Structural and optical characterization of indium and gallium indium sulfide films prepared by modulated flux deposition

Article abstract of DOI:10.1002/pssa.200723178

Indium and gallium indium sulfide thin films were deposited on soda-lime glass substrates at 350°C, in a wide range of sulfur delivery conditions, by Modulated Flux Deposition (MFD). Indium sulfide films were highly crystalline β-In₂S₃ (tetragonal) with E_g = 2.68 eV. Depending on the availability of sulfur, gallium indium-poor sulfide films consisted on metal-rich hexagonal β-GaS layers or stoichiometric amorphous films with E_g = 3.0-3.3 eV. Finally, gallium indium-rich sulfide films formed a mixture of β-In₂S₃ and β-GaS that became near amorphous as more sulfur was supplied; a bandgap energy of E _g = 2.44 eV was found.

Full text of DOI:10.1002/pssa.200723178

Original
Paper
phys. stat. sol. (a) 204, No. 10, 3367–3372 (2007) / DOI 10.1002/pssa.200723178
Structural and optical characterization of indium
and gallium indium sulfide films
prepared by modulated flux deposition
*
C. Sanz , C. Guillén, and M. T. Gutiérrez
Energy Department, CIEMAT, Avda. Complutense 22, Madrid 28040, Spain
Received 4 May 2007, revised 15 June 2007, accepted 18 June 2007
Published online 2 August 2007
PACS 68.55.Jk, 68.55.Nq, 78.40.Fy, 78.66.Li, 81.15.Ef
Indium and gallium indium sulfide thin films were deposited on soda-lime glass substrates at 350 °C, in a
wide range of sulfur delivery conditions, by Modulated Flux Deposition (MFD). Indium sulfide films we-
re highly crystalline β-In S (tetragonal) with E = 2.68 eV. Depending on the availability of sulfur, gal-
2
3
g
lium indium-poor sulfide films consisted on metal-rich hexagonal β-GaS layers or stoichiometric amor-
phous films with E = 3.0–3.3 eV. Finally, gallium indium-rich sulfide films formed a mixture of β-In S
3
g
2
and β-GaS that became near amorphous as more sulfur was supplied; a bandgap energy of E = 2.44 eV
g
was found.
©
2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction
Nowadays, there are a variety of chemical routes to obtain III–VI materials. Thin films of these semi-
conductors attract a broad interest due to their optoelectronic and energy potential applications. Cur-
rently, polycrystalline In S thin films are widely investigated as an attractive substitute for the CdS
2
3
buffer layer in Cu(In,Ga)(S,Se) (CIGS) solar cells [1–3]. This material can be obtained by a good num-
2
ber of dry and wet methods [4–10].
Polycrystalline GaS films are a possible substitute for CdS in GaS/CdTe heterojunction solar cells
[
11]; moreover, it might be applied to CIGS and GaAs solar cells. Also, GaS films become effective
surface passivation layers in GaAs-based optoelectronic devices [12–14]. Gallium sulfide films (GaS
and Ga S ) are usually obtained by single-source metal-organic chemical vapor deposition (MO-CVD)
2
3
[15, 16]. Some studies have been done in vacuum thermal evaporation [17] and radio-frequency reactive
magnetron sputtering [18]. However, despite indium sulfide films may be easily attained by chemical
bath deposition (CBD), the literature does not mention any success on gallium sulfide film deposition
from aqueous solutions, which has the handicap of the extensive hydrolysis undergone by gallium (III)
salts [19].
The bandgap energy of In S thin films may be shifted with regard to the bulk In S value
2 3 2 3
(E = 2.0 eV) [15] in a certain degree, due to a variety of effects (quantum size effect, sulfur deficiency,
g
oxygen and sodium doping), up to a maximum value of 2.8 eV [8]. In contrast, hexagonal β-GaS and
monoclinic α-Ga S are reported to have direct bandgaps of 3.05 eV and 3.42 eV, respectively [15]. As
2 3
gallium sulfides exhibit wider bandgap energies than indium sesquisulfide, it might be possible to de-
velop Ga–In–S films with improved properties for optoelectronic and photovoltaic applications. Kim
et al. have reported In Ga S films prepared by spray pyrolysis at 350 °C [20]. Starting from crystalline
2–x
x 3
*
Corresponding author: e-mail: carlos.sanz@ciemat.es, Phone: +34 913 466 672, Fax: +34 913 466 037
©
2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3368
C. Sanz et al.: Structural and optical characterization of In and GaInS films
β-In S films, the gradual addition of gallium reduced their crystallinity. Amorphous films were obtained
2 3
above x = 0.6, the minimum reported E being 2.0 eV. Amorphous films corresponding to InGaS had a
g 3
bandgap energy E = 2.3 eV.
