Ultraviolet LEDs for Solid-State Lighting
Investigating Organization
Cermet
Principal Investigator(s)
Jeff Nause
Subcontractor
None
Funding Source
Small Business Innovation Research
Award
DOE Share: $99,694
Contract Period
7/1/03 - 4/30/04
Two approaches are emerging as viable techniques for the production of solid state white light: visible wavelength LEDs coupled with modified phosphor compositions and UV emitters coupled with traditional, highly efficient YAG phosphors. The latter approach has the advantage of producing light with familiar color temperatures (warmth), which will greatly enhance the adoption rate of the light source by the public. However, UV (340 nm and 280 nm) semiconductor emitters with sufficient power required to stimulate YAG phosphors are not available.
The goal of this program was to develop the technology necessary to enable commercial production of high-quality (In,Al,Ga)N epitaxial materials and high-performance UV LEDs on AlN substrates for solid state lighting applications. Three major areas were targeted for a successful program through a Phase II effort: 1) development of production grade bulk AlN wafers using Cermet's Vapor Growth Process; 2) development of better quality materials, which include p- and n-type doped AlGaN and InAlGaN-based multi-quantum wells; 3) and introduction of novel LED device structures.
Project Results
In the nine-month Phase I program, Cermet and Georgia Tech's efforts focused on two of the three major technological barriers in the development of UV emitters. The first effort focuses on development of high-quality, bulk AlN crystals to eliminate dislocations, which can be a major contributor to efficiency roll-off at high drive current densities. Bulk AlN minimizes thermal expansion cracking in high Al-content emitters. Aluminum nitride substrates are transparent in the UV portion of the spectrum, allowing through-wafer emitter designs into the deep UV. Lastly, AlN has a significant thermal conductivity, enabling effective power management of large area power (>0.5 watt) LEDs for SSL needs.
Two-inch-diameter bulk AlN was grown using Cermet's process. The materials exhibited an etch pit density of 1 x 10 5 cm-2 and X-ray peak widths as low as 76 arc seconds. Polished surfaces of 5.8 angstroms (rms) were achieved on this material. The second effort focused on development of high n-type doping level in AlxGa1-xN alloys. Initial AlGaN layers and multiple quantum wells were grown on AlN substrates, with excellent structural results obtained.