Surface Science 600 (2006) 2163–2170
www.elsevier.com/locate/susc
Electron stimulated desorption of cesium atoms
from germanium-covered tungsten
V.N. Ageev a, Yu.A. Kuznetsov a, T.E. Madey
b
b,*
a
A.F. Ioffe Physical-Technical Institute, Russian Academy of Science, 194021 St. Petersburg, Russia
Laboratory for Surface Modification, Department of Physics and Astronomy, Rutgers, The State University of New Jersey,
Piscataway, NJ 08854-8019, USA
Received 14 October 2005; accepted for publication 17 March 2006
Available online 18 April 2006
Abstract
The electron stimulated desorption (ESD) yield and energy distributions for Cs atoms from cesium layers adsorbed on germaniumcovered tungsten have been measured for different Ge film thicknesses, 0.25–4.75 ML (monolayer), as a function of electron energy and
cesium coverage H. The measurements have been carried out using a time-of-flight method and surface ionization detector. In the majority of measurements Cs is adsorbed at 300 K. The appearance threshold for Cs atoms is about 30 eV, which correlates well with the Ge 3d
ionization energy. As the electron energy increases the Cs atom ESD yield passes through a wide maximum at an electron energy of
about 120 eV. In the Ge film thickness range from 0.5 to 2 ML, resonant Cs atom yield peaks are observed at electron energies of 50
and 80 eV that can be associated with W 5p and W 5s level excitations. As the cesium coverage increases the Cs atom yield passes through
a smooth maximum at 1 ML coverage. The Cs atom ESD energy distributions are bell-shaped; they shift toward higher energies with
increasing cesium coverage for thin germanium films and shift toward lower energies with increasing cesium coverage for thick germanium films. The energy distributions for ESD of Cs from a 1 ML Ge film exhibit a strong temperature dependence; at T = 160 K they
consist of two bell-shaped curves: a narrow peak with a maximum at a kinetic energy of 0.35 eV and a wider peak with a maximum at a
kinetic energy of 0.5 eV. The former is associated with W level excitations and the latter with a Ge 3d level excitation. These results can be
interpreted in terms of the Auger stimulated desorption model.
2006 Elsevier B.V. All rights reserved.
Keywords: Electron stimulated desorption (ESD); Adsorption; Tungsten; Cesium; Germanium
1. Introduction
Irradiation of solid surfaces with electrons can lead to
desorption of charged and neutral particles in ground
and excited states. Desorption can result either from thermal heating of the surface by the incident electron flux or
from direct transformation of the potential energy of an
electronic excitation localized near the particle – surface
bond into the kinetic energy of the desorbing particle. In
*
Corresponding author.
E-mail address: [email protected] (T.E. Madey).
0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2006.03.022
the latter case desorption is called electron-stimulated
desorption (ESD) [1].
At present, much is known about many aspects of the
ESD mechanisms for positive and negative ions. However,
there is a lack of reliable measurements of the ESD fluxes
for neutral particles though they are the main component
of the ESD yield in many cases. Therefore an elucidation
of the ESD mechanisms for neutral particles is particularly
important for a detailed understanding of the various physical processes underlying ESD, which are observed to occur
in many vacuum devices where electron beams are used.
Surface perturbations by ESD must be considered when
employing methods for surface diagnostics involving electron irradiation [2].
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V.N. Ageev et al. / Surface Science 600 (2006) 2163–2170
Alkali metals are often applied as coatings for thermoelectric, photoelectric and spin-polarized emitters, including emitters with negative electron affinity, to reduce their
work function. They are indispensable in thermionic energy
converters, ion propulsion systems, sources of negative
ions, and as promoters in heterogeneous catalysis [3]. In recent years they have found new applications, arising from
their exceptional properties in semiconductor surface oxidation, nitridation and other reactions. Moreover, alkali
metal adsorption plays an important role in Schottky barrier formation and in stabilizing band alignment at heterojunctions as well as insulator–semiconductor interfaces
using low temperature oxidation [4,5].
Alkali metal films adsorbed on a metal surface are
rather stable under electron irradiation due to the short
lifetime of excited states in such systems. However, the
deposition of a insulating or semiconductor film (even
monolayer thickness) between the metal substrate and the
adsorbed alkali metal hinders electron exchange and the
ESD yield of alkali metal particles increases by a large factor [2]. Such systems seem to be appropriate for studying
the ESD mechanisms of alkali metal atoms because alkali
metal fluxes can be detected by a surface ionization method
with high efficiency [6,7].
