Nanoholes Formed by Reverse VLS Mechanism


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.

Fig. 1. Cross-sectional SEM images of [011] oriented hole in GaAs (111)A.
Fig. 2. Cross-sectional SEM images of [111]B oriented hole in InP (111)A.

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