X-ray-absorption spectroscopy of CoSi2

Article abstract of DOI:10.1103/PhysRevB.53.16510

X-ray-absorption near-edge structure (XANES) spectra of thin-film CoSi₂ were measured at the Si K edge and Co L₃ edge using the total electron yield mode. The Si K-edge results for CoSi₂ showed a dramatic reduction of intensity in the first broad feature accompanied by a rise in a relatively strong and sharp feature at higher binding energies when compared to XANES spectra for crystalline Si. We attribute these two features to the Si 1s photoelectron excitations to a broad Si 3p nonbonding band and a relatively narrow band of hybridized Si p-Co 3d antibonding states, respectively. Analysis of the Co L₃-edge white line spectra for CoSi₂ reveals the appearance of a triple structure, which can be attributed to excitations to the unoccupied Co 3d nonbonding states and hybridized antibonding Co (3d,4s)-Si p states.

Full text of DOI:10.1103/PhysRevB.53.16510

PHYSICAL REVIEW B
VOLUME 53, NUMBER 24
15 JUNE 1996-II
X-ray-absorption spectroscopy of CoSi2
W. F. Pong and Y. K. Chang
Department of Physics, Tamkang University, Tamsui, Taiwan 251, Republic of China
R. A. Mayanovic
Department of Physics and Astronomy, Southwest Missouri State University, Springfield, Missouri 65804
G. H. Ho and H. J. Lin
Synchrotron Radiation Research Center (SRRC), Hsinchu, Taiwan 300, Republic of China
S. H. Ko*
Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180
P. K. Tseng
Department of Physics, Tamkang University, Tamsui, Taiwan 251, Republic of China
C. T. Chen†
AT&T Bell Laboratories, Murray Hill, New Jersey 07974
A. Hiraya‡
Institute for Molecular Science, Myodaiji, Okazaki 444, Japan
M. Watanabe
Research Institute for Scientific Measurements, Tohoku University, Aoba, Sendai 980, Japan
͑Received 9 June 1995͒
X-ray-absorption near-edge structure ͑XANES͒ spectra of thin-film CoSi were measured at the Si K edge
2
and Co L3 edge using the total electron yield mode. The Si K-edge results for CoSi2 showed a dramatic
reduction of intensity in the first broad feature accompanied by a rise in a relatively strong and sharp feature
at higher binding energies when compared to XANES spectra for crystalline Si. We attribute these two features
to the Si 1s photoelectron excitations to a broad Si 3p nonbonding band and a relatively narrow band of
hybridized Si p–Co 3d antibonding states, respectively. Analysis of the Co L -edge white line spectra for
3
CoSi reveals the appearance of a triple structure, which can be attributed to excitations to the unoccupied Co
2
3
d nonbonding states and hybridized antibonding Co (3d,4s)–Si p states. ͓S0163-1829͑96͒04924-7͔
I. INTRODUCTION
been reported recently. In this paper we present an analysis
of Si K-edge and Co L -edge XANES spectra of thin-film
3
The electronic properties of thin-film transition-metal
CoSi2 grown by a thermal reaction process. The major goal
is to gain a deeper understanding of the chemical bonding in
͑
TM͒ silicides such as CoSi have attracted great attention
2
thin-film CoSi . We discuss the results of XANES spectra
over the last decade because of their potential for widespread
applications in electronic devices. A detailed understanding
of the electronic structure of this material is important for
technological applications. The electronic structure of
2
of thin-film CoSi and compare our findings with other ex-
2
perimental data and theoretical calculations of DOS from
earlier studies.
