Vertical Architecture Cranks Up UV Output
These thin-film devices that feature a vertical injection architecture deliver a continuous wave output of 5.5 mW at 280 nm. “We soon expect to double [the output power]", adds Khan.
The researchers’ effort will aid the development of ultra-violet LEDs that could replace mercury lamps for air, water, and food purification; for biomedical treatments; and for polymer curing. Fabrication of ultra-violet LEDs begins with the growth of a 0.3 μm thick AlN layer, a 10 period AlN/AlGaN superlattice and an active region with five quantum wells that is sandwiched between n-type and ptype layers. These layers are deposited on a sapphire substrate by a combination of pulsed atomic layer epitaxy and MOCVD.
Focusing an excimer laser through the substrate leads to dissociation at the AlN/sapphire interface. “The entire epilayer peels off from the sapphire substrate," explains Khan. A Ni/Au layer forms the LED’s pcontact, and the n-contact is added by first exposing the n-type AlGaN layer by reactive ion etching, before defining a Ti/Al grid on this surface.
An unsaturated, continuous-wave output of 5.5 mW was realized at a 250 mA drive current. This corresponds to a current density of 25 mA cm-2, a relatively low value that leads to minimal self-heating and thermal degradation. Light output is uniformly distributed over the emitting surface, thanks to the absence of current crowding.
No appreciable change in output power was observed when the device was driven at a current density of 25 mA cm-2 for 210 hours. Extrapolating the output power as a function of time led to an estimate of device lifetime of well over 2000 hours. “Our next series of improved devices are being put on lifetest, and we will keep measuring them for a few months to get the actual numbers, rather than extrapolations," explains Khan.
“Our next target is to monolithically combine these vertical LEDs, and see how much total power we can get." Other goals include the roughening of the n-type AlGaN layer, which could lead to a tripling of the chip’s output power, and an increase in the overall efficiency of electrical to optical conversion.