news feature these LEDs. They used an oxidation reaction Bagneux, part of the CNRS, France's nation to eliminate much of the bulk-like non-con- al research agency. In addition, the device has fined silicon that remains in the porous net to be cooled with liquid nitrogen. I thinkthe work-which otherwise transports most of largest problem lies in achieving room-tem the charge carriers. Koshida's group has now perature operation, Gennser says. QCLs aised theefficiency to about 1%, an improve also emit too narrow a range of frequen ment of five orders of magnitude over the for use in long-distance optical telecommu earliest results. This is fine for display screens, nications ys Canham. But to provide the photonic It remains unclear which approach will signals needed to transmit information emerge as the best contender for lighting between chips will require a further 10-fold up silicon chips. But given the recent acceler increase And switching speeds are still about ation of progress, those intent on unitin the worlds of photonic and electronic infor mation technology can afford to be opti Dot comms mistic. At the very least, silicon-based Using a network of silicon wires is not the ptoelectronics is starting to seem less of an only way to make the element glow. The oxymoron same helpful quantum effects operate if the Shining example: networks of porous silicon Phillp Ball is a consultant editor of Nature. porous silicon is divided into nanometre- nanowires emit light in the visible spectrum. L.Canham, L. T. ApPL. Phys. Left. 57. 1046-1048(1990). sized particles known as nanocrystals or 2. Mizuno, H, Koyama, H. Koshida, N. Appl. Phys. Lett. quantum dots. Last year, Munir Nayfeh of data so that they can assess these claims. the University of Illinois at Urbana-Cham- But there might be another path to devel-3上x如mBLD甲sE&hh paign and his colleagues used ultrasound to oping a silicon-based laser. When Ulf 4. Gelloz, B, Nakagawa, T& Koshida, NAppl Phys. Lett. The smallest of these particles(about 1 Institute in Villigen, Switzerland, he and his 5. Ackakitr, Oet al Appl Phys. Lett. 76, Ix- O, science nanometre across)emit blue light olleagues described a quantum-cascade 287, 1471-1473(2000 Other teams have reported similar suc- laser(QCL) consisting of alternating laye aesi, L-, Dal Negro, L, Mazzoleni, C, Franzo, G. Priolo, E. cesses. In February last year, Brian Korgel of of silicon and a germanium-silicon alloy. &. belting. ct al a(ne 290.27-2780(2001 Texas at Austin and his col- These devices contain severa eague described how to, grow single blocks of light-emitting units, stacked one"出m)影如m nanowires that emit blue light. Unlike on top of the other. Electrons can tunnel' 10 Wang. T, Moll, N, Cho, K. Joannopoulos, J D. Pis. Rev. porous silicon, which consists of a mesh of between individual layers, emitting a photon nanowires, Korgel's nanowires are discrete in the process I1.Zhang, P, Crespi, V H, Chang, E. Louie, S G& Cohen, M.L threads of silicon just 4-5 nm wide Under the right conditions, this should orenzo Pavesi of the University of Tren- lead to stimulatedemission of coherent light, to in Italy has used an alternativeapproach to but the Swiss team has so far managed to pre create silicon nanocrystals. By firing high- duce only electroluminescence, not laser ergy silicon ions into quartz(silicon diox- emission. "There remain many obstacles de), and then heating the material to 1, 100 before we have a working laser,admits C, Pavesi and his colleagues generated sili- Gennser, who is now at the Laboratory of con particles about 3 nm across that were Microstructures and Microelectronics in embedded in the quartz. Last November, these researchers showed that not only do the nanocrystals emit red light when energized with a laser beam, but they can also amplify a probe beam of the same wavelength as the emission. Known as optical gain, this henomenon is one of the fundamental features flaser emission Although the Italian team has taken the first steps towards creating a silicon laser, light amplification is not the same as laser action. In a laser beam the light is coherent: all the photons are in phase. To achieve this, emitted photons must stimulate the emis- sion of others- the stimulated photons merge in step with those that induced them. This stimulated emission is achieved by acing the emitting material in an optical cavity bounded by mirrors which let phe tons bounce back and forth Tantalizingly, at the Materials Research Society meeting in Boston last December, Nayfeh reported optical gain and stimulated emission from his blue-light-emitting nanocrystals. Others in the field are now aiting for Nayfeh to publish quantitativ Light speed: single silicon nanowires(inset ) might supply photonic signals to fibre-optic cables. 