Nuclear Instruments and Methods in Physics Research B 157 (1999) 116±120
www.elsevier.nl/locate/nimb
Electrostatic charging e€ects in fast H‡ interactions with thin Ar
®lms
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 have studied electron emission, luminescence and sputtering from thin Ar ®lms excited by 2 MeV protons. We
varied the voltage, Va , of the anode surrounding the target, which extracts electrons from the ®lm and results in unbalanced positive charges. The resulting large internal electric ®elds alter sputtering and luminescence. At the beginning
of irradiation of a freshly deposited ®lm, we observe that a positive anode voltage of a few hundred volts produces a
large, catastrophic increase in sputtering which gradually disappears as irradiation progresses. We discuss the results in
terms of dielectric breakdown induced by macroscopic charging associated with deep charged defect sites. Ó 1999
Elsevier Science B.V. All rights reserved.
PACS: 79.20.Rf; 34.50.Dy; 77.22.Jp
1. Introduction
Charging of insulators is a common problem in
ion interactions with insulators. Charging occurs
due to the inability to drain away excess positive
charges that originate from implanted ions and
emitted secondary electrons. Some outstanding
questions of this phenomenon are: what conditions determine the degree of charging, what are
the charge-trapping sites and how are they distributed in the insulator. Rare-gas solids are good
model systems to study charging due to ion irra-
*
Corresponding author. Tel.: +1-804-982-2907; fax: +1-804924-1353; e-mail: [email protected]
diation because they are relatively simple, being
made up of a single element and they can often be
treated as a dense gas, whose properties under irradiation are well known.
Trapped positive charge results in an electrostatic potential in the solid that has in¯uence on
other phenomena. The electric ®eld may separate
electrons and holes generated by ionizing collisions, hindering their recombination, and thus
lowering the luminescence and sputtering they
produce [1,2]. The electrostatic potential has two
origins. A microscopic and transient potential is
formed along the ionization track produced by the
ion, which decays in a time of the order of 0.5 ns
in Ar [3], due to recombination. For insulating
®lms on a metal substrate, there is an additional,
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 4 2 9 - 2
D.E. Grosjean et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 116±120
position-dependent, decay time due to the migration of unbalanced positive charge to the substrate,
where it is neutralized by electron transfer. Unbalanced charges result from the initial loss of
energetic electrons into vacuum (electron emission)
and into the substrate. The remaining electrons,
degrading in energy due to collisions, distribute
around the positive charges partially screening
them. The unbalanced charges, together with their
images, set up strong electric ®elds in the ®lms. For
solid Ar, the mobility of a hole is so large (1000
cm2 sÿ1 Vÿ1 ), that a hole located 30 nm from the
substrate, which experiences an initial image ®eld
of 3 kV/cm, will reach the substrate before it can
self-trap into an excimer Ar‡
2 (1±10 ps) [4]. After
self-trapping, the hole-mobility drops by nearly
®ve orders of magnitude but, for the ®lm thickness
typically used (50±200 nm), the time to reach the
substrate and quench (in the absence of recombination) is still shorter than 20 ls, for electric ®elds
>100 V/cm. In contrast to these short times, we
observe e€ects due to large macroscopic potentials
with relaxation times of seconds [1,5,6]. These long
times then indicate that there exist in the ®lm less
mobile, or ``trapped'', charges that accumulate
with irradiation time. For the case of 33 keV H‡
incident on Ar ®lms, we found that a charge density builds up to a maximum value: r 1012 q/cm2
for the surface charge, and q 1017 q/cm3 for the
bulk charge [6], where q is the elementary positive
charge. The persistence of these charged traps
means that they are tightly bound, possibly at
voids or other defect sites, and at the surface.
Here we describe experimental manifestations
of electrostatic charging of thin ®lms of solid argon, during irradiation with 2 MeV protons. They
include modi®cations of electron emission, luminescence, and sputtering, and irradiation dose effects. We observe evidence of dielectric breakdown
for ®lms much thinner than previously reported
[5,6]. The experimental methods have been described in detail before [1,2,4]. They involve the
growth of pure Ar ®lms by vapor deposition in
ultrahigh vacuum onto a clean Au substrate
cooled to 8 K and irradiation with 2 MeV ions.
