Effect of Annealing on Microstructure in (Doped and Undoped) Hydrogenated Amorphous Silicon Films
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Effect of Annealing on Microstructure in (Doped and Undoped) Hydrogenated Amorphous Silicon Films W. Beyer1, 2, W. Hilgers2, D. Lennartz2, F.C. Maier2, 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 Laser heating and annealing of hydrogenated amorphous silicon (a-Si:H) films is of interest for improved material properties. Due to the variety of possible laser treatments with regard to wavelength, pulse duration, scan time etc., the definition of laser impact on the material is a challenge which we try to approach by comparing properties of laser and oven treated materials. Here we report on the effect of oven heat treatment (up to TA= 575°C) on microstructure and hydrogen content of hydrogenated amorphous silicon films, as detected by measurements of infrared absorption and of effusion of hydrogen as well as of implanted helium. The latter technique has been found to measure isolated voids (cavities) of the size of silicon divacancies and larger. Undoped as well as phosphorus and boron doped plasma-deposited aSi:H films of various hydrogen content (< 15 at.%) were investigated, including undoped device grade a-Si:H. The results show little indication for void-related microstructure in the asdeposited and annealed state for material with a concentration of silicon bonded hydrogen below 5 at. %. At higher hydrogen concentration, evidence is found that hydrogen out-diffusion due to annealing causes isolated voids in concentrations up to about 1020 cm-3. A possible mechanism for the annealing induced (micro-)void generation is discussed. INTRODUCTION Annealing effects in amorphous silicon (a-Si) are of interest for various reasons. On the one hand any deposition at elevated temperatures involves annealing effects of underlying layers or parts of the film. On the other hand, large area laser annealing may open possibilities for change and improvement of deposited films and devices, e.g. a-Si based solar cells. However, due to the variety of possible laser treatments with regard to wavelength, pulse duration, repetition rate, scan time etc., an improved understanding of laser annealing related changes in the material (along with a thorough characterization of the laser annealed state) are required. We try to approach this task by comparing laser-treated material with oven annealed material. We focus on annealing effects on void-related microstructure, which is an important property of a-Si materials. We characterize the microstructure by measurements of infrared absorption as well as effusion of implanted helium. Since helium does not react with the silicon network, helium effusion is known to be sensitive to microstructure and, in particular, isolated voids are detected since they trap diffusing helium [1]. Note, however, that only cavities equa
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