Fabrication of a nanometric Zn dot by nonresonant near-field optical chemical-vapor deposition

Article abstract of DOI:10.1063/1.1394955

We demonstrate a technique for the deposition of nanometric Zn dots by photodissociation of gas-phase diethylzinc using an optical near field under nonresonant conditions. The observed deposited Zn dot was less than 50 nm in size. The photodissociation mechanisms are based on the unique properties of optical near fields, i.e., enhanced two-photon absorption, induced near-field transition, and a direct excitation of the vibration-dissociation mode of diethylzinc.

Full text of DOI:10.1063/1.1394955

Fabrication of a nanometric Zn dot by nonresonant near-field optical chemical-vapor
deposition
Tadashi Kawazoe, Yoh Yamamoto, and Motoichi Ohtsu
Citation: Applied Physics Letters 79, 1184 (2001); doi: 10.1063/1.1394955
View online: http://dx.doi.org/10.1063/1.1394955
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APPLIED PHYSICS LETTERS
VOLUME 79, NUMBER 8
20 AUGUST 2001
Fabrication of a nanometric Zn dot by nonresonant near-field optical
chemical-vapor deposition
Tadashi Kawazoea)
Exploratory Research for Advanced Technology, Japan Science and Technology Corporation,
687-1 Tsuruma, Machida, Tokyo 194-0004, Japan
Yoh Yamamoto and Motoichi Ohtsub)
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
͑Received 16 May 2001; accepted for publication 27 June 2001͒
We demonstrate a technique for the deposition of nanometric Zn dots by photodissociation of
gas-phase diethylzinc using an optical near field under nonresonant conditions. The observed
deposited Zn dot was less than 50 nm in size. The photodissociation mechanisms are based on the
unique properties of optical near fields, i.e., enhanced two-photon absorption, induced near-field
transition, and a direct excitation of the vibration-dissociation mode of diethylzinc. © 2001
American Institute of Physics. ͓DOI: 10.1063/1.1394955͔
Optical near fields have been applied to high-resolution
optical microscopy, high-density optical memory, atom ma-
nipulation, and so on.1 Their application to nanostructure
fabrication has the potential to make high-density
nanometric-integrated photonic devices possible.2 Recently,
we demonstrated the feasibility of chemical-vapor deposition
͑CVD͒ of Zn dots using optical near-field techniques.3–5 In
our previous research, we utilized the high spatial resolution
capability of the optical near field to deposit Zn wires with a
width of 20 nm ͑Refs. 3 and 4͒ and Zn dots 60 nm in size.5
The size of the objects deposited by conventional far-field
optical CVD techniques was found to be diffraction limited.
Conventional optical CVD utilizes a two-step process: pho-
todissociation and adsorption. For photodissociation, the far-
field light must resonate the reacting molecular gasses in
order to excite molecules from the ground to the excited
electronic state.6,7 The Frank–Condon principle claims that
this resonance is essential for excitation.6 The excited mol-
ecules then relax to the dissociation channel, and the disso-
ciated Zn atoms adsorb to the substrate surface. However, for
near-field optical CVD ͑NFO CVD͒, photodissociation can
take place even in nonresonant conditions, due to the inher-
ent properties of the optical near field. In this letter, we ex-
amine the NFO CVD of nanometric Zn dots in nonresonant
conditions. This technique makes it possible to use various
light sources and gas sources for the deposition of a variety
of nanometric materials.
sources. The fiber probe used for NFO CVD was a high-
throughput single tapered UV fiber probe, which was fabri-
cated by pulling and etching a pure silica core fiber.1 The
cone angle of the fabricated fiber probe was 30° and its apex
diameter was 30 nm. In order to investigate the deposition
effect of nonresonant far-field light, a fiber probe without the
usual metal coating, i.e., a bare fiber probe, was used for the
deposition. Therefore, the optical far field was generated by
light leaking through the circumference of the fiber probe,
while the optical near field was generated at the apex, as
shown in Fig. 1. The separation between the fiber probe and
the sapphire substrate was controlled to within several na-
nometers by using a shear-force technique.1 The laser output
power from the fiber probe was measured with a photodiode
placed behind the sapphire substrate. The sizes of the depos-
ited Zn dots were measured using a shear-force microscope.
