engleski

The properties of magnetron sputtered AZO thin films annealed in a H2 atmosphere

The effect of annealing in a hydrogen atmosphere on the structural, optical and electrical properties of Al-doped ZnO (AZO) thin films, obtained by DC magnetron sputtering, was examined. After deposition on a non-heated quartz substrate, the samples were isochronally annealed for one hour in a hydrogen atmosphere at temperatures of 200, 300 or 400 °C. The influence of such treatment on the structural properties was analysed by GIXRD and correlated with UV-Vis, photoluminescence (PL) and impedance spectroscopy (IS) measurements. The improved conductivity and the role of point defects regarding the conductivity of AZO films was investigated. The most relevant defect among the point defects in intrinsic ZnO is probably the zinc vacancy (VZn), and it is suggested that it forms complexes with donors limiting their doping activity. Furthermore, one of the limiting factors for achieving minimum resistivity in AZO thin films appears to be the deactivation of the Al dopant by a vacancy-type defect. The GIXRD results show that annealing reduces the strain in the material, and the volume of the crystal lattice decreases due to a decrease of the unit cell parameters. Also, the crystallite size grows from 9.7 nm for the as-deposited sample, to 17.2 nm for the sample heated at 400 °C. The changes in the crystal lattice parameters towards structural ordering could be a consequence of annihilation of point defects, where interstitial atoms recombine with vacancies or simply diffuse out on the crystal surface, as concluded from our PL measurements. By measuring the UV-Vis transmittance, it was shown that heating increases the optical gap (from 2.85 eV to 3.07 eV) and nullifies defects related to interstitial atoms. The increase of the optical gap is consistent with the increase of conductivity, if the dominant contribution comes from the increase of the free carrier concentration since these electrons occupy the lowest energy levels in the conduction band. This kind of annealing may change the conductivity of the AZO thin films by almost 9 orders of magnitude as a result of the increased mobility and concentration of free charge carriers due to the reduced number of defects and the activation of the dopants as shallow donors. In doing so, the most probable process is the diffusion of interstitial atoms, along with the annihilation of vacancies, and the passivation of defects at the grain boundaries. Acknowledgments This work has been supported by Croatian Science Foundation under the project IP- 2014-09-9419.

Al-doped zinc oxide ; Hydrogen atmosphere ; Transparent conducting oxide ; Magnetron sputtering ; Conductivity ; Point defects.

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