CoSi has been studied theoretically by several investigators
2
1
–4
using band-structure calculations. Their calculations indi-
cate that Si p–TM d hybridization contributes substantially
to the density of states ͑DOS͒ near the Fermi level. Photo-
II. EXPERIMENT
Thin-film CoSi2 was grown by rapid thermal annealing of
Co sputtered on a crystalline Si͑100͒ substrate. Thin-film
Si/Co was first annealed at 500 °C for 60 sec, leading to
emission spectroscopy5 and soft-x-ray emission which
give useful information regarding the valence-band DOS of
solids, are in good agreement with calculated band structures
concerning the nature of Si p–TM d hybridization as well as
Si s–TM d bonding at or near the valence-band edge in TM
silicides. X-ray-absorption near-edge structure ͑XANES͒ can
provide complementary information concerning unoccupied
,6
7,8
compound formation of coexisting CoSi/Co Si, and then
2
again at 675 °C for 60 sec, forming the metallic CoSi phase
2
with a thickness of Х2560 Å . The reaction path of the top
layer appears to have proceeded in the following manner: Co
→Co Si→CoSi→CoSi . Details of the preparation proce-
2
2
4,9
11
states just above the Fermi level. Several Si K-edge and
dure have been described elsewhere. In addition, a sample
4
,10
Co L -edge
XANES investigations of bulk CoSi2 have
of Si/Co was prepared by depositing Co ͑thickness р100 Å͒
3
0
163-1829/96/53͑24͒/16510͑6͒/$10.00 53
16 510
© 1996 The American Physical Society

53
16 511
X-RAY-ABSORPTION SPECTROSCOPY OF CoSi2
FIG. 1. Normalized Si K near-edge absorption spectra. The
threshold region of crystalline Si is inset on a magnified scale.
FIG. 2. Normalized Co L3 near-edge absorption spectra. The
inset represents the experimental absorption spectrum ͑a͒ and the
convolution of theoretical DOS with a Gaussian broadening ͓͑b͒
and ͑c͔͒ of CoSi2 ͑from Ref. 10͒.
onto a Si͑100͒ substrate. The metal concentration profile and
homogeneity of CoSi2 were verified by Rutherford back-
scattering spectrometry and electron-probe microanalysis,
and its structure was characterized by x-ray diffraction. The
results from this analysis, along with the O K-edge XANES
nantly reflect the electronic and local structure environments
of the outermost layers ͑on the order of 100 Å͒ of CoSi2 ͑in
Si/CoSi2), Co ͑in Si/Co͒, and Si ͓in crystalline Si͑100͔͒, re-
spectively. For x-ray energies in the XANES region, the ex-
cited photoelectron undergoes transition from a core state to
an unoccupied state; the final state is determined in relation
to the initial state by the dipole selection rule. Zero energy
for the Si K edge ͑1839.3 eV͒ XANES spectra in Fig. 1 was
selected at the inflection point of the threshold in crystalline
Si. In the case of crystalline Si, the characteristic double-
peak feature near 1.5 and 2.6 eV above the edge and its
approximate 1.1-eV energy separation were observed. Al-
though calculated partial density of states for p symmetry is
unavailable, Woicik et al. assigned this double peak to the Si
1s to first two Si p unoccupied conduction-band states.13
This splitting of the Si K-edge XANES spectrum for crystal-
line Si is also in agreement with earlier published
͑Ref. 12͒ and Co K-edge extended x-ray absorption fine-
structure ͑EXAFS͒ measurements presented below, show
negligible oxidation of CoSi2 .
Si K-edge x-ray-absorption spectra of thin-film Si/CoSi2
and crystalline Si͑100͒ were obtained at beamline 1A at
the Ultraviolet Synchrotron Orbital Radiation Facility
͑UVSOR͒, Institute for Molecular Science, Okazaki, Japan.
The spectra were measured in the total electron yield ͑TEY͒
mode with an electron multiplier, using a double-crystal
InSb͑111͒ monochromator. Photon energies were calibrated
using the M5 edge of a Au photocathode. Co L -edge
3
XANES of Si/CoSi2 and Si/Co were measured using the
AT&T Bell Laboratories Dragon beamline at the National
Synchrotron Light Source ͑NSLS͒, Brookhaven National
Laboratory. XANES spectra were obtained in the TEY mode
with a channeltron detector, using the 1200-1/mm grating of
the monochromator. The energy scale was calibrated using
the well-known spectrum of a CaF2 thin film. The energy
resolution was set at ϳ1.0 eV in the Si K edge for InSb͑111͒
studies.1
4,15
Hitchcock et al. interpreted the two peaks as
15
the Si 1s to unoccupied Si 3p conduction-band excitations,
based on the idea of crystal symmetry.
and ϳ0.4 eV in the Co L edge for the grating monochro-
mator, respectively. All measurements were made at room
temperature.
