Electrical Properties of Sn-Doped α-Ga₂O₃ Films on m-Plane Sapphire Substrates Grown by Mist Chemical Vapor Deposition

1 Introduction

Gallium oxide (Ga₂O₃) is a promising semiconductor material for various kinds of applications such as transparent conducting layers,1 thin-film transistors,2 ultraviolet sensors,3 and high-power devices4-6 due to its large bandgap energy reported to be around 5.0 eV.7-10 Among the five different types of Ga₂O₃ polymorphs, β-gallia-structured Ga₂O₃ (β-Ga₂O₃) is the most thermally stable.11 Due to the availability of high-quality β-Ga₂O₃ single-bulk substrates grown by conventional melting growth technique such as floating zone (FZ) or edge-defined film-fed growth (EFG) or Czochralski (CZ) method,12-15 the devices based on β-Ga₂O₃ have largely progressed in recent years, resulting in the successful demonstration of Schottky barrier diodes (SBDs)16, 17 and metal–oxide–semiconductor field-effect transistor (MOSFET).18-20 In contrast, it is reported that some of the metastable phase of Ga₂O₃ polymorphs can be obtained on foreign substrates. One example is corundum-structured Ga₂O₃ (α-Ga₂O₃) grown on sapphire substrates using mist chemical vapor deposition (mist-CVD)7, 21 or halide vapor phase epitaxy (HVPE).22 α-Ga₂O₃ is able to form alloys with other corundum-structured oxides, such as α-Al₂O₃ and α-In₂O₃, the alloy offering a bandgap energy tunable in wide range of 3.7–8.7 eV.23, 24 The feature is promising for heterostructured device application, such as hetero-FET (HFET). In addition, the use of sapphire substrates is advantageous because they are available at low cost. We previously reported the successful n-type doping of α-Ga₂O₃ films grown on c-plane sapphire by Sn-doping.25, 26 However, the electron mobility of α-Ga₂O₃ films in these reports was as low as 24 cm² (V s)⁻¹. In single-bulk crystal of β-Ga₂O₃, the mobility at room temperature as high as 150 cm² (V s)⁻¹ was reported.27 Furthermore, based on first-principle calculation, the effective mass of electron in α-Ga₂O₃ is reported as 0.274m₀, as light as the value in β-Ga₂O₃,28 0.328m₀, where m₀ is the electron mass. Therefore, intrinsic mobility in α-Ga₂O₃ is much higher than 24 cm² (V s)⁻¹ and comparable to that of β-Ga₂O₃. Present low mobility in α-Ga₂O₃ films was attributed to poor crystallinity of the films. For example, α-Ga₂O₃ grown on sapphire substrates contain high density of dislocations, whose density was estimated as 7 × 10¹⁰ cm⁻² for the films grown on c-plane sapphire,29 because of lattice mismatch between α-Ga₂O₃ and sapphire (3.5% and 4.8% for c-axis length and a-axis length, respectively). Therefore, it is important to reduce the density of crystal defects and enhance the electron mobility. In this article, we show the fabrication of conductive Sn-doped α-Ga₂O₃ films on m-plane sapphire substrates by mist-CVD method and electrical properties of them. Some groups already reported the successful growth of α-Ga₂O₃ films on m-plane sapphire by laser molecular beam epitaxy (MBE) and mist-CVD.30, 31 However, there are no reports about the fabrication of conductive α-Ga₂O₃ films on m-plane sapphire and the characterization of electrical property of them. α-Ga₂O₃ films grown on m-plane sapphire exhibited mobility as high as 65 cm² (V s)⁻¹, much higher than previously reported value of 24 cm² (V s)⁻¹ in the films grown on c-plane sapphire, suggesting that the growth direction is important factor to obtain high mobility in α-Ga₂O₃ films.

2 Experimental Section

We fabricated Sn-doped and undoped α-Ga₂O₃ films on m-plane sapphire by mist-CVD method. Gallium(III) acetylacetonate and tin(II) chloride dehydrate were used as gallium (Ga) and tin (Sn) precursors, respectively. As a reaction source, we used a solution of these precursors in water with the addition of small amount of hydrochloric acid to dissolve these precursors completely. Water also acted as an oxygen source. As a carrier gas, nitrogen was used. Substrate temperature was set at 550 °C. To control carrier concentration of films, we varied Sn/Ga concentration ratio in reaction source. The growth speeds of films were controlled by changing gallium precursors concentration in the source solution. The concentrations of gallium precursor in the source solutions were set at 0.050 and 0.025 mol L⁻¹, resulting in the growth speed around 10 and 20 nm min⁻¹, respectively.

