Nanoscience researchers create LEDs in different colours

Now that incandescent light bulbs are a thing of the past, energy-saving Light-Emitting Diodes (LEDs) are supposed to illuminate our surroundings instead. But they often provide a bluer, more unpleasant light; Physics professor Lars Samuelson and his research colleagues at Lund University’s Faculty of Engineering (LTH) want this to change.

Last year, they received a total of SEK 50 million from the EU and the Foundation for Strategic Research to develop light-emitting diodes capable of reproducing all the visible wavelengths of blue, red, yellow and green. Currently the industry is only able to generate shortwave blue light. The white light which is produced in spite of these limitations is achieved by adding a sort of “light powder” (phosphor coating, not to be confused with the chemical element phosphorus), over the diode.

”But it doesn’t produce good light. The colours we see appear to be wrong. The light also disperses, which is not good as the eye is extremely sensitive to colour shifts. Several studies show that a stable and authentic white light, which resembles daylight, is important to our well-being”, says Lars Samuelson.

The solution starts with nanowires, the fortunate discovery which opened the way for a new method of constructing the smallest structures in electronics and optoelectronics. The breakthrough came in 2002.

Today the leading research in the field is underway in the universities of Lund, Lund, Berkeley and Harvard, among others. The specialism of the Lund researchers is to develop the physical and electrical properties of the nanowires – which benefits resource-efficient and better transistors, solar cells, light diodes and other electrical equipment we now consider an integral part of our lives. Recently, articles have been published in both Science and Nature (see also interview in the previous feature with Reine Wallenberg).

With the help of these nanowires, Samuelson and his colleagues will now build LED lamps whose light will be indistinguishable from ordinary sunlight to the naked eye.

Since last year, blue light has been produced in Sunnyvale, California, where the spinoff company Glo AB has its subsidiary with 60 employees (the parent company is in Lund). Development is taking place in close cooperation with companies that will use the LEDs in their LCD screens. Researchers have also succeeded in producing green light and, thanks to the grant from the EU and the Strategic Research Foundation, they are now working on building nanowires that emit the longer wavelengths of yellow and red.

But research into nanowire diodes is not only ongoing in Lund and in the US. Among others, European company Osram and Korean Samsung are involved in extensive research in the field.
“I believe that Glo has a great opportunity to be the first to get nanowire light-emitting diodes out into the market, especially as we hold many of the most basic patents, which others will find it very difficult to get around”.

A total of around twenty researchers are involved in light-emitting diode research at LU. Besides Lars Samuleson, another key figure in Bo Monemar, currently professor emeritus in semiconductor physics and one of the world’s foremost scientists in the field of gallium nitride. Others involved include Magnus Borgström, Anders Gustafsson, Mats-Erik Pistol and Dan Hessman.
Kristina Lindgärde

Nanowire diodes
The new light-emitting diodes are built up like a carpet of nanowires in which each thread emits photons within a particular wavelength spectrum. In the human eye, the colours combine into white. The colour emitted is determined by the semiconductor material of the nanowires; blue light is rendered by gallium nitride (GaN) with so-called quantum wells of gallium indium nitride (GaInN), produced on sapphire substrate. For green light, more indium is added to the gallium nitride. This approach does not work with current LED technology as more indium generates too much tension in traditional technology. Researchers are not yet ready to reveal how they intend to produce the yellow and red light.

A further advantage of the nanowire LEDs in comparison with the current versions is that the so-called p-n junctions can be oriented in a crystal direction which offers significantly better wavelength stability and higher efficiency.

Diode production may be based in Skåne
The new nanowire diodes may be produced in Skåne. This would create new job opportunities and have a symbolic value for the materials research facilities ESS and MAX IV.
But the establishment of production here would require financial support from the regional authorities. Otherwise, production will probably end up in the US, China or Taiwan, where factories for the production of semiconductors are ready for use.

“It would be great and satisfying to see production established here,” says Lars Samuelson.
Although he is a member of Glo’s board, the final decision is up to the investors. The majority of them are Scandinavian, but for people who have invested a total of half a billion Swedish crowns in Glo and would like to see a return within the foreseeable future, any patriotic feelings are a low priority. Within a year, Lars Samuelson estimates that the Glo board will have taken a decision on where production is to be based.

“Regardless of where production ends up, or whether some international giant buys up the company, the licensing revenues will still benefit Skåne. All the basic patents are here as well.”