Improved Crystallinity of Microcrystalline Silicon Films Using Deuterium Dilution
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Improved Crystallinity of Microcrystalline Silicon Films Using Deuterium Dilution
Susumu Suzuki, Michio Kondo, and Akihisa Matsuda Thin Film Silicon Solar Cell Superlab, Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan ABSTRACT Deuterium dilution instead of hydrogen has been studied in microcrystalline silicon growth using plasma enhanced chemical vapor deposition with monosilane. It was found that the crystallinity for D2 dilution is significantly improved as compared to that for H2 dilution at the same growth rate. Optical emission spectroscopy measurement shows that the electron temperature of SiH4 + D2 plasma is lower than that of SiH4 + H2, indicating that the bombarding ion energy is reduced for D2 dilution. It was also found that the H-D exchange reaction on the surface has a certain threshold number of events and that microcrystalline formation occurs only above the threshold. The role of atomic hydrogen originating from a diluent in crystal formation is discussed. INTRODUCTION Microcrystalline silicon (µc-Si) films grown at low temperatures by plasma-enhanced chemical vapor deposition (PECVD) are considered to be a promising material for thin film silicon solar cells with high conversion efficiency and long term stability[1-3]. Because of the indirect band gap of crystalline silicon, film thickness of more than 1 µm is necessary for sufficient absorption of sun light and therefore high-rate growth of well crystallized µc-Si films is required for low cost commercial production. Recent results have shown that good crystallinity µc-Si films can be deposited at several nm/s by a high pressure depletion method[4,5]. In this method, a great amount of atomic hydrogen generated in the plasma owing to the depletion of SiH4 is considered to promote crystallization without increasing ion bombardment under a high deposition pressure condition[4,5]. It has been pointed out that uniform hydrogen coverage of film-growing surface is important for the growth of highly crystallized films[6]. A role of impinging atomic hydrogen flux, however, is still unclear. In the present study, we have deposited µc-Si films by PECVD using D2 diluted SiH4 source gas and investigated effects of atomic deuterium generated from D2 gas on the deposition rate and crystallinity. As a result, we could obtain better crystallinity by using deuterium dilution at the same growth rate. EXPERIMENTAL Samples were deposited under a high pressure of 2 Torr using VHF-PECVD (f = 60 MHz) as reported previously[4,5]. The electrode size and distance were 128 cm2 and 17 mm, respectively. Synthetic silica (0.5 mm thick), Corning 7059 glass (0.7 mm thick) and (111) oriented crystalline silicon wafer (0.5 mm thick) were used as substrates. Several series of
A19.5.1
Table I . Si films deposited in the present study series gas flow rate(sccm) power(W) I SiH4/D2(H2) 10/90, 30/270, 50/450 3−135 II SiH4/D2 5 10/180−10/540 10/90 III SiD4/H2 15−135 10/90 IV SiH4/D2 15−135
substrate temp.(˚C) 250 250 250 250, 350, 450
samples were deposite
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