Highly Charged Ion Bombardment of Silicon Surfaces
Jason E. Sanabia, Scott N. Goldie, Laura P. Ratliff, Lori S. Goldner, and John D.
Gillaspy
Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899
Abstract. Visible photoluminescence from Si(100) surfaces irradiated by highly charged ions has recently
been reported [1]. In an attempt to reproduce these results, highly charged ion-irradiated silicon samples
were prepared at the Electron Beam Ion Trap at the National Institute of Standards and Technology. Two
highly sensitive fluorescence detection schemes were employed, both using ultraviolet light from an argonion laser for excitation. In the first detection scheme, the Xe44+-Si(100) samples were excited by the
ultraviolet light while a spectrograph equipped with a liquid nitrogen-cooled charge-coupled device camera
detected the fluorescence. The second detection scheme was a high throughput laser-scanning confocal
microscope equipped with a photon-counting photomultiplier tube. We characterized the sensitivities in
each detection scheme, allowing the assessment of the photoluminescence efficiency of Xe44+-Si(100). No
photoluminescence was detected in either setup.
describe visible light emission from porous silicon is
the quantum confinement model [7,8]. In this model,
confinement of electron and hole wave functions
inside the silicon nanocrystals widens the silicon
bandgap into the visible part of the light spectrum and
makes a direct bandgap. Ion accelerators can also treat
silicon to give it favorable optoelectronic properties by
implanting impurities or introducing defects in silicon
to make visible and infrared light emitting materials
[9,10].
INTRODUCTION
Highly charged ions (HCIs) are being explored for
possible applications in the fabrication of
nanostructures. Due to a rapid reneutralization that
begins as the HCI approaches the surface, the ion’s
potential energy (50keV for Xe44+) is released within
10 nm of the surface [2]. A single HCI on an
atomically flat mica surface results in nanometer-sized
damage to the surface [3]. The damage volume scales
with HCI potential energy (charge state) from 10 nm3
for Xe30+ to 150 nm3 for Xe50+. On the other hand,
HCI kinetic energy plays a small role in the damage
scale: two orders of magnitude variation in HCI
kinetic energy results in very little change in the size
of the damage site [4].
Hamza, Newman, Thielen, Lee, Schenkel,
McDonald, and Schneider reported that areas of
silicon exposed to highly charged ions (Xe44+) emitted
visible light when exposed to ultraviolet light [1]. The
emission contained a number of sharp peaks between
540 nm and 570 nm. They hypothesize that highly
charged ions form nanometer sized impact sites on the
silicon surface and that quantum confinement occurs.
Hence, highly charged ions have directly fabricated
light emitting nanostructures on a silicon surface.
Bulk silicon has an indirect bandgap in the
infrared, and is therefore a poor emitter of visible and
infrared light. Visible light emitting devices must
therefore employ exotic semiconductors, like GaN,
that can be expensive and toxic. Pure silicon can be
treated to give it favorable optoelectronic properties.
For example, photoluminescent porous silicon can be
fabricated by wet chemical etching [5] or spark
processing of silicon [6]. One of the models used to
Here we seek to reproduce and expand upon the
work of Hamza, et al. We describe experiments
designed to measure the quantum yield and spectra
from individual ion impact sites. The narrowness of
the emission spectrum observed in Ref. [1] suggests
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
2003 American Institute of Physics 0-7354-0149-7/03/$20.00
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that photoluminescent sites have the same emission
spectrum, however, it may be the case that the
emission spectrum is the sum of spectrally-distinct
impact sites. One way to determine this is by
measuring the photoluminescence from individual
impact sites [11].
Pa. The surface is then allowed to reach room
temperature.
The prepared Si(100) surface is exposed to a
continuous beam of Xe44+ ions from the EBIT [13].
The target chamber base pressure is 3×10-7 Pa. A
beam of isotopically pure 136Xe44+ ions is extracted
from the EBIT. Before exposure, a Faraday cup
measures the ion beam current (typically 10 pA)
through a 3 mm aperture. The Si(100) is translated into
the ion beam path between the aperture and the
Faraday cup.
The kinetic energy is 350 keV.
Exposures of several hours with a stable ion beam flux
are possible. The two samples reported here have total
doses of 1.3×1011 ions⋅cm-2 and 2.6×1011 ions⋅cm-2.
We have constructed two highly sensitive
fluorescence detection schemes to investigate silicon
samples exposed to highly charged ions at the Electron
Beam Ion Trap (EBIT) at the National Institute of
Standards and Technology. In the first scheme, a
fluorescence spectrometer samples a relatively large
area of the sample. The second scheme is a confocal
microscope capable of single fluorophore detection.
