letters to nature 29. Crone, B et al. Large-scale complementary integrated circuits based on organic Sature loops introduce a local strain field, which modifies the band structure and provides spatial confinement of the charge carriers. 31. verthamer, N. R, Helfand, E. Hohenberg. P. C. Temperature and purity dependence of the It is this spatial confinement which allows room-temperature superconducting field, Hez, Ill. Eledron spin and spin-orbit effects. Phys Rev. 147, 295-303 electroluminescence at the band-edge. This device strategy is highly compatible with ULSI technology, as boron ion implanta tion is already used as a standard method for the fabrication of silicon devices We thank E A Chandross, B Crone, H E. Katz, H Y Hwang, A J Lovinger and T Siegrist There have been great efforts over the past decade to obtain for discussions, and E Bucher for the use of equipment. useful, that is, technologically viable and efficient, light emission Correspondence and requests for materials should be addressed to L.H.S from silicon both in the visible and infrared regions of the spectrum. (e-mail:hendrik@lucent.com In the visible regions, porous silicon and other quantized systems, such as silicon/silicon dioxide superlattices and silicon nanopreci- pitates in silicon dioxide, have been the main emphasis. In the infrared region, systems such as erbium in silicon,, silicon/ and, more recently, iron disilicide offer potential routes. No approach has so far been applied commercially. The An efficient room-temperature reasons for this are a combination of the lack of genuine or silicon-based light-emitting diode in the case of infrared emitters, high thermal quenching giving very Wai Lek Ng, M. A Lourenco, R. M. Gwilliam*, S. Ledaint, G Shot Here we use a new approach-dislocation engineering, using K P. Homewood* conventional ULSI technology-that gives efficient light emission in silicon at room temperature. School of Electronic Engineering, Information Technology e- mathemati Because of its indirect bandgap, silicon is fundamentally a poor t School of Mechanical and Materials Engineering, University of Surrey, Guildford, Surrey, GU2 7XH, UK emitter of light. The main reason for this is that fast non-radiative recombination routes dominate the slower radiative route in this material. Indeed, in bulk silicon, at room temperature, radiative There is an urgent requirement for an optical emitter that is emission is normally entirely absent. However, if recombination ompatible with standard, silicon-based ultra-large-scale integra- through the non-radiative routes can be prevented, the radiative tion(ULSD) technology. Bulk silicon has an indirect energy emission could in principle be enhanced. Non-radiative recombi- bandgap and is therefore highly inefficient as a light source, nation is the result of diffusion of carriers to point defects in the necessitating the use of other materials for the optical emitters. silicon where efficient non-radiative recombination then occurs However, the introduction of these materials is usually incompat- Despite the low defect concentrations in good quality silicon this ible with the strict processing requirements of existing ULSI non-radiative route is always completely dominant. A way of echnologies. Moreover, as the length scale of the devices enhancing the radiative efficiency would be to prevent the carrier decreases, electrons will spend increasingly more of their time diffusion. If silicon can be formed as clusters then strong band-edge in the connections between components; this interconnectivity photoluminescence is possible,, but thus far this has only been problem could restrict further increases in computer chip proces- achieved by incorporating these clusters inside large bandgap sing power and speed in as little as five years. Many efforts have insulating oxides. However, the insulating matrix prevents efficient therefore been directed, with varying degrees of success, carrier injection, making devices difficult to produce. Similar but to engineering silicon-based materials that are efficient light much weaker(with efficiencies up to about 8 x 10) band-edge emitters-. Here, we describe the fabrication, using standard emission has also been observed in laser-recrystallized silicon, but silicon processing techniques, of a silicon light-emitting diode no explanation of its origin was presented LED) that operates efficiently at room temperature. Boron is We have made use of the controlled introduction of dislocation mplanted into silicon both as a dopant to form a p-n junction, as loops using conventional ion implantation and thermal processing well as a means of introducing dislocation loops. The dislocation The dislocation loop array, if appropriately produced, introduces a strain field in three dimensions that modifies the bandgap of the OOOOOOO AuSb Voltage () 8a5 Figure 1 The t for the device measured at room tempe schematic of the light-emitting diode(ED)device. The top and bottom ohmic contacts are Figure 2 Plots of the integrated electroluminescence intensity as a functi formed by Al and AuSb respectively. The infrared light is emitted through the window left forward voltage at various temperatures: 80 K (diamonds), 180 K (trial (circles) 192 A@2001 Macmillan Magazines Ltd NaturEvOl410)8March2001www.nature.com
