Observation of a band of fast responding metastable deep traps for charge carriers in crystalline Si(n)/SiNx:H/amorphous Si:H/Al structures

Excess feedback charge capacitance versus applied bias characteristics of crystalline Si(n)/SiNx:H/amorphous Si:H/Al structures points to a band of metastable deep trapping levels in thin films of undoped amorphous hydrogenated Si (a-Si:H). The band of states is characterized by a fast component of the overall response to bias pulse excitation, the fast component apparent time constants approaching the region of microseconds at room temperature. The values are shorter than the dielectric relaxation time τD=ϵ/σdark≈100 μs in this temperature range. This band of states lying closely to the Fermi level or to the hydrogen chemical potential is expected to mediate a fast hopping conduction (dispersive transport) of charge carriers. The transition from the empty to the occupied state of the fast responding states upon scanning properly the bias (filling the states) is much slower than the emission of charge carriers from the occupied states. The transition is accompanied by both a peak of the `dynamicʹ and a dispersion of the steady-state excess capacitance. The complementary slow component of the feedback charge capacitance response, observed as a peak at approximately identical biases and not accompanied by the steady-state capacitance dispersion, is assigned to thermally induced transitions to the conduction band of the electrons liberated from dangling bond-related Dz states at the `freeʹ surface (a-Si:H/Al interface). Judging from the temperature dependence of the fast excess capacitance peak amplitude, the activation energy for hopping is 0.15 eV. A fast redox reaction at the top metal electrode, of some at least partially mobile hydrogen species in transport states cannot be excluded, admitting a power-law time dependence of the effective diffusion coefficient of the species. Irrespective of the microscopic mechanism of the fast relaxation, both the dispersion of the steady-state capacitance and the superimposed capacitance peak amplitude are plausibly simulated, derived from a phenomenological model that points to a correlation between the two quantities.