Microstructure Characterization of Hydrogenated Amorphous Silicon Films by Rare Gas Effusion Studies
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Microstructure Characterization of Hydrogenated Amorphous Silicon Films by Rare Gas Effusion Studies Wolfhard Beyer Institut für Photovoltaik, Forschungszentrum Jülich, D-52425 Jülich, Germany
ABSTRACT The effusion of the rare gases neon and helium, as well as of hydrogen, was studied for plasma deposited (boron-doped and undoped) hydrogenated amorphous silicon films, grown at various substrate temperatures. Rare gas atoms were incorporated into the material during the growth process or by ion implantation. The results suggest that helium and neon effusion spectra give information on the material microstructure. INTRODUCTION Recently the effusion of implanted rare gas atoms Ne and Ar was used for microstructure characterization of amorphous silicon-carbon (a-Si:C:H) and silicon-oxygen (a-Si:O:H) alloys [1,2]. Since rare gas atoms do not react with silicon, their (out-) diffusion can give information on the presence of voids or openings in the material [3]. Depending on preparation conditions, the effusion peak temperature e.g. of implanted neon was found to vary between T1000°C, thus being a sensitive probe for characterization of film microstructure. For compact a-Si:H, however, previous work [4] has shown effusion temperatures in the low temperature range only for implanted helium, while implanted Ne and Ar atoms were found to effuse at high temperatures (above the temperature of crystallization) and thus cannot give information on details of microstructure of as-deposited material. Aim of this work is to explore the applicability of the rare gas effusion method to the characterization of a-Si:H films. We discuss effusion results of implanted neon and helium as well as of neon incorporated into the material during the deposition process. EXPERIMENT The a-Si:H films containing Ne were deposited in an electron cyclotron resonance (ECR) plasma deposition system. In a remote plasma arrangement, neon was used as a carrier gas and silane (SiH4) as a process gas. The gas pressure was 10-2 mbar, the gas flows of Ne and SiH4 were 30 and 5 sccm, respectively and the ECR power was 400W. The a-Si:H films used for effusion measurements of implanted Ne and He were deposited by the standard glow-discharge technique at an rf power (13.56 MHz) of 10 W, a flow of 3 sccm and a pressure of 0.5 mbar. Undiluted silane was used as a process gas. For boron doping, flows of diborane (diluted in H2) were added. As a substrate, crystalline silicon or quartz platelets were used. Typical film thickness was 1.5 µm. The Ne+ and He+ ion implantations were performed at room temperatue using a mass separator. Implantation energies were 100 keV for the neon ions and 30-40 keV for the He ions, resulting in depth distributions of neon and helium with maxima at depths of approximately 0.2 µm and 0.3 –0.4 µm, respectively, estimated by the TRIM routine [5]. The implantation dose was 1016 cm-2. Effusion measurements were performed as described elsewhere [6] with a heating rate of 20 °C/min.
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RESULTS Neon Effusion Studies In Figs 1a and b, effu
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