Nanowires can now be controlled

How this can be done is described in an article to appear in the January issue of Nature Nanotechnology, with Philippe Caroff and Kimberly Dick as the main authors.

According to Professor Lars Samuelson, this is a breakthrough both in the development of nanowire growth, and in the understanding of the fundamental materials physics processes involved.

– The results achieved here establish our position in this area of science and technology and give our ambitions an increased credibility, he says. The useful applications will not be far away.

It has been known for a long time that most semiconductor materials used in nanowires, including the very interesting material Indium Arsenide (InAs) studied here, are affected by irregularities in the layer-by-layer stacking sequence. These affect the electronic and optical properties in uncontrolled ways, and are therefore undesirable.

But now Philippe Caroff and Kimberly Dick have shown that it is possible to control these variations in great detail, which can be used for the development of new functions in nanowires.

It is now possible not only to fabricate perfect, defect-free nanowires, but also to switch freely between different crystal types along the length of a single nanowire, to produce a, so-called, superlattice, but still using only one chemical compound (InAs).

– Two of the key parameters needed to control the crystal structure are nanowire diameter and the temperature at which they are fabricated. But there are in total at least 10-12 different parameters that must be controlled when producing the nanowires, says Kimberly Dick.

Although this result has been demonstrated primarily for the binary compound InAs, it is believed that the mechanisms controlling the nanowire structure can be generally applied to related semiconductor materials used in nanotechnology.

With this technique it is also possible to grow highly regular nanowires with a perfect periodic facetted character.

Electron microscopy images show that the arrangement of atoms in the nanowire crystal exactly matches theoretical simulations. The electronic and optical properties of these wires have not been investigated yet but will be in the focus of theoretical as well as experimental studies.

The nanowires in this study had a typical diameter of 10-100 nanometers (one nanometer is one-millionth of one millimeter) and length of a few micrometers (one-thousandth of one millimeter).

The wires are produced by “baking” in an oven with a supply material in gas form, and grow from small microscopic gold “seeds”. Kimberly Dick defended a PhD thesis last year containing many electron microscopy images of similar nanowires.

The researchers work within the Nanometer Structure Consortium (nmC) at Lund University to also find commercial applications for these nanowires in electronics and opto-electronics, such as for light-emission and solar cell applications.

Lars Samuelsson could be contacted at
e-mail: Lars.Samuelson@ftf.lth.se or +46-70 317 7679.
Pressofficer Mats Nygren Mats.Nygren@kansli.lth.se or +46-708 220187
The original article “Controlled polytypic and twin-plane superlattices in III-V nanowires” written by the Lund University scientists P Caroff, K A Dick, J Johansson, M E Messing, K Deppert and L Samuelson is available at www.nature.com/naturenanotechnology.

It is also reviewed in an article in Semiconductor Today: www.semiconductor-today.com/news_items/2008/DEC/LUNDUNIVERSITY_021208.htm