review research
rhombus4
Michigan: Auger causes LED droop
Modulation experiments with nitride lasers reveal that Auger is the cause of LED droop
Researchers at the University of Michigan - a time-resolved photoluminescence study They then concluded that Auger
are throwing their weight behind Philips by Lumileds. recombination is the primary cause of droop
Lumileds’ controversial claim that Auger by using this coefficient to calculate the
recombination is the cause of LED droop, The University of Michigan researchers internal quantum efficiency in In
0.1
Ga
0.9
N
the decline in device efficiency at higher performed large-signal modulation quantum wells, and finding that this value
drive currents. measurements on a 407 nm laser with four, agrees “remarkable well” with that
3 nm thick quantum wells that were measured in quantum well LEDs with an
Pallab Bhattacharya and colleagues have sandwiched between 15 nm GaN barriers. identical active region. Different forms of
performed modulation measurements on The devices were driven with 120 ns pulses Auger recombination have been proposed
nitride lasers, and extracted a coefficient for that had a 100 ps rise time, and the emitted as the cause of droop. Bhattacharya says
Auger recombination - a non-radiative light was recorded by an ultra-high-speed that his data indicates that the normal Auger
process involving the interaction of an GaAs photodetector. Turn-on delays of a process is quite strong, due to the high
electron, a hole, and a third carrier. The few ns were determined for drive currents of carrier density in the wells. “However, there
magnitude of this coefficient shows that 30 mA to 90 mA. is bound to be a defect-assisted process as
Auger is the dominant cause of LED droop. well.”
Armed with this information, plus values of
“Prior to our work there has only been one the capture cross-section and deep trap The researchers are now planning to
report of the experimental determination of density - which were both determined from measure large signal modulation in an
the Auger recombination coefficient in deep level transient spectroscopy InGaN/GaN green laser, and analyze the
quasi-bulk InGaN layers,” explains measurements with a UV laser – the efficiency droop in green LEDs.
Bhattacharya. According to him, the researchers performed simulations that
approach that he and his co-workers have yielded a Auger recombination coefficient of M. Zhang et al. Appl.
used is far more direct than the earlier work 1.5 x 10
30
cm
6
s
-1
at 300K. Phys. Lett 95 201108 (2009)
Holey laser could aid silicon photonics
European researchers claim to have made the laser emission threshold.
the first photonic crystal laser that provides
a light source for a silicon waveguide. Pumping the structure with a 1.18 µm laser
produced 1585 nm emission in the silicon-
Developing lasers for silicon circuits is a hot on-insulator wire. Thanks to the geometry of
topic in the III-V research community, the hybrid lasing structure, the researchers
because it could lead to low-cost fabrication could optically pump and collect light
of ultra-compact optical integrated circuits. through the same wire. A scanning electron microscopy image
reveals the triangular lattice and the line
The partnership between the Photonics and The team is also looking at electrically defect in the photonic crystal.
Nanostructures Laboratory in Marcoussis, injecting their lasers. Credit: CNRS
France, and the Belgium research institute
IMEC, realized 1.585 µm pulsed emission “It is a very hard task,” explains
from an InP-based, two-dimensional corresponding author Fabrice Raineri from
photonic crystal heterogeneously integrated the Photonics and Nanostructures
to silicon-on-insulator wires. Laboratory. Adding electrical contacts may
induce losses and impact the optical
The photonic crystal lattice, which features properties of the structure, and channeling
line-defect waveguides in a triangular lattice, the current is difficult due to the air holes.
is designed to cause interference between “That’s why it is important to also discuss
forwards and backwards waves, leading to optical pumping for applications – it may be
propagation of slow light. This is beneficial more straightforward.”
because it can increase the time that light
interacts with a gain medium. Ultimately, this Y. Halioua et al. Appl. Phys.
longer interaction time could lead to a cut Lett. 95 201119 (2009)
January/February 2010
www.compoundsemiconductor.net 41
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87