Application of MeV ion bombardment
to create micro-scale annealing of Silica-Gold films
A. Bouyard¹, X. Blanchet¹, D. ILA²*, C.I. Muntele², I.C. Muntele², R.L.
Zimmerman²
1
2
Université Claude Bernard Lyon 1, 43 boulevard du 11 novembre 69622 Villeurbanne Cedex, France
Center for Irradiation of Materials, Alabama A&M University, P.O Box 1447, Normal, AL 35762-1447, USA
Abstract: This project studies the production of nanoscale annealing using MeV Si ion beams. To test the
technique we produced thin films of Au-Silica by sequential deposition of Au and SiO2 on Suprasil substrates. We
measured the thickness of the deposited films with an interferometer and by using Rutherford backscattering
spectrometry (RBS). Using the measured thickness we calculated the concentration of Au in each film. TRIM
simulation was used to confirm our results. Since the localized annealing causes the formation of gold nanoclusters, we performed optical absorption photospectrometry (OAP) on all slides, before deposition, after
deposition, and after bombardment by MeV Si beams. Optical index changes are apparent in the sequentially
deposited multilayer samples that were not seen in Au-silica co-deposited samples with the same volume fraction of
gold
INTRODUCTION
During the last decade, metallic ion
implantation and thermal annealing have been used to
change the linear and the non-linear optical properties
near the surface of silica glass [1-8]. Bombardment of
ion implanted samples has been shown [9,10] to replace
thermal annealing as a method to form nanoclusters of
the implanted atoms. An attractive property of ion
implantation is that ions can be focused in a welldefined space in an optical device, to induce local
changes in its linear and nonlinear properties.
Moreover, the size distribution of the nanoclusters
formed by post implantation bombardment is
surprisingly narrow a requirement for future
applications of metallic nanocrystals as quantum dots.
It has long been known that small metallic
particles or colloids embedded in dielectrics produce
colors associated with optical absorption at the surface
plasmon resonance frequency [6,7], which depends on
the index of refraction of the host substrate and the
electronic properties of the colloids formed in the host
material. For clusters with diameters much smaller than
the wavelength of light, the theories of Mie [8] can be
used to calculate the absorption coefficient (cm-1) of the
composite:
α=
18πQn0
λ
3
(ε
ε2
1 + 2n0
) +ε
2 2
2
2
(1)
where Q is the volume fraction occupied by the metallic
particles, no, is the refractive index of the host medium,
and ε1 and ε2 are the real and imaginary parts of the
frequency-dependent dielectric constant of the bulk
metal. Equation (1) is a Lorentzian function with a
maximum value at the surface plasmon resonance
frequency (νp). Values of ε1 for the metals, as a function
of wavelength, are tabulated in [11]. Using the
measured no for various wavelengths, one can predict
from equation (1) the photon wavelengths for the
surface plasmon resonance frequencies of metallic
colloids in the photorefractive host materials. This
method provides the wavelength for the gold colloids in
silica, suprasil 300 as host material, to be near 530 nm
[7, 11-14].
To produce nanoclusters such as gold in a silica
medium, the implantations are done at high doses (1 to
2×1017 ions/cm2) to overcome the solubility of the
implanted species in the substrate. This is also why
there is a wide distribution of cluster sizes and wide
distribution depth of the clusters in depth produced by
the above techniques. To overcome this problem, since
1994, we initiated a series of investigations into how to
confine the implanted ions in a narrow layer. This was
done by generating chemical barriers, engineering
defect barriers on both sides of the implanted layer, as
well as combining two non-equilibrium processes: ion
implantation and post-implantation irradiation [12-14].
In our previous publications we have shown that the
phase change from implanted Au in silica at room
temperature to nano-crystals of Au is purely a function
of energy deposited by electronic excitation per ion in
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
643
the track in the Au implanted layer [12-14] and not a
function of fluence or dose. In this paper we produce
gold layers between silica layers by sequentially
depositing gold and silica by e-beam evaporation.
Subsequently thermal annealing is shown to produce
layers of nanocrystals with optical properties essentially
identical with nanocrystals formed thermally after gold
ion implantation. Finally, we report here the successful
formation of gold nanocrystals in deposited metal films
embedded between silica layers by bombardment
through the layers with silicon ions. The sequentially
deposited Au-SiO2 samples reported here differ
distinctly from the co-deposited Au-SiO2 samples
previously reported [12-14] because the gold atoms are
in a pure layer between pure SiO2 layers, not randomly
distributed in a SiO2 host.
EXPERIMENTAL PROCEDURES
The silica glass for both the substrate and as a
deposition source was Suprasil-300, a commercial
product of Heraeus Amersil, Inc. and is of known
purity.
Gold and silica were evaporated from
independently controlled e-beam evaporators whose
shutters were opened sequentially such that a layered
structure was formed on the substrate at room
temperature. Rutherford backscattering spectrometry
(RBS) was used to measure the total thickness of the
structure and the amount of gold in each layer. Figure 1
shows the RBS spectrum of a surface layer and five
layers of gold embedded between five layers of SiO2.
