Novel Devices ♦ news digest


Heer adds. “We are already able to steer these electrons and we can switch them using rudimentary means. We can put a roadblock, and then open it up again. New kinds of switches for this material are now on the horizon.”


Theoretical explanations for what the researchers have measured are incomplete. De Heer speculates that the graphene nanoribbons may be producing a new type of electronic transport similar to what is observed in superconductors.


“There is a lot of fundamental physics that needs to be done to understand what we are seeing,” De Heer continues. “We believe this shows that there is a real possibility for a new type of graphene-based electronics.”


Georgia Tech researchers have pioneered graphene- based electronics since 2001, for which they hold a patent, filed in 2003. The technique involves etching patterns into electronics-grade SiC wafers, then heating the wafers to drive off silicon, leaving patterns of graphene.


This work has been detailed in the paper, “Exceptional ballistic transport in epitaxial graphene nanoribbons,” by Jens Baringhaus et al in Nature 2013, published online on 5th February 2014.DOI:10.1038/nature12952


Controlling photons with InGaN quantum dots


By emitting photons from an indium gallium nitride quantum dot at the top of a GaN micropyramid, researchers have created a polarised light source. The device can be used in applications such as energy-saving computer screens and wiretap-proof communications


Polarised light - where all the light waves oscillate on the same plane - forms the foundation for technology such as LCD displays in computers and TV sets, and advanced quantum encryption.


Normally, this is created by normal unpolarised light passing through a filter that blocks the unwanted light waves. At least half of the light emitted, and thereby an equal amount of energy, is lost in the process.


A better method is to emit light that is polarised right at the source. This can be achieved with quantum dots - crystals of semiconductive material so small that they produce quantum mechanical phenomena. But until now, they have only achieved polarisation that is either entirely too weak or hard to control.


A semiconductive materials research group led by Per Olof Holtz, a professor at Linköping University, have now developed an alternative method.


The concept is based on InGaN QDs grown on top of elongated GaN hexagonal pyramids, by which the predefined elongation determines the polarisation vectors of the emitted photons from the QDs. This growth scheme should allow fabrication of ultra-compact arrays of photon emitters, with a controlled polarisation direction for each individual emitter.


With these, they have succeeded in creating light with a high degree of linear polarisation, on average 84 percent. The results are being published in the Nature periodical Light: Science & Applications.


“We’re demonstrating a new way to generate polarised light directly, with a predetermined polarisation vector and with a degree of polarisation substantially higher than with the methods previously launched,” Holtz says.


In experiments, quantum dots were used that emit violet light with a wavelength of 415 nm, but the photons can in principle take on any colour at all within the visible spectrum by varying the indium content.


Two ways of creating polarised light (Credit: Fredrik Karlsson, LiU)


“Our theoretical calculations point to the fact that an increased amount of indium in the quantum dots further improves the degree of polarisation,” says reader Fredrik Karlsson, one of the authors of the article.


The micropyramid is constructed through crystalline growth, atom layer by atom layer, of the semiconductive material GaN. A couple of nanothin layers where the metal indium is also included are laid on top of this. From the asymmetrical quantum dot thus formed at the top,


March 2014 www.compoundsemiconductor.net 163


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