Nanocrystals enhance LED chips

25 May 2005

US reasearchers make white LEDs more efficient by replacing external color-converting phosphors with nanocrystals.

Scientists from the Los Alamos and Sandia laboratories in New Mexico have fabricated the first LEDs that use an all-inorganic, nanocrystal-based architecture.

The devices, which have CdSe nanocrystals incorporated within a GaN-based light emitting region, circumvent the inefficiencies associated with the majority of today’s LEDs, which use phosphors to convert blue emission into yellow light.

Examples of these inefficiencies include losses associated with light capture of the phosphor, non-radiative carrier losses during the re-emission process and losses due to re-absorption of the color-converted photons by the phosphor.

Semiconductor nanocrystals offer high quantum efficiencies and size-tunable colors. However, LEDs based on the technology have been plagued with the difficulty of making direct electrical connections to the nanocrystals. The solution was to sandwich the quantum dots between the GaN injection layers explained project leader Victor Klimov from Los Alamos.

The US scientists’ devices are pin structures with an intrinsic layer containing nanocrystals with a CdSe core and a ZnS shell. A monolayer of nanocrystals was assembled onto a p-type GaN layer, before 100-400 nm of n-type GaN was grown over the top.

Standard GaN deposition methods could not be used because the high temperatures would have degraded the nanocrystals. Instead, the researchers used a new lower-temperature technique, which they call energetic neutral atom beam lithography/epitaxy (ENABLE). With this, they created films said to be as good as those produced by MOCVD.

To grow GaN with the ENABLE process, a neutral nitrogen atom beam with kinetic energies of 0.5-5.0 eV and electron-beam-evaporated gallium metal combine to form epitaxial layers on a sapphire substrate heated to less than 500 °C.

The researchers produced a variety of LEDs, including 573 nm and 619 nm emitters with nanocrystal core diameters of 3.6 nm and 5.2 nm, respectively.

The devices showed no degradation in emission performance after 72 hours of continuous operation, but their external quantum efficiencies were only 0.001-0.01%. However, the researchers say that these low values can be improved significantly by optimizing the structure, and are not the result of their novel injection method.

From Compound Semiconductor.