The highest performance CdS/CdTe thin film solar cells are generally completed with a Cucontaining back contact. The copper appears to be critical for achieving heavy p-type doping of the CdTe at the contact to permit the formation of a low resistance contact. In previous extended x-ray absorption fine structure (EXAFS) work we have inferred that most of the Cu in CdTe films resides as Cu2O at the boundaries of CdTe grains in films that have received a chloride treatment in the presence of oxygen, a critical step needed to improve the performance of all CdTe thin-film cells. This has suggested a mechanism for grain boundary passivation in thinfilm CdTe solar cells. We believe most of the diffused Cu decorates grain boundaries as oxides, consistent with the low doping densities typically observed in CdTe solar cells. The significance for grain boundary passivation will be discussed. We also find evidence that the grain-boundary Cu2O in CdCl2 treated CdTe films is unstable and tends to transform to CuO under some stress conditions.

We have used the fine structure in the Cu K-edge x-ray absorption spectrum to help elucidate the lattice location of Cu in polycrystalline, thin-film CdTe solar cells. In particular, we have studied how the typical CdCl2 vapor treatment in dry air changes the local environment of the Cu in CdTe. We find the Cu absorption spectrum to be similar to that of Cu2Te in the as-deposited CdTe film but to convert to a spectrum similar to Cu2O environment after the vapor CdCl2 treatment.

The valence band and the Si 2p core level of electrochemically etched light emitting porous silicon samples prepared with different etching parameters and upon thermal annealing have been studied. The core level shows appreciable broadening as chemical etching time increases. Upon annealing, the core level linewidth decreases and the valence band develops spectral features. The photoluminescence intensity, also, decreases upon annealing. However, impurity species are found to be present only in trace amounts. Our data is consistent with a photoluminescence mechanism involving a silicon species that degrades, decomposes, or desorbs upon annealing.

Heteroepitaxial growth of Ge on Si(100) and Si on Ge(100) surfaces with Sb as a surfactant has been investigated by in situ high resolution photoemission and low energy electron diffraction (LEED). Our results show that an ordered monolayer of Sb atoms saturate the surface dangling bonds and consequently lower the surface free energy. Deposition of Ge or Si on the Sb/Si(100) or Sb/Ge(100) surfaces either at room temperature, followed by mild annealing or deposition at elevated temperature, result in an epitaxial layer of Ge or Si on the substrate, respectively. We provide clear experimental evidence that the deposited Ge or Si atoms changes position with the surface Sb atoms in this process. Ge or Si atoms occupy the epitaxial sites previously occupied by the Sb atoms. The Sb atoms in turn segregate to the surface and form a new ordered layer. The Bi-assisted growth process is also discussed.