Appl. Phys. A 71, 461–464 (2000) / Digital Object Identifier (DOI) 10.1007/s003390000584
Applied Physics A
Materials
Science & Processing
Rapid communication
Physical properties of the Ba-adsorbed Si(111) surface at elevated
temperatures
J.R. Ahn, H.W. Kim, K.D. Lee, J.W. Chung
Physics Department and Basic Science Research Institute, Pohang University of Science and Technology, San 31 Hyoja Dong, Pohang 790-784, Korea
Received: 16 May 2000/Accepted: 17 May 2000/Published online: 16 August 2000 –  Springer-Verlag 2000
Abstract. We have investigated the adsorption of Ba on
the Si(111) surface at elevated temperatures by using highresolution electron-energy-loss spectroscopy, low-energyelectron diffraction, and photoelectron spectroscopy with
synchrotron photons. We found two new ordered phases
2 × 1a and 2 × 1b with increasing Ba coverage in addition to
other ordered phases reported earlier. All the ordered surfaces
were found to remain semiconducting with a hybridization
band gap of ∼ 1.1 eV almost independent of Ba coverage. We
discuss evidence for the evolution of a Ba s − s hybridization
band for Ba coverage beyond 0.5 monolayers and propose
structural models for the three ordered phases, which are
quite consistent with our experimental data.
PACS: 73.20.-r; 71.20.Dg; 68.35.Bs; 79.60.-i
Recently the adsorption of alkali earth metals on semiconductor surfaces has become a research subject of growing
interest [1, 2]. Other than the anticipation of finding a variety of interesting surface phenomena as observed on the
alkali metal adsorbed semiconductor surfaces [3–7], investigation of divalent alkali earth metal overlayers on semiconductor surfaces will provide a systematic way to compare
these with monovalent alkali metal overlayers in order to understand the essential nature of metal–semiconductor surface
interactions. A previous study reported four sub-monolayer
surface reconstructions by annealing the Ba/Si(111) surface adsorbed at room temperature and found several interesting properties including additional lowering in work
function beyond the first layer, increasing oxidation rate
with increasing Ba content, and the formation of BaSi2 silicide [1]. In this paper, we report results from a combined
study of a high-resolution electron-energy-loss spectroscopy
(EELS), low-energy-electron diffraction (LEED), and photoelectron spectroscopy (PES) using synchrotron radiation for
the Ba/Si(111) surface adsorbed at elevated temperatures.
This system turns out to be a good example that demonstrates not only the adsorbate–substrate hybridization but also
the adsorbate–adsorbate hybridization. We propose structural
models for the three ordered Ba-induced structures, 2 × 1a,
2 × 8, and 2 × 1b, based on our experimental observations.
Experiments were carried out in two different ultrahigh
vacuum chambers, one for EELS and another for LEED
and PES measurements. The EELS chamber was equipped
with a Leybold-Heraeus ELS-22 spectrometer with an optimum resolution of 8 meV and a half-acceptance angle
of 2◦ . The base pressure of the chamber was maintained below 3 × 10−11 mbar throughout the measurements. The PES
chamber was installed at a synchrotron beamline 2B1, and
a Pohang light source, and was used to obtain valence band
and Si 2 p core level spectra using unpolarized photons of
energy 75 and 130 eV, respectively. The base pressure of
the PES chamber was better than 6 × 10−11 mbar and the
optimum energy resolution of the analyzer was 140 meV.
A silicon sample cut from a boron-doped Si(111) wafer
( ∼ 1.5 Ω cm) was cleaned by flashing at 1470 K for about
10 s and then annealed at 1070 K for about 1 min. The resulting clean surface produced a well defined 7 × 7 LEED
pattern. A commercial SAES dispenser was used as a Ba
evaporation source. The chamber pressure was maintained
below 1 × 10−10 mbar during Ba evaporation.
