Preferential Co-Si bonding at the Co/SiGe(100) interface

Article abstract of DOI:10.1063/1.119436

The initial stages of the reaction of Co with Si_0.79Ge_0.21(100) were studied in situ with extended x-ray absorption fine structure spectroscopy and reflection high energy electron diffraction. The Si:Ge ratio in the first coordinatio

Full text of DOI:10.1063/1.119436

Preferential Co–Si bonding at the Co/SiGe(100) interface
B. I. Boyanov, P. T. Goeller, D. E. Sayers, and R. J. Newmanich
Citation: Applied Physics Letters 71, 3060 (1997); doi: 10.1063/1.119436
View online: http://dx.doi.org/10.1063/1.119436
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/71/21?ver=pdfcov
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Preferential Co–Si bonding at the Co/SiGe„100… interface
B. I. Boyanov,a),b) P. T. Goeller,c) D. E. Sayers,a) and R. J. Newmanicha),c)
North Carolina University, Raleigh, North Carolina 27695
͑Received 25 August 1997; accepted for publication 23 September 1997͒
The initial stages of the reaction of Co with Si0.79Ge0.21(100) were studied in situ with extended
x-ray absorption fine structure spectroscopy and reflection high energy electron diffraction. The
Si:Ge ratio in the first coordination shell of Co in sub-monolayer Co films was found to increase
with film thickness and annealing temperature, indicating preferential formation of Co–Si bonds.
The impact of the observed preference for Co–Si bonding on the morphology of epitaxial
CoSi2 /Si1ϪxGex heterostructures is discussed. © 1997 American Institute of Physics.
͓S0003-6951͑97͒00247-7͔
Due to its low resistivity and good thermal stability, co-
balt disilicide (CoSi2) is considered an attractive contact ma-
terial for submicron Si devices. Since CoSi2 and Si have
similar crystal structures and are closely lattice-matched, it is
possible to achieve epitaxial growth of CoSi2 on both Si͑100͒
and Si͑111͒ substrates.1,2 However, in order to achieve
single-crystal growth on Si͑100͒, the nucleation of CoSi2 at
the Co/Si interface must be carefully controlled through the
use of CoSi2 templates layers.1–3 The studies indicate that at
Ͻ2 monolayer ͑ML͒ coverage, as-deposited Co/Si͑100͒ ex-
hibits a 2ϫ1 reconstruction and occupies surface sites with
twofold symmetry and sevenfold Co–Si coordination, as
shown in Fig. 1.4 In single-crystal CoSi2 /Si͑100͒ films, the
Co atoms at the interface exhibit sixfold coordination with
twofold symmetry, and occupy sites that would normally be
occupied by Si atoms if the Si͑100͒ lattice is extended across
the interface.5,6 Evidence for additional displacement7 and
compositional8 modulation at the interface has also been
found.
In comparison with the Co/Si͑100͒ system, results for
the Co/Si1ϪxGex(100) system are few and incomplete. The
reaction of Co with ͑100͒-oriented epitaxial Si1ϪxGex films
has been shown to lead to formation of pure CoSi2, accom-
panied by complete Ge segregation.9 Attempts to suppress
Ge segregation through co-deposition of co and Si resulted in
faceted epitaxial CoSi2 islands.10 The structure of the
CoSi2 /Si1ϪxGex(100) interface has not been determined.
The experiments reported in this letter were performed in
order to gain an understanding of the initial stage of the
reaction of Co with Si1ϪxGex , and its effect on the nucle-
ation and morphology of CoSi2 template layers.
Ϯ0.2 ML͒ by Rutherford backscattering ͑RBS͒. Extended
x-ray absorption fine structure ͑EXAFS͒ data were collected
in situ at the Co K edge ͑7709 eV͒ in total electron yield
mode at beamline X-11A at the National Synchrotron Light
source ͑NSLS͒. Data was collected both from as-deposited
films, and after annealing at 450 °C for 10 minutes. The ac-
quisition time for each sample was 6–8
h at 3
ϫ10Ϫ10 Torr. Surface contamination was monitored peri-
odically with reflection high energy electron diffraction
͑RHEED͒. No change in the RHEED patterns was observed
during the EXAFS measurements. Approximately 20
EXAFS scans were acquired for each sample at an incidence
angle of 75°Ϯ3° with respect to the sample surface. Data
were acquired in two or three orientations within 3° of each
other. Regions from each orientation free from Bragg reflec-
tions ͑glitches͒ were merged to produce a single data set that
was used for further processing. Structural parameters were
determined from nonlinear fits with ab initio standards.11
Confidence limits for the fit parameters were estimated as the
deviations of the parameters from their best-fit values which
increase the fit residue by 25% when all other parameters are
floated.
The freshly prepared Si0.79Ge0.21 films exhibited a sharp
2ϫ8 RHEED pattern. Immediately after Co deposition was
initiated a 2ϫ1 RHEED pattern was observed. After anneal-
ing for 10 min at 450 °C, a (3ͱ2ϫͱ2) surface diffraction
pattern identical to that reported by Stalder et al.12 for the
The samples used in this work consisted of approxi-
mately 0.7 and 1.7 ML of Co deposited at room temperature
on 800-Å-thick strained epitaxial Si0.79Ge0.21 films grown at
550 ° on p-type Si͑100͒ substrates. Atomically clean sur-
faces were prepared by spin-etching the Si͑100͒ substrates
with a 1:1:10 HF:H2O:ethanol solution, followed by in situ
thermal desorption at 900 °C, and deposition of a 200-Å-
thick homoepitaxial Si buffer layer at 550 °C. Cobalt cover-
age was confirmed ex situ ͑with an estimated accuracy of
FIG. 1. Surface adsorption site for Co/Si͑100͒, as proposed by Meyerheim
et al. ͑Ref. 4͒. The open dashed circles correspond to Si atoms at 2.35 Å,
and the hatched circles correspond to Si atoms at 2.72 Å. The adsorption site
for Co/Si0.79Ge0.21 /Si͑100͒ is shifted from that position by 0.2–0.4 Å, re-
sulting in the relaxation of the Co–Si and Co–Ge bonds to their natural
length, and the appearance of a long ͑3.16 Å͒ Co–Si͑Ge͒ bond.
a͒
Department of Physics.
Electronic mail: boyan_boyanov@ncsu.edu
b͒
c͒
Department of Materials Science.
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3060 Appl. Phys. Lett. 71 (21), 24 November 1997 0003-6951/97/71(21)/3060/3/$10.00 © 1997 American Institute of Physics
137.149.200.5 On: Sat, 29 Nov 2014 15:34:19

