X-ray photoelectron spectra and electronic structure of quasi-one-dimensional SbSeI crystals
The paper presents the X-ray photoelectron spectra (XPS) of the valence band (VB) and of the principal core levels from the (110) and (001) crystal surfaces for the quasi-one-dimensional high permittivity SbSeI single crystal isostructural to ferroelectric SbSI. The XPS were measured with monochroma...
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Інститут фізики конденсованих систем НАН України
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irk-123456789-1180652017-05-29T03:04:01Z X-ray photoelectron spectra and electronic structure of quasi-one-dimensional SbSeI crystals Grigas, J. Talik, E. Adamiec, M. Lazauskas, V. The paper presents the X-ray photoelectron spectra (XPS) of the valence band (VB) and of the principal core levels from the (110) and (001) crystal surfaces for the quasi-one-dimensional high permittivity SbSeI single crystal isostructural to ferroelectric SbSI. The XPS were measured with monochromatized Al Ka radiation in the energy range of 0–1400 eV at room temperature. The VB is located from 1.6 to 20 eV below the Fermi level. Experimental energies of the VB and core levels are compared with the results of theoretical ab initio calculations of the molecular model of the SbSeI crystal. The electronic structure of the VB is revealed. Shifts in the core-level binding energies of surface atoms relative to bulk ones, which show a dependency on surface crystallography, have been observed. The chemical shifts of the core levels (CL) in the SbSeI crystal for the Sb, I and Se states are obtained. Дана стаття представляє рентгенiвськi фотоелектроннi спектри (РФС) валентної зони (ВЗ) i остовних рiвнiв кристалiчних поверхонь (110) i (001) квазiодновимiрного монокристалу SbSeI з високою проникливiстю, який iзоструктурний сегнетоелектрику SbSI. РФС вимiряно з використанням монохроматичного випромiнювання Al K в енергетичному дiапазонi 0–1400 еВ при кiмнатнiй температурi. ВЗ розташована вiд 1.6 до 20 еВ нижче рiвня Фермi. Експериментальнi енергiї ВЗ та остовних рiвнiв порiвняно з результатами теоретичних розрахункiв “з перших принципiв” для молекулярної моделi кристалу SbSeI. Встановлено електронну структуру ВЗ. Виявлено зсуви остовних енергiй зв’язку поверхневих атомiв вiдносно об’ємних, якi демонструють залежнiсть вiд кристалографiї поверхнi. Отримано хiмiчнi зсуви остовних рiвнiв у кристалi SbSeI для Sb, Se та I. 2007 Article X-ray photoelectron spectra and electronic structure of quasi-one-dimensional SbSeI crystals / J. Grigas, E. Talik, M. Adamiec, V. Lazauskas // Condensed Matter Physics. — 2007. — Т. 10, № 1(49). — С. 101-110. — Бібліогр.: 11 назв. — англ. 1607-324X PACS: 79.60.-i, 82.80.Pv, 87.64.Lg DOI:10.5488/CMP.10.1.101 http://dspace.nbuv.gov.ua/handle/123456789/118065 en Condensed Matter Physics Інститут фізики конденсованих систем НАН України |
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The paper presents the X-ray photoelectron spectra (XPS) of the valence band (VB) and of the principal core levels from the (110) and (001) crystal surfaces for the quasi-one-dimensional high permittivity SbSeI single crystal isostructural to ferroelectric SbSI. The XPS were measured with monochromatized Al Ka radiation in the energy range of 0–1400 eV at room temperature. The VB is located from 1.6 to 20 eV below the Fermi level. Experimental energies of the VB and core levels are compared with the results of theoretical ab initio calculations of the molecular model of the SbSeI crystal. The electronic structure of the VB is revealed. Shifts in the core-level binding energies of surface atoms relative to bulk ones, which show a dependency on surface crystallography, have been observed. The chemical shifts of the core levels (CL) in the SbSeI crystal for the Sb, I and Se states are obtained. |
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Article |
| author |
Grigas, J. Talik, E. Adamiec, M. Lazauskas, V. |
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Grigas, J. Talik, E. Adamiec, M. Lazauskas, V. X-ray photoelectron spectra and electronic structure of quasi-one-dimensional SbSeI crystals Condensed Matter Physics |
| author_facet |
Grigas, J. Talik, E. Adamiec, M. Lazauskas, V. |
| author_sort |
Grigas, J. |
| title |
