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Uncovering the Surface States of Semiconductor Nanowires

7 October 2013

The miniaturization of microelectronic components enables new innovation, with nanowires made from semiconductors currently receiving great attention in research and development.  Previously, controversy has prevailed over the nature of the surface structure of gallium nitride nanowires.  Researchers from Jülich, Dusseldorf, Berlin and Lille, France, have now succeeded in elucidating this surface structure in a combined experimental and theoretical paper.  Their findings, which appear on the front cover of the current edition of the international journal Applied Physics Letters, are also likely to be applicable to nanowires made from other semiconductors, providing fresh impetus for the development of miniature optoelectronic components.

Uncovering the Surface States of Semiconductor Nanowires The image illustrates the so-called density of states at the surface of a gallium nitride crystal (below) under vacuum, and thus the probability that the electrons take on certain energy levels (red = high, green = low).
Source: Forschungszentrum Jülich

Semiconductor nanowires possess highly interesting optoelectronic properties, such as the ability to produce light. Through their ability to self-organize under controlled conditions, they can also be manufactured with almost exactly the same shapes and sizes.

Energy-efficient light-emitting diodes (LEDs) and lasers are fabricated from optoelectronic semiconductor materials. Their optoelectronic properties arise from the electronic structures of the materials.  For instance, blue and green light can be produced using gallium nitride, one of the most widely-used semiconductor materials in optoelectronics.  Although the electronic structure of gallium nitride is well-known, until now this knowledge did not extend to nanowires made of this material, which have diameters that vary from just a few nanometres to a few hundred nanometres and lengths of up to several micrometres.  As their surface-to-volume ratio is extremely high, their electronic surface structure takes on a much more important role, differing significantly from the known volume structure.

The approach of the research team was different from that of earlier attempts, in that the nanowires themselves were not studied. Instead, material samples were cleaved so that larger surfaces, which had the same structures as the nanowires, could be examined.  With the help of scanning tunneling microscopy, the researchers could measure the probability with which the electrons can adopt specific energy levels.  The calculations of the research team confirmed the measurement results.

Original publication:

Hidden surface states at non-polar GaN (1010) facets: Intrinsic pinning of nanowires;
L. Lymperakis et al.;
Appl. Phys. Lett. 103, 152101 (2013), published online 7 October 2013, DOI: 10.1063/1.4823723



Peter Grünberg Institute – Microstructure research (PGI-5)