Fabrication and Characterization of Low-Dislocation-Density GaN Epilayers on Sapphire Substrates

• Main Authors: Hsueh-Wei Wu, 吳學維
• Other Authors: 武東星
• Format: Others
• Language: zh-TW
• Online Access: http://ndltd.ncl.edu.tw/handle/04977246129853093453

碩士 === 國立中興大學 === 材料科學與工程學系 === 96 === Material properties of GaN epilayers grown on the wet-etched protruding, truncated and invert-pyramidal recessing patterned sapphire substrates (PSSs) as well as planar substrates , are in detail investigated, along with its relationship to their optical characterization. High-resolution x-ray diffraction (XRD) results reveal different dislocation configurations on the PSSs. The scanning electron microscopy images show that the etched pits in the case of the invert-pyramidal recessing PSS exhibit a regular distribution, less etched pits densities (EPDs) or threading dislocation densities onto the recessing pattern area than those onto the sapphire mesas. While no regular distribution for the etched pits onto the former is observed. Additionally, crystal quality of the GaN grown onto the invert-pyramidal recessing PSS is better than that onto the protruding PSS. The screw and edge dislocation density of GaN grown onto the invert-pyramidal recessing PSS is about 107 and 108 cm-2 (common 109 cm-2) respectively determined by etched pits method. The calculated density is a little lower than that derived from the XRD method. The reason to interpret this difference is proposed in this study. From photoluminescence (PL) spectra, the GaN grown onto the invert-pyramidal recessing PSS exhibits the narrowness among all GaN epilayers grown the four substrates mentioned above. Moreover, we demonstrate a tendency that the PL integrated intensity of all GaN epilayers decreases with an increment of the density of the dislocations with edge component. The results indicate that the edge-type dislocations act as the dominant nonradiative centers in GaN epitaxial layers. These results could reflect that the GaN grown on the invert-pyramidal recessing PSS is a much better template for the regrowth of GaN using a maskless technique as compared with the protruding PSS. The GaN after regrowth shows that EPDs associated with screw dislocation are around 7.3 × 105 cm-2 and those with edge dislocations are around 3.1 × 106 cm-2. As compared with the GaN directly grown onto the invert-pyramidal recessing PSS, the EPDs corresponding to edge dislocations of the regrown GaN on this template decrease a factor of two orders (from 108 cm-2 to 106 cm-2).