X-ray photoelectron spectra and electronic structure of quasi-one-dimensional SbSeI crystals |
| title_short |
X-ray photoelectron spectra and electronic structure of quasi-one-dimensional SbSeI crystals |
| title_full |
X-ray photoelectron spectra and electronic structure of quasi-one-dimensional SbSeI crystals |
| title_fullStr |
X-ray photoelectron spectra and electronic structure of quasi-one-dimensional SbSeI crystals |
| title_full_unstemmed |
X-ray photoelectron spectra and electronic structure of quasi-one-dimensional SbSeI crystals |
| title_sort |
x-ray photoelectron spectra and electronic structure of quasi-one-dimensional sbsei crystals |
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Інститут фізики конденсованих систем НАН України |
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2007 |
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http://dspace.nbuv.gov.ua/handle/123456789/118065 |
| citation_txt |
X-ray photoelectron spectra and electronic structure of quasi-one-dimensional SbSeI crystals / J. Grigas, E. Talik, M. Adamiec, V. Lazauskas // Condensed Matter Physics. — 2007. — Т. 10, № 1(49). — С. 101-110. — Бібліогр.: 11 назв. — англ. |
| series |
Condensed Matter Physics |
| work_keys_str_mv |
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2025-07-08T13:18:22Z |
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2025-07-08T13:18:22Z |
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1837084919958863872 |
| fulltext |
Condensed Matter Physics 2007, Vol. 10, No 1(49), pp. 101–110
X-ray photoelectron spectra and electronic structure of
quasi-one-dimensional SbSeI crystals
J.Grigas1, E.Talik2, M.Adamiec2, V.Lazauskas3
1 Faculty of Physics, Vilnius University, Sauletekio 9/3, LT–10222 Vilnius, Lithuania
2 Institute of Physics, Silesian University, Universytetska 4, 40–007 Katowice, Poland
3 Institute of Theor. Phys. and Astronomy, Vilnius University, Gostauto 12, LT–01108 Vilnius, Lithuania
Received September 4, 2006
The paper presents the X-ray photoelectron spectra (XPS) of the valence band (VB) and of the principal core
levels from the (110) and (001) crystal surfaces for the quasi-one-dimensional high permittivity SbSeI single
crystal isostructural to ferroelectric SbSI. The XPS were measured with monochromatized Al Ka radiation in
the energy range of 0–1400 eV at room temperature. The VB is located from 1.6 to 20 eV below the Fermi
level. Experimental energies of the VB and core levels are compared with the results of theoretical ab initio
calculations of the molecular model of the SbSeI crystal. The electronic structure of the VB is revealed. Shifts
in the core-level binding energies of surface atoms relative to bulk ones, which show a dependency on surface
crystallography, have been observed. The chemical shifts of the core levels (CL) in the SbSeI crystal for the
Sb, I and Se states are obtained.
Key words: SbSeI, electronic structure, XPS
PACS: 79.60.-i, 82.80.Pv, 87.64.Lg
1. Introduction
At present X-ray photoelectron spectroscopy (XPS) is widely used for investigating solids. Due
to the reduced co-ordination number, surface atoms have a potential different from the bulk atoms.
Surface CL shifts of the order of ∆Eb ≈ 0.5 eV towards both higher and lower binding energies
have been experimentally observed in different materials [1–4]. The magnitude and the sign of the
shift depend on electronic and structural properties of the bulk and surface atoms, as well as on a
surface structure. XPS studies also revealed a splitting of the CL in the incommensurate phase of
the ferroelectric semiconductor TlInS2 [5].
Antimony sulphoiodide, SbSI, is a well-known quasi-one-dimensional ferroelectric semicon-
ductor, which exhibits a number of prominent strongly coupled semiconductive and ferroelectric
properties [6]. XPS studies [7] of the SbSI single crystals revealed a huge crystallographic plane-
dependent splitting (∼ 3 ÷ 5 eV) of the CL due to the different valence state of surface and bulk
atoms. Theoretical ab initio calculations confirmed these findings. The valence band (VB) and CL
of this quasi-one-dimensional ferroelectric are extremely sensitive to the changes of the chemical
environment of atoms.
SbSeI is isostructural to the ferroelectric SbSI [8]. At room temperature, SbSeI crystals possess
a high ionic dielectric permittivity (ε ≈ 300), which increases with the temperature decrease [6].
