News of MRS Members/Materials Researchers
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and on the bottom of the Si wafer. Afterwards, Au wire leads were attached to both Al layers with Ag paint. The current-voltage characteristics of over 100 diodes were measured. The EL of the devices was found to arise from the Si only. The efficiency of the EL varied between 10-6 and 10-4 at an injection current of 50 mA. These efficiency values are several orders of magnitude higher than those previously observed for MOS diodes. The rise and fall times of the diodes varied from 4.5 µs to 20 µs, and 1.5 µs to 15 µs, respectively. Longer rise times were found to correspond to longer fall times and improved EL efficiencies. The researchers attributed the variations in the measured parameters to variations
in the nonradiative recombination process in the devices. In devices with fewer nonradiative recombination, the rise and fall times are longer, and the fraction of recombination due to the radiative recombination process is larger. The researchers speculate that the variation in the radiative recombination state densities arise from variations in silicon quality across the wafer, with the recombination being most efficient for high-quality spots. The devices with larger rise and fall times also exhibited near-lasing-threshold behavior—a rapid output power increase at a threshold value of the input current. Threshold behavior was accompanied by the appearance of resonance modes in the EL spectrum. The researchers attribute the
near-lasing behavior to the improved luminescence efficiency coupled to the low-absorption cross section of the Si and the formation of a quasi-cavity by two reflective metal layers in the device. GREG KHITROV
FOR MORE RESEARCH NEWS ON MATERIALS SCIENCE . . . . . . access the Materials Research Society Web site: www.mrs.org/gateway/ matl_news.html
News of MRS Members/Materials Researchers Robert Langer Langer Receives Receives the the 2002 2002 Charles Charles Stark Stark Draper Draper Prize Prize Robert
Robert Langer, Langer, Kenneth Kenneth J.J. Germeshausen Germeshausen Professor Professor of of Robert Chemical and Biomedical Biomedical Engineering Engineering at at the the MassaMassaChemical and chusetts Institute of of Technology, Technology, has has been been awarded awarded the the chusetts Institute Charles Stark Draper Draper Prize Prize for for his his invention invention of of medical medical Charles Stark technologies that help help prolong prolong lives lives and and ease ease suffering. suffering. technologies that Credited with developing developing biocompatible biocompatible polymer polymer techtechCredited with nologies that control control the the release release of of medicine medicine over over time— time— nologies that from weeks to to years—Langer’s years—Langer’s contributions contributions have have signifisignififrom weeks cantly cantly advanced advanced the the controlled controlled drug-delivery drug-delivery industry. industry. His engineering engineering of of polymer polymer plastics plastics isis now now allowing allowing delivery delivery of of medicine me
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