Out of the Lab: Light From Silicon

Scientists in the U.K. have taken a major step towards the development of a silicon-based laser. In research published today in the journal Nature, Kevin Homewood and his colleagues at the University of Surrey describe a silicon-based light-emitting diode (LED) that operates with good efficiency at room temperature.

The driving force for this research is a problem that's expected to crop up in about four or five years' time. As transistors shrink in size, the electrons that power them spend proportionately more time in the wires and connections between components. In the end, speed and processing power hit a brick wall.

Of course, we all know that photons are the information carriers supreme. And just as light carries information down an optical fiber, it can be used to transport signals around electronic circuitry. Instead of having multiple layers of metal to transport electrons from one part of the chip to another, light could travel along optical "highways" to connect critical processing functions. "It's like waving across a crowded room," says Homewood. "You get the information across without having to fight the crowds".

To make this a viable proposition, a light source (laser) and a detector are needed. The only problem is, bulk silicon doesn't emit light, so other materials have to be brought into play -- and that makes the processing far too complicated.

For the past ten years, researchers have tried to coax light out of silicon, with varying degrees of success. "There are lots of things that appear to do the job, but in actual fact they don't, because either they don't operate at room temperature, or they're not compatible with standard ultra-large-scale integration [ULSI] manufacturing in silicon," says Homewood. Surrey's technology meets both requirements, he adds.

The emission wavelength is about 1100 nanometers, which lies outside the low-loss regions of optical fiber. But for the short distances involved in optical interconnects, it doesn't matter.

There's a second, and potentially more explosive agenda. Homewood believes he could tweak the wavelength of the silicon so that it lases in the telecom bands around 1550nm. Ultimately, because silicon is so plentiful and so cheap, this might result in silicon lasers replacing today's transmitters. He doesn't want to say much about this, because it's commercially sensitive. But he adds "There's no reason, if we get a [silicon] laser working, that it shouldn't be better [than today's telecom lasers]."

But the sceptics still need convincing. Breakthroughs in light-emitting silicon have been announced countless times before. To understand why this one is different entails going back to the basics of how light is emitted in a semiconductor.

Semiconductors contain two types of charge carrier: electrons, which are negative, and holes (locations in the crystal that ought to contain electrons, but don't), which are positive. To get light out of a semiconductor, an electron needs to meet a hole. It pops into the hole, restoring electrical neutrality and giving up energy as a photon.

As noted, bulk silicon doesn't emit light. It has an "indirect" band gap. That means that the electron can't drop into the hole without also changing its momentum, and that's not very likely to happen. Instead, electrons wander around the crystal until they find defects, where they can give up energy more easily as heat, not light. Even though there are very few defects in single crystal silicon, the heat-generating process for recombination dominates over the light-generating one.

To get light out of silicon, it's necessary to prevent electrons from wandering 'round the crystal, so that they can't meet up with the defects. Cooling everything down helps because it slows down the electrons. Physically confining the electrons, by manufacturing the silicon as tiny particles or wires (porous silicon) also works, but that's not compatible with semiconductor processing.

Surrey's approach is to implant ions in the silicon. That means shooting ions at the silicon with enough energy so they get embedded below the surface. Each ion creates a "dislocation loop" around it -- rather like a ripple around a pebble thrown in a pond -- which creates local stress in the material, and that's what localizes the electrons.

In the work described in Nature, boron ions were used both for creating dislocation loops and for doping to create the LED. Boron implantation is a standard technique used in the fabrication of silicon integrated circuits. Crucially, though the dislocation loops hamper the long-range movement of electrons, "there's no problem with current flow," according to Homewood. Proof of this lies in the fact that the silicon-based LED has an efficiency that's comparable to existing gallium-arsenide LEDs, he says.

Surrey University filed a patent on this work a few weeks ago. It claims that the principle can be applied to other materials, like germanium or silicon carbide, to coax light out of them.

— Pauline Rigby, senior editor, Light Reading http://www.lightreading.com

Tinkerbell 12/4/2012 | 8:42:11 PM
re: Out of the Lab: Light From Silicon >>And what well known UK firm specializes in >>developing integrated optical >>solutions using silicon based processes?


And where did Andrew Rickman, founder of Bookham Technology, study before he started the company?..........

