Researchers tunnel to a new light source

With concerns over moving to a clean energy platform worldwide with electric vehicles and renewables, the energy we consume, or should we say do NOT consume, is as important as the green energy we produce. Thus, solid state lighting, more efficient than incandescent and fluorescent bulbs, based upon light emitting diodes (LED) is touted as the solution. However, LEDs struggle to deliver high brightness for the shorter-wavelength end of lighting needs. And emitted short wavelengths facilitate white light through known phosphor downconverters.

In the October 24, 2017 issue of Light: Science & Applications, currently online, the Ohio State University researchers, with scientists at Wright State University and Naval Research Laboratory, describe a promising new semiconductor LED made with GaN-based materials that could boost wallsocket efficiency by reducing energy losses and self-heating.

If this new technology can be harnessed for large light output, the breakthrough could enhance LED solid state lighting without a significant change to the existing LED manufacturing facility.

The new LEDs could provide more light with less voltage and resistance than in conventional GaN LEDs, thereby boosting the overall lumens per watt output and avoiding the efficiency droop that plagues high brightness LEDs.

One way the team overcomes this problem is by completely removing all p-type doping in gallium nitride, which historically is hard to dope and leads to a high series resistance.

The key to the team's discovery is the ability to create "holes" for radiative recombination with electrons by quantum-mechanical tunneling, not by p doping. The tunneling occurs by the Zener mechanism, delivering the holes to the zone of recombination, mitigating the need for clumsy p-type ohmic contacts and resistive p-type semiconductor injectors.

The team includes Paul R. Berger and Tyler A. Growden at Ohio State University; Elliott R. Brown and Weidong Zhang at Wright State University; and David F. Storm and David J. Meyer at the Naval Research Laboratory.

Their discovery was made while advancing resonant tunneling diodes (RTD) in the gallium nitride system for the Office of Naval Research under program manager Dr. Paul Maki. As reported in the August 2016 issue of Applied Physics Letters, their effort also established a stable GaN-based RTDplatform for high microwave power generation and potentially terahertz sources.

The fundamental science behind this advancement is the utilization of the extremely highelectric fields induced by the polarization effects within wurtzite GaN based heterostructures. These high fields allow the new device to not only inject electrons across a classic RTD double-barrier structure in the conduction band, but alsosimultaneously inject holes by Zener tunneling across the GaN band gap into the valence band. Thus, the new LED uses only n-type doping, but includes bipolar tunneling charges to create the new LED light source.

To be useful for commercialization, the team is working to balance the injected electron and hole ratio to create and therefore deliver up to one emitted photon for each injected electron.

Credit: 
Changchun Institute of Optics, Fine Mechanics and Physics