Journal of Non-Crystalline Solids 137&138 (1991) 911-914 North-Holland
STRUCTURAL AND ELECTRONIC PROPERTIES SILICON-SULFUR-SELENIUM ALLOYS
NON-CS iS0U OF
M. HAMMAM, S.M. AL-ALAWI, B. AL-ALAWI, S. A L - D A L L A L and S. ALJISHI Physics Department, University of Bahrain, P.O. Box 32038, Isa Town, BAHRAIN M. STUTZMANN Max-Planck-Institut ffir Festk6rperforschung, Heisenbergstr. 1, D-7000 Stuttgart 80, Federal Republic of Germany
Hydrogenated amorphous silicon-sulfur-selenium ternary alloys are grown by rf glow discharge. Results show that alloying a-Si:H with S and Se results in an increase in the optical gap from a value of 1.85 eV in unalloyed films to approximately 2 eV in films grown with a chalcogenide gas fraction of 0.8. IR absorption spectra reveal a clear Si-S stretching vibrational mode which grows in intensity with increasing S content. In addition, the spectra display a pronounced shoulder in the region of 700 to 760 cm -1 which is attributed to a superposition of $2, S e 2 a n d SiS vibrational modes. Most of the bonded hydrogen in the alloy films occurs in the form of polyhydrides. PDS measurements of the subgap optical absorption in 2.0 eV Eopt material reveal a broad exponential Urbach tail with a slope of approximately 92 meV. The tail saturates into a defect absorption shoulder at a value of approximately 102 cm -1, indicating that the deep defect density is on the order of 1017 to 1018 cm -3. The alloys display substantial photosensitivities as the dark conductivity values range between 10-12 and 10-1° S/cm and the photoconductivities, measured at 100 mW/em2 illumination intensity, lie at approximately 10-8 S/em.
There has been much activity over the past few years in
temperature was kept at 230°C and the process pressure
the search for stable and low defect density high and low
was maintained at approximately 0.25 torr. The rf power
bandgap thin films for use in tandem a-Si:H based
density used in the growth of all films was about 110
photovoltaic devices. One of the most promising materials on the low gap side to date, has been CulnSe2, a ternary
mW/cm z. A compositional variation series was prepared by changing the H2S flow rate while maintaining the HzSe
On the high bandgap side of a-Si:H, the carbon
and Sill 4 flows constant at 10 and 12 sccm, respectively.
alloys (a-Si,C:H) have received the most interest, although
In this work we shall define the gas volume ratio R~ as
there has been substantial work recently in the study of
such alternatives as a-Si,S:H 1,2 and a-Si,Se:H3 alloys.
P~ = [H2S+H2Se]/[SiH4 ]
These alloys are of particular interest because they offer the opportunity to display a juxtaposition of chalcogenide and Si based elemental semiconductor properties.
A compositional variation series was prepared with Rv spanning the range from 0 (a-Si:H) to 0.8. Growth rates were found to hover at about 2 ~/s.
In this work we report on the structural, optical and
electronic properties of ternary hydrogenated amorphous silicon-sulfur-selenium alloys (a-Si,S,Se:H) grown by
Infrared absorption measurements were carried out in the
conventional capacitively coupled RF glow discharge.
range of 400-4000 cm -1 . Figure 1 presents typical spectra
The alloys were deposited on Coming 7059 glass and highly resistive c-Si substrates from a mixture of SiH4, HzS and H2Se gas diluted in helium. The substrate
of a-Si, S,Se:H alloys in the 400-600 cm -1 range. A band at 480 cm -1 is discovered to grow in intensity with increasing R~ . This band, which is degenerate with the
0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.
M. Hammam et aL/ Ternary hydrogenated amorphous silicon-sulfur-selenium alloys
Si-Si TO-like phonon mode, has been ascribed to the Si-S
tentatively attributed to the vibrations of isolated S 2 and
stretching vibrational mode.
embedded in microvoids within the
amorphous network. Likewise in a-Si,Se:H, a mode is measured at 730 cm -1 .
We believe therefore that the
pronounced feature observed near 715 cm -1
in the IR
spectra of a-Si,S,Se:H alloys results from a superposition of the above bands.
la-Si,S,Se :H #,,-,,,,,
OJ t.J r'I'U ..12 tO
..Q ¢_.. 0
600 500 /+00 Wavenumber (cm-I ) FIGURE 1 IR absorption spectra of a-Si,S, Se:H alloys in the range of 400 to 600 cm "1 .