As shown in Fig. 1, the feature A1 ͑occurring between
Ϫ1 and 5 eV͒ found in the spectrum of CoSi2 is signifi-
cantly broader than in the Si XANES spectrum and contains
some weak structures at the threshold jump region. Its inten-
sity above the edge is also reduced dramatically, while the
main inflection point shifts toward higher energies with re-
3
III. RESULTS AND DISCUSSION
Figures 1 and 2 show the Si K-edge XANES spectra for
Si/CoSi and crystalline Si͑100͒, and the Co L -edge white
spect to crystalline Si. In addition, feature B located near
8.2 eV above the Si K edge is significantly enhanced for
1
2
3
line spectra of Si/CoSi2 and Si/Co. After pre-edge back-
ground subtraction, the spectra were normalized using the
CoSi compared to the spectrum for crystalline Si. In com-
2
paring our Si K-edge spectrum for CoSi to those previously
2
reported,4 we notice a discrepancy in feature B . Our data
,9
incident beam intensity I and scaled to the maximum of the
0
1
peak heights. Due to the surface sensitivity of TEY measure-
ments, we believe that the spectra in Figs. 1 and 2 predomi-
reveal a much stronger and sharper B feature than that pre-
sented by Lerch et al. and Weijs et al. Their spectra for
1
4
9

16 512
W. F. PONG et al.
53
CoSi showed that the energy dependence in the region be-
2
tween Ϫ1 and 4 eV was similar to that observed for our
data, with a lower relative intensity. However, contrary to
our data, their results displayed a long plateau between 4 and
12 eV above the edge, which slowly decreased as a function
of energy. In addition, the spectrum of Lerch et al. also ex-
hibited additional weak oscillating structure in the plateau
region. We have no conclusive explanation for these differ-
ences.
As mentioned above, the threshold in the Si K-edge
XANES spectrum of CoSi is shifted toward higher binding
2
energies with respect to the spectrum of crystalline Si. This
reflects the fact that the chemical state of the absorbing Si
atoms is changed in CoSi , as exhibited in the details of the
2
electronic structure of the valence bands, relative to its state
in the crystalline Si. As is well known, silicon has the dia-
mond cubic structure in which strong mutual interaction be-
3
tween the sp orbitals results in the formation of tetrahe-
FIG. 3. Schematic energy diagram showing respective atomic
drally coordinated sp3 hybrid bonds. Conversely, Si-Co
bonding in CoSi2 is much less covalent and substantially
more metallic in nature. It is therefore not surprising that the
electronic properties as well as the local structure surround-
3
levels forming Co 3d,4s and Si sp bands of CoSi . The labels
2
A1g , T2u , E , etc. indicated the irreducible representations of the
g
cubic point group. AB and B denote antibonding and bonding
states, respectively. ↑ and ↓ denote states for spin up and spin
down, respectively ͑not to scale͒.
ing the Si atom are quite different between CoSi and crys-
2
talline Si. Experimental evidence derived from systematic
6
photoemission studies performed by Franciosi et al. indi-
3.2, and 4.8 eV, respectively͒, as opposed to a single peak in
the case of Co. Furthermore, feature A2 in CoSi2 has an
approximate 0.4-eV energy shift to higher energies relative
to the main peak of thin-film Co. We infer, as for the Si
K-edge shift, that the shift in the main region of the Co
L -edge XANES of CoSi is primarily due to the effect of
cates that the binding energies of silicides shift with different
9
stoichiometries. Core-hole effects and charge transfer
2
,16
effects
may also influence the energy shift of the thresh-
old feature in CoSi , although there still exists some contro-
2
versy concerning the effects of charge transfer between Si
3
2
and metal in TM silicides.1 Provided that features A and
,17
change in the electronic configuration of the absorbing atom
͑Co͒. We note that the CoSi2 spectrum presented here not
only shows better resolution, but is also in good agreement
with the experimental data and the theoretical DOS obtained
by Eisebitt et al.10 The theoretical DOS was calculated by
1
B are related to the bonding states, then their interpretation
1
in terms of Si p orbitals and hybridized Si p and Co 3d
orbitals contributing to the DOS of CoSi2 follows accord-
ingly.
