Effect of Off-Axis Angle of C-Plane Sapphire Substrate for Cubic In₂O₃(111) Single-Crystal Layer Growth by Halide Vapor Phase Epitaxy

In recent years, due to the growing interest in energy-saving, there is a demand for the development of wide-bandgap semiconductor materials for next-generation electronic devices with low energy loss. Indium-oxide (In₂O₃) is a wide-bandgap semiconductor material with a bandgap energy of approximately 3 eV, and is widely studied as a base material for transparent electrodes such as tin-doped In₂O₃ (ITO) due to its high visible-light transmittance and low resistivity. However, in order to apply In₂O₃ in the active layer of electronic devices with low energy loss, it is essential to prepare high-quality single-crystal In₂O₃ layers with a low carrier density and a high carrier mobility. So far, the growth of single-crystal layers of cubic In₂O₃ (<i>c</i>-In₂O₃), which is a thermally stable phase with the bixbyite structure, has been attempted using various growth methods. Our research group has been reported the high-speed growth of thick <i>c</i>-In₂O₃ layers on <i>c</i>-plane sapphire substrates using an atmospheric-pressure halide vapor phase epitaxy (HVPE) system that uses indium monochloride (InCl) and oxygen (O₂) as source gases. However, the grown layers tended to be mixed layers of (100)- and (111)-oriented domains.<br/>In the present work, high-temperature HVPE growth of <i>c</i>-In₂O₃ layers on <i>c</i>-plane sapphire substrates with various off-axis angles (<i>Δ</i>_a) of 0 to 10° in the [11-20] direction is attempted in order to grow twin-free (111)-oriented single-crystal layers. The input partial pressures of InCl and O₂ were fixed at 1.0×10^-3 and 1.0×10^-2 atm, respectively, i.e., the input VI/III atomic ratio was fixed at 20. These precursors were separately transported by purified N₂ carrier gas to the growth zone that was maintained at 1000 °C. The thickness of the grown layers was evaluated by cross-sectional observations using field-emission scanning electron microscopy. The in-plane crystal orientations were evaluated by pole-figure measurements using high-resolution X-ray diffraction (XRD). Furthermore, the density and type of dislocations in the grown layers were determined by weak-beam dark-field (WBDF) imaging with cross-sectional transmission electron microscopy. In order to investigate the activation energy and scattering mechanism of carriers, van der Pauw Hall-effect measurements were performed in the range of 80–350 K to determine the carrier density and mobility of the grown layers.<br/>The growth rate increased monotonically as <i>Δ</i>_a increased, which is thought to be due to the increase of step density on the growing surface as <i>Δ</i>_a increased. The XRD pole figure revealed that a polycrystalline <i>c</i>-In₂O₃ layer consisting of a mixture of (111)-oriented twinned domains and (100)-oriented domains grew on the nominally just (<i>Δ</i>_a = 0.21°) substrate. However, as <i>Δ</i>_a increased, the (100)-oriented growth and (111)-oriented twinning were suppressed, and finally a twin-free <i>c</i>-In₂O₃(111) single-crystal layer was grown when <i>Δ</i>_a was 5° or higher. Therefore, it is suggested that the steps on the growing surface can constrain the in-plane rotation of the grown crystal. The WBDF images of a single-crystal layer grown with <i>Δ</i>_a = 5° clarified dislocation densities of 1.2×10⁹, 4.9×10⁹, and 1.3×10¹⁰ cm^-2 for screw, edge, and mixed dislocations, respectively. The Hall electron density and mobility of the single-crystal layer at room temperature were 1.4×10¹⁶ cm^-3 and 232 cm² V^-1 s^-1, respectively. The electron mobility increased as temperature decreased according to optical phonon scattering with <i>hω</i>_LO = 47 meV, and reached a maximum value of 2193 cm² V^-1 s^-1 at 90 K.