A.c. loss in sputtered hydrogenated amorphous germanium measurements at around liquid-nitrogen temperatures

Abstract

Audio frequency a.c. loss measurements in sputtered hydrogenated amorphous germanium films made over the temperature range 30–150 K are described. The measurements show similar features to those previously reported for evaporated films, namely a frequency exponent which decreases with temperature and a low-frequency loss peak which has a simple activated temperature dependence. The activation energy W’ of the loss peak increases with the hydrogen content of the films, and to a first approximation all features of the a.c. loss scale with temperature and hydrogen content according to the variation of the parameter W'/kT. The data is found to be most readily explicable in terms of carrier hopping between states in the films around which polaron wells are formed. The main effect of hydrogen incorporation is to increase the dielectric constant in the neighbourhood of the wells and thereby increase the polaron activation energy. The d.c. conductivity is found to scale with hydrogen content in a similar manner to the a.c. conductivity and the implications of this observation are discussed.