Nuclear Instruments and Methods in Physics Research B 230 (2005) 455–459
www.elsevier.com/locate/nimb
Kinetic electron emission in the interactions of slow
ions with MgO surfaces
P. Riccardi a,b,*, P. Barone a, A. Bonanno a, A. Oliva a,
P. Vetrò a, M. Ishimoto b,c, R.A. Baragiola b
a
b
Laboratorio IIS, Dipartimento di Fisica, Universitá della Calabria, and INFM Unità di Cosenza,
87036 Arcavacata di Rende, Cosenza, Italy
Laboratory for Atomic and Surface Physics, University of Virginia, Charlottesville, VA 22904, USA
c
Fujitsu Laboratories Ltd., Akashi, Hyougo 674-8555, Japan
Abstract
We report experimental energy distributions and yields of electrons emitted from MgO surfaces under the impact of
slow noble gas and sodium singly charged ions at varying incident energies.
At impact energies below 1 keV, electron spectra are nearly independent of ion type and energy. A tail of highenergy electrons is observed to grow at higher impact energies.
The results are explained in terms of promotion of oxygen-2p electrons during binary projectile-oxygen collisions
populating continuum and excitonic states. Excitons can significantly contribute to electron emission due to the negative electron affinity of the surface.
Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction
Ion-induced electron emission from solids is
due to the two main processes of potential and kinetic electron emission [1,2]. In potential electron
emission, electron excitation results from the conversion via Auger processes of the potential energy
*
Corresponding author. Address: Laboratorio IIS, Dipartimento di Fisica, Universitá della Calabria, and INFM Unità di
Cosenza, 87036 Arcavacata di Rende, Cosenza, Italy.
E-mail address: riccardi@fis.unical.it (P. Riccardi).
carried by the incoming ions when they are neutralized by electron capture from the surface. In kinetic electron emission, excitation results from the
transfer of kinetic energy of the incoming ion.
These excitation mechanisms have been extensively studied using metal target, while insulator
materials received considerable less attention, due
to experimental difficulties arising because the surface charges up during ion bombardment.
Among the insulator materials which are currently the subject of an intense scrutiny, magnesium oxide is one of the most interesting. This is
0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.nimb.2004.12.083
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P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 455–459
because it is a prototypical ionic insulator and because of advantageous properties of this material
for use in plasma display panels: a large ion
induced electron yield (and therefore a low discharge voltage) and high stability under ion
bombardment.
Experiments of electron emission from MgO
samples by slow ions have been recently reported
by us and other groups [3–7]. A consistent observation reported in many of this works is that the
energy distributions of emitted electrons resulted
independent on the type of projectiles.
Different interpretation were proposed to explain this remarkable finding such as Auger neutralization (AN) [1] of incoming ions, whose
ionization potential is shifted in front of the crystal
surface [4], or decay of collisionally-excited (MgO)
pair [7]. Thus, the important question that has to
be answered is whether the excitation mechanism
is driven by the potential or by the kinetic energy
of incoming projectiles.
In this work, we extend our measurements of
energy distribution of electrons emitted from
MgO surfaces by slow ions to higher impact energies than those used in previous research [4]. We
observe that electron spectra do not change at impact energies below 1 keV and are very similar
for different projectile ions.
At higher impact energies, spectra of electrons
emitted by He+ and Ar+ ion bombardment broaden due the growth of a continuous tail of high-energy electrons.
The results are consistent with a recently proposed model [4] in which excitation occurs when
oxygen-2p electrons are promoted during binary
projectile-oxygen collisions to continuum and excitonic states. Excitons can decay into vacuum due
to the negative electron affinity of the MgO
surface.
The surface of the samples was normal to the
ion beam and electrons were collected at angle of
78° with respect to surface normal.
Sodium ions were produced in thermal ionization source. Noble gas ions were produced in an
electron impact source, which was operated at
electron energy of 58 eV. Measurements performed at an electron energy of 30 eV in the ion
source showed that contamination of the ion beam
with doubly charged ions was negligible.
