Tailoring the Structure of Low-Temperature-Deposited Microcrystalline Silicon Films by Biasing the Substrate
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Tailoring the Structure of Low-Temperature-Deposited Microcrystalline Silicon Films by Biasing the Substrate Mario Birkholz 1, Burkhardt Selle, Walther Fuhs, Don L. Williamson 2 Hahn-Meitner-Institut Berlin, Silizium-Photovoltaik, Kekuléstr. 5, 12489 Berlin, GERMANY. 1 present address: Fraunhofer Institut für Schicht- und Oberflächentechnik, Bienroder Weg 54E, 38108 Braunschweig, GERMANY. E-mail: [email protected]. 2 Colorado School of Mines, Department of Physics, Golden, CO 80401, USA. ABSTRACT Biasing the substrate during deposition and the substrate’s surface morphology may both have major effects on the structural properties of thin films. We present the results of structural investigations (Raman and FTIR spectroscopy, XRD, SAXS) of thin silicon films that were prepared at low temperatures by electron-cyclotron resonance (ECR) chemical-vapor deposition. The effect of substrate bias during the deposition was investigated for positive DC susceptor biases VB ranging from 0 to 45 V. For stainless steel substrates with an artificially enlarged surface roughness (smart substrates), an increase of the crystallinity could be observed with Raman spectroscopy. Films prepared under a susceptor bias of +15 V exhibited a texture inversion of preferential (220)- to (111)- oriented grains, which was accompanied by an increase in grain size from 18 to 42 nm. Small-angle X-ray scattering (SAXS) revealed the films as deposited on Al foil to exhibit significant free volume fractions (microvoids). The ability of tailoring the structure of thin Si films by applying a bias is discussed in terms of controlling the energy and intensity of ion flux to the surface of the growing film. This can efficiently be achieved in an ECR system, where the mean free path of gas particles exceeds the thickness of the plasma sheath. INTRODUCTION The electronic properties of poly- and microcrystalline thin Si films are significantly affected by structural features and concentrations of distinct lattice faults. The dimensions of silicon grains in microcrystalline silicon, µc-Si, films are typically of the order of some 10 nm which has to be improved in order to arrive at the required values of charge carrier mobilities for electronic applications. On the other hand, for large-area applications the deposition temperature should not exceed a limit of about 800 K, above which glass or other inexpensive substrates are harmed or electronically deleterious impurities are introduced via interdiffusion. For film growth, the crucial physical quantity is the surface diffusion coefficient of the film-forming species, which should preferably attain a higher value than allowed for by the deposition temperature. Within plasma-assisted thin-film technology, use is often being made of a bias voltage that is applied to the susceptor. By application of this technique ions impinge on the film surface with a kinetic energy of some eV, causing a significant increase of effective temperature due to their thermalization. Advantageously, the effect is combined with an impr
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