41-42 Research Review v2 10/9/09 13:33 Page 42
research review
rhombus4
Progressing to color displays will require the addition of blue and
green LEDs. The team has already had success in this area,
according to corresponding author John Rogers from UIUC,
and a paper is now being written on that topic
hydrofluoric acid etched away the produced with the processes
vast majority of the AlAs layer. An developed by Rogers and his co-
automated printing tool with workers. “Options to licenses to the
elastomeric stamp then pulled arrays necessary IP have been obtained
of these tiny LED chips from the from Semprius by a separate startup,
substrate, thanks to a sufficiently CoolEdge, that will pursue lighting
strong Van der Waals interaction, and exclusively, starting with systems for
printed them onto another surface. general illumination.”
Fabrication of red displays was S.-I. Park et al. Science 325 977 (09)
completed by adding electrical
interconnects with planar processing HVPE ups GaN resistivity
methods. Scientists from Nanjing University,
China, have used HVPE to produce
Progressing to color displays will undoped GaN epilayers with a room-
require the addition of blue and temperature resistivity of 10
9
Ω cm.
green LEDs. The team has already
had success in this area, according Such high resistivities in GaN films -
to corresponding author John Rogers which are needed to provide the
from UIUC, and a paper is now necessary isolation from the
being written on that topic. substrate during the production of
powerful, high-frequency III-nitride
If this technology is to be transferred transistors - are normally realized with
Credit: UIUC to the manufacturing arena it will iron or carbon doping. However,
have to demonstrate acceptable additional impurities can lead to leaky
Displays based Desktop monitors, home theatre yields. “The yield of printed, working devices, inferior transport properties,
on an array of systems and instrument gauges devices is very close to 100%,” says and possibly reduced reliability.
red LED pixels could all be produced with the US Rogers, “ and the yield for The Chinese team grew their high
have been team’s technology. Wearable health interconnected arrays interfaced to resistivity GaN epilayers on a
built by a monitors offer another opportunity, computer control systems is in the commercial, free-standing GaN
partnership which would require the mounting of 85% range.” Most of these failures substrate, which was also produced
led by the LED chips on a flexible platform. are associated with external by HVPE.
University of connections to ribbon cables.
Illinois at The researchers built a range of Cathodoluminescence measurements
Urbana- small, red displays by first growing Similar etching and printing methods revealed a dislocation density for the
Champaign sacrificial 1.5 µm-thick AlAs layers are also being employed by a partner film of 6 x 10
6
cm
-2
. Resistivity was
and Semprius and GaInP-based LED structures on in this research, Semprius, which is evaluated by fabricating a Schottky
GaAs substrates. Square islands making small GaAs solar cells diode, and taking current-voltage
containing the LED epistructure coupled to concentrating optics. measurements.
and having a length of just 50 µm “They have built working modules,
were defined on the wafer by a and are on the verge of sending out Treating the GaN substrate and
combination of photolithography research demos at the 1-10 kW level epilayer as two resistors connected
and inductively-coupled plasma, for customer evaluation,” says in series led the researchers to
reactive-ion etching. Rogers. deduce a resistivity for the undoped
film of 3.6 x 10
9
Ω cm.
Immersion of the processed wafer in Lighting systems could also be H. Shi. Electron. Lett. 45 910 (09)
42 www.compoundsemiconductor.net September 2009
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