Improved crystallinity and surface morphology of a-plane AlN grown on high temperature annealed AlN/sapphire template by pulsed-flow mode metal-organic vapor phase epitaxy

Preparing high quality non-polar aluminum nitride (AlN) templates is the key to improving the performance of non-polar deep-ultraviolet light-emitting diodes. In this study, we investigated the effect of buffer layer on the crystallinity and surface morphology of a-plane AlN films regrown by pulsed-flow mode (PM) metal-organic vapor phase epitaxy (MOVPE). Three buffer layers were compared including low-temperature AlN buffer layer grown by MOVPE (MO-buffer), sputtered AlN buffer layer (SP-buffer), and high-temperature annealed sputtered AlN buffer layer (HTA-buffer). It is found that the (11-20) plane x-ray rocking curve-full width at half maximum (XRC-FWHM) values of a-plane AlN films are significantly reduced after the regrowth process. Thanks to the high crystalline quality of HTA-buffer, AlN regrown on HTA-buffer exhibits the smallest (11-20) plane XRC-FWHM values of 1260/1440 arcsec along the [0001]/[1-100] direction. The mosaic tilt and basal plane stacking fault density are estimated to be 0.41. and 1.76 x 106 cm(-1), respectively. The surface shows a uniform stripe-like pattern with the lowest root mean square value of 0.82 nm. Furthermore, the a-plane AlN epilayer regrown on HTA-buffer displays a weak in-plane stress anisotropy and high optical transmittance in the ultraviolet-visible region. Our work suggests that combining the PM regrowth with HTA-AlN buffer layer is a promising route to prepare high-quality a-plane AlN templates for efficient non-polar deep-ultraviolet light emitters.