Structure change of Ni(1 ML)/Si(1 1 1) by post-annealing observed by atomic force microscopy, ion scattering and photoelectron spectroscopy

Article abstract of DOI:10.1016/S0039-6028(02)01557-1

Structure change of Ni(1 ML: 7.83 × 10¹⁴ atoms/cm²)/Si(1 1 1) by post-annealing was observed by reflection high energy electron diffraction, atomic force microscopy (AFM), medium energy ion scattering (MEIS) and photoelectron spectroscopy. The AFM observation showed dramatic change of the surface morphology after the Ni deposition at room temperature (RT) followed by annealing at 400, 600, 700 and 800 °C for 2 min in an ultrahigh vacuum. MEIS using 70 keV He⁺ ions analyzed the depth profiles of Ni and the crystallographic structure of the Ni-composites formed by annealing. The valence band and the Si-2p and Ni-3p core level analyses using synchrotron-radiation light showed that the NiSi phase appeared by 1 ML-Ni deposition at RT and both NiSi and NiSi₂ islands were formed by annealing at 400 °C. Annealing at 600 and 700 °C led to growth of the B-type NiSi₂ islands with height of four and six Si-Ni-Si triple layers. After annealing at 800 °C, three-fourth of the deposited Ni atoms were dissipated from the surface and the dominant surface structure was the 1 × 1-ring clusters accompanied by a small amount of √19 × √19 phase. The present analysis clearly showed the structure change of Ni (1 ML)/Si(1 1 1) by post-annealing and provided the information about the kinetics for the Ni-Si system.

Full text of DOI:10.1016/S0039-6028(02)01557-1

Surface Science 511 (2002) 112–120
www.elsevier.com/locate/susc
Structure change of Ni(1 ML)/Si(111) by
post-annealing observed by atomic force microscopy,
ion scattering and photoelectron spectroscopy
a
Y.Hoshino , T.Nishimura , Y.Taki , Y.Asami , K.Sumitomo , Y.Kido
a
a
a
b
a,
*
a
Department of Physics, Ritsumeikan University, Kusatsu-shi Noji-higashi, Shiga-ken 525-8577, Japan
b
NTT Basic Research Laboratory, Atsugi, Kanagawa-ken 243-0198, Japan
Received 31 October 2001; accepted for publication 8 March 2002
Abstract
Structure change of Ni(1 ML: 7:83 ꢀ 1014 atoms/cm2)/Si(1 1 1) by post-annealing was observed by reflection high
energy electron diffraction, atomic force microscopy (AFM), medium energy ion scattering (MEIS) and photoelectron
spectroscopy.The AFM observation showed dramatic change of the surface morphology after the Ni deposition at
room temperature (RT) followed by annealing at 400, 600, 700 and 800 °C for 2 min in an ultrahigh vacuum.MEIS
using 70 keV Heþ ions analyzed the depth profiles of Ni and the crystallographic structure of the Ni-composites formed
by annealing.The valence band and the Si-2p and Ni-3p core level analyses using synchrotron-radiation light showed
that the NiSi phase appeared by 1 ML-Ni deposition at RT and both NiSi and NiSi2 islands were formed by annealing
at 400 °C.Annealing at 600 and 700 °C led to growth of the B-type NiSi2 islands with height of four and six Si–Ni–Si
triple layers.After annealing at 800 °C, three-fourth of the deposited Ni atoms were dissipated from the surface and the
pffiffiffiffiffi pffiffiffiffiffi
dominant surface structure was the 1 ꢀ 1-ring clusters accompanied by a small amount of 19 ꢀ 19 phase.The
present analysis clearly showed the structure change of Ni (1 ML)/Si(1 1 1) by post-annealing and provided the in-
formation about the kinetics for the Ni–Si system. Ó 2002 Published by Elsevier Science B.V.
Keywords: Medium energy ion scattering (MEIS); Photoelectron spectroscopy; Atomic force microscopy; Silicides; Semiconducting
films; Surface structure, morphology, roughness, and topography
1. Introduction
of deposition techniques have been developed.A
key parameter to form defect free epitaxial films is
the lattice match of the silicide to the single crystal
Si substrate.As well known, NiSi 2/Si has the
smallest lattice mismatch of 0.4%. Of course, prior
to the application, there are many requirements to
be cleared such as low resistivity, thermal stability
and good adhesion etc.Among many metal sili-
cides, NiSi2, CoSi2 and TiSi2 are the most prom-
ising candidates as the MOS-FET gate electrodes.
