Thin film passive solar windows produced by reactive evaporation of In-Sn

Abstract

The semiconductor indium tin oxide (ITO) when present as a thin film has been shown to be transparent to visible radiation while opaque to IR radiation. Sputtering, chemical vapor deposition and other coating methodologies have been used to prepare ITO thin films. Reactive evaporation is an alternative coating technique, which has as its major advantage technical simplicity. Our prepation of ITO thin films (30–90 nm) for passive solar windows included the reactive evaporation of In-Sn alloys (In-5wt.%Sn, In-10wt.%Sn and In-20wt.%Sn) onto commercial soda-lime glass held between 25 and 300 °C. The reactive evaporation atmosphere consisted of oxygen at partial pressures from 1 × 10 -4 to 1 × 10 -3 Torr in residual nitrogen. In selected instances ultrathin palladium nucleating layers were evaporated onto the glass substrates prior to the deposition of the ITO. This was done in order to minimize initial alloy-glass agglomeration effects, thus decreasing the final overall ITO film thickness while increasing the visible transmission properties. The film's visible and IR spectral transmission properties were examined using ratio recording spectrophotometry. The agglomeration, nucleation and growth phenomena of the films were extensively investigated by transmission electron microscopy. The agglomeration was found to be a function of the film thickness, with increasing agglomeration for thinner films. Surface analysis by scanning Auger microscopy (SAM), electron spectroscopy for chemical analysis (ESCA), scanning electron microscopy and energy-dispersive analysis of X-rays was also extensively carried out to determine our particular film properties. SAM indicated that only indium, tin and oxygen were present. No tungsten from the evaporation filament or elements from the glass were found. ESCA indicated that ITO was indeed present on the surface. Such work definitely indicated that ITO can be prepared by reactive physical vapor deposition and that the resultant films have the properties commonly found in sputtered ITO films.