Surface roughness of GaN and thin AlGaN layers grown by molecular beam epitaxy

Surfaces of GaN and AlGaN, intended for AlGaN/GaN heterostructures, were investigated by in situ reflection high energy electron diffraction and atomic force microscopy. Layers were grown by plasma-assisted molecular beam epitaxy on sapphire substrates. The GaN buffer layer thickness was about 74 nm while the Al-composition and thickness varied from x ¼ 0 to 1 and 0 to 7.2 nm, respectively. An empirical formula showing the lattice mismatch for Al0.4Ga0.6N/GaN as a function of film thickness is deduced. As a result, we find the Al atoms have the effect to smoothen the film surface. # 2003 Elsevier B.V. All rights reserved.