Oxidation and surface states

Amorphous silicon oxidizes upon exposure to air, at a rate which depends on the microstructure of the film. Columnar material (see Section 2.1.1) oxidizes rapidly along the surface of the columns because the intercolumnar regions are porous. The surface area of the columns is so large that an effective oxygen concentration of 10-20 at% 

Initially the growth rate is lower than on crystalline silicon. It is known that hydrogen has the ability to passivate the crystalline silicon surface and so its presence on the a-Si H film is probably the reason for the slower rate. The different oxidation properties of PVD and CVD films are a dramatic illustration of the different a-Si H structures that can be produced. 

The surface states are detected by ESR, PDS and photoemission experiments. In the first two cases, the surface state density is deduced from the thickness dependence of the defect density (Jackson, Biegelson, Nemanich and Knights 1983). When there are cm surface states and Njj cm bulk states, then the total number of defects in a film of thickness d, is 

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