Previously, we have measured the ESD yield [8–13] and
energy distributions [8,11] for alkali metal atoms from layers adsorbed on oxidized tungsten and molybdenum surfaces. It has been found that the ESD yield of alkali
metal atoms is initiated by the formation of a hole in the
oxygen 2s level. As the O 2s hole is filled in an Auger decay
process, the Auger electron emitted from the oxygen 2p
level causes the neutralization of an adsorbed alkali metal
ion. The yield is determined by the competition between
the re-ionization of alkali metal atoms via electron transfer
to the adjacent positive oxygen ion and the relaxation of
the positively charged oxygen ions by electrons from the
substrate conduction band [12].
An additional channel for the ESD yield of alkali metal
atoms from oxidized Mo has been attributed to resonant
ionization of Mo core levels [13,14]; these resonances are
characterized by narrow, sharp peaks in ESD yield vs electron energy. Resonances are also seen in ESD of rare earth
atoms (Eu, Sm) from oxidized W, and attributed to ionization of W core levels [15,16].
This paper reports measurements of the ESD yield and
energy distributions for cesium atoms adsorbed on germanium-covered tungsten surfaces. There are several interesting aspects of the results, in comparison with previous data
for ESD of Cs from O-covered and Si-covered W. First,
unlike Si with its relatively high 2p binding energy of
100 eV, Ge has its most shallow core level (Ge 3d, binding energy 30 eV) located slightly deeper than the O 2s at
25 eV. The difference permits one to clarify how the relative binding energy of substrate core levels affects the ESD
yield for alkali metal atoms. The position of the core level
determines the kinetic energy of electrons released in an
Auger decay process, and affects the neutralization proba-
bility of the Cs+ ion (the lower the electron kinetic energy,
the higher the neutralization probability). We find here that
ESD of neutral Cs occurs readily from Ge-covered W (low
Auger electron energy), in contrast to previous observations on Si-covered W (high Auger electron energy)
[17,18] where only alkali ions and no neutrals were seen
in ESD. Second, under certain conditions of Ge and Cs
coverages on W, clear evidence for electronic resonances
is observed for ESD of Cs atoms. The resonant features
are associated with W 5p and 5s core excitations, and the
formation of core excitons.
2. Experimental
The instrument and the procedures used in preparing
the sample for measurements are described in detail elsewhere [8]. In short, the experiments are carried out in a
bakeable stainless chamber with a residual gas pressure
below 5 · 10 10 Torr. A textured tungsten ribbon measuring 70 · 2 · 0.01 mm3 is used as the sample. To produce
a predominantly (1 0 0) oriented surface the ribbon is
heated at T = 2000 K and at a residual gas pressure of
about 10 9 Torr for 5 h by alternating current [19]. The
sample is cleaned of carbon by annealing in an oxygen
atmosphere [P(O2) = 10 6 Torr] at T = 1800 K for 3 h.
The sample contamination is checked by Auger electron
spectroscopy in separate experiments. The sample cleaning
is completed by annealing in ultrahigh vacuum at
T = 2200 K for 3 min after pumping oxygen.
Germanium is dosed onto the W ribbon at T = 300 K
from a directly heated evaporator made of a tungsten tube
containing pieces of Ge with an impurity contamination
less than 1016 at/cm3. The tube is arranged parallel to the
ribbon and has several holes to provide for uniform germanium deposition rate along the ribbon. The procedure for
determining Ge coverage on W is described below in the results section. Originally, we tried to install a flap in front of
the Ge evaporator to better control the deposition of Ge,
but it affected the electron and ion optics of the measurement system. This problem prevented us from comparing
the ESD yields for alkali metal atoms from oxidized tungsten and from Ge-covered tungsten. Since the main objective of the present work was to check the validity of our
Auger-stimulated desorption model, small uncertainty in
the determination of Ge coverages is not a problem. In
fact, reproducibility of Ge coverage measurements is a
few per cent, as judged by the reproducibility of the ESD
yields of Cs atoms.
