Lowtemperature Alinduced crystallization of hydrogenated amorphous Si1xGex 0'2x1 thin films Shanglon - PowerPoint PPT Presentation

Title: Lowtemperature Alinduced crystallization of hydrogenated amorphous Si1xGex 0'2x1 thin films Shanglon


1
Low-temperature Al-induced crystallization of
hydrogenatedamorphous Si1-xGex (0.2x1) thin
filmsShanglong Peng, Xiaoyan Shen, Zeguo Tang,
Deyan He Department of Physics, Lanzhou
University, Lanzhou 730000, ChinaReceived 14
August 2006 received in revised form 20 June
2007 accepted 17 July 2007Available online 25
July 2007

• Adviser Dr. Hon-Kuan
• Reporter
  Jheng-Jie Syu
• Date
  11/11/2008

2
Outline

• Introduction
• 2. Experimental details
• 3. Results and discussions
• 4. Conclusions

3
1.Introduction

• 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
  techniques2.3.

3-(2003)-Laser-crystallized microcrystalline
SiGe alloys for thin film solar cells
2-(1999)-Solid-phase crystallization of
amorphous SiGe films deposited by LPCVD on SiO2
and glass
Fig. 4. TEM bright field images of LIC SiGe films
deposited at 4500C and laser crystallized at (a)
250C (b) and 7400C.
Fig. 3. Plain-view TEM images of the samples with
(a) x 0 and (b) x 038 crystallized at 5500C
on silicon dioxide.
4
Outline

• Introduction
• 2. Experimental details
• 3. Results and discussions
• 4. Conclusions

5
2. Experimental details

• Al films (200300 nm thick) were firstly
  deposited by vacuum thermal evaporation on
  corning 7059 glass substrates.
• Hydrogenated a-Si1-xGex films (10001200 nm
  thick) were then grown on the Al-coated glass
  substrates using a radio frequency (13.56 MHz)
  capacitively-coupled PECVD system.
• The reaction gases were SiH4 (Ar dilution), Ar
  and GeH4 (H2 dilution) with a total flow in the
  range of 4050 sccm. The substrate temperature
  was fixed at 250 C. The base pressure and the
  deposition pressure were 3.010- 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.

6
Outline

• Introduction
• 2. Experimental details
• 3. Results and discussions
• 4. Conclusions

7
3. Results and discussions

• 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
(x0.2) film as-deposited and annealed at several
temperatures for 3 h.
8
3. Results and discussions

• 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 x0.5 (a) and x0.2 (b) annealed at
400 C for 3 h.
9
3. Results and discussions

• 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 AlGe).

Fig. 3. Cross-section SEM images of hydrogenated
a-Si1-xGex (x0.2) film annealed at 300 C (a)
and 400 C (b) for 3 h.
10
3. Results and discussions

• The three Raman broad peaks located at 300, 400
  and 500 cm- 1 can be clearly seen, which
  respectively correspond to the GeGe, SiGe and
  SiSi stretching mode s.
• The Raman peaks of the GeGe and SiGe modes
  shift to higher frequency (blue shift) with the
  increase of the Ge fraction, however, the peak of
  the SiSi 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 x1, 0.5, 0.4,
0.33 and 0.2.
11
3. Results and discussions

• It was reported that the increase of the
  crystalline phase leads to high-frequency shifts
  of GeGe and SiGe 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 SiSi,
  SiGe and GeGe bonds scales like (1-x)2,2x(1-x)
  and x2 and therefore the relative intensities17.

Fig. 5. Raman spectra of hydrogenated a-Si1-xGex
films with x0.5, 0.4, 0.33 and 0.2 annealed at
350 C for 3 h.
12
3. Results and discussions

• The Raman shift of the SiSi,SiGe and GeGe
  phonon modes for the unstrained Si1-xGex layer
  varies linearly with the Ge fraction according to
  the following relationships 1921

• 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 SiSi (a), SiGe (b) and
GeGe (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.
13
Outline

• Introduction
• 2. Experimental details
• 3. Results and discussions
• 4. Conclusions

14
4. Conclusions

• 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 GeGe and SiGe 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.

15
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Thanks for your attention.