IEEE Spectrum November, 2011 - 20

"When i pick a project on which to work, i don't generally look for a problem to solve.
rather, i look for a place to use something i can do very well."

Explaining LEDs'
Diminishing Returns

New concept pulls popular theories
together, drives researchers apart

T

electrostatic attraction, and
he LED community
recombine to emit light.
is ensnared in a
In red LEDs, these
long-running and
quantum wells are just
contentious debate over the
like those in an elementary
origin of a phenomenon
quantum-mechanics
called droop, the decline
textbook, and the probability
in the efficiency of blue
of finding electrons and holes
and white emitters as their
in the trenches is very high.
current is cranked up.
But blue and white LEDs are
Solving this mystery will
plagued by strong internal
enable the design of droopelectric fields. Chow explains
busting LED architectures
that when the current is
that will make brighter,
merely trickling through
cheaper solid-state lighting.
the LED, electrons and holes
The droop debate has
rarely form the bound states
recently heated up, with
needed for light emission:
a handful of competing
"The field just rips them off."
conjectures winning
In one model, as you
significant backing from
crank the current up, electron
groups of optoelectronics
and hole populations rise,
experts. Now a theorist at
Sandia National Laboratories, partially offsetting the internal electric field. However,
in Albuquerque, has found
even though most of the
a way to draw the models of
light emission together. Weng carriers are now in the well,
droop sets in because the
Chow calculates the behavior
internal field is still strong
of LEDs from their band
structure, which specifies the enough to yank electrons to
one side of the trench and
energies that charge carriers
holes to another, hampering
can have within devices and
light emission. Cranking the
the likelihood of finding
current even higher in this
carriers with those energies.
model negates the effects of
Every LED, regardless of
the internal field, improving
its color, operates by injecting
emission efficiency and leadelectrons into the device from
ing to a recovery from droop, a
one side and driving in their
positive counterparts-holes- phenomenon that contradicts
what is seen in real LEDs.
from the other. Both types
Blue and white LEDs
of carriers meet in a narrow
are riddled with defects in
trench, known as a quantum
their crystal lattices, which
well. Here they are trapped,
are more prevalent in their
bind together through
20

nA * Ieee Spectrum * november 2011

indium gallium nitride
quantum wells. When Chow
includes this detail in his
model, it can quash droop
recovery and mimic the real
behavior of these devices.
In this form, Chow's model
unites two leading conjectures
for the cause of LED droop-
defects and inefficient carrier
recombination in quantum
wells. But, crucially, he took
it one step further: Chow
incorporated the most
popular droop theory of all-
Auger recombination, a nonlight-emitting interaction of
three charge carriers that
propels either an electron or
a hole to a higher energy state.
With this addition, Chow's

theoretical predictions
for LED efficiency mirror
the major trends seen in
experimental results.
According to Ümit
Özgür, from Virginia
Commonwealth University,
Chow's work warrants attention. He believes that one of
its strengths is that it models
real physical phenomena
known to take place during
light emission. A simpler, very
widely used model to explain
LED behavior and account for
droop fails to do this. Özgür
now wants Chow to fit experimental data from a range of
real devices to his model.
Manos Kioupakis and
Chris Van de Walle from
the University of California,
Santa Barbara, are more
critical of Chow's work. They
point out that in Chow's
model, droop is sensitive to
temperature and argue that
this finding is inconsistent
with experimental results.
The Sandia researcher
counters by claiming this
inconsistency does not exist,
because his model is not as
sensitive to temperature as
the UCSB team suggests.
He also points out that
work by Jörg Hader and
his colleagues from the
University of Arizona can
replicate LED behavior
at various temperatures
with a model, like Chow's,
that includes defects.
What is clear is that the
droop debate shows no signs
of abating. While Chow's
work is drawing together
several leading theories,
it's failing to unite all the
theorists responsible for them.
-Richard Stevenson
spectrum.ieee.org

LuIs moLIna/Istockphoto

-Wilson Greatbatch (1919-2011), inventor of the implantable pacemaker
http://spectrum.ieee.org
Table of Contents for the Digital Edition of IEEE Spectrum November, 2011