During deposition, the partial pressure of DEZn was 100
mTorr and the total pressure in the chamber was 3 Torr.
Details of the Zn deposition procedures have been reported
in a previous work.5 It should be noted that the deposition of
Zn on the fiber probe and the resultant decrease in the effi-
ciency of optical near-field generation are negligible because
the deposition time is sufficiently short, as has been pointed
out elsewhere.5,8
Figure 1 shows the experimental setup for NFO CVD.
Ultra-high-purity argon ͑Ar͒ was used as a buffer gas and
diethylzinc ͑DEZn͒ as a reacting molecular gas source. The
second harmonic (␭ϭ244 nm) of an Arϩ laser was used as a
light source that resonates the absorption band of DEZn.7
The fundamental frequencies of Arϩ(␭ϭ488 nm) and
He–Ne(␭ϭ633 nm) lasers were used as nonresonant light
a͒
Electric mail: kawazoe@ohtsu.jst.go.jp
Also at: Exploratory Research for Advanced Technology, Japan Science
b͒
and Technology Corporation, 687-1 Tsuruma, Machida, Tokyo 194-0004,
Japan.
FIG. 1. Experimental setup for chemical-vapor deposition using an optical
near field.
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0003-6951/2001/79(8)/1184/3/$18.00 1184 © 2001 American Institute of Physics
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Appl. Phys. Lett., Vol. 79, No. 8, 20 August 2001
Kawazoe, Yamamoto, and Ohtsu
1185
FIG. 3. Cross-sectional profiles of the deposited Zn patterns. Dashed and
solid curves represent profiles along the dashed lines on the right-hand side
of Figs. 2͑a͒ and 2͑b͒, respectively.
2͑b͒. The dashed curve represents the cross-sectional profile
of the Zn dot deposited at ␭ϭ244 nm, taken across the
dashed line in Fig. 2͑a͒. These curves confirm that Zn dots
with a full width at a half maximum of 30 nm were deposited
in the region where the optical near field is dominant. The
dashed curve has tails 4 nm high on both sides of the dot.
These tails correspond to the deposition by the leaked far-
field light. The solid curve has no tails; thus, it is clear that
the leaked 488 nm far-field light did not deposit a Zn layer.
This result agrees with previous work using conventional
optical CVD for Zn deposition with a far-field light with ␭
ϭ300 nm.11 It should be noted that the 30 nm Zn dot without
tails was deposited under a nonresonant condition, despite
the presence of leaked far-field light.
We now discuss the possible mechanisms of DEZn dis-
sociation by the nonresonant optical near field. Figure 4
shows potential curves of an electron in the DEZn molecular
orbital drawn as a function of the internuclear distance of a
C–Zn bond, which is involved in photodissociation.7 The
relevant energy levels of the molecular vibration mode are
also represented by horizontal lines in each potential curve.
When using a conventional far-field light, photoabsorption
͑indicated by a white arrow in Fig. 4͒ dissociates DEZn.12 In
contrast, there are three possible mechanisms of photodisso-
ciation using a nonresonant optical near field. The first is the
two-photon absorption process, as indicated by the two ar-
rows ቢ in Fig. 4, due to the high-energy density of the
FIG. 2. Shear-force topographical images before and after NFO CVD at
wavelengths of ␭ϭ244 ͑a͒, 488 ͑b͒, and 633 ͑c͒ nm. The image sizes are
300ϫ300 nm. The observed laser output power and the irradiation time for
deposition were 1.6 ␮W and 60 s ͑a͒, 150 ␮W and 75 s ͑b͒, and 240 ␮W and
300 s ͑c͒.