letters to nature 192 NATURE | VOL 410 | 8 MARCH 2001 | www.nature.com 29. Crone, B. et al. Large-scale complementary integrated circuits based on organic transistors. Nature 403, 521–523 (2000). 30. Mooij, J. E. et al. Josephson persistent-current qubit. Science 285, 1036–1039 (1999). 31. Verthamer, N. R., Helfand, E. & Hohenberg, P. C. Temperature and purity dependence of the superconducting field, Hc2, III. Electron spin and spin-orbit effects. Phys. Rev. 147, 295–303 (1966). Acknowledgements We thank E. A. Chandross, B. Crone, H. E. Katz, H. Y. Hwang, A. J. Lovinger and T. Siegrist for discussions, and E. Bucher for the use of equipment. Correspondence and requests for materials should be addressed to J.H.S. (e-mail: hendrik@lucent.com). ................................................................. An efficient room-temperature silicon-based light-emitting diode Wai Lek Ng*, M. A. Lourenc¸o*, R. M. Gwilliam*, S. Ledain†, G. Shao† & K. P. Homewood* * School of Electronic Engineering, Information Technology & Mathematics; † School of Mechanical and Materials Engineering, University of Surrey, Guildford, Surrey, GU2 7XH, UK .................................. ......................... ......................... ......................... ......................... ........ There is an urgent requirement for an optical emitter that is compatible with standard, silicon-based ultra-large-scale integration (ULSI) technology1 . Bulk silicon has an indirect energy bandgap and is therefore highly inefficient as a light source, necessitating the use of other materials for the optical emitters. However, the introduction of these materials is usually incompatible with the strict processing requirements of existing ULSI technologies. Moreover, as the length scale of the devices decreases, electrons will spend increasingly more of their time in the connections between components; this interconnectivity problem could restrict further increases in computer chip processing power and speed in as little as five years. Many efforts have therefore been directed, with varying degrees of success, to engineering silicon-based materials that are efficient light emitters2–7. Here, we describe the fabrication, using standard silicon processing techniques, of a silicon light-emitting diode (LED) that operates efficiently at room temperature. Boron is implanted into silicon both as a dopant to form a p–n junction, as well as a means of introducing dislocation loops. The dislocation loops introduce a local strain field, which modifies the band structure and provides spatial confinement of the charge carriers. It is this spatial confinement which allows room-temperature electroluminescence at the band-edge. This device strategy is highly compatible with ULSI technology, as boron ion implantation is already used as a standard method for the fabrication of silicon devices. There have been great efforts over the past decade to obtain useful, that is, technologically viable and efficient, light emission from silicon both in the visible and infrared regions of the spectrum. In the visible regions, porous silicon2 and other quantized systems, such as silicon/silicon dioxide superlattices3 and silicon nanoprecipitates in silicon dioxide4 , have been the main emphasis. In the infrared region, systems such as erbium in silicon5 , silicon/ germanium6 and, more recently, iron disilicide7 offer potential routes. No approach has so far been applied commercially. The reasons for this are a combination of the lack of genuine or perceived compatibility with conventional ULSI technology and, in the case of infrared emitters, high thermal quenching giving very poor room-temperature efficiencies. Here we use a new approach—dislocation engineering, using conventional ULSI technology—that gives efficient light emission in silicon at room temperature. Because of its indirect bandgap, silicon is fundamentally a poor emitter of light. The main reason for this is that fast non-radiative recombination routes dominate the slower radiative route in this material. Indeed, in bulk silicon, at room temperature, radiative emission is normally entirely absent. However, if recombination through the non-radiative routes can be prevented, the radiative emission could in principle be enhanced. Non-radiative recombination is the result of