......Surrey University
Dr. Freud 12/4/2012 | 8:46:47 PM
re: Out of the Lab: Light From Silicon This is an excellent article and thread but this sort of technology has not yet been factored into the supplier's product development curve.

There is a lot of work going on to develop OC 768, improving transmission medium performance, as well as work on passive and other sorts of amplification.

If there is a VC out there reading thread, you should consider soliton wave development, because there is a feeling out there that achievements in this field could result in super longhaul applications, closer packing of optical channels, and would require less power to push the signal down the fiber.

Pauline Rigby 12/4/2012 | 8:46:47 PM
re: Out of the Lab: Light From Silicon I'm not surprised someone mentioned Bookham.

But Bookham doesn't make it's own electronics, does it?

Unless it could make both the optics and the electronics in-house, it wouldn't be able to take advantage of the monolithic integration opportunities offered by a silicon laser.

Do you agree, elmo?

If anyone from Bookham is out there, have you any thought on this?

Pauline Rigby
[email protected]
elmo 12/4/2012 | 8:46:50 PM
re: Out of the Lab: Light From Silicon And what well known UK firm specializes in developing integrated optical solutions using silicon based processes?


womble 12/4/2012 | 8:47:21 PM
re: Out of the Lab: Light From Silicon The room temperature efficiency of these devices is impressive. The emission wavelength corresponds to the band gap of silicon and I am not sure how they are going to tweak to 1500 nm. This change in wavelength is surely not a tweak, it is a huge jump!
Pauline Rigby 12/4/2012 | 8:47:23 PM
re: Out of the Lab: Light From Silicon I've addded to the story, to try and explain why this breakthrough is different. Yesterday's story didn't have enough detail.

I think you will see that this time the processing has the potential to be really simple. It hinges on boron ion implantation, which I'm told is a standard process for making ICs. I agree that there's quite a lot of work to do on the integration issues, but this is the best bet for silicon lasers so far, I reckon.

If it works, then the fact that silicon is so plentiful and so cheap will start to matter.

The Surrey team still has plenty of work to do. First, they need to make a laser, which requires a lot of engineering, but shouldn't require any major new breakthroughs, they tell me. They also want to tweak the wavelength.

[email protected]
gea 12/4/2012 | 8:47:26 PM
re: Out of the Lab: Light From Silicon I thought the major application for the technology as it now stands is free space optical interconnection. If it perfroms as claimed, this could have an impact on how quickly high end hardware (not necessarily telecom hardware) can operate. Free space optical interconnection is a hot topic in some areas, but it's not like a PC is going to be using this any time soon. It is only useful in very advanced architectures, probably like in military applications. It would take a while for this to impact the telecom world significantly.
redface 12/4/2012 | 8:47:47 PM
re: Out of the Lab: Light From Silicon Silicon is an indirect bandgap material and it does not naturally emit light when excited. Even if silicon can be coerced into emitting (by making silicon porous for example), it will probably have pretty compromised performance. Breakthroughs in silicon light emitting have been announced COUNTLESS times without any fruit, and it is hard to say whether it will be any different this time. For the emitting light to be used to transmit signal, the light needs to go into silicon waveguides. Then there is the process integration problems. There will be a lot of engineering issues involved.
The inventor's claim that silicon is a cheaper material than what's being currently used does not make a lot of sense, since it's not the original material but the processing that makes things expensive.
While getting silicon to emit light is cute, don't get too excited about it because it will be many years before one can see any possible application for it, if at all.
Tudy 12/4/2012 | 8:47:52 PM
re: Out of the Lab: Light From Silicon One of the only materials to date for integrated passive/active components is InP. Unfortunately, this material is notoriously hard to work with. If todays semiconductor processing equipment can use silicon to make active components -the potential is huge. Lots of companies are making active or passive components. Hardly any are making INTEGRATED active/passive components. If they are trying to make these integrated components, they're probably using InP and probably running into problems technically and economically. The use of silicon won't make things more efficient but it will allow greater integration since most ics are made of silicon today. In addition to this "familiar material", the economics of silicon could potentially kill type III-IV active material for lots of applications.
sandy 12/4/2012 | 8:47:52 PM
re: Out of the Lab: Light From Silicon Can anyone comment on the implications of this apparent breakthrough on integrated optics companies. Will the licensing of this technology by integrated optics companies have the potential of making their technologies more efficient and/or allow greater integrated functions or will it compete directly with the integrated optics industry? thanks for any reply.
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