letters to nature ture. The integrated PL also increases with temperature in the same 6. Vescan, L& Stoca, T Room-temperature SiGie light-emitting diodes. / Laminesenec 80, 48 manner as the EL, indicating that the temperature dependence is intrinsic to the recombination, rather than the injection mecha- 7. Leong, D, Harry, M, Reeson, K.I.& Homewood, K.P. A silicon/iron disilicide light-emitting diode ting non-radiative routes-the band-to- 9. Sweinbjornsson, E.O. Webe temperature electroluminescence from dislocation rich band transition is ly temperature-independent. In our case 10 Hirth, 1. R.& Lothe, 1. Theory of Dislocations 2nd edn, 63 (ohn wiley & Sons, New York, 1982). spatial localization of the radiative carrier population decouples it Correspondence and requests for materials should be addressed to K.P.H. from any non-radiative recombination occurring elsewhere, thus (e-mail: khomewood@eim. surrey. acuk) eliminating the luminescence quenching. A full understanding of the form of the weak increase of the integrated intensity with temperature seen requires further investigation; it could be asso- ciated with increased scattering within the confined carrier popula tions and the increase in the effective density of band states with temperature. We have also carried out EL frequency- resolved……… measurements that give a device response time, at room tempera- Dating of the oldest continental ture,of18±2μs.T drive current respectively meaning that the sediments from the emile depends nce f he wu mirnesctenee einten ar es an rt ted. Himalayan foreland basin to a shift in our case, no such shift is observed. Yani Najman"s, Malcolm Ringlet, Laurent Godin*s& Grahame Olivers The external quantum efficiency of our unpackaged plana has been measured, based only on the light emitted just the back window At a 100 ma forward current the University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N IN4 light is 19.8uw giving an external quantum efficiency of f Scottish Universities Environmental Research Centre,Scottish Enterprise (2.0+0.1)x10-4at room temperature. We also obtain significant Technology Park Rankine Avenue, East Kilbride g75 0oE uk edge emission from the device, of a further 80 uw, but this is not #Department of Earth Sciences, Oxford University, Parks Road, Oxford OXI 3PR, luded in our estimate of the quantum efficiency above. Taking it into account, our device has a quantum efficiency of 10-3. The Crustal Geodynamics Group, School of Geography and Geosciences, Purdie I emitted power from the face of the device was measured by placing uildin he University, St Andrews, Fife KY16 9ST, UK it immediately adjacent to a large-area calibrated power meter. The power meter is an Ophir Laser Star fitted with a PD300 IR head; the A detailed knowledge of Himalayan development is important for instrument is certificated and calibrated with standards traceable to our wider understanding of several global processes, ranging from the same efficiencies as the back emitting devices. For comparison, climate. Continental sediments 55 Myr old found in a foreland commercially available GaAs infrared LEDs have typical external basin in Pakistan'are, by more than 20 Myr, the oldest deposits efficiencies of 10. However, commercial packing of LEDs, in thought to have been eroded from the Himalayan metamorphic particular by minimizing the considerable internal reflection mountain belt. This constraint on when erosion began has losses of the planar device and by collecting the edge emission, influenced models of the timing and diachrony of the India can improve the external efficiency by up to a factor of about Eurasia collision, timing and mechanisms of exhumation" and Our first, non-optimized devices are therefore already within a uplift", as well as our general understanding of foreland basin factor of 3 or so of the efficiencies achieved in conventional dyna But the depositional age of these basin sediments was optimized led devices based on biostratigraphy from four intercalated marl units.Here We believe that the device demonstrated is the most likely we present dates of 257 detrital grains of white mica from this andidate currently for implementing efficient light sources in succession, using the Ar-Ar method, and find that the largest silicon. Such a device would also form the basis for the development concentration of ages are at 36-40 Myr. These dates are incompat- of an injection laser based on the same principles but with the ible with the biostratigraphy unless the mineral ages have been incorporation of an optical cavity. The approach itself is not limited reset, a possibility that we reject on the basis of a number of lines to silicon but could be applied to other materials, particularly other of evidence. A more detailed mapping of this formation suggests