TABLE I. Structural parameters for the 0.7 and 1.7 ML Co/Si0.79Ge0.21
films: edge shift E0 , coordination number N, and bond length R. A single
edge shift E0 was used for all shells in the fits. The Debye–Waller factor for
all shells was fixed at ␴2ϭ0.0020 Å2, as determined from a single-shell
Co–Si fit of a similarly prepared 1.7 ML Co/Si͑100͒ films, annealed to
450 °C for 10 min. The forward transform range is ⌬kϭ3.0–10.1 ÅϪ1
.
The fit range is ⌬Rϭ1.0–2.8 Å for the annealed films, and ⌬Rϭ1.0–3.5 Å
for the as-deposited sample. The number of independent parameters are 11
and eight for the fits of the as-deposited and annealed films, respectively.
The actual number of fitting parameters used are seven and five, respec-
tively.
Sample/Shell
E0 ͑eV͒
N
R ͑Å͒
0.7 ML, as-deposited
Co–Si
Co–Ge
Ϫ2.1 Ϯ 2.7
3.5 Ϯ 0.4
1.1 Ϯ 0.5
1.1 Ϯ 0.8
2.30 Ϯ 0.03
2.56 Ϯ 0.03
3.16 Ϯ 0.07
Co–Si
0.7 ML, annealed
Co–Si
Co–Ge
0.8 Ϯ 1.8
Ϫ2.9 Ϯ 1.9
Ϫ3.0 Ϯ 0.4
6.0 Ϯ 0.4
1.1 Ϯ 0.4
2.30 Ϯ 0.02
2.58 Ϯ 0.03
1.7 ML, annealed
Co–Si
Co–Ge
7.6 Ϯ 0.6
1.0 Ϯ 0.6
2.30 Ϯ 0.02
2.59 Ϯ 0.05
FIG. 2. Normalized and k2-weighted EXAFS data for the samples used in
this study. The top trace is the data for a 730-Å-thick CoSi2 /Si͑100͒ film.
730 Å CoSi2
Co–Si
8.0 Ϯ 0.3
2.31 Ϯ 0.00
CoSi2 Si-rich surface was observed for both the 0.7 and 1.7
ML samples. The corresponding EXAFS data are shown in
Figs. 2 and 3. Data for a 730-Å-thick CoSi2 /Si͑100͒ film is
also included in these figures for comparison. Numerical re-
sults for multi-shell fits to the EXAFS data are given in
Table I. The results for the as-deposited 0.7 ML film indicate
a first shell consisting of approximately 4:1 Si and Ge atoms
at distances similar to the natural Co–Si and Co–Ge bond
lengths ͑2.32 and 2.50 Å, respectively͒. The presence of a
higher shell ͑perhaps mixed with Ge͒ may be interpreted as a
Ge-induced average displacement of the Co atoms from the
Si͑100͒ adsorption site in Fig. 1 by 0.4–0.6 Å, resulting in
the relaxation of the nearest-neighbor distances and the ap-
pearance of a Co-͑Si,Ge͒ shell at Ϸ3.16 Å. It should be
noted that the first-shell Si:Ge ratio of the as-deposited films
is similar to the stoichiometry of the Si0.79Ge0.21 substrate.
Annealing of the 0.7 ML film at 450 °C for 10 min resulted
in the formation of a structure similar to that of CoSi2 ͑Fig.
3͒, with sixfold Co–Si coordination in the first shell. The
transition to a CoSi2-like structure is accompanied by an in-
crease of the Si:Ge ratio in the first shell to approximately
6:1. The sixfold Si coordination of the Co atoms at the Co/
SiGe interface is identical to that of Co atoms at the
CoSi2 /Si͑100͒ interface.5,6 The average Co–Si coordination
in the 1.7 ML annealed films is 7.6Ϯ0.6, which is similar to