g
Compared to indium sulfide, there is still little research in gallium and gallium-indium sulfide films.
Previous investigations on the In–S system reported the formation of tetragonal β-In S thin films by
2 3
Modulated Flux Deposition (MFD) [21]. However, there is not any available information about Ga–S
and Ga–In–S thin films prepared by this method yet. This work is focused on the effect of sulfur on the
crystallinity and optical properties of Ga–In–S films with different metal atomic ratios. The deposition
parameters have been adjusted in order to achieve films within a range of thickness (around 100 nm) that
provides a high transmission and wide bandgap energies.
2
Experimental
This vacuum thin film deposition technique, named Modulated Flux Deposition (MFD), has been
described in detail elsewhere [22]. Various evaporation sources, which are focused on different points of
the chamber, operate at the same time while the substrate follows a cyclical circular path, so the vapors
from the different metal sources condense alternatively on the heated substrate within a reactive chalco-
gen atmosphere.
Three series of samples were prepared: indium sulfide films (named set A), using a pure indium
evaporation source; gallium indium-poor sulfide films (set B), heating a 90 at% Ga – 10 at% In alloy in
another evaporation source; and finally gallium indium sulfide films (set C), with an intermediate Ga/In
atomic ratio, using both metal evaporation sources.
While the temperatures of evaporation of the metal sources remained constant, a broad interval of
temperature was used for the sulfur source (130 °C < T < 210 °C). The samples were deposited on soda-
S
lime glass (SLG) substrates which rotated at 30 r.p.m. and were hold at the same maximum temperature
–5
(TSubst = 350 °C). The base pressure was around 5 × 10 Pa.
The structure of the samples was analyzed by X-ray diffraction (XRD) using a Phillips X’Pert appara-
tus with nickel-filtered Cu K radiation. Optical properties were determined by measuring transmittance
α1
and reflectance at normal incidence, from 300 nm to 1500 nm, with a Perkin–Elmer Lambda 9
UV/VIS/NIR spectrophotometer. The thickness of the films was determined by means of a DEKTAK
3030 surface profilometer. The chemical composition was determined by Energy Dispersive X-ray
analysis (EDX) using a Hitachi S-2500 Scaning Electron Microscope (SEM); Corning 7059 glass sub-
strates were used for this purpose, in order to avoid the detection interferences of sodium from SLG
substrates.
The absorption coefficient (α) of the films was calculated using their transmittance T, reflectance R
and thickness d from Eq. (1):
4
2
Ê (1 - R)
ˆ
(1 - R)
T
2
+
4R
-
Á
2
˜
¯
Ë
T
-
αd
e
=
.
(1)
2
2
R
3
Results and discussion
As an overview, Fig. 1a shows the variation of the chalcogen/metal atomic ratio as a function of the
sulfur source temperature (T ) for the three mentioned groups of films. From this graph, some prelimi-
S
nary conclusions can be drawn.
Indium sulfide layers (curve A) reached a constant S/In ratio, attributable to In S , at intermediate
2 3
values of T , while it appears that lower and higher T values provided a deficiency and an excess of
S S
sulfur, respectively. Regarding to gallium indium-poor sulfide films (curve B), the S/(Ga + In) ratio
increased as T was risen until a nearly constant value corresponding to GaS was reached. Finally, galli-
S
um indium sulfide films (curve C), which were sulfur deficient at low T values, reached chalcogen/metal
S
©
2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.pss-a.com

Original
Paper
phys. stat. sol. (a) 204, No. 10 (2007)
3369
Fig. 1 a) Variation of the chalcogen/metal atomic ratio with respect to the sulfur source temperature within the sets
of films A–C. b) Absorption coefficient (α) of high-transmission sulfide layers selected from the sets of films A–C.
atomic ratios between M S and MS as T was increased; therefore, it might be interpreted as the forma-
2 3 S
tion of a mixture of the two proposed sulfides (In S and GaS).
