The influence of thermal annealing on the structural, optical and electrical properties of AZO thin films deposited by magnetron sputtering

Thin Al-doped ZnO (AZO) films were deposited by magnetron sputtering on a non-heated quartz substrate. As deposited samples have a nanocrystalline structure, a high transparency in the visible part of the spectrum, but a relatively low conductivity. After deposition, the films were isochronally annealed for one hour in hydrogen atmosphere at 200, 300 or 400 degrees C. The influence of such treatment on the structural properties was analysed by GIXRD and correlated with UV-Vis, photoluminescence and impedance measurements. The structural investigation demonstrated that the heat treatment reduces the strain in the material, the volume of the crystal lattice decreases and the crystal size grows. By measuring the optical properties it was shown that heating increases the optical gap and gradually reduces the number of point defects, mostly related to interstitial atoms. As a result of this process, the conductivity at room temperature increased >9 orders of magnitude due to an enhancement of the mobility and the concentration of free carriers. The activation energy for defect annihilation was estimated to be about 1 eV and corresponds to the diffusion of interstitial atoms with the annihilation of vacancies. The concentration of free carriers increases due to the activation of the dopants that act as shallow donors. (C) 2017 Elsevier B.V. All rights reserved.