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

Abstract 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-(AlxGa1-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-(AlxGa1-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-(AlxGa1-x)2O3 layers. Moreover, owing to dislocations annihilation in the graded α-(AlxGa1-x)2O3 layers, the calculated density of threading dislocations in α-Ga2O3 films with a graded α-(AlxGa1-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-(AlxGa1-x)2O3 layer. This study provides an attractive approach for obtaining high-quality epitaxial α-Ga2O3 thin films for high voltage power devices.