Characterization of nonpolar a-plane GaN epi-layers grown on high-density patterned r-plane sapphire substrates
Abstract
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 × 1010 and 1.5 × 109 cm-2, respectively. This knowledge will contribute to the further development of a-GaN epi-layers for high-performance devices.
- Publication:
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Journal of Crystal Growth
- Pub Date:
- February 2018
- DOI:
- 10.1016/j.jcrysgro.2017.12.036
- Bibcode:
- 2018JCrGr.484...50J
- Keywords:
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- A1. Atomic force microscopy;
- A1. Characterization;
- A1. X-ray diffraction;
- A3. Metalorganic vapor phase epitaxy;
- B1. Gallium compounds;
- B1. Nitrides