Epitaxial metal silicides have many applications
in the areas of Si-based semiconductor devices [1–
5].In general, forming epitaxial silicide thin films
with good quality can be very difficult and variety
* Corresponding author.Tel:. +81-77-561-2710; fax: +81-77-
561-2657.
E-mail address: ykido@se.ritsumei.ac.jp (Y.Kido).
0039-6028/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V.
PII: S0039-6028(02)01557-1

Y. Hoshino et al. / Surface Science 511 (2002) 112–120
113
In order to obtain good quality of silicide films
with an abrupt interface, it is essential to control
the initial stage of the silicidation process at the
atoms during the cooling process [17,20].Despite
the several reports mentioned above, the systematic
picture is still insufficient concerning the morphol-
ogy, diffusion kinetics, the phase and crystallogra-
phic structure of the reaction products and so on.
In this study, it is intended to obtain clearer
pictures of the atomic and electronic structures of
ultra-thin Ni deposited Si(1 1 1) surface before and
after post-annealing.Structure change by post-
annealing associated with the growth kinetics of
silicidation and surface reconstruction is particu-
larly concerned.For this purpose, we employed
RHEED, photoelectron spectroscopy (PES) with
synchrotron radiation (SR) light, medium energy
ion scattering (MEIS) and an atomic force micro-
scope (AFM).The PES analysis identifies the
chemical bonds of reaction products and MEIS
gives absolute amount of Ni near surface regions
together with the crystallographic structure of the
Ni-composites.The Ni deposition was made at
RT with a Knudsen cell (K-cell) and an infra-red
radiation (IR) heating system was used for anneal-
ing.Except for AFM observation, all the measure-
ments were performed in situ under an ultra-high
vacuum (UHV) condition ( 6 2 ꢀ 10À10 Torr).
interface.For example, metal silicides with CaF
2
structure tend to form A- and B-type epitaxial
layers on Si(1 1 1) [6,7].The former is a continua-
tion of the Si lattice planes and the latter results
from a single stacking fault caused by a 180°
rotation of the silicide with respect to the Si sub-
strate.Significant difference of the Schottky bar-
rier height between the A- and B-type NiSi2 was
reported by Tung [8].The orientation of epitaxial
NiSi2 layers depends upon annealing procedure,
deposition rate and temperature, initial amount of
Ni deposited and wafer preparation [7,9].Prefer-
ential growth of B-type NiSi2 by solid phase epi-
taxy (SPE) of submonolayer Ni deposited at room
temperature (RT) is due to kinetic and energetic
advantages but the detailed mechanism is still
unknown.
There are some reports on the atomic structure
and morphology of the Si(1 1 1) surface onto which
Co or Ni with submonolayer or less was deposited
[10–20].Van Loenen et al.[11] reported that the
Ni2Si islands grew for Ni-deposition more than
2.5 ML at RT and Si-rich silicides were formed
for fewer Ni coverage.According to Porter et al.
