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Research review
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GaN offers promise for thermoelectrics
One of the most promising nitride-based thermoelectrics has been built by
researchers at the University of California, Santa Barbara.
Researchers at the University of California, The UCSB team, which includes Hiroaki
Santa Barabara (UCSB), have fabricated a Ohta, Steven DenBaars and Shuji
GaN-based thermoelectric that can produce Nakamura, produced the thermoelectric by
2.1 µW from a 30 K temperature difference. first depositing a 3.5 µm thick layer of
This is roughly half of the power produced silicon-doped GaN on sapphire.
by an InN/AlInN thermoelectric fabricated by
Japanese researchers several years ago, but Conventional lithography, dry etching with
in that case the temperature difference was an inductively coupled plasma and the
far higher - 332K. formation of metal contacts led to the
creation of a series of devices with 1, 5, 10
Temperatures differences across and 25 thermoelectric elements.
thermoelectrics lead to the generation of a
potential difference, and this generates an Applying a 30 K temperature gradient
electrical current. across the 25-element device produced a
maximum open circuit voltage of 0.3V and a USCB researchers fabricated a series of
Today the leading materials for fabricating peak output power of 2.1 µW. High- thermoelectric devices, including a
thermoeletrics are based on Bi
2
Te
3
. These temperatures tests revealed that the version that contain 10 elements.
materials are scare and toxic, and devices thermoelectric shows no signs of Credit: UCSB
made from them are limited to operating degradation up to 825K, the limit of the
temperatures of up to 150 degrees C. testing apparatus.
In comparison, GaN-based devices are Ohta says that this work is just a preliminary
capable of far higher operating study. “We will move on to alloys or more
temperatures, they are not toxic, and in complex materials to improve efficiency.”
addition to power generation, they could be
used to provide on-chip spot cooling for A. Sztein et al. Appl. Phys.
nitride-based LEDs, lasers and HEMTs. Express 2 111003 (2009)
Microscopy unveils UV LED aging mechanism
An international collaboration has uncovered distinguished areas that were several actually believed to stem from higher
an aging mechanism in UV LEDs that microns in diameter and produced emission currents through the spots, which leads
involves local increases in current, followed at longer wavelengths. The density of these to local heating and atom migration.
by heating and atom migration. This spots, which are seen in both aged and This decreases the local potential and
discovery resulted from efforts by fresh devices, is roughly one per 100 µm
2
. increases the current once more, creating a
researchers at the Royal Institute of cyclical process that continues to fuel itself
Technology, Sweden, to measure the The researchers found that these spots until LED failure.
electroluminescence from UV LEDs with a produced relatively high
scanning near-field optical microscope electroluminescence intensity, indicating a Interestingly, these spots are not observed
(SNOM) that could realize a spatial local increase in carrier injection. Studies in all types of UV LED. “Currently, we are
resolution of 150 nm. over several days revealed a gradual performing SNOM measurements on UV
increase in emission intensity from these LEDs emitting at different wavelengths, and,
US firm Sensor Electronic Technology spots and a red-shift in emission for instance, for a 335 nm emitting device
provided the 285 nm, flip-chip LEDs, which wavelength. no lower potential spots have been found,”
featured an AlN buffer, an AlN/AlGaN said Saulius Marcinkevicius from the Royal
supperlattice, and an active region with five Possible explanations for this behavior that Institute of Technology.
quantum wells. are based on temperature, strong internal
electric fields, and quantum well intermixing A. Pinos et al. Appl. Phys.
Optical measurements uncovered well- were all ruled out. Device behavior is Lett. 95 181914 (2009)
42 www.compoundsemiconductor.net January/February 2010
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