In situ self-release of thick GaN wafer from sapphire substrate via graded strain field engineering

Abstract

We present a scheme for in situ self-release of thick GaN 2-in. wafer from sapphire substrate by engineering the gradient of misfit strains. Release energies of a-, m-, and c-planes of wurtzite GaN are systematically calculated under different biaxial strains by using first-principles method. The results reveal that the c-plane separation will take place under graded strains around −2.8%, where a drastic transition interface of release energy may strongly reduce the strength of c-plane bonding. Based on this mechanism, uniform thick GaN epilayer (>450 μm) is grown on (0001) sapphire substrate by hydride vapor phase epitaxy and subjected to a graded compressive strain field by bowing, fulfilling the c-plane separation condition. As a result, high quality free-standing GaN wafer (350 μm) can be achieved by self-release simply during the cooling process.