Nuclear Instruments and Methods in Physics Research B 164±165 (2000) 891±896
www.elsevier.nl/locate/nimb
E€ects of under- and overlayers on particle and photon emission
from solid argon induced by 2 MeV protons
D.E. Grosjean a, R.A. Baragiola
a
a,*
, W.L. Brown
b
Laboratory for Atomic and Surface Physics, Engineering Physics, University of Virginia, Thornton Hall, Charlottesville,
VA 22901, USA
b
Bell Labs, Lucent Technologies, Murray Hill, NJ 07974, USA
Abstract
We studied electron emission, luminescence, and sputtering of thin condensed gas ®lms by 2 MeV protons. For thin
Ar ®lms, an external electric ®eld causes nearly complete extraction of excited electrons, hindering hole recombination.
This leads to a reduction in luminescence and sputtering. We have observed that overlayers of N2 annul the e€ects of
external electric ®elds and also that they decrease ion-induced luminescence from the bulk of the Ar ®lms. In related
experiments, 20 nm Krypton layers were grown between the metal substrate and the Ar ®lms. We found that these
underlayers change the dependence of particle and photon emission on Ar ®lm thickness and dramatically increase
electron emission. We discuss these new e€ects in terms of internal electric ®elds produced by unbalanced holes and of
changed reactions of excitons and holes at the interfaces. Ó 2000 Elsevier Science B.V. All rights reserved.
PACS: 79.20.)m; 34.50.Dy; 78.60.Hk; 71.35.)y
Keywords: Electron emission; Luminescence; Sputtering; Rare gas solids; Excitons
1. Introduction
Many of the unsolved fundamental questions in
atomic collisions in solids are in the area of radiation e€ects in insulators induced by electronic
excitations. Searching for answers to those questions is motivated not only by the need for fundamental insight but also because of the potential
implications in other areas such as radiation biol-
*
Corresponding author. Tel.: +1-804-982-2907; fax: +1-804924-1353.
E-mail address: [email protected] (R.A. Baragiola).
ogy, astronomy, semiconductor processing, and
nuclear energy production. A characteristic of the
e€ects of electronic energy deposition in insulators
is their strong dependence on the type of material.
Relevant properties include the chemical composition, the transport of electrons, holes, and excitons through the material and interfaces, and their
trapping at defects and at the surface. In view of
this complexity we have chosen to study rare gas
solids, the simplest insulators, with the aim of
isolating the important basic phenomena. These
materials have desirable properties, including being chemically inert, and having weak internal
electric ®elds in their ground state, so they behave
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 1 1 1 3 - 1
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D.E. Grosjean et al. / Nucl. Instr. and Meth. in Phys. Res. B 164±165 (2000) 891±896
like a dense gas. Electronic excitations in rare gas
solids have been described in several reviews [1±7],
and abundant information exists from gas-phase
experiments [8,9] which can be used in the search
for a microscopic understanding of processes in
the solid.
The basic events in electronic excitations by fast
ions and their subsequent decay are the following.
By multiple collisions, the projectiles produce a
track of predominantly single ionizations and excitations. The 2 MeV protons used in our study
have an electronic stopping cross-section of
6.5 ´ 10ÿ15 eV atom/cm2 , or 6.5 eV per atomic layer
[10]. The energy is mainly deposited into electron±
ion pairs and excited Ar (excitons). For a typical
75 nm Ar ®lm this translates into an average
electronic energy loss of 1.2 keV, which results in
44 ionizations and 21 direct excitations [11].
Electrons and ions can migrate by di€usion and
under the in¯uence of internal electric ®elds from
trapped charges, charges in the ionization track,
image charges induced in the substrate, and by
externally applied electric ®elds. The ions which do
not reach the substrate will self-trap as Ar cluster
ions [5] and later recombine with thermalized
electrons into the excimer Ar2 . Directly formed
excitons can di€use through the lattice and either
be quenched at the substrate, trapped at the surface where they can lead to desorption, or self-trap
into excimers in the bulk. Radiative decay of Ar2
to the repulsive part of the ground state potential
leads to 9.8 eV luminescence and to energetic Ar
atoms that can initiate a collision cascade that may
result in sputtering (desorption).
