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optoelectronics A roadmap for cool and lossless lasers, with Bismuth By Julien Happich From the 21st to the 23rd of July took place the 5th international workshop on Bismuth-containing semiconductors, unveiling promising III-V material combinations for the design of very efficient lasers and record-breaking multi-junction solar cells. In an interview with EETimes Europe, Chief Scientist of the Tyndall National Institute Professor Eoin O’Reilly who chaired this workshop told us what could be the implications of these new materials for the semiconductor industry. III-V semiconductor materials such as GaAs or GaN have a wide range of applications in LEDs and lasers, including thin film InGaAsP/InP lasers. But currently, most photonic components for telecommunications applications have major intrinsic losses, with around 80% of the electrical power used by a laser chip being emitted as waste heat. The losses heat up the devices whose efficiency is very temperature sensitive, and in many applications, the waste heat must be dissipated using thermo-electric coolers in an air-conditioned environment. “The research on Bismuth-containing semiconductors stemmed from theoretical findings, demonstrating that devices built with GaAs alloys containing minute amounts of Bismuth, would drastically reduce the intrinsic losses mostly due to Auger recombinations”, explained O’Reilly. “Theoretical findings show us that in a GaBiAs alloy containing from 12 to 15% of Bismuth, we eliminate Auger recombination completely. This would improve laser efficiency by 50% while drastically reducing device temperature sensitivity”, continues Prof. Eoin O’Reilly, Chief Scientist of the Tyndall National Institute. O’Reilly. “With their improved efficiency and no cooling required, GaBiAs telecom lasers would draw an order of magnitude less power than traditional cooled solutions” the researcher estimates. This would translate into huge power savings for data centres, 100G long distance communications, in silicon photonics, or for future 5G networking infrastructures. That’s for the theory, but growing such materials is easier said than done and this has been the focus of all the previous workshops on Bismuth-containing semiconductors. 18 months ago, researchers demonstrated a laser incorporating 2% of Bismuth, and at this year’s workshop, the same team was able to include 6% of Bismuth. Another paper discussed a LED containing between 9 and 10% of Bismuth. At these concentrations, there was no reduction of Auger recombination but these device fabrication results are promising. Growing these highly mismatched semiconductor alloys defect-free is difficult, yet careful studies using Metal Organic Vapor Phase Epitaxy (MOVP) or Molecular beam Epitaxy have yielded III-V materials with up to 10% and 20% of Bismuth respectively. Because the laser devices operate on very thin layers, the active layers are grown with thicknesses of a few nanometers only, and the stress induced by the lattice mismatch further improves the laser efficiency. Another area of interest for Bismide semiconductors is the design of multi-junction solar cells to maximize their efficiency through appropriate stacking and band gap selection. Stephen Sweeney at University of Surrey has a patent on the use of such materials in solar cells and O’Reilly is confident that they’ll find their way into efficiency record-breaking devices. Last year, Sweeney’s team from the University of Surrey published a paper on the modelling of the quaternary alloy GaAsBiN, exhibiting the potential to cover a wide range of band gaps below 1.42 eV, while grown completely lattice matched onto GaAs or Ge with controllable band offsets. The paper concludes that alloy band gaps from 0.3 to 1.4 eV can be achieved with Bi and N concentrations ranging from 0%-10% and 0%-5%, respectively with very low or zero strain. The workshop was sponsored by the EU FP7 project, BIANCHO (Bismide and Nitride Components for High Temperature Operation), a four-year, €2.19m European research project working to tackle the energy challenges presented by the exponential growth in internet traffic. The BIANCHO project included five leading European partners with complementary expertise in epitaxy, structural characterization of materials, device physics, band structure modelling, advanced device fabrication, packaging and commercialisation. Coordinated by the Tyndall National Institute (Ireland), materials’ development was led by Philipps Universitaet Marburg (Germany) and by the Semiconductor Physics Institute (SPI) of the Center for Physical Sciences and Technology (Lithuania). Marburg led MOVPE growth and SPI led MBE growth of bismuth-containing epitaxial layers and their characterization by optical, electrical, and ultrafast techniques. The University of Surrey (UK) provided unique characterisation facilities and modelling expertise, with industry input from Huawei through CIP Technologies (UK), an organisation with a long history of applied photonics innovation, particularly in the telecommunications sector. 38 Electronic Engineering Times Europe September 2014 www.electronics-eetimes.com


EETE SEP 2014
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