientation was not achieved, and the oxygen vacancy concentration increased in the film at that annealing temperature. Nevertheless, a better polycrystalline nature with a dominant β-Ga 2 O 3 (-402)-preferred crystal film was achieved from the annealed Ga 2 O 3 film at 900 • C. A follow-up study to grow a β-Ga 2 O 3 single crystal on the buffer layer of the c-plane sapphire substrate with improved crystal quality under the process conditions for mass production and confirm its characteristics will be required.