Solar Energy Materials and Solar Cells 27 (1992) 243-251
North-Holland
Solar Energy Materials
and Solar Cells
Sputtering of thin pyrite films
M. Birkholz, D. Lichtenberger, C. H 6 p f n e r and S. Fiechter
Bereich Photochemische Energieumwandlung, Abteilung Materialforschung, Hahn-Meitner-lnstitut,
W-IO00 Berlin 39, Germany
Received 1 December 1991
Argon and reactive ion beam sputtering of thin pyrite (FeS 2) and other iron sulfide films is
reported. Films were characterized by X-ray diffraction, Rutherford backscattering, measurements of the temperature-dependent conductivity and optical spectroscopy.
1. Introduction
The high production costs of conventional solar cells make it necessary to
investigate alternative production methods and materials with respect to their
suitability for solar energy conversion. Semiconductors with high absorption coefficients are favorite candidates because of their potential to reduce material costs by
using a thin film as active layer. One of these materials is pyrite, which is in the
ideal case of composition FeS 2 and has an extraordinary high optical absorption
coefficient ( a > ~ 6 x 105 cm -1 for h v > 1.3 eV) [1]. A pyrite film of 0.1 ~m
thickness absorbs more than 99% of the sunlight (AM 1.5) and makes the material
very promising for solar cell applications. The band gap energy of synthetic crystals
was measured [1,2] to be 0.92 to 0.95 eV and, at room temperature, electron
mobilities in the range from 100 to 360 cm2/V s were found [3].
Problems with the material arise in connection with the open circuit voltage Uoc
measured in photoelectrochemical cells [1] which are limited to 0.2 V until now. In
principle it should be possible to yield half the band gap energy as open circuit
voltage [4], i.e. ca. 0.45-0.5 eV for pyrite is expected. Since the efficiency of a solar
cell is proportional to Uo~ this quantity must be enhanced to make the material
valuable for technical applications. The small values measured for Uoc are perhaps
due to the small difference of Fermi levels between pyrite and the electrolyte used
(iodide-iodine). This assumption is supported by recent electro-reflectance measurements [5]. Another explanation concerns the strong non-stoichiometry in pyrite,
that can lead to a sulfur deficiency in the percentage range as shown in recent
works [6,7]. The sulfur defects were shown to be simple point defects for which
ligand field theory predicts the development of electronic defect states in the
Correspondence to: S. Fiechter, Bereich Photoehemische Energieumwandlung, Abteilung Materialforschung, Hahn-Meitner-Institut, W-1000 Berlin 39, Germany.
0927-0248/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
244
M. Birkholtz et al. / Sputtering of thin pyrite films
forbidden zone [8]. Such defect levels offer another explanation for the low open
circuit voltages of pyrite in photoelectrochemical cells.
The density of these sulfur vacancies in the pyrite crystal lattice can be
described as a function of the sulfur vapour pressure and a Boltzmann distribution
with the vacancy formation enthalpy Hf. This means that at a fixed sulfur pressure
the density of sulfur defects increases with increasing preparation temperature [6].
Therefore, pyrite of high electronic quality should be produced at low temperatures. Until now different methods to produce thin pyrite films were used: metal
organic chemical vapour deposition [9], chemical spray pyrolysis [10], sulfurization
of iron oxides [11] and plasma assisted sulfurization of thin-iron films [12]. This
paper reports for the first time results on thin film preparation of pyrite by ion
beam and reactive sputtering. The method is widely used as an industrial deposition technique and offers the possibility to produce thin films at low temperatures.
A subsequent paper will review pyrite technology and properties.
2. E x p e r i m e n t a l
For the sputtering process a Technics Micro Ion Mill M I M / T L A 5.5 was used,
equipped with a diffusion pump to evacuate the process chamber to background
pressures of 5.3 x 10 -4 Pa. Sputtering was done in the ion beam mode, i.e. gas
atoms were ionized and accelerated in a standard Kaufman source (beam diameter
5.5 cm) which was spatially separated from the process chamber containing the
target and substrate, see fig. 1. For further information concerning technical and
operational details of the system see ref. [13]. Deposition processes were done (i)
with argon gas (4.8 N, Linde), (ii) with H2S (2.5 N, Linde) and (iii) with mixtures of
inlet 1
substrat
holder with
heater
source
; ~ accelerated
iii ion"
A V A " water
process
chamber
I
gas
inlet 2 f-"
to vacuum system
Fig. 1. Schematicdrawingof the sputter systemused.