diffusion of carriers to point defects in the silicon where efficient non-radiative recombination then occurs. Despite the low defect concentrations in good quality silicon this non-radiative route is always completely dominant. A way of enhancing the radiative efficiency would be to prevent the carrier diffusion. If silicon can be formed as clusters then strong band-edge photoluminescence is possible8 , but thus far this has only been achieved by incorporating these clusters inside large bandgap insulating oxides. However, the insulating matrix prevents efficient carrier injection, making devices difficult to produce. Similar but much weaker (with efficiencies up to about 8 3 10 2 6 ) band-edge emission has also been observed in laser-recrystallized silicon9 , but no explanation of its origin was presented. We have made use of the controlled introduction of dislocation loops using conventional ion implantation and thermal processing. The dislocation loop array, if appropriately produced, introduces a strain field in three dimensions that modifies the bandgap of the –1.5 –1.0 –0.5 Voltage (V) 0.0 0.5 1.0 0 2 4 6 8 10 AuSb Al AuSb B (p+) c-Si (n) Current (mA) Figure 1 The current–voltage plot for the device measured at room temperature. Inset, a schematic of the light-emitting diode (LED) device. The top and bottom ohmic contacts are formed by Al and AuSb respectively. The infrared light is emitted through the window left in the bottom contact. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 4 8 12 16 Forward voltage (V) Integrated electroluminescence (a.u.) Figure 2 Plots of the integrated electroluminescence intensity as a function of applied forward voltage at various temperatures: 80 K (diamonds), 180 K (triangles) and 300 K (circles). © 2001 Macmillan Magazines Ltd
letters to nature silicon in such a way that the silicon itself can be used to provide nt it is less than about 10 meV. Consequently, the barrier spatial confinement in three dimensions. To minimize the number the onset of injection is not significantly changed. Any of process steps, in the device described, boron implantation has under steady sta been used both to introduce the dislocation loop array and as the p" direction dynamically replaced by others moving in the forward tve s to form the dislocati species such as the host silicon could be After implantation and annealing, ohmic contacts consisting of and subsequent doping to form the p-n junction can be achieved sample respectively. The ohmic contacts were sintered at 360C for independently. The device operates as a conventional light-emitting 2 min. The top contact was 1 mm in diameter and, as the silicon diode under forward bias ubstrate is still nearly transparent at the emission wavelength of c A simple diagram of the device is shown in the inset to Fig. 1. The 1, 150 nm, a window was left in the larger back contact to allow the 1 X 10cm -at an energy of 30 keV. The sample was subsequently been fabricated Current-voltage(I-V) measurements were made for 20 00C to between the back an to check that the devic form the dislocation loop array and activate the boron dopants. The behaving as a diode. An I-V plot, characteristic of the fully mplants were made into a device grade CZ(Czochralski)n-type processed and working device, is shown in Fig. 1 icon substrate of resistivity 2-40 cm. An important point to The device was mounted into a mechanical holder inside a iphasize at this stage is that all these process steps are entirely continuous flow liquid-nitrogen cryostat. Light from the device conventional and are therefore completely compatible with ULSI was focused into a conventional 0.5 m spectrometer and collected hnology, allowing immediate implementation on a standard by a liquid-nitrogen-cooled germanium p-i-n detector. EL fabrication line. The array of dislocation loops formed has been measurements were then taken from 80 K to above room tempera- observed using cross-sectional transmission electron microscopy. ture. The onset of El was observed as the diode turns on under The array is situated in a planar region parallel to, and around forward bias. No EL is observed under reverse bias. The integrated 100 nm from, the p-n junction. The dislocation loops are typically EL intensity as a function of applied forward voltage is shown in about 80-100 nm in diameter and are spaced around 20 nm apart. Fig. 2 for several temperatures. The turn-on voltage at room The strain field at the edge of the dislocation loop is high and falls off temperature is typical of a conventional silicon diode operating around each loop approxi with the inverse of distance. The under forward injection. The full EL spectra taken, as a function of magnitude and sign of the stress at the edge of the dislocation loop temperature, at a forward current of 50 mA, are