Evaluation of Microcrystalline Silicon Films Deposited by Ultrafast Thermal Plasma CVD
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EVALUATION OF MICROCRYSTALLINE SILICON FILMS DEPOSITED BY ULTRAFAST THERMAL PLASMA CVD Yongkee Chae, Hiromasa Ohno, Keisuke Eguchi, and Toyonobu Yoshida The University of Tokyo, Dept of Materials Engineering, Hongo 7-3-1, Bunkyoku, Tokyo 113-8656, JAPAN.
ABSTRACT This research is the first attempt at applying thermal plasma chemical vapor deposition (TPCVD) for the ultrafast deposition of Si films for solar cells. A conventional deposition process of Si films, such as plasma-enhanced chemical vapor deposition (PECVD), is capable of a maximum deposition rate of approximately 5 Å/s and it takes a relatively long time to deposit an intrinsic layer. In this paper we report a novel ultrafast deposition approach using dc-rf hybrid TPCVD. The extreme improvement of stability, controllability, and cleanliness of the process enabled the deposition of microcrystalline Si films at the ultrafast rate of over 1000 nm/s, which is about 2000 times faster than that by conventional CVD. Moreover, a minimum defect density of 7.2x1016 cm-3 was achieved by post-treatment of the film in 2 Torr H2/Ar plasma. Monte-Carlo simulation and step coverage analysis suggested that the precursor is an approximately 1 nm cluster with a sticking probability of about 0.6. The success of this research will lead to the development of commercially viable technology in a roll-to-roll system in the near future, and will fundamentally change the established concepts of Si deposition technology. INTRODUCTION Solar cells based on hydrogenated amorphous silicon (a-Si:H) offer specific advantages such as a low process temperature, low-cost substrate materials, and a high absorption coefficient of sunlight. The absorption of light inside a-Si:H film, however, creates more defects and increases the density of trapping and scattering states and reduces the efficiency of the solar cell [1]. Therefore, microcrystalline Si (µc-Si) has attracted considerable attention for overcoming the deterioration of a-Si:H solar cells. The thickness required for the intrinsic layer of a µc-Si solar cell with a p-i-n cell structure is about 1-3 µm, which is about three times thicker than that of a-Si:H. Moreover, because deposition rates using a conventional deposition process, such as plasma-enhanced chemical vapor deposition (PECVD), is very low, below 1nm/s, it takes a relatively long time to deposit the i layer. Recent research in this field, therefore, has been focused on increasing the deposition rate of Si films without impairing the optoelectronic properties and causing dust formation. Madan et al. [2] and Madan and Morrison [3] used the hot-wire CVD technique to obtain poly-Si film with a high deposition rate (~1.5 nm/s), and Kessels et al.[4] also deposited solar-grade a-Si:H at a rate of 10 nm/s using a cascaded direct-current (dc) arc. Even if these techniques increased the growth rate to 10 times higher than that of conventional PECVD, they may not be sufficient for actual A4.4.1
applications. In this paper we report a novel ultrafast deposition approach of using d
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