The defect density in hydrogenated amorphous material was studied, as a function of the Fermi level position, by using photothermal deflection spectroscopy. A marked increase in the defect density was observed as the Fermi level position shifted towards one or the other band edges, under doping with B or P. The behavior was similar to that of amorphous hydrogenated Si, and was explained in terms of a H-mediated equilibrium between weak bonds and dangling bonds. By fitting this model to the present data, the lowest limit on the defect density in amorphous hydrogenated Ge was estimated to be 2 x 1016/cm3. This was larger, by some 2 orders of magnitude, than that in amorphous hydrogenated Si. The difference was attributed to the smaller band gap of amorphous Ge and to the associated lower formation energies of dangling-bond defects than in the case of Ge.

B.Ebersberger, W.Krühler, W.Fuhs, H.Mell: Applied Physics Letters, 1994, 65[13], 1683-5