Optoelektronic Properties of CuGaSe 2 Thin Films Studied by Time-resolved Microwave Conductivity
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Optoelektronic properties of CuGaSe2 thin films studied by time-resolved microwave conductivity A. Meeder*, D. Fuertes Marrón, M. Ch. Lux-Steiner and M. Kunst Hahn-Meitner-Institut GmbH, Glienicker Str. 100, 14109 Berlin, Germany Abstract: Applying the (contactless) time-resolved microwave conductivity technique (TRMC) to CVD- and PVD-grown device grade CuGaSe2 thin films experimental data of hole mobility and lifetime are obtained. TRMC data of stoichiometric and Gallium-rich, front and back side illuminated thin films are presented. In the Gallium-rich samples an extended decay over several orders of magnitude is observed. In the stoichiometric samples the majority of excess carriers decays nearly instantaneously. Differences in charge carrier transport at the front and the back side are discussed in the framework of a two layer defect model, previously reported in such absorber layers. Introduction: The electronic quality of wide gap chalcopyrites, and particularly CuGaSe2 (CGSe), is mainly limited by their high bulk defect densities [1, 2]. Recent studies have demonstrated that this point can be minimised using certain sequential deposition processes, including post annealing steps [2, 3]. For this work device grade CGSe thin films are grown by common PVD- as well as CVD-techniques. Such films show a two layer defect structure namely a less defective, nearly stoichiometric bulk region, and a distorted, highly defective near surface region. This was clearly shown applying a combined PDS and CPM sub-band gap absorption analysis [3]. A correlation of this near surface defect region with a recently observed Cu-depleted film surface [4] is not unlikely. As mobility µ and lifetime τ of free carriers depend strongly on the film defect density, changes in defect density and distribution are expected to result in different values of the µτ-product. With respect to the former mentioned two layer defect structure, we expect smaller µτ values in the near surface region and higher µτ values in the bulk region. To the best knowledge of the authors, such data have not been published for device grade CGSe thin films. Commonly, mobility data are obtained by Hall-analysis. Beside the problem of making good ohmic contacts electrical measurements on CGSe thin films are not straight forward. In polycrystalline films, grain-boundary effects mask the obtained data. Furthermore, the conductivity of Cu(In,Ga)Se2 thin films is strongly influenced by the effect of persistent photoconductivity (ppc) [5], leading to non-stable conductivity during Hall measurements. This effect is enhanced in Gallium-rich CGSe, commonly used as absorber material in devices. In this study, hole mobility and carrier lifetime data of device grade CGSe thin films are recorded applying the non-contact time-resolved microwave conductivity technique (TRMC), used for optoelectronic analysis of a-silicon thin films [6]. In particular, we have studied the influence of the near surface defect layer on the photoconductance along the film thickness. Finally, expe
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