Optical properties of \textit{n}- and \textit{p}-type ZnO thin films -- two different approaches to the impurity distribution inhomogeneity

TL;DR

This study explores the optical properties of n- and p-type ZnO thin films, emphasizing impurity distribution inhomogeneity effects using simulation and fluctuation theories, and compares theoretical predictions with experimental results.

Contribution

It introduces two approaches to model impurity inhomogeneity effects in ZnO films and correlates these models with experimental optical and luminescence data.

Findings

Broadened PL band in n-ZnO exceeds theoretical predictions.

Asymmetrical line shape explained by vibronic model.

Inhomogeneity affects donor-acceptor luminescence and activation energy.

Abstract

We investigated the optical properties of epitaxial \textit{n}- and \textit{p}-type ZnO films grown on lattice-matched ScAlMgO$_4$ substrates. Chemical doping yielded a severe inhomogeneity in a statistical distribution of involved charged impurities. Two approaches are adopted to treat the inhomogeneity effects; Monte Carlo simulation technique for the \textit{n}-doped films, and the fluctuation theory for the \textit{p}-ZnO. The broadening of PL band of \textit{n}-ZnO was significantly larger than predicted by theoretical results in which the linewidths of each individual emissions have been determined mainly from the concentration fluctuation of donor-type dopants by the simulation. Moreover, the rather asymmetrical line shape was observed. To explain these features, a vibronic model was developed accounting for contributions from a series of phonon replicas. In case of \textit{p}-type ZnO:N, analysis of excitation-intensity dependence of the peak shift of donor-acceptor luminescence with a fluctuation model has also proven the importance of the inhomogeneity effect of charged impurity distribution, as in the case of ZnO:Ga. We extracted the inhomogeneity in the sample and acceptor activation energy prepared under the various growth conditions. It is shown that the theoretical results are in good agreement with the experimental 5-K time-resolved luminescence for the systems in a fluctuation field. Finally, localized-state distributions have been studied in N-doped ZnO thin films by means of transient photocurrent measurement.