Microstructure Effects in Amorphous and Microcrystalline Ge:H Films
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Microstructure Effects in Amorphous and Microcrystalline Ge:H Films W. Beyer1,2, F. Einsele1, M. Kondo3, T. Matsui3, F. Pennartz1 1. IEF5-Photovoltaik, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany 2. Malibu GmbH & Co. KG, Böttcherstrasse 7, D-33609, Bielefeld, Germany 3. National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan ABSTRACT The characterization of void-related microstructure in amorphous and microcrystalline Ge:H films is reported. Various methods are applied including effusion measurements of hydrogen and of implanted helium and neon, measurements of the infrared absorption of C-H bonds due to in-diffusion of contaminants and of the stretching modes of bonded hydrogen. Several microstructure effects like interconnected voids and isolated voids and a quite different material homogeneity are detected and are found to depend on the preparation conditions. Amorphous Ge:H can be prepared with a (largely) homogeneous structure while microcrystalline Ge:H tends to consist of compact grains surrounded by more or less open voids. Enhanced substrate temperatures (TS ≈ 250°C) favour the growth of more compact material. INTRODUCTION One major defect in hydrogenated silicon and related materials is a void-related microstructure involving voids of various shapes and sizes [1]. While in microcrystalline materials these voids are expected to be related to grain boundaries, in amorphous materials growth effects like shadowing [2] and incorporation of hydrogen clusters at positions interrupting the amorphous network [3] are considered to be the main reasons. Here we focus on void related microstructure effects in hydrogenated amorphous and microcrystalline germanium (Ge:H) films which are of interest for application in thin film silicon solar cells. These microstructure effects are studied by effusion measurements of hydrogen and of implanted helium and neon, by measurements of hydrogen-related infrared absorption and by the measurement of the post-deposition incorporation of contaminants. EXPERIMENT Several series of undoped Ge:H films were grown by plasma enhanced chemical vapour deposition (PECVD) using gas mixtures of GeH4 and H2 as process gases. For comparison, amorphous and microcrystalline Si:H films were also investigated. Deposition conditions are listed in Table 1. Crystalline Si and Ge wafers were used as substrates. The deposition rate ranged from 0.2 nm/s to about 0.5 nm/s. The crystallinity of µc-Ge:H was studied by Raman scattering by evaluating the crystalline Ge peak near 290 cm-1 with reference to the total Raman scattering signal. The infrared absorption of bonded hydrogen and the effusion of implanted helium (He) [4,5] and neon (Ne) [6] were used for void-related microstructure characterization. In order to avoid major implantation-related microstructure changes [4,5], He+ and Ne+ ions (40 and 100 keV, respectively) were implanted at doses ≤ 1016 cm-2 for the microstructure analysis. Gas effusion was performed as described elsewh
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