Micro-crystalline silicon-germanium thin films prepared by the multi-target RF sputtering system
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Micro-crystalline silicon-germanium thin films prepared by the multi-target RF sputtering system Toru Ajiki, Isao Nakamura and Masao Isomura Department of Electrical and Electronic Engineering, Tokai University 1117 kitakaname, Hiratsuka, Kanagawa 259-1292, JAPAN ABSTRACT Micro-crystalline silicon-germanium (µc-SiGe) films were prepared by the multi-target RF sputtering system using Ar and Ar-H2 mixture gases. The crystallization temperature of Si0.3Ge0.7 films is reduced from 600 ˚C to 400 ˚C by the introduction of H2 into the sputtering gases. The dark conductivity of 1.7x10-7 S/cm and one order of magnitude of photosensitivity are obtained with the H2/Ar flow ratio of 2. The activation energy of dark conductivity is 0.42 eV, which is a half of the energy gaps of Si0.3Ge0.7, and show that the films have intrinsic nature. Besides, the absorption coefficients are similar to those of single crystalline Si0.3Ge0.7. The results suggest that the H2 introduction is effective both to reduce the dangling bond defects and to decrease the crystallization temperature of the µc-SiGe films.
INTRODUCTION Next generation thin-film solar cells are required to achive higher efficiency, lower cost and larger area. At the moment, micro-crystalline silicon has been paid attention as a new material for thin film solar cells, and the two stack solar cells with amorphous silicon top cells are already produced [1,2]. However, the conversion efficiency of thin-film solar cells is not enough to satisfy the requirements described above. In order to improve the conversion efficiency of thin film solar cells, it is important to make more efficient use of the sunlight by optimizing multi-junction structures. In this report, we proposed micro-crystalline silicon-germanium (µc-SiGe) as a bottom cell material of multi-junction solar cells, which improve the sensitivity in longer wavelength region because of its narrow energy gap. The µc-SiGe films were mostly prepared by the plasma enhanced chemical vapour deposition (PE-CVD), using the SiH4, GeH4 and H2 gases [3,4]. These gases are toxic for the environment and require the disposal system to remove the toxicity. In order to pursue safety and lower-cost processes, we investigate a magnetron sputtering method without toxic gases. The magnetron sputtering method is expected to realize lower cost and lager area processes. EXPERIMENTAL SiGe thin films were prepared on corning #7059 substrates by a radio-frequency (RF) magnetron sputtering system. The sputtering system has two targets, Si and Ge, and the composition of SiGe films were controlled by varying the RF power ratio applied to the Si and Ge targets. Here, the Ge content of SiGe films was maintained around 70 %. The sputtering was
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done with Ar or Ar and H2 mixture gases and the substrate temperature was 400-600 ˚C. The composition was measured by the X-ray photoelectron spectroscopy (XPS). The crystallinity of SiGe films was evaluated by the Raman spectroscopy and the X-ray diffraction (XRD). The Raman spectroscopy measuremen
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