However, due to a higher atomic form factor, i.e. higher density of electrons of Se in SbSeI than of
S in SbSI, the SbSeI crystals are not ferroelectrics in the whole temperature range [9].
The purpose of this paper is to study XPS of the isostructural to SbSI non-ferroelectric SbSeI
crystal, to discuss the form and the electronic structure of the VB and to reveal how the CL
splitting is related to different valence states of the surface and bulk atoms, and to the quasi-one-
dimensionality of these crystals.
c© J.Grigas, E.Talik, M.Adamiec, V.Lazauskas 101
J.Grigas et al.
2. Experimental
The SbSeI crystals were grown from a melt. The obtained crystals were 20 mm long and
5 × 5 mm2 in area. They are composed of the chains along the [001] axis. The XPS valence
band and the principal core level spectra were measured with monochromatic Al Ka radiation
(1486.6 eV) using PHI 5700/660 Physical Electronics Spectrometer. The photoelectron spectra as
a function of kinetic energy were analysed in the energy range of 0–1400 eV using a hemispherical
mirror analyser with an energy resolution of about 0.3 eV. The FAT mode of the spectrometer
was used – the pass energy of the analyzer is held at a constant value and the transfer lens system
retards the given kinetic energy channel to the range accepted by the analyzer. The measurements
were performed on the (110) (broken lengthwise) and (001) (broken crosswise) crystal surfaces
cleaved in vacuum, in the low 10−10 torr range in order to obtain a clean surface. The surface
stoichiometry and quality of the both (110) and (001) cleavage planes were checked by measuring
the characteristic XPS spectrum (figure 1). The only contaminant was found to be carbon. No
other spectral features showing non-stoichiometry or impurities have been found. The size of the
x-ray spot was about several square microns. The angle between the X-ray source and the analyzer
was 90 deg. The electron emission angle, measured from the sample normal, was 45 deg. Carbon
C 1s (284.5 eV) peak was applied for calibration.
While investigating the XPS, it is important to determine the relative concentrations of various
constituents. The Multipak Physical Electronics program enables us to perform quantification of
the XPS spectra utilizing peak area and peak height sensitivity factors (5.334 for Sb 3d; 0.821
for Se 3d; 6.303 for I 3d). The standard atomic concentration calculation provides a ratio of each
component to the sum of the other taken into account the elements in the data. Only those
elements for which the specific line is clearly visible in the spectrum are considered. For those
lines the background is subtracted using the Shirley method. The limit of the region of the line
is individually selected and then the integration is done. The peak shapes were fitted after a
background subtraction, using the Voigtian function.
3. Ab initio calculation of the energy levels, bond order and the charge of
atoms
The method of calculation is based on Hartree-Fock-Roothaan (HFR) equations in the Lin-
ear Combinations of Atomic Orbitals (LCAO) approximation for the molecular orbitals (MO).
According to the Koopmans’ theorem, the one-particle energies obtained from the canonical HRF
equations correspond to the approximate energies of the ionisation potential. The molecular orbital
(MO) is a linear combination of atomic orbitals (AO). The MO (φi) can be expanded by the AO
(χµ (r)) base:
φi(r) =
M∑
µ=1
Ciµχµ(r), (1)
where µ is the number of the AO. The C matrix is obtained by solving Hartree-Fock matrix
equation
FC = SCε. (2)
The method of solving equation (2) as well as the binding energy, bond strength and atomic charge
calculation is described in [7]. It is supposed that the electron correlation energy is small. Also,
the quantum mechanical calculations do not lokalize the surface states. The calculations were
performed using the GAMESS program [10].
4. Results of the XPS measurements
Figure 1 shows the XPS of the SbSeI crystal in the energy range from 0 to 1400 eV below the
Fermi level without contamination with any gas and only with a small amount of carbon (C 1s
peak at 284.5 eV).
102
X-ray photoelectron spectra and electronic structure of SbSeI crystals
Figure 1. XPS spectrum of SbSeI in the en-
ergy range 0 to 1400 eV.
Figure 2. XPS of SbSeI crystal valence band.
Auger spectra of Se LMM as well as of Sb MNN and of I MNN are also seen. The background
is caused by secondary electrons. XPS did not show any traces of impurities. Only the carbon was
visible after the sample was cleft under ultrahigh vacuum conditions, in the low 10−10 torr range.