2 3
Figure 1b compares the absorption coefficient (α) of the most representative layers of these sets,
namely those that had chalcogen/metal atomic ratios higher than 1 and the highest transmission spectra.
Here, the selected gallium indium sulfide film (named C1) showed the shortest bandgap. In comparison,
the absorption edge of the indium sulfide film (named A1) was slightly shifted towards higher energies.
Finally, both selected gallium indium-poor sulfide films (named B1 and B2) exhibited the widest band-
gaps.
Assuming these semiconductor compounds to have a direct interband transition, their optical bandgap
2
energies (E ) were calculated by plotting (αhν) versus hν and making a linear interpolation near the
g
absorption edge region. Hence, the estimated values of E were the following: 2.44 eV (sample C1),
g
2.68 eV (A1), 2.97 and 3.30 eV (B1 and B2).
Next, each individual group of samples will be discussed in detail.
3.1 Indium sulfide films
Firstly, a set of indium sulfide layers was prepared at different T values. It is worth noting that these
S
In–S films (Fig. 2a) had quite more intense diffraction lines than the two remaining sets of films, as it
will be shown in Sections 3.2 and 3.3.
According to Fig. 2a, all the samples had a tetragonal β-In S structure (JCPDS 25-0390) and exhibited
2 3
a strong (103) orientation, as observed in previous investigations [21]. Together with Fig. 1a, we can
observe that the tetragonal structure was retained even in largely sulfur-deficient samples, despite it
might be expected from their stoichiometries the appearance of indium-richer phases (monoclinic In S ,
6
7
orthorhombic InS). The FWHM values of the (103) peak confirm that the layer obtained at the lowest
value of T (130 °C) had a certain degree of crystallinity, that it became sharply enhanced as T was
S
S
increased up to T = 190 °C (sample A1 in Fig. 1a and b), and that it decreased partially in the sulfur-
S
richest layer prepared at T = 210 °C.
S
Regarding to the optical properties, their transmission spectra are shown in Fig. 2b. The sample ob-
tained at T = 130 °C, which was the least crystalline of them, exhibited a rather low transmission. In
S
contrast, the increase on the sulfur temperature made the remaining samples to reach transmission
maxima higher than 80%.
In summary, β-In S MFD-films achieved a high transmission and a good crystallinity as soon as a
2 3
minimal amount of sulfur was available.
www.pss-a.com
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3370
C. Sanz et al.: Structural and optical characterization of In and GaInS films
Fig. 2 a) Diffraction patterns and b) transmission spectra of In–S films obtained at different sulfur source tempera-
tures. The vertical notations indicate the FWHM values of the (103) peak.
3.2 Gallium indium-poor sulfide films
Secondly, a series of gallium sulfide films with a low content of indium was prepared. In contrast to the
moderate influence of sulfur on the structural and optical properties of indium sulfide layers, the avai-
lability of sulfur played a critical role in these samples.
At low T values (130 °C and 150 °C), the diffraction patterns indicated the formation of hexagonal
S
gallium monosulfide (β-GaS) (JCPDS 84-499), which is a layered compound with the atomic layers
parallel to the (001) crystallographic plane. As only diffraction from (00n) planes was observed in these
polycrystalline films, it is clear that the atomic layers had a parallel orientation with respect to the surface
of the substrate. This preferential orientation was also observed in GaS films prepared by a sputtering
method [18]. The increase on T from 130 °C to 190 °C, corresponding to the incorporation of a larger
S
amount of sulfur into the film, reduced the intensity of the diffraction lines gradually, until amorphous
films were obtained (Fig. 3a). Further increase on sulfur temperature (T = 200 °C) provided an amor-
S
phous sample as well.