[12], the B-type NiSi2 was grown by 0.12 ML Ni
deposition at RT.It was also reported that RT-
2. Experiment
deposition followed by annealing produced NiSi2,
The experiment was carried out at the beamline
named SORIS, which was connected to the storage
ring of a 575 MeV electron synchrotron working
at Ritsumeikan University.The structure and
performance of the SORIS beamline were de-
scribed elsewhere [22,23].We employed n-type
Si(1 1 1) substrates with an off-set angle less than
0.1° and prepared clean 7 ꢀ 7 surfaces by usual
chemical treatment followed by heating at 1200 °C
for 15 s by IR.Temperature was measured with a
thermocouple covered with thin silica-based glass
to avoid the silicidation.The uncertainty of mea-
sured temperatures is estimated to be Æ30 °C at
most.The RHEED observation together with
Auger electron spectroscopy confirmed the clean
7 ꢀ 7 surfaces without any contaminations.Then 1
ML Ni was deposited onto the clean Si(1 1 1)-7 ꢀ 7
surface at RT with the K-cell at a rate of 0.13 ML/
min.Post-annealing was performed at 400, 600,
pffiffiffi pffiffiffi pffiffiffiffiffi pffiffiffiffiffi
NiSi-like islands, 3 ꢀ 3, 19 ꢀ 19 structures
and 1 ꢀ 1-ring cluster (RC) [17–21].According to
Parkin et al.[18], Ni deposition of 0.08 ML (1 ML:
7:83 ꢀ 1014 atoms/cm2) at RT followed by anneal-
ing at 860 °C and quenching produced 1 ꢀ 1-RC
pffiffiffiffiffi
phase and for Ni coverage of 0.2 ML the 19 ꢀ
pffiffiffiffiffi
19 structure appeared.For higher Ni coverage
they found the formation of NiSi2 islands from
Auger and reflection high energy electron dif-
fraction (RHEED) measurements.Using scan-
ning photoelectron spectromicroscopy (SPESM),
Gregoratti et al.[20,21] performed two-dimen-
sional mapping of the reaction products on the
submonolayer-Ni deposited Si(1 1 1) surface.The
characteristic phase formation after post-annealing
is due to the high solubility of Ni into the Si sub-
strate at temperatures higher than about 750 °C
and to the surface segregation of the dissolved Ni

114
Y. Hoshino et al. / Surface Science 511 (2002) 112–120
700 and 800 °C for 2 min by IR heating in the base
The sharp 7 ꢀ 7 spots became weak and fuzzy by 1
pressure of 4 ꢀ 10À10 Torr.The B-type NiSi layer
ML-Ni deposition at RT.After annealing at 400
2
pffiffiffi pffiffiffi
pffiffiffiffiffi pffiffiffiffiffi
ꢀ
with thickness 45 A grown on Si(1 1 1) was pre-
°C, the 1 ꢀ 1, 3 ꢀ 3 and 19 ꢀ 19 phases
appeared.The appearance of the latter two phases
is consistent with the result observed by a scanning
tunneling microscope (STM) [19].The weak 1 ꢀ 1
spots are probably due to the formation of a small
amount of NiSi2.Post-annealing at 600 and 700 °C
led to a clear 1 ꢀ 1 pattern originating from NiSi2
islands, whose formation would be confirmed later
pared for comparison.The temperature-raising
and cooling rates were 100 °C/min and 100–120
°C/min, respectively.This condition is similar to
that for the usual solid phase epitaxy of transition
metal silicides.The amount of Ni was measured by
MEIS with a toroidal electrostatic analyzer (ESA).
Rutherford backscattering (RBS) using 2.0 MeV
Heþ beams calibrated the absolute amount of Ni.
The as-deposited and annealed samples were trans-
ferred from the preparation chamber equipped
with the K-cells and the IR and RHEED systems
to the MEIS and PES chambers without exposure
to the air.
We performed the ultra-violet photoelectron
spectroscopy (UPS) and the Si-2p and Ni-3p core
level analyses with SR-photon energy of 40, 114
and 130 eV.The emitted photoelectrons were de-
tected with a hemispherical ESA with a mean ra-
dius of 137.9 mm. MEIS with 70 keV Heþ ions
analyzed not only the depth profile of Ni but also
the crystallographic structure of the reaction
products on the surface using the ion-channeling
and blocking effects.The surface morphology of
the as-deposited and post-annealed samples was
observed ex situ in the air by AFM.
by MEIS and UPS.The 1 ꢀ 1 pattern accompa-
pffiffiffiffiffi pffiffiffiffiffi
nied by weak 19 ꢀ 19 spots appeared by post-
annealing at 800 °C.
Fig.1 shows the AFM images observed for the
clean 7 ꢀ 7 surface, as-deposited, 400-, 600-, 700-
and 800 °C-annealed Ni(1 ML)/Si(1 1 1).For the
clean surface, regular steps together with step
bunching are clearly seen.The deposited Ni atoms
coalesced to make ridge-like stripes perpendicular
to steps and also formed fine clusters in some
ꢀ
places.The average height of the ridges is 15. A.