M. Birkholtz et aL / Sputtering of thin pyrite films
245
both gases. A target disk of 10 cm diameter and 0.6 cm thickness was produced
from pyrite powder (Cerac Inc., 4 N purity). The distance between target and
substrate was 5.5 cm.
Disks made of 0.55 mm thick AF45 glass (Desag, diameter 20 mm) were used as
substrates. Some films were deposited on 1 mm thick quartz-glass disks (Heraeus,
diameter 20 mm) and on silicon wafers (Virginia semiconductors, diameter 1 inch,
p- and n-type, 0.1-10 f~ cm). Since the lattice constant of pyrite (5.4187 ,~) [8] and
silicon (5.43102 ,~) [14] differ only by 0.2%, silicon substrates might be well suited
for growing epitaxial films. Prior to the deposition process, glass substrates were
etched in aqua regia (Merck, Selectipur quality) and rinsed four times in triply
destilled water, Si-wafers were etched in 40% H F for some minutes and dried in a
N2-gas stream. Film deposition was performed within 90 to 240 min at a deposition
rate of about 0.5 ,~/s.
To find the optimum sputtering conditions, a series of experiments were carried
out by systematically varying the deposition temperature, Tdep, from 20 to 400°C,
the accelerating voltage, Uacc, from 0.5 to 3 kV, the chamber pressure, p, from
3.3 × 10 -3 to 8 x 10 -2 Pa and the annealing procedure which followed the
deposition process. The best morphology was produced by sputtering with H2S
alone, under conditions as follows: Uacc = 1.5 kV, p = 1.1 × 10 -2 Pa, T0ep = 75°C.
Finally the films were annealed in a two-step process: at first annealing the film at
a temperature of 100°C for one hour and afterwards at 250°C for a further hour.
Especially, the last step was important to produce films that remained free of
cracks, which were otherwise frequently observed. All films produced under these
conditions were polycrystalline, a few thousand , ~ g s t r 6 m s thick, mirrorlike and
pin hole free. The latter point is one of the most advantageous properties of thin
film deposition by sputtering.
Thin films were characterized by X-ray diffraction (XRD), Rutherford backscattering (RBS), conductivity measurements and optical spectroscopy. X R D was done
with a Siemens D500 powder diffractometer in the usual 0 - 2 0 coupled mode with
Cu K a radiation. For RBS spectra 2.0 MeV He + ions and a scattering angle of
170° were used. Temperature-dependent conductivity was determined in a range
between 80 and 300 K with a system described in ref. [3]. For this purpose, ohmic
contacts were prepared on the roughened surface in the symmetrical van de Pauw
geometry by using Ag-epoxy. Measurements of the optical density were done with
a conventional double beam photometer (Bruins Instrument Omega-10) for wavelengths between 400 and 1800 nm. No attemps were made to check for n- or p-type
conducting behaviour of the films produced by simple measurements of the
Seebeck coefficient. This method proved to give no reliable results as was shown in
investigations [3] done earlier on synthetic single pyrite crystals.
3. Results and discussion
The binary iron sulfide system is characterized by many phases with different
stoichiometries. The JCPDF database lists 24 files of iron-sulfur compounds and
246
M. Birkholtz et al. / Sputtering of thin pyrite films
M
PY
-g
~.
Py
MM
i
i
i
20
M
M M
PY
M
U
30
40
Two-Theta
50"
60
(Degrees)
Fig. 2. X-ray diffractogram showing a Fe 1_xS film (pyrrhotite) produced by sputtering with argon gas
(Tdep = 250°C, Uacc = 2.5 kV and p = 3.3 × 10 -3 Pa). Bar lines indicate positions and relative intensities
of typical powder X-ray reflections (according to JCPDS data files 3-799 for markasite (M) and 20-535
for pyrrhotite 1C (Py)).
their corresponding X-ray diffractograms [15]. Therefore, X-ray diffractometry
(XRD) is an important tool to check the phase purity of the films. Fig. 2 shows a
diffractogram of a film, sputtered with argon and grown at Tjep = 250°C, Uacc = 2.5
kV and p = 3.3 × 10 -3 Pa. No pyrite was produced in this case and the bars
indicate the diffraction patterns of markasite (M), another modification of FeS2,
and pyrrhotite, Fe~_xS (Py). Missing reflexes were interpreted as due to an
oriented growth of crystallites. In general, no single phase pyrite films could be
obtained using argon alone as the sputter gas, but diffraction could be assigned to
different phases with a sulfur to iron ratio somewhat larger than one (containing
pyrrhotite Fe] _xS, smythite Fe9S]1 and greigite Fe3S4). After switching to reactive
sputtering with H2S we had most success by using it as pure sputter gas, mixing it
with no argon at all.
i
~
i
i
i
I
L
I
35
40
45
50
55
60
E
30
Two-Theta
(Degrees)
Fig. 3. X-ray diffractograms of thin pyrite films produced under the same conditions as in fig. 2 but
tempered for one hour after deposition at different temperatures (a) 200°C (b) 250°C (c) 300°C (d)
400°C. We interpret the vanishing of the reflections in terms of a loss of sulfur in the films.
