Magnetron-sputter deposition of Fe3S4 thin films and their conversion into pyrite
(FeS2) by thermal sulfurization for photovoltaic applications
Hongfei Liu and Dongzhi Chi
Citation: Journal of Vacuum Science & Technology A 30, 04D102 (2012); doi: 10.1116/1.3699022
View online: http://dx.doi.org/10.1116/1.3699022
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Magnetron-sputter deposition of Fe3S4 thin films and their conversion
into pyrite (FeS2) by thermal sulfurization for photovoltaic applications
Hongfei Liua) and Dongzhi Chi
Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology
and Research), 3 Research Link, Singapore 117602, Singapore
(Received 5 January 2012; accepted 14 March 2012; published 2 April 2012)
The authors report on the fabrication of FeS2 (pyrite) thin films by sulfurizing Fe3S4 that were
deposited by direct current magnetron sputtering at room temperature. Under the selected
sputtering conditions, Fe3S4 nanocrystal films are obtained and the nanocrystals tend to locally
cluster and closely pack into ricelike nanoparticles with an increase in film thickness. Meanwhile,
the film tends to crack when the film thickness is increased over 1.3 lm. The film cracking can
be effectively suppressed by an introduction of a 3-nm Cu intermediate layer prior to Fe3S4 deposition. However, an introduction of a 3-nm Al intermediate layer tends to enhance the film cracking.
By post-growth thermal sulfurization of the Fe3S4 thin films in a tube-furnace, FeS2 with high
phase purity, as determined by using x ray diffraction, is obtained. Optical absorption spectroscopy
was employed to characterize the resultant FeS2 thin films, which revealed two absorption edges at
0.9 and 1.2 eV, respectively. These two absorption edges are assigned to the direct bandgap
C 2012 American
(0.9 eV) and the indirect allowed transitions (1.2 eV) of FeS2, respectively. V
Vacuum Society. [http://dx.doi.org/10.1116/1.3699022]
I. INTRODUCTION
Semiconducting FeS2 (pyrite) has long been proposed to
have great potential for photovoltaic applications due to its
suitable bandgap energy (0.95 eV) and large optical absorption coefficient (a > 105 cm1 in the wavelength range of
visible light).1,2 For thin film photovoltaic applications, taking material availability, extraction/processing cost, energy
conversion efficiency, and eco-friendliness into account,
FeS2 has recently been predicted to hold the leading position
among the most plausible candidates such as Cu2S, Cu2O,
CuO, etc.3,4 Although many crystal synthesis methods have
been used to prepare FeS2, the resultant crystal qualities are
still far from device applications, mainly attributed to S deficiency (i.e., nonstoichiometry) and/or crystal phase impurity
of FeS2.5–7
In general, there are two types of methods reported in the
literature for synthesizing FeS2. One is direct growth/deposition of FeS2, for example, by metal-organic chemical vapor
deposition (MOCVD),8,9 molecular beam epitaxy (MBE),10
and reactive/magnetron sputtering.11 The other consists of
two steps, i.e., deposition of iron and/or iron oxide thin films
followed by sulfurization of such films into FeS2.7,12–14
Although MOCVD and MBE are the most accurate material
growth methods, they yield too many structural defects in
FeS2, leading to the low solar-energy conversion efficiency
(<3%). Sputtering deposition is low-cost and the growth can
be made on large area substrates. However, there is a great
challenge in direct depositions of FeS2 by sputtering due to
the loss of sulfur. On the other hand, an increase of sulfur
inclusions in iron sulfide target tends to make it powdering
during sputtering. Generally, a special sulfur source, e.g.,
a)
Author to whom correspondence should be addressed; electronic mail:
liuhf@imre.a-star.edu.sg
04D102-1
J. Vac. Sci. Technol. A 30(4), Jul/Aug 2012
H2S (extremely hazardous and not available for commonly
used sputtering setups), is required to control the stoichiometry of FeS2 during sputtering, making the process complex.11
The two-step methods, when compared to the direct
growth methods, are much simpler and cheaper, and they are
widely used for FeS2 synthesizes. It has been reported that
the structural, optical, and electrical properties of FeS2 thin
films synthesized by sulfuring Fe are dependent not only on
the sulfurization parameters but also on the structures of Fe,
e.g., grain size and film thickness.15,16 The main controlling
factors are the diffusion rates of Fe and S atoms and an intermediate layer of FeS is usually formed between the top FeS2
