InN films were grown on GaN buffer layer by plasma-assisted molecular beam epitaxy (PA-MBE). We found that an In layer insertion between GaN buffer and InN film significantly improves the crystal quality and optical property of InN. The comparison of the GaN buffers with different treatment indicates that GaN buffer without In deposition is easily etched by N plasma when InN growth starts, and a 1-2 monolayers (ML) In can effectively prevent GaN from etching, thus maintaining the flat buffer surface for InN growth. With the optimum coverage of In insertion of 1.8 ML, the subsequent InN epifilm shows an atomically flat surface with root-mean-square (RMS) roughness as low as 0.2 nm. The peak intensity of photoluminescence (PL) from the same sample is increased by a factor of similar to 20 compared with the InN film grown without In insertion. (c) 2006 Elsevier B.V. All rights reserved.