Abstract
In order to contribute to the understanding of the optoelectronics properties of hydrogenated nanocrystalline silicon thin films, a detailed study has been conducted. The samples were deposited by 13.56 MHz PECVD (Plasma-Enhanced Chemical Vapor Deposition) of silane argon mixture. The argon dilution of silane for all samples studied was 96% by volume. The substrate temperature was fixed at 200oC. The influence of depositions parameters on optical proprieties of samples was studied by UV-Vis-NIR spectroscopy. The structural evolution was studied by Raman spectroscopy and X-ray diffraction (XRD). Intrinsic-layer samples depositions were made in this experiment in order to obtain the transition from the amorphous to crystalline phase materials. The deposition pressure varied from 400 mTorr to 1400 mTorr and the rf power from 50 to 250 W. The structural evolution studies show that beyond 200 W, we observed an amorphous-nanocrystalline transition, with an increase in crystalline fraction by increasing rf power and working pressure. Films near the amorphous to nanocrystalline transition region are grown at reasonably high deposition rates (~10 /s), which are highly desirable for the fabrication of cost effective devices. The deposition rate increases with increasing rf power and process pressure. Different crystalline fractions (21% to 95%) and crystallite size (6 - 16 nm) can be achieved by controlling the process pressure and rf power. These structural changes are well correlated to the variation of optical proprieties of the thin films.
Highlights
For high quality solar cell applications, materials with high optical absorption, high carrier mobility and low fabrication cost are demanded
In order to contribute to the understanding of the optoelectronics properties of hydrogenated nanocrystalline silicon thin films, a detailed study has been conducted
The variation of deposition rate plotted as a function of argon dilution in silane as shown in Figure 1, with 1000 mTorr working pressure, 100 W rf power, 50 sccm silane flow and with different argon flow of 6, 50 and 1200 sccm
Summary
For high quality solar cell applications, materials with high optical absorption, high carrier mobility and low fabrication cost are demanded. On the other hand, hydrogenated amorphous silicon (a-Si:H) has high optical absorption, but it suffers from low carrier mobility, photo-induced degradation named StablerWronski effect [1,2] and, poor optoelectronic properties. Thin film hydrogenated nanocrystalline silicon (nc-Si:H) deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) emerged as a material for large-area electronics applications [3,4,5,6]. The fabrication cost for nc-Si:H optoelectronic applications is expected to be low, since thin films of nc-Si:H can be deposited directly over large-area substrates using the same fabrication facilities well established for a-Si:H devices. Plasma deposited hydrogenated nanocrystalline silicon (nc-Si:H) offers the possibilities of high carrier mobility and stability against Staebler-Wronski effects [6,7,8,9]
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