and the bottom Fe layers.17 The short path for sulfur diffusions provided by grain boundaries could also be closed by
the increase in grain sizes in the upper layer during the thermal processing/sulfurization, leading to the layered structures with S and Fe enriched in the regions close to the
original surface and interface, respectively.11,18 The phase
diagram of Fe–O–S at 350 C indicated clearly that FeS
tends to be formed when sulfur is slightly lower than that for
FeS2.14
Our previous studies revealed that Fe3S4 crystallite thin
films, having higher sulfur contents than that of FeS, are
readily formed on Si substrates at room temperature by
sputtering using a Fe0.95S1.05 target.19 In fact, Fe3S4 is
found to be a natural intermediate on the polysulfide
pathway to pyrite.20 It is certainly believed that sulfurization of Fe3S4, instead of Fe, could effectively suppress the
formation of FeS intermediate layer as well as the FeS
phase impurity in FeS2. Moreover, this approach has an
advantage in fabricating FeS2/Si hetero p-n junctions since
the diffusion of Fe atoms into Si can be suppressed due to
the presence of sulfur. Unfortunately, this approach has not
received much attention and its related knowledge is lacking in the literature. In the present work, we report on
0734-2101/2012/30(4)/04D102/5/$30.00
C 2012 American Vacuum Society
V
04D102-1
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04D102-2 H. Liu and D. Chi: Magnetron-sputter deposition of Fe3S4 thin films
04D102-2
studies of direct current (dc) magnetron-sputter deposition
of Fe3S4 nanocrystal thin films and their conversion into
FeS2 by post-growth thermal sulfurization in a tube-furnace
system.
II. EXPERIMENT
Fe3S4 nanocrystal thin films with various thicknesses
were deposited at room temperature on Si (100) substrates in
a dc-magnetron sputter chamber.19 Pure argon ( 99.999%)
was used as the working gas, with a pressure setting at
4 103 Torr automatically controlled by a throttle valve.
A 3-in. conductive Fe0.95S1.05 target (99.99%, supplied by
Super Conductor Materials, Inc.) was used as the source
material. The dc-power applied on the target was 100 W
(560 V and 0.18 A), which led to a growth rate of 0.2-nm/s
under certain conditions. The native oxide skin layer of the
Si substrates was removed by using buffered hydrofluoride
solution before loading into the magnetron-sputter deposition chamber.
Sulfurization of the obtained Fe3S4 thin films was performed by post-growth thermal annealing under sulfur ambient in a tube-furnace system. Sulfur flakes (99.99% put in a
crucible) and the Fe3S4 thin film were separated at 20 cm
along the tube length direction. Nitrogen (99.999%) gas was
fed into the tube from the sulfur crucible side while keeping
the chamber pressure at one atmosphere. The thermal sulfurization process was started with nitrogen purging for two
hours followed by ramping up the temperature to 400 C.
This temperature was kept for 2 h before cooling down.
The nitrogen feeding was not stopped until the tube was
completely cooled down to room temperature.
For structural characterizations of the iron sulfide thin
films before and after sulfurization, x ray diffractions (XRD)
(Cu-Ka1) were carried out with a general-area-detector diffraction system (GADDS, Bruker-D8), which has the great
advantage of being highly sensitive to crystal phase structures. To avoid the diffractions from the Si substrate [e.g., Si
(004) at 69.126 ], the x-scan was interrupted by stopping at
34 and restarted at 35 , so that the lifetime of the detector
can be lengthened. For data analysis, the background of the
spectra was automatically subtracted using the software
BRUKER EVA installed with the system. The surface morphology was imaged in a top-view configuration using a Nomarski microscope (NM) and a field-emission scanning-electron
microscope (SEM, JEOL JSM-6700). A scanning NIR-VISUV spectrophotometer (Shimadzu, UV-3101 PC) was
employed to measure the optical absorption spectra of the
Fe3S4 thin films before and after sulfurization. For these
measurements, clear-glass slides were used instead of Si as
the substrates for deposition and sulfurization of the thin
films.
III. RESULTS AND DISCUSSION
A. Sputtering deposition of Fe3S4 thin films
Figure 1 shows the evolution of XRD spectra collected
from the as-deposited iron sulfide thin films as a function of
FIG. 1 (Color online) XRD (Cu-Ka1) spectra collected from the asdeposited Fe3S4 thin films with the thickness of 20 nm, 100 nm, 500 nm, and
1.3 lm for samples A, B, C, and D, respectively. The pattern of JCPDS 160713 (Fe3S4) is also presented for easy comparisons.
