Nanowires Laterally Grown on GaAs Substrates in the Vapor-Liquid-Solid Mode
Guoqiang Zhang, Kouta Tateno, Hideki Gotoh, and Tetsuomi Sogawa
Optical Science Laboratory
Semiconductor nanowires (NWs) have attracted much attention because of their exceptional versatility and potential for a wide range of applications, from electronics and photonics to biochemistry and medicine . There are two main approaches to the integration of NW-based devices: (1) using freestanding NWs epitaxially grown on a patterned substrate; (2) using an ex-situ assembly method designed to align NWs removed from the grown substrate and dispersed on another substrate. The former approach is preferable because of the contamination that occurs during the ex-situ assembly process with the latter approach. However, a freestanding structure is very difficult to use as regards electrode formation. The most attractive way to integrate and extend the applications of NW-based devices is to grow the NWs laterally on the substrates with an in-situ process. In the current work, we developed a technique for growing lateral GaAs NWs on GaAs (311)B and (001) substrates in the vapor-liquid-solid (VLS) growth mode and demonstrated its high controllability in terms of size, site, and composition [2, 3].
We used Au nanoparticles to catalyze the NW growth in the VLS mode. First, we investigated lateral GaAs NWs grown on a (311)B substrate under optimized growth conditions in a metalorganic vapor phase epitaxy system. Figure 1 shows a cross-sectional transmission electron microscopy (TEM) image of a lateral GaAs NW. There is a Au nanoparticle at the tip of the NW, indicating that the NW is grown in the Au-catalyzed VLS mode. This enables us to control their size and density by using size- and density-selective Au colloidal nanoparticles. We grew the site-specific lateral NW array by using a lithographically defined Au dot array. Figure 2 shows an atomic force microscopy (AFM) image of the array. To form heterostructures for bandgap engineering in the lateral NWs, we incorporated indium (In) during the growth and confirmed the realization of lateral InGaAs NWs . We expect that the planar NW structure will provide new opportunities and new functionalities for the development of electronic devices and highly efficient photonic devices.
 L. Samuelson, Mater. Today 6 (2003) 22.
 G. Zhang, K. Tateno, H. Gotoh, and H. Nakano, Nanotechnology 21 (2010) 095607.
 G. Zhang, K. Tateno, H. Gotoh, and T. Sogawa, Appl. Phys. Express 3 (2010) 105002.
Fig. 1. Cross-sectional TEM image of lateral GaAs NWs on (311)B substrate.
Note that there is a Au particle at the lateral NW tip. The arrow in the
inset, which is a top-view SEM image of lateral NWs, indicates a Au
particle at the NW tip.
Fig. 2. AFM image of lateral GaAs NWs.
Note that the Au particles moved
from the initial position, indicated
by the white arrows, to the NW tips
after the growth.
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