Multicolor LEDs Get a Boost from Quantum Dots
Anne L. Fischer
Much attention in research communities is focused on generating light using semiconductor nanocrystals, also known as quantum dots. One benefit of this method is that emission color can be controlled simply by changing the size of the dots.
An LED structure consisting of a monolayer of CdSe nanocrystals, capped with ZnS and coated with TOPO/TOP ligands, is shown sandwiched between P- and N-type GaN layers. Hole and electron injection from the P- and N-type GaN layers, respectively, results in radiative electron-hole recombination in the nanocrystals.
A traditional approach to color-selectable LEDs involves the use of InGaN LEDs in combination with color-converting phosphors. The LEDs produce light in the blue region, and the phosphors convert this color to different wavelengths. Researchers at Los Alamos National Laboratory in Los Alamos and at Sandia National Laboratories in Albuquerque, both in New Mexico, have demonstrated an alternative, producing multicolor light emitters by directly incorporating semiconductor nanocrystals into a PN junction formed from semiconducting GaN injection layers.
The goal of the study was to find a way to incorporate color-selectable chromophores (in this instance, semiconductor nanocrystals) directly into the LED structure without adversely affecting their light-emitting properties. To accomplish this, the group employed a deposition technique called energetic neutral-atom-beam lithography/epitaxy, which employs a beam of energetic neutral nitrogen atoms and a flux of evaporated gallium, allowing a low-temperature growth of high-quality GaN layers.
Using monolayers of TOPO/TOP-coated CdSe/ZnS nanocrystals encapsulated into GaN, the researchers demonstrated efficient electroluminescence with the emission wavelength controlled by the diameter of the nanocrystals. They also demonstrated two-color operation using bilayers of nanocrystals.
They found no degradation in emission after 72 hours of operation and encountered no problems in device performance after months of shelf life. Current problems that they face are spatial nonuniformity of the emission intensity and relatively low external quantum efficiencies. To optimize these parameters, the researchers are continuing to work on engineering improvements — specifically, methods for applying electrical contacts.
Nano Letters, June 2005, pp. 1039-1044.
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