Thermally Induced Nano-Structural and Optical Changes of nc-Si:H Deposited by Hot-Wire CVD

Nanoscale Research Letters, Apr 2009

We report on the thermally induced changes of the nano-structural and optical properties of hydrogenated nanocrystalline silicon in the temperature range 200–700 °C. The as-deposited sample has a high crystalline volume fraction of 53% with an average crystallite size of ~3.9 nm, where 66% of the total hydrogen is bonded as ≡Si–H monohydrides on the nano-crystallite surface. A growth in the native crystallite size and crystalline volume fraction occurs at annealing temperatures ≥400 °C, where hydrogen is initially removed from the crystallite grain boundaries followed by its removal from the amorphous network. The nucleation of smaller nano-crystallites at higher temperatures accounts for the enhanced porous structure and the increase in the optical band gap and average gap.

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Thermally Induced Nano-Structural and Optical Changes of nc-Si:H Deposited by Hot-Wire CVD

Nanoscale Res Lett Thermally Induced Nano-Structural and Optical Changes of nc-Si:H Deposited by Hot-Wire CVD C. J. Arendse 0 1 2 3 G. F. Malgas 0 1 2 3 T. F. G. Muller 0 1 2 3 D. Knoesen 0 1 2 3 C. J. Oliphant 0 1 2 3 D. E. Motaung 0 1 2 3 S. Halindintwali 0 1 2 3 B. W. Mwakikunga 0 1 2 3 0 B. W. Mwakikunga School of Physics, University of the Witwatersrand , Private Bag 3, P.O. Wits, Johannesburg 2050 , South Africa 1 C. J. Arendse T. F. G. Muller D. Knoesen C. J. Oliphant D. E. Motaung S. Halindintwali Department of Physics, University of the Western Cape , Private Bag X17, Bellville 7535 , South Africa 2 C. J. Arendse (&) G. F. Malgas (&) C. J. Oliphant D. E. Motaung B. W. Mwakikunga CSIR National Centre for Nano-Structured Materials , P.O. Box 395, Pretoria 0001 , South Africa 3 B. W. Mwakikunga Department of Physics, University of Malawi , The Polytechnic, Private Bag 303, Blantyre , Malawi We report on the thermally induced changes of the nano-structural and optical properties of hydrogenated nanocrystalline silicon in the temperature range 200700 C. The as-deposited sample has a high crystalline volume fraction of 53% with an average crystallite size of *3.9 nm, where 66% of the total hydrogen is bonded as :Si-H monohydrides on the nano-crystallite surface. A growth in the native crystallite size and crystalline volume fraction occurs at annealing temperatures C400 C, where hydrogen is initially removed from the crystallite grain boundaries followed by its removal from the amorphous network. The nucleation of smaller nano-crystallites at higher temperatures accounts for the enhanced porous structure and the increase in the optical band gap and average gap. Hot-wire CVD; Quantum size effects; Nano-crystallite; Optical band gap Introduction Hydrogenated nanocrystalline silicon (nc-Si:H) has been the subject of intense scientific and technological interest over the past decade, mainly due to its reduced photoinduced degradation [ 1 ], efficient visible photoluminescence [ 2 ], tailored optical band gap [ 3 ], increased conductivity and greater doping efficiency [ 4 ]. It has been highlighted that these unique features are a direct cause of the quantum size effects of the silicon nano-crystallites. These improvements make nc-Si:H a potential candidate for application in photovoltaic and opto-electronic devices [ 5, 6 ]. The hot-wire chemical vapour deposition (HWCVD) technique, based on the catalytic decomposition of the precursor gasses by a heated transition metal filament, has been established as a viable deposition technique for nc-Si:H thin films [ 6, 7 ]. The structural and opto-electronic properties of the thin films are dependent on the deposition parameters, of which the hydrogen dilution and substrate temperature are the most crucial. It has been established that the etching effect of atomic hydrogen, created by the catalytic decomposition of H2, is responsible for the termination of weak Si–Si bonds from the surface and subsurface regions and that the nucleation of the nano-crystallites are improved by increasing the hydrogen dilution [ 7–10 ]. It has also been reported that the hydrogen dilution during deposition determines the concentration and the distribution of hydrogen in nc-Si:H, which is closely related to the nano-structural features; i.e. crystallite size and crystalline volume fraction [ 11–14 ]. These nano-structural features eventually determine the optical properties of the material. In particular, the quantum size effects of the Si nano-crystallites and the hydrogen concentration have a strong correlation with the optical band gap [ 15, 16 ]. An investigation into the role of hydrogen in nc-Si:H is therefore crucial for the understanding of its relation to the nano-structure and the optical properties. In this contribution, we investigate the effects of the hydrogen concentration and bonding configuration in nc-Si:H deposited by HWCVD on the nano-structural features and the optical properties. The hydrogen concentration and bonding configuration were controlled by post-deposition isochronal annealing. Experimental The nc-Si:H thin film was deposited by the HWCVD process simultaneously on single-side polished h100i crystalline silicon and Corning 7059 glass substrates, using a mixture of 4 sccm SiH4 and 26 sccm H2 decomposed by seven parallel tungsten filaments, 15 cm apart and 36 cm away from the substrates. A detailed description of the experimental set-up is given elsewhere [ 17, 18 ]. The filament temperature, substrate temperature and deposition pressure were fixed at 1600 C, 420 C and 60 lbar, respectively. The as-deposited nc-Si:H thin film was *1140 nm-thick, as measured using a Veeco profilometer. Subsequent annealing was performed under high-purity, flowing N2 gas in a tube furnace at annealing temperatures (TA) ranging from 200 to 700 C in 100 C increments. The N2 flow rate, heating rate and dwell time for all temperatures amounted to 3 (...truncated)


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CJ Arendse, GF Malgas, TFG Muller, D Knoesen, CJ Oliphant, DE Motaung, S Halindintwali, BW Mwakikunga. Thermally Induced Nano-Structural and Optical Changes of nc-Si:H Deposited by Hot-Wire CVD, Nanoscale Research Letters, 2009, pp. 307, Volume 4, Issue 4, DOI: 10.1007/s11671-008-9243-0