Abstract: The present invention provides methods and apparatus for creating insulating layers in Group III-V compound semiconductor structures having aluminum oxide with a substantially stoichiometric compositions. Such insulating layers find applications in a variety of semiconductor devices. For example, in one aspect, the invention provides vertical insulating layers separating two devices, such as photodiodes, formed on a semiconductor substrate from one another. In another aspect, the invention can provide such insulating layers as buried horizontal insulating layers of semiconductor devices.