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Making SiGe core fibres a reality

Laser recrystallisation can create functional materials for faster transistors, optical transmission

Researchers has created glass fibres with single-crystal SiGe cores. The process used to make these could assist in the development of high-speed semiconductor devices and expand the capabilities of endoscopes says Ursula Gibson (pictured above), a physics professor at the Norwegian University of Science and Technology and senior author of the paper.

"This paper lays the groundwork for future devices in several areas," Gibson said, because the germanium in the silicon core allows researchers to locally alter its physical attributes.

Gibson  collaborated with scientists at the KTH Royal Institute of Technology in Sweden, Newcastle University and the University of Southampton in the UK and Clemson University in the USA, on the project. The article, "˜Laser recrystallization and inscription of compositional microstructures in crystalline SiGe-core fibres' was published in Nature Communications on October 24.

Melting and recrystallizing

When silicon and germanium two substances are combined in a glass fibre, flecks of germanium-rich material are scattered throughout the fibre in a disorderly way because the silicon has a higher melting point and solidifies, or "˜freezes' first. These germanium flecks limit the fibre's ability to transmit light or information. "When they are first made, these fibres don't look very good," Gibson said.

But rapidly heating the fibre by moving it through a laser beam allowed the researchers to melt the semiconductors in the core in a controlled fashion. Using the difference in the solidification behaviour, the researchers were able to control the local concentration of the germanium inside the fibre depending upon where they focused the laser beam and for how long.

"If we take a fibre and melt the core without moving it, we can accumulate small germanium-rich droplets into a melt zone, which is then the last thing to crystalize when we remove the laser slowly," Gibson said. "We can make stripes, dots... you could use this to make a series of structures that would allow you to detect and manipulate light."

An interesting structure was produced when the researchers periodically interrupted the laser beam as it moved along their SiGe fibre. This created a series of germanium-rich stripes across the width of the 150-micrometer diameter core. That kind of pattern created a Bragg grating, which could help expand the capability of long wavelength light-guiding devices. "That is of interest to the medical imaging industry," Gibson said.

Another key aspect of the geometry and laser heating of the SiGe fibre is that once the fibre is heated, it can be cooled rapidly as the fibre is carried away from the laser on a moving stage. Controlled rapid cooling allows the mixture to solidify into a single uniform crystal the length of the fibre - which makes it ideal for optical transmission.

Previously, people working with bulk SiGe alloys have had problems creating a uniform crystal that is a perfect mix, because they have not had sufficient control of the temperature profile of the sample.

"When you perform overall heating and cooling, you get uneven composition through the structure, because the last part to freeze concentrates excess germanium," Gibson said. "We have shown we can create single crystalline SiGe at high production rates when we have a large temperature gradient and a controlled growth direction."

Transistors that switch faster

Gibson also says the laser heating process could also be used to simplify the incorporation of SiGe alloys into transistor circuits.

Traditionally, Gibson said, electronics researchers have looked at other materials, such as GaAs, in their quest to build ever-faster transistors. However, SiGe, allows electrons to move through the material more quickly than they move through pure silicon, and is compatible with standard integrated circuit processing.

"SiGe allows you to make transistors that switch faster" than today's silicon-based transistors, she said, "and our results could impact their production."

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