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New Top Down Approach for Fabrication of Nano-wires - ACT Node

February 2012

During the last half of 2011, the ANFF ACT Node (through EME) hosted a trainee student from Germany, Mareen Gläske. Mareen's work with the node staff focused on the research topic of GaAs nanowires. Normally nanowires are grown in MOCVD reactor on GaAs templates covered with gold colloidal particles - Mareen investigated another route using micro/nano-fabrication techniques to manufacture the nanowires. If successful, this technique offers the advantage of making nanowires in adequately doped hetero-structures for device applications. Two methods were used:

The work with the PS spheres was done in collaboration with Dr Qin Li from Curtin University (now with Griffith University) who have extensive experience with PS spheres. The group from Curtin Uni fabricated the PS patterning on bare GaAs wafers and GaAs/AlAs epi-structures. Both types of wafers were first coated with 500nm of SiOX via PECVD to serve as an etching mask for the compound semiconductor materials. Subsequently O2-plasma was used to trim the PS spheres in a controlled manner followed by ICP etching of the SiOX mask and GaAs as shown in Figure 1 (below left).

Figure 1 Figure 2a Figure 2b
Figure 1: Schematics of the process chart for etching GaAs using PS spheres. Figure 2a and 2b: Closely packed PS spheres spread on GaAs and the final etched SiOX mask and GaAs.

Figures 2a and 2b (above) show the coverage of a GaAs sample with PS spheres of 560nm nominal diameters (left) and the etched SiOX mask and GaAs. The results confirm that this technique has the potential of achieving nano-wires in GaAs based structures using this cheap patterning technology. A certain amount of refinement is still required with respect to the trimming of the PS spheres and to optimising the sidewall smoothness. Another possibility is to use two sizes of PS spheres, as shown in Figure 3 (below left), with a combination of 560nm and sacrificial 277nm PS spheres and using O2-plasma to trim the largest balls to sizes around 280nm while the small spheres would have completely disappeared. This would enable larger spacing of the PS spheres to facilitate the dry etching of the semiconductor material.

Figure 3 Figure 4
Figure 3: 560nm and 277nm diameter PS spheres
combined on a GaAs sample.
Figure 4: Etching result of pillars using EBL patterning
of 1µm circle shapes.

The second technique used in this work was direct patterning using e-beam lithography (EBL) to define circles. Figure 4 (above right) shows the etching results with various sizes of circles/pillars. The ICP etching involved a gas combination of five gases (Cl2/H2/Ar/BCl3/CH4) required to obtain smooth sidewalls in the GaAs/AlAs stacked layers. Though expensive, this technique is showing promise of obtaining well defined and arranged pillars/nanowires. The technique requires further fine-tuning to enhance the aspect ratio through an optimisation of the masking technique (three level masking with Cr/SiOX on the semiconductor material). The results of this research were presented by the ACT Node Facility Manager, Dr Fouad Karouta, in an oral presentation at ICONN 2012 held recently in Perth held 5-9 February.

Ref: M. Gläske, F. Karouta, E. Eftekhari, Q. Li, J. Tian, K. Vora, X. Li, H.H. Tan and C. Jagadish,
"Top-down Approach for Fabricating GaAs-based Nanowires",
International Conference on Nanoscience and Nanotechnology, Perth, 5-9 February 2012, paper #974.


Mareen Gläske has since returned to Germany, but not before enjoying some of highlights of Australia and the 'down under' experience (see photos below) and her research has also received the the 'Avi' treatment from our 'resident' cartoonist Avi Shalav.

cartoon Mareen Glaske with python Mareen at Uluru
Scientist makes breakthrough discovery in use of self assembled
nano-sized polystyrene spheres - nano beanbags! © Avi Shalav
Mareen Gläske with an olive python (left) and enjoying an evening meal at Uluru (Ayres Rock), Central Australia