shown in Fig 3. The which determines the confining potential can be calculated using low-temperature EL spectrum from the device shows the main the standard elastic theory of dislocations and the known values of features expected from emission at the silicon band edge, including, We obtain a value for the maximum stress at the outside edge (1.2A at 1, 190 nm, the phonon replica of the main peak at 1, 130 nm. The the Poisson's ratio, 0.42, and Youngs modulus, 113 GPa, for silicon room-temperature EL spectrum has the main peak at 1, 160 nm with out)of the interstitial dislocation loop of 25-50 GPa, which will a full-width at half-maximum of 80 nm. No EL or photolumines cause an increase in bandgap energy at the edge of 325-750 mevA cence(PL) is observed at any other wavelengths, even at low omplete description of the stress field across the device, which is temperatures-a result that we again attributed to the close spatial Ist the superposition of the stress field of all the dislocations, is confinement of carriers within regions of bulk silicon. The tempera- complicated and will vary in detail across and between samples. ture dependence of the integrated EL intensity is shown in Fig 4. In However, given the distance dependence of the stress around most systems, demonstrated for light emission in silicon. 4.the EL the loops, the local stress at any point is entirely dominated by the quenches very strongly with increasing temperature, making prac- superposition of the stress field of only the closest dislocations. As tical room-temperature devices problematic. Here, it can be seen carriers are injected across the junction they encounter a blocking that the el intensity actually increases as we go up in temperature. a potential that varies in height depending on the direction of their similar behaviour has been observed but not explained in lase instantaneous velocity, from about 750 meV in the direction of and recrystallized silicon. The device is therefore, as required, most across a loop, down to about 20 me Vat a point equidistant between efficient at room temperature and above. Inset in Fig. 4 is a plot of two adjacent dislocations. It is this blocking potential that confines the integrated PL intensity as a function of measurement tempera- carriers close to the junction region. The varying potential falls away approximately as inverse distance back towards the junction at 80K 9s8m 260K Temperature(K 230 measurement temperature. Inset, a plot of the integrated photoluminescence intensity as Figure 3 Spectra of the electroluminescence intensity against wavelength at various a function of temperature. The solid lines are provided as a guide to the eye. The temperatures. The device was operated at a forward current of 50 ma for all photoluminescence was excited by the 488 nm line of an argon laser at a power of temperatures. NatuReVol4108March2001www.nature.com A@2001 Macmillan Magazines Ltd
letters to nature NATURE | VOL 410 | 8 MARCH 2001 | www.nature.com 193 silicon in such a way that the silicon itself can be used to provide spatial confinement in three dimensions. To minimize the number of process steps, in the device described, boron implantation has been used both to introduce the dislocation loop array and as the ptype dopant to form a p–n junction in an n-type silicon substrate. However, another implant species such as the host silicon could be used to form the dislocations so that the dislocation engineering and subsequent doping to form the p–n junction can be achieved independently. The device operates as a conventional light-emitting diode under forward bias. A simple diagram of the device is shown in the inset to Fig. 1. The device reported here was made by implanting boron at a dose of 1 3 1015 cm 2 2 at an energy of 30 keV. The sample was subsequently annealed in a nitrogen atmosphere for 20 minutes at 1,000 8C to form the dislocation loop array and activate the boron dopants. The implants were made into a device grade CZ (Czochralski) n-type silicon substrate of resistivity 2–4 Q cm. An important point to emphasize at this stage is that all these process steps are entirely conventional and are therefore completely compatible with ULSI technology, allowing immediate implementation on a standard fabrication line. The array of dislocation loops formed has been observed using cross-sectional transmission electron microscopy. The array is situated in a planar region parallel to, and around 100 nm from, the p–n junction. The dislocation loops are typically about 80–100 nm in diameter and are spaced around 20 nm apart. The strain field at the edge of the dislocation loop is high and falls off around each loop approximately with the inverse of distance. The magnitude and sign of the stress at the edge of the dislocation loop which determines the confining