review research
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Vertical architecture cranks up UV output
A partnership between Asif Khan’s group at the University of South Carolina and its spin-off, Nitek,
claim to have developed ultra-violet LEDs with a record output for a single chip.
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 bio-
medical 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 p-
type layers. These layers are
deposited on a sapphire substrate by
a combination of pulsed atomic layer
epitaxy and MOCVD.
Focusing an excimer laser through uniformly distributed over the emitting
the substrate leads to dissociation at surface, thanks to the absence of
the AlN/sapphire interface. “The current crowding.
entire epilayer peels off from the
sapphire substrate,” explains Khan. A No appreciable change in output
Ni/Au layer forms the LED’s p- power was observed when the
contact, and the n-contact is added device was driven at a current
by first exposing the n-type AlGaN density of 25 mA cm
-2
for 210 hours.
layer by reactive ion etching, before Extrapolating the output power as a
defining a Ti/Al grid on this surface. function of time led to an estimate of
device lifetime of well over 2000
An unsaturated, continuous-wave hours. “Our next series of improved
output of 5.5 mW was realized at a devices are being put on lifetest, and
250 mA drive current. This we will keep measuring them for a
corresponds to a current density of few months to get the actual
25 mA cm
-2
, a relatively low value that numbers, rather than extrapolations,”
leads to minimal self-heating and explains Khan.
thermal degradation. Light output is
“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 Top: Ultra-violet lamps are widely used for water
the chip’s output power, and an purification, but LEDs could replace this technology if
increase in the overall efficiency of their efficiencies are improved. CREDIT: Hanovia.
electrical to optical conversion. Bottom: Ultra-violet LEDs with a vertical carrier
injection architecture promise to unlock the door to
V. Adivarahan et al. single chip output powers of tens of mW.
Appl. Phys. Express 2 092102
October 2009
www.compoundsemiconductor.net 41
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