We did not find noticeable change of the surface composition with time at fixed temperature as
well as with illumination time. The strongest peaks of Sb 3d, Sb 4d, I 3d, I 4d and Se 3d were
chosen to investigate the peculiarities of the crystallographic plane-dependent core-level XPS in
this quasi-one-dimensional semiconductor.
Figure 3. XPS of SbSeI crystal spin-orbit dou-
blet Sb 4d. The chemical shift is + 1.7 eV.
Figure 2 shows the valence band (VB) spec-
trum. The spectrum is referred to the Fermi level
(EF). The EF was defined with the accuracy
of 0.3 eV. The VB is separated by the gap of
about 1.2 eV from the Fermi level and it is lo-
cated to about 20 eV below EF. Above 16 eV,
the VB is hidden by the Sb 4d band. In these
crystals, the optical band gap is about 1.6 eV at
300 K. It nearly corresponds the value obtained
from the XPS measurements. The shape of VB
is crystallographic-plane dependent and it will be
discussed in section 6.
Figure 3 shows the spectrum of the nearest to
VB spin-orbit doublet of Sb 4d from the planes
broken lengthwise (110) and crosswise (001). The
core-level binding energy is referred (in all fig-
ures) to the Fermi level with the calibration ac-
curacy of 0.3 eV. The peaks of Sb 4d5/2 and 4d3/2
are located at about 33.8 and 35 eV, respectively.
The binding energy increases 1.7 eV in compar-
ison with Eb in pure Sb (Eb = 32.1 eV for 4d5/2
and Eb = 33.3 eV for 4d3/2).
The peaks of Sb 3d5/2 and 3d3/2 are located
at about 529.9 and 539 eV, respectively (figure 4).
The shift of the binding energy is equal
to 1.6 eV in comparison with pure Sb (Eb =
528.3 eV for 3d5/2 and Eb = 537.4 eV for 3d3/2).
The position, pinned by calibration, of I 3d5/2 is
103
J.Grigas et al.
at Eb = 619.2 eV and of I 3d3/2 at Eb = 630.5 eV (figure 5).
The chemical shift is about −0.1 eV (broken crosswise) and −0.2 eV (broken lengthwise) to
lower values (for pure I 3d5/2 is at Eb = 619.3 eV), I 3d3/2 is at Eb = 630.7 eV).
Figure 4. XPS of SbSeI crystal spin-orbit dou-
blet Sb 3d. The chemical shift is + 1.6 eV.
Figure 5. XPS of SbSeI crystal spin-orbit dou-
blet I 3d. The chemical shift is about -0.1 eV
(broken crosswise) and -0.2 eV (broken length-
wise) sample.
Figure 6 shows the spectrum of the spin-orbit doublet of I 4d. The peaks of I 4d5/2 and 4d3/2
are located at about 49 and 51 eV, respectively. The crosswise spectrum is broader and split into
two components.
The position of the overlapping Se 3d5/2 and Se 3d3/2 peaks was obtained at 54.5 eV and
55.5 eV (figure 7). For selenium, the chemical shift is also in the opposite direction in relation to
Sb, i.e. to the lower binding energy about -1.1 eV (for pure Se 3d5/2 Eb = 55.6 eV and for 3d3/2
Eb = 56.6 eV).
Thus, the electronic structure measurements revealed the chemical shift of Sb states of +(1.6−
1.7) eV to the higher binding energy, and I states of −(0.1 − 0.2) eV and Se states of −1.1 eV to
the lower binding energy. This shift suggests charge transfer from Sb to Se and I – more to Se than
to I. In SbSI, the chemical shift of Sb states is of +1 eV to the higher binding energy, and I states
of −0.8 eV and S states of −3 eV to the lower binding energy [7]. Thus, the chemical shifts in SbSI
and SbSeI crystals are quite different.
The spin-orbit doublets of all the crosswise spectra from the perpendicular to the chains (001)
surface are split into two components, which are clearly seen after resolving the spectra into Gaus-
sian contributions. The split components are separated by about 0.8 to 1.2 eV. The larger split
was obtained in ferroelectric SbSI crystals [7].
