Evaluation of dielectric properties of nanocrystalline ZnO films at sub-ambient temperatures

Nanocrystalline ZnO films were prepared on glass substrates via the Pechini method and the spin-coating technique. X-ray diffraction patterns showed that the films were single phase and free from impurities or secondary phases within the sensitivity limit of the conventional diffractometer used in this investigation. A crystallite size of ~ 30 nm and a lattice microstrain of 0.7 × 10 –3 were determined from the X-ray diffraction patterns, using the Williamson–Hall model. Impedance spectroscopy studies were performed in the frequency range 40 Hz to 5 MHz at temperatures as low as 90 K. The complex impedance spectra showed only one semicircle, suggesting that the dielectric response mainly stemmed from a single capacitive element, corresponding to bulk grains. This was confirmed by means of the electric modulus approach. The real and imaginary parts of the dielectric permittivity decreased with increasing frequency and decreasing temperature. From the evolution of the imaginary part of the modulus with the temperature, an activation energy of ~ 20 meV was determined. The hopping of small polarons between localized states was identified as the physical mechanism governing the conduction process in the films studied. The dielectric losses were characterized by the imaginary component of the capacitance. The dependence of the DC resistivity on the temperature showed typical semiconductor behavior. Interestingly, the value of the activation energy, obtained from the analysis of the DC resistivity, agreed well with that estimated from the imaginary part of the electric modulus. This finding was interpreted as a sign of the common thermal origin of the phenomena involved in the motion of the charge carriers.