the eightfold coordination of Co in bulk CoSi2. The Si:Ge
ratio in the first shell of the annealed 1.7 ML film is approxi-
mately 8:1.
The increase in the Si:Ge ratio in the first coordination
shell of the annealed films indicates a preference for the for-
mation of Co–Si bonds. Such preference may be explained
in terms of a thermodynamic model previously developed by
Aldrich et al.13 The large difference between the enthalpies
of formation of CoSi2 ͑Ϫ99 kJ/mole͒ and CoGe2 ͑Ϫ36 kJ/
mole͒ dictates that creation of Co–Si bonds at the expense of
Co–Ge bonds is energetically favorable, resulting in CoSi2
formation and Ge segregation. The preference for Co–Si
bonding has significant consequences for the control of mis-
oriented grains in CoSi2 films grown on ͑100͒-oriented
Si1ϪxGex substrates, since it implies that interaction between
the metal layer and the SiGe substrate should be avoided. If
Ge is uniformly distributed on the Sige surface, as indicated
by the EXAFS results for the as-deposited 0.7 ML film, the
preference for Co–Si bonding will result in Ge enrichment of
portions of the Co/SiGe interface, and islanding of the CoSi2
template. Assuming sixfold coordination in the first ML,
eightfold coordination in the second ML, and Si-rich surface
termination, the average Co–Si coordination of a 1.7 ML
FIG. 3. Fourier-transformed k2-weighted EXAFS data ͑solid line͒ and fit
results ͑dashed line͒ for the samples used in this study. The top trace is the
data for a 730-Å-thick CoSi2 /Si͑100͒ film. The transform range is ⌬k
ϭ3.0–10.1 ÅϪ1
.
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Appl. Phys. Lett., Vol. 71, No. 21, 24 November 1997 Boyanov et al. 3061
137.149.200.5 On: Sat, 29 Nov 2014 15:34:19

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In conclusion, we have studied the initial stages of the
reaction of Co with Si0.79Ge0.21. At room temperature Co is
adsorbed at displaced surface sites, resulting in approxi-
mately stoichiometric Si:Ge ratio in the first coordination
shell, and the appearance of long ͑3.16 Å͒ Co–Si͑Ge͒ bonds.
The Si:Ge ratio in the first coordination shell of Co in the
annealed films has been found to increase rapidly with film
thickness, to 6:1 in 0.7 ML films and 8:1 in 1.7 ML films,
indicating preferential formation of Co–Si bonds.
Funding for this work was provided by the DOE under
Contract Nos. DE-FG05-93ER79236 ͑D.E.S., instrumenta-
tion͒, DE-FG05-89ER45384 ͑D.E.S., X-11 Operations at
NSLS͒, and by the NSF under Grant No. DMR-9633547
͑R.J.N.͒. Research was carried out in part at beamline X-11
at the National Synchrotron Light Source ͑NSLS͒,
Brookhaven National Laboratory. The NSLS is supported by
the U.S. Department of Energy, Division of Materials Sci-
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