Nanowire advance into two dimensions

We've shown that you can pull individual nanofins out, lay them flat, and then make some complex structures with them. In particular, we can make Hall bars, which were the old fundamental structure for studying quantum devices on.Adam Micolich

Researchers from the University of New South Wales and Australian National University have demonstrated a way to grow nanowires to produce arrays of identical high-aspect ratio nanofins with good geometric control. Although nanowires have potential applications in the next generation of transistors, light-emitting devices, photovoltaics, sensors, and also in topological quantum computing, here they used mechanical transfer to lay them flat on a separate device substrate for processing into more complex device structures.

Nanofins have a high aspect ratio, which allows them to be used where a 1D conduction channel with high surface-to-volume ratio is needed. However, this can also be a limitation, since the ratio imposes restrictions on the devicesand circuits that can be produced. As reported in Nano Letters [Seidl et al. Nano Lett. (2019) DOI: 10.1021/acs.nanolett.9b01703], in this study the team wanted identify a further spatial dimension to work with, showing the growth of free-standing rectangular InAs nanofin structures that can be mechanically transferred to give a flat 2D structure that more complex devices can be fabricated upon.

The research developed from a team member exploring if other geometries can used rather than small circular openings for nanowires. Growth was performed by selective-area epitaxy (SAE), a technique involving a patterned dielectric mask used to template nucleation and growth on a crystalline substrate, a method used to produce arrays of 1D nanowires as a ‘catalyst-free’ alternative to the vapor–liquid–solid (VLS) method.To produce nanowire arrays by SAE involves a crystalline substrate coated in an amorphous layer, with openings then made – with the crystal surface in the openings catalysing the growth – rather than using gold nanoparticles or Ga droplets to catalyse growth.

The key innovation was to change from small circular holes in the dielectric mask, typically used to grow nanowire arrays, to long narrow rectangular slots. With slots that are sufficiently narrow and oriented in a certain way with regard to the crystalline substrate, tall thin 2D nanofins can be grown in regular arrays with high yield. Nanowires have been grown both vertically and horizontally with the SAE approach, but this is the first time both aspects have been combined along the substrate and also directly up to get a wall rather than a horizontal or vertical wire.

The work brings an understanding of what's required in terms of control over the template mask and growth conditions, and that you can make more complex structures, such as a Hall bar, where the electrode gaps for Hall measurements are intrinsically tiny and contacts tend to interfere with electron conduction along the channel,and which the basis for most quantum device structures.