Bulk and surface states on hydrogenated amorphous silicon

Abstract

Large depletion and strong accumulation layers can be induced and studied at the surface of amorphous silicon in contact with an electrolyte. We show that in the accumulation range, surface states dominate the induced space charge layer, the contribution of bulk states being insignificant throughout. On an untreated film we find that there is an oxide layer 2–3 monolayers thick, as expected. The total surface‐state density on such films is around 1013 cm−2. The states are spread out in energy over ∼0.3 eV and centered about 0.3 eV below the conduction band edge. Treatment of the film’s surface with concentrated HF and immediately immersing the sample into the electrolyte containing 1% HF, results in an oxide‐free surface. The surface‐state density, however, is not affected significantly. This is in marked contrast with the situation in crystalline Si surfaces, where addition of a minute amount of HF to the electrolyte reduces the surface‐state density by one to two orders of magnitude.