For all experiments, cesium is deposited on the target at
T = 300 K from a directly heated evaporator by thermal
decomposition of cesium chromate. The cesium concentration on the target surface is determined from the deposition
time under constant flux. The cesium flux intensity is determined by measuring the total cesium ion current during
surface ionization at the sample surface heated to the
current saturation temperature. The cesium concentration corresponding to a Cs monolayer (H = 1) on the
V.N. Ageev et al. / Surface Science 600 (2006) 2163–2170
3. Results
Electron bombardment of a cesium layer adsorbed at
T = 300 K on a Ge-covered tungsten surface results in
desorption of Cs atoms. The yield of Cs atoms depends
on the Cs coverage and Ge film thickness. Fig. 1 shows a
plot of the Cs atom ESD yield from a cesium monolayer
adsorbed at T = 300 K on a Ge-covered tungsten surface
as a function of Ge deposition time. The electron energy
Ee is fixed at 120 eV. The delay of the Cs atom yield
appearance is connected with the dead time of the Ge evaporator. After an initial nonlinear period due to the warmup time of the Ge evaporator, the plot becomes linear in
the range 300–400 s. The measurements start with a cold
Ge evaporator, then the surface with Ge is dosed up to a
specific coverage, then the dose is stopped and a Cs monolayer is deposited. Finally a Cs atom ESD yield measurement is made. After the sample cleaning the whole
process is repeated for another Ge dose followed by another 1 ML Cs dose. We varied the sequence of specific
Ge doses and the Ge deposition rate but did not find any
significant changes in the Ge deposition time dependence
of the Cs atom ESD yield. The measurement of a single
data set took about 1 h, however the stability and reproducibility of the measurements are good and are indicated
by the sizes of data points in each figure. The yield passes
6.0
Θ Cs = 1 ML
5.5
5.0
Atomic Cs yield, arb. units
germanium-covered tungsten surface is taken to be
N0 = 5 · 1014 at/cm2, since the ESD yield of cesium atoms
reaches a maximum at this value.
The sample can be cooled by flowing gaseous nitrogen
through hollow current leads via a copper tube immersed
into liquid nitrogen. The sample temperature is varied in
the temperature range 160–300 K by changing gas flow
rate, and the temperature measurements in this range are
based on the sample resistance. In addition, the sample
can be heated resistively and the sample temperature is
determined with an optical micropyrometer in the high
temperature range; a linear extrapolation of the temperature dependence of the heating current is used to determine
temperature between 300 K and incandescence.
For ESD measurements of Cs atoms, the electron bombardment current is typically less than 5 · 10 6 A, and the
measurement time per data point is 50 s. The desorbed cesium atoms are ionized in a surface ionization detector consisting of a textured tungsten ribbon heated to T = 1800 K
[8]. The desorbing cesium ions are retarded by applying a
positive potential between the target and retarding
electrode located in the drift space. The ESD energy distributions for Cs atoms are measured by means of a time-offlight method by pulsing the electron beam using gating
pulses of duration of 1 ls and repetition frequency of
1 kHz. Electron-beam amplitude modulation with subsequent synchronous detection of the signal at the detector
output is employed to increase the signal-to-noise ratio.
The ion current signal is amplified by an electron multiplier
and a wide band amplifier.
2165
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
200
400
600
800
1000
1200
1400
Ge deposition time, sec
Fig. 1. ESD yield of Cs atoms from cesium monolayer adsorbed on
germanium film covered tungsten as a function of germanium deposition
time. Substrate temperature T = 300 K. Electron energy Ee = 120 eV.
through a maximum with increasing Ge deposition. We
suggest that the maximum corresponds to the formation
of a Ge monolayer on the tungsten surface and the yield
saturates after more than two monolayers of Ge cover
the surface. Henceforth we use the terms ‘‘thin and thick
films’’ meaning that the thin film has a thickness less than
one monolayer and the thick film has a thickness more than
two monolayers.
Fig. 2 presents plots q = f(Ee) of the ESD yield for Cs
atoms from a cesium monolayer adsorbed at T = 300 K
on Ge-covered tungsten for various Ge film thicknesses
as a function of electron energy. The yield q is expressed
as desorbed atoms per incident electron, in arbitrary units,
and the data are displayed in two frames (a, b) for clarity.
