PII:
Acta mater. Vol. 47, No. 5, pp. 1497±1511, 1999
# 1999 Acta Metallurgica Inc.
Published by Elsevier Science Ltd. All rights reserved
Printed in Great Britain
S1359-6454(99)00028-2
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ION IMPLANTATION-INDUCED NANOSCALE PARTICLE
FORMATION IN Al2O3 AND SiO2 VIA REDUCTION
E.M. HUNT1 and J.M. HAMPIKIAN2{
Columbian Chemical Company, Marietta, Georgia, U.S.A. and 2School of Materials Science and
Engineering, Georgia Institute of Technology, 778 Atlantic Drive, Atlanta, GA 30332-0245, U.S.A.
1
(Received 23 July 1998; accepted in revised form 6 January 1999; accepted 10 January 1999)
AbstractÐA novel method for creating nano-dimensional metallic precipitates in oxide materials using the
technology of ion implantation is reported. The reduction of single-crystalline alumina to Al and fused
silica to Si is induced by ion implantation with ions which are selected in accordance with the laws of thermodynamics. The Al and Si resulting from reduction subsequently cluster and react with other elements to
form nano-dimensional precipitates. The implantation of 150 keV Y+ and Ca+ into alumina to a ¯uence
of 5 1016 ions/cm2, results in Al particles with an average diameter of 12.5 nm and 8.0 nm, respectively.
Alumina implanted with Mg+ at the same ion energy and ¯uence forms MgAl2O4 platelets ranging from 5
to 10 nm in width and between 15 and 40 nm in length. The implantation of silica with 160 keV Zr+ ions
to a ¯uence of 1 1017 Zr+/cm2, results in the formation of ZrSi2 particles ranging in size between 1 and
17 nm. Consistent with thermodynamic predictions, control implants of Cr+ and Si+ in alumina and Cr+
in silica do not result in the formation of particles that contain elements originally present in the substrate.
# 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.
1. INTRODUCTION
Dielectric materials with nonlinear optical properties have a wide variety of potential applications,
including use in opto-electronic systems [1±4]. The
desirable properties are the result of a ®ne dispersion of nano-sized particles (less than 10 nm in
diameter) which are embedded in the oxide substrate. Nanoparticle formation via ion implantation
has been studied extensively in the last decade
because ion implantation is ideally suited for the
fabrication of planar devices. In this study, the formation of metallic nano-dimensional particles in
two di€erent oxide systems, alumina (Al2O3) and
silica (SiO2), has been achieved through a novel
synthesis route, the ion implantation-induced reduction of the host matrix.
The appearance of colloidal features in ceramic
materials due to ion implantation has been reported
by a number of researchers for ion ¯uences ranging
between approximately 1 1016 and 1 1017 ions/
cm2 [5±11]. In this type of colloid formation, ion
implantation is used to create a supersaturated solution near the surface of the substrate which is followed by precipitation of the implanted ion and the
formation of nanocrystals [11]. In alumina, the precipitation of the implanted material is usually
induced by holding the samples at an elevated temperature during implantation or by performing a
{To whom all correspondence should be addressed.
post-implantation annealing treatment. This type of
colloid formation in alumina has been reported for
implantations of Au [5], Ag [6], Cu [7], Fe [8],
Ni [9], Mn [10], Si and Ge [11]. Compound semiconductor crystals can also be formed through this
implantation and precipitation process. White et
al. [11] have demonstrated the formation of SiGe,
GaAs, InAs, GaP, InP, CdS, CdSe and GaN via
sequential implantation of stoichiometric, overlapping doses of the elements comprising the compound.
The type of particle formation described in this
work relies on a di€erent mechanism in which the
particles formed are comprised (in part or in
entirety) of the cation of the host matrix instead of
the implanted ion. Previous research has shown
that the implantation of 5 1016 Y+/cm2 into
alumina at an accelerating energy of 150 keV results
in the formation of spherical metallic Al particles
with an average diameter of 010 nm [12].
Thermodynamic calculation of the Gibbs free
energy of the reduction reaction involving Y and
Al2O3 shows that this reaction is possible with a
free energy of formation of approximately ÿ230 kJ/
mol at 300 K, as illustrated in Fig. 1. The calculation of the Gibbs free energy of reaction may also
be used to predict in what other ion±substrate combinations similar reduction reactions may occur. In
this paper, the occurrence of particle formation via
ion implantation-induced reduction is presented and
explored.
1497
1498 HUNT and HAMPIKIAN: ION IMPLANTATION-INDUCED NANOSCALE PARTICLE FORMATION
implanted substrate surface in relation to unimplanted material. Optical spectrometry was used to
detect particles in the oxide matrix and TEM, electron energy-loss spectroscopy (PEELS) and energy
®ltered TEM (EFTEM) were used to characterize
the particles.
3. RESULTS
3.1. Alumina substrates
Fig. 1. Gibbs free energy as a function of temperature.
2. EXPERIMENTAL
Single-crystalline R-plane (02
21) Al2O3 substrates
which were donated by Sapphikon Inc. as well as
high-purity fused silica substrates were used in this
study. The alumina substrates were annealed prior
to implantation for 80 h at 15008C to ensure surface
crystallinity. Ion implantation treatments were
carried out in the Surface Modi®cation and
Characterization Facility in the Solid State Division
of Oak Ridge National Laboratory at a vacuum in
the range of 10ÿ4 Pa in an Extrion implantation system which employs the Freeman ion source con®guration. The 1 cm 1 cm 0.07 cm alumina
substrates and the 2 cm 2.5 cm 0.1 cm silica substrates were uniformly implanted over a portion of
their surface. Beam current densities (in the range of
03 mA) were kept low for the alumina substrates in
order to avoid excessive beam heating, while the
silica implantations were carried out at much higher
beam currents (050 mA) in order to promote di€usion during the implantation. A portion of each
implanted silica substrate was also post-implantation annealed at 11008C for 1 h in ¯owing
Ar + 4% He. Plan-view thin sections for transmission electron microscopy (TEM) were prepared
using standard techniques, with ®nal thinning being
carried out in a Gatan Precision Ion Polishing
System using 4 keV Ar ions incident on the sample
surface at approximately 58.
