Evolution of Metallic Conductivity in Epitaxial ZnO Thin Films on Systematic Al Doping

The metal-like behaviors and metal-semiconductor transition (MST) of highly conducting Zn1-x Al (x) O (x = 1 at.% to 10 at.%) thin films deposited by cosputtering on a-Al2O3 have been investigated. The temperature-dependent transport properties reveal that the Zn1-x Al (x) O films were highly degenerate. The MST temperature (T (MST)) varied from 190 K to 260 K with Al doping from x = 2 at.% to 10 at.%. A simple degenerate band model is used to explain the observed shift in the metal-like behaviors upon Al doping. An empirical approach is used to analyze the resistivity functional below T-MST, taking into account the contributions from both the weak localization and Coulomb interactions in explaining the MST. Analysis by least-square fittings of measured data shows excellent agreement. The optical bandgap increases with carrier concentration as n (Hall) (2/3) , which is interpreted as the Burstein-Moss shift for a nonparabolic effective mass. Such nonparabolicity is scrutinized by quantitative comparisons of the plasma edge values n (optical) versus the n (Hall) values.