ON THE MECHANISM OF DOPING AND DEFECT FORMATION IN A-SI-H

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 equilibrium 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 atomic 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.