The implanted substrates were examined with a
variety of techniques in order to fully characterize
the resulting morphology. Knoop microhardness
measurements over a range of loads were carried
out in accordance with the ASTM standard E 38489 in order to determine the crystalline state of the
In this paper, it will be demonstrated that equilibrium thermodynamics may be used to predict the
end state of ion-implanted materials. Figure 1 presents the Gibbs free energy curves for the reduction
reactions of Y, Ca and Mg with Al2O3, showing
that the free energy of their reaction is negative
over a wide range of temperatures. Thus, these
implant ions are thermodynamically capable of
reducing the substrate. In the sections below, the
experimental results of the implantation of Y+ into
Al2O3 are ®rst presented, followed by the results of
the Ca+ and Mg+ implantations into Al2O3.
3.1.1. Y+ implantations. Ion implantation of Y
into [02
21] Al2O3, to a dose of 5 1016 Y+/cm2,
administered at an energy of 150 keV and at ambient temperature, produces an amorphous surface
region which is approximately 119 2 6 nm thick as
determined by RBS and RBS-C measurements
(Fig. 2). Knoop microhardness measurements
demonstrate absolute softening of the implanted
surface, consistent with the presence of an amorphous phase [12]. This is not an unexpected result
for this implantation, as it exceeds the criterion for
the critical energy density of amorphization
reported by Burnett and Page [13, 14]. TEM examination of and electron di€raction data taken from
these substrates con®rms the amorphous nature of
the near-surface material. TEM examination also
reveals the presence of nano-sized crystalline precipitates embedded within the amorphous matrix
material. These particles have a face centered cubic
(f.c.c.) structure, an experimentally determined lattice parameter of 0.412 2 0.002 nm, and an average
particle size of 12.5 2 0.3 nm (see Fig. 3). EDS
measurements indicate that the particles are chemically Al-rich and O-poor with respect to the surrounding matrix material. However, the EDS data
have fairly low resolution compared to the size
scale of the particles being investigated here, prohibiting its use for the ®ne-scale investigation of their
chemistry. The crystallographic structure of the particles and the relatively small lattice parameter lead
to the conclusion that these particles may be metallic, possibly metallic Al, which is also f.c.c., having a lattice parameter of 0.40497 nm.
The identi®cation of the particles present in the
150 keV Y-implanted sample as a form of metallic
Al is veri®ed using parallel-detection electron
energy-loss spectroscopy (PEELS). The energy-loss
HUNT and HAMPIKIAN: ION IMPLANTATION-INDUCED NANOSCALE PARTICLE FORMATION 1499
Fig. 2. Random and aligned RBS spectra for alumina substrates implanted with 5 1016 Y+/cm2 at
ambient temperature and at 150 keV.
spectra resulting from oxidized Al and metallic Al
in the low-loss region (1±40 eV) are suciently
di€erent to allow di€erentiation between the two
signals. The low-loss energy spectra from these materials standards, shown in Fig. 4, show that the
valence-loss signal characteristic of alumina has a
broad feature at approximately 24 eV, while the
plasmon-loss characteristic of metallic Al has a relatively sharp peak at 15 eV. The presence of the
implanted species must also be considered. As the
yttria (Y2O3) standard spectrum in Fig. 4 shows,
yttria also has a loss feature at 15 eV as well as a
broad feature at 040 eV. The latter feature does not
occur in the energy-loss spectra from Al and
alumina. In addition to the di€erences between the
low-loss energy spectra of the materials, PEELS can
be performed at a reasonably high resolution by
using the smallest spectrometer entrance aperture to
physically select the area from which the signal is
taken. In the 150 keV Y+-implanted sample, it was
possible to isolate columns of matrix material containing individual particles with the aperture, and
subsequently to isolate nearby columns of material
that did not contain particles, in order to compare
Fig. 3. TEM bright ®eld image of 150 keV Y+-implanted alumina showing spherical nano-sized particles embedded in an amorphous matrix.
1500 HUNT and HAMPIKIAN: ION IMPLANTATION-INDUCED NANOSCALE PARTICLE FORMATION
the energy-loss signals from material with particle
and matrix-loss contributions and material with
matrix-loss contributions only. The resulting spectra, seen together in Fig. 5, both show the single
scattering energy-loss signal characteristic of
alumina at approximately 25 eV. The spectrum
from the particle-bearing material shows a sharp
loss feature at 15 eV, an energy-loss characteristic
of both Al and yttria. Note that the particle-bearing
spectrum does not contain the similarly prominent
feature at 040 eV, which is also characteristic of
yttria. This result shows that the particles contain
material that causes incident electrons to lose 15 eV,
and that this material is not present in the surrounding material. This result is consistent with the
identi®cation of these particles as Al from their
electron di€raction pattern. This Al was formed by
reduction of the substrate, consistent with thermodynamic prediction.
Energy ®ltered TEM (EFTEM) was utilized in
order to show graphically where the Al resides in
each sample [15±18]. Using the PEELS spectra as a
guide, three primary energy losses were selected
with which to image using a 5 eV energy window:
the characteristic energy loss of Al and yttria
(15 eV), the characteristic energy loss of oxidized Al
(25 eV) and the secondary loss feature characteristic
of yttria (40 eV). Presented in Fig. 6 is a series of
TEM/EFTEM images from the same area of a
150 keV Y+-implanted sample. Figure 6(a) contains
a conventional bright ®eld image; particles which
are oriented in a condition of strong di€raction
with respect to the incident electrons appear dark.