Figure 1 shows full width at half maximum (FWHM) values in X-ray powder diffraction (XRD) ω-scan profiles for the diffraction from symmetric (30–30) plane of α-Ga₂O₃ films grown on m-plane sapphire as a function of film thickness with different growth speeds around 10 and 20 nm min⁻¹. As shown in Figure 1, irrespective of growth speed, FWHM values decreased with the increase in film thickness and the films grown with lower growth speed of 10 nm min⁻¹ exhibited lower FWHM values compared with films with higher growth speed of 20 nm min⁻¹. These results suggested that growth speed and film thickness were important factors for fabricating highly crystalline α-Ga₂O₃ films on m-plane sapphire.

Crystallinity of α-Ga₂O₃ film on m-plane sapphire can be improved with decrease in growth speed and increase in film thickness. However, if we fabricate thick α-Ga₂O₃ films with low growth speed, growth time becomes large. Therefore, we tried to fabricate α-Ga₂O₃ films on m-plane sapphire with two-step growth method. Figure 2a shows schematic structure of the samples grown with two-step growth method and Figure 2b shows XRD 2θ/θ-scan profiles of the samples. At first, we grew thin template α-Ga₂O₃ layer on m-plane sapphire with low growth speed of 10 nm min⁻¹. Typical thickness of low-speed grown α-Ga₂O₃ under layers was around 100 nm. Then, we grew thick α-Ga₂O₃ films upon them with high growth speed of 20 nm min⁻¹. To investigate the thickness dependence of crystal quality of α-Ga₂O₃ films, total thicknesses of α-Ga₂O₃ layers were set at around 1 or 2 μm. In Figure 2b, the peaks derived from m-axis-aligned α-Ga₂O₃ were observed. Irrespective of film thickness, no other crystalline phases were observed, suggesting the existence of heterogeneous phases were negligibly small. Table 1 shows FWHM values of XRD ω-scan profiles for the diffraction from both symmetric (30–30) plane and asymmetric (10–14) plane of α-Ga₂O₃ films grown by two-step growth procedure and α-Ga₂O₃ films grown by one-step growth procedure with growth speed around 20 nm min⁻¹. By applying two-step growth procedure, FWHM values of XRD ω-scan profiles were largely decreased from the films with one-step growth procedure, which suggested the improved crystallinity of the films with two-step growth procedure. As for films grown by two-step procedure, FWHM values for α-Ga₂O₃ films with the thickness of 2 μm were smaller than that for films with thickness of 1 μm, which suggested the improved crystallinity in thicker α-Ga₂O₃ films. FWHM values of XRD ω-scan profiles of α-Ga₂O₃ films grown by two-step growth procedure were as low as 1200 arcsec for both symmetric plane diffraction and asymmetric plane diffraction. In contrast, as for α-Ga₂O₃ films on c-plane sapphire in our previous report,26 FWHM values of XRD ω-scan profiles were around 30 arcsec for symmetric plane diffraction and 1000 arcsec for asymmetric plane diffraction. Therefore, because of larger FWHM values for both symmetric and asymmetric plane diffractions, crystallinities of α-Ga₂O₃ films on m-plane sapphire were inferior than those of films on c-plane sapphire. In addition, the sharpness of XRD ω-scan profiles was influenced by dislocation density and their type. The profiles for symmetric plane diffractions are mainly broadened by screw components, whereas the profiles for asymmetric plane diffractions are broadened by both screw and edge components. Because both of the FWHMs values for symmetric and asymmetric diffractions were as large as 1200 arcsec, α-Ga₂O₃ films on m-plane sapphire contain both screw and edge dislocations with high density. In this respect, α-Ga₂O₃ films grown on m-plane sapphire are different from α-Ga₂O₃ films on c-plane sapphire. For the films on c-plane sapphire, the densities of screw dislocations were quite low, whereas the edge dislocation densities were high, due to the small FWHM value for symmetric diffraction and large FWHM value for asymmetric diffraction.

To confirm epitaxial alignment of α-Ga₂O₃ films to sapphire substrates, we conducted pole figure analysis. Figure 3 shows pole figures of diffraction from (10–12) plane for α-Ga₂O₃ films and m-plane sapphire substrates (α-Al₂O₃). Note that both α-Ga₂O₃ and α-Al₂O₃ are corundum-structured crystals, which have threefold symmetry along c-axis. Three spots of diffraction observed in both pole figures of α-Ga₂O₃ and α-Al₂O₃ are reflecting threefold symmetry of corundum-structured crystal. No rotational domains were observed from pole figure of α-Ga₂O₃. Diffraction spots of α-Ga₂O₃ and α-Al₂O₃ were located at same in-plane directions and same tilt angles, indicating both of the crystals, α-Ga₂O₃ and α-Al₂O₃, were aligned in the same in-plane direction as well as out-of-plane direction.

