Breakthrough for nano threads

Europe is one step ahead of the US in the development of a new type of semiconductor structure consisting of incredibly thin nano threads. A Swedish team headed by Professor Lars Samuelson at the LTH, the Lund Institute of Technology, Lund University, has taken the lead in this field of research. “In nano threads, we can combine semiconductor materials that no one has previously been able to grow. This results in entirely new electrical properties: a single electron can be monitored and made to run a unidimensional electronic steeplechase,” says Professor Samuelson.

The question of who came in first has been exciting since this is such a hot scientific innovation. The article from the LTH team was submitted one month before an article from UC Berkeley and about seven weeks ahead of another article on the subject from a group at Harvard. The Lund team was also the first to be published in Applied Physics Letters (Vol 80, 1058, 2002), followed by two articles from the Berkeley team and Samuelson’s team, who published jointly in the February issue of Nano Letters. The Harvard team’s article appeared this week in Nature. The new findings have also recently been commented on as a news bulletin in Science (News of the Week).

In other words, its looks like the Americans are hot on the heels of the Swedish team. But this is an illusion. Now that all the material has been published it appears that the LTH scientists have mastered the technological processes with a precision that the Americans have yet to attain.

The classic way to form small structures on an electronic chip is to work in two dimensions. The material is placed in sheets on top of each other, and in the interface between the different semiconductor materials interesting electrical properties arise. But certain materials only permit partial unions with each other. This is the case with indium arsenide and indium phosphide. After an initial layer the crystalline process becomes uneven and defective when one material is to grow on the other. The new technique combines materials that have never been used together before.

For a couple of years leading research teams have been focusing on threadlike structures and have learned to form such threads. Nano threads have a diameter of 10-70 nanometers and can be a thousand times longer than their diameter. (One nm=one billionth of a meter) To make a thread as thick as a strand of hair would mean bundling together at least ten million nano threads! In semiconductor research there has also been a great deal of interest in so-called nano tubes of carbon, and last year scientists were able to present carbon nano tubes containing transistors and nano threads that both contained transistors and functioned as logical circuits.

But until now it has only been possible to construct carbon nano tubes and nano threads of one consistent material, which does not provide any interesting electronic effects. The present breakthrough involves a nano thread containing segments of different materials, indium arsenide and indium phosphide, for instance. The sharper the border is between these segments, the better the electric current going through the wire can be controlled. It is in this respect that the results from LTH are clearly superior. The Americans have attained a transitional zone of 15-20 nm, whereas the LTH team has brought that zone down to the equivalent of a single layer of atoms. What’s more, unlike the Americans, Samuelson’s team has managed to meter the electrical behavior of the new threads. It is no mean feat of technology to attach the relatively bulky connections used in conventional electronics to these delicate threads.

Samuelson’s research team—associated with the Nanometer Consortium at Lund University—start out with a nano-size particle of gold when they construct their threads. The particle can be placed on a base of indium, for instance. This is heated up, and the gold and indium form a melt. This process takes place in an ultra-vacuum. Arsenic is added, using a so-called molecular epitaxy beam, until the melt reaches saturation. When it is cooled, indium arsenide crystallizes under the gold. The gold is a catalyst that remains unchanged throughout the process. In the process of crystallization a pillar gradually grows under the gold. If phosphorous, for example, is now added to the indium melt, a new crystallization process starts, yielding indium phosphide.

“This opens the road to faster, more energy-efficient, and even tinier miniature electronics,” says Professor Samuelson. “Nano threads can also be constructed on top of two-dimensional structures and thus be incorporated in conventional ‘sandwich’ structures.”

“Entirely new materials will be produced. Of special interest is the fact that this new technology will probably make it possible to manufacture materials for magnetic storage, meaning that with these dimensions it will be feasible to attain terabit densities for storing information on a hard disc.”

“It will be possible to send an electrical impulse through a nano thread and thereby create a single photon for use in fiber-optical communication, for instance. This is a breakthrough in quantum optics and information transfer. Today it is possible to tap into an optical fiber and siphon off information without being discovered. But with digital ‘ones’ consisting not of clusters of photons but of a single photon, those on the receiving end will know immediately if they have been bugged, since the signal will disappear.”

“It will also be possible to construct exceedingly small light diodes and tiny and rapid light detectors. I can almost promise that by the end this year our lab will be able to create a single point source of light the size of, say, 25x25x25 nm. Such tiny light diodes can be of tremendous importance in the optical storage of information and for applications in medicine and biology,” says Professor Samuelson, adding: “Nano threads are not only of interest in electronics. They will play a role in the development of new materials and even in pure research in physics. Nano threads can be ‘test benches’ for how electrons and photons behave under new conditions.”