Growth of High-Crystalline α-Ga₂O₃ Thin Films on Sapphire Substrates Using Template Engineering for High Voltage α-Ga₂O₃ Power Devices

With the rapid expansion of the industry, the demand for high-efficiency and durable power semiconductors in harsh conditions such as high temperature and high voltage has increased. Gallium oxide (Ga₂O₃) is a potential material for next-generation power devices, with a bandgap energy of 4.8-5.3 eV, which is higher than Si, SiC, and GaN. Ga₂O_3,especially corundum structured α-Ga₂O_3, has gotten a lot of interest among Ga₂O₃ phases, owing to its special characteristics such as epitaxial growth on a cheap sapphire substrate and the presence of p-type materials with the same crystal structure. However, the mismatch of the lattice and thermal expansion coefficients between α-Ga₂O₃and sapphire, however, causes a high density of threading dislocations (TDs) and cracks in films. High-density TDs act as leakage current paths within the device, reducing efficiency and limiting lifetime. Solving these problems is essential for the development of high-performance devices based on α-Ga₂O₃.<br/>Herein, we proposed the template engineering for the high-quality and strain-free α-Ga₂O₃films by adopting the compositionally graded α-(Al_xGa_1-x)₂O₃layers and a thin sapphire nano-membrane.<br/>In the first chapter, compositionally graded α-(Al_xGa_1-x)₂O₃layers are adopted to reduce threading dislocations for high-quality epitaxial α-Ga₂O₃ films. the evolution of strain relaxation and the inclination of threading dislocations in graded α-(Al_xGa_1-x)₂O₃layers are confirmed by reciprocal space mapping (RSM) and transmission electron microscopy (TEM). Through RSM and TEM studies, we confirmed that compressive strain enhances the inclination of dislocations, and therefore, the dislocations merge and annihilate in the graded α-(Al_xGa_1-x)₂O₃layers. The calculated density of threading dislocations in α-Ga₂O₃ films with a graded a-(Al_xGa_1-x)₂O₃buffer layer is reduced by 64.9% compared with that of α-Ga₂O₃ films deposited directly grown on a bare sapphire substrate. Furthermore, a fabricated lateral-structure Schottky diode reveals enhanced breakdown voltages and forward current density due to the improved crystalline quality by using the graded α-(Al_xGa_1-x)₂O₃buffer layer.<br/>In the second chapter, the properties of the α-Ga₂O₃thin films selectively grown on the thin substrates, stripe patterned sapphire nanomembrane, were investigated. First, the microstructure of α-Ga₂O₃was investigated about the pattern direction, leading to the conclusion that it is preferable to construct the pattern in the [100] direction for device development. The thin substrate absorbed the stress applied to the thin film due to the strain partitioning effect, resulting in a 13.6% drop in misfit edge dislocation density of α-Ga₂O₃ and an 18.7% decrease in the 104 FWHM value of the XRD ω-rocking curve. The thin film's stress was analyzed using reciprocal space mapping, and it was proven that the thin substrate was responsible for the highest stress reduction of 51.6%, due to the strain partitioning effect.<br/>This work will pave the way for high-quality α-Ga₂O₃ growth and its related application development, with follow-up research predicted to lead to the advancement of high-performance power devices.