From an optical point of view (Fig. 3b), the polycrystalline sulfur-poorest film (T = 130 °C) exhibited
S
a really low transmission spectra. It may seem that the optical behavior of the films changed at
TS = 190 °C. Thus, an amorphous structure and a high transmission were reached at the same time. More-
Fig. 3 a) Diffraction patterns and b) transmission spectra of gallium indium-poor sulfide films deposited at
different sulfur source temperatures.
©
2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.pss-a.com

Original
Paper
phys. stat. sol. (a) 204, No. 10 (2007)
3371
over, the additional increase on T provided an upper maximum of transmission as well as a shift of the
S
absorption edge towards higher energies. In Fig. 1a and b, these amorphous layers prepared at
TS = 190 °C and 200 °C are named B1 and B2, respectively.
It remains to elucidate if the small content of indium of these films played a significant role in their
properties, or if they are essentially inherent to the gallium sulfide material obtained by this technique. In
this sense, some investigation focused on the preparation of pure GaS films at the same substrate tem-
perature is under way.
3.3 Gallium indium-rich sulfide films
Eventually, a collection of films was deposited using both metal evaporation sources, in order to obtain
films with significant amounts of gallium and indium.
In Fig. 4a, films deposited at low sulfur temperatures (160 °C and 170 °C), having a clear chalcogen
deficiency according to Fig. 1a, showed diffraction lines corresponding to the sulfides (001)-oriented
β-GaS and (103)-oriented β-In S . On increasing the sulfur temperature (180 °C and 190 °C), which
2
3
provided layers with chalcogen/metal ratios between MS and M S , the diffraction lines decreased and
2 3
near amorphous films were obtained.
In contrast to the previous collection of gallium sulfide films (Section 3.2), whose low content of
indium did not allow the formation of β-In S , this set of samples with a significant amount of indium
2
3
showed some In S diffraction peaks. However, under no circumstances they became intense, as it could
2 3
be expected taking into account the excellent crystallinity observed in the In–S binary system (Sec-
tion 3.1).
According to Fig. 4b, the samples obtained at low T values (160 °C and 170 °C) exhibited a mo-
S
derate transmission. The maximum transmission was achieved by the sample prepared at T = 180 °C
S
(named C1 in Fig. 1a and b), having a stoichiometry near to MS (Fig. 1a). Also, the transmission de-
creased for the film prepared at T = 190 °C, whose chalcogen/metal ratio (also between MS and M S )
S 2 3
was higher.
Therefore, it has not been observed the formation of any ternary compound. On the contrary, it ap-
pears that a kind of competition for sulfur was established between gallium and indium at low T values,
S
resulting in a mixture of both binary sulfides. Even more, it would seem that the gallium played an inter-
fering role on the indium sesquisulfide formation and, as a consequence, it reduced its crystallinity
markedly. In any case, as T was increased, both polycrystalline compounds tended to form an amor-
S
phous mixed sulfide.
Fig. 4 a) Diffraction patterns and b) transmission spectra of gallium indium-rich sulfide films obtained at different
sulfur source temperatures.
www.pss-a.com
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3372
C. Sanz et al.: Structural and optical characterization of In and GaInS films
4
Conclusions
Indium sulfide thin films were deposited on glass substrates at 350 °C by Modulated Flux Deposition
MFD). Good-quality tetragonal β-In S films with (103) orientation and E = 2.68 eV were easily
(
2
3
g
obtained. As soon as a minimal amount of sulfur was delivered, their crystallinity as well as their
transmission enhanced strongly. However, gallium sulfide films with a low content of indium had a more
complex behavior, the availability of sulfur being critical. A low delivery of sulfur provided (001)-
oriented hexagonal β-GaS films, but it was associated to a gallium excess that provided low transmission
films. Larger amounts of sulfur formed amorphous films with transmission maxima higher than indium
sulfide layers; their estimated bandgap energy values (E ) varied between 3.0 eV and 3.3 eV. Finally,
g
gallium indium sulfide films were prepared; X-ray diffraction did not show any ternary compound, but a
mixture of β-GaS and β-In S was formed. When the sulfur content was increased, the mixture of crystal-
2
3
line sulfides tended to form an amorphous mixed sulfide with a higher transmission; the bandgap energy
was E = 2.44 eV, slightly lower than for the indium sulfide films.
g
Acknowledgments This work has been supported by the FOTOFLEX-CM Project (S-0505/ENE-O123) of IV
PRICIT by Comunidad de Madrid, SENFOVOL Project (CIT-120000-2005-6), “Células fotovoltaicas ligeras y
flexibles” Project by Fundación Ramón Areces and CONSOLIDER GENESIS FV Project (CSD2006-04).
