Effect of Al Concentration Analyzed by ICP-OES on the Structural, Electrical and Optical Properties of Co-sputtered ZnO:Al Films

We prepare co-sputtered Al doped zinc oxide (ZnO:Al) films and investigate the effect of elemental concentrations on the electrical, optical and structural film properties by varying the rf-power applied to the Al target PAl. To obtain the Zn and Al concentrations Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) is used, a novel and relatively simple approach for a high throughput investigation of elemental concentrations in ZnO films. It is found, that while the Al concentration cAl rises exponentially from 1.4·1020 to 7.2·1020 cm-3 with increasing PAl, the charge carrier density increases only linearly from 2.9·1019 to 1.2·1020 cm-3. For PAl > 150 W this can be attributed to an increased doping induced defect density. For PAl > 250 W, the enhanced bombardment of the growing surface with highly energetic particles from the plasma leads to more structural disorder and reduced crystallinity, which results in a decrease of mobility and of charge carrier density. Atomic force microscopy (AFM) images show a correlation between crystallite size and surface roughness of the films, suggesting that larger crystallite sizes promote a smoother film surface. The optical bandgap widens from 3.26 to 3.37 eV with increasing Al concentration due to the Burstein-Moss-Effect, which causes a blue-shift of the absorption edge. In the long wavelength range (λ > 1000 nm), the optical transmission decreases with increasing Al concentration due to enhanced free carrier absorption. An Al rf-power of 200-250 W is found for films with the best combination of low resistivity and high transmission.