MgO samples were prepared by electron beam
deposition on a highly doped Si substrate and their
thickness was approximately 100 nm. They were
polycrystalline with the grains oriented such that
they present an oxygen terminated (1 1 1) surface,
as determined by X-ray diffraction. Samples were
sputter cleaned for about 1 h by 1 keV Ar+ ion
bombardment at a current density of 5 lA/cm2
and the cleaning was monitored by Auger electron
spectroscopy (AES) and electron energy loss spectroscopy (EELS). Weak features due to band-gap
states appeared in the EELS spectra before cleaning [8]. AES and EELS spectra for the sputter
cleaned MgO surfaces were in excellent agreement
with well-known spectra reported in literature [9–
11]. Distortion of the electron energy distributions
due to surface charging was prevented by neutralizing the positively charged sample with an electron flood gun right before the measurements.
The energy of the Auger transition observed in
the AES spectra after this procedure ensured neutrality of the sample.
The ion-induced spectra were acquired with the
sample biased at a negative voltage Vb = 4.9 V.
During the acquisition of the ion induced spectra,
the ion current density was kept in the range of
10 1 lA/cm2 and the acquisition time was 20 s.
Repeated acquisitions showed that surface charging and damage had no significant effects over this
time interval.
2. Experiments
3. Results and discussion
The experiments were performed in UHV
(10 10 Torr) using a double pass cylindrical mirror energy spectrometer. The spectrometer was
operated at constant pass energy of 50 eV and a
resolution of 0.2 eV.
Fig. 1 reports electron emission yields c for
He+, Ne+, Na+ and Ar+ ion bombardment of
MgO surfaces as a function of incident ion energy.
These yields were obtained from the current mea-
P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 455–459
457
1.6
He +
Na +
1.4
Yield (electrons/ion)
Ne +
1.2
Ar+
1.0
0.8
0.6
0.4
0.2
0.0
0
500
1000
1500
2000
2500
3000
Ion Energy (eV)
Fig. 1. Energy dependence of the total electron yields from a
MgO(1 1 1) surface at normal incidence. Also shown are
representative statistical uncertainties.
sured on the sample under positive and negative
bias, with an uncertainty of 30%.
Fig. 2 shows energy distributions of electrons
emitted by 500 eV Ne+ and Na+ impact. The spectra have been normalized to the same height to
compare line shapes. Fig. 2 show a remarkable
finding of our investigations: at low impact energies, the shape of the energy distributions is nearly
the same for different ions. This result is consistent
with experiments of electron emission from MgO
surfaces by slow ions, recently reported by us
[3,4] and other groups [5,7]. On the other hand,
as discussed in [4], charging of the surface during
ion impact can explain inconsistent results reported in [6].
The very similar electron energy distributions
and yields measured for Na+ and Ne+ projectiles
is a strong argument against the idea that Auger
neutralization (AN) dominates electron emission
from MgO, which has been often invoked in previous works [4,6,12]. The fact, that electron spectra
do not depend on the ionization energy of incoming projectiles, strongly suggests that electron
emission is due to decay of some excited state
intrinsic to the solid (i.e. not involving the electronic properties of the projectiles) populated by
the transfer of kinetic energy of incoming ions.
In previous research [4], we have shown that this
results are consistent with a model in which excita-
Fig. 2. Normalized energy distribution of electrons ejected
from a MgO surface under impact of 500 eV Ne+ and Na+ ions.
The sample was biased at 4.9 eV.
tion occurs by electron promotion, when the valence shell of the projectile and the oxygen anion
interpenetrate during a close atomic collision, forming a transient ‘‘quasi’’ molecule. Collisions with
oxygen anions promote oxygen-2p electrons along
quasi-molecular orbitals (MOs), above a threshold
projectile energy which has been determined to be
50 eV for the analogous case of Na+ exciting oxidized Al [13]. The O-2p electrons can be promoted
by all the projectiles studied, as follows from calculations for these and similar systems [14,15], and
from Barat–Lichten MO correlation rules [16].
