Characteristics of InAs quantum dot density on a GaNAs buffer layer grown by metalorganic chemical vapor deposition were investigated using atomic force microscopy, focusing on the growth rate of the buffer layer as a significant growth parameter. High dot density of 1.7 x 10(11) cm(-2) was achieved by using the GaNAs buffer with the growth rate of 1.45 mu m/h. It is found that Surface roughness of the buffer layer is a key factor of increasing dot density. Suppression of coalescent dots that are one of the causes of emission intensity degradation was also quantitatively obtained. These results are effective for quantum dot lasers with high modal gain. (C) 2008 Elsevier B.V. All rights reserved.