conference report technology
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s a fast-growing and dynamic country, Korea with a plenary talk on devices made using semipolar and
was an ideal host for the most recent nonpolar crystal orientations, which can help reduce or
International Conference on Nitride Semiconductors eliminate the piezoelectric fields that plague conventional
(ICNS-8). This popularity of this conference series is c-plane (polar) nitride devices. The audience was
clearly on the up: almost 1000 delegates flocked to the especially keen to hear his views on the best film
meeting, and 27 different industrial exhibitors made their orientation for making green laser diodes.
way to Jeju Island from across the globe, continuing the
trend of growth seen from previous conferences in Las DenBaars began by recapping recent achievements
Vegas (2007) and Bremen (2005). throughout the globe. The leading successes on c-plane
material are a 526 nm laser from Osram and 515 nm
Despite the recent economic recession, the leading LED version from Nichia. For non-polar lasers, UCSB and
manufacturers - such as Cree, Osram, Philips Lumileds, Rohm are leading the way with lasing at around 492 nm
and Samsung - dominated the conference program and 500 nm, while Sumitomo Electric recently produced a
presentations. Their strong presence also provided further 531 nm laser on the unusual semipolar 2021 plane.
evidence of the robust nature of the nitride-based device
market, which is expected to grow strongly, thanks in part Thanks to the rapid pace of progress using all of these
to the emergence of solid-state lighting (SSL). Similarly, orientations, no clear winner has emerged just yet;
sustained worldwide governmental support for the however, DenBaars says that the alternative orientations
development of energy-efficient SSL ensured that a high do offer some significant advantages. For example,
proportion of European, American and Asian academics although incorporating high concentrations of indium is
were able to attend. still difficult, the UCSB group has shown that the absence
Thanks to these shared goals and interests, it was no
Thanks to these shared goals and interests, it
surprise to see that the majority of contributions from
academia and industry focused on light-emitting devices.
was no surprise to see that the majority of
Key themes that emerged during the week were the
unsolved problems of ‘efficiency droop’ and the ‘green
contributions from academia and industry
gap’.
focused on light-emitting devices. Key themes
Although LED efficiencies initially tend to rise with
increasing current densities, they subsequently level off
that emerged during the week were the
and then drop as the operating current increases. This
unsolved problems of ‘efficiency droop’ and the
‘roll-over’ and the ‘droop’ that follows it are major
obstacles to the development of high-efficiency, high-
‘green gap’
brightness LEDs for low-cost SSL. Unfortunately, while
droop isn’t such a problem for near-UV devices, it
becomes increasingly important for longer-wavelength of fields across the light-emitting InGaN quantum wells
blue and green emitters, which contain high proportions allows them to be grown thicker, and this in turn enables
of indium in the InGaN active regions of the device. This the elimination of the AlGaN cladding layer. Getting rid of
increase in indium content tends to be correlated with this layer is a big plus point, because it means that the
drastic reductions in device efficiencies even at the growth time for a 492 nm blue m-plane laser can be
optimum operating current, limiting device performance as slashed from 12 hours to just 2 hours. Additionally, LEDs
emission wavelengths move into the green and yellow grown on semipolar orientations - where indium
spectral regions. incorporation is easier - could help close the green gap:
UCSB’s first unoptimized 562 nm LEDs showed a
Unfortunately, coming from the other end of the spectrum, surprisingly high external quantum efficiency of 13.4 %.
the efficiencies of AlInGaP devices - traditionally used to
create red, yellow and orange emitters - also falls off as Droop: the mysterious malady
wavelengths shorten and move towards the green, hence However, UCSB’s nonpolar and semipolar LEDs still
the term ‘green gap’. It’s a gaping hole that desperately showed unexplained efficiency droop, despite the
needs to be plugged, especially if devices such as green reduction of piezoelectric fields. Summarising the debate
lasers for projection applications are to reach the market. on droop, Shugo Nitta from Toyoda Gosei discussed the
possible candidate mechanisms: Auger recombination,
With this in mind, Steve DenBaars of the University of related to high current densities; electron overflow from
California, Santa Barbara (UCSB) opened the conference the active region; poor hole injection; polarisation effects,
November / December 2009 www.compoundsemiconductor.net 23
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