The Effect of Hydrogen Dilution on the Hot-Wire Deposition of Microcrystalline Silicon

Abstract

The growth of amorphous (a-Si:H) and microcrystalline (pc-Si) silicon by hot-wire chemical vapor deposition (HWCVD) has been studied by combining in-situ ellipsometry, atomic force microscopy (AFM), and Raman spectroscopy. Generally a dense nucleation layer is formed during a-Si:H HWCVD, containing nuclei about 0.8 nm high and 10 to 20 nm in diameter. The surface roughness gradually increases with film thickness and settles at a root mean square (RMS) value of 1.6 nm at about 200 nm thickness. For hydrogen dilution at gas flow ratios x=[H2]/[SiH4] of 15 to 120 microcrystalline material was obtained. The grain size and nucleation layer, however, are strongly dependent on x. Low H2 dilution enhances the formation of an amorphous-like interface layer from which the μc-Si:H growth eventually starts. Increasing x promotes the etching of amorphous regions and the surface diffusion of precursors, resulting in larger nuclei. X = 30 yields extended μc-Si nuclei (30 nm height, 90 nm diameter) and a pronounced increase in surface roughness for thicker films, but suppresses the formation of the amorphous-like nucleation layer. A further increase in x remarkably lowers the growth rate, but smoother surfaces at comparable film thickness and larger lateral dimensions of the grains occur. This is interpreted as incipient etching of the crystallites.

Optimisation of doped microcrystalline silicon films deposited at very low temperatures by hot-wire CVD

“…This behaviour indicates a compensation of the unwanted n-type character of as grown undoped c-Si:H, usually associated to oxygen contamination. Similar results have been obtained by the group at IPE-Stuttgart [6] for similar boron concentrations (N D~1 0 19 cm -3 ) with a high vacuum HWCVD set up. By contrast, in the case of c-Si:H obtained by Very High Frequency CVD, the compensation was achieved at much lower concentrations (N D~1 0 16 cm -3 ) [11].…”

“…The approach of polycrystalline silicon with a large grain size either requires expensive substrates compatible with high temperature processes [3] or is very time consuming in recrystallization techniques [4]. Recently, much advances have been reported on the deposition of high quality intrinsic [5] and doped [6] c-Si:H by the HWCVD technique. Furthermore, the possibility to fabricate thin film devices such as solar cells [7] or field effect transistors [8] by this technique has also been demonstrated.…”

Optimization of Processing and Modeling Issues for Thin Film Solar Cell Devices Including Concepts for The Development of Polycrystalline Multijunctions: Annual Report; 24 August 1998-23 August 1999

“…The role of particular species in the formation of crystalline films is still unclear. While it has been accepted that SiH 3 , having the lowest sticking probability [30] , may promote ordered or crystalline films, there is also evidence of atomic hydrogen as the agent of crystalline film formation [31,32]. It is also possible that the factor determining crystalline fraction is not a single species, but the ratio of silane radicals to the atomic hydrogen flux to the growing film surface.…”

“…Consequently, the crystallinity of the films can be improved by increasing the substrate temperature and thus the mobility of surface species. [31,32]. This conclusion stems from the argument that higher hydrogen dilution leads to higher atomic hydrogen production at the wire.…”

Investigations on doping of amorphous and nanocrystalline silicon films deposited by catalytic chemical vapour deposition

“…Whereas the first advantage is not relevant to device fabrication, since doped layers are usually very thin, the others could result in improvements in the performance of devices fabricated by Cat-CVD. In spite of these potential advantages, only a few studies on doped silicon thin films deposited by this technique have been published [3][4][5][6][7][8], and more effort in the study of doped material produced by Cat-CVD should be done.…”