The Cs atom appearance threshold does not depend on the
Ge film thickness and is about 30 eV taking into account
the emitter work function. This value is close to the ionization energy of the Ge 3d level [20]. Fig. 2(a) (0.25 ML) and
(b) (4.75 ML) show that the yields of Cs atoms from a
cesium monolayer adsorbed on a Ge film with a low
(<0.5 ML) and a high thickness (>2 ML) grow approximately linearly with increasing electron energy up to an
electron energy of 90 eV and then smoothly reach a
broad maximum at 120 eV. In the intermediate range of
Ge coverage in the plots q = f(Ee), resonant-like Cs atom
yield peaks are observed (cf. Figs. 2(a) and (b)) at electron
energies of 50 and 80 eV. The intensity of each of the resonant peaks passes through a maximum with increasing
germanium film thickness and depends on the Cs coverage
H; this is illustrated in Fig. 3 for a thin Ge film. The 50 eV
peak appears at a Cs coverage H > 0.25 while the 80 eV
peak is distinctly seen only at H > 0.5. The positions of
these peaks and Cs atom yield appearance threshold are
independent of the Cs coverage indicating that the electron
energy does not depend on the target work function. The
electron energy values for the resonant peaks correlate well
V.N. Ageev et al. / Surface Science 600 (2006) 2163–2170
6.0
5.5 (a)
5.0
4.5 ΘCs = 1 ML
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
20
40
6.0
5.5
ΘGe = 1.25 ML
Atomic Cs yield, arb. units
Atomic Cs yield, arb. units
2166
0.75
0.25
ΘGe = 1 ML
120 eV
5.0
4.5
80 eV
4.0
3.5
70 eV
3.0
2.5
2.0
50 eV
1.5
1.0
0.5
60
80
100
120
140
0.0
0.0
160
0.2
0.4
0.6
6.0
5.5 (b)
5.0 Θ = 1 ML
Cs
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
20
40
1.5
4.75
80
100
120
140
160
Ee, eV
Fig. 2. (a, b) ESD yield of Cs atoms from cesium monolayer adsorbed on
germanium film of different thickness deposited on tungsten as a function
of electron energy. Germanium film thickness in monolayers (0.25–4.75) is
indicated on each curve. Substrate temperature T = 300 K.
6.0
Atomic Cs yield, arb. units
5.5
1.0
1.2
Fig. 4. ESD yield for Cs atoms from cesium layer adsorbed on germanium
monolayer covered tungsten as a function of cesium coverage for electron
energies 50, 70, 80 and 120 eV.
ΘGe = 1.25 ML
60
0.8
ΘCs , ML
ΘGe = 1 ML
ΘCs = 1 ML
5.0
4.5
4.0
3.5
0.75
3.0
gies as a function of cesium coverage. The yield grows
almost linearly with H up to H 0.9 and then passes
through a smooth maximum. The slope of each plot increases with increasing electron energy and is almost independent of the Ge film thickness, for Ge coverages greater
than 1 ML.
Fig. 5 shows plots of the ESD yield q for Cs atoms from
a Cs monolayer adsorbed on a Ge monolayer-covered
tungsten surface as a function of electron energy for substrate temperatures from 160 to 300 K. The yield decreases
with decreasing temperature in the electron energy range
30–160 eV. However, the appearance threshold and the position of resonant peaks in electron energy scale do not
depend on temperature, and the peak intensity is almost
independent of the substrate temperature.
Fig. 6 demonstrates the normalized Cs atom ESD energy distributions from a Ge monolayer-covered tungsten
surface for different Cs coverages H at an electron energy
of 50 eV, corresponding to a resonant peak. The energy
distributions are slightly asymmetric, and are extended
2.5
0.5
2.0
6.0
1.5
5.5
ΘGe = 1 ML
1.0
5.0
ΘCs = 1 ML
0.25
0.5
0.0
0
20
40
60
80
100
120
140
160
Ee, eV
Fig. 3. ESD yield of Cs atoms from different cesium coverage adsorbed on
germanium monolayer covered tungsten as a function of electron energy.
Cesium coverage in monolayers (0.25, 0.50, 0.75, 1.0) is indicated on each
curve. Substrate temperature T = 300 K.
Atomic Cs yield, arb. units
Atomic Cs yield, arb. units
Ee, eV
300 K
4.5
4.0
3.5
240 K
3.0
2.5
2.0
160 K
1.5
1.0
0.5
with the ionization energies of the tungsten 5p and 5s core
levels, respectively [20].
Fig. 4 illustrates plots of the ESD yield for Cs atoms
from a Cs layer adsorbed on a 1 ML Ge film deposited
on a tungsten ribbon at T = 300 K for four electron ener-
0.0
0
20
40
60
80
100
120
140
160
Ee, eV
Fig. 5. ESD yield for Cs atoms from cesium monolayer adsorbed on
germanium monolayer covered tungsten as a function of electron energy
for three substrate temperatures: 160, 240 and 300 K.
