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Low-temperature Al-induced
crystallization of hydrogenated
amorphous Si1−xGex (0.2≤x≤1) thin
films
Shanglong Peng, Xiaoyan Shen, Zeguo Tang, Deyan He *
Department of Physics, Lanzhou University, Lanzhou 730000, China
Received 14 August 2006; received in revised form 20 June 2007; accepted 17 July 2007
Available online 25 July 2007
Adviser : Dr. Hon-Kuan
Reporter : Jheng-Jie Syu
Date
: 11/11/2008
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Outline
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1. Introduction
2. Experimental details
3. Results and discussions
4. Conclusions
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1.Introduction
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Low-temperature formation of microcrystalline (mc-) or polycrystalline (poly-)
Si1−xGex films on inexpensive substrates such as glass has been expected to
realize advanced systems in displays and three-dimensional ultra large-scale
integrated circuits.
Some low-temperature approaches such as solid phase crystallization and
laser annealing have been carried out to crystallize amorphous (a-) Si1−xGex
films. However,poly-Si1−xGex films with small grain (∼1 μm) were often obtained
by these techniques[2.3].
[2]-(1999)-Solid-phase crystallization of
amorphous SiGe films deposited by LPCVD on
SiO2 and glass
Fig. 3. Plain-view TEM images of the samples with (a) x = 0 and
(b) x = 0:38 crystallized at 5500C on silicon dioxide.
[3]-(2003)-Laser-crystallized microcrystalline SiGe alloys
for thin film solar cells
Fig. 4. TEM bright field images of LIC SiGe films deposited at
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4500C and laser crystallized at (a) 250C (b) and 7400C.
Outline
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1. Introduction
2. Experimental details
3. Results and discussions
4. Conclusions
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2. Experimental details
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Al films (200–300 nm thick) were firstly deposited by vacuum thermal
evaporation on corning 7059 glass substrates.
Hydrogenated a-Si1−xGex films (1000–1200 nm thick) were then grown on the
Al-coated glass substrates using a radio frequency (13.56 MHz) capacitivelycoupled PECVD system.
The reaction gases were SiH4 (Ar dilution), Ar and GeH4 (H2 dilution) with a
total flow in the range of 40–50 sccm. The substrate temperature was fixed at
250 °C. The base pressure and the deposition pressure were 3.0×10− 4 Pa and
150 Pa, respectively. The applied radio frequency power was 30 W.
The annealing temperatures were 300, 350, 400 and 450 °C,respectively, and
the annealing time was kept constant for 3 h.
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Outline
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1. Introduction
2. Experimental details
3. Results and discussions
4. Conclusions
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3. Results and discussions
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The peaks at 28°, 47° and 55° can be seen when the sample was annealed at
a temperature of 350 °C ,which correspond to the diffraction from (111), (220),
and (311) planes of the crystallized Si1−xGex films, respectively.
Further increase in the peak intensity and reduction in the full width at half
maximum can be found with the increase of the annealing temperature,
indicating an enhancement in the film crystallinity.
28°
47°
55°
25°
Fig. 1. XRD patterns of hydrogenated a-Si1−xGex (x=0.2) film asdeposited and annealed at several temperatures for 3 h.
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3. Results and discussions
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We found that the three XRD peaks of (111), (220), and (311) in Fig. 2(a) are
much stronger than
those in Fig. 2(b), confirming that the Ge-rich sample is easier to be crystallized
at the same annealing temperature.
The small shift of the peak position was observed, which results from the
increase of the lattice constant with the increase of the Ge fraction.
Fig. 2. XRD patterns of hydrogenated a-Si1−xGex
films with x=0.5 (a) and x=0.2 (b) annealed at 400
°C for 3 h.
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3. Results and discussions
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Increasing the annealing temperature to 400 °C, not only the Al layer
disappeared, but also the structure of the SiGe film changed dramatically. We
believe that Al atoms diffuse into the SiGe film and induce the film to crystallize
by forming a mixture of Al and SiGe.
In the following phase the Si grains go on growing laterally only until they
touch adjacent grains and form a continuous poly-Si film on the glass substrate.
It was reported that crystallization of a-SiGe needs much longer annealing
time at low annealing temperature below 420 °C (the eutectic temperature of
binary of Al–Ge).
Fig. 3. Cross-section SEM images of hydrogenated a-Si1−xGex (x=0.2) film annealed at 300 °C (a) and 400 °C (b) for 3 h.
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3. Results and discussions
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The three Raman broad peaks located at 300, 400 and 500 cm− 1 can be
clearly seen, which respectively correspond to the Ge–Ge, Si–Ge and Si–Si
stretching mode s.
The Raman peaks of the Ge–Ge and Si–Ge modes shift to higher frequency
(blue shift) with the increase of the Ge fraction, however, the peak of the Si–Si
mode shifts to low frequency (red shift) in the Ge composition range under
study.
Fig. 4. Raman spectra of the as-deposited
hydrogenated a-Si1−xGex films with x=1, 0.5,
0.4, 0.33 and 0.2.
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3. Results and discussions
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It was reported that the increase of the crystalline phase leads to highfrequency shifts of Ge–Ge and Si–Ge peaks.
Furthermore, it is known that the intensities of the peaks for crystallized
Si1−xGex alloys depend on the composition x because the number of Si–Si, Si–
Ge and Ge–Ge bonds scales like (1−x)2,2x(1−x) and x2 and therefore the
relative intensities[17].
Fig. 5. Raman spectra of hydrogenated a-Si1−xGex films
with x=0.5, 0.4, 0.33 and 0.2 annealed at 350 °C for 3 h.
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3. Results and discussions
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The Raman shift of the Si–Si,Si–Ge and
Ge–Ge phonon modes for the unstrained
Si1−xGex layer varies linearly with the Ge
fraction according to the following
relationships [19–21]:
si - si(x)  520.2 - 29.7x
si - Ge(x)  400  29x
Ge  Ge( x)  282.5  12x
(1)
(2)
(3)
The little deviations may be due to the
presence of tensile strain and the
interaction of lattice phonons caused by
residual Al-doping (Fano interaction).
Fig. 6. Experimental frequency shifts (squares and dashed lines) of the Si–Si (a), Si–Ge (b) and Ge–Ge (c) modes as a
function of Ge fraction x for the hydrogenated a-Si1−xGex films annealed at 350 °C for 3 h. The corresponding calculated
frequency shifts (lines) for fully unstrained SiGe films (Eqs. (1)–(3)) are presented in comparison with the experimental data.
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Outline
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1. Introduction
2. Experimental details
3. Results and discussions
4. Conclusions
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4. Conclusions
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 It is shown that the crystallization of hydrogenated a-Si1−xGex films begins
at the Al/a-Si1−xGex interface and the Al-induced layer exchange significantly
promotes the crystallization of the films.
The Ge–Ge and Si–Ge peaks shift to a higher frequency with the increase of
the Ge fraction.
With the increase of the Ge fraction and annealing temperature,there is an
enhancement in film crystallinity and grain size.
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Thanks for your attention.
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