Formation of Sn nanocrystals in thin SiO2 film using low-energy ion implantation

Article abstract of DOI:10.1063/1.120470

This letter reports on a simple technique for fabricating Sn nanocrystals in thin SiO₂ film using low-energy ion implantation followed by thermal annealing. These Sn nanocrystals have excellent size uniformity and position controllability. Their average diameter is 4.8 nm with a standard deviation of 1.0 nm. Most of the Sn nanocrystals reside at the same depth. The lateral edge-to-edge facing between neighboring Sn nanocrystals is fairly constant: about nm. A narrow as-implanted ion distribution profile and the effect of the SiO₂-Si interface are considered to contribute to the size uniformity and position controllability. The features of these nanocrystals will open up new possibilities for novel devices.

Full text of DOI:10.1063/1.120470

Formation of Sn nanocrystals in thin SiO 2 film using low-energy ion implantation
Anri Nakajima, Toshiro Futatsugi, Naoto Horiguchi, and Naoki Yokoyama
Citation: Applied Physics Letters 71, 3652 (1997); doi: 10.1063/1.120470
View online: http://dx.doi.org/10.1063/1.120470
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Formation of Sn nanocrystals in thin SiO2 film using low-energy ion
implantation
Anri Nakajima,a) Toshiro Futatsugi, Naoto Horiguchi, and Naoki Yokoyama
Fujitsu Laboratories Limited, 10-1 Morinosato–Wakamiya, Atsugi 243-01, Japan
͑Received 20 March 1997; accepted for publication 21 October 1997͒
This letter reports on a simple technique for fabricating Sn nanocrystals in thin SiO2 film using
low-energy ion implantation followed by thermal annealing. These Sn nanocrystals have excellent
size uniformity and position controllability. Their average diameter is 4.8 nm with a standard
deviation of 1.0 nm. Most of the Sn nanocrystals reside at the same depth. The lateral edge-to-edge
spacing between neighboring Sn nanocrystals is fairly constant: about 3 nm. A narrow as-implanted
ion distribution profile and the effect of the SiO2–Si interface are considered to contribute to the size
uniformity and position controllability. The features of these nanocrystals will open up new
possibilities for novel devices. © 1997 American Institute of Physics. ͓S0003-6951͑97͒01451-4͔
Metal nanocrystals are expected to exhibit unique physi-
cal properties due to the single electron effect1 and their
large nonlinear optical properties.2–4 The possibility of using
these structures not only for studying various aspects of
physics but also for creating new devices is stimulating much
research in this field. To create such devices, we need precise
control over the size and position of the nanocrystals. Fur-
thermore, the ideal technique would be compatible with con-
ventional large-scale integration device fabrication methods.
This way, we could use a proven, mature Si technique to
further investigate the possibilities of new devices.
Metal or semiconductor nanocrystals in barrier materials
with low dielectric constants have the advantage of exhibit-
ing the single electron effect at high temperatures, since the
total capacitance of the dots decreases in such materials. The
advantage of using metal instead of semiconductor nanocrys-
tals is that a large number of electrons can enter metal nano-
crystals because the energy separation of the quantum levels
in such nanocrystals is negligible. This negligible quantum-
level separation also results in a constant addition energy. In
addition, metal nanocrystals embedded in dielectric materials
exhibit a large third-order susceptibility and a picosecond
response time. Their structure is also favorable for applica-
tions to optical waveguides and other optical devices.2–4
Ion implantation is a promising fabrication technique for
forming nanocrystals in dielectric materials. This technique
can enable the controlled depth distribution of a desired spe-
cies and is extensively used in semiconductor technology. To
our knowledge, the formation of Cu,5 Ag,6,7 Au,4 and Fe
͑Ref. 8͒ metal nanocrystals in implanted glass has so far been
reported.
substrate was heavily doped with Sb. The calculated pro-
jected range was at the center of the SiO2 layer, and the
projected standard deviation was 2.0 nm. Subsequent thermal
annealing was carried out in a N2 ambient at 900 °C for 10
min.
We studied the size and position of Sn dots using trans-
mission electron microscopy ͑TEM͒ with a Hitachi H-
9000NAR operating at 300 kV. The chemical composition of
the dots was determined with a field-emission analytical
electron microscope operating at an acceleration voltage of
200 kV. An energy dispersive x-ray ͑EDX͒ spectrometer
with a solid angle of 0.12 sr was attached to the microscope.
The Sn dots were analyzed using an electron beam about 1
nm in diameter.
