CPM and PDS - A critical interpretation of experimental results

CPM and PDS spectra of a-Si:H yield identical shape of the Urbach tail, while the defect absorption measured by PDS differs significantly from CPM. In this work an analysis of CPM and PDS spectra of annealed and degraded films is presented. Numerical simulations of CPM and PDS data, taking into account optical transitions, capture and emission processes as well as the Fermi level, yield information on the energy distribution and the charge state of the defects. The simulations reveal the coexistence of defects in the D-, D+ and D-0 states. The defect distribution is dominated by charged states as predicted by the defect-pool model. Good agreement between measured and simulated PDS and CPM spectra can be obtained in the case of a homogeneous defect density. It is shown that differences between CPM and PDS are due to different sensitivities of the techniques to charged and neutral defect states. Microscopic inhomogeneities may cause significant additional differences.