Effects of target composition on the optical constants of DC sputtered ZnO: Al thin films
Al-doped ZnO thin films were deposited from ZnO:Al ceramic and Zn:Al metal alloy targets in Ar and Ar + O2 atmospheres respectively, using Direct Current (DC) magnetron sputtering. The samples exhibited transmittance T > 80% in visible region with good NIR shielding. The results indicated that, band gap energy ranged from 3.34 to 3.44 eV and 3.39 to 3.46 eV for films prepared from alloy and ceramic targets, respectively. Films obtained from alloy target at a substrate temperature of 200 oC showed low electrical sheet resistance of 10 Ω/sq, and highest values of mobility (15.9 cm2/Vs) and carrier concentration (2.98 × 1021 cm-3). However, films prepared from ceramic target at a substrate temperature of 300 oC revealed the highest sheet resistance of 32 Ω/sq, with lower values of mobility (14.1 cm2/Vs) and carrier concentration (1.92 × 1020 cm-3). The increase in sheet resistance and decrease in mobility as well as carrier concentration might be due to increased scattering centers for carriers, resulting to increased sheet resistance. Optical spectra of the films were fitted to SCOUT software in order to determine the refractive index, n and extinction coefficient, k. Generally, the calculated n and k in the visible part of the solar spectrum for different samples, ranged from 1.59 to 2.2 and 0.00013 to 0.0194 respectively, which are in agreement with results obtained using other methods. In general, the findings of this study shows that alloy target is suitable for deposition of ZnO:Althin films for devices/applications where low deposition temperature is required.
Keywords: DC Magnetron Sputtering, Optical Constants, Transparent Conducting Oxides (TCO)
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