Properties of Zinc Oxide Films Cosputtered with Aluminum at Room Temperature

Abstract

The electrical, structural, and optical properties of ZnO films cosputtered with Al at room temperature using a radio frequency magnetron cosputtering system were systematically investigated. The increase in carrier concentration was initially drastic with increasing cosputtered Al content and gradually alleviative for the cosputtered films behaving as an n-type degenerated semiconductor. The Hall mobility of these degenerated samples was deeply affected by both the crystal grain size and the degree of the growth orientation, becoming a dominant factor for obtaining the film with the lowest resistivity. The shifts of the ZnO(002) diffraction peak of these cosputtered films were affected by the Al3+ substitute for the lattice Zn2+ sites as well as the excess Al content existing in the interstitials. In terms of the associated optical properties, all these cosputtered films exhibited excellent transmittance around visible wavelengths. In the ultraviolet wavelengths, an apparent blue-shift at the absorption edge due to the increase in carrier concentration correlated to the Burstein–Moss shift was also observed from these degenerated cosputtered Al–ZnO films. The depth distribution of the Al impurities was more uniform than a conventional sputtered Al-doped ZnO film at a similar Al doping level. The carrier concentration and Hall mobility of the cosputtered Al–ZnO film were found to be apparently increased due to the more effective activation of the Al–O chemical bonds and less metallic Al and Zn atoms existed in the interstitials analyzed from X-ray photoelectron spectroscopy (XPS) investigations.