Shape stability of InAs self-assembled islands on vicinal GaAs(0 0 1) substrates

We have grown InAs self-assembled islands on vicinal GaAs(0 0 1) substrates. Atomic force microscopy and photoluminescence studies show that the islands have a clear bimodal size distribution. While most of the small islands whose growth is limited by the width of one multi-atomic step have compact symmetric shapes, a large fraction of the large islands limited by the width of one step plus one terrace have asymmetric shapes which are elongated along the multi-atomic step lines. These results can be attributed to the shape-related energy of the islands at different states of their growth.