The influence of arsenic species (As-2 or As-4) on cornpositionally graded InAsxP1-x buffer layers (x = 0.15 - 0.8) grown by molecular beam epitaxy on InP is investigated. It was found that As-2 has a higher incorporation rate than As-4. Anisotropic strain relaxation occurs for samples grown with As-4, with higher relaxation along [110] and lower relaxation along [1-10]. Relatively high and isotropic strain relaxation occurs for buffers grown with As-2. The observed mosaic broadening is much greater when using As-2. Atomic force microscopy morphological features are consistent with the strain relaxation results. Strong corrugations along [1-10] dominate the surface of films grown with As-4, while grainy surfaces occur with As-2. The use of As4 increased the overall surface roughness. The authors interpret these results with a simple model: the higher incorporation rate of As-2 enables In to incorporate more uniformly in the two perpendicular (110 directions, while the lower incorporation rate of As-4 decreases this uniformity. The different incorporation rates of In in the [110] and [1-10] directions may result in diverse morphologies and strain relaxation induced by arsenic spices. (c) 2007 American Vacuum Society.