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Article

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

by
Sakal Pech
†,
Sara Kim
† and
Nam-Hoon Kim
*
Department of Electrical Engineering, Chosun University, Gwangju 61452, Korea
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Coatings 2022, 12(2), 140; https://doi.org/10.3390/coatings12020140
Submission received: 27 December 2021 / Revised: 20 January 2022 / Accepted: 23 January 2022 / Published: 25 January 2022
(This article belongs to the Special Issue Semiconductor Thin Films)
Browse Figures
Versions Notes

Abstract

:
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.

1. Introduction

Gallium oxide (Ga2O3) 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-Ga2O3 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-Ga2O3 at temperatures above 750 °C [1,3]. β-Ga2O3 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 (EC) strength of 8 MV/cm, an excellent electron mobility of 300 cm2/V∙s, and an exceptional Baliga figure of merit (BFOM) of 3444 [1,4].
Polycrystalline β-Ga2O3 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-Ga2O3 with high crystalline quality is complicated and costly. β-Ga2O3 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-Ga2O3 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].
Notably, various substrates such as sapphire (Al2O3), magnesium oxide (MgO), yttria-stabilized zirconia (YSZ), gallium arsenide (GaAs), and Si have been employed for the heteroepitaxy of β-Ga2O3 [8,9]. In particular, the c-plane (0001) sapphire has been widely employed as a substrate for β-Ga2O3 because of its six-fold symmetry. Crystals of (−201)-oriented β-Ga2O3 also exhibit six-fold symmetry and similar thermal expansion coefficients as β-Ga2O3 (αc = 3.15 × 10−6 K−1) and sapphire (αc = ~4.3 × 10−6 K−1) [6,10].
For the heteroepitaxy of β-Ga2O3, 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-Ga2O3 epitaxial growth, the buffer layer using some metal alloys decreased threading dislocation density significantly and eliminated strain accumulation at the α-Ga2O3–sapphire interface [14,15,16]. Because β-Ga2O3 for power device applications requires significantly fewer defects than for optical device applications, to produce a high-quality monoclinic-phase β-Ga2O3 epitaxial layer for power device applications, high-quality polycrystalline β-Ga2O3 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-Ga2O3 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-Ga2O3 buffer layer.
Radio frequency (RF) magnetron sputtering deposition was selected to prepare high-quality polycrystalline β-Ga2O3 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-Ga2O3 films are universally obtained at elevated substrate temperatures during film deposition because β-Ga2O3 crystallization can only be realized at relatively high temperatures [18]. Another method is to fabricate polycrystalline β-Ga2O3 films through a two-step method by post-annealing the as-deposited Ga2O3 films at room temperature. Here, the β-Ga2O3 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 Ga2O3 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-Ga2O3 buffer layer at high temperatures, the residual stress of the β-Ga2O3 films should be studied intensively as a function of the temperature.

2. Experimental Details

Ga2O3 films were deposited on 1 × 1 cm2 c-plane (0001) sapphire substrates using an RF magnetron sputtering system (IDT Engineering Co., Gyeonggi, Korea) at room temperature [21], with a Ga2O3 (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 N2 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 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.