Figure 2 shows the shear-force topographical images be-
fore and after NFO CVD on the sapphire substrate with
atomic-level steps9 for ␭ϭ244 ͑a͒, 488 ͑b͒, and 633 ͑c͒ nm.
For Fig. 2͑a͒ (␭ϭ244 nm), the laser power was 1.6 ␮W and
the irradiation time was 60 s. Atomic-level step structures 0.4
nm high on the sapphire substrate are clearly observed, as
indicated by the dashed lines in the left-hand side of Fig. 2͑a͒
͑before the NFO CVD͒. However, after the NFO CVD
͑right-hand side͒, the atomic-level steps disappear and a de-
posited Zn dot less than 50 nm in diameter appears at the
center of the image. This occurs because the optical near
field deposited the Zn dot directly under the apex of the fiber
probe. Furthermore, since the bare fiber probe also leaked
strong far-field light, a Zn layer that covered the atomic-step
structures was deposited.
For Fig. 2͑b͒, the laser power was 150 ␮W and the irra-
diation time was 75 s. The photon energy at this wavelength
(␭ϭ488 nm) is higher than the dissociation energy of DEZn,
but it is still lower than the absorption edge of DEZn.7,10
Therefore, it is not absorbed by the DEZn gas. A Zn dot less
than 50 nm in diameter appears at the center of the broken
circle on the right-hand side of Fig. 2͑b͒. The atomic-level
steps in Fig. 2͑b͒ are still observed, despite the leakage of
far-field light from the bare fiber probe.
In Fig. 2͑c͒ (␭ϭ633 nm), the laser power was 240 ␮W
and the irradiation time was 300 s. Despite the higher power
and longer irradiation time, there was no Zn deposition. This
shows that the thermal effect of laser irradiation is negligible
for Zn deposition.
FIG. 4. Potential curves of an electron in DEZn molecular orbitals. The
relevant energy levels of the molecular vibration modes are also represented
by horizontal lines.
In Fig. 3, the solid curve is a cross-sectional profile of
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the Zn dot deposited at 488 nm across the dashed line in Fig.
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1186
Appl. Phys. Lett., Vol. 79, No. 8, 20 August 2001
Kawazoe, Yamamoto, and Ohtsu
optical near field at the apex of the high-throughput fiber
probe. The second is the transition, shown by arrow ባ, to
the intermediate-energy level ͑dotted curve͒ induced by the
fiber probe and successive relaxation to the dissociative trip-
let state. Such an induced transition is due to energy transfer
between two localized dipole oscillators, i.e., the near-field
probe and the DEZn molecules. Similar energy transfer has
been observed between optically forbidden levels in dye
molecules.13 The third mechanism involves the transition to
an excited state of a molecular vibration mode whose energy
is higher than the dissociation energy ͑2.26 eV͒, as indicated
by the dashed line and arrow ቤ in Fig. 4. Although this
transition is prohibited by the wave-number conservation law
for conventional far-field light,14 it is allowed in the present
case because of the large uncertainty in the wave number of
the sub-wavelength-size optical near field. The three mecha-
nisms mentioned above are not adopted when ␭ϭ633 nm
light is used, because its photon energy is lower than the
dissociation energy of DEZn and its two-photon energy is
also lower than the energy at the absorption edge. This is
confirmed by Fig. 2͑c͒.
In conclusion, we demonstrated the deposition of a 30
nm Zn dot using NFO CVD with nonresonant light and dis-
cussed the possible mechanisms of deposition. Details of
these mechanisms are under investigation, which involves
evaluating the dependence of the deposition rate on the op-
tical near-field energy density, the probe–substrate separa-
tion, photon energy, and so on.
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The experimental results and the suggested mechanisms
described above show numerous potential advantages, i.e.,
this technique not only increases spatial selectivity, but also
makes it possible to use various light sources and gas sources
not previously used in conventional far-field optical CVD.
Thus, nonresonant NFO CVD is a very useful nanofabrica-
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