Growth of Semi-Insulating GaN by Using Two-Step AlN Buffer Layer

Semi-insulating GaN is grown by using a two-step AlN buffer layer by metalorganic chemical vapour deposition. The sheet resistance of as-grown semi-insulating GaN is dramatically increased to 10¹³Ω/sq by using two-step AlN buffer instead of the traditional low-temperature GaN buffer. The high sheet resistance of as-grown GaN over 10¹³Ω/sq is due to inserting an insulating buffer layer (two-step AlN buffer) between the high-temperature GaN layer and a sapphire substrate which blocks diffusion of oxygen and overcomes the weakness of generating high density carrier near interface of GaN and sapphire when a low-temperature GaN buffer is used. The result suggests that the high conductive feature of unintentionally doped GaN is
mainly contributed from the highly conductive channel near interface between GaN and the sapphire substrate, which is indirectly manifested by room-temperature photoluminescence excited by an incident laser beam radiating on growth surface and on the substrate. The functions of the two-step AlN buffer layer in reducing screw dislocation and improving crystal quality of GaN are also discussed.