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

α-Ga 2 O 3 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-Al 2 O 3 ) substrate with the same crystal structure. However, the lattice mismatch (4.3%) and the different thermal expansion coefficients between α-Ga 2 O 3 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-Ga 2 O 3 films grown directly on the substrate. Herein, to circumvents these issues, compositionally graded α-(Al x Ga 1-x ) 2 O 3 layers are adopted to reduce threading dislocations for a high quality of epitaxial α-Ga 2 O 3 films. The evolution of strain relaxation and the inclination of threading dislocations in graded α-(Al x Ga 1-x ) 2 O 3 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 x Ga 1-x ) 2 O 3 layers. Moreover, owing to dislocations annihilation in the graded α-(Al x Ga 1-x ) 2 O 3 layers, the calculated density of threading dislocations in α-Ga 2 O 3 films with a graded α-(Al x Ga 1-x ) 2 O 3 layer is reduced by 64.9% compared with that of α-Ga 2 O 3 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 x Ga 1-x ) 2 O 3 layer. This study provides an attractive approach for obtaining high-quality epitaxial α-Ga 2 O 3 thin films for high voltage power devices.