Better control over the onset of microcrystallinity in fast-growing silicon network
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In view of obtaining a Si:H network at the onset of microcrystallinity at a high deposition rate, we have adopted an intelligent approach to find out a tricky plasma condition in radio frequency (rf) plasma-enhanced chemical vapor deposition that provides a better control on growth introducing retarded microcrystallization. The deposition parameter includes a combination of high electrical power applied to the (SiH4+H2)-plasma and high gas pressure in the reaction chamber. High rf power increases the number density of film-forming precursors as well as atomic H density in the plasma, which helps to increase the film deposition rate and to promote microcrystallinity, respectively. In addition, high pressure helps not only to increase the film-growth rate by producing a dense plasma but also retards the microcrystallization process by increasing significantly the gas phase collision frequency and consequently reducing the effective reactivity of atomic H on the surface of a fast-growing Si:H network. A combination of high-power and high-pressure plasma conditions provides a reasonably wide range of H2 dilution to work with and better control in producing a Si:H network at the onset of microcrystallinity, while increasing the film-growth rate.
I. INTRODUCTION
Hydrogenated silicon (Si:H) films, both in amorphous and microcrystalline form, are very useful in thin film technologies, e.g., in the fabrication of devices like solar cells and thin film transistors. The c-Si:H is a mixed phase material with a composition of crystalline grains embedded in an amorphous matrix and the degree of microcrystallization is determined by the crystalline volume fraction of the network. Because of the favorable combination of high carrier mobility and high doping efficiency c-Si:H thin films are in significant use as component layers in solar cells. However, when used as intrinsic absorber layer in solar cell, it is required in large thickness of about 2–3 m because of its low absorption of solar radiation. Moreover, the stringent condition required for the fabrication of c-Si:H films by conventional plasma-enhanced chemical vapor deposition (PECVD) contributes a very low deposition rate, and that stands against the cost-effective throughput of the device using this material. On the contrary, the most stable aSi:H based solar cells have been reported to be obtained
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0338 J. Mater. Res., Vol. 19, No. 9, Sep 2004
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at just before the onset of microcrystallinity,1,2 and that has initiated a lot of work dealing with the material in the neighborhood of such structural transition.3–7 Accordingly, in the present work, we concentrate on the development of Si:H network at the onset of microcrystallinity and simultaneous increase in the film deposition rate with a goal of extending the material in the fabrication of cost-effective and stable solar cells. II. EXPERIMENTAL
The hydrogenated silic
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