ndirect materials and silicon alloys. For example, going from that the marl units are structurally intercalated with the con- germanium to silicon to silicon carbide could produce devices, tinental sediments and accordingly that biostratigraphy cannot be sing the same approach, that could span the near-infrared region, used to date the clastic succession. The oldest continental foreland including the 1.3 um and 1.5 um wavelength regions of the spec- basin sediments containing metamorphic detritus eroded from trum, important for optical fibre communications, and up to the the Himalaya orogeny therefore seem to be at least 15-20 Myr ultraviolet region. u younger than previously believed, and models based on the older age must be re-evaluated Received 23 October 2000: accepted 29 January 2001. European Commission Techmaog Roudmap-optoelectronic intercommee 3. Lu, Z... Lockwood, D. 1. Baribeau, L-M. Quantum confinement and light emission in Sio /s Formation(Figs I and 2)and consists of a >8-km-thick fossil-free recipitatesin SO2 formed by ion implantation Nuc Instrum. Methods B 96, 387-391(1995 5. Zheng. B et al. Room-temperature sharp line electroluminescence at A=1. um from an erbium. Edinburgh EH9 3/W UK(YN): Department of Eart doped, silicon light-en diode. AppL Phys. Lett 64, 28-42-28-44(1994) A@2001 Macmillan Magazines Ltd NaturEvOl410)8March2001www.nature.comletters to nature 194 NATURE | VOL 410 | 8 MARCH 2001 | www.nature.com ture. The integrated PL also increases with temperature in the same manner as the EL, indicating that the temperature dependence is intrinsic to the recombination, rather than the injection mecha￾nism. The strong temperature quenching of PL and EL in most semiconductors is primarily the result of the strong temperature dependence of the competing non-radiative routes—the band-to￾band transition is relatively temperature-independent. In our case spatial localization of the radiative carrier population decouples it from any non-radiative recombination occurring elsewhere, thus eliminating the luminescence quenching. A full understanding of the form of the weak increase of the integrated intensity with temperature seen requires further investigation; it could be asso￾ciated with increased scattering within the confined carrier popula￾tions and the increase in the effective density of band states with temperature. We have also carried out EL frequency-resolved measurements that give a device response time, at room tempera￾ture, of 18 6 2ms. The PL and EL are both superlinear with excitation power and drive current respectively, meaning that the device improves further as we drive the device harder. In ref. 9 a similar dependence of the luminescence intensity is attributed, owing to a shift in emission wavelength, to sample heating. However, in our case, no such shift is observed. The external quantum efficiency of our unpackaged planar device has been measured, based only on the light emitted just through the back window. At a 100 mA forward current the emitted light is 19.8mW giving an external quantum efficiency of ð2:0 6 0:1Þ 3 10 2 4 at room temperature. We also obtain significant edge emission from the device, of a further 80 mW, but this is not included in our estimate of the quantum efficiency above. Taking it into account, our device has a quantum efficiency of 10−3 . The emitted power from the face of the device was measured by placing it immediately adjacent to a large-area calibrated power meter. The power meter is an Ophir Laser Star fitted with a PD300 IR head; the instrument is certificated and calibrated with standards traceable to the National Institute of Standards. Our front emitting devices have the same efficiencies as the back emitting devices. For comparison, commercially available GaAs infrared LEDs have typical external efficiencies of 10−2 . However, commercial packing of LEDs, in particular by minimizing the considerable internal reflection losses of the planar device and by collecting the edge emission, can improve the external efficiency by up to a factor of about 15. Our first, non-optimized devices are therefore already within a factor of 3 or so of the efficiencies achieved in conventional optimized LED devices. We believe that the device demonstrated is the most likely candidate currently for implementing efficient light sources in silicon. Such a device would also form the basis for the development of an injection laser based on the same principles but with the incorporation of an optical cavity. The approach itself is not limited to silicon but could be applied to other materials, particularly other indirect materials and silicon alloys. For example, going from germanium to silicon to silicon carbide could produce devices, using the same approach, that could span the near-infrared region, including the 1.3mm and 1.5mm wavelength regions of the spec￾trum, important for optical fibre communications, and up to the ultraviolet region. M Received 23 October 2000; accepted 29 January 2001. 