X-ray photoelectron spectroscopy spectra measured on copper sulfide (CuxS) films showed that a thin surface reaction product containing Cu in the +2 valence state was formed on CuxS films exposed to air for 46 h at 40 °C and 90% relative humidity. An entirely different CuxS surface reaction product layer was formed in dry air at 170 °C for 30 min and it contained sulfur in the +6 valence state. The copper (Cu) valence state in CuxS was not found to be +2 even when the x value was less than 1.9. When the argon sputter-cleaned surface of CuxS or CuxS/CdS films was exposed to room-temperature air for 10 min, cadmium (Cd) atoms appeared on the CuxS surface. X-ray powder diffraction patterns showed that CuO and CdS reacted at 500 °C in flowing nitrogen to form Cu2S and CdO. This cation exchange between CdS and copper oxide may explain the surface Cd on the CuxS films. The standard free energy of reaction between CuO and CdS is positive while that between Cu2O and CdS is negative. These results indicate a method for stabilizing CuxS/CdS solar cells against degradation.
Momentum, heat and mass transfer processes were studied in a vertical cylinder reactor for the epitaxial growth of Si from SiC1 4 in H 2 by chemical vapor deposition. An analytical solution to the problem of heat and mass transfer in a tapered annulus is presented based on constant transport properties and fully-developed laminar tlow. The mean gas-phase temperature and deposition rate distribution of silicon are calculated within the reactor using the developing temperature model. Results of experimental studies of silicon deposition from SiC1 4 in H 2 at 1200°C in a vertical cylinder reactor are compared with the analytical results, and with other models of diffusion-controlled chemical vapor deposition. This study provides an analytical basis for epitaxial deposition rate distributions in vertical cylinder reactors, and for reactor design to improve the yield and uniformity of epitaxial growth • * This report was done with support from the United States Energy Research and Development Administration.• Any conclusions or opinions expressed in this report represent solely those of the authors and not necessarily those of The Regents of the University of California, the Lawrence Berkeley Laboratory or the United States Energy Research and Development Administration.-1-I.
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