TEM and XANES investigations and optical properties of SnO nanolayers

TEM, XANES and optical investigations show that tin film oxidation starts with the formation of tetragonal tin monoxide, which is converted to unstable orthorhombic tin dioxide with the increase of annealing temperature, transferring, in its turn, into stable tetragonal tin dioxide. In our experimental conditions, no complete transition of the orthorhombic dioxide phase into the tetragonal one is observed up to a temperature of 750 degrees C. According to XANES, data oxidation in the air of tin film surfaces starts at room temperature with the formation of an amorphous Sn-O layer. Probably, this surface layer corresponds to an amorphous membrane, which is formed at the initial stage of interaction at the phase boundaries. After 1 h annealing at temperature intervals of 170-240 degrees C this surface layer (similar to 5 nm) mainly consists of monoxide phase. Annealing at 450-750 degrees C leads to only tin dioxide formation in the surface layer. At least two edges can be observed at the fundamental absorption region of oxide film obtained under annealing at a temperature below 600 degrees C, which corresponds to two values of band gaps: E-g1 = 2.5-2.7 eV and E-g2 similar to 3.4 eV. With an increase of the annealing temperature up to 600 degrees C, only one absorption edge can be observed with a greater band gap of E-g2 similar to 3.4 eV. This absorption edge corresponds to that of tin dioxide with a composition of SnO2 for both modifications observed in our experiments. The second edge with the narrower band gap corresponds to tin monoxide, SnO. and is observed at a temperature below 550 degrees C. Copyright (C) 2006 John Wiley & Sons, Ltd.