Thermal oxide on CdSe

X-ray photoelectron spectroscopy (XPS) has been used to characterize the thin thermal oxide film grown on single crystal CdSe(0001) and polycrystalline CdSe by exposure to O${}_2$ (dry air) at 350 $°$ C. SeO${}_x$ species, where $\text{x=2,3}$ , are clearly identified by a 5 eV shift of the Se 3$d_{\text{3/2,5/2}}$ peaks to higher binding energy. A very weak shift to lower binding energy is observed for the Cd peaks. The positions of the Cd and O peaks do not match those found for the known cadmium oxides, CdO and CdO${}_2$ . Instead, it is proposed that the Cd bound oxygen atoms occupy substitutional Se sites. The presence of Cd bound oxygen can also be inferred from the intensities of the ${\mathrm{SeO}}_x,$ Cd, and O peaks. Raman spectroscopy confirms the existence of O in Se substitutional sites. Angle-resolved XPS is used to determine the thickness of the oxide and the relative amount of SeO${}_x$ and Cd bound oxygen. The XPS data are consistent with an 8–9 Å thick oxide where $\sim$ 60% of the oxygen is bound to Se and $\sim$ 40 is bound to Cd. The data show that the oxide structure contains two layers; a passivation layer made of the SeO${}_x$ species and, underneath, a layer containing oxygen in Se substitutional sites.