976 Macmillan Magazines L NatuRevOl40922february2001www.nature.comthese LEDs. They used an oxidation reaction to eliminate much of the bulk-like ‘non-con￾fined’ silicon that remains in the porous net￾work — which otherwise transports most of the charge carriers4 . Koshida’s group has now raised the efficiency to about 1%,an improve￾ment of five orders of magnitude over the earliest results. This is fine for display screens, says Canham. But to provide the photonic signals needed to transmit information between chips will require a further 10-fold increase. And switching speeds are still about two orders of magnitude too slow. Dot comms Using a network of silicon wires is not the only way to make the element glow. The same helpful quantum effects operate if the porous silicon is divided into nanometre￾sized particles known as nanocrystals or ‘quantum dots’. Last year, Munir Nayfeh of the University of Illinois at Urbana-Cham￾paign and his colleagues used ultrasound to shatter porous silicon into nanocrystals. The smallest of these particles (about 1 nanometre across) emit blue light5 . Other teams have reported similar suc￾cesses. In February last year, Brian Korgel of the University of Texas at Austin and his col￾leagues described how to grow single nanowires that emit blue light6 . Unlike porous silicon, which consists of a mesh of nanowires, Korgel’s nanowires are discrete threads of silicon just 4–5 nm wide. Lorenzo Pavesi of the University of Tren￾to in Italy has used an alternative approach to create silicon nanocrystals. By firing high￾energy silicon ions into quartz (silicon diox￾ide), and then heating the material to 1,100 7C, Pavesi and his colleagues generated sili￾con particles about 3 nm across that were embedded in the quartz. Last November, these researchers showed that not only do the nanocrystals emit red light when energized with a laser beam, but they can also amplify a ‘probe’ beam of the same wavelength as the emission7 . Known as optical gain, this phenomenon is one of the fundamental features of laser emission. Although the Italian team has taken the first steps towards creating a silicon laser, light amplification is not the same as laser action. In a laser beam the light is coherent: all the photons are in phase. To achieve this, emitted photons must stimulate the emis￾sion of others — the stimulated photons emerge in step with those that induced them. This ‘stimulated emission’ is achieved by placing the emitting material in an optical cavity bounded by mirrors which let pho￾tons bounce back and forth. Tantalizingly, at the Materials Research Society meeting in Boston last December, Nayfeh reported optical gain and stimulated emission from his blue-light-emitting nanocrystals. Others in the field are now waiting for Nayfeh to publish quantitative data so that they can assess these claims. But there might be another path to devel￾oping a silicon-based laser. When Ulf Gennser was working at the Paul Scherrer Institute in Villigen, Switzerland, he and his colleagues described a quantum-cascade laser (QCL) consisting of alternating layers of silicon and a germanium–silicon alloy8 . These devices contain several five-layer blocks of light-emitting units, stacked one on top of the other. Electrons can ‘tunnel’ between individual layers,emitting a photon in the process. Under the right conditions, this should lead to stimulated emission of coherent light, but the Swiss team has so far managed to pro￾duce only electroluminescence, not laser emission. “There remain many obstacles before we have a working laser,” admits Gennser, who is now at the Laboratory of Microstructures and Microelectronics in Bagneux, part of the CNRS, France’s nation￾al research agency.In addition,the device has to be cooled with liquid nitrogen.“I think the largest problem lies in achieving room-tem￾perature operation,” Gennser says. QCLs also emit too narrow a range of frequencies for use in long-distance optical telecommu￾nications. It remains unclear which approach will emerge as the best contender for lighting up silicon chips. But given the recent acceler￾ation of progress, those intent on uniting the worlds of photonic and electronic infor￾mation technology can afford to be opti￾mistic. At the very least, silicon-based optoelectronics is starting to seem less of an oxymoron. ■ Philip Ball is a consultant editor of Nature. 1. Canham, L. T. Appl. Phys. Lett. 57, 1046–1048 (1990). 2. Mizuno, H., Koyama, H. & Koshida, N. Appl. Phys. Lett. 69, 3779–3781 (1996). 3. Hirschman, K. D., Tsybekov, L., Duttagupta, S. P. & Fauchet, P. M. Nature 384, 338–341 (1996). 4. Gelloz, B., Nakagawa, T. & Koshida, N. Appl. Phys. Lett. 73, 2021–2023 (1998). 5. Ackakir, O. et al. Appl. Phys. Lett. 76, 1857–1859 (2000). 6. Holmes, J. D., Johnston, K. P., Doty, R. C. & Korgel, B. A. Science 287, 1471–1473 (2000). 7. Pavesi, L., Dal Negro, L., Mazzoleni, C., Franzò, G. & Priolo, F. Nature 408, 440–444 (2000). 8. Dehlinger, G. et al. Science 290, 2277–2280 (2000). 9. Curry, R. J., Gillin, W. P., Knights, A. P. & Gwilliam, R. Appl. Phys. Lett. 77, 2271–2273 (2000). 10.Wang, T., Moll, N., Cho, K. & Joannopoulos, J. D. Phys. Rev. Lett. 82, 3304–3307 (1999). 11.Zhang, P., Crespi, V. H., Chang, E., Louie, S. G. & Cohen, M. L. Nature 409, 69–71 (2001). news feature 976 NATURE|VOL 409 | 22 FEBRUARY 2001 |www.nature.com Shining example: networks of porous silicon nanowires emit light in the visible spectrum. Light speed: single silicon nanowires (inset) might supply photonic signals to fibre-optic cables. ▲ B. KORGEL DERA ALFRED PASIEKA/SPL © 2001 Macmillan Magazines Ltd