Figure 1 shows the changes in work function ∆Φ and
the corresponding changes in LEED pattern as a function
of Ba exposure with the Si(111) substrate at two different annealing temperatures Ta = 840 ◦ C (Fig. 1a) and Ta =
750 ◦ C (Fig. 1b). Because of the relatively high substrate temperatures, the saturation values of ∆Φ, − 1.4 eV (Fig. 1a) and
− 1.6 eV Fig. 1b, are significantly lower than − 2.5 eV from
the adsorption at room temperature reported earlier [1]. The
series of ordered phases observed by Weitering, however, also
appears
√ in√the data presented in Fig. 1. The only exception is
the 3 × 3R30◦ phase, which indicates that no BaSi2 silicide is formed on our sample surfaces [1]. Among the ordered
phases shown in Fig. 1, the presence of two ordered 2 × 1
phases is new in this work. Similar ordered phases have also
been observed for other divalent metals of Ca [8], Sr [9], and
Yb [10]. The values of ∆Φ are ∼ −1.0, −1.2, −1.3, −1.4,
and − 1.7 eV for the 3 × 1, the 5 × 1, the 2 × 1a, the 2 × 8,
and the 2 × 1b phases, respectively. Assuming that the Ba
coverage, θ, of the saturated 2 × 1b phase at Ta ∼ 750 ◦ C is
462
A
B
+
C
C
+
D1
E
0.0
o
T ~ 840 C
-0.5
-1.0
-1.5
2
4
6
8
10
12
14
16
18
Ba deposition time (min)
(a)
?Ë (eV)
A
A
+
B
D1
B +
+ B D1 E
w C w+C
B
0.0
D2
o
T ~ 750 C
-0.5
-1.0
-1.5
0
2
4
6
8
10
12
14
16
Ba deposition time (min)
(b)
Fig. 1a,b. The changes in work as a function of Ba exposure time on the
Si(111) surface at a 840 ◦ C and b 750 ◦ C, respectively. The progressive development of the ordered phases is shown at the upper part of each figure,
where A, B, C, D1 , E, and D2 denote 7 × 7, 3 × 1, 5 × 1, 2 × 1a, 2 × 8, and
2 × 1b phases, respectively. The wC phase is a weakly developed C-phase
a monolayer (ML), the coverage of the 2 × 1a phase is estimated to be 1/2 ML.
A series of representative EEL spectra from the ordered
phases are presented in Fig. 2, where one immediately no-
EELS Ei=10 eV
Þin=Þout=60
o
P g'
Phase
Intensity (arb. units)
12
21b
Pg
6
0
Ei = 75eV
28
o
Þin = 45 Þout=0
1
o
1.8
Eb (eV)
0
tices the semiconducting property from the band gaps of
about 1.1 eV. It is interesting to note that the band gap is
almost unaltered with increasing Ba coverage. The spectra
were obtained using an electron beam of energy 10 eV under
a specular scattering geometry with the incident and outgoing
angles both at 60◦ . Because of the close similarity between
the loss spectrum from the 3 × 1 phase and that from the
AM/Si(111)-3 × 1 surface, we ascribe the two characteristic peaks P1 and P2 to interband transitions from a common
filled Ba–Si s– p hybridized band to an empty s– p band (P1 )
and to a Ba-derived empty band (P2 ) as assigned for the
AM/Si(111) surface [3, 10, 11]. Such interband transitions
seem to persist in all the ordered phases although their relative energies and intensities vary with coverage slightly; one
finds that while P1 weakens in intensity with increasing θ,
P3 becomes visible at ∼ 2.0 eV when the 3 × 1 phase disappears completely. The most prominent feature, however,
is the presence of loss peaks Pg and Pg at ∼ 0.7 eV and
∼ 0.3 eV inside the band gaps of the 2 × 8 and the 2 × 1b
phases, respectively. In order to understand where these loss
peaks come from, we have measured the changes in the valence band (Fig. 3) and in the Si 2 p surface core level (Fig. 4)
with θ(≥ 0.5 ML) using synchrotron photons of energy 75 eV
and 130 eV, respectively. The valence bands in Fig. 3 are assumed to consist of a bulk state B and a surface state S
as for the Na/Si(111)-1 × 1 surface [12]. By fitting the valence bands with B and S, we find that S shifts towards
E F significantly (by about 0.3 eV) with increasing θ, which
has an important implication as discussed in the following
paragraphs.