V.N. Ageev et al. / Surface Science 600 (2006) 2163–2170
1.0
ΘGe = 1 ML
ΘCs = 1.0 ML
1.0
ΘCs = 0.5 ML
dN / dE, arb. units
dN / dE, arb. units
ΘGe = 3 ML
ΘCs = 0.25 ML
0.6
0.4
0.2
0.0
0.0
0.1
0.2
ΘCs = 0.25 ML
ΘGe = 1 ML
ΘCs = 0.75 ML
0.8
2167
0.3
0.4
0.5
0.6
0.8
0.6
0.4
0.2
0.0
0.0
0.7
0.1
0.2
0.3
Fig. 6. Normalized ESD energy distributions for Cs atoms from cesium
layer adsorbed on germanium monolayer covered tungsten for cesium
coverages 0.25, 0.5, 0.75 and 1.0 ML. Substrate temperature T = 300 K.
Electron energy Ee = 50 eV.-
1.0
0.5
0.6
0.7
Fig. 8. Normalized ESD energy distributions for Cs atoms from cesium
layer adsorbed on 1 and 3 ML germanium films deposited on tungsten for
cesium coverage 0.25 ML. Substrate temperature T = 300 K. Electron
energy Ee = 120 eV.
1.0
ΘGe = 3 ML
ΘCs = 1 ML
ΘGe = 1 ML
ΘCs = 1 ML
dN / dE, arb. units
ΘCs = 0.25 ML
0.8
dN / dE, arb. units
0.4
Ee, eV
Ee, eV
0.6
0.4
0.8
160 K
300 K
0.6
0.4
0.2
0.2
0.0
0.0
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.1
0.2
0.3
0.7
Ee, eV
forward to higher energies. They also shift toward higher
energies without any change in shape with increasing Cs
coverage. For a thick Ge film, the shift does not exceed
0.04 eV in the Cs coverage range from 0.25 to 1 (Fig. 7),
for electron energy 120 eV. Their shape and position in energy do not depend on Ge film thickness for coverages
close to H = 1. However, the energy distributions shift toward higher kinetic energies by 0.05 eV at H = 0.25 in
going from a thin to thick Ge film deposited on tungsten
(Fig. 8), whereas the peak shape does not change. Note
that all data shown in Figs. 6–8 are for substrate
T = 300 K.
Fig. 9 presents normalized ESD energy distributions for
Cs atoms from a Cs monolayer adsorbed on a thin Ge film
deposited on tungsten for T = 300 K and T = 160 K. The
slightly asymmetric energy distribution at T = 300 K splits
into two almost symmetric peaks at T = 160 K: a narrow
peak with a maximum at an energy of 0.35 eV and a wide
one with a maximum at an energy of 0.5 eV. The electron
energy dependences of the peak intensities are quite differ-
0.5
0.6
0.7
Fig. 9. Normalized ESD energy distributions for Cs atoms from cesium
monolayer adsorbed on 1 ML germanium film deposited on tungsten for
substrate temperatures 160 and 300 K. Electron energy Ee = 50 eV.
2.0
Atomic Cs yield, arb. units
Fig. 7. Normalized ESD energy distributions for Cs atoms from cesium
layer adsorbed on 3 ML germanium film deposited on tungsten for cesium
coverage 0.25 and 1.0 ML. Substrate temperature T = 300 K. Electron
energy Ee = 120 eV.
0.4
Ee, eV
1.8
ΘGe = 1 ML
1.6
ΘCs = 1 ML
2
1.4
1.2
1.0
0.8
0.6
1
0.4
0.2
0.0
0
20
40
60
80
100
120
140
160
Ee, eV
Fig. 10. ESD yield for Cs atoms from cesium monolayer adsorbed on
1 ML germanium film deposited on tungsten as a function of electron
energy for Cs atom energy distributions in Fig. 9 with a maximum at a
kinetic energy of 0.35 eV (curve 1) and with a maximum at a kinetic energy
of 0.5 eV (curve 2). Substrate temperature T = 160 K.
ent (Fig. 10). The high-energy peak dependence has a
threshold at an electron energy of 30 eV and passes
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V.N. Ageev et al. / Surface Science 600 (2006) 2163–2170
through a wide maximum at Ee 120 eV, while the low-energy peak dependence has a resonant character with peaks
at electron energies of 50 and 80 eV.
Note that Fig. 10 curve 1 differs significantly from the
corresponding ESD yield curves (for 1 ML Cs on
1 ML Ge) in Figs. 2 and 3. Fig. 10 shows the partial
ESD yields at the peaks of the energy distributions in
Fig. 9, whereas the ESD yields of Figs. 2, 3 are the total
ESD yields for atoms of all kinetic energies. The yields of
Figs. 2, 3 are essentially the sum of data similar to
Fig. 10, curves 1 and 2.
