engleski

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

Zinc oxide is a material which is suitable for many applications due to the large band gap, (3.37 eV) high carrier mobility and the relatively high exciton binding energy (60 meV). The combination of high conductivity and transparency for visible light makes this material a possible candidate for front electrodes in thin film solar cells. For this purpose, it is important to have deposition method that is reproducible and can be easy up scaled. Thin ZnO:Al films were deposited by magnetron sputtering on non-heated quartz substrate. As deposited samples have nano-crystalline structure, high transparency in visible part of spectrum but relatively low conductivity. After deposition, films were isochronal annealed by one hour in hydrogen atmosphere at 200, 300 and 400 °C. The influence of such treatment on structural properties was analyzed by GISAXS and GIWAXS and correlated with uv-vis transmittance, luminescence and conductivity. Structural investigation demonstrated that heat treatment reduces the stress 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 point defects, mostly related to interstitial atoms. As result of this process, the room temperature conductivity increased for more than 7 orders of magnitude. The increase in conductivity was found as a result of enhancement of mobility and concentration of free carriers. The mobility increase due to passivisation of defects at the grain boundaries and due to reducing the number of defects in bulk of material. The activation energy for defect annihilation was estimated to be 1 eV and corresponds to the diffusion of interstitial atoms with the annihilation of vacancies. The concentration of free carriers increase as result of the activation of the dopands that act as shallow donors. .

ZnO thin films; thermal annealing; magnetron sputtering; structural; electrical; optical properties

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