Three fine particles seen are SiC (confirmed by
RHEED), which tend to be created when the
vacuum is worse ( P 5 ꢀ 10À9 Torr) during the
surface cleaning process at high temperatures.By
400 °C-annealing, Ni atoms coalesced to form is-
lands on upper terrace sides at step edges.The
ꢀ
height of relatively large islands is 6.5 A, corres-
ponding to two Si–Ni–Si-triple layers of NiSi2.In
fact, Vrijmoeth et al.[15] showed that the NiSi
(1 1 1) surface was terminated by a Si–Ni–Si triple
2
3. Results and discussion
layer.At this stage, both NiSi (smaller) and NiSi
(larger) islands were formed.From the shape and
location, the larger islands are expected to be a
2
Table 1 shows the RHEED patterns observed
for the as-deposited and post-annealed samples.
Table 1
Results obtained by RHEED, AFM, PES and MEIS for Ni(1 ML)/Si(1 1 1), as deposited and annealed at 400, 600, 700 and 800 °C for
2 min
Sample
RHEED patterns
7 ꢀ 7 (weak)
pffiffiffiffiffi pffiffiffiffiffi
AFM
PES
NiSi
MEIS
ꢀ
As-deposited
Ridge-like stripes with height of 1.5 A
accompanied by small islands
Amorphous Ni: 1.0 ML
400 °C
600 °C
700 °C
800 °C
1 ꢀ 1 þ 19 ꢀ 19 þ
pffiffiffi pffiffiffi
Small islands plus large ones with height NiSi þ NiSi2
of 2-triple layers
Partially epitaxial
Ni: 0.81 ML
3 ꢀ 3 (weak)
1 ꢀ 1
Triangular-shaped islands with height of NiSi2
4-triple layers
Epitaxial B-type NiSi2
Ni: 0.53 ML
Epitaxial B-type NiSi2
1 ꢀ 1
Triangular- and polygon-shaped islands
with height of 6-triple layers
Very fine clusters
NiSi2
pffiffiffiffiffi pffiffiffiffiffi
1 ꢀ 1 þ 19 ꢀ 19
NiSi2 Polycrystal-like
(small amount) Ni: 0.27 ML
(weak)

Y. Hoshino et al. / Surface Science 511 (2002) 112–120
115
Fig.1.AFM images (2 lm ꢀ 2 lm) observed for clean 7 ꢀ 7 surface, as-deposited, 400-, 600-, 700- and 800 °C-annealed samples.
precursor state of the NiSi2 islands observed for
the 600 °C-annealed sample.Both phase formation
will be confirmed by UPS analysis later.Thus it is
reasonable to consider the smaller islands located
still on the terraces to be the NiSi phase.Anneal-
ing at 600 °C led to epitaxial growth of atomically
flat NiSi2 islands with a triangular shape.These
islands are just the B-type NiSi2(1 1 1), which is
determined by MEIS, as will be shown later.The
bases of the triangles are located along step edges
RHEED pattern suggests the formation of the
1 ꢀ 1-RC phase.It must be noted that the amount
of Ni at the surface is 0.27 ML (see Table 1) and
pffiffiffiffiffi pffiffiffiffiffi
the Ni coverages of the 19 ꢀ 19 and 1 ꢀ 1-RC
structures are 0.16 and 0.14 ML [17,18], respec-
tively, if the surface is terminated completely with
each corresponding structure.Therefore, the resid-
ual Ni atoms of about 0.1 ML probably exist as fine
NiSi2 clusters.Recently, Gregoratti et al.[20]
showed that two types of islands of NiSi and NiSi2
in coexistence with the 1 ꢀ 1-RC phase were formed
for Ni deposition of 1 ML at RT then annealed at
877 °C for 15 s and subsequently at 677 °C for 45 s
followed by slow cooling to RT (100 °C/min).
