The thickness of low temperature AlGaN buffer layers grown on r-sapphire substrates was found to directly affect the crystalline structure of the buffer layer as well as the structural and optical properties of subsequently grown a-plane GaN films. A buffer layer with a thickness of 30nm results in randomly distributed fine domains without extended defects. Increasing the thickness to 90nm led to a uniform and largely coalesced crystalline structure, with well-defined stacking faults. GaN films grown on the thinner buffer layer contain a lower density of larger stacking faults, and exhibited brighter stacking-fault luminescence as compared to films grown on thicker buffer layers. These studies indicated that the optimum buffer layer thickness for growth of a-plane GaN was about 30nm.

Role of the Buffer Layer Thickness on the Formation of Basal Plane Stacking Faults in a-Plane GaN Epitaxy on R-Sapphire. Z.H.Wu, A.M.Fischer, F.A.Ponce, T.Yokogawa, S.Yoshida, R.Kato: Applied Physics Letters, 2008, 93[1], 011901