High-Efficiency Nitride-Based Solid-State Lighting
Investigating Organization
University of California, Santa Barbara
Principal Investigator(s)
Shuji Nakamura
Subcontractor
Lighting Research Center at Rensselaer Polytechnic Institute
Funding Source
EE Science Initiative
Award
DOE Share: $2,995,155
Contract Period
09/28/01 - 04/30/05
This project is focused on developing efficient white-light-emitting luminaires via a combination of novel GaN-based blue light emitting diodes (LEDs) and conventional YAG:Ce-based yellow phosphors. The blue LEDs 'pump' the yellow phosphor, and white light results from proper color mixing.
Typical (In,Al)GaN LEDs are composed of thin (< 0.1 micron) stacked layers with varying composition and doping (i.e., electrical conductivity), which are processed in a cleanroom to etch a defined mesa structure and deposit metal contacts. Unfortunately, due to the relatively high index of fraction of these materials, only a little light (< 8% per face), generated within the chip, escapes from it. Thus, a significant enhancement in light extraction (and, therefore, overall efficiency) is needed.
The novel LEDs studied in this project are termed Microcavity LEDs (MC-LEDs), whose total thickness is a fraction of a conventional LED. This reduced cavity thickness (ideally about 0.5 micron or less) causes the formation of optical modes within the structure and their accompanying directional emission from the structure. This directional emission is calculated to lead to high light extraction efficiency (> 40%), given that we can carefully control the thickness and composition of the various device layers, which, in the case of a quantum well are, as thin as 10 nanometers. In addition, microcavity formation requires precise control of device thinning after the as-grown film has been detached from its substrate. Lastly, the electrical contacts and mirror(s) on either side of the structure need to be properly formed, both requiring significant processing optimization.
We are also developing luminaire designs that are tailored for directional emission from MC-LEDs. These luminaires must first be designed using ray-tracing and other optical modeling software, since the internal and external geometry of an 'optimal' luminaire design is often not inherently obvious. In addition, the placement of the yellow phosphor and the composition/refractive index of its medium must be properly chosen, since they directly affect overall luminaire efficacy. Once these factors have been considered and a prototype is constructed, we place an MC-LED in the luminaire to experimentally verify the efficiency and uniformity of light emission.