Ion-assisted Sputter Deposition of Microcrystalline Silicon Films with Pulsed-DC Plasma Excitation
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Ion-assisted Sputter Deposition of Microcrystalline Silicon Films with Pulsed-DC Plasma Excitation P. Reinig, F. Fenske, B. Selle, W. Fuhs Hahn-Meitner-Institut Berlin, Abt. Silizium-Photovoltaik, Kekuléstr.5, D-12489 Berlin, Germany ABSTRACT We have applied a novel Ion-Assisted Sputter Deposition (IASD) method to deposit microcrystalline silicon (µc-Si) thin films with high deposition rates. An unbalanced magnetron sputter source together with an asymmetrical bipolar pulsed-DC plasma excitation operating in the frequency range 50-250 kHz was used for realizing high ion fluxes to the growing film. µc-Si films of high crystallinity are obtained at T > 400 °C with growth rates of up to 90 nm/min. The crystallinity of the films is characterized by the thickness-independent ratio of the optical thickness n⋅d determined by FTIR measurements and the atomic area density N⋅d as given by RBS analysis. INTRODUCTION In the field of silicon thin film solar cells there is a increasing demand of high rate deposition processes of polycrystalline films. The major problem consists in the necessity to achieve high crystallinity with reasonably high deposition rates at the low temperatures compatible with the glass substrates. A possible way to overcome this restriction is the promotion of low-temperature (< 500 °C) growth by a suitable bombardment of the growing film with hyperthermal particles. For instance it was shown that silicon homoepitaxy at low temperatures can be enhanced by controlling the energy of Si+ ions1 in a narrow temperature dependent energy window of 20±10 eV. Similar results have been obtained in case of bombardment by rare gas ions2. Ion bombardment of the growing film is inherent to sputtering processes. Under controlled conditions it can be used in a beneficial way for low temperature growth of epitaxial silicon3. The key feature is to realize high ion flux densities to the growing film while keeping the ion energy in an optimum regime. Possible incorporation of Ar is a typical drawback of the sputter deposition process. Recently there has been considerable interest in time-modulated deposition processes. Pulsed magnetron sputtering shows an enhanced plasma density4 in comparison to continuous plasma excitation. There are reports5 on metal ion plasma densities exceeding 1012 cm-3 in case of pulsed magnetron sputter deposition. Furthermore, bipolar pulsing of a sputter target can be used to prevent target charging effects6. A special magnetic field configuration of the sputter source, the so-called „unbalanced magnetron“7 results in higher ion flux densities to the substrate. In this paper we report on compositional and structural properties of amorphous and microcrystalline (µc) silicon films prepared with a novel sputtering technique. We focused on the characterization of the structure (degree of crystallinity), which we studied by Rutherford backscattering (RBS), infrared (IR) and Raman spectroscopy. As was already reported in a detailed study on optical constants of sputtered a-Si8, the accuracy of the measu
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