Analysis of dislocation defects in compositionally step-graded α-(Al Ga_1- )₂O₃ layers

The ultra-wide bandgap semiconductor α-Ga₂O₃ can be heteroepitaxially grown on a sapphire substrate. However, due to a lattice mismatch of about 4.6% with a sapphire substrate, many dislocation defects occur in α-Ga₂O₃ films. To reduce the dislocation density, compositionally step-graded α-(Al Ga_1- )₂O₃ layers were fabricated on a c-plane sapphire substrate using mist CVD. TEM measurements revealed few dislocations in the initial layer of α-(Al_0.96Ga_0.04)₂O₃, but numerous dislocations were observed in the subsequent layer of α-(Al_0.84Ga_0.16)₂O₃. However, the step-graded α-(Al Ga_1- )₂O₃ layers exhibited bending of the dislocations under both compressive and tensile strains due to compositional differences of α-(Al Ga_1- )₂O₃, resulting in about 50% reduction of the dislocation density in the high-Ga-composition layer of α-(Al Ga_1- )₂O₃. The introduction of multiple 50 nm α-Ga₂O₃ layers into the compositionally step-graded α-(Al Ga_1- )₂O₃ layers resulted in a notable reduction in dislocation defects at the interface between the sandwiched α-Ga₂O₃ layers. It is assumed that the dislocations were bent by the strain caused by the composition change, resulting in a decrease in the number of dislocations. It is anticipated that further reduction of dislocation density will be achieved by optimizing the composition change and thicknesses of layers that provide effective strain for dislocation bending, and by stacking these layers.