Effect of substrate misorientation angle on the structural properties of N-polar GaN grown by hot-wall MOCVD on 4H-SiC(0001̄)

Abstract

The effects of substrate misorientation angle direction and degree on the structural properties of N-polar GaN grown by a novel multi-step temperature epitaxial approach using hot-wall metal–organic chemical vapor deposition (MOCVD) on 4H-SiC (000$\bar{1}$) substrates is investigated. The surface morphology and X-ray diffraction (XRD) rocking curves (RCs) for both symmetric and asymmetric Bragg peaks of the multi-step temperature N-polar GaN are compared to a material obtained in a two-step temperature process. In the latter the temperature in the second step was varied so that it corresponds to the growth temperatures in each of the steps of the multi-step process. Different step-flow patterns are obtained on the substrates with a misorientation angle of 4° depending on whether its direction is towards the a-plane or the m-plane. In contrast, for a misorientation angle of 1° towards the m-plane, the surface morphology of N-polar GaN is dominated by hexagonal hillocks when using the 2-step temperature process and a step meandering growth mode is observed when employing the multi-step temperature process. These results are discussed and explained in terms of kinetic and thermodynamic considerations. As the growth temperature of the GaN layer in the 2-step temperature process increases from 950 °C to 1100 °C, the surface roughness and RCs widths decrease for the three types of substrates indicating improved crystal quality at higher temperature. The multi-step epitaxial approach is shown to be beneficial for achieving smooth surface morphology and low defect density of N-polar GaN layers grown on C-face SiC substrates with a misorientation angle of 4° and an RMS value of 1.5 nm over an area of 20 $\mu$m $\times$ 20 $\mu$m is attained when the substrate mis-cut is towards the m-plane.