film thickness, which was increased from 20 nm to 100 nm,
500 nm, and 1.3 lm, labeled as samples A, B, C, and D,
respectively. It is clearly seen that diffraction peaks became
more pronounced with an increase in film thickness. The
spectra of samples A and B, having the thickness equal to or
smaller than 100 nm, do not show any diffraction peaks. A
comparison between the diffraction spectra of samples C and
D indicates that the peak intensities are increased by increasing the film thickness. The columnar pattern, shown together
with the experimental spectra in Fig. 1, is of JCPDS 16-0713
(Fe3S4). The excellent match between JCPDS 16-0713
(Fe3S4) and the experimental spectra, in terms of the peak
positions and the peak intensity ratios, provides clear-cut
evidence that Fe3S4 crystals with relatively high phasepurity were obtained under the selected sputtering conditions. A careful spectrum analysis of samples C and D
revealed that although there is a significant increase in the
intensity of the diffraction peaks from samples C to D due to
the increased film thickness, there is; however, no significant
change in linewidth of the diffraction peaks. A simple estimation from the linewidth of the diffraction peaks, in terms
of the Scherrer’s formula, reveals that the crystallite sizes of
Fe3S4 in samples C and D are 24 and 22 nm, respectively,
almost the same.
Figures 2(a)–2(d) present the SEM images recorded from
samples A–D, respectively. Figure 2(a) does not exhibit any
structural features while fine particles with diameters smaller
than 20 nm are seen in Fig. 2(b). It is seen in Fig. 2(c) that an
increase in film thickness to 500 nm leads to the formation of
ricelike particles with the short and long diameters in the
ranges of 25–45 nm and 100–130 nm, respectively. These
ricelike nanoparticles are randomly, hierarchically, and
loosely packed on the substrate forming nanoscale pores
and/or gaps. In connection with the XRD spectra evolutions
in Fig. 1, it is believed that these ricelike nanoparticles are
consisted of Fe3S4 nanocrystals. It has to be noted that
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04D102-3 H. Liu and D. Chi: Magnetron-sputter deposition of Fe3S4 thin films
FIG. 2. Top-viewed SEM images recorded from (a) sample A (20 nm), (b)
sample B (100 nm), (c) sample C (500 nm), and (d) sample D (1.3 lm).
average particle sizes, especially the long diameters,
observed by SEM in Fig. 2(c) are much larger than that of
24 nm obtained from XRD linewidth via Scherrer’s formula.
The sizes of the nanoparticles were further increased by an
increase in film thickness to 1.3 lm while the ricelike shape
was kept intact [see Fig. 2(d)]. However, the XRD results do
not show any increase in crystallite size from sample C
(24 nm) to sample D (22 nm). A closer look into the image in
Fig. 2(d) revealed that the individual ricelike nanoparticles
are, in fact, consisted of even smaller structures. From the
crystallite sizes point of view, a combination of the XRD
and SEM observations (presented in Figs. 1 and 2) suggests
that the smaller structures must be Fe3S4 nanocrystals (in the
range of 20 nm in terms of XRD estimations). During the
sputtering deposition, when the film thickness is increased to
larger than 100 nm, the Fe3S4 nanocrystals tend to locally
cluster and closely pack, in a similar behavior of self-assembly, forming the observed ricelike Fe3S4 nanoparticles.
B. Film cracking and its suppression of Fe3S4 thin
films
Figure 3(a) shows the optical microscopic image recorded
by NM from a Fe3S4 film deposited under the same conditions as those of samples A–D but with an increased film
thickness of 1.5 lm. A network of connected cracks emerged
clearly on the surface. The inset in Fig. 3(a) shows a typical
microstructure of the cracks, where the ricelike morphology
of the Fe3S4 nanoparticles is the same as those observed in
Fig. 2(d). The cracks typically generated and propagated in
between the ricelike nanoparticles that consist of Fe3S4
nanocrystals. In general, the cracking of thin films, when deposited on a substrate, is usually related to the magnitude of
residual stress, the film thickness, the fracture resistance of
the substrate and the interface, as well as the flaw distributions at/near the interface.21,22 The morphological evolution
as a function of film thickness in Figs. 2 and 3(a) indicates
that the critical thickness for cracking of the roomtemperature deposited Fe3S4 nanocrystalline thin film is in
the range of 1.3–1.5 lm. On the other hand, as revealed in
04D102-3
FIG. 3. (Color online) Microscopic images recorded by Nomarski microscope from 1.5-lm thick Fe3S4 films grown (a) directly on Si, (b) on Si with
a 3-nm Cu intermediate layer, and (c) on Si with a 3-nm Al intermediate
layer. (d) Macroscopic image recorded by a digital camera, showing the
overall cracking and detachment of the 1.5-lm thick Fe3S4 film grown on Si
with a 3-nm Al intermediate layer.
