Growth of thick GaN layers on c‐plane sapphire substrates using stress absorbing layer (SAL)

Abstract

We investigate the effects of inserting defective stress absorbing layer (SAL) in between sapphire substrate and thick GaN layer. A two-step growth technique, in which GaN layers grow at different growth rates in each step, is adopted for growing high-quality GaN layer (NL) at low growth rate and SAL at high growth rate by hydride vapor phase epitaxy (HVPE). SALs grown directly on c-sapphire show hexagonal columnar shaped rough morphology with low crystalline quality. Nevertheless, strain in SAL is more relaxed compared with high quality NL. SAL contains majority of high-quality layers and minority of highly defective regions. The highly defective regions are strongly in-plane compressed by thermal stress and absorbs thermal stress. Overgrown NLs on SALs show lower residual strain compared to NL on NL sample. These results show that bending and microcracks can be suppressed in free-standing GaN substrates by adopting SALs in GaN growth.