The next generation of light-emitting diodes, solar cells and batteries could be based on semiconductor nanowires. Researchers at Sweden’s Lund University have now shown the possibility for the structures to grow from freely suspended nanoparticles of gold in a flowing gas, instead of beginning with a silicon wafer or other substrate, as is standard today.
Lars Samuelson, Lund University Professor of Semiconductor Physics, believes the technology will be geared up for commercialisation in two to four years, and a prototype for solar cells is expected to be completed in two years.
“When I first suggested the idea of getting rid of the substrate, people around me said ‘you’re out of your mind, Lars; that would never work’. When we tested the principle in one of our converted ovens at 400°C, the results were better than we could have dreamt of”, said Samuelson.
“The basic idea was to let nanoparticles of gold serve as a substrate from which the semiconductors grow. This means that the accepted concepts really were turned upside down!”
In a recently published article in the journal Nature, the researchers demonstrate how the growth can be controlled using temperature, time and the size of the gold nanoparticles. Since first thought of, the technology has been refined, patents have been obtained and further studies have been conducted.
Baked Nanowires: Hold the Wafers
They have recently built a prototype machine with a specially built oven. Using a series of ovens, the researchers expect to be able to ‘bake’ the nanowires, as the structures are called, and thereby develop multiple variants, such as p-n diodes. A further advantage of the technology is avoiding the cost of expensive semiconductor wafers.
“In addition, the process is not only extremely quick, it is also continuous. Traditional manufacture of substrates is batch-based and is therefore much more time-consuming”, says Professor Samuelson.
The researchers are currently working to come up with a practical method to capture the nanowires and make them self-assemble in an ordered manner on a specific surface. The surface could be glass, steel or another material suited to the purpose.
The reason why no one has tested this method before, in the view of Professor Samuelson, is that today’s method is so basic and obvious. Such things tend to be difficult to question. But, the Lund team do have a head start, thanks to concurrent research based on a new method in the manufacture of nanowires on semiconductor wafers, known as epitaxy.
The researchers have chosen to name the new method aerotaxy. Instead of sculpting structures out of silicon or another semiconductor material, the structures, referred to as nanowires or nanorods, are instead allowed to develop, atomic layer by atomic layer, through controlled self-organisation.
Semiconductors are what we call materials that neither conduct electricity as well as metals, nor stop a current as effectively as insulators; silicon and germanium are two such examples. These properties may not sound attractive, but in fact they are highly desirable, because we can influence the conductive capacity of the materials.
For instance impurity atoms can be introduced, this is known as doping. Materials with different types of doping can be combined to manufacture products such as transistors, solar cells or LEDs.
Magnus Heurlin, Martin H. Magnusson, David Lindgren, Martin Ek, L. Reine Wallenberg, Knut Deppert & Lars Samuelson
Nature (2012) doi:10.1038/nature11652