Hopping conduction in insulating rf-sputtered zinc oxide films.

Abstract

The dielectric constant and ac conductivity of rf-sputtered insulating zinc oxide films has been measured in the metal-insulator-metal configuration over a wide temperature (90--500 K) and frequency (0.1--100 KHz) range. The films sputtered from oxide targets in argon were c-axis oriented having a resistivity of \ensuremath{\simeq}${10}^{12}$ \ensuremath{\Omega} cm. The dielectric constant shows a strong frequency dispersion at temperatures above 330 K which is the intrinsic behavior of the films. The dielectric dispersion data has been fitted to (i) Debye-type relaxation (with a distribution of relaxation times), (ii) a random-free-energy-barrier model (which assumes that for hopping conduction, the dielectric dispersion is correlated to the dc conductivity), and (iii) the dielectric modulus (which assumes that microscopic inhomogeneities and the distribution of conductivity relaxation times is responsible for dielectric dispersion). The origin of dielectric dispersion has been discussed in the light of these models. It has been found that hopping conduction dominates in ZnO films up to 450 K.