Behavior of threading dislocations in SAG-GaN grown by MOVPE

Cross-sectional transmission electron microscope (TEM) study has been carried out to reveal the microstructures in selective-area-growth (SAG) GaN, with special reference to the influences of carrier gas species and morphology of masks. The layers of SAG-GaN were deposited over an a-SiO2 mask of stripe-type by a metalorganic vapor phase epitaxy (MOVPE) method with trimethylgallium and ammonia. The carrier gas was either nitrogen (N-2) or hydrogen (H-2) of ambient pressure. In the case of N-2 carrier gas, the regions of GaN overlying the mask terrace, or the epitaxial-lateral-overgrowth (ELO) region, tilt the crystallographic orientation gradually toward the center of the mask terrace. The tilting is attributed to low-angle grain boundaries. In the case of H-2 carrier gas, the ELO regions show no tilt of c-axis. In the specimen with the mask of W/T = 3/3 (mu m), threading dislocations of both a-type and [a + c]-type run straight upwards, and regions without threading dislocations are then left over the mask-terrace, where WIT is the widths of windows and terraces. It can be expected to obtain a layer of GaN with a very low density of threading dislocations by adopting a double mask, the upper one of which has terraces over the windows of the lower one.