Figure 1 shows a cross-sectional TEM micrograph of
nanoscale Sn dots in a thin SiO2 layer formed by low-energy
implantation. It is evident that spherical Sn dots with excel-
lent size uniformity exist in the thin SiO2 layer. The average
diameter of 28 measured dots was 4.8 nm with a standard
deviation of 1.0 nm. The Sn nanocrystals’ position shifted
slightly from the center of the thin SiO2 layer to the SiO2–Si
interface. Figure 2 is a high-resolution TEM micrograph of
the Sn dots. Clear lattice images indicate that the Sn dots are
indeed crystalline.
To confirm the chemical composition of these nanocrys-
tals, we took EDX measurements ͑Fig. 3͒. In Fig. 3, the
circles represent measurements on the core region of the
nanocrystals, and the squares are those on a region far from
the nanocrystals. The obvious Sn peaks at the core region
In this letter, we report on a fascinating method to fab-
ricate metal nanocrystals in extremely thin SiO2 layers by the
low-energy ion implantation of Snϩ ions followed by ther-
mal annealing. With this method, we formed an array of Sn
nanocrystals with excellent size uniformity and position con-
trollability.
The low-energy implantation of Snϩ ions was done at an
energy of 10 keV and a dose of 5ϫ1015 ions/cm2 to a 15 nm
thick SiO2 layer thermally grown on a Si substrate. The Si
FIG. 1. Cross-sectional TEM micrograph of Sn nanocrystals in a thin SiO2
layer fabricated using low-energy implantation.
a͒
Electronic mail: nakajima@qed.flab.fujitsu.co.jp
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FIG. 2. High-resolution cross-sectional TEM micrograph of Sn nanocrystals
in a thin SiO2 layer fabricated using low-energy implantation.
FIG. 4. EDX line analysis for a sample with Sn nanocrystals in a thin SiO2
layer. An electron beam intersected both the Sn nanocrystal in the SiO2 layer
and that at the SiO2–Si interface. Si intensity was monitored as a reference.
SiO2–Si interface position is shown by the bar.
reveal that the nanocrystals include Sn ions. The almost
identical peak ratio of Si to O in both regions indicates that
most of the oxygen combined with Si in both regions. This
implies that hardly any oxygen combined with Sn ions and
suggests that the formed nanocrystals are metal Sn.
surement, an electron beam scanned along the depth direc-
tion with the EDX intensity being monitored as a function of
the depth position. The scanned line intersected both the Sn
nanocrystal in the SiO2 layer and that at the SiO2–Si inter-
face. As a reference, the signal intensity of Si was also moni-
tored. In Fig. 4, the position of the SiO2–Si interface is
shown by the bar. The intensity of Si essentially increases
with depth, as indicated by the broken lines. The slopes of
this line for the SiO2 region near the surface and that for the
Si substrate region are almost the same. This increase of
intensity is considered to correspond to the thickness of the
sample, which increases with depth. In spite of this thickness
increase with depth, the intensity of Sn is reduced to the
noise level in the Si substrate region below about 2 nm from
the SiO2–Si interface. This indicates that the Sn concentra-
tion in this region is almost negligible. On the contrary, in
the SiO2 layer, strong Sn peaks were observed in the region
whose center depth had slightly shifted from the center of the
SiO2 layer to the interface. This is consistent with the posi-
tion of Sn nanocrystals in Figs. 1 and 2. It should be noted
that we can see a small but nevertheless existing Sn peak
occurs around the SiO2–Si interface region. This indicates
that a small amount of Sn exists at this interface region.
However, this existing Sn region is limited to within about a
2 nm depth from the interface. Judging from the lattice im-
age of the dots at the interface in the TEM micrographs of
Figs. 1 and 2, Sn ions may have formed an alloy with Si at
the interface of the Si substrate. For the above-mentioned
reasons, we can safely say that an excellent depth control of
Sn nanocrystals was achieved by our process.
To our knowledge, the size uniformity of such fabricated
Sn nanocrystals is the best among those ever reported for
metal nanocrystals. For example, Ag nanocrystals formed in
glass using ion implantation with 60 keV at a dose of 4
ϫ1016/cm2 have an average diameter of 4.2 nm with a stan-
dard deviation of 3.7 nm.7 For further comparison purposes,
we referred to the reported average diameter of 9.8 nm and
the standard deviation of 3.4 nm for Si dots fabricated by
electron-beam lithography and subsequent dry and wet
etchings.9 The standard deviation of the Sn dots fabricated in
this study was better than that of such artificially fabricated
Si dots. Furthermore, the technique we propose offers excel-
lent control over dot positioning. Although we can observe
the existence of nanoscale dots at the SiO2–Si interface in
Figs. 1 and 2, most of the Sn nanocrystals in the SiO2 layer
reside at the same depth. In the lateral direction, the edge-to-
edge spacing between neighboring Sn nanocrystals in the
SiO2 layer is also fairly constant: about 3 nm.
