Comparative Study of Solid-Phase Crystallization of Amorphous Silicon Deposited by Hot-wire CVD, Plasma-Enhanced CVD, an
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0989-A16-04
Comparative Study of Solid-Phase Crystallization of Amorphous Silicon Deposited by Hotwire CVD, Plasma-Enhanced CVD, and Electron-Beam Evaporation Paul Stradins1, Oliver Kunz2, David L. Young1, Yanfa Yan1, Kim M. Jones1, Yueqin Xu1, Robert C. Reedy1, Howard M. Branz1, Armin G. Aberle2, and Qi Wang1 1 National Renewable Energy Laboratory, Golden, CO, 80401 2 The University of New South Wales, Sydney, Australia ABSTRACT Solid-phase crystallization (SPC) rates are compared in amorphous silicon films prepared by three different methods: hot-wire chemical vapor deposition (HWCVD), plasma-enhanced chemical vapor deposition (PECVD), and electron-beam physical vapor deposition (e-beam). Random SPC proceeds approximately 5 and 13 times slower in PECVD and e-beam films, respectively, as compared to HWCVD films. Doping accelerates random SPC in e-beam films but has little effect on the SPC rate of HWCVD films. In contrast, the crystalline growth front in solid-phase epitaxy experiments on (100)-oriented silicon wafer substrates propagates at similar speed in HWCVD, PECVD, and e-beam amorphous Si films. This strongly suggests that the observed large differences in random SPC rates originate from different nucleation rates in these materials while the grain growth rates are relatively similar. The larger grain sizes observed for films that exhibit slower random SPC support this suggestion.
INTRODUCTION Solid-phase crystallization (SPC) of amorphous Si (a-Si) has become an important process in the thin-film electronics and the solar cell industry [1]. In these applications, obtaining large (at least a few microns), high-quality grains is essential. Thermal annealing of a-Si films typically produces submicron grains with a broad size distribution [2]. Laser crystallization [3] can produce uniform, larger grains but not with an inexpensive batch process. The electronic quality of thermally crystallized grains can be significantly improved by Rapid Thermal Processing (RTP) and subsequent H-passivation [4], enabling inexpensive large-scale production of devices with reasonable efficiencies [5]. Nevertheless, a deeper understanding of thermally induced nucleation and subsequent growth in solid phase, as well as the role of impurities and network microstructure is necessary for further progress. We have previously found that a-Si:H films deposited by hot-wire (HW) chemical vapor deposition (CVD) and plasma-enhanced CVD (PECVD) have different SPC rates at a given temperature [6, 7]. This indicates that the network microstructure might play an important role in SPC. In this work, we compare SPC of a-Si prepared by HWCVD, PECVD, and electron beam physical vapor deposition. By comparing the random SPC and solid-phase epitaxy (SPE) in the same films, we demonstrate that it is most likely the nucleation that leads to the observed differences in overall random SPC rates. The role of dopants in the SPC of these films is also addressed.
EXPERIMENTAL The HWCVD and PECVD a-Si:H films were grown at NREL on Corning 1737 glass substr
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