news feature verely degrade the layer's electrical cor a,Aa result, semiconductor laser diodes Searching for silicon's perfect partner with silicon circuits, they are using a which hinders the constant drive within the computational methods to devise electronics industry to reduce the size of its semiconductors with the desired rcuitry. Separate lasers introduce further problems when chips are connected together, lattice constant and the band gap because achieving and maintaining precise (see main text needed to produce alignment between the chips and the light light of useful wavelengths sources becomes more complicated. It would John Joannopoulos and be far better if light-emitting devices could b colleagues at the Massachusetts integrated directly with silicon chips That would be simple if silicon itself calculated that a specific blend of emitted light. But normal bulk silicon does zinc, silicon, phosphorus and not, for a subtle reason. Semiconductors arsenic should have a lattice emit light when their electrons jump constant only 0.08% smaller than between energy levels. The size of the energ. that of silicon and should emit gap between these states- known as the light at the ideal frequency for band gap-determines the wavelength of telecommunications. The only the photon emitted This modelled blend of carbon, silicon and tin should emit light snag is that it is likely to be Silicon's band gap corresponds to light and match up with silicon's lattice spacing. extremely hard to make. with a wavelength of 1 um, which would be Others have been searching fine for fibre-optic transmission. But silicon As squeezing light out of silicon silicon substrate pass through the for alloys that should be more has what solid-state physicists call an in- has tended to bring the words first organic layer to the erbium- amenable to synthesis. Vincent direct 'band gap, meaning that any electron 'blood and stone to mind, containing layer, where they meet Crespi and his colleagues at moving between the two energy states must physicists are always on the negative charge carriers provided Pennsylvania State University change its momentum.This makes the tran- lookout for new ways to integrate by a top contact of aluminium As have teamed up with Arizona sition less likely to occur. ptical devices with silicon chips. the charge carriers recombine, State University's John is search has led William Gillin they emit infrared radiation. Kouvetakis, a materials scientist Porous potential of Queen Mary College in London But these OLEDs have a long who specializes in synthesizing Despite these problems, researchers trying down an unusual path-he has way to go before they are mplex inorganic compounds. to persuade silicon to glow are currently in gone organic. practical. The voltage needed to Crespi's team"has modelled an upbeat mood. Silicon, they have found, switch the volts-is blends of carbon, tin, germanium Des emit light if chopped down into tiny based) materials sit happily on a massive by industry standards, and silicon that have lattice pieces just a few nanometres wide. silicon surface because they are and their efficiency, just 0.01%, constants within 1% of silicons, The first evidence for this remarkable amorphous, rather than leaves a lot to be desired Gillin is and optimal wavelengths for change in silicon's behaviour emerged in crystalline-there is no lattice optimistic that it will be possible fibre-optic transmission 990from Canhami's lab, then based at the mismatch because there is no to get his devices to work at as ouvetakis is now trying to Defence Research Agency(DERAs earlier crystal lattice. But the trick is to little as 3 volts. They also have make these materials.So far, he incarnation)at Malvern in the English Mid-find such light-emitting the advantage of being relatively has produced alloys of tin and lands. Canham used hydrofluoric acid to substances that are sufficiently easy to make and can emit light germanium that are stable up to dissolve silicon, etching away much of the conducting to function in of different wavelengths if th 400C and in which, crucially, the materialtoleavea poroussubstancemadeup electronic devices. Gillin and his erbium is replaced with different tin atoms seem to occupy the of a network of silicon nanowires". These colleagues have created tiny threads confine mobile electrons to silicon-based organic light- Other physicists are hoping to separates out into clusters in narrowchannels.As thespaceavailable tothe emitting diode(OLED)using a sidestep the problem of lattice these materials, destroying the electrons shrinks, quantum confinement,layer of erbium tris(8 mismatch by rational design. crystal structure in the process. effects come into