Our previous experiments demonstrated that
the availability of electrons determines the amount
of luminescence and the sputtering yield, because it
a€ects the ability of the ions to recombine into
excitons and excimers [12]. For low ionization
densities, like those caused by 2 MeV protons, a
thin ®lm can be depleted of free electrons by a
suciently high, externally applied, electric ®eld.
This essentially complete electron extraction is
made possible by two factors. An electrostatic ®eld
inside the ®lm, produced by trapped charges,
prevents electrons from reaching the substrate and
redirects them towards the surface where they can
be extracted by the external electric ®eld if it is
large enough [13]. There the electrons do not ®nd a
surface potential barrier because Ar has a negative
electron anity that allows even thermalized
electrons to escape the solid. The importance of
the negative electron anity is con®rmed by the
observation that solid Kr and Xe, which have
similar properties to Ar but positive surface barriers for electrons, yield much lower electron
emission than Ar [13]. The sensitivity to the surface barrier suggests that the behavior of Ar can be
changed drastically by modifying its surface with
thin deposits of other materials. In this work we
deposited thin layers of nitrogen with the idea of
changing the surface barrier but kept the thickness
low enough so that the attenuation of electrons in
this layer is unimportant [14]. As we describe below, we observed that thin nitrogen overlayers
cause a simultaneous reduction of electron yield
and increase of luminescence, as well as the disappearance of the in¯uence of external electric
®elds.
2. Experiments
The experiments were done in an ultrahighvacuum chamber attached to the 3.75 MV Van de
Graa€ accelerator at Bell Laboratories, Lucent
Technologies. Fig. 1 shows the setup schematical-
Fig. 1. Experimental setup.
D.E. Grosjean et al. / Nucl. Instr. and Meth. in Phys. Res. B 164±165 (2000) 891±896
ly; details have been published [3,11,15]. Condensed-gas ®lms are grown by vapor deposition of
99.9995% pure gas onto an Au substrate cooled to
8 K. The ®lms are grown at 100 nm/min and are
expected to be polycrystalline [16]. Surrounding
the target are heat shields cooled with LN2 ; the
inner one acts also as an electrode (anode) to apply
an electric ®eld to the target surface and collect
electrons emitted from the target. The base pressure around the target is 1 ´ 10ÿ10 Torr. To ensure a clean substrate surface, a fresh gold ®lm is
evaporated onto the substrate before growing a
condensed gas ®lm.
In the experiments, we measured sputtering,
UV luminescence and electron emission during
irradiation by 2 MeV protons. Excimer luminescence (9.8 eV) was detected by a vacuum ultraviolet spectrometer. Sputtering was measured with a
quadrupole mass spectrometer calibrated against
the decrease of ®lm thickness measured with
Rutherford backscattering spectroscopy [17].
Electron emission was determined from target
currents. The yields of electrons, sputtered Ar and
luminescence photons are denoted by the symbols
c, Y, and L, respectively. c and Y are expressed in
number of particles ejected per projectile ion and L
is the number of photons produced in the ®lm [11].
In the experiments where we study the e€ect of
external electric ®elds, the anode voltage Va was
scanned linearly from 0 up to +750 V and back
down while recording luminescence, sputtering
and electron emission simultaneously. During the
measurements, the projectile ¯ux was kept low so
that the total sputtering during a voltage scan was
less than a monolayer.
3. Results and discussion
Fig. 2 shows the e€ect of the anode voltage on
the particle and light emission from a 75 nm thick
Ar ®lms induced by 2 MeV ions. Electron emission
saturates for Va > 200 V and a hysteresis is observed when scanning Va up and down [12,18].
This contrasts with the case of a bare Au substrate,
where there is no hysteresis, and electron emission
saturates at a few volts. The reason for the di€erences is the production of long-lived, positively
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Fig. 2. Electron yield c, luminescence L, and sputtering yield Y
for 2 MeV H‡ on a 75 nm Ar ®lm (*) and on 75 nm Ar ®lms
covered with an overlayer of nitrogen 5 nm thick (open symbols) and 20 nm thick (closed symbols). The yields are given as
a function of anode voltage. Some representative error bars
pertaining to a given sample are shown. In addition there is a
10% sample-to-sample variation, so di€erences in luminescence
at high anode voltages for di€erent N2 overlayer thicknesses are
not signi®cant. For the case of a 20 nm N2 overlayer the
sputtering of Ar atoms is undetectable above the background.
