POROSITY AND OXIDATION OF AMORPHOUS SILICON FILMS PREPARED BY EVAPORATION, SPUTTERING AND PLASMA-DEPOSITION

The amount of water adsorbed by amorphous silicon films upon exposure to normal humidity levels has been measured with a quartz-crystal microbalance. Films electron-beam evaporated on room temperature substrates absorb about 11 mol% H2O on the internal surfaces of an interconnected void structure. We estimate that nearly 15% of the silicon atoms lie at internal void surfaces. The water causes internal oxidation and cannot be removed by drying. It produces an increase in resistance and a shift of the optical absorption edge to higher photon energies. Hydrogenated amorphous silicon prepared by sputtering at 200°C or by rf plasma deposition at 25 and 150°C, on the other hand, is not porous. Exposure to 60% relative humidity at 300 K produces a 5 Å thick oxide layer in about 20 h. Water adsorption strongly affects the conduction in the space charge layer near the surface of plasma-deposited films but has little or no effect on the conductivity of sputtered films. Long term relaxation effects are observed in the conductivity of freshly plasma-deposited amorphous silicon-hydrogen films prepared at room temperature. Based on the very low hydrogen content found in chemical-vapor-deposited (CVD) amorphous silicon we conclude that the CVD films are impervious to water. © 1979.