1
21
Now let us turn to the Co L -edge x-ray-absorption spec-
Tersoff and Hamann and by van Leuken. A convolution of
the two theoretical DOS with the instrumental and core-hole
lifetime broadening leads to better agreement with our ex-
perimental results ͑see inset of Fig. 2͒, regarding features
A2 , B2 , and C2 of the Co L3-edge spectrum. ͑In spite of
encouraging agreement, there was still a slight shift of the
relative energy position of the triple structures in comparison
between theoretical and experimental data.͒ We infer that
features A2 , B2 , and C2 of the Co L3-edge spectrum are
directly related to hybridization between the Co 3d,4s and
3
tra for thin-film CoSi and Co. Shown in Fig. 2, the white
2
line peaks at the Co L edge are attributed to the excitation
3
of electrons from Co 2p3/2 core states to final unoccupied Co
3
d states above the Fermi level. Zero energy for the Co L3
edge ͑778.7 eV͒ was selected at the central point of the
jumping threshold in Co. In general, the L white line peak
3
position and shape for Si/Co are in agreement with the ab-
sorption spectrum of Co grown on Cu͑100͒.18 However,
there are several satellite structures observed in our data,
indicated in Fig. 2 by vertical dotted lines near Ϫ0.6, 2.0,
and 4.4 eV. It is likely that these satellite structures reveal
the chemical states of the interfacial Co atoms at the Co-Si
substrate interface, for which there may be two contribu-
tions. One source may be the interdiffusion of Co into Si at
very low coverage producing a region of different
3
Si sp orbitals. This can be explained with greater clarity
using Fig. 3, which shows a schematic representation of the
3
Co 3d, Co 4s, and Si sp bands of CoSi derived from their
2
respective atomic levels. Our model most closely resembles
a model derived from electronic structure calculations for
‘‘bulklike’’ CoSi2 by van den Hoek, Ravenek, and
1
9
22
3
stoichiometry. A second source may be the formation of
Baerends. In this model, the Si sp states become hybrid-
2
0
interface states for Co atoms located close to the interface
between the Co layer and the Si surface. We suspect these
satellite structures may also be associated with partial oxida-
tion of the top Co layer,12 which was primarily caused by
leaving the sample surface open to atmosphere.
ized with Co 3d and Co 4s orbitals in CoSi , resulting in a
2
band structure having two nonbonding E ͑Co 3d) and T
g
2u
͑Si p) states, as well as hybridized Co 3d–Si p and Co
4s–Si s states, which are represented by T2g and A1g , re-
spectively.
It is quite obvious from Fig. 2 that the Co L -edge white
It is important to stress that an understanding of the
‘‘bulklike’’ characteristics ͑i.e., Young’s modulus, electrical
conductivity, etc.͒ requires knowledge of the local atomic
3
line region of the CoSi2 spectrum is different from that of
Co, as its main features clearly display three structures,
marked in the figure as A , B , and C ͑located near 1.4,
structure of the thin-film CoSi . In a separate study, total
2
2
2
2

53
16 513
X-RAY-ABSORPTION SPECTROSCOPY OF CoSi2
photoelectrons originating from p states become excited to
the first unoccupied d-like and s-like states above the Fermi
level, the most reasonable conclusion is that features A and
2
B correspond to 2p electrons making transitions to both
2
3/2
partially unoccupied nonbonding E states and empty anti-
g
bonding T2g states. A similar assignment for features A and
2
B in the Co L -edge white line region of the CoSi spec-
2
3
2
10
trum was given by Eisebitt et al. The E states have mainly
g
Co 3d character, constitute metallic bonds, and have charac-
teristics similar to pure Co metal. These states are located in
energy between the bonding and antibonding states. The an-
tibonding T2g states are formed from hybridized Co 3d–Si
p states, for which the degree of hybridization is determined
by the extent of Co 3d and Si p orbital wave-function over-
lap, which in turn is reflected in the DOS. The crystal-field
splitting of the T2g and E states is 1.8 eV, which is well
g
characterized by Tersoff and Hamann in terms of a quasigap
in the density of states near the Fermi level separating bond-
ing and antibonding states. In addition, feature C2 can be
assigned to the Co 2p3/2→ antibonding A transition. The
1g
antibonding A1g states are derived from Co 4s orbitals inter-
acting with Si s orbitals.
