Studies on temperature dependent semiconductor to metal transitions in ZnO thin films sparsely doped with Al

TL;DR

This study investigates how varying Al doping levels in ZnO thin films influence the temperature-dependent semiconductor to metal transition, revealing a decrease in transition temperature and eventual metallic behavior at higher doping concentrations.

Contribution

It provides a detailed experimental and theoretical analysis of SMT in Al-doped ZnO films, highlighting the effects of doping concentration and grain boundary scattering.

Findings

SMT temperature decreases from ~270 K to ~50 K with increasing Al from 0.02% to 0.25%.

At higher doping (~0.5-2%), ZnO remains metallic across all temperatures.

A theoretical model explains SMT behavior based on thermal activation and scattering mechanisms.

Abstract

For a detailed study on the semiconductor to metal transition (SMT) in ZnO thin films doped with Al in the concentration range from 0.02 to 2%, we grew these films on (0001) sapphire substrates using sequential pulsed laser deposition. It was found that the Al concentration in the films increased monotonically with the ratio of ablation durations of the Alumina and ZnO targets used during the deposition. Using X-ray photo electron spectroscopy it was found that while most of the Al atoms occupy the Zn sites in the ZnO lattice, a small fraction of the Al also gets into the grain boundaries present in the films. The observed SMT temperature decreased from ~ 270 to ~ 50 K with increase in the Al concentration from 0.02 to 0.25 %. In the Al concentration range of ~ 0.5 to 2 % these doped ZnO films showed metallic behavior at all the temperatures without undergoing any SMT. A theoretical model based on thermal activation of electrons and electron scatterings due to the grain boundaries, ionic impurities and phonons has been developed to explain the observed concentration and temperature dependent SMT.