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Selective Area Epitaxy of InP Nanowires - ACT Node

September 2013

Semiconductor nanowires are currently of great research interest due to their unique properties. Axial and radial heterostructure nanowires (NWs) have been proposed as nano-building blocks for next generation electronic and photonic devices. These devices are expected to revolutionise our technological world in the way of new devices and components. The unique properties stem from their large surface to volume ratio, very high aspect ratio, and carrier and photon confinement in two dimensions. Most semiconductor nanowires are grown by catalyst-assisted vapour-liquid-solid (VLS) process due to its versatility. However, precise site control of nanowires using this technique is difficult. Also, incorporation of the metal catalysts into the nanowires may create deep level traps which could degrade their optical and electrical properties. Selective-area metal-organic vapour-phase epitaxy (SA-MOVPE) has the advantages that the nanowires can be grown without any catalyst, and allows the controlled positioning of the nanowires.

Qian Gao, a PhD student working at the Department of Electronic Materials Engineering (EME), Research School of Physics and Engineering, ANU has successfully grown high quality indium phosphide (InP) nanowires using SA-MOVPE. Various facilities of the ANFF ACT Node have been used in fabricating these InP nanowires and characterising their properties. To fabricate the patterned substrate for growing nanowires, 30 nm thick SiOx was deposited on an indium phosphide substrate by Plasma Enhanced Chemical Vapour Deposition (PECVD), and ZEP was spin-coated onto the film as the electron beam resist.

Using the electron beam lithography (EBL) at the ANFF ACT Node, followed by wet chemical etching, a series of patterns with holes of various diameters and pitches was transferred to the resist and then onto the SiOx layer. The substrate with the pattern was then cleaned and loaded into the MOVPE system for nanowire growth. Figure 1 (below) shows a typical scanning electron microscopy (SEM) image of the patterned substrate (a) and InP nanowires after the epitaxial growth (b-d). The images show that nanowire diameter increases with increasing pitch for the same window diameter. This is due to the larger collection area around the openings which allows for more adatoms to diffuse towards the NWs and contribute to lateral growth.

Figure 1
Figure 1: SEM images of (a) patterned substrate and InP nanowires with the spacing of (b) 500 nm, (c) 1 µm, (d) 2 µm.
The designed diameter is 170 nm. The scale bars are 1 µm.

To further study the crystal structure of the nanowires, transmission electron microscopy (TEM) was employed. The Focused ion beam (FIB) at ANFF ACT Node was used to prepare TEM lamella with a thickness less than 100 nm. These nanowires were first planarised by BCB for support. Figure 2 shows a lamella after the FIB milling and polishing processes. High resolution TEM images and diffraction patterns of the InP nanowires indicates the nanowires have a wurtzite crystal structure without any stacking faults.

Figure 2 Figure 3
Figure 2: SEM image of a TEM lamella. Figure 3: (a) TEM image of InP nanowires. (b) High-resolution TEM image
and (c) selected area electron diffraction (SAED) pattern are shown in the
right side of the figure.

With these high quality InP nanowires, we will be fabricating high efficient devices such as photo-detectors and solar cells. These devices are exciting because of the large surface area of nanowires to absorb light and efficient carrier transport properties that could improve device efficiency. Since nanowires are inherently non-planar structures, the fabrication processes can be challenging and new processes have to be developed and optimised using ANFF facilities and with the assistance from the highly skilled ANFF staff.

Story courtesy of Qian Gao, Department of Electronic Materials Engineering,
Research School of Physics & Engineering, The Australian National University.