Transient photoconductivity, density of tail states and doping effect in amorphous silicon

The Transient Photoconductivity (TPC) is studied in the pre-recombination time range on the basis of a Multiple Trapping (MT) model which uses a transient occupation function to account for simultaneous interactions of all the tail states with the conduction band. A direct inversion method for the extraction of the Density Of States (DOS) from the TPC data in amorphous semiconductors is derived. Application of this transient spectroscopy to n-type a-Si:H results in a very narrow band tail (tail width of 14 meV) starting around 0.16 eV below the mobility edge and leading to low DOS. This finding is in favour of the defect pool concept, and involves some doping effects at the donor level.