The Si K-edge spectrum for thin-film CoSi can be satis-
2
factorily interpreted using the energy-level diagram in Fig. 3.
The main feature of the Si K-edge XANES spectrum for
FIG. 4. ͑a͒ Normalized Co K-edge EXAFS oscillations ␹(k)
2
bulk CoSi was previously attributed to the unoccupied an-
weighted by k for CoSi . ͑b͒ Magnitude of Fourier transform of
2
2
9
3
Ϫ1
tibonding Si p states. More specifically, referring to Fig. 3,
the EXAFS k ␹ data from kϭ3.4 to 12 Å . Structural parameters
of two nearest neighbors obtained by fitting the Co K-edge EXAFS
we infer that feature B of our Si K-edge XANES spectrum
1
2
spectrum: N Co-Siϭ8, RCo-Siϭ2.32Ϯ0.01 Å,
␴
ϭ(5.10
for thin-film CoSi can be attributed mainly to Si 1s→ an-
Co-Si
2
Ϫ3
2
2
Ϯ0.07)ϫ10
ϭ(8.14Ϯ0.03)ϫ10
Å . N
ϭ12, RCo-Coϭ3.80Ϯ0.02 Å, ␴
tibonding T transitions. Since feature B is relatively nar-
Co-Co
Co-Co
2g
1
Ϫ3
2
Å . N is the coordination number, R the
row, the antibonding T2g band has a relatively narrow energy
2
nearest-neighbor distance, and ␴ the mean-square vibrational am-
width. If feature B is attributed to transitions to the anti-
1
plitude.
bonding T2g states, then the question arises of how to ascribe
feature A in terms of the chemical bonding in DOS. The
1
electron yield EXAFS measurements23 were performed at
nonbonding T2u states have Si p symmetry and theoretically
have the highest probability to be considered as the states in
connection with feature A1 . However, we do not expect to
see any peaks in the Si K-edge XANES spectrum provided
that the nonbonding T2u states are completely filled ͑see Fig.
3͒.
the Co K edge on our thin-film CoSi . The normalized
2
2
EXAFS oscillation ␹(k) weighted by k for Co K edge and
3
the corresponding Fourier transforms of the k ␹ data are
2
4
shown in Fig. 4. Further analysis involved use of the com-
bination of the multiple-scattering EXAFS computer pro-
gram FEFF5 ͑Ref. 25͒ and the nonlinear least-squares fitting
computer program FEFFIT.26 As shown, the quality of the fit
in Fig. 4 is quite good. Structure results from fitting indicate
that the absorbing central Co atom is surrounded by eight
nearest-neighbor Si atoms at 2.32Ϯ0.01 Å and twelve next-
nearest-neighbor Co atoms at 3.80Ϯ0.02 Å, which is consis-
tent with CoSi2 having the fluorite structure. Our thin-film
Regarding feature A1 in Fig. 1, we see clearly that the
threshold of the XANES for CoSi is at the same energy as
2
for crystalline Si. This implies that feature A is associated
1
with unoccupied Si 3p states, in agreement with the conclu-
sions of Hitchcock et al.15 In fact, linear augmented-plane-
wave band-structure calculations for NiSi ͑Ref. 5͒ suggest
2
that the Si 3p free-electron-like band, which is principally
responsible for the metallic character of the material, is
spread over a wide energy range in comparison with the Ni
3d band, which has a relatively narrow energy width in
sample is indeed similar to that of ‘‘bulklike’’ CoSi re-
2
ported in the literature.27 This justifies the use of the elec-
tronic structure model, which is very similar to the one de-
veloped by van den Hoek, Ravenek, and Baerends, to
explain the XANES of thin-film CoSi2 .
NiSi . Furthermore, it has been observed previously that in
2
free-electron-like metallic systems, one obtains additional
weak structures at the threshold of XANES as the result of
valence electron excitations induced by the electron gas-core
We now discuss the XANES spectra of CoSi in detail in
2
relation to the electronic structure model shown in Fig. 3.
Referring to Fig. 3, we can see that all the antibonding states
are empty, but the bonding and the nonbonding states are
filled except for a vacancy for one electron ͑indicated by
hole interactions.28 Therefore, we conclude that feature A is
1
due to Si 1s→ nonbonding T2u ͑Si 3p) transitions.
