Multi-colour LEDs

A team of University of California scientists at Los Alamos National Laboratory have developed the first completely inorganic, multi-colour light-emitting diodes (LEDs) based on colloidal quantum dots encapsulated in a gallium nitride (GaN) semiconductor.

The work is said to represent a new “hybrid” approach to the development of solid-state lighting, which offers the advantages of reduced operating expenses, lower energy consumption and more reliable performance.

In research published in the current issue of the scientific journal Nano Letters, the team reports on the first successful demonstration of electroluminescence from an all-inorganic, nanocrystal-based architecture where semiconductor nanocrystals are incorporated into a p-n junction formed from semiconducting GaN injection layers. The new LEDs utilise a novel type of colour-selectable nanoemitters, colloidal quantum dots, and makes use of emerging GaN manufacturing technologies.

According to Dr. Victor Klimov, who leads the nanocrystal-LED research effort, “numerous technologies could benefit from energy efficient, colour-selectable solid-state lighting sources ranging from automotive and aircraft instrument displays to traffic signals and computer displays.

“Semiconductor nanocrystals, known also as quantum dots, are attractive nanoscale light emitters that combine size-controlled emission colours and high emission efficiencies with chemical flexibility and excellent photostability. The use of nanocrystals in light-emitting technologies has, however, always been hindered by the difficulty of making direct electrical connections to the nanocrystals. By putting the quantum dots between GaN injection layers, we’ve gotten around this difficulty.”

The secret to making the electrical connection to the quantum dots is the use of a technique developed at Los Alamos by Mark Hoffbauer and his team that utilises a beam of energetic, neutral nitrogen atoms for growing GaN films.

The technique, called ENABLE (Energetic Neutral Atom Beam Lithography/Epitaxy), allows for the low-temperature encapsulation of nanocrystals in semiconducting GaN without adversely affecting their luminescence properties.

By encapsulating one nanocrystal layer or two layers of nanocrystals of different sizes, the researchers have demonstrated that their LEDs can emit light of either a single colour or two different colours. The two colour-operation regime is an important step toward creating devices that produce white light.