When the sample is imaged using 15 eV electrons,
the particles imaged appear bright [Fig. 6(b)], indicating that the electrons which pass through those
regions of the sample experience an energy loss of
15 eV. When the same area is imaged with 25 eV
loss electrons, the matrix appears bright, while the
particles are dark [Fig. 6(c)]. Imaging with 40 eV
electrons also causes the matrix to appear brighter
Fig. 5. High-resolution PEELS from a 150 keV Y+implanted alumina substrate showing that the particlebearing region exhibits a strong energy-loss feature at
015 eV.
than the particles [Fig. 6(d)]. These three images
[Fig. 6(b)±(d)] demonstrate that the particles are
composed of Al and con®rm that they do not contain alumina or yttria. In addition to imaging with
the low-loss electrons associated with oxidized and
metallic Al, it is also possible to create an elemental
map using a series of images formed using core-loss
electrons. When the oxygen in the sample is
mapped in this manner the resulting image shows
dark particles, indicating that they are de®cient in
oxygen with respect to the surrounding matrix
(Fig. 7). If the same technique is used to map Y
using the M3 core-loss excitation, the result is a
dim and featureless image (not shown). This result
indicates that the implanted Y is not collected into
discrete regions of the sample, but rather is dispersed throughout the substrate. The lack of brightness might also be explained by the relatively small
amount of Y present compared to the other elements present.
The presence of nanoscale particles in the
150 keV Y implantation suggests the possibility of
performing optical absorption measurements in
order to assist in the identi®cation of particle-bearing material. It is known that nanoscale particles
which are suspended in an insulating matrix will
result in absorption of incident electromagnetic
radiation. According to the Mie theory [19], the
amount of incident radiation absorbed will peak at
a wavelength which is related to the surface plasmon resonance of the particle material and the
dielectric and optical properties of the matrix material as shown in the equation
lpeak ˆ
Fig. 4. Standard Al, alumina and yttria PEELS spectra
from the low energy-loss region.
1=2
2pc ÿ
1 ‡ 2n20
op
1†
where op is the plasmon frequency of the bulk
metal, c is the speed of light, and n0 is the index of
refraction of light in the dielectric medium.
HUNT and HAMPIKIAN: ION IMPLANTATION-INDUCED NANOSCALE PARTICLE FORMATION 1501
Fig. 6. Series of EFTEM images taken from a 150 keV Y+-implanted alumina substrate. Image (a) is a
bright ®eld image, (b) a 15 eV-loss image, (c) a 25 eV-loss image and (d) a 40 eV-loss image. The arrow
indicates the location of Al particles.
When di€erential absorption measurements were
carried out on the 150 keV Y+-implanted material,
the resulting absorption peak maximum was located
at approximately 240 nm (see Fig. 8). Equation (1)
yields a wavelength for maximum absorption due to
Al particles embedded in an alumina matrix of
lpeak=221 nm, using values of n0=1.76 and
op=2.28 1016/s [19]. This value is slightly lower
than the measured peak of 240 nm; however, this
equation can result in predicted values up to 50 nm
lower than the experimentally measured peak
positions [20, 21].
3.1.2. Ca+ and Mg+ implantations. According to
the Gibbs free energy calculations presented in
Fig. 1, the implantation of Ca+ and Mg+ ions
under conditions similar to the Y+ implantation
conditions can result in reduction of the substrate,
and may therefore also result in the formation of
similar Al particles in the matrix. These conditions
consist of implantation with a low current density
beam of singly charged ions to a ¯uence of
5 1016 ions/cm2 into an alumina substrate held at
ambient temperature. The incident ion energy for
each of these experiments was chosen so that the
1502 HUNT and HAMPIKIAN: ION IMPLANTATION-INDUCED NANOSCALE PARTICLE FORMATION
Fig. 7. Bright ®eld image (a) and oxygen jump ratio image (b) of 150 keV Y+-implanted alumina showing the oxygen de®ciency of the particle region.
resulting ion concentration pro®les would approximate those of the previous Y implantations, as predicted by PROFILE [22].
Accelerating energies of 50 and 70 keV were used
for two separate Ca+ implantations. When examined with RBS and RBS-C, both implantations
resulted in substrates which contain dechanneled
regions indicative of a heavily damaged lattice.
These damaged regions occur in the area of the
spectrum representing the substrate surface and
appear qualitatively thinner than similar dechanneled regions resulting from similar Y implantations. To quantify the extent of this damage,
Knoop microhardness measurements were carried
out and the resulting values demonstrate absolute
softening, an indication that the surface is amorphous.
Fig. 8. Relative optical absorption spectra from 150 keV
Y+-, 70 keV Ca+- and 45 keV Mg+-implanted alumina
substrates.
Di€erential optical absorption measurements carried out on the Ca+-implanted substrates result in
an absorption spectrum with a peak at approximately 242 nm for the 70 keV implantation (see
Fig. 8). This peak is consistent with the presence of
nano-sized particles in the substrate material. Recall
that the calculated value of the wavelength at which
the maximum absorption due to metallic Al particles embedded in alumina will occur is 0221 nm,
and that an experimental peak at 0240 nm is
observed from alumina samples containing metallic
Al particles. The appearance of an obvious peak in
the absorption spectrum for these samples indicates
that there are nano-sized particles present in the
matrix, and the peak wavelength is consistent with
the presence of particles composed of metallic Al.
TEM examination of back-thinned, plan-view
50 keV and 70 keV Ca-implanted substrates reveals
embedded particles ranging in size from 6 to 10 nm
with average sizes of 7.49 2 1.35 nm and
8.76 2 1.23 nm, respectively (see Fig. 9). Electron
di€raction shows that these particles are f.c.c. with
a lattice parameter of 0.411 2 0.002 nm. This agrees
well with the lattice parameter determined from the
particles present in the Y-implanted alumina
(0.412 20.002 nm), and both are close to the lattice
parameter of pure Al, 0.40497 nm. These particles
are somewhat smaller than previously examined
particles, with average sizes 4±5 nm smaller than
the Y-induced particles. It can be seen in Fig. 9
that many of the particles in the Ca-implanted substrates also demonstrate a ring-like contrast not
previously observed in the Y-induced particles. The
contrast could be due to either chemical or di€raction contrast mechanisms. It is possible that this
HUNT and HAMPIKIAN: ION IMPLANTATION-INDUCED NANOSCALE PARTICLE FORMATION 1503
Fig. 9. Bright ®eld TEM images of (a) 50 keV and (b)
70 keV Ca+-implanted alumina substrates showing spherical Al particles embedded in the amorphous matrix.
contrast is a result of compositional variation of Al
as a function of particle radius. Unfortunately, the
very small (2±4 nm) size scale of the contrast features severely limits the techniques that can be used
to examine their origin.
