Obtainment of the density of states in the band tails of hydrogenated amorphous silicon

International audiencen this work, we present two new pairs of formulas to obtain a spectroscopy of the density of states(DOS) in each band tail of hydrogenated amorphous silicon (a-Si:H) from photoconductivity-basedmeasurements. The formulas are based on the knowledge of the small-signal recombination life-times0, the characteristic decay time of the concentration of trapped carriers generated in excess bythe illumination, and that can be measured by methods like the Oscillating Photocarrier Grating(OPG) or Moving Grating Technique (MGT). First, we deduce the formulas and test their accuracyby numerical simulations using typical a-Si:H parameters. Next, we characterize an a-Si:H sampleusing well-known methods, like Fourier transform photocurrent spectroscopy to evaluate thevalence band tail and modulated photoconductivity to measure the conduction band tail. We alsoperformed measurements of steady-state photoconductivity, steady-state photocurrent gratingand MGT, for a range of generation rates. Fromthese measurements—and taking typical val-ues for the capture coefficients, the extended states mobilities and the DOS at the bandedges—we apply the new formulas to get the band tails. We find that the results obtained fromthe application of our formulas are in good agreement with those found with the traditionalmethods for both band tails. Moreover, weshow that MGT/OPG measurement to gets0can beavoided if one of the band tails is measured by one of the traditional methods, since the knownband tail can be used to evaluates0with one pair of equations, and then the other pair can beapplied to get the other band tai