The confocal microscope samples a relatively small
area of the sample and was developed to measure
fluorescence from individual ion impact sites. We
bombarded Si(100) surfaces with Xe44+ using the same
procedures as Hamza, et al. Unfortunately, we have
not detected fluorescence from our samples with either
optical setup, but our procedure places a limit on the
absorption cross section of an individual ion impact
site in Xe44+-Si(100).
After exposure, the Xe44+-Si(100) samples are
removed from the EBIT target chamber and exposed
to atmosphere for approximately 1 hour during transfer
to the vacuum chambers in which the fluorescence
measurements are performed.
Fluorescence Spectrometer
The fluorescence spectrometer is schematically
depicted in Figure 1. The Xe44+-Si(100) sample is held
at 1 Pa in a vacuum chamber behind a fused quartz
viewport at room temperature. The vacuum chamber
is mounted on a platform that translates in the x, y, and
z directions and rotates about the z-axis parallel to the
sample plane.
An argon ion laser supplies a
continuous 364 nm (UV) excitation beam. This laser
line is chosen because it is close to the wavelength of
the pulsed 379 nm excitation that Hamza, et al. used.
The UV excitation is focused to an ellipse on the
surface of the sample with a minor axis diameter of 20
µm (diffraction limited). The UV excitation was
polarized linearly in the plane of incidence and was
incident at 56o (Brewster’s angle) to minimize ghost
reflections of the excitation beam on the vacuum
viewport.
EXPERIMENTAL
Xe44+-Si(100) Sample Preparation
A 13 mm × 3 mm × 1 mm Si(100) sample is cut
from a float zone Si(100) wafer that was sent to us by
Hamza, et al.
The sample is mounted in a
molybdenum fixture and inserted into a heating
assembly in an ultrahigh vacuum system. The base
pressure is 3×10-8 Pa. The sample, fixture, and heating
assembly are outgassed overnight by passing current
through a tungsten filament heater such that the
sample temperature is held at 870 K. The sample
temperature is measured with an optical pyrometer.
The silicon annealing procedure that we use has been
shown to drive off adsorbates and the native oxide
from the silicon surface and leave a clean, wellordered Si(100)-(2×1) reconstructed surface [12]. The
sample is heated by passing a current directly through
the sample. In the annealing procedure, the sample is
heated rapidly to 1520 K until either the pressure rises
above 3×10-7 Pa or 60 seconds has elapsed. Next, the
sample is rapidly cooled to 1170 K and held there for
60 seconds. Finally, the sample is cooled to below
870 K at a rate of 2 K⋅s-1. This annealing cycle is
repeated until the sample can remain at 1520 K for 60
seconds at a vacuum chamber pressure less than 3×10-7
The fluorescence from the sample is collected by
an off-axis parabolic mirror with a 300 mm focal
length and an estimated numerical aperture of 0.25.
The solid angle of the parabolic mirror collects 1.7 %
of the fluorescence from the sample, assuming
isotropic emission. The fluorescence then passes
through an uncoated pellicle beam splitter and reflects
from two metallic steering mirrors. A 300 mm lens
focuses the fluorescence onto the entrance slit of the
liquid nitrogen cooled charge coupled detector
(LNCCD) spectrograph. A 400 nm long pass filter is
placed in front of the entrance slit of the LNCCD
spectrograph to filter out the scattered UV excitation.
569
The beamsplitting mirror above the objective lens
transmits 1 % of the UV reflected from the sample.
SAMPLE IN VACUUM
LIQUID NITROGEN-COOLED
CCD SPECTROGRAPH
PELLICLE BS
MIRROR
The solid angle of the objective collects 10 % of
the fluorescence from the sample, assuming isotropic
emission. The fluorescence then passes back through
the dichroic mirror, which blocks most of the UV
beam and transmits the longer wavelength
fluorescence. A holographic notch filter blocks the
remaining UV and transmits the fluorescent light
(80 % at 500 nm). A lens focuses the fluorescence
through a 150 µm pinhole to filter out background that
does not originate from the focal spot on the sample.
Behind the pinhole, a photon-counting photomultiplier
tube (34 counts·s-1 dark noise) detects the
fluorescence. This photomultiplier has a quantum
efficiency of 25 % at 400 nm and 5 % at 550 nm. We
estimate that the confocal microscope has a detection
efficiency of 0.1 % at 550 nm. This estimate is based
on estimated transmission/reflection losses for each of
the mirrors, lenses, the dichroic, and the pinhole
between the sample and the detector.
400 nm LONG
PASS FILTER
633 nm
ALIGNMENT
LENS
MIRROR
LENS
PARABOLIC
LENS
364 nm
EXCITATION
FIGURE 1. Schematic of the fluorescence spectrometer.
The LNCCD spectrograph has a 0.3 m focal length.
