Optical properties and electrical conduction mechanisms of electron beam evaporated Cu-GeO2 thin cermet films
Optical, DC, and Hall effect measurements were performed on a number of Cu-GeO2 thin cermet films with the aim of obtaining information about DC conduction mechanism. Optical absorption studies showed that incorporation of Cu in the matrix of GeO2 introduces defect states leading to a reduction in the optical energy gap. The DC conductivity results revealed that above a certain temperature Tc, conductivity increases sharply with activation energy lying in the range 0.66 to 0.77 eV. Below Tc, the conductivity is weakly activated with activation energy lying in the range 0.10 to 0.25 eV. A sign reversal in Hall mobility was observed for all the samples. Furthermore, the Hall mobility showed a maximum at a critical temperature (Tc) identical to that of the DC conductivity. This suggests that the DC conductivity is dominated by a mixed conduction process and the small polaron model best describes the conduction mechanism. From the combined knowledge of optical absorption, DC conductivity and Hall effect results a room-temperature band diagram (for 30 at.wt% Cu films) is proposed in which the mobility gap is calculated to be 3.78 eV. Above Tc, the mobility gap reduces to 2.62 eV. The frequency response of dielectric loss and the AC conductivity showed striking minima around a cut-off frequency (fm≈10⁵ Hz) indicating that a single universal power-law cannot describe the conduction mechanism for the entire frequency range. Instead, a two power-law hypothesis is advanced. Below 10⁵ Hz, the small polaron tunnelling model best describes the conduction mechanism, while above 10⁵ Hz, the conductivity is identified with photon-activated resonant processes.