Fig. 2, the onset of clustering and closely-packing of the
Fe3S4 nanocrystals occurred when the film thickness is larger
than 100 nm. This result implies that the film/substrate interface plays a key role in the film cracking observed in Fig.
3(a). In this regard, we have further engineered the Fe3S4/Si
interface by introducing an ultrathin metallic intermediate
layer.
Figure 3(b) shows the NM images of a Fe3S4 nanocrystal
thin film grown with the same parameters as those of the
sample shown in Fig. 3(a) but with a 3-nm Cu layer deposited in the same magnetron-sputter chamber just before the
deposition of Fe3S4. It is clearly seen that the film cracking
is completely suppressed. In a sharp contrast, the film cracking of Fe3S4 is significantly enhanced by an incorporation of
a 3-nm Al intermediate layer at the interface of Fe3S4/Si.
The NM image presented in Fig. 3(c) and the digital-camera
recorded image in Fig. 3(d) were taken from a Fe3S4/Al/Si
sample which has exactly the same growth parameters and
thickness as those of the samples shown in Figs. 3(a) and
3(b). One can see in Figs. 3(c) and 3(d) that the Fe3S4 film
completely cracked in to small pieces and detached from the
substrate. A detailed spectroscopic analysis (not shown for
the sake of brevity) revealed that after Fe3S4 detachment the
ultrathin Al layer still remained on the Si surface, free of any
cracks. These results provide clear-cut evidence that the
fracture resistance of the Fe3S4/Si interface plays an important role in the film cracking of the sputtering deposited
Fe3S4 nanocrystalline thin films.
C. Thermal sulfurization of Fe3S4 thin films
Samples A, B, C, and D were further selected for postdeposition thermal sulfurization and the samples after sulfurization are labeled as SA, SB, SC, and SD, respectively.
The thicker film with Cu-intermediate layer was not
employed in this study to avoid the effect caused by Cu diffusion during the thermal processing. Figure 4 presents the
XRD spectra collected from samples SA-SD as well as the
JVST A - Vacuum, Surfaces, and Films
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04D102-4 H. Liu and D. Chi: Magnetron-sputter deposition of Fe3S4 thin films
FIG. 4. (Color online) XRD (Cu-Ka1) spectra collected from the FeS2 thin
films (by post-deposition thermal sulfurization of Fe3S4) with the thickness
of 20 nm, 100 nm, 500 nm, and 1.3 lm for samples SA, SB, SC, and SD,
respectively. The pattern of JCPDS 06-0710 (Pyrite) is also presented for
easy comparisons. The asterisks indicate the crystal phase impurity
remained in the FeS2 film with a larger thickness.
pattern of JCPDS 06-0710 (Pyrite). When compared with the
as grown Fe3S4 samples, the crystal qualities of the sulfurized samples are significantly improved as indicated by the
increased XRD peak intensity together with the narrowed
peak linewidth. The diffraction angles of the XRD peaks,
especially those of samples SA-SC, agree well with the patter of JCPDS 06-0710 (Pyrite), indicating that FeS2 crystals
with relatively high phase purity are obtained by the postgrowth thermal sulfurization. However, the XRD spectrum
of sample SD exhibits some very weak diffraction peaks
other than those of FeS2 (indicated by the asterisks in Fig.
4). It is difficult to determine the impurity phase structures
because of the relatively weak intensities and the small peak
numbers. Since these diffraction peaks are completely absent
from the spectra of samples SA-SC which have smaller film
thickness, we believe that they are most likely to originate
from the deep regions within the film of sample SD, where
the sulfurization of Fe3S4 is insufficient due to the sulfurpoor condition caused by a limited sulfur diffusion depth.
Nevertheless, in case of thin film photovoltaic applications,
such a limitation of sulfur diffusion depth will not cause any
problem. This is because the film thickness of FeS2 required
for photovoltaic applications, and thus the film thickness of
Fe3S4, is always much smaller than that of sample SD.
Figures 5(a) and 5(b) show the SEM images recorded
from samples SB and SC, respectively. The inset images are
of samples B and C, respectively; they were taken with the
same magnifications as those of samples SB and SC for easy
comparisons. Basically, both the films exhibit an improved
surface smoothness after sulfurization except for those large
white features on the surface of sample SB, which are probably sulfur condensate formed during the sample cooling
down [see Fig. 5(a)]. The ricelike morphology of sample C
[see the inset of Fig. 5(b)] is completely disappeared;
04D102-4
FIG. 5. Top-viewed SEM images recorded from the sulfurized samples SB
(a) and SC (b). The insets in the images are recorded from their parent samples B and C with the same magnifications for easy comparisons.
instead, a smoother surface with an increased packing density emerged after sulfurization [Fig. 5(b)]. These morphological evolutions are consistent with the improvement of
crystal qualities as revealed by XRD. Grain sizes estimated
from the XRD diffraction peaks of the resultant FeS2 thin
films are in the range of 29–37 nm, similar as those obtained
by Fe sulfurization.11,17 However, the film surface of the
FeS2 obtained by sulfuring Fe3S4 in this study [e.g., see Fig.
