Effects of nitrogen flux and RF sputtering power on the preparation of crystalline a-plane AlN films on r-plane sapphire substrates

Abstract

A-plane aluminum nitride (AlN) with high quality is crucial to fabricate high-performance non-polar deep-ultraviolet optoelectronic devices. In this work, we prepared crystalline a-plane AlN films on r-plane sapphire substrates by combining reactive magnetron sputtering and high temperature annealing (HTA). The effects of N2 flux and radio frequency (RF) sputtering power on the crystal quality, the surface morphology and the in-plane stress state of a-plane AlN films were comprehensively investigated. The results suggest that the properties of high temperature annealed a-plane AlN (HTA-AlN) films positively depend on the initial states of the sputtered AlN (SP-AlN) films. Increasing the N2 flux or the RF sputtering power can improve the crystalline quality of SP-AlN films by reducing the kinetic energy of deposited particles, which facilitates a-plane AlN deposition. A higher N2 flux smoothens the surface morphology due to the relieved bombardment effect, which is confirmed by the enlarged in-plane tensile stress state. However, a higher sputtering power leads to a rougher surface because of the accelerated deposition rate. With optimized sputtering parameters, a high-quality a-plane HTA-AlN template was obtained with full width at half maximum values of (11–20) plane x-ray rocking curves as low as 1188 and 1224 arcsec along [0001] and [1–100] directions, respectively. The surface presents an ordered stripe-like morphology with a root-mean-square value of 0.79 nm. Our work provides a convenient and effective strategy to prepare high quality a-plane AlN templates and accelerate the versatile application of non-polar deep-ultraviolet light-emitting diode devices.