HIGH-ELECTRIC-FIELD TRANSPORT IN A-SI-H .2. DARK CONDUCTIVITY

Electric-field-dependent dc dark conductivity data measured over a broad temperature ( 10 less-than-or-equal-to T less-than-or-equal-to 300 K) and field regime (10(2) less-than-or-equal-to F less-than-or-equal-to 6 X 10(5)V/cm) in phosphorus- and boron-doped and intrinsic amorphous hydrogenated silicon (a-Si:H) are described. The data demonstrate the strong influence of the electric field on carrier propagation. Enhancements over six orders of magnitude in conductivity are achieved for fields greater than 10(5)V/cm, which changes a-Si:H films from highly insulating to very conductive at low temperatures. Below 50 K the conductivity is dominated by field-induced transport. The field dependence is described empirically by a power law sigma approximately F(gamma) with gamma in the range 10 less-than-or-equal-to gamma less-than-or-equal-to 17. The enhancement and gamma depend on doping level with significant differences between electron and hole currents. These results are confirmed by experiments on intrinsic a-Si:H where space-charge limited currents have to be taken into account. The strong field dependence of the carrier mobility prevents the buildup of an inhomogeneous carrier density in between the contacts. The field-dependent increase in conductivity measured on either intrinsic or doped a-Si:H is therefore not fundamentally different. The data are interpreted using a field-enhanced nearest-neighbor hopping model. The number of accessible states for tunneling increases rapidly with the field, a feature which is closely linked to the existence of band-tail states in a-Si:H. The description of the field dependence in terms of an effective temperature is discussed briefly.