5(b)] is much smoother than those obtained by sulfuring
Fe.7,12,18
D. Optical absorption spectra
For optical absorption spectra measurements, Fe3S4 thin
films of d ¼ 200-nm thick were deposited on clear-glass substrates under the same sputtering conditions as those discussed above. Both SEM and XRD could not reveal any
significant influences caused by changing the substrate (from
Si to glass) on the film deposition and the thermal sulfurization, mainly due to the low-temperature processes and the
small film thicknesses. Figure 6 shows the absorption coefficient, a ¼ ln½ð1 RÞ=T =d, obtained by measuring the transmittance, T, and reflectance, R, at room temperature from the
iron sulfide thin films before and after sulfurization (with the
same sulfurization parameters as those on Si substrates discussed above). The absorption coefficient of the as grown
Fe3S4 thin film increases slowly with photon energies in the
studied range, exhibiting no apparent absorption edge due to
its semimetallic natures.23 However, one can see that with
the increase in photon energies the increase in absorption
coefficient (in the range of 105 cm1) is significantly accelerated after the sulfurization, forming the apparent optical
band edge. The more developed fringes in the spectra at the
energies smaller than 1.0 eV indicate that the crystal quality,
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04D102-5 H. Liu and D. Chi: Magnetron-sputter deposition of Fe3S4 thin films
FIG. 6. (Color online) Optical absorption spectra measured from 200-nm
thick Fe3S4 films deposited on clear-glass substrates before and after the
post-deposition thermal sulfurization. The straight lines are linear fittings of
the spectrum in segments.
the thickness uniformity, and the surface/interface smoothness of the thin film are apparently improved during the thermal sulfurization, consistent with the XRD and SEM results.
Absorption edge of the FeS2 thin film (i.e., after sulfurization) was further analyzed by segmental linear fittings of the
spectrum (see Fig. 6). The results show two absorption edges
at the energies of 0.9 and 1.2 eV, respectively. The absorption edge of 0.9 eV is smaller but very close to the value of
0.95 eV which is reported as the direct bandgap energy of
semiconducting FeS2 in the literature.1,2,7 The slightly
smaller bandgap energy obtained here could be caused by
crystal defects of the resultant FeS2 and/or fitting errors due
to the unavoidable light interference fringes. The absorption
edge at 1.2 eV is attributable to the indirect allowed transitions of FeS2 with the assistance of phonons due to the
enhanced carrier-phonon coupling at the resonant photon
energies.24
IV. CONCLUSIONS
In conclusion, Fe3S4 nanocrystal films with thicknesses
ranging from 20 nm to 1.5 lm were deposited on Si and
clear-glass substrates by dc-magnetron sputtering at room
temperature. Structural and morphological evolutions of the
deposited thin films provided evidence that onset of locally
clustering and closely packing of Fe3S4 nanocrystals into
ricelike nanoparticles occurred when the film thickness is
larger than 100 nm. A film thickness close to or larger than
1.5 lm resulted in film cracking. The cracks created in
04D102-5
between adjacent ricelike nanoparticles and channeled
within the Fe3S4 film, forming connected crack networks. It
is found that such cracks can be completely suppressed by
an ultrathin (3-nm) Cu intermediate layer deposited prior to
the deposition of Fe3S4 but significantly enhanced by an Al
intermediate layer with the same thickness. A postdeposition thermal sulfurization of Fe3S4 with the film thickness smaller than 500 nm resulted in FeS2 with high crystal
purity. However, sulfurization of Fe3S4 with a larger thickness (1.3 lm) led to the incorporation of phase impurity,
most likely due to the limitation of sulfur diffusion depth.
Optical absorption measurements revealed two absorption
edges at 0.9 and 1.2 eV for the sulfurized FeS2. These two
absorption edges are attributed to the direct bandgap
(0.9 eV) and the indirect allowed transitions (1.2 eV) of
FeS2. The indirect allowed transition is believed to be
assisted by phonons. These results are important for further
developing FeS2-based thin film photovoltaic devices.
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