In order to identify the phase of reaction
products, we performed the PES analysis with 40,
114 and 130 eV photon beams.Fig.2 shows the
valence band (VB) spectra measured for the clean
Si(1 1 1)-7 ꢀ 7 surface, as-deposited, 400-, 600- and
800 °C-annealed samples.For comparison, the VB
ꢀ
and the height is almost constant (12.5 A) corres-
ponding to 4-triple layers.After annealing at 700
°C, large islands of NiSi2 were still alive and small
ones seem being dissolved and diffused into the
substrate.The diffused Ni atoms partly segregated
to the surface during slow cooling to RT and
probably formed the RC structure [17] and fine
NiSi2 clusters.Annealing at 800 °C dissolved the
NiSi2 island completely and surface segregation
occurred during the cooling process [17,20].The
ꢀ
segregated Ni atoms created a small amount of
the 19 ꢀ 19 structure and fine Ni-composite
spectrum for the B-type NiSi2 (45 A) grown on
pffiffiffiffiffi pffiffiffiffiffi
Si(1 1 1) was also measured.Three dashed lines in-
dicate the binding energies of the Ni2Si(À1:3 eV),
clusters (probably NiSi2).The dominant 1 ꢀ 1

116
Y. Hoshino et al. / Surface Science 511 (2002) 112–120
around À2 eV in the VB spectrum for the as-
deposited sample comes primarily from the NiSi
phase.Coexistence of the NiSi and NiSi phases
2
are seen after post-annealing at 400 °C and an-
nealing at 600 °C led to formation of the NiSi2
phase only.The broad peak in the VB spectrum
for the 400 °C-annealed sample is decomposed
into two components corresponding to those ob-
served for the as-deposited and 600 °C-annealed
samples.The spectrum for the 600 °C-annealed
ꢀ
sample is quite similar to that for NiSi2 (45 A)/
Si(1 1 1).The VB spectrum obtained for the 800
°C-annealed sample indicates that the primary
structure is no longer the NiSi2 phase, although a
small amount of the NiSi2 phase may still exist.
We also measured the Si-2p and Ni-3p core
levels at photon energy of 130 and 114 eV, re-
spectively.Background was subtracted by the
Shirley method [29].Fig.3 depicts the Si-2p spectra
Fig.2.UPS spectra measured with 40 eV SR-photons for clean
7 ꢀ 7 surface, as-deposited, 400-, 600-, 700- and 800 °C-an-
ꢀ
nealed samples.The B-type NiSi 2 layers with thickness of 45 A
was prepared by 15 ML-Ni deposition on Si(1 1 1)-7 ꢀ 7 surface
at RT followed by post-annealing at 650 °C for 5 min.Growth
of the B-type NiSi2(1 1 1) was confirmed by MEIS using the
blocking effect.S
(adatom dangling bonds), S2 (rest-atom
1
dangling bonds) and S3 (adatom back bonds) denote the surface
state levels for the Si(1 1 1)-7 ꢀ 7 [27].SR-photons were incident
along surface normal direction and photoelectrons emitted to
60° were detected.Binding energy values are scaled from Fermi
level.The VB spectrum observed for the 400 °C-annealed sample
is decomposed into two components (thin solid curves) from the
as-deposited (NiSi) and 600 °C-annealed (NiSi2) samples.
NiSi(À1:85 eV) and NiSi2 (À3:15 eV) [24–26].By
deposition of 1 ML-Ni at RT the 7 ꢀ 7 surface
states (S1, S2 and S3) disappeared and the NiSi
phase appeared.The surface states, S , S2 and S3
1
were assigned to the adatom dangling bonds, rest-
atom dangling bonds and adatom back bonds,
respectively [27].These surface peaks immediately
disappear by O2-exposure of 10 L(1 L: 106 Torr s)
[28].In fact, the 7 ꢀ 7-RHEED pattern became
very weak after the Ni-deposition.Thus the peak
Fig.3.Si-2p core level spectra observed for 130 eV photons
incident along the surface normal and emission angle of 60°. SA,
SB and SC denote the surface components of Si(1 1 1)-7 ꢀ 7 and
are assigned to the rest-atoms, the adatoms and the atoms
bonding to the adatoms, respectively [27].The Si-2p 3=2 peak for
Si(1 1 1)-7 ꢀ 7 is taken as the origin of the binding energy.

Y. Hoshino et al. / Surface Science 511 (2002) 112–120
117
measured at normal incidence and detection angle
of 60° with respect to the surface normal.The
Si-2p spectrum from the clean 7 ꢀ 7 surface is de-
composed into bulk and surface components (SA,
SB, SC).The peak (S A) with a 0.8 eV lower binding
energy than the bulk peak was assigned to the rest-
seen for 600 °C-annealed sample was slightly
shifted by 0.4 eV toward the higher binding energy
side compared with that observed for the as-
deposited and 400 °C-annealed samples.This bind-
ing energy shift is responsible for phase change
from NiSi to NiSi2.In fact, Gregoratti et al.