We assign the different components of the split spin-orbit doublets to the surface and bulk
104
X-ray photoelectron spectra and electronic structure of SbSeI crystals
Figure 6. XPS of SbSeI crystal spin-orbit dou-
blet I 4d.
Figure 7. XPS of SbSeI crystal spin-orbit
doublet Se 3d. The chemical shift is about
−1.1 eV.
contributions. Due to the reasons mentioned in Introduction the core-electron binding energies
should be different for surface and bulk atoms. However, the shift in the quasi-one-dimensional
SbSeI as well as SbSI [7] crystals is found only for (001) plane-surface. Thus, XPS in SbSeI as well
as in ferroelectric SbSI are crystallographic plane-dependent.
5. Model of SbSeI structure and calculated Koopmans’ energies
For the theoretical ab initio calculation of energy levels the molecular model of the SbSeI crystal
is needed. The model should be a cluster composed of an even number of molecules. The interaction
between the clusters is assumed to be weak.
A projection of the crystal structure of SbSeI on the (110) and (001) planes is shown in figure 8.
It contains double chains [(SbSeI)∞]2 consisting of two chains (SbSeI)∞ related by a two-fold screw
axis and linked together by short and strong Sb–Se bonds [6,8]. Weak van der Waals-type bonds
of d > 3.8 Å (bond strength 0.06) bind the double chains. The weakly bonded double chains may
be considered as non-interacting and one double chain may be taken as a molecular model of the
crystal for the calculation of binding energies. The double chain is formed of many simplified unit
cells. Such a simplified unit cell consists of six atoms.
From two to twenty of unit cells such as a molecular model of SbSeI crystal, have been considered
for the energy level calculations. Energy levels slightly change with the increase of the cluster.
The model of twenty SbSeI molecules is sufficient enough to qualitatively describe the electronic
spectrum of the crystal surface and bulk. It was found that the quantum-chemical calculations are
sensitive to variation of geometry.
105
J.Grigas et al.
After breaking the crystal under high vacuum conditions, strong bonds of the (001) surface
atoms become open. Due to the reduced coordination number, the (001) surface atoms experience
the potential different from the bulk atoms. As a result, the bond strength of the first two atomic
layers from the (001) surface change. The bond strength between the first and the second atomic
layers doubles at the expense of broken bonds: it increases in the first layer and decreases in the
second layer. Löwdin charges mainly change only electronegative atoms in the first three atomic
layers from the (001) surface, e.g. they become Sb+0.59 Se−0.32 I−0.38 in the first layer, Sb+0.59
Se−0.26 I−0.19 in the second layer and Sb+0.57 Se−0.33 I−0.27 in the third layer while the charges of
all other subsequent atomic layers in bulk are Sb+0.605 Se−0.31 I−0.29. The charges of the surface
electronegative atoms compensate electron density of the broken bonds along the [001]-axis. From
experimental viewpoint, a surface experiences different concentration of electronegative atoms, e.g.
from the XPS of I 3d5/2 we have found that the atomic concentration of I is 35.1 at % in the
broken crosswise crystal while it is 32.9 at % in the broken lengthwise crystal. The concentration
of Sb is nearly the same in crosswise and lengthwise planes. However, the core-level XPS do not
split and are not sensitive to the broken weak bonds along the x- and y-directions.
Figure 8. Crystal structure of SbSeI projected on
the (001) plane (upper part). The simplified unit
cell is formed by two molecules of SbSeI, i.e. by
6 atoms. Molecular chain of SbSeI crystal cluster
in the (100) plane (lower part). Dotted line shows
the xy-projection of the simplified unit cell. The
calculated bond strengths and atom’s charges
are shown in the picture. They are different in
different planes from the surface and reveal in the
VB electronic structure and core-level splitting.
Figure 9. Electronic structure of VB
in SbSeI crystal. Representation for the
peaks of the density of states and in-
tensity of the valence electrons for 16
molecules SbSeI cluster (top and mid-
dle); VB spectra approximation by the
Gaussian broadening method and their
comparison with experimental XPS (bot-
tom). Energy states broadening parame-
ter is = 0.54 eV.
Table 1 presents the theoretical values of the binding energies, calculated with MINI basis
set without taking into account the spin-orbit interaction, and experimentally obtained energies,
Eb exp. The quantum-mechanical method and the chosen model give negative core-level energies
higher than their experimental values are. Nevertheless, the model reflects well the electronic
structure of the crystal.
