Strain relief mechanisms in the growth of GaN/Ga2O3 heterostructures investigated via 4D-STEM

The strain effects and defects present in epitaxial GaN films grown on nitridated β-Ga2O3 substrates are studied using scanning transmission electron microscopy (STEM) techniques, including 4D-STEM. The results show that grains of an intermediate phase (GaOxN1−x) form along substrate cleavage planes during the nitridation. Tracking of the diffraction pattern across the interface further indicates the presence of the intermediate phase. Analysis of the results suggests that larger grains formed near the interface may be leading to a greater relief of strain between the regions, resulting in a decrease in dislocation density. In virtual dark-field imaging, a segregation is observed between film regions with primarily screw dislocations and regions with primarily edge dislocations. Shear strain is found to increase in regions where x-strain and y-strain diverge from each other, indicating that mixed-type dislocations with screw components may be forming to reduce strain energy accumulated from high edge-dislocation densities. The ratios of average values from 4D-STEM maps between the two samples are cross-validated with the ratios of edge-dislocation density, mosaic tilt, and mosaic twist obtained from previous x-ray diffraction measurements. To facilitate this, a dimensional scaling factor is implemented to account for the observed film segregation, showing good agreement. The results are discussed in the context of improving vertical diode performances by reducing threading dislocation densities.