Investigation of Band Tailing in Sputtered ZnO:Al Thin Films Regarding Structural Properties and Impurities
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Investigation of Band Tailing in Sputtered ZnO:Al Thin Films Regarding Structural Properties and Impurities Steffi Schönau1, Florian Ruske1, Sebastian Neubert2, Bernd Rech1 1
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Silicon Photovoltaics, Kekuléstr. 5, 12489 Berlin 2 Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, PVcomB, Schwarzschildstr. 3, 12489 Berlin ABSTRACT Thin films of pure aluminum doped ZnO and with addition of nitrogen, oxygen and hydrogen have been prepared by magnetron sputtering. The spectral absorption coefficient close to the band gap energy has been determined by spectrophotometry and analyzed regarding band tailing and creation of defect bands. We found, that an improvement of Raman crystallinity under O 2 rich growth conditions is not accompanied by a suppression of band tailing as expected. An additional absorption feature evolves for layers grown in N 2 containing atmosphere. Doping with hydrogen attenuates sub-band gap absorption. INTRODUCTION Doped zinc oxide is a key material in various optoelectronic devices. A suitable way to adjust electrical and optical properties, at least for sputtered, doped ZnO, is the application of thermal post-deposition treatments including at least one step where the ZnO:Al is protected by a thin protective layer. This method leads not just to a considerably enhanced mobility but to significantly reduced sub-band gap absorption, as well1,2,3. However, although absorption tails below the band gap are well known for ZnO:Al prepared by various deposition techniques, a deep understanding of the underlying mechanisms is still missing. In an earlier work we studied the appearance of such tails in non-reactively sputtered ZnO:Al thin films in respect to deposition conditions and applied annealing treatments3. In this work special emphasis was put on the influence of impurities on the Urbach energy E U . It is usually assumed, that optical transmission is heavily influenced by film stoichiometry and that increasing deposition temperature has a beneficial effect on transmittance due to the evaporation of excess zinc. Here, we therefore studied the influence of oxygen addition during the deposition process on band tailing, electrical properties and Raman activity of the ZnO:Al. Nitrogen on the other hand is always present in small quantities in production sputtering coaters and is known to lead to a yellowish to brownish coloring of ZnO when purposely added to the process. Therefore, nitrogen addition was investigated for various deposition conditions and post-deposition treatments. Finally, it was investigated whether a passivation of optically active defects can be reached by addition of hydrogen to the process. EXPERIMENTAL DETAILS
The ZnO:Al thin films were deposited on 1.1 mm thick Corning Eagle XG glass substrates with a dimension of 100 mm × 100 mm using rf-magnetron sputtering from a circular ceramic target of 250 mm in diameter and 1% wt. aluminum oxide. The target to substrate distance was fixed to 124 mm and the pro
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