106
X-ray photoelectron spectra and electronic structure of SbSeI crystals
Table 1. Theoretical and experimental values of 20 (SbSeI) molecules chain binding energies at
300 K (in eV).
State −Ebmin − Ebmax −Eb(exp)(5/2 − 3/2)
Sb 3s 909 – 910
I 3p 888 – 891
Sb 3p 786 – 787
I 3d 646 – 649 619 – 631
Sb 3d 558 – 559 530 – 539
Se 3s 234 – 23
I 4s 194 – 190
Se 3p 172 – 180
Sb 4s 162 – 163
I 4p 139 – 142
Sb 4p 117 – 119
Se 3d 60 – 62 54.5 – 55.5
I 4d 55 – 58 49.5 – 51
Sb 4d 42.5 – 43.5 34 – 35
VB Se 4s 18.5 – 22.5 hidden
(s–band) I 5s 14.5 – 17.5 13 – 16
VB [Se 4p, Sb 5p, I 5p]
(p–band) 1.2 – 9.3 1.2 – 8
6. Discussion
Theoretical HFR calculations give the ionization potential with its zero at the vacuum level.
The experimental binding energies are referred to the Fermi level. As the energy gap of SbSeI is
1.6 eV, in order to compare the experimental and theoretical binding energies and refer to the Fermi
level we decreased the calculated ionization potential by the work function φ = 6.5 eV. Theoretical
calculations qualitatively explain the experimental values of XPS. Nevertheless, the theoretical
eigenvalues of the VB and CL are higher than the experimental binding energies (table 1).
Some discrepancy takes place due to i) a limited basis set of AO for obtaining a molecular
orbital solution is used (20 SbSeI molecules as the crystal model), and ii) screening effects are not
taken into account. Despite the Koopmans’ theorem provides an invaluable tool in assigning XPS
it neglects the relaxation processes.
Calculations of the Ciµ coefficients (equation (1)) shows that VB consists of s- and p-bands
separated by 2 eV gap. The longer is the chain the smaller is the gap between the s- and p-bands.
The VB is located 1.6 to about 20 eV below the Fermi level. Figure 9 shows theoretically calculated
VB form and electronic structure of SbSeI crystal. The spectrum is referred to the Fermi level (EF).
The intensities of the XPS were described in three ways (see [11]): a) by the energy states band
εi from the characteristic equation (2); b) by the peaks of the density of states, and c) by the
Gaussian broadening method.
Figure 9 gives the comparison of the earlier discussed calculated SbSeI VB structure with the
experimental XPS spectrum up to16 eV. At higher energies the VB overlaps with Sb 4d core level
energy (figure 1) and is not shown in figure 9. Intensity of the bands depends on the density of
states D(ε). The top of figure 9 shows the representation of D(ε). Experimental XPS is the integral
picture of all electronic states. In the bottom of figure 9 the approximation of SbSeI bands spectra
by the Gaussian broadening method and their comparison with experimental XPS is presented.
The calculations give a fairly good description of the places and the widths of the VB energy
bands. The VB of SbSeI is composed of the s- and p-type bands separated by 2 eV forbidden gap.
The s-band is formed of the Sb 5s, Se 4s, and I 5s bands of SbSeI cluster, separated by narrow gaps.
Extending a chain of atoms in cluster, an interval between the I 5s and Sb 5s narrows and, finally,
disappears. However, the break between the Se 4s and I 5s states remains stable and equal to 1.15
107
J.Grigas et al.
eV. Among these bands, formed of the electronegative atom states, comes to light the degenerate
Se-level which does not mix up with the levels of other atoms of the cluster, while the Sb 5s and
I 5s states are disposed to the mixing.
Table 2. Population (in %) of VB levels in 4 (SbSeI) molecule cluster. VB consists of s- and
p-bands. The energy of levels is denoted by εi. The last column (D) shows the dominant atomic
plane from the (001) surface where photoemission dominates.