PEELS and EFTEM were performed in order to
con®rm these conclusions. Figure 10(a)±(c) presents
an energy-loss series from the 50 keV Ca+implanted alumina substrate beginning with a
bright ®eld image [Fig. 10(a)]. Figure 10(b) shows
that electrons interacting with the particle material
experience the 15 eV loss which is characteristic of
metallic Al. Figure 10(c) shows that electrons interacting with only the surrounding substrate experience the 025 eV loss which is characteristic of
alumina. The particle material is also not illuminated by the 035.5 eV loss which is characteristic of
calcia (not shown). The previous discussion of the
EFTEM results from the Y-implanted alumina indicates that these images are sucient to demonstrate
that the material comprising the particles in this
sample is Al.
The implantation of alumina substrates with
Mg+ is also predicted to result in the reduction of
the substrate material. The implantation of 35 keV
and 45 keV Mg+ results in heavy damage to the
substrate surface region as evidenced by dechanneled regions observed in the RBS and RBS-C spectra. The extent of this damage was investigated
using Knoop microhardness and the results again
demonstrate absolute softening on the surface of
the substrates (not shown).
A di€erential optical absorption spectrum for the
45 keV Mg+-implanted sample is presented in
Fig. 8. An absorption peak located at 205 nm
suggests that there are nano-sized particles present
in the matrix. However, this peak occurs at a wavelength approximately 40 nm lower than the peaks
previously seen resulting from the presence of Al
particles embedded in an alumina matrix. This discrepancy indicates that the particles causing the
absorption are markedly di€erent from those present in the Y- and Ca-implanted substrates. Optical
absorption measurements of the 35 keV sample
reveal similar absorption features.
TEM examination of the 35 keV Mg+-implanted
substrate reveals crystalline particles present in the
amorphous matrix [see Fig. 11(a)]. These particles
are identi®ed by the resulting di€raction pattern
(inset) as MgAl2O4. The di€raction pattern results
in a calculated lattice spacing of 8.080 2 0.002 nm,
matching the 8.083 nm lattice parameter of
MgAl2O4 very well. There are faint rings, partial
arcs and spots present in this di€raction pattern, indicating a variety of particle morphologies and
orientations. The rings indicate that there are aluminate particles which are randomly oriented in the
plane present in the matrix, in addition to more
oriented particles which result in the arcs and spots.
The randomly oriented particles are dicult to
image by themselves due to the strong contribution
from the oriented particles of the same family of
planes. When imaged in dark ®eld mode using the
{400}, {440} and {444}-type re¯ections, plate-like
particles typically 5±10 nm in width and 15±40 nm
in length are illuminated [see Fig. 11(b)]. Fine-scale,
1.5 nm Moire fringes are visible in both imaging
modes, but are especially evident in the dark ®eld
images. These fringes result from the slight misorientation among the plate-like particles which is
also responsible for the appearance of arcs in the
di€raction pattern. EDS con®rms the presence of
Mg in the areas containing particles.
In addition to the implantations of Ca+ and
Mg+, which were thermodynamically predicted to
be able to reduce the Al2O3 and possibly form particles containing Al, two implantations were selected
for implantation which were not predicted to produce such particles, namely Cr+ and Si+. Figure 1
indicates that the reduction reactions for both Cr
and Si with alumina have positive free energies, indicating that substrate reduction and Al particle
formation will not occur. Again, both of these implantations were carried out under conditions similar to the conditions for the original Y+
implantations, with incident ion energies tailored to
cause the local chemical pro®les to mimic those of
the original Y pro®les. The ¯uence for both implantations remained constant at 5 1016 ions/cm2.
1504 HUNT and HAMPIKIAN: ION IMPLANTATION-INDUCED NANOSCALE PARTICLE FORMATION
Fig. 10. Series of EFTEM images from a 50 keV Ca+-implanted alumina substrate. Image (a) is a
bright ®eld image, (b) a 15 eV-loss image, and (c) a 25 eV-loss image. The arrow indicates the location
of Al particles.
The implantation of 100 keV Cr+ does not result
in an amorphous surface layer as seen by Knoop
microhardness measurements (not shown). The formation of a buried amorphous layer, however,
would not be unexpected on the basis of Cr+ implantation results reported in the literature [10, 23].
The Knoop microhardness measurements for the
50 keV Si+-implanted substrate do show a small
degree of absolute softening, although the magnitude is much smaller than the softening seen in the
Y+-implanted substrates.
Optical absorption spectra from these substrates
show no evidence of a particle-induced absorption
maximum in the wavelength range from 200 to
800 nm [Fig. 12(a)]. TEM examination of the Cr+and Si+-implanted substrates con®rms that there
are no second-phase particles in these substrates
through the lack of both bright ®eld di€raction
contrast and a second-phase di€raction pattern.
The lack of a bright amorphous haze in the electron
di€raction pattern resulting from these substrates
supports the conclusion that there is no signi®cant
surface amorphization in either substrate, consistent
with the hardness measurements. Di€raction patterns from these substrates do demonstrate an unusual pattern of extra spots accompanying the
matrix spot pattern [Fig. 12(b)]. This pattern is the
result of implanted ion damage to the alumina substrate and was also observed in both Zr+- and Si+implanted alumina.