4. Discussion
Cesium adsorbed on a germanium surface lowers the
surface work function, and the dipole–dipole repulsion between Cs adparticles results in a random distribution of Cs
across the surface [21,22]. Cesium is adsorbed on Ge in partially ionic form and the Ge atoms acquire some negative
charge [23–26]. As coverage increases the dipole–dipole
repulsion between Cs adparticles leads to decrease of their
bonding energies to the surface; as h approaches 1, the film
has metallic character. Reconstruction of a Ge surface by
Cs adsorption has not been reported, though there are
many peaks at a monolayer coverage in the thermal
desorption spectra for monolayer Cs coverage [22]. The
Ge–W interface is very sharp and there is no evidence for
diffusion of Ge into tungsten [23].
The ESD yield of Cs atoms from a cesium layer adsorbed on a Ge film deposited on tungsten can be interpreted on the basis of the Auger-stimulated desorption
model that was developed to explain the ESD of alkali
metal ions and atoms from alkali metal layers adsorbed
on oxidized W and Mo surfaces [9–13]. According to this
model the main channel of adsorption bond excitation in
ESD of alkali ions and atoms from oxidized W is the formation of a hole in the oxygen 2s level with ionization energy of about 25 eV; this energy determines the appearance
threshold. The hole can be filled by an electron from the
oxygen 2p level causing an Auger process. The Auger electron can either leave the adsorption system or be captured
by an adsorbed alkali metal ion. As a result of the Auger
process the negative oxygen ion becomes a positive ion,
and if the Auger electron leaves the system the positive oxygen ion starts repelling the adjacent positive alkali metal
ion; this can lead to the ESD of an alkali ion. If the Auger
electron neutralizes an alkali ion and the O+ recovers its
negative charge by capturing electrons from the substrate
faster than the alkali atom becomes reionized, there is a
repulsive interaction between oxygen and the alkali atom,
so that the alkali atom escapes from the surface. The greater the overlap between valence orbitals of O and alkali, the
higher is the kinetic energy of desorbed atoms. As the alkali
metal coverage increases, the equilibrium distance between
the adparticles and the substrate also increases due to the
decrease in heat of adsorption with coverage, and the
kinetic energy of the desorbing Cs atoms decreases.
The ESD yield for Cs atoms from a cesium layer adsorbed on Ge-covered W (Figs. 2–4) is lower by a factor
of 2 or 3 than from that adsorbed on an oxidized W surface
[9], which is consistent with easier electron transport
through the Ge film. The appearance threshold for Cs
atoms is about 30 eV, which correlates well with the Ge
3d level ionization energy. It appears that the ESD mechanism for Cs atoms from a Cs layer adsorbed on a Gecovered W surface is analogous that described above for
the ESD of alkali metal atoms from a Cs layer adsorbed
on oxidized W and Mo surfaces. The Auger electron emitted after a hole in the Ge 3d level is filled can neutralize an
adsorbed Cs ion. If the excited positive Ge ion restores its
initial charge by capturing electrons from the substrate faster than the Cs atom is reionized, it starts repelling the Cs
atom that can leave the surface. Note, however, that capture of an energetic Auger electron by a free Cs+ ion is
not a likely process – the atom would need to be excited
electronically during the capture. However, if the ion is
close to the surface, the energetic Auger electron can scatter and lose considerable energy before being captured by
the Cs+ ion to form neutral Cs.
Another possible mechanism for ESD of neutral Cs
could be neutralization of desorbing Cs+ via a resonant
charge transfer (CT) process. However, there is much evidence against resonant CT as the dominant process in
the present experiments. One observation involves ESD
of Cs+ from Si/W, where Cs+ is seen at electron energies
>100 eV, above the Si 2p threshold [18]. A similar resonant
CT neutralization of Cs+ would be expected in that case
also, but no neutral Cs is seen in ESD from Si/W. The lack
of neutral alkali atoms from Si/W is attributed to the higher kinetic energy of the Auger electron resulting from decay
of the Si 2p hole [17,18]. Other evidence against resonant
CT is based on the kinetic energy distributions (KEDs).
If the KED for neutrals is similar to that of ions, perhaps
resonant electron capture occurs during desorption. If the
KEDs are different, another explanation seems reasonable.