1. European Commission Technology Roadmap—Optoelectronic Interconnects for Integrated Circuits (eds Forchel, A. & Malinverni, P.) (Office for Official Publications of the European Communities, Luxembourg, 1998). 2. Hirschman, K. D., Tysbekov, L., Duttagupta, S. P. & Fauchet, P. M. Silicon-based visible light-emitting devices integrated into microelectronic circuits. Nature 384, 338–341 (1996). 3. Lu, Z. H., Lockwood, D. J. & Baribeau, J.-M. Quantum confinement and light emission in SiO2/Si superlattices. Nature 378, 258–260 (1995). 4. Komoda, T. et al. Visible photoluminescence at room temperature from microcrystalline silicon precipitates in SiO2 formed by ion implantation. Nucl. Instrum. Methods B 96, 387–391 (1995). 5. Zheng, B. et al. Room-temperature sharp line electroluminescence at l = 1.54mm from an erbium￾doped, silicon light-emitting diode. Appl. Phys. Lett. 64, 2842–2844 (1994). 6. Vescan, L. & Stoica, T. Room-temperature SiGe light-emitting diodes. J. Luminescence 80, 485–489 (1999). 7. Leong, D., Harry, M., Reeson, K. J. & Homewood, K. P. A silicon/iron disilicide light-emitting diode operating at a wavelength of 1.5mm. Nature 387, 686–688 (1997). 8. Tybeskov, L., Moore, K. L., Hall, D. G. & Fauchet, P. M. Intrinsic band-edge photoluminescence from silicon clusters at room temperature. Phys. Rev. B 54, R8361–R8364 (1996). 9. Sveinbjo¨rnsson, E. O. & Weber, J. Room temperature electroluminescence from dislocation rich silicon. Appl. Phys. Lett. 69, 2686–2688 (1996). 10. Hirth, J. P. & Lothe, J. Theory of Dislocations 2nd edn, 63 (John Wiley & Sons, New York, 1982). Correspondence and requests for materials should be addressed to K.P.H. (e-mail: k.homewood@eim.surrey.ac.uk). ................................................................. Dating of the oldest continental sediments from the Himalayan foreland basin Yani Najman*§, Malcolm Pringle†, Laurent Godin‡§ & Grahame Oliver¶ * Fold and Fault Research Project, Department of Geology and Geophysics, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4 † Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride G75 0QF, UK ‡ Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK ¶Crustal Geodynamics Group, School of Geography and Geosciences, Purdie Building, The University, St Andrews, Fife KY16 9ST, UK .......................................... ......................... ......................... ......................... ......................... A detailed knowledge of Himalayan development is important for our wider understanding of several global processes, ranging from models of plateau uplift to changes in oceanic chemistry and climate1–4. Continental sediments 55 Myr old found in a foreland basin in Pakistan5 are, by more than 20 Myr, the oldest deposits thought to have been eroded from the Himalayan metamorphic mountain belt. This constraint on when erosion began has influenced models of the timing and diachrony of the India– Eurasia collision6–8, timing and mechanisms of exhumation9,10 and uplift11, as well as our general understanding of foreland basin dynamics12. But the depositional age of these basin sediments was based on biostratigraphy from four intercalated marl units5 . Here we present dates of 257 detrital grains of white mica from this succession, using the 40Ar–39Ar method, and find that the largest concentration of ages are at 36–40 Myr. These dates are incompat￾ible with the biostratigraphy unless the mineral ages have been reset, a possibility that we reject on the basis of a number of lines of evidence. A more detailed mapping of this formation suggests that the marl units are structurally intercalated with the con￾tinental sediments and accordingly that biostratigraphy cannot be used to date the clastic succession. The oldest continental foreland basin sediments containing metamorphic detritus eroded from the Himalaya orogeny therefore seem to be at least 15–20 Myr younger than previously believed, and models based on the older age must be re-evaluated. The Balakot Formation, located in the Hazara–Kashmir Syntaxis of Northern Pakistan, is a continental foreland basin sedimentary sequence that contains detritus eroded from the India–Eurasia suture zone and the metamorphic rocks of the Himalaya11. The Balakot Formation overlies the Palaeocene shallow marine Patala Formation (Figs 1 and 2) and consists of a .8-km-thick fossil-free § Present addresses: Department of Geology & Geophysics, Edinburgh University, West Mains Road, Edinburgh EH9 3JW, UK (Y.N.); Department of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby BC, V5A 1S6, Canada (L.G.). © 2001 Macmillan Magazines Ltd