The geometrical arrangement is that of a vacuum
diode; the ®lm is deposited on a grounded substrate and is surrounded by a cylindrical anode
117
that is biased with a variable positive voltage, Va .
Typically we record, as a function of anode voltage, the current to the substrate, bulk luminescence from the ®lm (excimer Ar2 ® 2Ar transition
at 9.8 eV) with an ultraviolet spectrometer and the
increase in Ar partial pressure in the target
chamber caused by sputtering, with a mass spectrometer.
2. Results and discussion
Fig. 1 shows the variation of electron emission,
luminescence and sputtering as a function of Va
Fig. 1. Variation of sputtering, photon and electron emission
yields with anode voltage. Notice the sharp increase of sputtering near 400 V (``catastrophic'' sputtering) and the fading of
the breakdown behavior with irradiation dose (``conditioning'').
Labels next to the curves indicate doses in units of 1014 ions/
cm2 . The voltage was scanned at 25 V/s, in the direction
shown by the arrows.
118
D.E. Grosjean et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 116±120
for a 75 nm ®lm irradiated by 2 MeV protons. The
saturation of electron emission requires Va larger
than 400 V, rather than the few volts needed in the
case of the bare metal substrate. The reason for the
need of high Va is that a positive potential develops at the beam spot (5 mm dia.) very close to the
much larger grounded substrate. This creates, in
front of the beam spot on the surface, a potential
barrier for electrons that only disappears at large
Va [6]. Electrons that cannot overcome the barrier
are returned to the sample. By analogy to our
studies on a similar apparatus [6], the voltage at
which the I±Va curves saturate indicate surface
potentials of 60 V. In contrast, experiments with
55 keV proton irradiation of 60 nm Ar ®lms
showed saturation at Va 20 V and beam spot
potential less than 1 V [6].
The seemingly contradictory results may result
from a number of di€erences between the experiments. Low energy protons deposit their charge
close to the surface, where they capture an electron. They then travel through the solid mostly as
neutral H. This is another scource of excess positive charge that that may be trapped at defect sites
at the outer regions of the ®lm. However, we believe that the most likely reason for the di€erence
between the experiments is the dissimilar ®lm
growth temperatures. In the experiments reported
here, we used 8 K, and therefore expect our ®lms
to contain a signi®cantly higher density of defect
sites, including pores, compared with those of the
keV experiments, where the ®lms were grown at 20
K.
Large electric ®elds may induce dielectric
breakdown in the ®lm. Evidence for breakdown is
the sharp increase in the Ar partial pressure in the
chamber when large anode potentials are applied.
Fast ¯uctuations or spikes accompany the pressure
increase. A possible reason for this increased
sputtering, which we term `catastrophic' is the heat
spike generated by dielectric breakdown, which
may also account for the enhanced sputtering of
solid D2 with high density of trapped charges [7].
For thicker ®lms, breakdown e€ects are stronger
and the increase in the sputtering yield can be
more than an order of magnitude. We notice that
the breakdown behavior is not very reproducible.
This observation strengthens the view that defects
are important in charging and breakdown, since
the quantity and type of defects are likely quite
variable for di€erent thin ®lm depositions.
Remarkably, the breakdown behavior fades
upon prolonged irradiation and eventually disappears (Fig. 1). We call this phenomenon `conditioningÕ. The irradiation doses required for
conditioning increase with ®lm thickness and decrease for high dE/dx projectiles, such as 2 MeV
Ne‡ . The ¯uctuations, or spikes, that accompany
the pressure bursts decrease with irradiation dose.
We note that the doses required for conditioning
only remove of the order of monolayers, thus ®lm
thinning by sputtering is not important. The disappearance of the breakdown behavior suggests a
decrease in the density of trapped charges. Therefore, we propose that conditioning is to due to
radiation-induced annealing of regions of the ®lms
with high concentration of defects.