charged traps in the insulator ®lm, which cause a
positive surface potential that hinders electron
collection at moderate Va [13]. Electron extraction
means that holes are left behind that cannot be
neutralized into excimers by recombination and
therefore luminescence drops. As stated above,
luminescence occurs in a transition from Ar2 to the
repulsive part of the Ar2 ground state. This repulsion produces energetic Ar recoils that can
initiate sputtering. Thus, the decrease of luminescence due to the removal of electrons leads to the
observed decrease in sputtering. The fact that
sputtering decreases less than luminescence when
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D.E. Grosjean et al. / Nucl. Instr. and Meth. in Phys. Res. B 164±165 (2000) 891±896
electrons are extracted is evidence of pathways to
sputtering that do not go through the excimer Ar2 .
3.1. Overlayers
To test the role of the surface on particle
emission and electric ®eld e€ects we deposited
layers of other condensed gases on top of 75 nm
Ar ®lms. A 10 nm layer of solid oxygen on top of
the Ar ®lm was found to eliminate the electric ®eld
e€ect on luminescence. Since oxygen is electronegative, and known to trap electrons, we did
subsequent studies with nitrogen overlayers, seeking a simpler situation. Fig. 3 shows the e€ects of
di€erent thicknesses of nitrogen overlayers on
75 nm Ar ®lms. Even for the thinnest nitrogen
coverage, 2 nm, the sputtering yield of Ar atoms is
less than 15% of that for an uncoated argon ®lm,
and is not shown in the ®gures. An external electric
®eld no longer a€ects the luminescence, but a
10 nm nitrogen overlayer reduces it to about 40%
of its N2 -free level. As nitrogen coverage increases,
the electron yield approaches the value c ˆ 1:5 of
pure N2 determined for a 200 nm ®lm, as shown in
Fig. 4. This value of c is much smaller than c 70
for an Ar ®lm of similar thickness [15]. Hysteresis
is also present, signaling the formation of deeply
trapped charges.
Fig. 4. Electron yield for 2 MeV H‡ incident on a 200 nm
nitrogen ®lm deposited on Au, as a function of anode voltage.
The observations shown in Fig. 3 indicate that a
N2 overlayer prevents the extraction of electrons
from an Ar ®lm and also indicate that the Ar/N2
interface quenches Ar excitons and/or holes, which
are precursors to luminescence. On the other hand,
we found that placing a 10 nm nitrogen layer under
a 75 nm argon ®lm, increases the Ar2 luminescence
to 1.2 times the value for the same Ar ®lm without
this underlayer. The two experiments indicate that,
although the N2 interface quenches holes or excitons, it does so less e€ectively than Au.
3.2. Kr underlayers
Fig. 3. Dependence of Ar excimer luminescence and electron
emission from 75 nm Ar ®lms on the thickness of nitrogen
overlayers. The projectiles are 2 MeV H‡ . Luminescence is
measured without applying an external electric ®eld.
To test the role of the substrate in hole neutralization and the dependencies of the yields on
the electric ®eld, we used a 20 nm Kr underlayer
between the Ar and the Au substrate. The lower
panel of Fig. 5 shows the Ar-thickness dependence
of particle and photon emission for ®lms grown on
Kr underlayers, and compares it with that of Ar
®lms grown directly on Au. In general, all the
yields are seen to increase with ®lm thickness but
luminescence is suppressed for very thin ®lms. The
reason for this behavior is the radiationless conversion of holes and excitons into ground state Ar
at the Au surface [3]. Holes can reach the substrate
by di€usion and at short distances to the substrate,
D.E. Grosjean et al. / Nucl. Instr. and Meth. in Phys. Res. B 164±165 (2000) 891±896
Fig. 5. Dependence of the yields cS , L0 and Y 0 and the fractions
cS =m on Ar ®lm thickness, for 2 MeV H‡ . The superscripts S, 0
indicate measurements taken at saturation and zero anode
voltage, respectively. The lines without symbols are for Ar ®lms
grown on a bare substrate and the lines linking symbols correspond to Ar ®lms grown on a 20 nm Kr layer on top of Au.
