Strain relaxation and dislocation annihilation in compositionally graded α-(AlxGa1-x)2O3 layer for high voltage α-Ga2O3 power devices

α-Ga₂O₃ of the corundum structure and the large bandgap of 5.3 eV has attracted great interest because it can be grown on a sapphire (α-Al₂O₃) substrate with the same crystal structure. However, the lattice mismatch (4.3%) and the different thermal expansion coefficients between α-Ga₂O₃ and the sapphire substrate induce crystalline defects and thermal strain, leading to a high density of threading dislocations and degraded electrical properties of the α-Ga₂O₃ films grown directly on the substrate. Herein, to circumvents these issues, compositionally graded α-(Al_xGa_1-x)₂O₃ layers are adopted to reduce threading dislocations for a high quality of epitaxial α-Ga₂O₃ films. The evolution of strain relaxation and the inclination of threading dislocations in graded α-(Al_xGa_1-x)₂O₃ layers 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)₂O₃ layers. Moreover, owing to dislocations annihilation in the graded α-(Al_xGa_1-x)₂O₃ layers, the calculated density of threading dislocations in α-Ga₂O₃ films with a graded α-(Al_xGa_1-x)₂O₃ 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 diodes reveals enhanced breakdown voltages and forward current density due to the improved crystalline quality using the graded α-(Al_xGa_1-x)₂O₃ layer. This study provides an attractive approach for obtaining high-quality epitaxial α-Ga₂O₃ thin films for high voltage power devices.