research review
rhombus4
developing a low-reflectivity, Al2O3 coating for one of the substrates and a 2µm thick GaN epilayer deposited on
facets, in order to increase wall plug efficiency. This them is below 5 x 10
4
cm
-2
; optical excitation experiments
adjustment, alongside the introduction of longer cavities, produce strong emission at 3.4 eV, indicating good
has led to the fabrication of a 1.1W QCL mounted on AlN optical quality; and reflectance measurements reveal the
that does not require cooling. strong hexagonal symmetry of the crystals.
Micro-pixelated lamp combats This ammonothermal approach also has the edge over
current crowding HVPE in terms of the growth process, according to
Nitek and Asif Khan’s group at the University of Ammono’s Robert Dwilin´ski: “ In HVPE, gases are flowing
Ammono South Carolina have fabricated a 42 mW lamp emitting through the open reactor and only a small fraction of the
produces at 280 nm. raw materials is converted into the product. But with the
hexagonal GaN ammonothermal method, thanks to recrystallization of
crystals with This ultraviolet lamp is a promising source for air and polycrystalline GaN feedstock in a closed system, almost
excellent optical water purification and polymer curing. “42 mW is certainly 100 percent of the raw material can be converted into the
quality by the enough to purify water at the tap at a rate of about a final product.”
ammonothermal gallon per minute,” says Khan.
method. Dwilin´ski says that other strengths of the ammonothermal
Credit: Ammono One of the challenges of making the chip was to approach include lower growth temperatures that reduce
overcome lateral current crowding, and this was energy consumption, easier reactor maintenance, and the
addressed with a monolithic chip that contains 1600 scaleable nature of the process, which means that it is
micropixels, each with a diameter of 20 µm. Driven at an possible to grow hundreds of crystals in one run.
output power of 22 mW, this lamp has a lifetime of over
1500 hours. Ammono aims to lead the world in the development of
ever-larger high-quality, non-polar substrates. “We can
Khan says that the team should be able to double the easily keep this position because our growth method can
output power of the chip by optimizing the surface be scaled up to any thickness,” says Dwilin´ski.
roughening of the chip.
Calculations confirm the benefit of
Ammonothermal approach yields semi-polar planes for green emitters
non-polar substrates Calculations by a US researcher have revealed why semi-
A Polish partnership has employed an ammonothermal polar planes of gallium nitride offer the best platform for
growth method to produce non-polar GaN substrates with growing green semiconductor lasers.
incredibly low threading dislocation densities (TDDs).
John Northrop from the Palo Alto Research Center
Development of high-quality non-polar substrates is seen performed first-principles calculations based on the
as an important goal in the nitride community, because it chemical potential, which show that indium will
enables the fabrication of optoelectronic devices that are incorporate in higher concentrations on the semi-polar
free from the large internal electric fields that hamper (1122) surface than the non-polar (1010) surface.
electron-hole recombination in conventional LEDs and
lasers. Northrop says that he considered the chemical potential -
which is the free-energy per atom for an atomic species -
Led by the Warsaw firm Ammono, this Polish team has because it allows comparisons of energy between
produced m-plane substrates up to 11 mm by 22 mm in surfaces that have differing numbers of atoms.
References size that have a TDD below 5 x 10
4
cm
-2
. These substrates
V. Adivarahan have fewer defects and are slightly larger in size than He reached his conclusions by employing “fairly complex”
et. al. 2009 typical pieces produced by the leading commercial code developed at the Fritz Haber Institute, Berlin, and
Appl. Phys. supplier of m-plane GaN, Mitsubishi Chemical, which calculating the indium chemical potential of (1122) and
Express 2 produces its material by HVPE. (1010) layers with various degrees of indium incorporation.
102101 These calculations included a low hydrogen chemical
Ammono’s substrate production begins by dissolving potential, a condition that is typically found in an MOCVD
R. Kucharski et GaN-containing feedstock in ammonia in one zone of a growth chamber.
al. 2009 Appl. high-pressure autoclave. A temperature gradient drives
Phys. Lett. 95 material to a second zone, leading to crystallization of So far, the longest emission wavelength for a nitride laser
131119 (2009) GaN on native seeds, due to supersaturation of the has been realized on the (2021) plane.
solution. Measurements indicate that the quality of
Northrop Appl. Ammono’s material is excellent: X-ray diffraction spectra Northrop is interested in these results that were produced
Phys. Lett. 95 have peaks with a full-width half maximum below 20 arc by Sumitomo, but he has not performed calculations for
133107 (2009) seconds; optical microscopy reveals that the TDD in the that particular plane.
42 www.compoundsemiconductor.net November / December 2009
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