Preparation, Structure And Properties Of Silicon Serial Nanocrystalline Composite Films

In this thesis, the research history and status of hydrogenated nanocrystalline silicon films (nc-Si:H) are reviewed. Preparation methods, growth mechanism, structural, electrical and optical properties are produced in detail.In the present work, nanocrystalline silicon films (nc-Si:H), nanocrystalline silicon-carbon composite films (nc-SiC_x:H), nanocrystalline silicon-oxygen composite films (nc-SiO_x:H) and boron/phosphor doped nanocrystalline silicon films are deposited respectively, using SiH₄, C₂H₄, N₂O, B₂H₆, PH₃ and H₂ as resource gases. In the preparation progress, different deposition parameters, such as composition of reaction gases, substrate temperature, RF power, are adopted to obtain various films of silicon serious. AFM, XRD, TEM, IR, UV, XPS, Raman and PL are applied to characterize the structure and physical properties of the films. The relation between deposition conditions and film structure, properties is analyzed. A series of important conclusions and original results are revealed.Nanocrystaline silicon films of good quality in structure and optical, electrical properties are obtained by PECVD. The nature of the structure of nc-Si:H films is a two-phase composite framework, one of which is crystalline, another is amorphous. The Si crystals, prefer to be in Si(111) phase and 6¹0nm in size, are embedded in the amorphous matrix. The distribution of the crystals is random, and the crystalline proportion in the films can reach 60-70%.A theory about growth center is proposed to qualitatively explain the deposition process of silicon films, in the view of thermodynamics reaction. Large numbers of growth centers are generated on the substrate surface, due to the energy provided by plasma and the substrate temperature. Some reaction would occur, such as atoms gathering and nucleation, and thus films are formed. The concentration of growth centers is determined by the deposition parameters such as SiH₄/H₂ ratio, substrate temperature and RF power, and based on our experiments, proper deposition conditions are determined for nc-Si:H films. Nanocrystalline structure cannot be obtained if the density of growth centers is too large or too small. The density can beraised by or increasing SitLj/Fb ratio or RF power. The substrate temperature would provide energy for moving of the atoms on the growing surface, and the degree of crystallization can be enhanced by higher substrate temperature.Deposition progress of silicon films is quantitatively studied using molecular dynamics (MD) simulation. The deposited atoms occupy the majority of the dynamic energy in the reaction system, thus the diffusion of substrate atoms is blocked. The deposited atoms can form films, or be implanted into the substrate surface and bonded with the substrate atoms. The implantation will influence the Si-0 network in the substrate, reducing the bond angel of O-Si-0. The dynamics temperature (DT) of substrate is also decreased.Many properties, such as broadening of optical gap and increase of conductivity, are caused by small size. With crystal size increasing, the special properties of silicon films disappear gradually. Two kinds of transmission mechanics exist in the nc-Si:H films: crystalline and non-crystalline. With the increasing of crystalline portion, the electrical conductivity can be increased above 6 orders of magnitude.Successfully, B, P doped nc-Si:H films are directly prepared by PECVD method using B2H6 and PH3 as doping gas respectively. Controllable doping of nc-Si:H films of high efficiency is achieved. Introduction of phosphor atoms into silicon films is favorable for crystallization of silicon films, while boron atoms would hinder it. Boron or phosphor doping can both improve physical properties of nc-Si:H films, increasing the conductivity. As to doping efficiency, P is better than B evidently, because P atoms can enter silicon crystal network and displace the position of Si atoms. The conductivity of doped nc-Si:H films can reach 4.0X10"1D"1cm"1 when B2H6/SiH4 = 0.5%, and almost l'Q'W"1 when PH3/SiH4 = 0.5%, the increase extent reaching above 6 orders of magnitude.Nanocrystalline silicon-carbon composite films (nc-SiCx:H) are deposited by mixed gases of S1H4, C2H4 and H2. The structure nature of nc-SiCx:H films is quite similar to that of nc-Si:H films. The only difference is that there are carbon atoms in the amorphous network. The crystals are still Si, and no SiC crystals are observed.Introduction of C atoms can prevent crystallization of silicon films, and with increasing of CiHVSiH* ratio, the films gradually become amorphous from nanocrystalline. While C atoms enter silicon films, the conductivity will be decreased, while the optical gap will be increased evidently, which can reach 2.15eV in maximum.For the first time, it is proposed that nanocrystalline silicon-oxygen composite films (nc-SiOx:H) can be directly synthesized by PECVD method, using S1H4, N2O and H2 as resource gases, and photoluminescence (PL) phenomenon is observed from the as-deposited films without any post treatment,, with a center at about 530nm (2.34eV). Nano-scale silicon crystals, about 8nm in size, are embedded in the SiOx matrix. With N2O/SiH4 ratio increasing, content of oxygen in the films become higher. At the same time, the intensity of photoluminescence becomes stronger, and the peaks move towards shorter wavelength a little. Increase of substrate temperature can promote the crystallization of the films, and increase the O concentration in the films, raising the PL intensity. But the PL intensity will be stable and not sensitive to deposition in the region of higher substrate temperature, because Si crystals will grow to a large size at higher substrate temperature, decreasing the quantum effects. It is considered that the PL originates mainly from the defects at the interface and in the SiOx network, while it is also influenced by quantum effects of nano-scale Si crystals. To obtain films with good photoluminescence, it is essential to control the size of Si crystals, the content of Si crystals and the amount of oxygen related defects.