The hysteresis seen in Fig. 1, and previously
reported is related to the time required for the
buildup of charged traps, a few seconds at our
current densities. The characteristic time in the
hysteresis curve is inversely related to the beam
current density [1]. As positive charges build up,
they increase the electric ®eld they generate which
in turn moves them faster to the substrate, a selflimiting process. The electric ®eld also redirects
electrons that might otherwise reach the substrate
to the surface, where they can be ejected even if
they have thermal energies, since Ar has a negative
electron anity. The internal electric ®eld also
forces the positive holes to the substrate, where
they are neutralized, thereby decreasing the potential of each ionization track. Thus, charging has
two opposite e€ects: it enhances electron emission
by moving electrons inside the ®lm towards the
surface and decreasing the positive potential, and
it decreases collection of the emitted electrons by
building up a potential barrier in front of the beam
spot, for insuciently large Va . Hysteresis is also
seen in luminescence and sputtering because these
phenomena depend partially on the availability of
electrons. When electrons are extracted, they cannot participate in recombination with holes and
thus decrease that part of luminescence and sputtering which is started with a recombination into
the Ar2 excimer [1].
D.E. Grosjean et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 116±120
119
Fig. 2 shows transient breakdown phenomena
induced by switching Va for a partially conditioned 77 nm Ar ®lm. It can be seen that breakdown does not occur instantly after the anode is
switched positive, but after several seconds, consistent with the idea that it takes time to build up
the charges and hence the internal electric ®eld. We
note that when the anode voltage is switched to
zero, there is a small tail in the electron emission
current that decays slowly. In a previous study
with low energy ions, we found that electron
emission current under breakdown condition will
remain a few seconds after the ion beam is interrupted [5]. We attributed this e€ect to ®eld emission from the substrate into (and through) the ®lm
that persists until a sucient fraction of the excess
charge in the ®lm is neutralized by part of the ®eld
emission current.
Fig. 3 shows similar data for a 75 nm Ar ®lm
that has been fully conditioned. Charging phenomena are still noticed in the transient in luminescence and, on a smaller scale, in sputtering.
However, this charging does not result in breakdown phenomena. We notice that, unlike the case
of Fig. 2, when Va is switched to zero (in 0.1 s),
the electron current returns abruptly to zero and
electrons are able to neutralize the trapped charges
very fast, as judged by the fast increase in luminescence. A 10% spike is seen in the sputtering
yield but not in bulk luminescence and may be
related to surface recombination processes. When
Va is then switched from 0 to +500 V (in 0.1 s),
the electron current saturates immediately indicating that the ®elds needed to saturate electron
redirection inside the ®lm are achieved rapidly.
Luminescence and sputtering decrease rapidly,
Fig. 2. Transients induced by switching the anode voltage, for 2
MeV H‡ on a 77 nm Ar ®lm that shows breakdown behavior.
Vertical scales for the sputtering yield (S), luminescence (L) and
electron emission yield (c), are in arbitrary units.
Fig. 3. Same as Fig. 2, but for a ``conditioned'' 75 nm Ar ®lm.
The arrows direct to transient behavior or its absence, when
switching anode voltage.
120
D.E. Grosjean et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 116±120
since removal of electrons in the ®lm means less
recombination into Ar2 , a precursor state for these
emissions. As electrons leave, deep charged traps
accumulate slowly, creating a macroscopic electric
®eld inside the ®lm. The increase of both luminescence and sputtering with time in each cycle,
with a steady electron emission current, suggests
that electrons are replenished in the ®lm, most
likely by ®eld emission from the substrate.
In summary, we have observed new breakdown
phenomena and transient e€ects in the irradiation
of thin Ar ®lms with 2 MeV ions, which are
manifestations of electrostatic charging. The high
voltages needed for saturation of electron currents
and the hysteresis in the I±Va curves are indicative
of macroscopic charging, which we attribute to the
population of deep traps at defect sites. In some
cases, especially for thick ®lms, charging can be so
severe that dielectric breakdown ensues. This is
accompanied by a large increase of the sputtering
yield by more than an order of magnitude. The
defect sites may be annealed by prolonged irradiation under breakdown conditions, which cause a
gradual disappearance of the breakdown behavior.
Further accounts of our studies will be presented
in a future publication.
Acknowledgements
We acknowledge support from NSF, Division
of Materials Research.
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