Implantation-Induced Nanocavities and Au
Nanoparticles in Si and SiO2
Supakit Charnvanichborikarn†, James S. Williams, and Martin J. Conway
Department of Electronic Materials Engineering
Research School of Physical Sciences & Engineering
The Australian National University
Email: †spc109@rsphysse.anu.edu.au
Abstract—In this study, implantation-induced nanocavities have
been formed in Si by H irradiation and annealing. Further Si
irradiation was conducted in selected samples to shrink the cavity
size and narrow the size distribution. Au was introduced into the
near surface region with a dose of 1×1015 cm−2 followed by an
anneal at 900-950°C to successfully precipitate Au in the cavities.
The annealing caused limited Oswald ripening of precipitates.
Wet oxidation and formation of a buried oxide (BOX) were
then performed to encapsulate precipitates in an SiO2 layer.
Results indicate that it is possible to form Au nanoparticles at a
precise depth in SiO2 but it is difficult by the present methods to
control their size.
Keywords-component; implantation-induced nanocavity, Au
nanoparticle
I.
INTRODUCTION
A thin layer of nanoparticles in particular substrates can
exhibit a specific direct band gap and when it is electrically or
optically stimulated may emit light of a particular wavelength
depending on nanparticle size. In this paper, we examine Au
nanoparticles in SiO2 and investigate methods of generating a
narrow band of such particles of controlled size. Recently,
implantation-induced cavities in Si have been reported to be
particularly efficient sinks for several metallic impurities such
as Au, Cu, and Fe [1]. Cavities can be filled with some species
(such as Au) and the resultant precipitate size is constrained to
the original cavity size. The work of Ruault el al. [2] has
further demonstrated the shrinking of cavities by Si
implantation. This paper uses precipitation in cavities to first
obtain a band of controlled size precipitates in Si. Various
methods are then tried to embed such nanoparticles in SiO2
including direct implantation into SiO2 and wet oxidation of Si
containing Au precipitates.
II.
EXPERIMENTS
Two types of sample have been used in this study: <100>
Boron doped Czochralski (CZ) silicon and SIMOX
(Separation by Implantation of Oxygen), whose Si and SiO2
thicknesses are 180 nm and 200 nm, respectively. H and Au
were implanted into a Si sample (A) to a dose of 3×1016cm−2
and 1×1015cm−2 with energies of 12 and 80 keV, respectively,
followed by annealing at 850°C for 30 minutes. Au was
successfully gettered into the cavities to form Au or Au
silicide precipitates. For Ostwald ripening studies, Si samples
(B and C) were implanted with 15 keV H and 80 keV Au to
the same doses as sample A. Both samples were annealed at
850°C for 1 hour. Sample C was further annealed in Ar flow at
900°C for 6.5 hours. Implantation of 125 keV O was also
performed into Si (D) at 500°C to a dose of 1.8×1018cm−2 to
create a BOX layer to a depth of 284 nm. Sample D was
subsequently implanted with H and Au to the same doses and
energies as sample A followed by annealing to form Au
nanoprecipitates in the Si region. Wet oxidation was
performed for 6.5 hours to create a SiO2 layer on Si (E).
Sample E was afterwards implanted with 12 keV H and 100
keV Au to the same doses as A and annealed. SIMOX samples
(F and G) were implanted with 10 keV H (at 60° incidence for
sample G) to the same dose as sample A followed by
annealing at 850°C for 30 minutes and then irradiation of Au
to the same dose as sample A. Samples F and G were further
annealed in Ar flow at 950°C for 1 hour. Finally, all samples,
excluding E, were wet oxidised at 900°C from 3 to 8 hours.
III.
1.
RESULTS AND DISCUSSION
Samples A, B and C
Figure 1: XTEM images of samples B (left) and C (right)
For sample A, the effect of oxidation was to segregate the
precipitates behind the growing oxide so that they were not
incorporated into SiO2. Hence, direct oxidation does not work
as a means of incorporating Au nanoparticles into SiO2.
For samples B and C, Figure 1 illustrates that the number
of small precipitates was significantly reduced after an
extended 6.5 hours anneal at 900°C, while the overall sizes of
precipitates and cavities slightly increase, explained by
Ostwald ripening. Hence, it can be concluded that long
annealing does not significantly affect the size of large
precipitates. Only the number of small precipitates is reduced
and the size distribution is significantly narrowed.
2.
Sample D
From fig. 2, a narrow band of Au was formed between the two
oxide layers (the underlying BOX layer and the surface
thermal oxide) in sample D. However, the original shape of
precipitates is significantly changed during thermal oxidation.
We believe that this occurred as a result of the indistinct
boundary between the BOX layer and overlaying Si (initially a
SixOy layer before oxidation) and the fact that Au silicide
precipitates are molten above 363 °C.
IV.
CONCLUSIONS
This study shows that Au nanoprecipitates can be formed in Si
by decoration of cavities to provide a tight size distribution. It
is possible to imbed these precipitates in SiO2 but further work
is needed to optimise the process.
Figure 3: RBS spectrum of sample E
Figure 2: RBS spectrum of sample D
3.
Sample E
As shown in fig. 3, Au particles in SiO2 were not relocated to
cavities after annealing in sample E. XTEM confirms the
RBS result and shows a wide band of Au precipitates and a
large distribution of precipitate size. We note that Ntsoenzok
et al. [3] showed that, regardless of implantation temperature,
forming cavities in SiO2 by He ion irradiation is impossible.
This was ascribed to the very high mobility of He in SiO2. We
believe the same is true for H-implantation into SiO2.
4.
Figure 4: XTEM images of sample F reveal incomplete oxide layer
formation and semi-spherical precipitates on the front interface of BOX
layer. The samples were oxidized for 3 hrs (left), 6 hrs (middle), and 8
hrs (right).
Samples F and G
SIMOX sample (F) was used to obtain a more uniform BOX
layer but unusual precipitate shapes also occur, presumably as
a result of Au silicide formation close to the SIMOX interface
in this case and wetting phenomena on SiO2/Si surfaces.
To try to overcome this, in sample G, Au silicide
precipitates were formed away from the SIMOX interface
before thermal oxidation. Fig. 5 shows that precipitates retain
their circular shape but there still remains a thin Si layer
between the two oxides.
The results from samples D, F, and G suggest that an
alternative low temperature oxidation should be employed in
order to retain the shape and size of precipitates.
Figure 5: XTEM images of sample E before oxidation (left)
and after oxidation (right).
REFERENCES
[1] Y.-Y. J.Wong-Leung, “The Gettering of Metals in Silicon to Defects
Induced by Ion Implantation,” Ph.D. dissertation, 1997.
[2] M.-O. Ruault, F. Fortuna, H. Bernas, M. C. Ridgway, and J. S. Williams,
“How nanocavities in amorphous Si shrink under ion beam irradiation: An in
situ study,” Applied Physics Letters, vol. 81, no. 14, 2002.
[3] E. Ntsoenzok, H. Assaf, and M. Ruault, “Bubbles and cavities induced by
rare gas implantation in silicon oxide,” Mater. Res. Soc. Symp. Proc., vol.
864, 2005.