Kouta Tateno, Hideki Gotoh and Hidetoshi Nakano
Optical Science Research Laboratory
There are several kinds of nanohole fabrication techniques suitable for
nano-scale devices. Anodic oxidation of aluminum produces an elegant array
of highly ordered nanoholes with a high aspect ratio. For semiconductors
in nanohole fabrication, electron beam lithography and reactive ion etching
(RIE) have been researched, especially for silicon-on-insulator (SOI) substrates
for two-dimensional photonic crystals. Here, we report a new nanohole fabrication
method using Au particles. This method is based on the reverse VLS (vapor-liquid-solid)
mechanism.
The VLS mechanism is well known as the growth mechanism for free-standing
semiconductor nanowires. The point of the VLS growth is to use the liquid-state
metal alloy particles as catalysts at low temperature around 400 - 500℃.
In the case of GaAs using Au particles as catalysts, the liquid Au particles
maintain constant Ga and As concentrations in a thermodynamically balanced
vapor-liquid-solid system so that the growth occurs when Ga and As are
supplied and super-saturation occurs in the Au alloy particles. The concept
of the reverse VLS process (etching) as follows: Ga and As are removed
from the particles by supplying an etching source gas, at the boundary
between the Au particle and the substrate, Ga and As dissolve continuously
from the substrate to maintain their concentrations in the particle, and
it turns out that Au particles dig into the substrate to make holes. We
were able to successfully demonstrate smooth holes in GaAs and InP substrates
by the reverse VLS mechanism for the first time [1].
The etching was carried out in a low-pressure (76 Torr) horizontal MOVPE
reactor. Carbon tetrabromide (CBr4) was the etching source (5×10-6 mol/min). AsH3 and PH3 were the group-V sources (2×10-3 mol/min) and were supplied around the etching temperature. Au-deposited
GaAs and InP substrates were used for etching. For the InP samples, PH3 was also supplied during the etching to reduce the etching rate.
The nanohole formation tends to proceed in the [111]B direction. For
GaAs, straight holes sometimes appear in the [011] (as shown in Fig. 1)
and [211]B directions. This is due to the stable {111}B facets, which block
the etching. For InP, many straight holes are seen in the [111]B direction
as shown in Fig. 2. For both materials, direct etching of the surface also
occurs. It is therefore necessary to find the optimum etching conditions
for high selectivity to fabricate nanoholes. This hole-fabrication method
is very promising for application to various types of nano-devices and
is expected to contribute to nano-science and -technology.
[1] K. Tateno et al., Jpn. J. Appl. Phys. 44 (2005) L428.
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