Charge transfer from adsorbates to the bulk in a–Si:H

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Abstract
The mechanism of charge transfer from adsorbates to the bulk in hydrogenated amorphous silicon (a-Si:H) has been studied as a function of ambient pressure and surface oxide thickness by measuring the time dependence of the planar conductance. The initial surface condition of the films was characterized by the effect of adsorbed water on the conductance. In a high-quality vacuum, the surface was found to have a hole accumulation layer caused by a negatively charged oxide layer. Upon exposure to a gaseous ambient, the conductance slowly decreases as charge is transferred into the bulk from the surface. The conductance decrease occurs over a period of several hours in some cases. A model is presented to account for the kinetics of the charge transfer involving a tunnelling of electrons from the adsorbates into localized gap states in the silicon. The initial surface condition was found to have a large effect upon the charge transfer in the case of an oxygen ambient.