Researchers tunnel to more efficient light
By removing p-doping and using quantum-mechanical tunelling, US team boosts lumens per watt in GaN-based LEDs
In the October 24, 2017 issue of Nature's Light: Science & Applications, currently online, 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 wall socket 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 GaN, 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 tunnelling, not by p doping. The tunnelling 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 tunnelling diodes (RTD) in the GaN system for the Office of Naval Research under program manager Paul Maki. As reported in the August 2016 issue of Applied Physics Letters, their effort also established a stable GaN-based RTD platform for high microwave power generation and potentially terahertz sources.
The fundamental science behind this advancement is the use of the extremely highe lectric fields induced by the polarisation effects within a 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 also simultaneously inject holes by Zener tunnelling across the GaN band gap into the valence band. Thus, the new LED uses only n-type doping, but includes bipolar tunnelling charges to create the new LED light source.
To be useful for commercialisation, 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.
'Near-UV Electroluminescence in Unipolar-Doped, Bipolar-Tunneling (UDBT) GaN/AlN Heterostructures' by Tyler A. Growden et al; Nature's Light: Science and Applications