play. These affect the band hydroxyquinoline) deposited on Rather than conducting trial-and-"The next challenge,"says Crespi, gap, making it much easier for the electrons another organic compound error searches for the right is to get the proper ratios of tin, o move between energy states. Positive charge carriers from a The benefits are impressive. Illuminate silicon nanowires with laser light to create pairs of negative and positive charge carriers yellow light is reasonably straightforward to chips. Before porous silicon came along, and suddenly porous silicon glows with visi- make. After this, things get tricky because the researchers trying to integrate LEDs into ble light. n as photoluminescence, this tiny wires become increasingly fragile. But chips had been hampered by the same prob process is 10,000 times more efficient in the porous silicon that emits green and blue light lems affecting lasers: the incompatibility of nanowires than in normal silicon has been reported Ordinarily, silicon would emit infrared The prospects for silicon-based opto Fauchet's silicon-based LED is a proof of light, but Canham reasoned that the band electronic devices were boosted in 1996, concept, but it is not yet technologically gap is increased by the quantum confine- when Philippe Fauchet of the University of viable. The energy efficiency and switching menteffect. Asa result, the wavelength of the Rochester in New York created a porous-sili- speeds-the rate at which the LED can be emitted light gets shorter as the nanowires con light-emitting diode(LED)integrated turned on and off still need to be get thinner. Because increasing the etching onto a chip. LEDs are not bright enough for improved But progress is being made. time produces narrower wires, the system long-distance optical telecommunications, Nobuyoshi Koshida and his colleagues can be tuned to produce light of different but they can be used for communicatingover the Tokyo University of Agriculture and Tech colours. Silicon that emits red, orange or the short distances between and within nology have greatly increased the efficiency of NaturEvoL40922FeBruAry2001www.nature.com Macmillan Magazines L 975severely degrade the layer’s electrical con￾ductivity. As a result, semiconductor laser diodes must be kept separate from silicon circuits, which hinders the constant drive within the electronics industry to reduce the size of its circuitry. Separate lasers introduce further problems when chips are connected together, because achieving and maintaining precise alignment between the chips and the light sources becomes more complicated. It would be far better if light-emitting devices could be integrated directly with silicon chips. That would be simple if silicon itself emitted light. But normal ‘bulk’ silicon does not, for a subtle reason. Semiconductors emit light when their electrons jump between energy levels. The size of the energy gap between these states — known as the band gap — determines the wavelength of the photon emitted. Silicon’s band gap corresponds to light with a wavelength of 1 mm, which would be fine for fibre-optic transmission. But silicon has what solid-state physicists call an ‘in￾direct’ band gap, meaning that any electron moving between the two energy states must change its momentum. This makes the tran￾sition less likely to occur. Porous potential Despite these problems, researchers trying to persuade silicon to glow are currently in an upbeat mood. Silicon, they have found, does emit light if chopped down into tiny pieces just a few nanometres wide. The first evidence for this remarkable change in silicon’s behaviour emerged in 1990 from Canham’s lab, then based at the Defence Research Agency (DERA’s earlier incarnation) at Malvern in the English Mid￾lands. Canham used hydrofluoric acid to dissolve silicon, etching away much of the material to leave a porous substance made up of a network of silicon ‘nanowires’1 . These tiny threads confine mobile electrons to narrow channels.As the space available to the electrons shrinks, quantum ‘confinement’ effects come into play. These affect the band gap, making it much easier for the electrons to move between energy states. The benefits are impressive. Illuminate silicon nanowires with laser light to create pairs of negative and positive charge carriers and suddenly porous silicon glows with visi￾ble light. Known as photoluminescence, this process is 10,000 times more efficient in the nanowires than in normal silicon. Ordinarily, silicon would emit infrared light, but Canham reasoned that the band gap is increased by the quantum confine￾ment effect.As a result, the wavelength of the emitted light gets shorter as the nanowires get thinner. Because increasing