X-ray photoelectron spectroscopy studies of aging effects on the surface of Au - a-Si:H - Sb-Cr Schottky diodes

During investigations of Au - a-Si:H - Cr-Sb photovoltaic Schottky diodes, it was observed that photoconversion parameters (Isc,, Voc, η), improved markedly with time for samples stored for several weeks in ambient air. This was always accompanied by apparent color changes in the area under the top (Au) electrode, from gold to deep purple, and by evolution of its surface conductivity σ from a highly conducting to an insulating state. Profilometry indicated that the colored area rose about 80 nm above the original surface during these changes. These diodes have been examined using depth-profiling surface analytical techniques, namely secondary-ion mass spectroscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy (XPS), but primarily the latter. The XPS studies of the entire layer thickness on the gold electrode were carried out using erosion by Ar+ ion bombardment. The XPS line positions were used to infer electrical properties of silicon and gold constituents. The results show that Si atoms first diffuse through the gold electrode, where they react with atmospheric constituents to form a thick layer composed mainly of SiO2. this layer is responsible for the observed changes in color, σ, and Isc. The latter change, which leads to a maximum rise in η of about 60%, is felt to result from the fact that the SiO2 layer acts as an antireflection coating. Gold from the electrode layer also diffuses outward, mixed intimately with the silicon oxide. Further aging results in a degradation of the electrical continuity of the Au electrode, which is believed to be responsible for the observed slow drop in σ. (author). 14 refs., 9 figs