Whereas we have not measured KEDs for Cs+ from Ge/W,
we find that ESD of Cs and Cs+ from O/W have substantially different KEDs [9,11,17]; this argues against the resonant CT mechanism. Moreover, the ESD cross section of
alkali metal ions from oxidized tungsten decreases exponentially with increasing ion mass while that of atoms is
almost independent of the mass [17]. Also, the concentration dependences of the ESD yield for alkali metal atoms
and ions are quite different [27]. Taken together, these
effects indicate that the resonant CT mechanism is not a
major process in the present work; the experimental observations for ESD of Cs from Ge/W support the Auger-stimulated desorption mechanism proposed here.
Only two Ge monolayers appear to shield completely
from the W substrate a third Ge layer in contact with the
adsorbed Cs layer, causing the Cs atom ESD yield to
saturate for a Ge coverage of 3 ML (deposition time
t > 800 s, see Fig. 1). The Ge deposition time dependence
of the ESD yield for Cs atoms at H = 1 passes through a
V.N. Ageev et al. / Surface Science 600 (2006) 2163–2170
maximum, indicating that the lifetime of a positive Ge ion
in a Ge monolayer adsorbed on tungsten is shorter than
that of a neutral Cs atom formed via an Auger process
above the positive Ge ion [9].
The ESD yield for Cs atoms from a Cs layer adsorbed
on a thin Ge film deposited on W increases linearly with
Cs coverage up to H = 1 (Fig. 4). Hence, the lifetime of a
positive Ge ion is independent of the Cs coverage. The linear dependence of the Cs atom ESD yield on Ge deposition
time after reaching a constant Ge atom flux (300 < t <
400 s, Fig. 1) supports the assumption of a random distribution of Cs atoms across the surface at T = 300 K.
The energy of Auger electrons causing the neutralization
of adsorbed alkali metal ions is very important for realizing
the Auger-stimulated desorption mechanism of ESD. Indeed, the ESD yield for Cs atoms from a Cs layer adsorbed
on oxidized W initiated by the formation of a hole in the O
2s level with an energy of 25 eV is higher than that from a
Cs layer adsorbed on Ge film-covered tungsten initiated by
the formation of a hole in the Ge 3d level with an energy of
30 eV. In contrast, ESD from a cesium layer adsorbed on
silicided tungsten initiated by the formation of a hole in
the Si 2p level with an electron energy of 100 eV has
not been observed [18]. A possible explanation for this
trend is that the neutralization probability for adsorbed
ions via electron capture is higher for slower Auger
electrons.
The Cs atom ESD energy distributions shift toward
lower energies with increasing Cs coverage adsorbed on 3
ML Ge-covered W (Fig. 7), as well as on oxidized W. This
behavior is consistent with an initial-state effect, viz., an increase of the distance between the adsorbed Cs layer and
the substrate due to a decrease in heat of adsorption with
increasing Cs coverage, arising from lateral repulsive interactions between adsorbed Cs ions [2]. On the other hand,
the Cs atom ESD energy distributions for thin Ge films
shift toward higher energies with increasing Cs coverage
(Fig. 6). This is counter to the behavior anticipated for
the purely initial-state effect indicated above. We suggest
that there is a counterbalancing final-state effect, for which
the gradient of repulsive potential in the excited Cs antibonding state increases as the concentration of adsorbed
Cs increases. The reason for this effect is not clear. We have
made a similar observation (shift of peak in kinetic energy
distribution to higher energy with increasing coverage) for
the ‘‘split’’ high energy peak in Fig. 9, corresponding to
ESD of Cs at 160 K. These data are described in a separate
paper [28].
We can elaborate on the factors that influence Cs energy
distributions. The lateral repulsive interaction between
adsorbed Cs ions reduces the bond energy between the
adsorbed ions and the surface, and the ion-surface distance
increases slightly. The onset of metallization is associated
with overlapping valence orbitals of neighbouring Cs; the
distance from ion core to surface reaches a limiting value
as Cs coverage increases, and the layer becomes metallic.
We have observed manifestations of this effect in studying
2169
ESD of alkali metal atoms from oxidized tungsten [11]. The
ESD kinetic energy distribution of alkali atoms is determined by the bond distance between adsorbate and surface
(initial-state effect) and the gradient of repulsive potential
in the repulsive excited state (final-state effect). Generally
speaking, the heat of chemisorption decreases with increasing adsorbate coverage. In this case the atom ESD energy
distribution should shift toward lower energies. However,
if a compound is formed between the adsorbate and the
substrate, the desorption activation energy may grow as
the coverage increases and, correspondingly, the energy
distribution shifts to higher energies. These effects occur
if the gradient of repulsive potential is independent of the
coverage. If the gradient depends on the coverage it is difficult to predict the direction of the shift.