potential can be calculated using the standard elastic theory of dislocations10 and the known values of the Poisson’s ratio, 0.42, and Young’s modulus, 113 GPa, for silicon. We obtain a value for the maximum stress at the outside edge (1.2 A˚ out) of the interstitial dislocation loop of 25–50 GPa, which will cause an increase in bandgap energy at the edge of 325–750 meV. A complete description of the stress field across the device, which is just the superposition of the stress field of all the dislocations, is complicated and will vary in detail across and between samples. However, given the inverse distance dependence of the stress around the loops, the local stress at any point is entirely dominated by the superposition of the stress field of only the closest dislocations. As carriers are injected across the junction they encounter a blocking potential that varies in height depending on the direction of their instantaneous velocity, from about 750 meV in the direction of and across a loop, down to about 20 meVat a point equidistant between two adjacent dislocations. It is this blocking potential that confines carriers close to the junction region. The varying potential falls away approximately as inverse distance back towards the junction at which point it is less than about 10 meV. Consequently, the barrier height and the onset of injection is not significantly changed. Any carriers moving back across the junction are, under steady state conditions, dynamically replaced by others moving in the forward direction. After implantation and annealing, ohmic contacts consisting of AuSb eutectic and Al were applied to the back and front of the sample respectively. The ohmic contacts were sintered at 360 8C for 2 min. The top contact was 1 mm in diameter and, as the silicon substrate is still nearly transparent at the emission wavelength of 1,150 nm, a window was left in the larger back contact to allow the electroluminescence (EL) through. Front window devices have also been fabricated. Current–voltage (I–V) measurements were made between the back and front contacts to check that the device is behaving as a diode. An I–V plot, characteristic of the fully processed and working device, is shown in Fig. 1. The device was mounted into a mechanical holder inside a continuous flow liquid-nitrogen cryostat. Light from the device was focused into a conventional 0.5 m spectrometer and collected by a liquid-nitrogen-cooled germanium p–i–n detector. EL measurements were then taken from 80 K to above room temperature. The onset of EL was observed as the diode turns on under forward bias. No EL is observed under reverse bias. The integrated EL intensity as a function of applied forward voltage is shown in Fig. 2 for several temperatures. The turn-on voltage at room temperature is typical of a conventional silicon diode operating under forward injection. The full EL spectra taken, as a function of temperature, at a forward current of 50 mA, are shown in Fig. 3. The low-temperature EL spectrum from the device shows the main features expected from emission at the silicon band edge, including, at 1,190 nm, the phonon replica of the main peak at 1,130 nm. The room-temperature EL spectrum has the main peak at 1,160 nm with a full-width at half-maximum of 80 nm. No EL or photoluminescence (PL) is observed at any other wavelengths, even at low temperatures—a result that we again attributed to the close spatial confinement of carriers within regions of bulk silicon. The temperature dependence of the integrated EL intensity is shown in Fig. 4. In most systems, demonstrated for light emission in silicon5,6,7 the EL quenches very strongly with increasing temperature, making practical room-temperature devices problematic. Here, it can be seen that the EL intensity actually increases as we go up in temperature. A similar behaviour has been observed but not explained in laserrecrystallized silicon9 . The device is therefore, as required, most efficient at room temperature and above. Inset in Fig. 4 is a plot of the integrated PL intensity as a function of measurement tempera- 1,030 1,080 1,130 1,180 1,230 0 4 8 12 16 Wavelength (nm) 80 K 180 K 300 K 110K 260 K Electroluminescence intensity (mV) Figure 3 Spectra of the electroluminescence intensity against wavelength at various temperatures. The device was operated at a forward current of 50 mA for all temperatures. 50 100 150 200 250 300 0 2 4 6 8 10 12 Temperature (K) 50 150 250 4 8 12 Temperature (K) Integrated electroluminescence (a.u.) Integrated photoluminescence (a.u.) Figure 4 A plot of the integrated electroluminescence intensity as a function of measurement temperature. Inset, a plot of the integrated photoluminescence intensity as a function of temperature. The solid lines are provided as a guide to the eye. The photoluminescence was excited by the 488 nm line of an argon laser at a power of 150 mW. © 2001 Macmillan Magazines Ltd