References
[1] S. Gall, N. Barreau, S. Harel, J. C. Bernède, and J. Kessler, Thin Solid Films 480/481, 138 (2005).
[2] N. Naghavi, S. Spiering, M. Powalla, B. Cavana, and D. Lincot, Prog. Photovolt., Res. Appl. 11, 437 (2003).
[3] E. B. Yousfi, B. Weinberger, F. Donsanti, P. Cowache, and D. Lincot, Thin Solid Films 387, 29 (2001).
[4] S. Kosaraju, J. A. Marino, J. A. Harvey, and C. A. Wolden, Sol. Energy Mater. Sol. Cells 90, 1121 (2006).
[5] A. Timoumi, H. Bouzouita, R. Brini, M. Kanzari, and B. Rezig, Appl. Surf. Sci. 253, 306 (2006).
[6] M. Calixto-Rodríguez, A. Tiburcio-Silver, A. Ortiz, and A. Sanchez-Juarez, Thin Solid Films 480/481, 133
(
2005).
7] N. Barreau, J. C. Bernède, H. El Maliki, S. Marsillac, X. Castel, and J. Pinel, Solid State Commun. 122, 445
2002).
[
(
[
8] J. George, K. S. Joseph, B. Pradeep, and T. I. Palson, phys. stat. sol. (a) 106, 123 (1988).
9] M. G. Sandoval-Paz, M. Sotelo-Lerma, J. J. Valenzuela-Jáuregui, M. Flores-Acosta, and R. Ramírez-Bon, Thin
Solid Films 472, 5 (2005).
[
[10] C. D. Lokhande, A. Ennaoui, P. S. Patil, M. Giersig, K. Diesner, M. Muller, and H. Tributsch, Thin Solid Films
3
40, 18 (1999).
[
[
[
[
[
[
11] E. Cuculescu, I. Evtodiev, M. Caraman, and M. Rusu, J. Optoelectron. Adv. Mater. 8, 1077 (2006).
12] A. N. MacInnes, M. B. Power, A. R. Barron, P. P. Jenkins, and A. F. Hepp, Appl. Phys. Lett. 62, 711 (1993).
13] X. A. Cao, H. T. Hu, Y. Dong, X. M. Ding, and X. Y. Hou, J. Appl. Phys. 86, 6940 (1999).
14] N. Okamoto and H. Tanaka, Jpn. J. Appl. Phys. 37, 484/485 (1998).
15] M. Lazell, P. O’Brien, D. J. Otway, and J.-H. Park, J. Chem. Soc. Dalton Trans. 4479 (2000).
16] S. Schulz, E. G. Gillan, J. L. Ross, L. M. Rogers, R. D. Rogers, and A. R. Barron, Organometallics 15, 4880
(1996).
[
[
[
17] S. Suh and D. M. Hoffman, Chem. Mater. 12, 2794 (2000).
18] M. Ohyama, H. Ito, and M. Takeuchi, Jpn. J. Appl. Phys. 44(7A), 4780 (2005).
19] D. S. Boyle, K. Govender, R. L. Hazelton, and P. O’Brien, 203rd Meeting of the Electrochem. Soc. (Paris,
2003).
[
[
[
20] W.-T. Kim, H.-S. Kim, Y.-G. Kim, and S.-R. Hahn, J. Mater. Sci. Lett. 6, 479 (1987).
21] C. Sanz, C. Guillén, and M. T. Gutiérrez, Thin Solid Films 511/512, 121 (2006).
22] T. García, C. Guillén, J. Herrero, M. T. Gutiérrez, and F. Briones, Proceedings of Photovoltaic in Europe: from
Photovoltaic Technology to Energy Solutions, Rome, Italy, November 2002, p. 412.
©
2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.pss-a.com