Band Atomic state −ε1(eV ) Sb(%) Se(%) I(%) D
VB(s) Se 4s 21.6 26 69 5 Se2
20.2 22 77 2 Se2
18.5 13 82 5 Se1
18.4 17 82 1 Se1
I 5s 17.2 18 8 74 I2
16.7 6 6 88 I1
14.7 23 8 69 I1
14.6 25 6 70 I1
Sb 5s 13.5 55 20 25 mixed
13.3 43 10 47 mixed
12.1 63 32 5 mixed
10.9 53 36 11 mixed
Se 4p 8.1 31 55 14 Se2
7.9 36 46 18 Se2
7.2 43 51 6 bulk
6.5 33 41 26 bulk
5.9 39 48 13 bulk
5.9 38 34 28 bulk
5.5 39 39 22 bulk
5.3 21 49 30 bulk
5.2 27 31 42 bulk
5.0 31 34 35 bulk
Sb 5p 4.8 31 45 24 bulk
4.1 14 58 28 bulk
VB(p) 4.0 22 15 63 bulk
3.7 23 18 59 bulk
3.5 11 41 38 bulk
3.3 1 52 48 bulk
3.2 10 43 47 bulk
2.7 16 31 53 bulk
2.5 9 32 59 bulk
I 5p 2.3 3 2 95 I1
2.1 2 1 97 I1
2.0 1 11 88 I1
1.7 14 16 70 I1
1.2 13 21 66 I1
The form of the VB can be explained by the analysis of MO population. Knowing the MO
coefficients Ciµ (equation (1)), one can evaluate the contribution of A atom electrons for εi state:
piA =
M∑
µ∈A
C2
0iµ . (3)
There C0 = S
−1/2
C is the matrix of MO coefficients orthogonolized according to Löwdin. The
obtained results are shown in table 2, i.e. the population of the VB states for 4 (SbSeI) cluster.
108
X-ray photoelectron spectra and electronic structure of SbSeI crystals
All binding energies in table 2 and all figures are referred to EF . From the table 2 one can see
that the sharp ionization side (near EF ) of the most intensive p-band of the VB is formed by I
atom (66%) of the first plane from the surface perpendicular to the z-axis (see figure 8) as well as
by Se (21%) and Sb (14%) electrons. Since the Sb 5p contribution is small, the VB edge is sharp.
The highest MO of the VB is mainly composed of I 5p of the first surface atomic plane. When the
atomic chain is long, this level degenerates. The high-energy p-band side of the VB is formed of
the Se 4p (55%) states of the second plane from the surface, as well as by the Sb 5p (31%) and
I 5p (14%) states. The lower-energy side of the s-band is formed by the Se (36%), Sb (53%) and
I (11%) mixed surface and bulk states. The higher-energy side of the s-band is formed by the Se
(69%) states from the second plane from the surface, as well as Sb (26%) and I (5%) states. There
are no states in the VB composed of pure Sb states MO. Interaction of Sb atoms with other atoms
increases the width of the VB. Sb atoms also interact with the atoms of the neighbouring chains
(figure 8) and also increase the p-band. The conduction band (CB) edge is formed almost 100%
by the Sb 5p states from the first surface plane. Therefore, the donor level is of Sb 5p states and
the acceptor level is of I 5p states.
The form of the VB is well approximated by the Gaussian smearing with σ = 0.1, and this is
nearly in accordance with the spectral resolution of the employed X-ray photoelectron spectrometer.
Bond strengths and charges of surface atoms in (001) plane (figure 8) differ considerably from
bulk atoms. Bonds become stronger on the surface. The charge of surface Se and I atoms is smaller
than of bulk atoms. Breaking of the strong bonds should cause large shifts of the binding energies
of the (001) surface atoms. Such changes should cause the splitting of the spin-orbit doublets.
Using the HFR method we have calculated the core-level density of states of surface and bulk
atoms. HFR method does not take into account the spin-orbit interaction and instead of spin-
orbit doublets we have single bands about 3 eV (figures 3–7). The calculated average energies are
smaller than the experimental ones (table 1) due to the reasons discussed above. All the CL have
the structure similar to VB: electrons of bulk atoms form the middle of the bands, electrons of
the first Se and I atoms layer from the (001) surface form the more positive side while electrons of
the second atom layer form the more negative side of the Se 3d, I 3d and I 4d bands. Electrons of
the second Sb layer from the (001) surface form the more positive side of Sb 3d and Sb 4d bands,
while the electrons of the first and even of the third layer atoms form the more negative sides.