(- - - -) Electron emission, (Ð) luminescence, ( ) sputtering.
their motion to the Au surface is enhanced by drift
due to the strong image electric ®eld [19]. If the
holes reach the interface before the ionized electrons can cool down suciently to allow recombination, they can neutralize by an Auger process
[3,20,21]. Excitons can also reach the interface by
di€usion, where they can be quenched by resonance ionization followed by Auger neutralization,
or by Auger deexcitation [20]. The increase of luminescence and sputtering when Ar is grown on a
Kr overlayer can be attributed to the less ecient
quenching of Ar excitons and holes at the Ar/Kr
interface, similar to what we observed for the Ar±
N2 interface. The reason is that Auger quenching
processes are blocked in wide band-gap materials
like Kr and N2 .
In addition to the luminescence quenching, it is
clear in the lower panel of Fig. 5 that sputtering,
895
luminescence, and electron emission were all
higher for the Ar/Kr/Au case than for Ar/Au. We
propose that this di€erence is also related to the
di€erence in neutralization/deexcitation processes
at the Ar/underlayer interface. In the Ar/Au case
for no applied anode voltage, there is a ``dark
zone'' extending about 35 nm from the interface in
which holes tend to be neutralized at the substrate
due to the image force attraction. By adding the
intervening underlayer of Kr, this region is no
longer such a sink for Ar holes. The most dramatic
di€erence this creates is the enhancement of the
thin-®lm sputtering yield. For very thin ®lms (<50
nm), the dark zone is a large portion of the entire
®lm, removing most of the holes and excitons that
can cause sputtering. The intervening Kr layer
removes part of that dark zone from the Ar, and
provides a slight re¯ecting surface for di€using
excitons. Note that as the Ar ®lm thickness increases, it appears that the sputtering yield may
approach the thick-®lm yield for Ar/Au, which
would be expected for this surface process. A
similar argument is used for the enhanced luminescence yield for thin ®lms; in that case, also, the
yields look as if they may be similar for thicker
®lms. The electron yield for the Kr underlayer is
enhanced by the additional electrons created in the
Kr layer by the projectile and by the Ar excitons,
described below.
In the top panel of Fig. 5 we show ratios cS =m as
a function of thickness, for 2 MeV H‡ bombardment. Here the superscripts 0 and S indicate that
measurements were made with zero or saturation
anode voltage, respectively. The ratio cS =m gives
the fraction of the total number of electron±hole
pairs produced in the ®lm, m, that can be extracted.
To obtain m, we divide the energy loss in the ®lm,
calculated with known electronic stopping crosssection [10], by W ˆ 27 eV, the mean energy spent
per pair in Ar [1]. For Ar on Au, cS =m 1 and
consistent with previous results for thin ®lms under condition of weak ionization density [12,13].
As stated above, ecient electron extraction results from the negative electron anity of Ar and
the internal electric ®eld produced by trapped
charges, which pushes electrons to the surface and
away from the substrate. The Kr underlayer causes
a substantial increase of cS =m, which is larger by a
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D.E. Grosjean et al. / Nucl. Instr. and Meth. in Phys. Res. B 164±165 (2000) 891±896
factor of 1.4 than in the case of Ar on Au, even
when the electron±hole pairs created in the Kr
layer are included in m. A likely mechanism for the
enhanced electron yield is the ionization of the Kr
layer by Ar excitons di€using to the interface. The
process is near resonant since the Ar excitation
energy (12.1 eV) is very close to the ionization
energy (band gap) of Kr, 11.9 eV [1]. An e€ect of
this type has been observed in the case of Kr impurities in an Ar matrix, which are eciently
ionized by di€using Ar excitons [22]. The values
of LS /L0 and YS /Y0 and their thickness dependencies are similar to those for the clean Au substrate, and consistent with the variation of electron
extraction.
4. Conclusions
The use of N2 overlayers and Kr underlayers
have allowed us to test models of the dynamics of
holes and electrons in thin Ar ®lms produced in
dilute ionizations. Overlayers as thin as 5 nm of
oxygen or nitrogen greatly reduced the electron
yield, and prevented the applied ®eld from reducing luminescence. This was taken as con®rmation
that electron removal is needed to produce the
excess charges that set up an internal electric ®eld
responsible for complete charge separation. Electron emission from Ar is dramatically increased by
Kr underlayers because Ar excitons are converted
into electron±hole pairs at the Ar/Kr interface.
Acknowledgements
This work was supported by NSF, Division of
Materials Research.
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