Transcript STRUCTURAL CHANGES STUDIES OF a

STRUCTURAL CHANGES STUDIES OF a-Si:H FILMS DEPOSITED BY
PECVD UNDER DIFFERENT HYDROGEN DILUTIONS USING
VARIOUS EXPERIMENTAL TECHNIQUES
Veronika Vavruňková1 Jarmila Müllerová2 Rudolf Srnánek3, Pavol Šutta1
1
Department of Materials and Technology, New Technology Research Centre, University of West Bohemia, Univerzitní 8, Plzeň, 306 14, Czech Republic
2
Department of Engineering Fundamentals, Faculty of Electrical Engineering, University of Žilina, ul.kpt. J. Nálepku 1390, 031 01 Liptovský Mikuláš, Slovakia
3
Department of Microelectronics, Slovak University of Technology, Ilkovicova 3, Bratislava, 812 19, Slovak Republic
Corresponding author: [email protected]
Introduction
 thin films of a-Si:H prepared using the hydrogen dilution in silane source gas by
plasma enhanced chemical vapour deposition (PECVD) belong to promising
materials for low-cost solar cells
 this deposition technique has been used to yield material with good optoelectrical properties, such as light stability (less light induced degradation known as
Staebler-Wronski effect), higher optical band gap
 the structure of films is necessary to understand and improve properties of the
a-Si:H films which is significant for the application in solar cells
 in this work, we focus on the effects of hydrogen dilution ration on the structural
quality and evolution of c-Si in amorphous network
 protocrystalline silicon constitutes the evolution from amorphous to
nanocrystalline silicon and contains the medium-range order (MRO)
Experimental material
 a series of a-Si:H thin films were deposited using rf-PECVD under increasing
dilution of silane plasma by hydrogen
 the hydrogen to silane dilution ratio R = H2/SiH4 was varied from 5 to 40
 the samples were deposited on Corning glass 1737 substrates and on [100]
oriented c-Si wafers
 a reference sample was deposited using pure silane (R = 0)
 the thickness of all films was approximately 300 nm
X-ray diffraction
 the XRD patterns were recorded using automatic power diffractometer X´pertPro
with the Cu X-ray tube ( = 0.154178 nm) and a thin film attachment
Si (111)
Corning glass 1737
R=30 film on glass
R=33 film on glass
R=35 film on Si
R=0 film on Si
Intensity (a.u.)
400
Laue peaks from substrate
300
Si (220)
200
Si (311)
100
Si (220)
0
20
30
40
50
60
2D detector
2  (degrees)
R = 30
Raman spectroscopy
 the Raman spectra were excited with a He-Ne laser generating the wavelength
of 632.8 nm
88,43
  520,7
LR 
 grain size
300
Si = 519.7 cm
-1
5.0
250
4.5
4.0
3.5
200
150
492
LR (nm)
amorphous peak
from glass
R = 25
TEM images of 1 m thick a-Si:H films deposited at different dilution R [TU Delft]
Raman intensity (a.u.)
500
 substrate and dilution influence
 MRO was determined from thin
films deposited on c-Si
 lower value of FWHM indicates
that films and contain MRO ~ 5-7
nm in size
 evolution of the crystalline
phase with increasing dilution on
samples on glass substrate
R = 20
489
dilution
3.0
2.5
2.0
1.5
1.0
R = 40
100
grain size from crystalline Si
grain size from amorphous phase
0.5
0.0
487
30
32
34
R = 33
50
36
38
40
Dilution
R = 30
FTIR spectrometry
0
200
0
300
25
CH (%)
600
700
40.5
800
1
40.0
Raman shift (cm )
2
39.5
3
 mapping from the surface 5 x 5 μm
 integral intensity of Si crystalline peak
39.0
4
38.5
0.5 µm
5
38.0
0
1
2
3
4
5
Length X (µm )
Conclusions
ATR
20
CH (%), ATR
500
-1
 IR absorbance spectra were performed in the range of 650 – 4000 cm-1 using the
ATR accessory with trapezoidal silicon crystal with a bevelled edge of 45
 stretching vibrations of silicon
hydrides SiH (2000cm-1) and SiH2
(2090cm-1)
400
15
10
5
0
0
10
20
30
40
Dilution (-)
 microstructure factor and hydrogen concentration
 the hydride configurations are agreeing with contribution to vacancies and void
surfaces
 formation of the polycrystalline phase with increasing dilution – samples on glass
 c-Si substrate - amorphous containing medium-range order
 medium-range order determined from the films deposited on c-Si ~ 5-7 nm
 grain sizes calculated from Raman spectrometry increase with increasing dilution
about ~ 3-5 nm
 films - quite homogeneously
 microstructure factor ~ 12 % in average,
 material - slightly porous
References
[1]
[2]
[3]
[4]
Van Elzakker, G.; Šutta, P.; Tichelaar, F.; Zeman, M. Phase control and stability of thin silicon films
deposited from silane diluted with hydrogen. In MRS Symposium Proceedings. Vol. 989.
Warrendale : 2007. ISBN 978-1-55899-949-7.
Park, Y. - B., Rhee, Y. – B. Microstructure and initial growth characteristics of the low temperature
microcrystalline silicon films on silicon nitride surface J. Appl. Phys. 90, 217 – 221 (2001).
Stannowski, B., Schropp, R.E.I. Thin Solid Films, 383 (2001) p.125 – 128.
Vavruňková, V., Müllerová, J., Šutta, P. AEEE. 6 (2007), p. 108-111.