STM and XPS study of filtered vacuum arc deposited Sn-O films

Highly conductive, transparent films were fabricated by filtered vacuum arc deposition, followed by rapid thermal annealing (RTA) at 300°C for 30 s. Low temperature measurements of resistance versus temperature demonstrated metallic conduction in the films. Scanning tunneling microscopy/spectroscopy (STM/SPS) showed that the films obtained under optimal conditions were highly homogeneous and had degenerate semiconductor characteristics. However, if the arc had even short periods of unstable operation, then non-degenerate semiconductor regions were also found. The average heights of the crests on the as-deposited films depended on their thickness, being 5, 2 and 0.6 nm for the 50, 350 and 450 nm thick layers, respectively. RTA decreased the crest heights. Well-separated clusters of amorphous tin oxide having typical lateral dimensions of 10 × 4 nm and a height of 6 nm were observed in the thin 50 nm films. The STM topographic images of films after annealing showed clusters with lateral dimensions of 40 × 40 nm and a height of 3 nm, probably originating from the merging of the previously described narrower grains. If non-degenerate semiconductor clusters with wide band gaps were in the as-deposited film, they remained after RTA. It is presumed that these non-degenerate clusters were SnO rather than SnO₂, and that they are responsible for the poor conductivity of these films. X-ray Photoelectron Spectroscopy (XPS) showed that the oxygen concentration of the film did not change after annealing. There were many occupied states in the band gaps of both the as-deposited and annealed samples (in the energy range 0-5 eV), which, however, had different energy distributions. The RTA induced relaxation increased the short range ordering, and led to cluster growth in the films. These changes freed trapped electrons, and increased their mobility.