3. 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 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. 87-1901). 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 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 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 DD–S = 0.94 λ/ω·cosθ and Williamson–Hall equation DW–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 15.30 to 20.50 nm for DD–S and 18.97 to 22.98 nm for DW–H with an increase in temperature from 600 to 900 °C, as shown in Figure 2b. In comparison with DD–S and DW–H, the overall DW–H exhibited large values, showing a similar tendency to increase, while the slopes of the increase in DW–H were relatively lower than those in DD–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 DW–H compared to those of DD–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]. The lattice constants of the monoclinic β-Ga2O3 are generally a = 1.223 ± 0.002 nm, b = 0.304 ± 0.001 nm, and c = 0.580 ± 0.001 nm, and the angle between the a- and c-axes is β = 103.7 ± 0.3° [1,43,44]. Above 600 °C, the annealed film exhibited a typical monoclinic crystal structure with two inequivalent Ga sites and three inequivalent O sites. The lattice constants slightly changed with an increase in the annealing temperature from 600 to 900 °C, while the lattice constants of the Ga2O3 film at 600 °C were a = 1.2219 nm, b = 0.3035 nm, c = 0.5803 nm, and β = 104.06°. There were slight fluctuations in the a value (1.2190–1.2220 nm) and c value (0.5803–0.5821 nm) when the b value varied between 0.3019 and 0.3035 nm. The β value showed a tendency to decrease from 104.06° at 600 °C to 103.86° at 800 °C and suddenly returned to 104.06° at 900 °C. This variation in the lattice constants is shown in Figure 3a–d with an increase in temperature, indicating that the lattice constants of the strained lattice of Ga2O3 are far from their bulk values. The volume of a unit cell was obtained from the expression for monoclinic systems: V = abc∙sinβ. The volumes of the unit cell in all annealed films were in the range of 208.70 × 10−3–208.83 × 10−3 nm3, which was less than the corresponding bulk value of 209.63 × 10−3 nm3 at all temperatures [8,33], as shown in Figure 3e, indicating that the Ga2O3 films compressively strained the unit cell of the films within the range of 600–900 °C. There are two distinct Ga sites: the Ga(I) atoms are bonded to four neighboring O atoms in a tetrahedral arrangement, while the Ga(II) atoms are octahedrally arranged and bound to six neighboring O atoms [44]. The wave function of the conduction band bottom generally comprises 4s orbitals of Ga3+ ions in octahedral sites [29]. A compressive strain may lead to an increased octahedral occupancy by Ga3+ ions, forming a compact structure in the unit cell, whereas a tensile strain in the film may lead to an influx of Ga3+ 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 Ga2O3 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 Ga3+ 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 Ga3+ 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/D2, 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 × 1015 line/m2 along the (–402) orientation was obtained at 900 °C when the dislocation density decreased from the highest value of 4.2730 × 1015 line/m2 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 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 3d5/2) and 20.31 and 19.92 eV (Ga 3d3/2) for the annealed Ga2O3 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 3d5/2 peaks centered at the high binding energies of 21.06 and 20.39 eV represented the Ga3+ oxidation state associated with the Ga–O bond expected in the Ga2O3 [54]. The Ga 3d3/2 peaks at the low binding energies of 20.31 and 19.92 eV may be related to the Ga+ or Ga2+ oxidation state in the GaOx 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 Ga2O3 and O(II) at 533.11 and 532.24 eV, respectively, associated with defected oxygen sub-lattice such as oxygen-related vacancies in the Ga2O3 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 Ga2O3 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 Ga2O3 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-Ga2O3 were observed [56]. The TEM image of the films at 800 °C shows Moiré fringes due to several minute β-Ga2O3 (–402) nanocrystals with a pseudomorphic coherence for (–603) and (–201) orientations, which may have originated from the partial symmetry mismatch between the monoclinic β-Ga2O3 and hexagonal ε-Ga2O3 [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 Ga2O3 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-Ga2O3 (–402) plane with similar XRD results in Figure 3f. Figure 5d shows that the annealed Ga2O3 films at 900 °C exhibit a polycrystalline nature with β-Ga2O3 (–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 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.

4. 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 was consistently relaxed with an increase in the annealing temperature to 900 °C. Although the dislocation density in the annealed Ga2O3 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-Ga2O3 (–402)-preferred crystal film was achieved from the annealed Ga2O3 film at 900 °C. A follow-up study to grow a β-Ga2O3 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.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings12020140/s1, Figure S1: Cross-sectional field emission scanning electron microscope (FESEM) images of (a) as-deposited and annealed Ga2O3 films at different post-annealing temperatures: (b) 500, (c) 600, (d) 700, (e) 800, and (f) 900 °C.

Author Contributions

Conceptualization, S.P., S.K. and N.-H.K.; methodology, S.P. and N.-H.K.; investigation, S.P., S.K. and N.-H.K.; data curation, S.P.; writing—original draft preparation, S.P., S.K. and N.-H.K.; writing—review and editing, S.K. and N.-H.K.; supervision, N.-H.K.; project administration, N.-H.K.; funding acquisition, N.-H.K. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by a research fund from Chosun University, 2021.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article or Supplementary Materials.