The layers examined in Figure 1 were on a carbon
substrate used as a witness during simultaneous
deposition on the Suprasil research samples. When
account is taken for the higher Rutherford cross section
of nitrogen ions backscattered from deep in the
structure, the five subsurface gold layers each had
within 15% equal thickness of 6 nm. Compared with
the total thickness of the structure, measured optically
as well as by RBS as 580 nm, the atomic ratio of Au to
SiO2 is 14%. The volume fraction Q is 5% and may be
used in Eq. 1 if 100% of the gold layer is transformed to
spherical nanocrystals.
Si ions were used to induce the formation of
nanocrystals following methods described in earlier
work [12-14] with ion implanted gold layers. The
energy of the bombarding Si particles (5.0 MeV) was
selected such that the Si ions stopped well beyond the
depth of the Au deposited layers, such that the energy
deposited in the gold-containing layers was due mainly
to the electronic energy loss. Fluences of silicon
between 5×1015 ions/cm2 and 2×1017 ions/cm2 were for
gold nanocluster formation. In all of the above postimplantation bombardments the temperature of the
silica host was kept at 300K.
The optical absorption photospectrometry
(OAP), FTIR and RAMAN spectra were obtained and
analyzed.
RESULTS AND DISCUSSION
Higher ion beam fluences and higher atomic
numbers of the bombarding ions resulted in more
damage to the silica glass, and a subsequent change of
the optical properties [7, 11-14]. This was observed by
OAP during all bombardments. These effects are
reduced with heat treatments above 970K [11,12].
Optical absorption measurements, together
with Equation 1, rather than electron microscopy, a
destructive process, were used to monitor the formation
of nanocrystals. Figure 2 shows the optical absorption
0.25
0.20
Au
100
197
Absorbance(A.U.)
Counts per Channel
Multilayer Au:SiO2 on GPC
0.15
0.10
0.05
0
200
400
600
Channel Number
0.00
200
800
300
400
500
600
700
800
900
Wavelength(nm)
Figure 2: Optical Absorption spectrum of the Multilayer
Au-SiO2 structure on a Suprasil substrate.
Figure 1: 5 MeV N14 ion RBS spectrum of Au-SiO2
multilayer on a carbon substrate produced by sequential ebeam deposition
644
of the multilayer Au-SiO2 sample before Si ion
bombardment. The periodic oscillating variation in
optical absorption is an interference effect owing to the
different index of refraction of the substrate and
deposited material.
After bombardment by 4 MeV Si ions at
5x1016 cm-2, Figure 3 shows the optical absorption in
the region where Equation 1 and the published values of
the parameters for gold and silica would predict optical
absorption.
Figure 3 reveals an absorption band centered at
490 nm and an increased general absorption that has
obscured the interference effects displayed in the as
prepared sample. Higher Si ion fluence has previously
been shown [14] to further increase the damage without
increasing the evidence for nanocrystal formation.
Figure 4 is the same sample after annealing for
1 hour at 1100oC. The optical evidence of damage is
reduced, especially at shorter wavelengths.
The
interference effects on the transmission of light through
Absorbance(A.U.)
0.2
0.1
0.0
300
400
500
600
700
800
Wavelength(nm)
Figure 3: Optical absorption of Au SiO2 multilayer after 4
MeV Si ion bombardment.
0.2
o
Annealed at 1100 C
o
Annealed at 1100 C and
17
-2
Optical Density
4 MeV Si ions 1x10 cm
420 nm
0.1
280 nm
0.0
200
400
600
800
Wavelength (nm)
Figure 5: Comparison of a multilayer Au-SiO2 device after
thermal annealing and then after silicon ion bombardment
the thin film have returned. That these effects do not
appear in other samples indicates that the effective
index of refraction is different in the multilayered
structure.
Figure 5 is the optical absorption of a thermally
annealed multilayered Au-SiO2 film sample compared
with the same sample that has in addition 4 MeV silicon
ion bombardment. Simulation shows [15] that energy is
transferred preponderantly only to electrons in the film
and that the ions stop in the SiO2 substrate well beyond
the 580 nm film..
Although no TEM micrographs are yet available
for the samples reported here, the similarity of the
optical absorption data with previous similarly post
bombarded samples for which TEMs were taken are
consistent with equally uniform gold precipitates.
CONCLUSIONS
A comparison of the results obtained from post
bombardment of Au implanted silica with results
obtained from bombardment of co-deposited Au-silica
films indicates the possibility of producing multi
layered devices using a sequential deposition technique.
0.20
Absorbance(A.U.)
0.15
0.10
ACKNOWLEDGEMENTS
0.05
0.00
300
400
500
600
700
800
Wavelength(nm)
Figure 4: Optical absorption of Au-SiO2 multilayer after 4MeV Si ion bombardment and annealing for 1 hour at
1100oC.
This project was supported by the Center for
Irradiation of Materials at Alabama A&M University.
The work at ORNL was sponsored by the Division of
Materials Science, U.S. Department of Energy, under
Contract DE-AC05-96OR22464 with Lockheed Martin
Energy Research Corp.
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*
Corresponding author: Tel (256) 851-5866, Fax (256) 851-5868,
e-mail [email protected].
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