The Si 2 p core level spectra shown in Fig. 4 obtained
from each of the three ordered phases were analyzed by fitting
the spectra with three components, BP, SP1 , and SP2 , contributed from bulk Si atoms and two different types of surface
Si atoms (Fig. 4). The fittings were performed using spinorbit-split Voigt functions with a Lorentzian full width at half
maximum (FWHM) fixed at 0.11 eV and the Gaussian optimum FWHMs of 0.3 and 0.35 eV for BP and for both SP1 and
S
1.6
1.4
21a
P3
P 2'
P 1'
P2
P1
21a +
51
51 +
wk 31
31
Intensity (arb. units)
?Ë (eV)
B
+
B
A
21a 28 21b
Ef
21b
28
B S
21a
0
1
2
3
4
5
6
Loss energy (eV)
Fig. 2. The evolution of EEL spectra with Ba coverage on the Si(111) surface at 750 ◦ C. Notice that all the phases remain semiconducting with band
gaps of ∼ 1.1 eV. Note also the loss peaks Pg and Pg present inside the band
gaps of the 2 × 8 and 2 × 1b phases
4
2
0
-2
-4
Eb (eV)
Fig. 3. Valence band spectra obtained from the three ordered phases above
0.5 ML. The spectra were analyzed by a bulk state B and a surface state S.
The weak dotted curves are background intensities. Inset shows the changes
in binding energy E b of the surface state
463
Þin = 0
o
Þout = 45
o
-0.30
º E b(eV)
Ei = 130 eV
-0.35
o
o
3.8A
4.4 A
SP1
-0.40
-0.45
Intensity (arb. units)
-0.50
2x1a 2x8 2x1b
21b
28
(a)
(b)
Top view
21a
SP2 BP SP1
2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5
Fig. 4. The Si 2 p surface core level spectra obtained from the three ordered
phases above 0.5 ML. The spectra were fitted with three components,
B (solid curves), SP1 (dotted curves), and SP2 (dashed curves) representing
the contributions from bulk, the first-layer, and the second-layer Si atoms,
respectively. The relative binding energy (∆E b ) is referenced to the binding
energy of B. Inset shows the change in ∆E b of SP1
SP2 , respectively. Naturally SP1 and SP2 are attributed to surface Si atoms directly bonded to Ba adsorbates and Si atoms
in the second layer, respectively. With increasing θ, SP1 is
found to shift towards the higher binding energy side up to
∼ 0.05 eV (see inset) while SP2 remains almost fixed in binding energy.
In Fig. 5, we propose structural models for the 2 × 1a (1/2
ML), the 2 × 8 (3/4 ML), and the 2 × 1b (1ML) phases. Here
we assumed the hollow site as a binding site for Ba, which
was also favored for Mg [13], although the ontop site and the
bridge site were preferred for Sn on the Si(111)-7 × 7 surface
[14] and for Yb on the bulk-terminated Si(111)-1×1 surface
[15], respectively. Lin et al. showed that the orbital mixing
between Sn 5 p and Si 3 p orbitals results in a bonding and
antibonding combination of surface bands with a band gap
E g ∼ 1.1 eV for the Sn/Si(111) surface [14]. Northrup also
found a band gap of E g ≥ 1.8 eV induced by Na at θ = 1.0
ML for the Na/Si(111) surface [16]. In line with these observations, we ascribe the band gap E g ∼ 1.1 eV in Fig. 2 to
the Ba 6s − Si 3 p hybridization, which opens a hybridization band gap as schematically illustrated in Fig. 6a. Since
the two valence electrons from Ba adatoms saturate all the
dangling bonds at 0.5 ML for the 2 × 1a phase as for the 1.0
ML of Na/Si(111) surface [17, 18], the excess Ba atoms at
coverages beyond 0.5 ML (Fig. 5b and c) would form a s–
s hybridization with Ba atoms already bonded directly to Si
atoms in the first layer. The close similarity of all the spectra (see Figs. 2–4) obtained from each of the three ordered
phases above 0.5 ML implicates quite similar, albeit not identical, local atomic bonding structures as suggested in Fig. 5.
