Two-step growth of crack-free 5 <i>μ</i>m-thick Al0.2Ga0.8N on sapphire substrate with sputtered AlN nucleation layer

For AlGaN-based near ultraviolet laser diodes (UVA LDs) with emission wavelength below 365 nm, there is no such suitable substrate for homoepitaxy to the required AlGaN films. In this study, we proposed a two-step growth (TSG) method of thick Al0.2Ga0.8N layer grown on the sapphire substrate with a sputtered AlN nucleation layer. The influence of growth rate and V/III ratio on the growth mode of an AlGaN layer has been studied in detail. It is found that AlGaN films exhibit larger island size and lower island density with low growth rate and low V/III ratio during the 3D growth stage, which can effectively relax the compressive stress and suppress the dislocation formation of the subsequent thick AlGaN layer. Therefore, a crack-free high-quality 5 μm-thick Al0.2Ga0.8N layer with 85% biaxial relaxation is grown on a sapphire substrate by using the TSG method, revealing the threading dislocation density decreases from 1.2 × 1010 to 6.6 × 108 cm−2. This work paves the way for the realization and improvement of high-performance AlGaN-based UVA emitters.