Subgap density of states spectroscopy using steady-state photoconductivity-based experiments

Abstract

We solve the general equations for a semiconductor of photoconductivity dominated by one type of carrier and obtain two pairs of analytical formulas for a density of state (DOS) spectroscopy inside the band gap from the measurement of the diffusion lengths at different temperatures and generation rates. The equations are tested initially with a numerical simulation and then experimentally for unintentionally-doped hydrogenated amorphous silicon due to the extended consensus about its DOS shape. We estimate the diffusion lengths of the photocarriers using the steady-state photocurrent and the steady-state photocarrier grating. The energy dependence of the DOS below the Fermi energy is estimated. We extract the characteristic temperatures of the valence band tails and their hole capture coefficients, which are in perfect agreement with the bibliographical consensus. For the first time, we provide a consistent explanation of the free-hole concentration decrease with temperature, observed at low temperatures in this amorphous material.