technology conference report
rhombus4
between the InGaN quantum wells and the p-type layer.
This improved hole injection efficiencies at high current
densities. However, Fred Schubert from Rensselaer
Polytechnic Institute showed that p-doped electron
blocking layers allowed good hole injection, while
compositional control could be used to reduce the
polarisation mismatch between the LED’s quantum wells
and the barriers that separate them. This combination
reduced the carrier leakage, thereby reducing droop.
However, the strong droop observed in UCSB’s nonpolar
LEDs suggested that polarisation couldn’t be the whole
story.
No matter what, there were no reports of droop-free
LEDs, regardless of the active layer design, film
orientation, defect densities or measurement temperature
– leading Jong-In Shim of Hanyang University and many
others came to the conclusion that droop must be
intrinsic to III-nitride materials, although it wasn’t clear
why. At the packed rump discussion on this topic, most
agreed that some loss mechanism occurred at high carrier
densities, but the simple rate equation used to describe it
might be inappropriate to describe all the complex, poorly
Delegates could which encourage carrier leakage; and problems related to understood recombination processes that seem to occur
walk for just a few defects and InGaN material composition. in the nitrides.
minutes from the
conference to Aurelien David confirmed Philips Lumileds’ existing However, leading the discussion away from droop
marvel at position on droop, presenting both electrical and mechanisms, James Ibbetson of Cree said it was “time to
magnificent views luminescence measurements that could be explained in move on from the name-calling” and instead think about
of Jeju Island terms of a composition-independent Auger process. “what comes next”, arguing that high LED operating
Consistent with this finding, many groups reported temperatures produced an equally important droop effect.
success by reducing carrier densities in the active region: Christian Fricke presented Osram’s route towards better
the Fraunhofer Institute, Germany, opted for wide thermal management, involving drilling tiny holes in the
quantum wells and low dislocation densities, while Cree’s chip and creating buried p-contacts, permitting uniform
“engineering solution” was to increase the chip area. current injection and light extraction.
Taking a different approach, the group of Seong-Ju Park at However, he also emphasised the need to control costs
Gwangju Institute of Science and Technology, which is and to concentrate on LED reliability and light quality,
working in association with Samsung LED, reduced droop rather than limiting the focus simply to high-brightness
by removing the AlGaN electron blocking layer that lies devices, which are not appropriate for every application.
Slashing LED costs
At the packed rump discussion on this topic,
One way to reduce costs is to increase the substrate size,
cutting down on the wasted edge material and thus
most agreed that some loss mechanism
boosting yields. However, it can be harder to get uniform
occurred at high carrier densities, but the
layers on larger substrates, due to wafer bow. With an eye
on future trends, Aixtron demonstrated a new reactor
simple rate equation used to describe it might
configuration for 6-inch wafers, capable of producing
GaN-on-sapphire epilayers with a reproducible thickness
be inappropriate to describe all the complex,
uniformity of 1%. Building up to full devices, the University
of Cambridge group presented 455 nm LEDs with a high
poorly understood recombination processes
internal quantum efficiency of 58 % fabricated on 6-inch
silicon, while IMEC showed considerable progress with
that seem to occur in the nitrides their green LEDs on 4-inch sapphire.
24 www.compoundsemiconductor.net November / December 2009
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