conference report technology
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An alternative route towards lowering costs is to increase the high defect densities were linked to high forward
LED efficiencies. This can be realized by reducing the voltages, low output powers and highly resistive p-type
high densities of dislocations – typically 10
8
cm
-2
– that material. Elsewhere at ICNS-8, only modest progress was
are produced when nitride films are grown on foreign reported on the reduction of defect densities in
substrates. While there were few surprises in this heteroepitaxial AlGaN and AlN films, which is needed for
established area, the University of Cambridge did present high external quantum efficiencies and output powers in
data showing that dislocations could move and react the UV.
within GaN films at growth temperatures, suggesting a
future for suitably chosen annealing-based dislocation Low defect density, insulating GaN and AlN substrates
reduction processes. are also promising for GaN-based electronics, as
dislocations are thought to lead to high leakage currents
Of course, low defect density bulk substrates would be an and degradation in GaN-based HEMTs, which are used in
ideal solution to the dislocation problem, but their costs high-frequency and power switching applications.
need to fall substantially. Fortunately, considerable Reflecting these concerns, many groups presented device
progress towards commercially viable substrates is being reliability studies. One clear trend is towards the use of The 8th
made using several competing approaches: both the InAlN, and even InAlGaN, in order to circumvent some of International
Chinese Academy of Sciences (with Suzhou Nanowin) the strain and defect issues associated with AlGaN-based Conference on
and Unipress/TopGaN announced plans for mass transistors. New designs were also presented, including Nitride
production of substrates grown by hydride vapour phase the ‘normally-off’ transistors based on nonpolar crystal Semiconductor
epitaxy (HVPE), the latter using high-pressure solution orientations that were developed by Umesh Mishra’s was held
growth on top of a HVPE-grown ‘seed’ crystal. group at UCSB, while Cree announced a series of in Jeju
improvements to their high-power, high-frequency International
In contrast, Osaka University, Japan, announced transistor range, bringing the prospect of high-power Convention
breakthroughs in bulk GaN grown by the sodium-flux near-terahertz applications for the nitrides within reach. Center, Jeju
method, reporting inclusion-free 2-inch c-plane GaN Island, Korea,
wafers with dislocation densities of around 3 x 10
4
cm-2 It will be interesting to see how much progress is made between 18
and announcing their plans to scale up to 4-inch substrate on this front, and also on the quests for an understanding and 23
production. However, the most impressive data came from of the green gap and LED droop when the nitride October. 940
the Polish company Ammono Ltd., who reported flat 2- community reconvenes for the ninth meeting of this delegates from
inch c-plane GaN substrates grown by the series in Glasgow in 2011. If this meeting is anything 35 countries
ammonothermal method with dislocation densities as low like the previous affairs, it promises to be a have a real attended this
as 5 x 10
3
cm-2. Despite the low growth rates associated buzz about it. event
with this method, high-volume production and commercial
release of these 2-inch substrates is scheduled to occur,
once a sufficient stock of seed crystals has been built up.
All of these methods also have the potential to lead to the
commercialisation of bulk nonpolar and semipolar GaN.
Additional progress on low defect density c-plane bulk
AlN was also announced by several groups, including the
Russian start-up Nitride Crystals, who reported the
sublimation growth of c-plane AlN using AlN-on-SiC
seeds in a tungsten crucible. The resulting 2-inch
substrates have a ‘block’ structure that enables the
realization of areas with dislocation densities as low as
10
2
cm
-2
. Substrate sales will begin once seed crystal
stock has been built up.
Although the availability of all large-area native substrates
is currently limited and the cost is high, this route towards
low defect density material looks set to be especially
important for UV-LEDs. Although deep-UV LEDs were
announced based on nonpolar (NTT Corporation),
semipolar (University of South Carolina with NITEK) and
c-plane (RIKEN and Sensor Electric) heteroepitaxial films,
November / December 2009 www.compoundsemiconductor.net 25
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