A.c. loss in amorphous germanium

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Abstract
A.c. loss measurements on thin films of amorphous germanium prepared by thermal evaporation and by r.f. sputtering are reported. The measurements were made using sandwich geometry over the audio-frequency range and at temperatures between 4 and 150 K. At high frequencies power-law loss was generally observed, with an exponent s which was less than one but increased with decreasing temperature. Below a cut-off frequency the a.c. loss tended to decrease more rapidly. The cut-off frequency was a strong function of temperature, generally being activated with an activation energy of 40-75 meV. Some correlation of the a.e. loss with the parallel d.c. conductivity was observed. The data is not in accord with the relationship expected for bulk tunnelling between states widely distributed in space and energy, and is interpreted in terms of a two-particle activated transition between reconstructed sites on internal surfaces within the film. With reasonable assumptions about the physical parameters involved, this model can explain all our observations, including the temperature dependence of the cut-off, which is associated with the distribution of states in space, and the magnitude of the a.c. loss.