Formation of conductive CdO layer on CdS thin films during air heating

A thin layer of CdO is formed on a chemically deposited CdS thin film throu gh reaction with atmospheric oxygen during heating in air for 5-240 min at 370-500 degreesC. The sheet resistance of the film drops from about 10(13) Omega -3.5 k Omega (370 degreesC) and 470 Omega (500 degreesC). From optica l transmittance and reflectance spectra, the thickness of the CdO layer was found to be about 10 nm; the crystalline grain size is 12-25 nm, depending on the temperature and duration of heating, and the estimated electrical r esistivity is 10(-4)-10(-3) Omega cm. The optical bandgap of the CdS-CdO la yer is effectively that of the underlying CdS thin film. about 2.45 eV in t he annealed film. The CdS, which remains under the conductive CdO top layer , is photosensitive, with a photo-to-dark current ratio of 10(3) and crysta lline grain diameter of about 10 nm in the case of a film heated at 500 deg reesC for 5 min. These results are discussed in the context of window layer s in solar cells.