Microstructure Characterization of Amorphous Silicon Films by Effusion Measurements of Implanted Helium
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Microstructure Characterization of Amorphous Silicon Films by Effusion Measurements of Implanted Helium W. Beyer1, 2, 3, W. Hilgers2, D. Lennartz2, F.C. Maier3, N.H. Nickel1, F. Pennartz2, P. Prunici3 1 Institut für Silizium-Photovoltaik, Helmholtz-Zentrum Berlin für Materialien und Energie, Kekuléstrasse 5, D-12489 Berlin, Germany 2 IEK5-Photovoltaik, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany 3 Malibu GmbH & Co.KG, Böttcherstrasse 7, D-33609, Bielefeld, Germany ABSTRACT An important property of thin film silicon and related materials is the microstructure which may involve the presence of interconnected and isolated voids. We report on effusion measurements of implanted helium (He) to detect such voids. Several series of hydrogenated and unhydrogenated amorphous silicon films prepared by the methods of plasma deposition, hot wire deposition and vacuum evaporation were investigated. The results show common features like a He effusion peak at low temperatures attributed to He out-diffusion through a compact material or through interconnected voids, and a He effusion peak at high temperatures attributed to He trapped in isolated voids. While undoped plasma-grown device-grade hydrogenated amorphous silicon (a-Si:H) films show a rather low concentration of such isolated voids, its concentration can be rather high in doped a-Si:H, in unhydrogenated evaporated material and others. INTRODUCTION Void-related microstructure is known to be an important property of thin film silicon materials. Voids, i.e. volume parts which are empty or have a strongly reduced mass density may cause electronic defect states like dangling bonds or strained Si-Si bonds as they act as internal surfaces. Effusion measurements which detect gases leaving a given material on heating can often distinguish between compact material which may contain isolated voids and material with interconnected voids [1]. Interconnected voids, i.e. channels of reduced mass density throughout the material may provide rapid out-diffusion paths for hydrogen molecules as well as indiffusion paths for oxygen or water molecules. Reduced electronic quality of amorphous silicon (a-Si) and related alloys has often been associated with the presence of clustered hydrogen and hydrogen-related interconnected voids [2]. Isolated voids, on the other hand, may trap gas atoms or molecules and high pressure molecular hydrogen in such voids (of 10-20 Å diameter) has been reported [3]. Here we use effusion of implanted helium to detect and characterize voidrelated microstructure. He effusion curves are expected to give information on material imperfections since He does not react with the silicon host material [4]. Low temperature He effusion has been associated with interconnected voids [4]. A high temperature He effusion stage has been attributed to the presence of isolated voids and a parameter F* was defined to estimate the isolated void concentration [5]. We report on effusion measurements of hydrogen and implanted helium for series films of hydrogenated amorphous silic
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