[21] also observed a similar binding energy shift
between NiSi and NiSi2 islands by SPESM.By
post-annealing at 800 °C, the Ni-3p peak was
broadened and strongly weakened in spite of the
residual Ni amount of 0.27 ML estimated from
MEIS, as will be shown below.
The MEIS measurement was carried out using
70 keV Heþ ions under two scattering geometries
with the same scattering angle (54.74°), (i) aligned
[0 0 1]-incidence and ½111Š-emergence and (ii)
random incidence and emergence.Fig.5(a) and (b)
shows the aligned and random MEIS spectra for
as-deposited, 400-, 600- and 800 °C-annealed
atom [27].The other surface components S and
B
SC correspond to the adatoms and the atoms
binding to the adatoms, respectively [27].Theses
surface components are still seen for the as-de-
posited sample, although the intensities are sig-
nificantly weakened.In fact, as mentioned earlier,
the weak 7 ꢀ 7 RHEED pattern was observed.
Formation of the NiSi2 phase created a new peak
with a 0.7 eV lower binding energy than the bulk
ꢀ
peak (NiSi2 (45 A )/Si(1 1 1)).This NiSi signal is
2
clearly seen for the 600 °C-annealed sample as
well.A small amount of NiSi is also seen for the
400- and 800 °C-annealed samples.The Ni-3p
spectra were observed at the same incidence and
2
detection geometry (Fig.4).The Ni-3p
peak
3=2
þ
Fig.5.(a) Double alignment MEIS spectra for 70 keV He
incident along ½001Š-axis and backscattered to ½111Š-axis and
(b) random MEIS spectra.Scattering angle and plane are 54.74 °
and (110), respectively.In case (b), scattering plane was rotated
by 5.5° off from ð110).
Fig.4.Ni-3p core level spectra observed with photon energy of
114 eV for as-deposited, 400-, 600- and 800 °C-annealed sam-
ples.Binding energy values are scaled from Fermi level.

118
Y. Hoshino et al. / Surface Science 511 (2002) 112–120
samples.Here, the energy scale of 1 keV corre-
ꢀ
sponds to the depth scale of 9 A for Si.The total
amounts of Ni located from the top surface down
ꢀ
to 25 A in depth are determined from the random
spectra to be 1:0 Æ 0:02, 0:81 Æ 0:04, 0:53 Æ 0:02,
0:27 Æ 0:024 ML for as-deposited, 400-, 600- and
800 °C-annealed samples, respectively.The long
tail of the Ni spectrum seen for the 600 °C-an-
nealed sample is responsible for the formation of
ꢀ
NiSi2 islands with height of 4-triple layers (12.5 A),
which was observed by AFM.Apparently the tail
is reduced in the aligned spectrum compared with
that in the random one owing to the ion shadow-
ing effect for the epitaxial NiSi2 layers.In the case
of 400 °C-annealing, a small tail is also seen in the
random spectrum and is reduced in the aligned
spectrum.It indicates the existence of thin NiSi
2
islands (2-triple layers), which was expected from
the AFM observation.In order to determine
which type of epitaxial layer was formed, A-type
or B-type, we performed polar scans around the
½111Š- and ½115Š-axis in the ð110Þ plane of the
Si(1 1 1) substrate for 600- and 700 °C-annealed
samples (see Fig.6).Here, the incident angle was
fixed to 62° with respect to surface normal (ran-
dom incidence) and the detection angle was varied
around the above axes.Fig.7(a) shows the scat-
tering yields from the second layer Ni atoms (in-
cluding a small contribution from the fifth layer
Ni) and from the Si substrate as a function of
polar angle around the ½111Š-axis with respect to
the surface normal ([1 1 1]-axis).It must be noted
that our toroidal ESA has an excellent energy
resolution of DE=E ¼ 9 ꢀ 10À4, which allows a
layer-by-layer analysis near surface regions [30,31].
If the A-type NiSi2 islands are grown, the ½111Š-
blocking dip would be seen for the scattering
components from the second layer Ni for the po-
lar-scan spectrum around the [111]-axis of the Si
substrate.On the other hand, growth of the B-type
NiSi2 would yield the ½111Š-blocking dip in the
½115Š direction of the Si substrate.In the case of
the polar scan around the ½111Š-axis, we have no
blocking dip, as shown in Fig.7(a).On the other
hand, the polar scan around the ½115Š-axis of the
Si substrate gives the clear ½111Š-blocking dip (Fig.