the etching time produces narrower wires, the system can be tuned to produce light of different colours. Silicon that emits red, orange or yellow light is reasonably straightforward to make.After this,things get tricky because the tiny wires become increasingly fragile. But porous silicon that emits green and blue light has been reported2 . The prospects for silicon-based opto￾electronic devices were boosted in 1996, when Philippe Fauchet of the University of Rochester in New York created a porous-sili￾con light-emitting diode (LED) integrated onto a chip3 . LEDs are not bright enough for long-distance optical telecommunications, but they can be used for communicating over the short distances between and within PEIHONG ZHANG/PENN STATE NATURE|VOL 409 | 22 FEBRUARY 2001 |www.nature.com 975 news feature chips. Before porous silicon came along, researchers trying to integrate LEDs into chips had been hampered by the same prob￾lems affecting lasers: the incompatibility of materials. Fauchet’s silicon-based LED is a proof of concept, but it is not yet technologically viable. The energy efficiency and switching speeds — the rate at which the LED can be turned on and off — still need to be improved.But progress is being made. Nobuyoshi Koshida and his colleagues at the Tokyo University of Agriculture and Tech￾nology have greatly increased the efficiency of As squeezing light out of silicon has tended to bring the words ‘blood’ and ‘stone’ to mind, physicists are always on the lookout for new ways to integrate optical devices with silicon chips. This search has led William Gillin of Queen Mary College in London down an unusual path — he has gone organic. Gillin’s organic (carbon￾based) materials sit happily on a silicon surface because they are amorphous, rather than crystalline — there is no lattice mismatch because there is no crystal lattice. But the trick is to find such light-emitting substances that are sufficiently conducting to function in electronic devices. Gillin and his colleagues have created a silicon-based organic light￾emitting diode (OLED) using a layer of erbium tris(8- hydroxyquinoline) deposited on another organic compound9 . Positive charge carriers from a silicon substrate pass through the first organic layer to the erbium￾containing layer, where they meet negative charge carriers provided by a top contact of aluminium. As the charge carriers recombine, they emit infrared radiation. But these OLEDs have a long way to go before they are practical. The voltage needed to switch them on — 33 volts — is massive by industry standards, and their efficiency, just 0.01%, leaves a lot to be desired. Gillin is optimistic that it will be possible to get his devices to work at as little as 3 volts. They also have the advantage of being relatively easy to make and can emit light of different wavelengths if the erbium is replaced with different metals. Other physicists are hoping to sidestep the problem of lattice mismatch by rational design. Rather than conducting trial-and￾error searches for the right crystalline material to integrate with silicon circuits, they are using computational methods to devise semiconductors with the desired properties — both in terms of lattice constant and the band gap (see main text) needed to produce light of useful wavelengths. John Joannopoulos and colleagues at the Massachusetts Institute of Technology have calculated that a specific blend of zinc, silicon, phosphorus and arsenic should have a lattice constant only 0.08% smaller than that of silicon10, and should emit light at the ideal frequency for telecommunications. The only snag is that it is likely to be extremely hard to make. Others have been searching for alloys that should be more amenable to synthesis. Vincent Crespi and his colleagues at Pennsylvania State University have teamed up with Arizona State University’s John Kouvetakis, a materials scientist who specializes in synthesizing complex inorganic compounds. Crespi’s team11 has modelled blends of carbon, tin, germanium and silicon that have lattice constants within 1% of silicon’s, and optimal wavelengths for fibre-optic transmission. Kouvetakis is now trying to make these materials. So far, he has produced alloys of tin and germanium that are stable up to 400 7C and in which, crucially, the tin atoms seem to occupy the correct sites. Normally, tin separates out into clusters in these materials, destroying the crystal structure in the process. “The next challenge,” says Crespi, “is to get the proper ratios of tin, silicon and germanium.” Searching for silicon’s perfect partner This modelled blend of carbon, silicon and tin should emit light and match up with silicon’s lattice spacing. ▲ © 2001 Macmillan Magazines Ltd