3.2. Silica substrates
Reduction by appropriately chosen implanted
ions should occur for any oxide material. Silica is
another important material in waveguide and integrated optics applications and may also be susceptible to Si particle formation via reduction under
appropriate circumstances. Calculations show that
Zr is thermodynamically capable of reducing silica
(see Fig. 13). The implantation of silica with Si and
other ions and subsequent particle formation has
been well studied [1, 24±44], although Zr has
received little attention as an implant ion from
HUNT and HAMPIKIAN: ION IMPLANTATION-INDUCED NANOSCALE PARTICLE FORMATION 1505
Fig. 11. (a) Bright ®eld TEM of a 35 keV Mg+-implanted alumina substrate and (b) dark ®eld TEM of
a 35 keV Mg+-implanted alumina substrate showing the oriented MgAl2O4 crystals embedded in the
alumina matrix.
other researchers investigating nanoparticle formation in silica.
The silica implantations were carried out with
ambient substrate temperatures, a ¯uence of
5 1016 ions/cm2 and a current density of 50 mA;
this current density is at least 40 mA higher than
that used in the alumina implantations. The
implanted substrates were annealed for 1 h at
11008C in an atmosphere of ¯owing Ar + 4% He.
The higher current density and the post-implantation anneal are necessary due to the much lower
di€usivity of Si+ in silica as compared to Al+ in
alumina [44]. In addition to the Zr+ experiment a
control implantation was carried out with Cr+, an
ion thermodynamically incapable of reducing silica
according to free energy calculations (see Fig. 13).
The Cr+ implantations were carried out under conditions similar to those outlined for the Zr implantations, including high current density implantation
and post-implantation annealing.
Di€erential optical absorption spectra taken from
as-implanted and annealed silica substrates
implanted with 5 1016 and 1 1017 160 keV Zr+/
cm2 are shown in Fig. 14. These spectra show that
neither of the as-implanted substrates demonstrates
an absorption peak indicative of the presence of
nanoscale particles. The lower-dose implantation
may not form particles large enough to be detected
by these measurements owing to their low-wavelength absorption [36]; however the larger-dose implantation should produce sucient free Si to form
particles large enough to absorb at the higher wavelengths detected in these measurements. Despite the
higher overall absorption which is attributable to
the greater amount of implantation damage resulting from the larger-dose implantation, no absorption peak resulting from particle formation is
evident. Figure 14 also shows the di€erential optical
absorption spectra from these samples after postimplantation annealing at 11008C in Ar + 4% He
for 60 min. These annealed samples do exhibit an
absorption peak indicative of the presence of nanoparticles in the substrate at approximately 260 nm.
TEM examination of the large-dose, annealed
Zr+-implanted silica reveals the presence of a fairly
continuous background of 1±3.5 nm particles interspersed with larger particles ranging in size from 5
to 17 nm (see Fig. 15). The larger particles have an
average size of 11 2 3.0 nm. All particles present in
this substrate appear to contribute to a polycrystalline di€raction pattern that is consistent with their
identi®cation as ZrSi2, an orthorhombic Si±Zr
alloy.
1506 HUNT and HAMPIKIAN: ION IMPLANTATION-INDUCED NANOSCALE PARTICLE FORMATION
Fig. 14. Relative optical absorption spectra from Zr+ asimplanted and annealed silica substrates.
Fig. 12. (a) Relative optical absorption spectra from
100 keV Cr+- and 50 keV Si+-implanted alumina substrates showing the absence of absorption features indicative of the presence of nanoparticles. (b) Di€raction
pattern from the Cr+ implantation, with extra di€raction
spots arrowed indicating lattice damage.
In addition to the Zr+ implantations, which were
expected to result in the reduction of the silica substrate, an implantation of Cr+ was carried out as a
control experiment. A di€erential optical absorption
spectrum from the as-implanted substrate seen in
Fig. 16 shows that there is no absorption peak indicative of particle formation. The annealed substrate, however, does show an absorption feature at
approximately 236 nm. The discontinuities in the
spectra at 380 nm and 560 nm are artifacts resulting
from the spectrophotometer shutter.
TEM examination of the Cr+-implanted and
annealed substrates con®rms the presence of particles in the implanted region of the substrate.
There is a general background of roughly equiaxed
particles which range in size from 3 to 8 nm with an
average size of 5 2 1.13 nm (Fig. 17). In addition,
there are larger, faceted particles present, possibly
at the substrate surface. The polycrystalline di€raction pattern resulting from these particles, seen in
the inset of Fig. 18, has been identi®ed as that of
Cr, consistent with Cr+ implantations forming Cr
particles and not reducing SiO2, as expected.
4. DISCUSSION
4.1. Alumina substrates
Fig. 13. Gibbs free energy vs temperature for the reduction of silica by various ions.
The open literature contains a large body of
work regarding ion implantation of a variety of
ions into alumina substrates, yet the formation of
particles as a result of the reduction of the alumina
substrate material has not been previously reported.
This is a result of both the fact that most elements
are not capable of reducing the alumina substrate,
and that experimentally, Al particles are dicult to
detect. Al particles in this size range embedded in
alumina exhibit very little di€raction contrast in
conventional TEM and result in a faint di€raction
pattern that can be partially or entirely masked by
the amorphous haze that results from the amorphous layer which concurrently forms at the surface
of the substrate [12]. The use of PEELS and
HUNT and HAMPIKIAN: ION IMPLANTATION-INDUCED NANOSCALE PARTICLE FORMATION 1507
Fig. 15. TEM of a 1 1017 Zr+/cm2 implanted and annealed silica substrates showing particles which
are identi®ed by their electron di€raction pattern as being ZrSi2.
EFTEM in this work was critical to the identi®cation of the Al particles which formed.
