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

Abstract

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 TMST, 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.