Electrical and optical properties of ion beam sputtered ZnO:Al films as a function of film thickness

Abstract

Electrical and optical properties of as-deposited, ion beam sputtered, Al-doped ZnO films have been studied as a function of film thickness and carrier concentration. Hall effect measurements reveal that the bulk electrical resistivity of the film generally decreases with increasing film thickness. Additionally, it is observed that the rate of decreasing resistivity depends on the particular film thickness regime. For thinner films (100–200 nm), the resistivity decreases rapidly with increasing film thickness and is due to increases in both carrier concentration and Hall mobility. However, for thicker films, the resistivity decreases more slowly with increasing film thickness and approaches a nearly constant value at a thickness of 1100 nm. In this thickness regime, the slight decrease in resistivity with increasing film thickness is found to be due to an increase in carrier concentration alone. The above observations suggest the presence of at least two scattering mechanisms. It is speculated that grain boundary and ionized impurity are the most likely mechanisms. These issues, together with observed optical properties of the films, are discussed.