Band gap states in a-SiGe:H alloys determined from charge DLTS experiments

We used the charge deep level transient spectroscopy (DLTS) to determine the midgap density of states (DOS) distribution in undoped a-SiGe:H layers. By applying a bias voltage of -5 V and 0 V on the MIS structures and annealing them 10 minutes at equilibrium temperature of 490 K we moved the position of the Fermi level, E/sub F/, in the a-SiGe:H material, which has led to the different equilibrium distributions of the DOS. In case of applying -5 V, which moves E/sub F/ towards the valence band edge, the DOS distribution is dominated by a peak at 0.61 eV below the conduction band edge and in case of 0 V, when E/sub F/ is around midgap, a peak in the DOS distribution is formed at 0.80 eV. In both cases light soaking leads to an increase in the DOS and a formation of a subband at 0.90 eV.