Auger electron spectroscopy analysis of oxidation states of Te in amorphous CdTe oxide thin films Ultrathin oxygenated amorphous CdTe (a-CdTe:O) films are prepared by rf sputtering of CdTe in a background of argon or argon/nitrogen/oxygen mixtures. Atomic force microscopy ͑AFM͒ is used to characterize the films and shows that they have an island structure typical of most sputtered thin films. However, when sufficiently low powers and deposition rates are employed during sputtering, the resulting films are remarkably smooth and sufficiently thin for use as barrier layers in inelastic electron tunneling ͑IET͒ junctions. Four terminal current-voltage data are recorded for Al/a-CdTe:O/Pb tunnel junctions and conductance-voltage curves are derived numerically. WKB fits to the conductance-voltage curves are obtained using a two-component trapezoidal plus square ͑TRAPSQR͒ model barrier potential to determine values for the tunnel barrier parameters ͑height, shape, and width͒; these parameters are consistent with AFM topological measurements and values from similar devices reported in the literature. IET spectra are presented which confirm that electrons tunnel through ultrathin regions of the a-CdTe:O films, which contain aluminum oxide subregions in a manner consistent with the TRAPSQR barrier model. Because tunneling occurs predominantly through these ultrathin regions, IET spectroscopic data obtained are representative of states at, or within a few tenths of nanometers from, the surface and confirm that the a-CdTe:O surface stoichiometry is very sensitive to changes in the argon/oxygen/nitrogen concentration ratios during film growth. Full IET spectra, current-voltage, and conductance-voltage data are presented together with tunnel barrier parameters derived from ͑WKB͒ fits to the data. The results presented here indicate that inelastic electron tunneling spectroscopy is a useful tool for characterizing the surface states of a-CdTe:O and possibly other photovoltaic materials.
Having completed the inaugural WaterQuality Matters column series focusing on the theme "Hot Topics in Water Quality," initiated in the April 2019 issue of Journal AWWA, the AWWA Water Quality and Technology Division's committees extend the conversation in this recurring column by expounding on a common question, "What keeps you up at night?" The column in this issue has been prepared by the Water Quality Laboratory Committee.
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