Non-polar <i>a-</i> plane UV emission from GaN micropillar arrays for UV devices

Abstract

This work reports the UV-B emitters based on a-plane core–shell GaN/AlGaN micropillar arrays. Using high-aspect-ratio a-faceted GaN micropillars fabricated via top-down etching as templates, we perform a radial growth of GaN/AlGaN multiple quantum wells on GaN micropillar arrays to target UV emission on a-facets. The regrowth process revealed a distinct transition from hexagonal to dodecagonal geometries with the formation of m-plane facets on the edges. This 12-fold geometry is driven by anisotropic kinetics, where the growth rate on non-polar a-planes significantly exceeds that on m-planes by a factor of about 2. We compared two heterostructure designs to address the lattice mismatch inherent in Al-rich shells: a simple core–shell structure and a modified design incorporating a compositionally graded AlGaN spacer. Structural analyses demonstrate the high quality of core–shell GaN/AlGaN growth on both a- and m-plane facets, but only a-plane facets exhibit intense UV emission. The presence of a graded spacer reduces the horizontal crack density by 2 but also selectively enhances the radiative efficiency of the m-plane facets. Furthermore, by optimizing the V/III ratio with high ammonia flux, we successfully enhance the m-plane UV emission. Thanks to time-correlated cathodoluminescence spectroscopy, we confirm a distinct carrier dynamic, where m-plane emission exhibits faster decay times than a-planes (even with increasing ammonia flux), related to the presence of non-radiative defects. From these measurements, the a-plane facets exhibit a better efficiency of UV emission, compared to m-plane facets, highlighting the potential of the non-polar a-faceted templates for UV optoelectronics.