M. Birkholtz et al. / Sputtering of thin pyrite films
I
I
I
J
I
247
I
-g
8
"E
25
30
35
40
45
50
Two-Theta (Degrees)
55
60
Fig. 4. X-ray diffractogram of a thin film a s d e p o s i t e d on a Si-wafer at optimum sputtering parameters:
Tdep = 75°C, Uacc = 1.5 kV and p = 5 . 3 × 1 0 -3 Pa. The occurrence of all pyrite powder reflection
indicates that no epitaxial growth occurred although the unit cell edges of pyrite and silicon differ only
by 0.2%.
Fig. 3 shows the influence of annealing at different temperatures for thin films
deposited on glass. The bars in the plot indicate the X-ray diffraction pattern of
pyrite powder (JCPDF card No. 6-710). By increasing the annealing temperature,
the intensity of the pyrite reflections decreases which can be interpreted as a loss
of sulfur in the films. The optimum procedure for increasing the size of crystallites
without loosing sulfur is a two-step annealing process with tempering for one hour
at 100°C followed by an additional hour at 250°C.
Fig. 4 shows an X-ray diffractogram of a film deposited on a Si(100) wafer. The
bars in the diagram again indicate positions and intensities of reflections of pyrite
powder. There is also a small amount of marcasite detectable. Although the unit
cell edges of pyrite and silicon match very well, the presence of all pyrite powder
reflections in the range measured clearly indicates that no epitaxial growth took
place. The figure also shows some deviations from the pyrite reflections compared
to the JCPDF data typical for many films produced. First, the positions of
reflections do not match exactly the bars from the JCPDF database. This could be
due to a height misalignment of the sample during measurements. But this effect
was experimentally excluded. Therefore, we take it as indication for a higher
lattice constant and a decreased density in our samples. The unit cell edge a
calculated from the shift of reflections was maximally 1% larger than the theoretical value, causing the density to be maximally 97% of the theoretical one (5.013
g/cm3). Such an effect of decreased density is not unusual in sputtered films and
has also been observed for other materials [16]. Secondly, the relative intensities of
the powder reflections did not agree with the JCPDF data. The reflection at
20 = 33° was clearly too strong in relation to the others. This effect is partly due to
the film's low thickness which causes a damping of intensities with increasing 20,
but this can not fully account for it. The increased intensity of the reflection at 33°
will be due to a preferred orientation of crystallites with the (100) planes parallel
to the surface of the substrate.
M. Birkholtz et aL / Sputtering of thin pyrite films
248
12000
[ eJ ~ H e *
2.0 MeV
,°oo
1
, oo
o
Fe
I
o
si
4800
2400
0
0.2
0.6
1
1.4
1.8
Backscattedng Energy / M e V
Fig. 5. Typical Rutherford backscanering (RBS) spectrum of a pyrite film deposited on quartz glass.
The resulting stoichiometry is S : Fe = 2.0 + 0.05.
Rutherford backscattering measurements (RBS) were carried out to determine
the stoichiometry and the thickness of the sputtered films. Fig. 5 shows the RBS
spectrum of a typical pyrite film on quartz glass produced by H2S sputtering. The
peak at an energy of about 1.45 MeV in the spectrum represents the Fe atoms in
the layer, the one at 1.18 MeV the S atoms. The two steps below those energies
are caused by the fused silica substrate. By comparison of the scattering cross
section corrected peak areas for Fe and S one obtains the composition of the film
as FeS2. 0_+0.05. Fitting the spectrum with the usual theoretical functions as supplied
by the program RUMP [17] gives a film thickness of 480 ,~. Films which were
sputtered with Ar and A r / H z S mixtures show a F e / S ratio from 1.0 to 1.8, but
never a value near 2.0.
A measurement of the temperature-dependent conductivity is given in fig. 6.
The curve shows typical semiconducting behaviour. The slope of the curve at room
.
.
.
.
I
.
.
.
.
l
.
.
.
.
I
.
.
.
.
o
0.1
8
0.01
....
0
I ....
10
I ....
20
1000 T ' I /
I ....