Such states are available for photoabsorption transitions
because the Si 3p free-electron-like band has strong DOS
contributions extending up to the Fermi level, causing elec-
tron transfer from the Si 3p orbitals to the Co ‘‘atomic’’
empty brackets͒ in the nonbonding E states. Subsequently,
g
our assignments for features A , B , and C of the CoSi
2
2
2
2
spectrum in Fig. 2 can be made more specific. Since the

16 514
W. F. PONG et al.
53
states. This causes the nonbonding T2u states to be partially
unoccupied above the Fermi level and thus gives rise to a
smooth and broad feature extending ϳ5 eV above the edge
in the spectrum. The nature of electron charge transfer from
Si to metal atom sites in TM silicides has been discussed
previously by Lambrecht and co-workers.2
The considerably weaker intensity and broadness of fea-
ture A1 in comparison to B1 can be attributed to a small
number of unoccupied states in the broad low-lying non-
bonding T2u band and a relatively larger number of unoccu-
pied states in the antibonding T2g band. On the whole, the
former shows that the Si 3p-symmetry final density of states
is small and spreads over a wide energy range. Meanwhile,
the latter reflects the increasing Co 3d density of states
above the Fermi level, in agreement with the natural charac-
teristics of 3d symmetric states, giving rise to the narrow
energy width. Finally, we would like to point out that due to
charge transfer effects, the unoccupied nonbonding T2u band
is situated above the Fermi level. As a consequence, we lo-
cate the Fermi level in the region between the nonbonding
and Co (3d,4s)-derived orbitals in thin-film CoSi . Three
2
distinct peaks appear in the Co L -edge white line region,
3
which are attributed to photoelectron transitions to the non-
bonding Co 3d states and hybridized Co (3d,4s)–Si p an-
tibonding states. We find very good agreement between Co
L3-edge XANES spectra measured on our thin-film CoSi2
and ones measured previously on ion-beam-synthesized
CoSi . In the Si K-edge XANES spectrum of CoSi , a rela-
,16
2
2
tively narrow feature B is associated with photoelectron ex-
1
citations to hybridized Si p–Co 3d antibonding states. Fea-
ture B1 is significantly more pronounced and sharper in our
Si K-edge XANES spectrum for thin-film CoSi in compari-
2
son to the same measured previously from bulk CoSi
.
Fi-
2
nally, we speculate that the threshold region feature A of the
1
same spectrum is due to transitions to the nonbonding T2u
͑Si 3p) states which are partially unoccupied due to charge
transfer from Si to Co in CoSi2 .
T2u states and the bonding T states, whereas van den
Hoek, Ravenek, and Baerends suggested that the Fermi
2g
2
2
ACKNOWLEDGMENTS
level was mainly in the region of nonbonding E and T
states.
g
2u
One of the authors ͑W.F.P.͒ acknowledges support by the
National Science Council of the R.O.C., under Contract No.
NSC84-2112-M-032-010. This work was performed on
beamline 1A at UVSOR, the Institute for Molecular Science
͑IMS͒, and AT&T Bell Laboratories Dragon beamline at
NSLS.
IV. CONCLUSION
In summary, our analysis of the XANES spectra indicates
significant effects due to photoelectron excitations to Si p-
1
1
*
Present address: Applied Materials, Santa Clara, California
S. H. Ko, S. P. Murarka, and A. R. Sitaram, J. Appl. Phys. 71,
5892 ͑1992͒.
9
5054.
†
12
Present address: Synchrotron Radiation Research Center ͑SRRC͒,
Hsinchu, Taiwan 300, Republic of China.
We have recently measured the O K-edge XANES spectra of
CoSi , Si, and Co. The spectra of CoSi and Si are similar to
2
2
‡
Present address: Department of Material Science, Hiroshima Uni-
versity, Hiroshima 739, Japan.
that of pure molecular oxygen ͓see M. W. Ruckman et al., Phys.
Rev. Lett. 67, 2533 ͑1991͔͒ suggesting predominantly O-O
bonding of molecular oxygen on the surfaces of the samples.
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these characterization studies show negligible oxidation of
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