Dc properties of ZnO thin films prepared by rf magnetron sputtering

In the development of ZnO-based varistors the electrical properties of ZnO/Bi2O3 junctions and of the two individual oxides are being investigated. Following our recent work on a.c. conductivity in Al-ZnO-Al sandwich structures we currently report d.c. measurements. The structures were prepared by r.f. magnetron sputtering in an argon/oxygen mixture in the ratio 4:1. Capacitance-voltage data confirm that the Al/ZnO interface does not form a Schottky barrier and measurements of the dependence of capacitance on film thickness indicate that the relative permittivity of the films is approximately 9.7. With increasing voltage the current density changed from an ohmic to a power-law dependence with exponent n approximate to 3. Furthermore measurements of current density as a function of reciprocal temperature showed a linear dependence above about 240 K, with a very low activation energy below this temperature consistent with a hopping process. The higher temperature results may be explained assuming a room-temperature electron concentration no and space-charge-limited conductivity, dominated by traps exponentially distributed with energy E below the conduction band edge according to N=N-0 exp(-E/kT(t)), where k is Boltzmann's constant. Typical derived values of these parameters are: n(0)= 7.2 X 10(16) m(-3), N-0 - 1.31 X 10(45) J(-1) m(-3) and T-t= 623 K. The total trap concentration and the electron mobility were estimated to be 1.13 x 10(25) m(-3) and (5.7-13.1) X 10(-3) m(2) V-1 s(-1) respectively.