Carrier Transport in Homo- and Heteroepitaxial Zinc Oxide Layers
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1109-B07-05
Carrier Transport in Homo- and Heteroepitaxial Zinc Oxide Layers
Klaus Ellmer Helmholtz-Zentrum für Materialien und Energie, dept. Solare Energetik (SE5), Glienicker Str. 100, 14109 Berlin, Germany
ABSTRACT Homo- and heteroepitaxial ZnMgO:Al, ZnCaO:Al and ZnO:Ga films have been grown on sapphire and ZnO substrates by RF (13.56 MHz) reactive magnetron sputtering from oxidic targets. The films grow epitaxially, i.e. with a preferred in-plane and out-of plane orientation. However, the heteroepitaxial films on sapphire exhibit a much higher crystallographic defect density, compared to the homepitaxial films. The ZnMeO films (Me – metal)on a-plane sapphire exhibit a lower defect density leading to higher Hall mobilities. Both, homo- and heteroepitaxial ZnO:Ga films with carrier concentrations N>1020 cm-3 exhibit the same mobility values, which increase with increasing carrier concentration. This behaviour is typical for electrical grain barrier limited transport, as decribed recently for polycrystalline ZnO:Al(Ga) films on glass. For the ZnCaO:Al films, deposited at similar conditions as the ZnO:Ga films, much lower carrier concentrations were measured, both for sapphire as well as for ZnO substrates. The mobilities of the ZnCaO:Al films on ZnO are much higher than that on the sapphire single crystals. The measured Hall mobilities are compared to single crystalline ZnO transport data. Additionally, the work functions of the ZnMeO layers have been measured by X-ray and ultra-violet photoelectron spectroscopy. As expected, the work functions are lower compared to unalloyed ZnO, which can be used for ZnO band gap and band alignment engineering.
INTRODUCTION Since more than a decade zinc oxide (ZnO) and its derivatives see a renaissance (the second one!), due to its prospective use for opto-electronic devices in the blue and ultra-violet spectral range [1]. One of the main research topics is p-type doping of ZnO in order to build pn junctions. ZnO and its derivatives are also of interest for transparent, conductive electrodes, especially for thin film solar cells [2]. In this field high transparency and at the same time low resistivity are the main goals. Also, alloying of ZnO is interesting for improving the band alignement in the heterojunction between the absorber and the window and contact layer in a thin film solar cell. Though many investigations on electrical transport in ZnO can be found in literature (for a review see [3]), a systematic comparison between homo- and heteroepitaxial films has not yet been performed. In this article the transport properties of homo- and heteroepitaxial ZnMexOy films are compared. A series of deposition techniques, including chemical vapour deposition [4], pulsed laser ablation [5], magnetron sputtering [6-8], molecular beam epitaxy [9,10], and ion-beam assisted deposition [11] were used to grow ZnO thin films. In flat panel displays and thin films solar cells the zinc oxide films are degenerately doped with carrier concentrations up to 1021 cm-3, which leads to
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