Conflicts of Interest

The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Coatings 12 00140 g001 550
Figure 1. Top-view field emission scanning electron microscopy (FESEM) surface images of (a) as-deposited and annealed Ga2O3 films at different post-annealing temperatures: (b) 500, (c) 600, (d) 700, (e) 800, and (f) 900 °C.
Figure 1. Top-view field emission scanning electron microscopy (FESEM) surface images of (a) as-deposited and annealed Ga2O3 films at different post-annealing temperatures: (b) 500, (c) 600, (d) 700, (e) 800, and (f) 900 °C.
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Figure 2. (a) X-ray diffraction (XRD) patterns of the as-deposited and annealed Ga2O3 films with different annealing temperatures of 500, 600, 700, 800, and 900 °C [31]. (b) Full widths at half maximum (FWHM) and crystallite size along the (–402) plane of the annealed Ga2O3 films with the annealing temperature from 600 to 900 °C. (c) Average grain sizes under the same conditions.
Figure 2. (a) X-ray diffraction (XRD) patterns of the as-deposited and annealed Ga2O3 films with different annealing temperatures of 500, 600, 700, 800, and 900 °C [31]. (b) Full widths at half maximum (FWHM) and crystallite size along the (–402) plane of the annealed Ga2O3 films with the annealing temperature from 600 to 900 °C. (c) Average grain sizes under the same conditions.
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Figure 3. Lattice characteristics of the annealed Ga2O3 films with different annealing temperatures of 600, 700, 800, and 900 °C. (a–c) Lattice constants of a, b, and c, respectively. (d) Angle between the a- and c-axes of β. (e) Volume of a unit cell obtained from the expression V = abc∙sinβ, for monoclinic systems. The bulk values of the lattice constants are a = 1.2222 nm, b = 0.3041 nm, c = 0.5809 nm, and β = 103.85°; therefore, the bulk value of the volume of the unit cell is V = 209.6265 × 10–3 nm3 [36]. The range of the reference values of lattice constants was shaded with a = 1.223 ± 0.002 nm, b = 0.304 ± 0.001 nm, c = 0.580 ± 0.001 nm, and β = 103.7 ± 0.3° [1,43,44]. (f) Strain (ε) and d-spacing along the (–402) plane.
Figure 3. Lattice characteristics of the annealed Ga2O3 films with different annealing temperatures of 600, 700, 800, and 900 °C. (a–c) Lattice constants of a, b, and c, respectively. (d) Angle between the a- and c-axes of β. (e) Volume of a unit cell obtained from the expression V = abc∙sinβ, for monoclinic systems. The bulk values of the lattice constants are a = 1.2222 nm, b = 0.3041 nm, c = 0.5809 nm, and β = 103.85°; therefore, the bulk value of the volume of the unit cell is V = 209.6265 × 10–3 nm3 [36]. The range of the reference values of lattice constants was shaded with a = 1.223 ± 0.002 nm, b = 0.304 ± 0.001 nm, c = 0.580 ± 0.001 nm, and β = 103.7 ± 0.3° [1,43,44]. (f) Strain (ε) and d-spacing along the (–402) plane.
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Figure 4. X-ray photoelectron spectra of (a,d) Ga 2p3/2 (blue) and Ga 2p1/2 (red) of Ga 2p, (b,e) Ga 3d5/2 (red) and Ga 3d3/2 (green) of Ga 3d, and (c,f) O 1s obtained from the annealed Ga2O3 films at (a–c) 800 °C and (d–f) 900 °C.
Figure 4. X-ray photoelectron spectra of (a,d) Ga 2p3/2 (blue) and Ga 2p1/2 (red) of Ga 2p, (b,e) Ga 3d5/2 (red) and Ga 3d3/2 (green) of Ga 3d, and (c,f) O 1s obtained from the annealed Ga2O3 films at (a–c) 800 °C and (d–f) 900 °C.
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Figure 5. (a,d) Transmission electron microscopy (TEM) images, (b,e) selected area electron diffraction (SAED) patterns, and (c,f) inverse fast Fourier transform (IFFT) images of the annealed β-Ga2O3 films at the annealing temperatures of (a–c) 800 °C and (d–f) 900 °C.
Figure 5. (a,d) Transmission electron microscopy (TEM) images, (b,e) selected area electron diffraction (SAED) patterns, and (c,f) inverse fast Fourier transform (IFFT) images of the annealed β-Ga2O3 films at the annealing temperatures of (a–c) 800 °C and (d–f) 900 °C.
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Pech, S.; Kim, S.; Kim, N.-H. Magnetron Sputter-Deposited β-Ga2O3 Films on c-Sapphire Substrate: Effect of Rapid Thermal Annealing Temperature on Crystalline Quality. Coatings 2022, 12, 140. https://doi.org/10.3390/coatings12020140

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Pech S, Kim S, Kim N-H. Magnetron Sputter-Deposited β-Ga2O3 Films on c-Sapphire Substrate: Effect of Rapid Thermal Annealing Temperature on Crystalline Quality. Coatings. 2022; 12(2):140. https://doi.org/10.3390/coatings12020140

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Pech, Sakal, Sara Kim, and Nam-Hoon Kim. 2022. "Magnetron Sputter-Deposited β-Ga2O3 Films on c-Sapphire Substrate: Effect of Rapid Thermal Annealing Temperature on Crystalline Quality" Coatings 12, no. 2: 140. https://doi.org/10.3390/coatings12020140

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