Although the formation of particles via ion implantation-induced reduction of an oxide has not
been previously reported, electron or neutron irradiation has been shown to result in the formation
of metal particles from the cation of a host
lattice [45]. However, the energies involved
(01 MeV for electrons and 14 MeV for
neutrons) [46] suggest that charged and neutral lattice defects are responsible for the formation of the
particles. In this research, the incident ion energy is
relatively low, on the order of 100 keV, and the operative mechanism is simply reduction, via, for
example
the chemical formulae of the oxides of the
implanted ions, CaO and Y2O3. First, a number
of assumptions must be made for simpli®cation; it
is assumed that upon reduction of the alumina
the implanted ion forms a roughly stoichiometric
form of its own oxide (e.g. Y2O3 or CaO); and it
is also assumed that the same fraction of
implanted ions reacts with the substrate in both
cases. For a given ion dose, therefore, a Caimplanted sample will contain less free Al as compared with the free Al contained in an Yimplanted substrate. With less Al available to subsequently grow particles, the resulting particles
would therefore be smaller. This is consistent with
experimental observations.
2Y ‡ Al2 O3 ˆ 2Al ‡ Y2 O3 ;
DG ˆ ÿ228 kJ=mol …298K†
2†
The results of the four ion implantations into
alumina substrates, summarized in Fig. 18, demonstrate that particle formation via substrate reduction may be understood by examination of the
thermodynamic free energies of the implanted element with the substrate material.
Implantation of Ca into single-crystalline
alumina under the stated conditions results in the
amorphization and reduction of the alumina substrate material. The free Al created by the reduction is clustered together in crystalline particles
with an average size of 08 nm. The small size of
these particles relative to the particles seen in substrates implanted with Y (average size 08 nm)
may be partially explained by the examination of
Fig. 16. Relative optical absorption spectra from Cr+ asimplanted and annealed silica substrates showing the
development of an absorption feature during the annealing
process.
1508 HUNT and HAMPIKIAN: ION IMPLANTATION-INDUCED NANOSCALE PARTICLE FORMATION
Fig. 17. Bright ®eld TEM of a Cr+-implanted silica substrate and the electron di€raction pattern associated with the particles present in the matrix.
4.2. Volume fraction
As a ®nal check on the validity of the proposed
mechanism, the total experimental volume fraction
of Al particles present in the implanted substrates
has been calculated (VFED) and compared to the
volume fraction of Al that is theoretically possible
(VFT), assuming that each incident ion produces reduction of the substrate and that all of that Al is
consolidated into particles [47]. It should not be
possible for the ratio of these volume fractions
(R = VFED/VFT) to exceed unity within the framework of the proposed mechanism. The ratio, R, of
the experimental and theoretical volumes of metallic
Al may be calculated using particle size measurements from EFTEM images and certain parameters
of the ion implantation as determined by
PROFILE:
2 b 3
X 4
3
Ni 7
pR
6
6 iˆ1 3 i
7
6
7
6 Rp …Area† 7
4
5
Rˆ
VFED
ÿ
ˆÿ
VFT
DoseR = DensityAl
ÿ Rp
3†
where VFED is the experimentally determined particle volume fraction, Ri is the radius of the ith
bin, Ni is the number of particles whose radius
lies within the radius range of the ith sorting bin,
b is the number of 2 nm bins required to sort particle sizes, Rp is the projected range of implanted
ions as determined by PROFILE, VFT is the
maximum theoretical particle volume fraction,
DoseR is the retained ion dose as determined by
PROFILE, DensityAl is the atomic density of
f.c.c. Al, and Area is the area measured of the
image.
The term in the denominator, the maximum
theoretical volume fraction of Al, assumes that
100% of the implanted ions react with the substrate
to form free Al and that the Al particles formed
have the standard lattice parameter. The numerator,
the experimentally determined volume fraction,
relies on 15 eV energy-®ltered images, used because
these images show contrast from all the consolidated metallic Al, not just those particles which
may be di€racting for a given imaging condition.
The following is an example of the volume fraction
calculation for the 150 keV, 5 1016 ions/cm2 implantation:
ÿ
ÿ
3:52 10ÿ23 m3 = 50:2 10ÿ9 m 9:31 10ÿ15 m2
0:075
ˆ 0:50
Rˆ
ˆ
ÿ
0:149
4:51 1020 ions=m2 =6:02 1028 atoms=m3
50:2 10ÿ9 m
4†
HUNT and HAMPIKIAN: ION IMPLANTATION-INDUCED NANOSCALE PARTICLE FORMATION 1509
Fig. 18. Summary of ion implantations into alumina.
A similar calculation performed for the 50 keV,
5 1016 Ca+/cm2 implantation yields R = 0.31.
These calculations show, in both cases, that the
ratio does not exceed unity. This indicates that it is
possible for the reported doses of implanted ions to
produce the amount of free Al required to form the
particles imaged in these substrates, which is consistent with the proposed mechanism. In addition,
the smaller ratio produced for the Ca implantations
is consistent with the smaller expected amount of
free Al produced by the implantation of this monoxide-former compared to the Y implantations, as
discussed earlier.
The formation of MgAl2O4 spinel particles in the
Mg-implanted alumina is not an unexpected result
when the formation of the spinel, MgAl2O4 is considered thermodynamically (Fig. 1). There are two
reaction routes through which spinel particles may
be formed. The ®rst involves the reduction of the
Al2O3 by the Mg ions and the subsequent reaction
of the resulting MgO with the remaining Al2O3 to
form MgAl2O4. These separate reactions have free
energies of ÿ127.3 kJ/mol and ÿ22.4 kJ/mol, respectively. The second route is a reaction between
the Mg and Al2O3 resulting directly in the formation of MgAl2O4 and a small amount of excess
Al. This reaction has a free energy of ÿ194.5 kJ/
mol. Either reaction involves reduction of the
alumina substrate, consistent with the thermodynamic model. The failure of the Cr+ and Si+
implants to reduce the alumina is also consistent
with the thermodynamic model. The examination of
the free energies of the reduction reactions predicts
this outcome in both cases. This ®nding is consistent
with
the
work
of
other
researchers [10, 23, 48, 49].
