On the mechanism of doping and defect formation in a-Si: H

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Abstract
We present experimental data on the correlation between the defect density and the Fermi-level position in hydrogenated amorphous silicon (a-Si: H) films doped substitutionally with phosphorus or boron, or interstitially with lithium. In particular the results obtained by in- and out-diffusion of lithium suggest that doping and defect formation in a-Si: H are controlled by an intrinsic process in which both defect creation and annihilation occur. We compare our data with the predictions of two hydrogen-based models for the thermal eauilibrium state of a-Si:H. Reasonable agreement is obtained with a defect reaction in which the transfer of hydrogen from Si-H bonds to weak bonds is limited to distances comparable to one atomie spacing. We also present results on the density-of-states distribution N(E) in the gap of n-type and p-type films derived from the subgap optical absorption. The surprising result that N(E) peaks below midgap in n-type films but above midgap in p-type samples is also explained in terms of the thermal equilibrium picture.