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.com
letters 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 mechanism. 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-toband 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 associated with increased scattering within the confined carrier populations 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 temperature, 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 spectrum, 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 erbiumdoped, 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 incompatible 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 continental 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
letters to nature agnostics. P. Gottig and R Ramachandran hel corrections hank G. Bourenkov and H. bartunik. and g. leonard with synchrotron data collection at DESY BW6(Hamburg) and ESRF ID14-4(Grenoble), respectively. Competing interests statement Self-assembled monolayer organic he authors declare that they have no competing financial interests. field-effect transistors ence should be addressed to H B. (e-mail: hbsepbiochem. mpg. de).The ates of the tricorn protease have been deposited in Protein Data Bank under Jan Hendrik Schon, Hong Meng &Zhenan Bao on code 1K32 Nare4l3,713-716(2001) The values of the transconductance in table l and in the tey 715, second paragraph) are incorrect. The values should be addendum by ten. The data plotted in Figs 2 and 3 are correct conclusions are not affected An efficient room-temperature silicon-based light-emitting diode Wai Lek Ng, M. A Lourenco, R. M. Gwilliam, S Edain, G shao Ordered nanoporous arrays of carbon K P Homewood supporting high dispersions of Silicon light-emitting diodes (led) show light emission at the platinum nanoparticles bandgap energy of silicon with efficiencies approaching those of standard Ill-Vemitters: 0. 1% for planar devices(our Letter) and sang Hoon Joo, Seong Jae Choi, lwhan Oh, Juhyoun Kwak, about 1% when total internal reflection is minimized by surface Zheng Liu, Osamu Terasaki& Ryong Ryoo turing. We point out here an additional example of a silico device also show t emission at the bandgap the authors described devices made by the SACMos-3 a plas. However, and focus the We inadvertently omitted to cite an earlier reference bulk of the paper on visible emission under reverse (G Che, B Lakshmi, E R. Fisher and C. R. Martin No they also report briefly on a device operated under forward bias 349; 1998), which was published in 1995(and not 20 inted) ving efficiencies of around 0.01%, although no explanation of the Also, our suggestion that using the pores in a microporous material mechanism is given. It is now becoming clear that crystalline silicon, as templates could be a way in which to produce nanoscale materials when appropriately engineered, is capable of supporting efficient has been discussed before(see, for example, C. R. Martin Science light emission, opening up many significant applications. O 266, 1961-1966(1994)and J. C Hulteen C R. Martin J Mater. I. Green, M. A Shao, l, Wang, A, Reece, P. L.& Gal, M. Efficient silicon light-emitting diodes. Natur Chem.7,1075-1087(1997) 412,805-808(2001) Kramer, I. et aL Light-emitting devices in ind 527-533(1993) erratum The timing of the last deglaciation in North atiantic climate records Warm tropical sea surface Claire Waelbroeck, Jean-Claude Duplessy, Elisabeth Michel, Laurent Labeyrie, Didier Paillard Josette Duprat temperatures in the Late cretaceous Nature412,724-727(2001) and Eocene epochs We directly used the observed leads of sea surface temperature wi Paul N Pearson, Peter W. Ditchfield, Joyce Singan Katherine g. harcourt-Brown christ aspect to air temperature( dated in calendar years), whereas the air Richard K. olsson Nicholas j Shac Mike A ha with reservoir ages computed as the difference between marine and atmospheric ages. Taking this into consideration, apparent Nature413,481-487(2001) surface-water ages are 1, 180+ 630 to 1, 880+ 750 years at the end of the Heinrich I surge event(14, 500 years BP)and 930+ 250 to In this Article, the temperature scale in Figure 3i should have been 1,050+ 230 years at the end of the Younger Dryas cold episode. This the same as in Figure 3g s not change the discussion and conclusions 470 A@2001 Macmillan Magazines Ltd ATURE VOL 414/22 NOVEMBER 20011