A ®nal discussion point involving particle formation via reduction in the alumina substrates concerns the accompanying amorphization which
occurs with Al particle formation. For all systems
examined in which substrate reduction occurs, the
matrix which surrounds the Al particles is amorphous. Conversely, for control samples which do
not produce substrate reduction, amorphization is
not seen in the particle-bearing region. In this
study, the implantation of Y and Ca into alumina
has been documented to cause the formation of
crystalline Al nanoparticles embedded in an amorphous alumina matrix which also contains the
implant ion. However, the implantation of Cr and
Si, although imparting radiation damage which is
comparable to the Y and Ca implants, does not
result in the amorphization of the alumina. Thus, it
appears that these processes, the reduction and the
amorphization, are linked in some manner; perhaps
the reduction process and the subsequent clustering
of Al cause the substrate to lose its long-range
order.
4.3. Silica substrates
The ion implantation of 1 1017 Zr+/cm2 into
silica substrates did not result in particle formation
in the substrate according to optical absorption
measurements. The failure to produce Si particles
with these ambient-temperature implantations may
be explained by the very low di€usivity of Si in
1510 HUNT and HAMPIKIAN: ION IMPLANTATION-INDUCED NANOSCALE PARTICLE FORMATION
SiO2. Previous research has shown that temperatures in excess of 900±10008C are required to cause
free Si (which is present as a result of ion implantation) in a silica substrate to form particles [35±
37, 44]. Post-implantation annealing treatments at
11008C in Ar + 4% He for 1 h does result in particle formation; however, the particles which form
are silicide in nature, i.e. ZrSi2. The ZrSi2 formation
may be explained by considering a two-step reaction. First, Zr must reduce silica to form free Si:
Zr ‡ SiO2 ˆ Si ‡ ZrO2 :
5†
Second, the free Si can then react with free Zr to
form ZrSi2:
2Si ‡ Zr ˆ ZrSi2 :
6†
The free energies for these reactions are depicted as
a function of temperature in Fig. 13.
5. SUMMARY
The experiments presented in this research
demonstrate the importance of equilibrium thermodynamics in determining the end state of ionimplanted materials. The implantation experiments
with alumina substrates show that the ions for
which the free energy of reduction is positive do
not result in reduction of the alumina substrate.
The prediction of reduction of Al2O3 by Y+, Ca+
and Mg+ are all accurate when the very low-energy
MgAl2O4 spinel phase is considered. The control
implants of Cr+ and Si+ do not form Al particles
in Al2O3, as expected.
The results of the experiments with silica substrates also support the proposed reduction mechanism. The very low di€usivity of Si+ in silica
complicates the particle-formation process, and
makes a post-implantation anneal necessary. The Zr
implantation, which is expected to result in silica reduction, assuming equilibrium, results in the formation of a Si±Zr alloy only after the postimplantation anneal. The control implantation of
Cr+, predicted to cause no substrate reduction,
results in the simple precipitation of Cr upon
annealing, as expected. Overall, both the alumina
and silica substrate implantations support the use
of equilibrium thermodynamics to predict the end
states resulting from the relaxation of the collision
cascade. Finally, this research has demonstrated
that a host cation can be forced into a precipitate
di€erent from the original matrix.
AcknowledgementsÐThis work has been supported by the
National Science Foundation under Grant No. DMR9624927. The authors wish to acknowledge the assistance
and support rendered by N.D. Evans and D.B. Poker, at
the Oak Ridge Institute for Science and Technology and
the Oak Ridge National Laboratory, respectively. The
support of the SHARE Program under contract DEAC05-76OR00033 with Oak Ridge Associated Universities
and that of the U.S. Department of Energy, under con-
tract DE-AC05-96OR22464 with Lockheed Martin Energy
Research Corp. is acknowledged.
REFERENCES
1. White, C.W., Zhou, D.S., Budai, J.D., Zuhr, R.A.,
Magruder, R.H. and Osborne, D.H., Mater. Res. Soc.
Symp. Proc., 1994, 316, 499.
2. Mouritz, A.P., Sood, D.K., St. John, D.H., McSwain,
M.V. and Williams, S.J., Nucl. Instrum. Meth., 1987,
B19/20, 805.
3. Haglund, R.F. Jr, Yang, L., Magruder, R.H., White,
C.W., Zuhr, R.A., Yang, L., Dorsinville, R. and
Alfano, R.R., Nucl. Instrum. Meth., 1994, B91, 493.
4. Allegre, J., Arnaud, G., Mathieu, H., Lefebvre, P.,
Granier, W. and Bondes, L., J. Crystal Growth, 1994,
138, 998.
5. Magruder III, R.H., Haglund, R.F., Yang, L., White,
C.W., Yang, L., Dorsinville, R. and Alfano, R.R.,
Appl. Phys. Lett., 1993, 62, 465.
6. Freire, F.L., Broll, N. and Mariotto, G., Mater. Res.
Soc. Symp. Proc., 1996, 396, 385.
7. Haglund, R.F., Yang, L., Magruder III, R.H., Wittig,
J.E., Becker, K. and Zuhr, R.A., Opt. Lett., 1993, 18,
373.
8. Farlow, G.C., Sklad, P.S., White, C.W. and
McHargue, C.J., J. Mater. Res., 1990, 5(7), 1502.
9. Sklad, P.S., McHargue, C.J., White, C.W. and
Farlow, G.C., J. Mater. Sci., 1992, 27(21), 5895.
10. Ohkubo, M., Hioki, T. and Kawamoto, J., J. appl.
Phys., 1986, 60(4), 1325.
11. White, C.W., Budai, J.D., Zhu, J.G., Withrow, S.P.,
Hembree, D.H., Henderson, D.O., Ueda, A., Tung,
Y.S. and Mu, R., Mater. Res. Soc. Symp. Proc., 1996,
396, 377.
12. Hunt, E.M. and Hampikian, J.M., J. Mater. Sci.,
1997, 32, 3393.
13. Burnett, P.J. and Page, T.F., Radiat. E€ects, 1986, 97,
3524.
14. Burnett, P.J. and Page, T.F., J. Mater. Sci., 1984, 19,
3524.
15. Hunt, E.M., Hampikian, J.M. and Evans, N.D.,
Microsc. Microanal., 1996, 96, 534.