900 800 7 0 0 600 Wavenumber -(cm-1) FIGURE 2 IR absorption spectra of a-Si,S, Se:H alloys in the range of 600 to 900 cm -1 . The stretching mode frequencies of Si-Se bonds are normally measured at 380 cm -1 . This is outside the range
Figure 2 shows the IR spectra in the 600-900 cm -a range.
of our current measurement system.
O f particular interest is the pronounced feature observed on the high wavenumber side of the dominant 640 cm -1 Si-H rocking and wagging mode 5. The feature appears as a distinct shoulder in high R v films. In a-Si,S:H, two modes appearing at about 710 and 760 cm -~ have been
The Si-H stretching mode region is shown in Figure 3. A peak is measured at about 2100 cm -1 , independent of R~. This indicates that most of the H is bonded in the form of polysilanes and high order hydrides 5,7.
M. Hammam et aL / Temaly hydrogenated amolphous silicon-sulfur-selenium alloys
a-Si,S, Se:H I
4= Q. 0 LIJ
S. Se.. ..''"S,Se .-"
a-Si, S : H . . . . a-Si,Se:H . a-Si,S,Se:H
1.8 - I 0.0
Rv FIGURE 4 Optical (Tauc) gap of a-Si,S:H, a-Si,Se:H and a-Si,S, Se:H alloys as a function of chalcogen gas ratio, R~.
2200 2100 2000 1900 Wavenumber (cm-I) FIGURE 3 IR absorption spectra of a-Si,S,Se:H alloys in the range of 1900 to 2200 cra -1 .
a-Si,S,Se:H PDS T__~.10~-
OPTICAL PROPERTIES A CARY spectrophotometer w a s used to measure the optical transmission of the alloy films in the range of 400 to 2000 nm.
The results were employed to obtain the
o 10" g ~-101
optical absorption spectra as well as the thickness, d, and index of refraction, n, of the individual samples. Values of n were discovered to range between 2.0 and 2.4, similar to values measured in a-Se.
I ..... n~,.59
Figure 4 shows the variation with R v of the optical gap, Eopt . The a-Si,S,Se:H data fall in between that of a-Si,S:H and that of a-Si,Se:H. Eopt varies from 1.85 eV in the unalloyed a-Si:H films to about 2.0 eV at P~ = 0 . 8 . .
FIGURE 5 Subgap optical absorption spectra measured by PDS. The two samples have an Eop t difference of 0.05 eV which is evidenced clearly by the shift at the top of the Urbach tails.
M. Hammam et aL / Ternaryhydrogenated amorphous sUicon-sulfur-selenium alloys
Subgap optical absorption spectra were measured using photothermal deflection spectroscopy (PDS).
shows sample spectra for two a-Si,S,Se:H alloy films. The
Hydrogenated ternary amorphous silicon-sulfur-selenium
Urbach energy, which corresponds to the characteristic
alloys were grown by rf glow discharge. The alloys
energy of the wider of the two band tails, falls at about 92
display evidence of both S and Se bonding. Alloying with
meV in both films. The exponential Urbach tail ends in a
the chalcogen atoms is found to produce an increase in the
defect absorption shoulder at an energy of 1.7 eV. The
Urbach energy from that measured in our unalloyed a-
saturation occurs at an a of 102 cm -1 indicating defect
Si:H (65 meV). This increase is linked to a broadening of
densities8 in the range of 1017 to 1018 cm -3 (assuming that
the band tails in the alloys. The defect absorption shoulder
the predominant defect is the Si dangling bond as in a-
saturates at a value of 102 cm -1, indicating a relatively high
photosensitivities ranging from 102 to 103, indicating that
these materials are of potential use in thin film The photo- and dark conductivities of the a-Si,S,Se:H
alloys are plotted in Figure 6. The photoconductivities were measured with unfiltered white light from a
tungsten-halogen lamp providing an intensity of 100 mW/cm2. The photoconductivities fall near 10.8 S/cm, a factor of 10e to 103 higher than the dark conductivities. There is no evidence of a clear trend with changing R,,.
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0.¢ 0.6 0.8
Rv FIGURE 6 Photo- and Dark conductivity as a function of Rv. The photoconductivities are measured with 100 mW/cm2 unfiltered tungsten-halogenlamp illumination.
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