Most of the above discussion has focused on an ESD
channel for Cs that is initiated by Ge 3d excitations. There
is also a second ESD channel identified in this work. That
is, the resonant Cs atom ESD yield peaks at electron energies of 50 and 80 eV are connected with resonant excitations of W 5p and 5s levels. This ESD channel may be
caused by the formation of W core level excitons. We have
observed similar resonant peaks in the Li and Na atom
ESD yield from lithium and sodium layers adsorbed on
oxidized molybdenum, respectively, [13,14] and in the
ESD of neutrals from samarium layers adsorbed on oxidized tungsten [16]. In the latter case some peaks have
been attributed to SmO molecules on the basis of the temperature dependence of ion current in the surface ionization detector [29]. A distinct correlation is observed
between the resonant peaks and the low energy tails in
the neutral ESD energy distributions from Li and Na layers adsorbed on oxidized Mo. These features have been
interpreted as evidence for the ESD of slow-moving LiO
and NaO molecules [13,14]. No tails have been found in
the ESD energy distributions for neutrals from Cs layers
adsorbed on Ge covered tungsten, and other evidence is invoked to suggest that CsGe may form in ESD. By analogy
with ESD of LiO, NaO, and SmO, based on the resonant
features in Figs. 2, 3, 10 for Cs on thin (1 ML) Ge, and
based on the existence of the ‘‘split’’ energy distribution
in Fig. 9, we propose that CsGe may form, and the resonant ESD features are due to CsGe. The excitation of W
core levels is transferred via the thin Ge layer to Cs, and
initiates desorption of a CsGe complex. This process turns
off for thicker Ge coverages >2 ML. The absence of lowenergy tails in the kinetic energy distributions may imply
that CsGe molecules are unstable and can dissociate rapidly at the surface, or that they form under specific conditions in ESD.
The electron energy dependences of the low-energy and
high-energy Cs atom ESD yield (Fig. 10) show that the
low-energy peak is due to the excitation of W 5p and 5s
levels while the high-energy peak is due to the excitation
of Ge 3d levels. As indicated previously, Ge films thicker
than 2 ML shield the tungsten excitations completely,
and prevent resonant W features from exciting ESD of Cs.
2170
V.N. Ageev et al. / Surface Science 600 (2006) 2163–2170
Under the conditions of the present experiments (Cs
coverages >0.25 ML, Ge film thickness 0.25–4.75 ML) we
have not observed a peak at an electron energy of 38 eV;
this feature was the most intense in the ESD of SmO molecules and was associated with the W 5p3/2 level excitation
energy. However, we have observed recently a resonantlike peak at 38 eV in ESD of Cs atoms from Cs layers
adsorbed on a Ge monolayer-covered W surface only at
h < 0.3 ML and at a substrate temperature T < 300 K
[28,30]. Apparently this effect may be attributed to the presence of the Ge 3d level which is rather close to the W 5p3/2
level and facilitates its relaxation.
Finally, we call attention to the reversible temperaturedependence in ESD of Cs exhibited in Fig. 9, and its association with resonant and non-resonant processes shown in
Fig. 10. There are a number of examples in the literature in
which temperature dependent phenomena have been reported in ESD; see [31] and references therein. In
particular, a reversible temperature dependence has been
observed for the ESD yields of neutral and ionic Na and
K from a lunar sample [31]. Those observations were
attributed to temperature-related variations in the steadystate population of different adsorption sites on the surface.
The dramatic temperature-dependent changes in energy
distributions seen in the present work (Fig. 9) have not
been reported previously, and merit further study.
5. Summary
This paper reports the ESD of neutral Cs species from a
cesium layer adsorbed on germanium covered tungsten.
The Cs atom yield is associated with excitations of Ge 3d
and W 5p and 5s levels. The W excitations have a resonant
character that may be attributed to the formation of tungsten core level excitons. The Cs atom ESD energy distributions shift toward lower and higher energies with increasing
cesium coverage depending on Cs coverage and Ge film
thickness. The direction of the shifts is determined by a
competition between initial and final state effects. The energy distributions for ESD of Cs on a 1 ML Ge film exhibit
a strong temperature dependence, and both resonant and
non-resonant processes can be identified.
Acknowledgements
The authors acknowledge valuable discussion with Dr.
B.V. Yakshinskiy. This work has been supported in part
by the Russian Foundation of Basic Research, Grant No.
03-02-17523, by the Russian Federal Agency on Science
and Innovations, State Contract No. 02.434.11.2027, and
by the US National Science Foundation, Grant No. CHE
0315209.
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