Analysis of dislocation defects in compositionally step-graded α-(Al Ga )O layers.

The ultra-wide bandgap semiconductor α-GaO 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-GaO films. To reduce the dislocation density, compositionally step-graded α-(Al Ga )O layers were fabricated on a -plane sapphire substrate using mist CVD. TEM measurements revealed few dislocations in the initial layer of α-(AlGa)O, but numerous dislocations were observed in the subsequent layer of α-(AlGa)O. However, the step-graded α-(Al Ga )O layers exhibited bending of the dislocations under both compressive and tensile strains due to compositional differences of α-(Al Ga )O, resulting in about 50% reduction of the dislocation density in the high-Ga-composition layer of α-(Al Ga )O. The introduction of multiple 50 nm α-GaO layers into the compositionally step-graded α-(Al Ga )O layers resulted in a notable reduction in dislocation defects at the interface between the sandwiched α-GaO 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.