Distribution of gap states in amorphous selenium thin films

Post-transit photocurrent analysis, based on the time-of-flight transient p hotoconductivity technique, was successfully carried out for a series of am orphous selenium (a-Se) thin films. The method allows a determination of th e density of gap states beyond the shallow tail states from the emission-do minated post-transit currents. Prominent hole and electron traps were resol ved some 0.4-0.5 eV above the valence-band edge and 0.55-0.65 eV below the conduction-band edge. These two traps represent the thermally accessible le vels of the D+ and D- intrinsic negative-U defects in a-Se. The tail end of the transient currents and an apparent temperature dependence below 0 degr ees C of the resolved densities can be interpreted as indications for a fur ther set of hole and electron traps close to the Fermi level, but experimen tal uncertainties and a simulation of the temperature dependence show that such assignment remains questionable. [S0163-1829(99)11615-1].