7(b)).In this polar scan, a small ½115Š-blocking dip
is seen for the scattering components from the Si
Fig.6.Cross sections of A-type NiSi 2(1 1 1)/Si(1 1 1) (upper)
and B-type-NiSi2(1 1 1)/Si(1 1 1) (lower) viewed from ½110Š-axis.
substrate.Here, we note that the plus sign of the
polar angle is taken as rotation toward the ½112Š-
axis of the Si substrate and the ½111Š- and ½115Š-
axes make a same angle of 70.53° with respect to
the [1 1 1]-axis (surface normal, see Fig.6).The
present polar scan analysis revealed the epitaxial
growth of the B-type NiSi2 islands by post-an-
nealing at 600 and 700 °C.The variation of NiSi
2
orientation as a function of deposited Ni thickness
was investigated in detail by Tung [9] by means of
low energy electron diffraction and transmission
electron microscopy.According to his report, thin
ꢀ
NiSi2 layers formed with 1–5 A Ni are predomi-
nantly type B oriented.
4. Summary
Structure change of Ni(1 ML)/Si(1 1 1) by post-
annealing was observed by RHEED, AFM, MEIS

Y. Hoshino et al. / Surface Science 511 (2002) 112–120
119
þ
Fig.7.70 keV He
incidence at 62° from surface normal (random) for 600 °C-annealed sample.(a) Polar scan around ½111Š-axis in
ð110Þ plane of Si substrate for scattering component from the second-layer Ni atoms (including a small contribution from the fifth-
layer Ni) and for scattering component from the Si substrate.(b) Polar scan around ½115Š-axis in ð110Þ plane of Si substrate for
scattering component from the second-layer Ni (including a small contribution from the fifth-layer Ni) and for scattering component
from the Si substrate.
ꢀ
and PES using SR-light.All the analyses were
performed in situ under UHV condition ( 6 2 ꢀ
10À10 Torr) except for the AFM observation.All
the results obtained by RHEED, AFM, PES and
MEIS are listed up in Table 1.The AFM obser-
vation revealed dramatic change of the surface
morphology after the Ni deposition at RT fol-
lowed by annealing at 400, 600, 700 and 800 °C
height of relatively large islands is 6.5 A, corre-
sponding to 2-triple layers of NiSi2.At this stage,
both NiSi (small) and NiSi2 (large) islands were
formed and the amount of the residual Ni atoms
was 0.81 ML. From the shape and location, the
larger islands seem to be a precursor state of the
NiSi2 islands observed for the 600 °C-annealed
sample.Annealing at 600 and 700 °C led to growth
of the B-type NiSi2 islands with average height of
4- and 6-triple layers, respectively.Nearly half the
deposited Ni atoms (0.5 ML) was lost from the
surface region by post-annealing at 600 °C for 2
min.After annealing at 800 °C, three-fourth of the
deposited Ni atoms were dissipated from the sur-
face and the dominant surface structure was the
þ
in UHV.MEIS using 70 keV He ions analyzed
the depth profiles of Ni and the crystallographic
structure of the Ni-composites formed by post-
annealing.The valence band and the Si-2p and
Ni-3p core level analyses were performed with SR-
photon energy of 40, 114 and 130 eV and identified
the phases formed by Ni(1 ML)-deposition at RT
and by post-annealing.The surface morphology
change studied by the present experiment is sum-
marized as follows.The Ni atoms deposited at RT
coalesced to make ridge-like stripes perpendicular
to steps and formed fine clusters in some places.
1 ꢀ 1-RC accompanied by a small amount of the
pffiffiffiffiffi pffiffiffiffiffi
19 ꢀ 19 phase.A small excess (about 01. ML)
of the Ni atoms which were segregated to the
surface probably formed fine clusters of NiSi2.
ꢀ
The average height of the ridges is about 1.5 A.
The UPS analysis suggests that the fine clusters
and/or ridges take the NiSi phase.By 400 °C-
annealing, Ni atoms coalesced to form Ni–silicide
islands on upper terrace sides at step edges.The
Acknowledgements
The authors would like to thank Prof.T.Nak-
ada for useful discussion and comments on the

120
Y. Hoshino et al. / Surface Science 511 (2002) 112–120
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