Characterization of nonpolar a-plane GaN epi-layers grown on high-density patterned r-plane sapphire substrates

To reduce the number of threading dislocations (TDs) in nonpolar a -plane GaN ( a -GaN) epi-layers grown on flat r -plane sapphire substrates ( r -FSS), we investigated the effects on the crystalline quality of the a -GaN epi-layers of high-density patterned r -plane sapphire substrates ( r -HPSS), the patterns of which were placed at intervals of several hundred nanometers. Two types of r -HPSS, the patterns of which had diameters and heights on the order of several hundred nanometers ( r -NHPSS) or several micrometers ( r -MHPSS), were prepared with conventional r -FSS. The effect of these r -HPSS on the a -GaN epi-layers was demonstrated by evaluating the surface morphology and the crystalline quality of the epi-layers. The surfaces of the a -GaN epi-layer grown on r -FSS and r -NHPSS were pit-free and mirror-like, whereas the surface of the a -GaN epi-layer grown on r -MHPSS was very rough due to the large, irregular GaN islands that grew on the patterns, mainly at the initial growth stage. The crystalline quality of the a -GaN epi-layer grown on r -NHPSS was better than that of the a -GaN epi-layer grown on r -FSS. We confirmed that there were fewer TDs in the a -GaN epi-layer grown on r -NHPSS than there were in the a -GaN epi-layer grown on r -FSS. The TDs propagating to the surface in a -GaN epi-layer grown on r -NHPSS were mainly generated on the flat sapphire regions between the patterns. Interestingly, it was also found that the TDs that propagated to the surface concentrated with a periodic pitch along the c -axis direction. The TD densities of a -GaN epi-layers grown on r -FSS and r -NHPSS were estimated to be approximately 5.0 × 10 10 and 1.5 × 10 9 cm −2 , respectively. This knowledge will contribute to the further development of a -GaN epi-layers for high-performance devices.