METASTABLE PHOTOINDUCED CHANGES IN COMPENSATED A-SI-H FILMS

The light-induced increase of the dark conductivity sigma-d and of the defect density N(d) was studied for two series of n-type a-Si:H films doped with 30 or 300 ppm of B2H6, respectively, and various volume parts of PH3. We analyse the data for the excess conductivity sigma-e in terms of the light-induced shift of the Fermi level, DELTA-E(f), which is determined by the density of centers providing the negative excess charge, N(e), and by the density of states at the Fermi level, g(E(f)). Both N(e) and g(E(f)) contribute to the experimentally observed dependence of DELTA-E(f) on the exposure time and on the Fermi-level position. The negative excess charge presumably arises from the light-induced activation of donors or deactivation of acceptors, respectively. A small density of such centers (congruent-to 10(15)cm-3) is sufficient to produce the observed shifts of E(f), since N(d) and thereby g(E(f)) is small in compensated a-Si:H films.