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technology nitrides
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GaN: better with defects?
It is taken for granted that lowering the defect density in GaN-based light emitters
improves their performance. But photoluminescence studies on GaN powders
suggests that defects might actually be a good thing, says Birgit Schwenzer from
the University of California, Santa Barbara.
T
here is no doubt that GaN-based light emitting markets. These include car headlamps, projectors, and
devices are a great success. Blue, green and the most lucrative sector of them all, general illumination.
white LEDs made with this material now backlight the But success in these markets will only come if the cost-
keypads and screens of billions of mobile phones, and per-lumen of the devices falls substantially.
provide light sources in bill-board displays, torches and
traffic lights. Meanwhile, GaN ultra-violet lasers are Researchers throughout the world are embarking on this
powering Blu-ray players and some of the latest games quest by increasing the efficiency of GaN devices,
consoles. particularly at the high drive currents needed to generate
the required lighting levels demanded by the emerging
However, if sales of these devices are to continue to applications. Many believe that brighter devices can be
grow, then their manufacturers will have to target new realized by reducing the defect density in these
structures. Although nitrides are incredibly resilient to high
defect densities - which would kill luminescence in other
compound semiconductor light emitters - several
theoretical and experimental studies are claiming that
defect-free nitrides would be far more efficient.
I decided to take a different path to cutting the cost-per-
lumen of nitride devices, which focused on reducing
expenditure on the growth tools used for manufacture.
However, through these efforts at developing a lower cost
synthesis method, I have turned one of the core pieces of
perceived wisdom about GaN on it head - I’ve discovered
that increasing the defect density in this material can
actual enhance its photoluminescence intensity in the
case of GaN nanoparticles.
Back in 2002 - under the guidance of Umesh Mishra and
Steven DenBaars at the University of California, Santa
Barbara (UCSB) - I started looking at methods to grow
nitride materials that could offer a more affordable
alternative to the widely used MOCVD approach that
employs multi-wafer reactors with million dollar price tags.
My work focused on a growth method known as
ammonolysis, which is also described as ammonothermal
synthesis. This process aimed to create highly crystalline,
chemically pure material by heating a gallium containing
starting material in an ammonia atmosphere. Wet-chemical
Figure 1. Ammonolysis is a relatively simple technique for producing group approaches to GaN synthesis were avoided, because
III-nitrides in a tube furnace. GaN powders were prepared at UCSB by they can lead to the addition of impurities such as carbon,
heating either metallic gallium or gallium oxide-based precursors under oxygen or hydrogen, which diminish the intensity of the
ammonia at temperatures of 900-1100
o
C band edge photoluminescence in GaN. These impurities
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www.compoundsemiconductor.net September 2009
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