research review VCSELs retain speeds at high temperatures
A GERMAN team claims to have broken the record for data transmission from an oxide- confined 980 nm VCSEL operating at 85 °C. Their device, which is capable of 25 Gbit/s operation at that elevated temperature, is an ideal source for very short optical links in high performance computers, according to the researchers from the Technical University of Berlin and VI Systems.
“Since temperatures inside computers are as high as 85 °C, or even higher, good temperature stability is indispensable for robust, inexpensive optical links,” says Dieter Bimberg, head of the research team at the Technical University of Berlin.
Today 850 nm is the standard wavelength for short-reach optical links and local and storage-area networks, but Bimberg believes there is a strong case for 980 nm sources in all these applications.
“980 nm has the crucial advantage of transparency of the GaAs substrate, so one can easily realize bottom-emitting devices, increasing and simplifying packaging density. This is very important, for example, in the case of a large number of VCSELs for parallel optical links.”
VCSELs are fabricated via MOCVD growth
The 980 nm VCSELs produced by researchers at the Technical University of Berlin and VI Systems have bit error rates at 25 Gbit/s of less than 10-12
.
of an epistructure containing 24 pairs of Al0.12
Ga0.88 As and Al0.90 Ga0.10 As layers for the
bottom mirror, and 37 pairs for the top mirror. Sandwiched between these mirrors is an active region with five compressively strained In0.21
Ga0.79 P0.12
4.2 nm thick, which are interlaced with 6 nm thick GaAs0.88
Ga0.02 As oxide apertures positioned just
As quantum wells that are tensile strained barriers.
Selective wet etching forms two 30 nm-thick Al0.98
above the microcavity, in the field intensity nodes in the first two periods of the upper mirror.
Output from this 10 µm-diameter oxide aperture VCSEL is 4.3 mW at 20 °C, falling
to 2.6 mW at 85 °C. This relatively small reduction in power stems from an intentional red-shift detuning of 15 nm between the quantum well gain peak and the cavity resonance.
Future targets for the team are to speed the 980 nm VCSELs to 40 Gbit/s and maintain this rate at 100 °C. “We will use an optimized active region to improve the temperature stability even further, and an optimized cavity design to increase the speed beyond 25 Gbit/s,” reveals Bimberg.
A. Mutig et al. Appl. Phys Lett. 97 151101(2010)
Modeling questions Auger’s contribution to droop
CURVE fitting with the standard equation for carrier recombination in an LED shows that Auger recombination cannot, by itself, account for droop, the decline in device efficiency at high drive currents. That’s the claim of a partnership between Rensselaer Polytechnic Institute (RPI), Sandia National Laboratories and Samsung LED.
Their effort involved fabricating a range of LEDs with varying numbers of quantum wells, device areas and quantum efficiencies; measuring external quantum efficiency at a range of currents; and fitting the data with the well-known “ABC” equation for carrier recombination. This equation describes non-radiative recombination at defects by a term that is proportional to the carrier concentration, and uses quadratic and cubic variants to cater for radiative and Auger recombination, respectively.
“It is impossible for us to just use the ABC model and get a good fit,” explains team member Fred Schubert from RPI, who says that the team’s experiment indicates that there are contributions from second order, third order and fourth order terms caused by carrier leakage. “Some of the samples have significant fourth order contributions.”
Inserting additional terms in the carrier recombination model has enabled the US- Korean partnership to fit the experimental data far better. To realize a good fit at all drive currents, it began by matching the ABC model to the data at low current densities. “This part of the curve is not in question,” says Schubert. “Everybody agrees that there is Shockley-Reed-Hall recombination and radiative recombination.”
Curve fitting was then extended to higher currents, where droop plays a significant
54
www.compoundsemiconductor.net November / December 2010
role. Here they found that at current densities of 111 A/cm2
the higher-than-third
order terms contribute 13 percent or more to the total recombination rate.
Using their model, the team extracted an coefficient for third order processes of 8 x 10-29
cm6 s-1 for the LEDs, which
is comparable to values obtained by other experimentalists, but far higher than those determined by first-principle theoretical calculations for Auger recombination.
The researchers point out that this striking difference could be due to one component of carrier leakage that, like Auger recombination, is proportional to the cube of the carrier density.
Q. Dai et al. Appl. Phys Lett. 97 133507 (2010)
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