On the dielectric properties and conduction mechanism in cuprous oxide thin films grown on (111)-oriented MgO substrates by means of non-reactive DC magnetron sputtering

Copper oxide thin films were grown on (111)-oriented MgO substrates via the DC magnetron sputtering technique. High-purity copper was used as target material. The films were grown in an atmosphere of pure argon at a pressure of 1.27 torr. The substrate temperature was maintained at 400 degrees C during the growth. After deposition, the films were annealed in-situ in an argon/oxygen atmosphere (80%/20%) at 450 degrees C for either two or four minutes. It was found that the annealing time was a sensitive process parameter for achieving single- or mixed-phase copper oxide thin films. The single-phase cuprous (Cu2O) oxide films were obtained with the shorter annealing time (two minutes). Longer annealing times led to destabilization of the Cu2O phase, concomitant with the formation of the cupric oxide (CuO) phase in the sample. From optical spectroscopy studies, a band gap value of 2.4 eV was determined for the Cu2O films. This value decreased for the mixed-phase films. Near-infrared spectroscopy spectra were characterized by the presence of troughs and peaks, probably stemming from volume and surface scattering. Furthermore, differences in the intensity of the absorption with the chosen annealing process were observed, which were probably associated with slight variations in the grain size and/or morphology of the films. The electrical behavior of the films was investigated by means of complex impedance spectroscopy and DC transport measurements. Strong dependence of the electrical properties on the frequency and temperature was observed. The impedance spectra were well modeled in terms of equivalent electrical circuits. As a result, it was found that the predominant contribution to the resistance and capacitance of the Cu2O films came from the bulk of the grains. It was also verified that the relaxation of the charge carriers was a thermally active process with an activation energy of similar to 0.17 eV. A semiconducting-like behavior in the 100-320 K temperature range was also verified. The electronic transport mechanism in the Cu2O films was satisfactorily described within the framework of the Mott variable range hopping model. From the fit of this model to the experimental data, values as high as 3.5 x 10(18) eV(-1) cm(-3) and similar to 12 nm were estimated for the density of states at the Fermi level and the hopping distance, respectively.