We next tried Sn-doping to α-Ga₂O₃ films grown on m-plane sapphire. Sequence of sample creation was based on the two-step growth method schematically shown in Figure 2a. Sn-doping was only applied to α-Ga₂O₃ upper layers. Therefore, thin α-Ga₂O₃ under layers remained undoped. Thicknesses of each α-Ga₂O₃ layers were same as those shown in Figure 2a. We conducted Hall measurements of Sn-doped α-Ga₂O₃ samples with van der Pauw configuration. Homemade Hall measurement system was used and magnetic fields as high as 1 T were applied for the measurements. Note that electrical properties of undoped α-Ga₂O₃ under layers were highly resistive. Therefore, only Sn-doped upper α-Ga₂O₃ layers were conductive. As contact electrodes, Ti/Au electrodes were fabricated at the corners of samples. Thickness of Ti and Au electrodes were 50 and 20 nm, respectively. As a deposition method of Ti/Au electrodes, we used electron beam evaporation. Figure 4a shows carrier concentration in α-Ga₂O₃ films at room temperature as a function of Sn/Ga concentration in source and Figure 4b shows the relationship between Hall mobility and carrier concentration at room temperature in α-Ga₂O₃ films grown on m-plane sapphire. In Figure 4b, we also show our previous results of films on c-plane sapphire reported by Akaiwa et al.26 for comparison. As shown in Figure 4a, carrier concentrations were almost proportionally varied with the change of Sn/Ga concentration ratio, indicating successful control of carrier concentration. At the same Sn/Ga concentration ratio, there were only small differences in carrier concentration between the films with different thicknesses of 1 or 2 μm, which suggested uniformity of carrier concentration along depth direction in Sn-doped α-Ga₂O₃ films. Table 2 shows the comparison of Sn atomic concentration and carrier concentration in two different samples evaluated by secondary ion mass spectroscopy (SIMS) and Hall measurement. Note that the samples were grown with two-step growth procedure and total thicknesses of the samples were 2 μm. Although there were slight differences, carrier concentrations and Sn atomic concentration in the films were comparable. We considered that carrier concentrations and Sn atomic concentration were differed as a result of unintentional incorporation of donor such as Si impurity or compensating defects. As shown in Figure 4b, Hall mobilities of films on m-plane sapphire become larger with the increase in film thickness at the same carrier concentration, partly due to the improved crystal quality of the films. Irrespective of film thickness and substrate orientation, Hall mobility at high carrier concentration increased with the decrease in carrier concentration due to the decrease in ionized impurity scattering, whereas Hall mobility at low carrier concentration decreased with the decrease in carrier concentration due to the increase in dislocation scattering. The mechanisms of dislocation scattering was also discussed for some materials such as germanium,32, 33 gallium nitride (GaN),34 in our previous report.26 α-Ga₂O₃ films on m-plane sapphire showed much higher mobility compared with those on c-plane sapphire. As a maximum value, Hall mobility of 65 cm² (V s)⁻¹ were obtained at carrier concentration of 1.2 × 10¹⁸ cm⁻³ in films with total thickness of 2 μm. The reason why higher mobility was obtained in α-Ga₂O₃ films on m-plane sapphire is controversial issue. It is reported that the effective mass of electron at the conduction band minimum of α-Ga₂O₃ is isotropic.28 Therefore, higher motility in α-Ga₂O₃ films on m-plane sapphire than that in films on c-plane sapphire was not resulted from the anisotropy of electron effective mass in α-Ga₂O₃, suggesting it can be attributed to the reduction of scattering caused by crystal defects. In contrast, as mentioned previously, crystallinity of α-Ga₂O₃ on m-plane sapphire examined by XRD ω-scan measurements was not superior than that of films on c-plane sapphire. Table 3 shows the concentration of several typical impurities incorporating in the films on m-plane and c-plane sapphire revealed by SIMS. Note that the concentration of each impurity contained 40% errors from true value. There were no large differences of concentrations in each impurity between the films on m-plane and c-plane sapphire except for the concentrations in Al and Cl elements, which showed the differences with one order of magnitude, suggesting that the scattering originating from impurities should be similar. Therefore, despite higher mobility, crystallinity and each impurity density of α-Ga₂O₃ on m-plane sapphire were not largely improved from α-Ga₂O₃ films on c-plane sapphire. Effort will be continued to clarify the reason for the differences in mobility between α-Ga₂O₃ films on m-plane and c-plane sapphire.

3 Conclusion

We showed the fabrication and characterization of conductive Sn-doped α-Ga₂O₃ films on m-plane sapphire. Although the crystallinity was still inferior in comparison with previously reported α-Ga₂O₃ films on c-plane sapphire, highly crystalline α-Ga₂O₃ films were obtained on m-plane sapphire substrates by applying two-step growth method. Carrier concentration of Sn-doped α-Ga₂O₃ films is controlled in the range of 10¹⁷–10¹⁹ cm⁻³ by changing the Sn/Ga concentration ratio in source solution. Compared with the previous report of α-Ga₂O₃ films on c-plane sapphire, much higher mobilities with a maximum value of 65 cm² (V s)⁻¹ were obtained in the films on m-plane sapphire with the film thickness of 2 μm grown by two-step procedure.

Conflict of Interest

The authors declare no conflict of interest.