AbstractThe oxidation of high Al content AlxGa1-xAs has received much attention due to its use in oxide-aperture, vertical-cavity surface emitting lasers (VCSELs) and for passivating AlAs against environmental degradation. We have recently identified the spinel, gamma phase of Al2O3 in layers laterally oxidized in steam at 450°C for =0.98 & 0.92 and have seen evidence for an amorphous precursor to the gamma phase. At the interface with the unoxidized AlxGa1-xAs , an ~17nm amorphous phase remains which could account for the excellent electrical properties of oxide-confined VCSELs and help reduce stress concentrations at the oxide terminus.