Structural Nature of Nanocrystalline Silicon
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STRUCTURAL NATURE OF NANOCRYSTALLINE SILICON WEIQIANG HAN, GAORONG HAN, JIANMIN QIAO*, PIYI DU, ZHISHANG DING, JING XU, AND ZHIJIAN SHEN Department of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China *Microelectronics Laboratory, Santa Clara University, Santa Clara, CA 95053
ABSTRACT Nanocrystalline silicon (nc-Si:H) films with an average grain size ranging between 2 and 10 nm are prepared in a modified plasma chemical vapor deposition system. X-ray diffraction, transmission electron diffraction, and high resolution electron microscope are used to elucidate the structural nature in the deposited nc-Si:H films. The lattice network of the deposited silicon films changes from disordered structure to ordered structure with the increasing of plasma energy. The results also show that the crystal lattice of the nc-Si:H film is distorted from those of microcrystalline silicon and crystalline silicon. In the observed x-ray diffraction, there are two anomalous peaks at 20 = 29.50 and 32.50 of Si besides the normal peaks at 20 = 28.50 of Si(11) and 20 = 47.30 of Si(220). By the high resolution electron microscopy study, a new crystallography structure with distorted Si(111) crystallites in ncSi:H films is found. Based on our results, a structure model of nc-Si:H films is proposed.
INTRODUCTION Nanocrystalline materials have been derived from the area of solid materials since 1984 [1], and nanocystalline silicon (nc-Si:H) film has been reported since 1986 [2-5]. Nc-Si:H films are composed of many ultra-fine silicon particles and each of them is a perfect crystallite with a size of about 10 nm. The electrical, optical, and structural properties of nc-Si:H are quite different from those of amorphous silicon (a-Si:H) and microcrystalline silicon (pc-Si:H) films, i.e., widening of the optical bandgap [6], visible photoluminescence effect [7], and resonant tunneling effect [8]. These properties are promising for applications to optoelectronics and ultra large scale integrated circuits. Although this material has been investigated for several years, some questions remain concerning its microstructure, growth mechanism, and nucleation, etc. In this paper, we have investigated the structural nature of the nc-Si:H films and their growth processes.
EXPERIMENTS The nc-Si:H films are prepared by a radio frequency (RF) grow discharge from a gas mixture of 3% SiH 4 and 97%H 2 in a modified plasma chemical vapor deposition system. The deposition parameters are: chamber pressure of 70 pa, Mat. Res. Soc. Symp. Proc. Vol. 297. '1993 Materials Research Society
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substrate temperature of 320 'C, RF power between 0.20 and 0.86 W/cm 2 , and gas mixture flow rate of 50 sccm. The crystallography structural property of the deposited films is determined by the X-ray diffraction (XRD) and transmission electron diffraction (TED). The microstructure of the deposited films is characterized by the high-resolution transmission electron microscopy (HREM). The samples are deposited on polished silicon(100) for XRD measure
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