Nonpolar a-plane GaN (a-GaN) grown on r-plane sapphire substrate is one of the promising materials for eliminating an internal field in III-nitride devices. Thus, a high performance light-emitting diode can be expected by using a high crystalline quality a-GaN. In our study, we realized a high crystalline quality a-GaN by using both patterned sapphire substrate (PSS) and sputtered AlN buffer layer (sp-AlN).
The PSS had conical patterns with a diameter of 900 nm and a height of 600 nm. The patterns placed with triangular arrangement and an interval of 1000 nm. The 30-nm-thick sp-AlN was deposited on the PSS at 300 oC. Approximately 3.5-um-thick a-GaN was grown by using metal-organic vapor phase epitaxy with optimized growth conditions. The crystalline qualities of the a-GaN were evaluated by X-ray rocking curves full width at half maximum (XRC-FWHM) for both on- and off-axis planes. Moreover, the growth behavior of a-GaN on PSS was characterized by in-situ reflectance and scanning electron microscope.
For the on-axis GaN (11-20) plane, the XRC-FWHM in the c-axis direction of the a-GaN was 462 arcsec, whereas it was 647 arcsec in the m-axis direction. For the off-axis GaN (10-12) plane, the XRC-FWHM was 990 arcsec. These XRC-FWHMs were significantly decreased compared with that of a-GaN grown on nitridated r-plane flat sapphire. It was suggested the density of defects in a-GaN were decreased by both PSS and sp-AlN. To clarify how to defects in a-GaN decrease by using the PSS and sp-AlN the transmission electron microscope observation was performed.
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