A 300 groves·mm-1, 500 nm blazed grating with an
average efficiency of 78 % at 550 nm disperses the
light onto the LNCCD detector. The LNCCD detector
is a 1340 × 100 pixel array with 90 % quantum
efficiency at 550 nm and greater than 60 % quantum
efficiency between 400 nm and 800 nm. The LNCCD
has very low thermal noise allowing spectra to be
recorded for at least 1000 seconds with no introduction
of thermal noise (above readout noise) to the signal.
We estimate that the parabolic setup has an overall
detection efficiency of 0.4 % at 550 nm. The detection
efficiency is defined as the fraction of the fluorescence
emitted from the sample that is detected by the
LNCCD. This estimate is based on assumed losses for
each transmission/reflection from optics between the
sample and the detector. The three mirrors inside the
spectrograph and have been included in this loss
estimate.
CCD VIDEO CAMERA
364 nm EXCITATION
EMISSION
SCANNING
MIRRORS
AXIS
LENS
BEAMSPLITTING
MIRROR
DICHROIC
BEAMSPLITTER
HOLOGRAPHIC
NOTCH FILTER
Confocal Microscope
SAMPLE HOLDER
LENS
The laser-scanning confocal microscope, similar in
design to one described elsewhere [14], is
schematically depicted in Figure 2. This microscope
can acquire the fluorescence image of a sample and is
sensitive to a single fluorescent molecule (see below).
The Xe44+-Si(100) sample is held at 1 Pa behind a 0.5
mm fused quartz window in a vacuum chamber at
room temperature. The vacuum chamber is mounted
on a platform that translates in the x, y, and z
directions. An argon ion laser supplies the 364 nm
(UV) excitation beam, which reflects from a dichroic
mirror and passes two rotating mirrors. A 0.6 NA
objective lens with high transmission in the UV
focuses the 364 nm light through the quartz window to
a spot with a diameter of 730 nm (diffraction limited).
The lens is corrected for the 0.5 mm window
thickness. The spot can be scanned across the surface
plane by rotating the scanning mirrors. For focusing, a
video camera and lens mounted above the objective
lens gives an image of the UV spot on the surface.
OBJECTIVE LENS
LENSES
LENSES
UV LASER
PINHOLE
PHOTOMULTIPLIER
FIGURE 2. Schematic of the confocal microscope.
RESULTS
Using the fluorescence spectrometer, we did not
detect any fluorescence for 10 W⋅cm-2 (60 µW into an
area of 5.6×10-6 cm2) of 364 nm excitation on the ionexposed region of the Xe44+-Si(100) sample. The
Xe44+-Si(100) sample had a dose of 1.3×1011 ions⋅cm-2.
The LNCCD exposure time was 100 s. There was no
background in the spectrum, only readout noise
averaging 1.4 counts.
We can impose an upper limit on the product of the
absorption cross section at 364 nm, σ, and the
fluorescence quantum yield, φ, of a single ion impact
site in Xe44+-Si(100). In general, the signal-to-noise
ratio, s/∆sn, is given by
570
s ∆s n =
nDφσPτ ( Ahν )
nDφσPτ ( Ahν ) + CPτ + N d τ + s r2
As in the case of the fluorescence spectrometer
measurement, we can impose an upper limit on φσ
using Equation 1. The term sr2 = 0, since we are not
using the LNCCD detector. The term CPτ + Ndτ =
220 counts·s-1 × 40 s = 8800 counts. The number of
ion impact sites sampled in the A = 4.2×10-9 cm2 is n =
1100 ions. The detection efficiency is D = 0.001.
This information gives the upper limit φσ = 9×10-22
cm2. The quantum yield is less than or equal to 1.
Therefore, the upper limit for the absorption cross
section is of order 10-21 cm2. Again, we conclude that
the product of the absorption cross section at 364 nm
and the fluorescence quantum yield for an individual
impact site on Xe44+-Si(100) is extremely low. The
value is 4 orders of magnitude higher for the confocal
microscope for three reasons: First, the fluorescence
spectrometer samples 730000 ion impact sites,
whereas the confocal microscope samples 1100.
Second, the photomultiplier is much noisier than the
LNCCD.
Finally, the optics in the confocal
microscope autofluoresce and contribute to the
detected background.
(1)
The numerator is the signal, s, in counts. n is the
number of fluorophores in the area A. D is the
detection efficiency. P is the incident beam power. τ
is the collection time. A is the incident beam area on
the sample. hν is the incident photon energy. The
denominator is the total noise uncertainty, ∆sn, which
is a propagation of the errors from the following noise
sources: the statistical noise in the signal, s ; the
statistical noise in the background, CPτ , where C is
the background count rate per incident power; the
statistical noise in the dark counts, N d τ , where Nd is
the dark count rate; and the readout noise of the
LNCCD camera, sr.
