Panasonic’s Non-polar LEDs Combine Efficiency And Power
Wide quantum wells that are free from internal electric fields combat droop and enable high efficiencies at high drive currents
Engineers at Panasonic’s Device Module Developement Centre have fabricated non-polar LEDs on m-plane GaN that deliver incredibly high efficiencies at high current densities.
These non-polar 408 nm LEDs, which have dimensions of 450 µm by 450 µm, operate at light output efficiency of 39.2 percent when driven at a current density of 1000 A cm-2. This corresponds to an output of 1.35 W.
“The efficiency is not a record at low current densities of around 200 A cm-2, but I think it is a record for high current densities of around 1000 A cm-2", says Panasonic’s Akira Inoue, who unveiled these results in a paper presented at the 2012 International Electron Devices Meeting in San Francisco in mid-December.
Inoue and his co-workers have also developed techniques to control the radiation pattern produced by the LED. They can adjust this by altering the surface of the LED packages and the height of the chips, and by introducing a striped texture on the top m-plane surface.
These efforts could help the uptake of m-plane LEDs in projection displays and automotive headlamps. “In these applications, engineers must design optical components, such as lens, mirrors, and prisms," explains Inoue. “The controlled radiation pattern is useful for the optical design.
LEDs produced by the team feature a silicon-doped, n-type GaN layer; an InGaN/GaN multiple quantum well; a magnesium-doped, p-type AlGaN electron-blocking layer; and a magnesium-doped, p-type GaN layer. After dicing of the wafer, chips were mounted face down on ceramic packages made from either AlN or alumina.
The LEDs feature a relatively large recombination region, with three quantum wells with a thickness of 15 nm, sandwiched between 30 nm GaN barriers. The engineers have selected this design to combat LED droop, the decline in device efficiency at high drive currents. They believe that the droop is caused by a combination of electron leakage and Auger recombination.
According to the team, thick wells reduce carrier density, and this minimises Auger recombination. Meanwhile, the lack of a polarization field within the active region diminishes the electron leakage current.
The LEDs, which have surface texturing on the top surface to enhance light extraction, produce a peak external quantum efficiency of 51.5 percent at 88 A cm-2. Cranking the current density up to 1400 A cm-2 propels the output power to 1.62 W.
Today, the vast majority of LED manufacture is carried out on sapphire and SiC substrates, with the use of silicon, which is much cheaper, tipped to grow. In comparison, m-plane GaN substrates are incredibly expensive, but Inoue believes that there will come a time when these are used for LED production.
“The price reduction of GaN substrate is going on every year", says Inoue, “and the size of m-plane GaN-LED is only one-third to one-fifth of that of conventional, high-power c-plane GaN-LED at the same rated current of around 1 A."
This reduction in chip size for a given output power trims costs. “We have a plan to begin the commercial production in 2015," says Inoue.
Panasonic’s 408 nm m-plane can deliver an LED output of more than 1.5 W
Panasonic’s LED exhibits very little droop. The researchers believe that this stems from thick wells that minimise Auger recombination and a lack of internal electric fields that reduce electron spill over into the p-side of the device.