letters to nature 470 NATURE | VOL 414 | 22 NOVEMBER 2001 | www.nature.com Acknowledgements We acknowledge ®nancial support of the Deutsche Forschungsgemeinschaft and of Roche Diagnostics. P. GoÈttig and R. Ramachandran helped with biochemical analyses. We thank G. Bourenkov and H. Bartunik, and G. Leonard for help with synchrotron data collection at DESY BW6 (Hamburg) and ESRF ID14-4 (Grenoble), respectively. Competing interests statement The authors declare that they have no competing ®nancial interests. Correspondence should be addressed to H.B. (e-mail: hbs@biochem.mpg.de). The coordinates of the tricorn protease have been deposited in Protein Data Bank under accession code 1K32. ................................................................. addendum An ef®cient room-temperature silicon-based light-emitting diode Wai Lek Ng, M. A. LourencË o, R. M. Gwilliam, S. Ledain, G. Shao & K. P. Homewood Nature 410, 192±194 (2001). .................................................................................................................................. Silicon light-emitting diodes (LED) show light emission at the bandgap energy of silicon with ef®ciencies approaching those of standard III±V emitters: 0.1% for planar devices (our Letter) and about 1% when total internal re¯ection is minimized by surface texturing1 . We point out here an additional example of a silicon device also showing light emission at the bandgap2 . The authors described devices made by the SACMOS-3 process and focus the bulk of the paper on visible emission under reverse bias. However, they also report brie¯y on a device operated under forward bias giving ef®ciencies of around 0.01%, although no explanation of the mechanism is given. It is now becoming clear that crystalline silicon, when appropriately engineered, is capable of supporting ef®cient light emission, opening up many signi®cant applications. M 1. Green, M. A., Shao, J., Wang, A., Reece, P. J. & Gal, M. Ef®cient silicon light-emitting diodes. Nature 412, 805±808 (2001). 2. Kramer, J. et al. Light-emitting devices in industrial CMOS technology. Sensors Actuators A37±A38, 527±533 (1993). ................................................................. erratum Warm tropical sea surface temperatures in the Late Cretaceous and Eocene epochs Paul N. Pearson, Peter W. Ditch®eld, Joyce Singano, Katherine G. Harcourt-Brown, Christopher J. Nicholas, Richard K. Olsson, Nicholas J. Shackleton & Mike A. Hall Nature 413, 481±487 (2001). .................................................................................................................................. In this Article, the temperature scale in Figure 3i should have been the same as in Figure 3g. M ................................................................. corrections Self-assembled monolayer organic ®eld-effect transistors Jan Hendrik SchoÈn, Hong Meng & Zhenan Bao Nature 413, 713±716 (2001). .................................................................................................................................. The values of the transconductance in Table 1 and in the text (page 715, second paragraph) are incorrect. The values should be divided by ten. The data plotted in Figs 2 and 3 are correct and the conclusions are not affected. M ................................................................. correction Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles Sang Hoon Joo, Seong Jae Choi, Ilwhan Oh, Juhyoun Kwak, Zheng Liu, Osamu Terasaki & Ryong Ryoo Nature 412, 169±172 (2001). .................................................................................................................................. We inadvertently omitted to cite an earlier reference alongside ref. 8 (G. Che, B. Lakshmi, E. R. Fisher and C. R. Martin Nature 393, 346± 349; 1998), which was published in 1995 (and not 2000 as printed). Also, our suggestion that using the pores in a microporous material as templates could be a way in which to produce nanoscale materials has been discussed before (see, for example, C. R. Martin Science 266, 1961±1966 (1994) and J. C. Hulteen & C. R. Martin J. Mater. Chem. 7, 1075±1087 (1997)). M ................................................................. correction The timing of the last deglaciation in North Atlantic climate records Claire Waelbroeck, Jean-Claude Duplessy, Elisabeth Michel, Laurent Labeyrie, Didier Paillard & Josette Duprat Nature 412, 724±727 (2001). .................................................................................................................................. We directly used the observed leads of sea surface temperature with respect to air temperature (dated in calendar years), whereas the air temperature calendar ages should have been converted into 14C ages, with reservoir ages computed as the difference between marine and atmospheric 14C ages. Taking this into consideration, apparent surface-water ages are 1,180 6 630 to 1,880 6 750 years at the end of the Heinrich 1 surge event (14,500 years BP) and 930 6 250 to 1,050 6 230 years at the end of the Younger Dryas cold episode. This does not change the discussion and conclusions. M © 2001 Macmillan Magazines Ltd