Nucleation Mechanism of Microcrystalline Silicon Studied by Real Time Spectroscopic Ellipsometry and Infrared Spectrosco
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Nucleation Mechanism of Microcrystalline Silicon Studied by Real Time Spectroscopic Ellipsometry and Infrared Spectroscopy Hiroyuki Fujiwara, Yasutake Toyoshima, Michio Kondo, and Akihisa Matsuda Thin Film Silicon Solar Cells Super Laboratory, Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305-8568, Japan ABSTRACT We have characterized a-Si:H initial layers for µc-Si:H nucleation by real time spectroscopic ellipsometry (SE) and infrared attenuated total reflection spectroscopy (ATR) to investigate the µc-Si:H formation mechanism. By performing Ar plasma treatment of a-Si:H layers, we confirmed a presence of a 2 monolayer thick sub-surface in a-Si:H layers. In the a-Si:H sub-surface that leads to the µc-Si:H nucleation, an important peak at ~1937 cm-1 assigned to the SiHn complex was found in the ATR spectra. From H2 plasma treatment experiments, we proposed that this SiHn complex is formed by H insertion into strained Si-Si bonds. The SiHn complex formed in the a-Si:H sub-surface showed a clear relationship with the µc-Si:H nucleation. From these results, we conclude that the µc-Si:H nucleation occurs by the formation of the chemically active and flexible SiHn complexes in the 2 monolayer thick a-Si:H sub-surface. INTRODUCTION Recently, microcrystalline silicon (µc-Si:H) fabricated by a plasma deposition process on inexpensive substrates, such as glass and stainless steel, has attracted a wide attention for a solar cell device application [1]. Generally, the µc-Si:H nucleation occurs from a hydrogenated amorphous silicon (a-Si:H) layer in a thickness of 5~500 Å on the substrates, when films were deposited by hydrogen dilution of SiH4 source gas. In order to control the µc-Si:H film structure for the device application, there is a critical need to understand the µc-Si:H nucleation mechanism. For this purpose, intensive research efforts have been made to clarify this mechanism in the last two decades [2-13]. Nevertheless, several models proposed for the µc-Si:H nucleation remain controversial [2-13], and the role of hydrogen dilution on the µc-Si:H nucleation has not been understood well. Real time measurements provide a desirable tool to monitor a complicated structural change induced by the µc-Si:H nucleation as a function of thin film thickness [9-13]. In particular, real time spectroscopic ellipsometry (SE) performed in ultraviolet/visible region has been applied successfully to determine microscopic µc-Si:H structures including surface roughness and bulk layer thickness [11,12]. For the purpose of deducing the possible role of hydrogen in the µc-Si:H nucleation, real time measurement of infrared spectra collected by attenuated total reflection spectroscopy (ATR) is most promising, owing to its measurement sensitivity exceeding a single monolayer [12-18]. The combination of real time SE with real time ATR, therefore, provides a greater capability to characterize the µc-Si:H nucleation process, since microscopic structures and SiHn (n=1~3) bonding states can be obtained simultaneously [12,14]. In
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