The promoted MO are 3dr (He–O), 3dp and 4fr
(Ne–O, Na–O) and 4fr (Ar–O). The promoted
MO cross-empty levels, such as the 3sr, 4sr for
He, Ne and Na projectiles and the 3dd, 4sr for
Ar, correlating with O-3s and O-4s excitons.
We have proposed that excitons can decay into
vacuum and thus contribute to electron emission.
Excitons are normally considered to be bound
states of solids but, in MgO (and in LiF), the negative electron affinity of the surface causes the exciton to be above the vacuum level [17]. The band
gap of MgO (7.8 eV in the bulk) drops at the
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P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 455–459
surface due to the decrease in the Madelung potential, by more than 1 eV, with variations among different crystal faces [18,19]. There is also a decrease
in the exciton energy from a bulk value of 7.7 eV
to a value which, for MgO(1 0 0), is 6.2 eV from
high resolution EELS measurements [17]. The surface exciton exists in a region of changing surface
potential, extending from outside the solid to the
first atomic layers. To establish the position of
the vacuum level, we determined the ionization
energy of our samples (energy difference between
the top of the valence band and the vacuum level)
by measuring the photoelectron threshold [4]. The
value of 5.3 eV is consistent with the minimum
energy loss measured in EELS [17] and with calculations [19]. The negative electron affinity of the
surface (vacuum level below the conduction band
minimum in the bulk) allows excitons to couple
to the continuum of states outside the solid, which
may contribute to the considerable width of surface excitons [17].
To gain further insight into the electron excitation mechanisms, we measured electron spectra for
Ar+ and He+ impact, at varying incident energies.
The spectra are reported in Figs. 3 and 4. We note
that the shape of the distribution do not change
for low impact energies, further reinforcing the
idea of the preferential excitation of a state intrinsic to the solid.
On the other hand, a tail of high-energy electrons is observed to grow at higher projectiles
energy and is enhanced for faster He ions. To explain this observation, we consider that, once promoted, the O-2p electrons can transfer to excited
states at the crossings of the promoted MO with
unfilled MOs that correlate to excitons and conduction band states. The population of the final
levels will decrease with excitation energy DE,
and therefore will favor excitonic to continuum
states. The probability for direct excitation to continuum states falls exponentially with the ratio
DE/v, where v is the relative collision velocity
[20]. We attribute the cause of the high-energy tails
in the spectra to those direct excitations, since
these tails decay near exponentially and are
enhanced for faster projectiles. This interpretation
of the high-energy tail is analogous to that used to
discuss similar experiments on LiF [21,22]. We dif-
Fig. 3. Normalized energy distribution of electrons ejected
from a MgO surface under impact of He+ ions at varying
impact energies.
Fig. 4. Same as Fig. 3 but for Ar+ ions.
fer, however, in the identification of electronic
states responsible for electron emission. For LiF
P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 455–459
it has been proposed that electron promotion leads
to an autoionizing doubly excited atomic state
F ** (2p43s2 embedded in the conduction band
of the solid, that can decay by electron emission
[22]; this state would release upon decay the same
energy as a free F **, leading to the production of
electrons of a few eV. Electron energy loss (EELS)
measurements and theoretical studies of LiF and
MgO do not show evidence of such doubly-excited
states in the solid; one can furthermore expect that
the excitation energy of this putative state in the
solid should be very different from that of a free
ion. A simpler explanation is that the decay of
the excitons into vacuum (or exciton break-up
[23]) produces electrons with an energy distribution peaked at the observed low energies, due to
the small (positive) difference in energy between
the exciton and the vacuum level.
In conclusion, the energy distribution of electrons emitted from MgO surfaces under the impact
of slow singly charged noble gas and sodium ions
shows a remarkable independence of the type of
incident ion and energy. This result shows that electron emission results from the decay of an intrinsic
excitation in the solid. We propose that excitons
produced by electron promotion during binary
projectile-oxygen collisions decay into vacuum
due to the negative electron affinity of the surface.
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