We used EDX line analysis to investigate the distribu-
tion of Sn ions after thermal annealing ͑Fig. 4͒. In this mea-
It should be pointed out that the slope of the broken line
for the SiO2 layer near the SiO2–Si interface is larger than
those of the other two regions. This indicates that the Si
concentration in the SiO2 layer near the interface, when nor-
malized by the thickness of the sample, is larger than that in
the SiO2 layer near the surface and smaller than that in the Si
substrate. This suggests the possible existence of a transition
region where the bonding changes from SiO to SiO2. It is
reported10 that the thermal oxide is under compression due to
FIG. 3. EDX spectra from the core region of Sn dots ͑circles͒ and from a
region far from the Sn dots ͑squares͒ in a thin SiO2 layer.
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Appl. Phys. Lett., Vol. 71, No. 25, 22 December 1997 Nakajima et al. 3653
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the density mismatch between Si and SiO2 in the transition
region.
region near the SiO2–Si interface. This will lead to an excel-
lent size and position uniformity of the formed Sn nanocrys-
tals. The escape of Sn ions from the SiO2 layer into the
gaseous ambient may also occur in the thin SiO2 sample.
This lowers the Sn concentration near the surface of the SiO2
and contributes to maintaining the narrow ion distribution
during thermal annealing.
Another possible reason, which would lead to the size
uniformity of Sn nanocrystals, is that the compressive strain
that exists in the SiO2 near the SiO2–Si interface due to the
density mismatch may be reduced by the forming of Sn
nanocrystals in that region. An appropriate nanocrystal size
that minimizes the strain energy will then exist. This effect
will also be closely related to the distribution of Sn nano-
crystals. The lateral and depth positions of Sn nanocrystals
will be those that minimize the strain energy.
In summary, we developed a simple technique for fabri-
cating Sn nanocrystals in thin SiO2 layers using low-energy
ion implantation followed by thermal annealing. Sn nano-
crystals formed by this method have excellent size unifor-
mity and position controllability. Structures consisting of
metal nanocrystals in an extremely thin SiO2 layer offer the
possibility of developing new electronic devices such as
high-temperature single electron tunnel diodes. A high
throughput in fabricating nanoscale structures is another at-
tractive feature offered by using this technique.
To investigate the effect of the SiO2–Si interface on the
size and position distribution of Sn nanocrystals, we took a
cross-sectional TEM micrograph of a thick thermally grown
SiO2 sample ͑0.5 ␮m͒ that was Sn implanted and subse-
quently annealed. The Sn implantation and annealing condi-
tions were the same as those for the thin SiO2 sample. A
trace of Sn dots can also be seen in the cross-sectional TEM
micrograph for the thick SiO2 sample. However, the size of
the Sn dots was much smaller ͑a typical diameter of 2 nm͒
and they were dispersed more along the depth direction
͑from about 10 nm to 50 nm from the surface͒ when com-
pared with those in the thin SiO2 sample. This dispersion, in
turn, indicates that the transfer of Sn ions from the SiO2 layer
to the Si substrate is suppressed in the thin SiO2 sample.
Several possible reasons can account for this suppres-
sion. One is that the diffusion of Sn ions is much faster in
SiO2 than in Si. Another is that the segregation coefficient m
of Sn is much smaller than unity, where m equals the impu-
rity equilibrium concentration in Si divided by that in SiO2.
A final reason is that the diffusion of Sn ions in the SiO2
layer near the SiO2–Si interface is much slower due to the
compressive strain in that region. Considering the gradual
decrease of Sn concentration with depth in the SiO2 layer
near the interface ͑Fig. 4͒, this last reason is the most plau-
sible since the former two reasons lead to an abrupt change
of Sn concentration at the SiO2–Si interface.
The authors thank Dr. Y. Tosaka for his helpful discus-
sions and Dr. T. Ito for his encouragement.
Also, in the cross-sectional TEM micrograph for the
thick SiO2 sample, no trace of Sn dots is observed in the
region from the surface to about 10 nm in depth. This sug-
gests that Sn ions are depleted in this region. This Sn deple-
tion is thought to be due to the escape of some Sn ions from
the SiO2 layer into ambient gas.
A number of comments can be made regarding the ex-
tremely narrow size and position distribution of Sn nano-
crystals in the thin SiO2 layer. It has been reported11 that the
diameter of Cu nanocrystals in SiO2 glass increases linearly
with the implanted Cu concentrations. Although subsequent
thermal annealing was not carried out in that report, the re-
sults suggest the strong correlation between nanocrystal size
and as-implanted ion concentration even when subsequent
annealing has been done. For thin SiO2 layers, a narrow as-
implanted ion distribution can be achieved with low-energy
ion implantation and maintained sufficiently during thermal
annealing due to the slow diffusion of Sn ions in the SiO2
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3654 Appl. Phys. Lett., Vol. 71, No. 25, 22 December 1997 Nakajima et al.
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