Contrasting transparent conductive properties of ZnO films on amorphous and crystalline substrates in view of thickness dependence

Abstract

The authors conducted comparative studies on ZnO films deposited on various substrates to elucidate how the different nucleation and crystallization processes affect their transparent conductive properties. The resistivity versus thickness curves of Ga-doped ZnO films deposited on a-SiNx:H films and glass substrates coincided within the experimental error. This result means that as long as the amorphous substrate is neither reactive with the deposited film nor providing crystalline seeds, resistivity is determined only by self-crystallization. In contrast, the resistivity of undoped ZnO films on sapphire c-planes was about half that on glass substrate even when the films were deposited at room temperature, indicating that the crystal template of sapphire stimulates local crystallization of ZnO films, though they are not epitaxial. With regard to the dependence on deposition temperature, a sudden drop in carrier concentration of undoped ZnO films was commonly observed between 200 and 300 °C for both glass and sapphire substrates, as a result of eliminating crystal disorder that facilitates holding donors. A significant difference was manifested between 300 and 500 °C; ZnO films on glass were nearly insulating, whereas those on sapphire were conductive, reflecting higher mobility and more reduced state. On sapphire substrates, the resistivity and sheet resistance versus thickness curves exhibited a monotonic decrease below 200 °C, whereas a plateau region appeared between 20 and 100 nm at 300 and 400 °C. This corresponds to the existence of electrically dead or inactive region near the interface probably because of depletion of carriers in the lattice-matched epitaxial layer. ZnO films became well conductive only when they were sufficiently thick.