As the charge of the first and second layer atoms most of all differ from the charge of the bulk
atoms, the split of the crosswise spin-orbit doublets may be assigned to the core-level shift by
1 − 1.2 eV of the surface layer atoms relative to bulk atoms. So, the results presented in figures 3
to 7 may be interpreted as the crystallographic plane-dependent core-level binding energy shifts of
surface atoms relative to bulk atoms. However, this shift is smaller than in ferroelectric SbSI [7].
High concentration of electrons or holes is needed for the screening of high ionic polarization (in
SbSI also the spontaneous polarization).
7. Conclusions
X-ray photoelectron spectra of the valence band and of the principal core levels of the quasi-
one-dimensional semiconductor SbSeI are presented in the energy range from 0 to 1400 eV. A
molecular model of the crystal is used for ab initio theoretical calculations of the binding energies.
The electronic structure of valence band is calculated and confirmed experimentally. The crystal is
more ionic according to Mulliken than to Löwdin charges but it is less ionic than the isostructural
ferroelectric SbSI crystal. Only three atomic layers from a surface along the [001]-axis may be
considered as the surface layers. From the fourth atomic layer, the charges and the bond strength
correspond to the bulk. The XPS studies revealed the crystallographic plane-dependent surface
atom core-electron binding energy shifts relative to bulk atoms. However, this shift is smaller than
in ferroelectric SbSI crystal.
109
J.Grigas et al.
8. Acknowledgement
This work was performed, in part, within the framework of the Lithuanian-Ukrainian project
“Development and studies of new materials for information technologies, acousto-, opto- and na-
noelectronics”.
References
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3. Chiang T.C., Kaindl G., Mendel T. Phys. Rev. B, 1986, 33, 695.
4. Irizawa A., Yamasaki A., et al. Solid St. Commun., 2002, 124, 1.
5. Grigas J., Talik E. Phys. Status Solidi (b), 2003, 237, 494.
6. Grigas J. Microwave dielectric spectroscopy of ferroelectrics and related materials. Gordon and Breach
Science Publ. OPA Amsterdam, 1996, 161–188.
7. Grigas J., Talik E., Lazauskas V. Lithuanian J. Phys., 2004, 44, 427.
8. Vots G.P., Rentreper P.J. Zeitshrift für Krystallography, 1982, 161, 111.
9. Kvedaravicius S., Audzijonis A., Mykolaitiene N., Grigas J. Ferroelectrics, 1996, 177, 181.
10. Schmidt M.W., Baldrige K.K., et all. J. Comput. Chem., 1993, 14, 1347.
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Рентгенiвськi фотоелектроннi спектри та електронна
структура квазiодновимiрних кристалiв SbSeI
Й.Грiгас1, Е.Талiк2, М.Адамєц2, В.Лазаускас3
1 Фiзичний факультет, Вiльнюський унiверситет, вул. Саулетекiо, 9/3, LT–10222 Вiльнюс, Литва
2 Iнститут фiзики, Сiлезький унiверситет, вул. Унiверситетська, 4, 40–007 Катовiце, Польща
3 Iнститут теоретичної фiзики i астрономiї, Вiльнюський унiверситет, вул. Гостауто, 12, LT–01108
Вiльнюс, Литва
Отримано 4 вересня 2006 р.
Дана стаття представляє рентгенiвськi фотоелектроннi спектри (РФС) валентної зони (ВЗ) i остов-
них рiвнiв кристалiчних поверхонь (110) i (001) квазiодновимiрного монокристалу SbSeI з високою
проникливiстю, який iзоструктурний сегнетоелектрику SbSI. РФС вимiряно з використанням моно-
хроматичного випромiнювання Al Kα в енергетичному дiапазонi 0–1400 еВ при кiмнатнiй темпера-
турi. ВЗ розташована вiд 1.6 до 20 еВ нижче рiвня Фермi. Експериментальнi енергiї ВЗ та остовних
рiвнiв порiвняно з результатами теоретичних розрахункiв “з перших принципiв” для молекулярної
моделi кристалу SbSeI. Встановлено електронну структуру ВЗ. Виявлено зсуви остовних енергiй
зв’язку поверхневих атомiв вiдносно об’ємних, якi демонструють залежнiсть вiд кристалографiї по-
верхнi. Отримано хiмiчнi зсуви остовних рiвнiв у кристалi SbSeI для Sb, Se та I.
Ключовi слова: SbSeI, електронна структура, РФС
PACS: 79.60.-i, 82.80.Pv, 87.64.Lg
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