H. Siekmann

Optimization of the electrical properties of magnetron sputtered aluminum-doped zinc oxide films for opto-electronic applications

Abstract Magnetron sputtered ZnO:Al films are promising candidates as front electrode in a variety of opto-electronic devices. Here we report on efforts to obtain highly conductive and transparent ZnO:Al films using different deposition conditions for RF, DC and MF (mid frequency) sputtering. Investigations were made to see the effect of target doping concentration (TDC), film thickness, sputter pressure and deposition temperature. RF sputtering from ceramic targets yields low resistivities between 3 and 5×10−4 Ω cm for target doping concentrations between 4 and 0.5%. With decreasing TDC to 0.5% carrier mobilities up to 44 cm2/Vs were obtained, accompanied by the extension of the region of high transmission to the near infrared, due to a reduction in free carrier absorption and corresponding shift in plasma wavelength. DC and MF sputtering from metallic targets yielded similar low resistivities at deposition rates up to 200 nm/min. An analysis of mobility (μ) data of all films as function of the corresponding carrier densities (N) showed that the μ–N values obtained in this study are in the vicinity to limits suggested in the literature.

Material Aspects of Reactively MF-Sputtered Zinc Oxide for TCO Application in Silicon Thin Film Solar Cells

Al-doped ZnO films were deposited on glass in an in-line system by reactive mid-frequency (MF) magnetron sputtering. The influence of substrate position on the film properties as well as the relation between static and dynamic deposition are studied. All films showed low resistivity ( -4 Ωcm) and excellent transparency (> 80 % in the visible region). The resistivity ρ for substrate positions above the sputter craters (race tracks) is up to a factor of two higher than on other positions where the smallest ρ is 1.9 x 10 -4 Ωcm. Major differences in statically deposited films as a function of the position on the substrate are found for the structural film properties as characterized by x-ray diffraction (XRD) and etching behaviour. The different surface textures obtained after etching are directly related to variations in the short-circuit current densities of amorphous silicon p-i-n solar cells prepared on these etched ZnO:Al films.