Setting s/∆sn = 1, we can solve for φσ. The terms
CPτ (background) and Ndτ (dark counts) are both
effectively zero for the LNCCD. The number of ion
impact sites in the sampled area, A = 5.6×10-6 cm2, is
7.3×105 sites. Assuming each impact site results in a
fluorescent center, n = 7.3×105. The detection
efficiency is D = 0.003. This information gives the
upper limit φσ = 5×10-25 cm2. The quantum yield is
less than or equal to 1, therefore, the upper limit for
the absorption cross section is σ = 10-25 cm2. For
comparison, the cross section for an organic
fluorescent molecule is of order 10-16 cm2. We
conclude that the product absorption cross section at
364 nm and the fluorescence quantum yield for a
single ion-impact site on Xe44+-Si(100) is extremely
low.
The confocal microscope can detect single
fluorescent molecules. To demonstrate this, we
prepared a sample of Stilbene 3 fluorophores dispersed
onto a clean quartz coverslip. We used spin-casting so
that each molecule was separated by distances greater
than the UV spot size. In this procedure, we dropped a
few 10 µL drops of a dilute solution of Stilbene 3 in
methanol onto a quartz coverslip spinning at 3000
revolutions per minute. During imaging, we used a
1.3 NA oil immersion objective lens to achieve a
tighter focus and to collect more emitted light. In
fluorescence images showing single Stilbene 3
molecules, the signal for each molecule was
approximately 5000 counts·s-1 over a background of
300 counts·s-1. The excitation intensity was 7700
W⋅cm-2 (7.0 µW into an area of 9.1×10-10 cm2) The
diameter of each molecule in the image was the
diameter of the UV spot, roughly 340 nm.
To further investigate these samples, confocal
microscope images were taken of the Xe44+-Si(100)
sample with a slightly higher dose of 2.6×1011
ions⋅cm-2. The sample was exposed to atmosphere for
up to 24 hours before the measurement. We did not
detect any fluorescence for 1200 W⋅cm-2 (5.1 µW into
an area of 4.2×10-9 cm2) of 364 nm excitation on the
ion-exposed region of the Xe44+-Si(100) sample. We
collected 100 µm × 100 µm fluorescence images over
a 13 mm length of the sample one image, translating
the Xe44+-Si(100) sample 1 mm between images. Each
image was collected in 40 seconds and showed no
features. Summing over the pixels in each image and
dividing by 40 seconds, the background in each image
was 220 counts·s-1; 190 counts·s-1 is due to
autofluorescence of the microscope optics and 30
counts·s-1 is due to dark noise in the photomultiplier.
Stilbene 3 has an absorption cross-section of
1.5×10-16 cm2 at 364 nm, an emission maximum at 430
nm, and a quantum yield of 15 % [15]. Multiplying
the excitation intensity by the absorption cross section
and converting to counts·s-1 using the energy of a 364
nm photon, we find that a single Stilbene 3 is excited
at a rate of 2.3×106 counts·s-1. A single Stilbene 3
molecule is therefore emitting photons at a rate of
3.5×105 counts·s-1. We estimate that 1.5 % of this
fluorescence is detected, given that 24 % of the
emission is collected by the oil immersion objective
(assuming isotropic emission) and that the
photomultiplier has a quantum efficiency of 25 % at
571
400 nm. Hence, we estimate a 5300 counts·s-1 signal
from a single Stilbene 3 molecule, in good agreement
with the measured 5000 counts·s-1.
silicon atoms with a metal atom at the center can have
a HOMO-LUMO transition at 520 nm [19].
ACKNOWLEDGEMENTS
DISCUSSION
We would like to thank Alex Hamza and Dieter
Schneider for openly discussing their results and for
providing the silicon wafer. We also acknowledge
Garnett Bryant and Kimberly Briggman for their
insightful discussions. JES and SNG are grateful for
the support of NRC/NIST Postdoctoral Research
Associateship.
We did not detect photoluminescence from our
Xe44+-Si(100) samples using two highly sensitive
detection schemes. Our measurements place an upper
limit of 10-25 cm2 for the 364 nm absorption crosssection of a single Xe44+ impact site on Si(100). On
the other hand, Hamza, et al. detected appreciable
fluorescence signal from their Xe44+-Si(100). We
followed as closely as possible the reported sample
preparation techniques and used similar ion beam
exposures. We are confident in the exposure of our
samples, based on our experience with scanning probe
microscopy imaging of mica and graphite surfaces,
where the distinct features at the ion impact sites make
the ion irradiated regions of the surfaces easily
recognizable [3,16]. We can only speculate that there
are differences between our optical detection schemes
and sample preparation techniques and those used by
Hamza, et al. that would explain the discrepancy.
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