Chips to go beyond silicon

If there is one element that defines the world of high technology, it is silicon. Without it there would be no micro-processors, no PCs and no Windows. Silicon is significant enough to give a valley its namesake, yet ironically not indigenous to the region. Regardless, its days as the driving force behind IT are numbered. Silicon’s semiconducting properties are destined to hit its physical limits sometime in the next decade as transistor gate lengths can be shrunk no more.

Researchers are spending a great deal of time and effort trying to find a semiconductor to replace it. While silicon facsimiles may provide some solutions, there is a great deal of hope that the very world from which we come – the organic realm – will offer the most viable path to the next generation of computing.

The idea of organic computing is not new, but advances in this the field have been slow, in part due to a myriad of natural hurdles incumbent to the organic world. Unlike their inorganic counterparts, organic materials are fragile, less stable and susceptible to degradation. But they will be cheaper to produce and are more flexible both physically and biochemically than inorganic materials.

“One of the really great benefits of organics is flexibility, they are not mechanically rigid like silicon,” said Rafik Loutfy, director of the Xerox Research Centre of Canada in Mississauga, Ont. But he added that instability has been a major reason, up until now, for little research in the area.

“Organic material tends not to be stable, particularly in air,” he explained. Organic materials degrade when exposed to (amongst other things) water, light and air.

Ye Tao, group leader for organic materials and devices at the National Research Council in Ottawa, agrees that organic materials’ fundamental sensitivity to both water and oxygen is a major hurdle to be overcome.

He and his team have been working on using organic materials to create light-emitting devices for display technology. Colours can all be produced with organic materials, but the issue is still their stability, he said.

Organic molecular structures are much more conducive to change than inorganic ones so it is easier to modify them to get exactly the colour or properties scientists desire. “Inorganic material has little range to change the structure to get certain light colours,” Tao explained.

Today compounds such as silicon carbide are used to create the blue we see in our monitors, but it is both very expensive to make and offers little colour latitude.

a longer life

The lifetime of organic material used in displays has increased dramatically over the past decade.

“In 1986 when we started, a lot of (these colours) would be stable for about four hours, now if you encapsulate and purify the material properly, green can work for 100,000s of hours,” Tao said.

With green more or less solved, scientists are trying to develop molecules to fill the two ends of the light spectrum: red and blue. Both colours, Tao explained, have some natural attributes which make their riddle more difficult to solve. For our eyes to read blue and red with the same light intensity as middle-spectrum green, they have to be much brighter.

“The (molecular) degradation is proportional to the current intensity put into the device,” he said. So a blue or red organic molecule of equal natural stability as green will degrade faster because more current has to be run through it for it to appear as bright.

The big upside for the consumer will be better colours. Though we may not see true organic displays on the market for a number of years, their inherent flexibility will add a new twist. Imagine a roll up computer screen many times larger than the one you use today with colours akin to those found in photographs.

from screens to computers

Years down the line the ultimate outcome may well be organic computers where carbon-based molecules replace silicon as the semiconductor of choice. As the distance between the transistor source and drain gets smaller, silicon’s inherent physical properties no longer allow it to act as a semiconductor.

Several laboratories are working on creating organic field effect transistors. One of them is at the Bell Labs, the research and development arm of Lucent Technologies Inc. in Murray Hill, N.J., Here they have succeeded in making ultra short channels for transistors, using a single molecular layer. They are one- to two-nanometres long. To put this in perspective, today’s traditional semiconductor channels (the distance from source to drain) are over 100 nanometres.

The organic solutions at Bell Labs are also not made using anything approaching traditional techniques. They are, in fact, grown.

Organic semiconducting molecules are designed which will bind to electrodes, in this case gold. But at this molecular level, the incredibly small size can be problematic.

Zhenan Bao, a chemist who is working on these organic transistors at the Bell Labs, explained the process.

“If we put the molecules onto the substrate (see diagram) they may not actually bridge the two electrodes,” she said. “We don’t deposit the source and drain electrodes at the same time, we fabricate one electrode first and use the actual molecule to separate the two electrodes,” Bao said.

“We have one electrode and the (organic) molecule will find the electrode and stay there and then we put a second electrode on top of the molecule,” using the thickness of the molecular film to put in the other contact, she said.

In essence they are creating a sandwich, with the filler (the organic semiconducting material) dictating the distance between the two pieces of bread.

“I don’t see it suddenly displacing silicon…you use molecular properties to (their) advantage,” said Paul Weiss, professor in the department of chemistry at Penn State in University Park, Penn.

And this is the key. Scientists have to start thinking outside the proverbial box.

“The goal with the molecule is not to make chips the way we make them today but instead take advantage the scale of the molecule. “It would seem sort of silly to only mimic silicon in organic molecules. I mean, ultimately I think that there has got to be a better way,” he said.

Xerox’s Loutfy agrees.

“Life itself is organic so I have the tendency to go and find [whether] we can do things organically, because the more you are doing organically the more you are working with nature.”