30
40
K "1
Fig. 6. Temperature-dependent conductivity of one of the pyrite films produced on quartz glass
(Tder, = 75°C, Uacc = 1.5 kV and p = 5 . 3 × 1 0 -3 Pa). The room temperature activation energy, EA,
equals 0.07 eV.
249
M. Birkholtz et aL / Sputtering of thin pyrite films
t e m p e r a t u r e corresponds to an activation energy of E A = 0.07 eV, which is
comparable to thin films produced by other methods [9,12], obtaining values of
0.14 and 0.1 eV, respectively. The fact that E A does not equal half the band gap
energy may be explained in several ways. For example the doping level, the film
structure or the stoichiometry deviation of the samples [8] can contribute to the
value measured. We believe that higher values could be obtained for films with
fewer grain boundaries. This hypothesis is supported by preliminary investigations
of our films with transmission electron microscopy (TEM) that showed particle
sizes of 100-150 ,~. It is worth mentioning that the resistivity of our films is high
( M l ) range) compared to those for n-type synthetic crystals [3]. This can tentatively be interpreted in terms of the low production temperature of this films
I
I
I
0.9
1.3
1.7
'
I
2.5
ta
e-
1.5
o
1
0.5
0
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i
0.5
2.1
2.5
Energy/eV
30
i
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2,5
/
25
/'I
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20
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-~15
<="
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7
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0.5
f
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,
0.5
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0.7
,
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~----+---;-"~
1.1
,
I
1.3
1.5
E ne rgy/eV
,
l
1.7
,
I
1.9
,
0
2.1
Fig. 7. (a) Spectrum of the optical density versus energy of a reactive sputtered pyrite film. The optical
density increases rather slowlywith increasing energy. (b) Plots of (OD × h~,)2 and (OD x hv) 1/2 versus
hv usually indicate direct and indirect optical transition, yielding 1.5 eV and 0.6 eV for the spectrum
given in (a). The applicabilityof this plotting procedure for pyrite is rather doubtful (see text).
250
M. Birkholtz et al. / Sputtering of thin pyrite films
resulting in a S : Fe ratio closer to 2 : 1 than in synthetic crystals produced at high
temperatures.
For measurements of the optical density (OD) the ratio recording technique was
used. By means of this method the wavelength dependency of the reflectivity does
not affect the measurement. Two films produced under the same conditions but of
different thicknesses were measured simultaneously. The signal recorded is due to
the optical density of the film thickness difference. Fig. 7a shows a typical
spectrum. Usually plots of (OD × h u ) 2 and (OD × h~') 1/2 versus h u are done to
determine the energy of a direct and indirect band gap. They are given in Fig. 7b.
The increase of optical density with decreasing wavelength in fig. 7a indicates
the electronic transition from the valence to the conduction band. The precise
value of the band gap Eg and the mechanism of this transition is still a point of
controversy (a compilation of Eg values is given in ref. [18]). The plots in fig. 7b
may be interpreted in terms of an indirect optical transition at 0.6 eV and a direct
transition at 1.5 eV. It should be kept in mind that this plotting procedure was only
developed for semiconductors having non-degenerate and parabolic bands. Band
structure calculations done for pyrite [19-21] indicate flat rather than parabolic
valence and conduction bands. Therefore it is not at all clear whether such plotting
procedures can provide the value of the energy gap for semiconducting pyrite.
Furthermore, it is known that small crystallites in polycrystalline films cause
Rayleigh scattering, which could also account for the small increase in optical
density. Finally it cannot be excluded that the optical density signal in the lower
energy range is due to electronic defects introduced by the sulfur deficiency as
proposed in ref. [8]. Our spectrum of the optical density shows that basic features
of the pyrite spectrum.
4. Conclusion
Successful production of this polycrystalline pyrite films at low temperatures by
reactive H 2S sputtering was reported. X-ray diffraction and Rutherford backscattering indicated single phase films with stoichiometries close to S / F e = 2/1. The
films were shown to be semiconducting with an activation energy of 0.07 eV at
room temperature. Typical pyrite optical density spectra were obtained. The
feasability of sputtering for production of thin pyrite films has been proven and
potential applications have to be investigated in subsequent work such as epitaxial
growth and production of pyrite heterojunctions.
Acknowledgements
We like to thank Chris B~irtels, Klaus Diesner, Jeffrey Erxmeyer, Michael
Giersig and Rainer Schieck for their support of this work. Furthermore, many
thanks are given to Siegfried Klaumiinzer for supplying the sputter system.
M. Birkholtz et aL / Sputtering of thin pyrite films
251
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