The s–s intermixing would then form a filled surface band Sm
Side view
º E b (eV)
Second layer Ba atom
First layer Ba atom
First layer Si atom
Second layer Si atom
(c)
Fig. 5a–c. Structural models for a the 2 × 1a, b the 2 × 8, and c the 2 × 1b
phases. The Si substrate was assumed to be a bulk truncated 1 × 1 Si(111)
surface
Fig. 6a,b. The schematic energy diagrams of the Ba adsorbed Si(111) surface (middle) derived from that of Ba (left) and of Si (right) for a θ ≤ 0.5
ML and for b θ > 0.5 ML. For θ ≤ 0.5 ML, valence electrons of Ba saturate the dangling bonds of the Si atoms to make the surface semiconducting.
However, for θ > 0.5 ML the excess Ba atoms form a s–s hybrid surface
band Sm near E F to produce the loss peaks Pg and Pg in Fig. 2
464
near E F as shown in Fig. 6b. We believe that the loss peak
Pg or Pg represents an excitation from Sm to the empty Ba
5d band. In Fig. 2, one notes that Pg shifts by about 0.4 eV to
become Pg with further increase in θ.
Now the question is why does Pg shift with increasing
coverage? Since the coordination number z (4 and 6 for the
2 × 8 and the 2 × 1b, respectively) and the wave function
overlap t between Ba atoms increases with increasing θ, the
band width W of Sm should increase accordingly because
W is proportional to zt. This explains the shift of Pg into
Pg as coverage increases due to the reduced band gap between Sm and Ba 5d bands as is typical for a band-crossing
transition [19]. One finds, however, that the overlap is not
strong enough to form a global metallic overlayer as seen by
the apparent band gaps in Fig. 2, almost unaltered from the
the initial value of ∼ 1.1 eV of the 3 × 1 phase. As s–s hybridization becomes enhanced with increasing coverage, the
Ba 6s–Si 3 p intermixing may become weak a result of the
depolarization effect due to the redistribution of charges as
observed theoretically for the K/Si(100) surface [20]. This
depolarization effect, primarily caused by the charge transfer
from Si back to Ba atoms, accounts not only for the reducing
intensity and the gradual shift of S surface band towards E F
in Fig. 3 but also for the increased binding energy of SP1 of
Si 2 p core level seen in Fig. 4.
In summary, we studied physical properties of the Ba adsorption on the Si(111) surface at elevated temperatures. We
observe two new Ba-induced 2 × 1 ordered phases in addition to the series of other ordered phases reported earlier. We
find evidence that the excess Ba adsorption beyond 0.5 ML
where all the Si dangling bonds are saturated induces s–s
hybridization between Ba atoms, which results in the characteristic loss peaks for the 2 × 8 and the 2 × 1b phases inside
the band gaps of the Ba 6s–Si 3 p hybridization bands. However, the s–s hybridization turns out not to be strong enough
to make the Ba-adsorbed surfaces metallic. As the s–s intermixing increases with increasing coverage, the redistribution
of charges, essentially from Si back to Ba, explains the behavior of the surface valence band as well as the changes in Si 2 p
core level spectra.
Acknowledgements. We acknowledge partial supports from the Korea Research Foundation (1998 and 1999).
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