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)