X-ray absorption spectra and conduction band structure of In 2S3

Article abstract of DOI:10.1016/j.ssc.2004.04.050

X-ray absorption spectra of the spinel In₂S₃ were recorded at the sulphur K and indium L₁ and L₃ absorption edges, scanning the unoccupied electronic states of S p, In p, and In (s,d) symmetry, respectively. Alignment of all spectra on a common scale and comparison with calculated densities of states allowed an interpretation of all observed features in terms of final state character, chemical bonding and local symmetry. A picture of the conduction band structure is established.

Full text of DOI:10.1016/j.ssc.2004.04.050

Solid State Communications 131 (2004) 257–260
www.elsevier.com/locate/ssc
X-ray absorption spectra and conduction band structure of In2S3
*
M. Womes, J.C. Jumas , J. Olivier-Fourcade
´ ´ ´ ´
Universite Montpellier II, Laboratoire des Agregats Moleculaires et Materiaux Inorganiques (CNRS UMR 5072), CC015,
Place Eugene Bataillon, F-34095 Montpellier Cedex 5, France
Received 14 July 2003; received in revised form 5 April 2004; accepted 14 May 2004 by H. Eschrig
Available online 2 June 2004
Abstract
X-ray absorption spectra of the spinel In2S3 were recorded at the sulphur K and indium L1 and L3 absorption edges, scanning
the unoccupied electronic states of S p, In p, and In (s,d) symmetry, respectively. Alignment of all spectra on a common scale
and comparison with calculated densities of states allowed an interpretation of all observed features in terms of final state
character, chemical bonding and local symmetry. A picture of the conduction band structure is established.
q 2004 Elsevier Ltd. All rights reserved.
PACS: 61.10 H; 71.20
Keywords: A. Semiconductors; D. Electronic band structure; E. X-ray absorption spectroscopy
1. Introduction
would be of particular importance for the study of variations
of the band gap.
In this letter we present an analysis of the conduction
band of the spinel In2S3 by X-ray absorption spectroscopy.
The atomic arrangement in this phase is identical to that in
In2S3 thin films [2–5]. The cation distribution can be written
as (InI0I.I67A0.33)[In2III]S4 (round brackets: tetrahedral or
A-site, square brackets: octahedral or B-site, A: unoccupied
site) with ordered (b-In2S3) or random (a-In2S3) distribution
of cations and vacancies on tetrahedral sites [8]. Sulphur is
bonded to three cations on B-sites and one cation on an
A-site when the nearest tetrahedral site is occupied. The
octahedral sites in the spinel lattice are strongly distorted by
the displacement of the anions in [111] direction away from
their crystallographic 32e position in space group Fd3m,
while the Td symmetry of the tetrahedral sites is rather well
conserved [9].
Indium(III)-sulphide, In2S3, is currently intensively
studied in view of applications in photovoltaic devices,
with the aim to replace CdS for environmental reasons
[1–5]. Thin films of In2S3 deposited on photo-conducting
metal oxides allow to expand the light-sensitive domain of
the latter [1]. Moreover, the optical band gap of the In2S3
films themselves can be varied by an appropriate choice of
the synthesis conditions [2–5]. This variation is due to a
shift of the lower edge of the conduction band while the top
of the valence band remains unaffected [5]. A comprehen-
sion of the observed phenomena needs a detailed knowledge
of the electron band structure of the material. Investigations
of the electronic states of In2S3, whether in crystalline
form or as thin films, are however, still rather scarce.
Lefebvre et al. presented a calculation of the indium and
sulphur s and p densities of states (DOS) in the spinel In2S3
between 215 and þ10 eV with respect to the Fermi energy
EF [6]. Ihara et al. studied the valence band states by X-ray
photoelectron spectroscopy [7]. No investigation of the
unoccupied electronic states is known so far, although this
X-ray absorption spectroscopy allows to scan the
unoccupied electronic states separately for the different
final state symmetries (s, p, d, …) and separately for each
element. By recording spectra at the sulphur K and indium
L1 and L3 X-ray absorption edges, corresponding to S
1s ! S 3p, In 2s ! In 5p, and In 2p3/2 ! In 5s, 5d
transitions, respectively, we scan the most relevant elec-
tronic states composing the bottom of the conduction band.
All spectra are aligned on a common energy scale with the
*
Corresponding author. Fax: þ33-4-67-14-33-04.
E-mail address: jumas@univ-montp2.fr (J.C. Jumas).
0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2004.04.050

