A model for GaN `ghostʹ islands

During submonolayer deposition of GaN by molecular-beam epitaxy (MBE), novel `ghostʹ islands are observed by scanning tunneling microscopy (STM). The island contrast depends on gap voltage of the STM, suggesting a different atomic structure of the islands than that of GaN terrace or normal islands. Formation of `ghostʹ islands is related to the presence of excess gallium atoms on surface during MBE. Island size distribution show no peak but a monotonically decaying curve, indicating a surfactant mediated process for nucleation of such islands. A model is put forward explaining the origin and the structure of such islands.