Scientists have been working on WLEDs for nearly 10 years. Ideally, they would like to make highly efficient, long lifetime, cheap solid-state WLEDs that could compete with the energy hungry incandescent light bulbs and fluorescent tubes that we use today. Although WLEDs with light efficiencies reaching 200 lm/W have been made, these devices are still very expensive compared with ordinary light bulbs.

Recently, various teams around the world, including the CRHEA-CNRS team, showed that monolithic WLEDs can be fabricated by stacking blue and yellow emitting (GaIn)N/GaN quantum wells inside a GaN p-n junction. Monolithic means that the LED is made in a single epitaxial growth run instead of from the more common two-step process (epitaxial growth plus deposition of specific luminophore) used to make conventional phosphor-based WLEDs. This should not only bring down costs but also improve device reliability.

Making white light
The CRHEA-CNRS team made devices by mixing quantum wells with different thicknesses in the electrically active zone to obtain blue and yellow light emissions. Combining blue and yellow light produces white light. The quantum wells were situated in the middle of the p-n junction in the device and white light is emitted when current is injected into the device.

But there is a snag to this design: the colour emitted is very sensitive to the injection current and the device's efficiency limited by the yellow-emitting quantum wells.

The team has have now shown that it can solve this problem by exploiting a structure in which the quantum wells are located outside the p-n junction. This means that the wells no longer need to be electrically injected but can be optically pumped by the blue photons instead. "This is of key importance because the approach is much more flexible than previous ones while keeping the monolithic aspect," Damilano told nanotechweb.org.

The researchers grew the structures by molecular beam epitaxy on sapphire substrates. They collected the electroluminescence through an optical fibre connected to a spectrometer. The injected current makes charge carriers (electrons and holes) recombine in the electrically active zone, which leads to blue photons being emitted. Some of the blue photons are absorbed by the yellow-emitting quantum wells so the researchers obtain yellow emission. As before, combining blue and yellow light gives white light emission.

Using WLEDs instead of conventional lighting could significantly reduce our electricity consumption, says Damilano.

The team now plans to test other light converters, such as quantum dots, which may be even more efficient than GaInN quantum wells for emitting yellow or red light. The researchers will also look at using silicon substrates to further decrease device fabrication costs.

The work, which was published in Applied Physics Letters, was partly funded by the French ANR-PNANO "DEMONI" project. The team also includes scientists from two French companies RIBER and LUMILOG.