EQUILIBRIUM DEFECT DENSITY IN HYDROGENATED AMORPHOUS-GERMANIUM

The defect density N-D of hydrogenated amorphous germanium (a-Ge:H) has been studied as a function of the Fermi level position E(F) using photothermal deflection spectroscopy. A strong increase of N-D is observed as E(F) is shifted towards either one of the band edges by doping with boron or phosphorus, respectively. This behavior is similar to that of amorphous silicon (a-Si:H) and can be explained by a hydrogen-mediated equilibrium between weak bonds and dangling bonds. From a fit of this model to our data the lowest limit of the defect density in a-Ge:H is estimated to be 2X10(16) Cm-3, which is larger by about two orders of magnitude than in a-Si:H. The reason for this difference is the smaller band gap of a-Ge:H which entails smaller formation energies of dangling-bond defects than in a-Ge:H.