Strain and luminescence properties of hexagonal hillocks in N-polar GaN

Owing to its unique properties, N-polar GaN offers several advantages over Ga-polar GaN, particularly for applications in high power electronics. However, the growth of high-quality N-polar material is challenging. One dominant issue is the increased surface roughness, due to the occurrence of hexagonal-shaped hillocks, referred to as hexagons, on the material’s surface. Although, there are different methods to reduce the density of these hillocks, such as the use of vicinal substrates or optimum growth conditions, the properties of such hillocks are not extensively studied. Here, we investigate the crystallographic and luminescence properties of these hexagonal features using the techniques of electron backscatter diffraction (EBSD) and cathodoluminescence (CL) hyperspectral imaging in the scanning electron microscope combined with micro-Raman mapping. CL revealed increased light emission from the top of the hexagons compared with the surrounding material. Additionally, dark spots in intensity images, associated with non-radiative recombination at threading dislocations, could be resolved on top of the hexagons, but not in the surrounding area, implying improved material quality of the hexagons. Extensive strain analysis using EBSD revealed that the hexagons are composed of equivalent triangular segments with tensile strain along symmetrically equivalent <11-20> directions. As the hexagons become larger, this strain increases with distance from the centre. This was confirmed by mapping the Raman E2 (high) mode. Overall this provides crucial insight into the strain state of these hexagonal features