258
M. Womes et al. / Solid State Communications 131 (2004) 257–260
help of the calculated DOS from Ref. [6]. This approach is
based on the assumption of a sufficient screening of the core
hole created by the absorption process. The presence of a
core hole affects mainly transitions in the vicinity of the
absorption threshold, where the kinetic energy of the photo-
excited electron is lowest. These transitions are shifted
towards lower energies, while spectral features at higher
transition energies are rather unaffected. In the present case
core hole effects on peak positions turned out to be
negligible so that the experimental spectra are directly
comparable to densities of states calculated for the
electronic ground state. This same procedure has previously
been applied with success to the spectra of FeS and FeS2
[10] and allows a coherent interpretation of all spectral
features and to establish an experimental picture of the
conduction band structure over a range of approximately
30 eV above EF:
2. Experimental
Polycrystalline In2S3 was synthesised by direct reaction
of a stoichiometric mixture of the elements in an evacuated
silica tube at 1050 8C during 2 h followed by a first
annealing at 800 8C during 4 days and a second annealing
at 450 8C for 3 h. The reaction led to a reddish powder which
X-ray powder diffraction showed to be the pure a-In2S3
spinel phase. X-ray absorption spectra were recorded at the
SuperACO (sulphur) and DCI (indium) storage rings of
LURE, Orsay. All spectra were recorded in transmission
geometry on finely ground powders sieved to 5 mm and
deposited on a microporous membrane. The deposits were
obtained by dispersing the powder in cyclohexane followed
by filtering the solvent through the membrane. The
beamlines were equipped with Si(111) double crystal
monochromators, which were calibrated by setting the
maximum of the S K edge white line in FeS2 to 2472.1 eV
[11] and the first inflection point on the K edge of a Ti metal
foil to 4966.0 eV [12]. The spectral resolution is limited by
the broadening caused by the monochromator (0.3 eV at
2.5 keV, 1.5 eV at 4 keV) and by the finite life time of the
core hole (0.59 eV for S 1s, 3 eV for In 2s and 2p [13]).
Fig. 1. X-ray spectra of In2S3 recorded at the S K, In L1 and In L3
absorption edges, aligned on a common energy scale together with
calculated DOS from Ref. [6].
the distinguishable bands in terms of atomic origin and final
state symmetry as deduced from the spectra and with their
position relative to the top of the valence band as determined
from Fig. 1.
The calculated DOS shows that the In 5p states are split
into two bands from octahedral indium, reflecting the
aforementioned distortion on the B-sites, plus a third, single
band from tetrahedral indium, in agreement with the local Td
symmetry on A-sites. The appearance of unoccupied In 5s
states is consistent with the formal electron configuration
[Kr] 4d10 5s0 5p0 of In3þ. The distribution of In 5s states is
not affected by distortions on lattice sites. The density of
vacant S 3p states shows two structures a and b. Comparison
with the In DOS allows to attribute a and b to sulphur 3p
states hybridised with In 5s and 5p states, respectively.
Structure b comprises S 3p states mixed with all three In p
bands. Their individual contributions are smeared out by the
Gaussian broadening function applied to the calculated
DOS. They give in their sum a single, large band, which is
asymmetrically broadened towards lower energies in
agreement with the shape and the position of the In p
bands. The experimental S K edge spectrum shows two
intense structures, a narrow peak at lower energy and a
broader, asymmetric peak at higher energy, which can be
attributed to transitions to bands a and b, respectively. Their
maxima are separated by 4.2 eV, in good agreement with the
calculated energy difference a 2 b of 3.9 eV. The good
agreement between the theoretical value, calculated for the
3. Results and discussion
Fig. 1 shows the S K, In L1, and In L3 edge X-ray
absorption spectra together with the calculated sulphur p
and indium s and p DOS, convoluted with a Gaussian
function to simulate experimental broadening [6]. The zero
of the common energy scale has been chosen at the top of
the valence band as determined in Ref. [7]. Table 1 lists all
spectral features discussed in the following with their
absolute energies at which they appear on the X-ray
absorption spectra. The table gives also the composition of

