INTRINSIC NANO-CRYSTALLINE SILICON THIN FILMS DEPOSITED BY HOT-WIRE
CHEMICAL VAPOUR DEPOSITION
C. J. Arendse 1 , D. Knoesen 1 , S. Halindintwali 1 , T. F. G. Muller 1 , C. J. Oliphant 1, 2 and G. F. Malgas 3
1 Dept of Physics, University of the Western Cape, Bellville, 2 National Metrology Institute of South
Africa, Pretoria, 3 CSIR National Centre for Nano-Structured Materials, Pretoria
Hydrogenated nano-crystalline silicon (nc-Si:H) has been identified as a potential candidate for
Furthermore, we have developed an analytical method to successfully model the optical thin film photovoltaic cells, mainly due to its reduced photo-induced degradation and increased conductivity 1 . Over the last decade UWC has been active in this research arena, with the main objective of understanding the growth process and optimizing the deposition conditions for devicequality nc-Si:H. In this contribution, we report on properties of amorphous/ nano-crystalline silicon and silicon-alloyed thin films. By adopting the effective medium approximation, a single optical model could be constructed that could model material that was in different structural phases 3 .
The modeled optical absorption spectra were correlated with that directly measured from the extension of the filament-lifetime during the hot-wire chemical vapour deposition (HWCVD) process, the growth process and the photoinduced stability of nc-Si:H.
The HWCVD process relies on the catalytic action of a tantalum filament to crack silane and hydrogen gas into SiH x
radicals and atomic hydrogen that impinge on a heated substrate where thin film growth will occur. Results show that pre- and post-deposition annealing of the Tafilament in a hydrogen ambient at its deposition temperature improves its lifetime substantially 2 .
This is due to the formation of an encapsulating silicon layer at the sensitive, low temperature regions (electrical contact points), which act as an photothermal deflection spectroscopy and the modeled absorption closely followed the same trends.
References
1.
V. Shah et al. (2003) Sol. Energy Mater.
Sol. Cells 78, 469.
2.
D. Knoesen et al. (2008) Thin Solid
Films 516, 822
3.
T. F. G. Muller et al. (2010) Phys. Status
Solidi C 7(3-4), 750 inhibitor for further silicide formation.
The influence of hydrogen dilution and deposition time on the evolution of the nano-structural properties of the nc-Si:H thin films was investigated to provide information of the growth process. The growth in the crystallite size is accompanied by a reduction in the compressive strain within the crystalline region and an improved ordering and reduction in the tensile stress in the amorphous network. Surface diffusion determines the growth in the amorphous regime, whereas the diffusion of hydrogen within the film controls the growth during the transition from amorphous to nanocrystalline silicon. Fig. 2 shows a cross-sectional scanning electron c-Si substrate
Figure 1. SEM micrograph of a nc-Si:H thin film. microscopy (SEM) micrograph of a nc-Si:H thin film, where the conical shape of the crystallites is evident.
The photo-induced stability of the nc-Si thin films was investigated by probing the changes in the photo-response as a function of light-soaking time. The stability has improved significantly as compared to that of hydrogenated amorphous silicon (a-Si:H). Corresponding author: cjarendse@uwc.ac.za