Zinc oxide (ZnO) research relives a third renaissance since about 15 years, due to its prospective use for opto-electronic devices in the blue and ultra-violet spectral range. Here, one of the main research topics is the p-type doping of ZnO in order to build pn junctions. In order to tune the wavelength, ZnO alloys, for instance with magnesium or cadmium, are investigated. ZnO and its derivatives are also of interest for transparent, conductive electrodes, especially for thin film solar cells [1]. In this field alloying of ZnO is interesting for improving the band alignment in the heterojunction between the absorber and the window and contact layer. In this paper the transport properties of polycrystalline and epitaxial ZnMe_xO_y films are summarized. The films habe been prepared by reactive magnetron sputtering from oxidic targets and by RF (13.56 and 27.12 MHz) plasma excitation. The carrier concentration in these films can be adjusted by small additions of oxygen or hydrogen to the Ar sputtering gas. X-ray diffraction was used to characterize the phases and the structural perfection (strain, grain sizes). By (temperature-dependent) Hall and conductivity measurements the electrical transport properties were analyzed. These data for epitaxial and polycrystalline ZnMe_xO_y films are compared to data from literature and with the reported data of single crystalline ZnO [2]. By photoelectron spectroscopy (PES) the work functions φ of the different ZnMe_xO_y layers have been measured. The expected decrease of φ by alloying with group II elements was observed. Its implications for a better band alignment in thin film solar cells will be discussed.

[1] "Transparent Conductive Zinc Oxide: Basics and Applications in Thin Film Solar Cells"; Edited by K. Ellmer, A. Klein and B. Rech (Springer, Berlin, 2008).

[2] K. Ellmer, J. Phys. D: Appl. Phys. 34 (2001) 3097.

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