M. Womes et al. / Solid State Communications 131 (2004) 257–260
259
Table 1
Composition of bands of unoccupied states and their position relative to the top of the valence band (Ref. [7]) determined from Fig. 1. Absolute
energies of the corresponding spectral features on the X-ray absorption spectra. All energies in eV
Spectral feature
a
b
c
d
e
Relative position
Band character
S K edge
2.8
7.0
10
17
30
S p þ In s
2471.2
–
S p þ In p, d
2475.4
4244.0
3739
In d, p þ S p
In d, p þ S p
2485.5
4254
In d þ S p
2498.2
–
–
In L1 edge
–
In L3 edge
3734.5
3742
3748
3762.3
electronic ground state, and the experimental value confirms
for the S K edge our assumption of negligible core hole
effects, which otherwise would have affected peak a to a
higher extent than peak b. The S K edge spectrum is placed
on Fig. 1 such that the two lowest absorption bands match
with structures a and b of the S p DOS. It can be noted that
the relative positions of a and b are predominantly
determined by chemical bonding to octahedrally co-
ordinated indium, three times as abundant as In on A-
sites. The large structure b comprises the unresolved
contributions from S 3p states hybridised with each of the
In p bands.
[16] on the band structures of 4d transition metals, In and Sn
lets conclude on contributions from In 4f states in the range
of structures d and e.
It is interesting to compare the X-ray spectra of In2S3 and
SnS2. The formal electron configurations of the cations and
anions are the same in both compounds, 4d10 5s0 5p0
and 3p6, respectively, but the compounds differ in structure.
Tin(IV)-sulphide has a lamellar structure of the CdI2 type
composed of SnS6-octahedra of almost ideal Oh symmetry
[17]. The Sn 5p states form a single narrow band in SnS2 [6].
The S K edge spectrum of SnS2 (Fig. 2) shows three narrow
peaks a, b, and c, which have been identified as transitions to
mixed S 3p–Sn 5s, S 3p–Sn 5p5d, and S 3p–Sn 5d states,
respectively [18]. The narrow band b reflects the weak
spread in energy of the Sn 5p states owing to the high local
symmetry on the Sn site while the In 5p states are more
dispersed in energy due to the lower local symmetry on
the spinel B-site and the presence of In on a second
At the In L1 edge we observe a white line, which is
consistent with the high density of unoccupied metal p states
at the bottom of the conduction band. Due to the limited
spectral resolution the three peaks of the In p DOS are not
resolved. The maximum of the white line is aligned with that
of the S K edge peak b. It can be noted that the spectral
feature b is asymmetric and broadened towards the high
energy side on both the S K and In L1 edge.
The In L3 edge consists of a series of peaks of which the
lowest in energy contains transitions to vacant In 5s states. It
is therefore matched with band a on Fig. 1. According to the
selection rules for dipolar electronic transitions, final states
of both s and d symmetry are accessible from the 2p3/2
ground state. Thus, some admixture of In d states to band a
by hybridisation cannot be excluded. However, the
transition matrix element is much greater for p-d than for
p-s transitions. Any d density contributing to the weak
spectral structure a in addition to the s band is, therefore,
expected to be negligible. The calculated DOS shows no
important In s density above band a so that all other
structures in the experimental spectrum can be interpreted as
p-d transitions. The alignment of the first In L3 edge peak
with a gives coincidence of the second peak with band b
which thus consists of hybridised S 3p-In 5p5d states.
Further In d states coincide around c with the high energy
tail of structure b of the S K and In L1 edge, indicating the
presence of an In d band with minor contributions from In p
and S p states. Two further bands of S 3p-In 5d character,
labelled d and e, appear clearly in the S K and In L3 spectra.
Minor contributions from In p states to d can be deduced
from a corresponding weak structure in the In L1 edge
spectrum. The work of Mu¨ller et al. [14,15] and of Sham
Fig. 2. Comparison of the S K edge X-ray absorption spectra of
SnS2 and In2S3.

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M. Womes et al. / Solid State Communications 131 (2004) 257–260
to Inþ on tetrahedral sites with consecutive filling of the S
3p–Intet 5s band, which represents the lowermost part of the
conduction band. This could be due to non-stoichiometries
caused by the synthesis process or by a charge transfer from
the support to the In2S3 film. A further mechanism could be
partial or complete ordering of the vacancies on tetrahedral
sites, which would change the local symmetry on the anion
sites and could also affect the band structure. Such effects
should be observable by X-ray absorption spectroscopy.
Acknowledgements
The authors are grateful to A.M. Flank, R.C. Karnatak
and J.M. Esteva of the LURE staff for their assistance in
recording X-ray absorption spectra.
Fig. 3. Comparison of the metal L3 edge X-ray absorption spectra of
SnS2 and In2S3.
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