16. Hunt, E.M., Hampikian, J.M. and Evans, N.D.,
Microsc. Microanal., 1997, 97, 413.
17. Hunt, E.M., Wang, Z.L., Evans, N.D. and
Hampikian, J.M., Microsc. Microanal., 1997, 97, 1001.
18. Hunt, E.M., Wang, Z.L., Evans, N.D. and
Hampikian, J.M., Micron, 1998, 29(2-3), 191.
19. Bohren, C.F. and Hu€man, D.R., Absorption and
Scattering of Light by Small Particles. John Wiley,
New York, 1983, chap. 9.
20. Hughes, A.E. and Jain, S.C., Adv. Phys., 1979, 28(6),
717.
21. Doyle, W.T., Phys. Rev., 1958, 111(4), 1067.
22. PROFILE Ion Implantation Code, Implant Sciences
Corp., 35 Cherry Hill Drive, Danvers, MA 09123,
U.S.A.
23. White, C.W., Farlow, G.C., McHargue, C.J., Sklad,
P.S., Angelini, M.P. and Appleton, B.R., Nucl. Inst.
Meth., 1985, B7/8, 473.
24. Atwater, H.A., Shcheglov, K.V., Wong, S.S., Vahala,
K.J., Flagan, R.C., Brongersma, M.L. and Polman,
A., Mater. Res. Soc. Symp. Proc., 1994, 316, 409.
25. Tyschenko, I.E., Kachurin, G.A., Zhuravlev, K.S.,
Pazdnikov, N.A., Volodin, V.A., Gutakovsky, A.K.,
Leier, A.F., FroÈb, H., Leo, K., BoÈhme, T., Rebohle,
L., Yankov, R.A. and Skorupa, W., Mater. Res. Soc.
Symp. Proc., 1997, 438, 453.
26. Pan, Z., Morgan, S.H., Henderson, D.O., Magruder,
R.H. and Zuhr, R.A., Nucl. Instrum. Meth., 1996,
B114(3-4), 281.
HUNT and HAMPIKIAN: ION IMPLANTATION-INDUCED NANOSCALE PARTICLE FORMATION 1511
27. Magruder, R.H., Weeks, R.A., Anderson, T.S. and
Zuhr, R.A., Mater. Res. Soc. Symp. Proc., 1997, 438,
429.
28. Battaglin, G., Nucl. Instrum. Meth., 1996, B116(1-4),
102.
29. Haglund, R.F., Mogul, H.C., Weeks, R.A. and Zuhr,
R.A., J. Non-Cryst. Solids, 1991, 130, 326.
30. Magruder III, R.H., Weeks, R.A., Zuhr, R.A. and
Whichard, G., J. Non-Cryst. Solids, 1991, 129, 46.
31. Perez, A., Treilleux, M., Capra, T. and Griscom,
D.L., J. Mater. Res., 1987, 2(6), 910.
32. Anderson, T.S., Magruder, R.H., Weeks, R.A. and
Zuhr, R.A., J. Non-Cryst. Solids, 1996, 203, 114.
33. Bertoncello, R., Glisenti, A., Granozzi, G., Battaglin,
G., Caccavale, F., Cattaruzza, E. and Mazoldi, P., J.
Non-Cryst. Solids, 1993, 162, 205.
34. Hosono, H., Japan. J. appl. Phys., 1993, 32(9A), 3892.
35. Zhu, J. and White, C.W., J. appl. Phys., 1995, 78,
4386.
36. White, C.W., Budai, J.D., Withrow, S.P., Zhu, J.G.,
Pennycook, S.J., Zuhr, R.A., Hembree, D.M. Jr,
Henderson, D.O., Macgruder, R.H., Yacaman, M.J.,
Mondragon, G. and Prawer, S., Nucl. Instrum. Meth.,
1997, B127-128, 545.
37. Cheong, H.M., Paul, W., Withrow, S.P., Zhu, J.G.,
Budai, J.D., White, C.W. and Hembree, D.M. Jr,
Appl. Phys. Lett., 1996, 68(1), 87.
38. Mutti, P., Ghislotti, G., Bertoni, S., Bonoldi, L.,
Cerofolini, G.F., Meda, L., Grilli, E. and Guzzi, M.,
Appl. Phys. Lett., 1995, 66(7), 851.
39. Shimizu-Iwayama, T., Nakao, S. and Saitoh, K., Nucl.
Instrum. Meth., 1997, B121, 450.
40. Shimizu-Iwayama, T., Nakao, S. and Saitoh, K., Appl.
Phys. Lett., 1994, 65(14), 1814.
41. Shimizu-Iwayama, T., Nakao, S., Saitoh, K. and Itoh,
N., J. Phys.: Condens. Matter, 1994, 6, L601.
42. Kachurin, G.A., Tyschenko, I.E., Zhuravlev, K.S.,
Pazdnikov, N.A., Volodin, V.A., Gutakovsky, A.K.,
Leier, A.F., Skorupa, W. and Yankov, R.A., Nucl.
Instrum. Meth., 1997, B122, 571.
43. Kachurin, G.A., Zhuravlev, K.S., Pazdnikov, N.A.,
Leier, A.F., Tyschenko, I.E., Volodin, V.A., Skorupa,
W. and Yankov, R.A., Nucl. Instrum. Meth., 1997,
B127-128, 583.
44. Nesbit, L.A., Appl. Phys. Lett., 1985, 46(1), 38.
45. Hughes, A.E., Radiat. E€ects, 1983, 74, 57.
46. Shikama, T. and Pells, G.P., Phil. Mag. A, 1983,
47(3), 369.
47. Hunt, E.M. and Hampikian, J.M., manuscript in preparation.
48. Townsend, P.D., Rep. Prog. Phys., 1987, 50, 501.
49. White, C.W., Budai, J.D., Withrow, S.P., Pennycook,
S.J., Hembree, D.M., Zhou, D.S., Vo-Dihn, T. and
Magruder, R.H., Mater. Res. Soc. Symp. Proc., 1994,
316, 487.