Effect of Strained Si-Si Bonds in Amorphous Silicon Incubation Layer on Microcrystalline Silicon Nucleation
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Effect of Strained Si-Si Bonds in Amorphous Silicon Incubation Layer on Microcrystalline Silicon Nucleation Hiroyuki Fujiwara, Michio Kondo, and Akihisa Matsuda National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba-shi, Ibaraki 305-8568, Japan ABSTRACT In order to investigate microcrystalline silicon (µc-Si:H) nucleation from the hydrogenated amorphous silicon (a-Si:H) phase, we performed a H2-plasma treatment of a-Si:H layers deposited at different temperatures. In the H-treatment experiment, the formation process of an infrared peak at ~1937 cm-1, assigned to SiHn (n=1~2) complex, is studied, as the SiHn complex is proposed to be a precursor for the µc-Si:H nucleation. With increasing the a-Si:H deposition temperature, the total amount of the SiHn complex formed by the H treatment increased. Nucleation of µc-Si:H under high H2-dilution conditions showed a clear relationship with the SiHn complex formed by the H treatment. The SiHn complex formation process, in terms of strained Si-Si bond breaking by H, is discussed. INTRODUCTION The phenomenon of microcrystalline silicon (µc-Si:H) nucleation from the hydrogenated amorphous silicon (a-Si:H) has been observed at low temperatures (50~450 oC) in plasma-enhanced chemical vapor deposition (PECVD) using SiH4 diluted by a large quantity of H2 [1-6]. Because µc-Si:H nucleation from the a-Si:H phase occurs at temperatures much lower than those required for solid phase crystallization (oC), the microscopic mechanism of µc-Si:H nucleation has been the subject of intensive research over the last two decades [1-6]. The importance of H in µc-Si:H nucleation is well recognized, as this process is very difficult to achieve at low temperature using pure SiH4. Although several models have been proposed to explain low-temperature H-induced µc-Si:H nucleation, the role of H in the µc-Si:H nucleation process remains highly controversial [1]. In our previous studies, we found a new infrared peak at 1937±5 cm-1 assigned to an SiHn (n=1~2) complex in a two-monolayer thick a-Si:H sub-surface region under µc-Si:H nucleation conditions [4-6]. We confirmed that the SiHn complex is created by insertion reaction of H into a strained Si-Si bond, and the SiHn complex shows a direct relationship with µc-Si:H nucleation. From these results, we proposed that the structural modification by H creates the chemically-active and flexible SiHn complexes in the a-Si:H sub-surface, which in turn induce µc-Si:H nucleation [5,6]. In this study, in order to expand our model for µc-Si:H nucleation, we investigated the formation process of the SiHn complex by a H2-plasma treatment. EXPERIMENTAL PROCEDURES We deposited a series of a-Si:H/µc-Si:H films at different deposition temperatures ranging from 180 and 370 oC by conventional rf PECVD [3-7]. All the films were deposited on crystal Si(100) substrates covered with 30 Å thick native oxides. The a-Si:H films (400 Å) deposited at the different temperature without using H2 dilution (R=[H2]/[SiH4]=0) were treated by the H2 p
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