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Everyday science

Battle of the elements: silicon builds digital revolution from sand

31 May 2019 Susan Curtis

Which is your favourite chemical element? To mark the International Year of the Periodic Table, our science journalists will be making the case for their pick from the 118 known elements. In this instalment, Susan Curtis argues that silicon derived from natural sand has driven a technological revolution that has transformed our daily lives.

Silicon
(Adapted from shutterstock/agsandrew)

Silicon is the element that driven the biggest changes in our modern lives. Silicon chips in computers, phones, and all sorts of other gadgets have revolutionized both the way we work and the way we play, while optical fibres made from silica criss-cross the globe to create the communication networks at the heart of our connected world.

Yet this digital revolution is only a very recent phenomenon. As a school student in the early 1980s, I remember the excitement surrounding our very first “computer suite” – a narrow room reclaimed from the back of a maths classroom that was equipped with a Commodore PET, two BBC Micros and a printer. It all seemed very futuristic, even if all we did was to experiment with infinite GOTO loops and play primitive games on the monochrome screens, but little did we know then how the computer chip would transform our lives.

Into the solid state

The power of silicon electronics stems, of course, from the element’s semiconducting properties, which allows solid-state devices to be created that can be switched repeatedly between  “on” and “off” states. But silicon is not the best electronic material – germanium, for example, supports faster electron transport, and was a serious rival to silicon when semiconducting transistors were being developed in the 1950s. Silicon, though, has important advantages for mass-market adoption: it is cheap and plentiful – with silicon being the second most abundant element in the Earth’s crust – and thin insulating layers needed for transistor structures are easy to make by heating silicon wafers in a furnace to form stable silicon dioxide.

The first silicon transistor to operate at faster speeds than germanium was demonstrated in 1961 by physicist Jean Hoerni of Fairchild Semiconductor, who added gold dopants to control and enhance silicon’s natural electronic properties. Hoerni’s work was funded by Cray, who went on to build the world’s first supercomputer from 600,000 individual transistors that were packaged in a specially designed module to minimize connection lengths.

But it was the emergence of silicon integrated circuits in the early 1960s that brought computers into the mainstream. The number of components that could be combined on a single chip first doubled every year, and then from 1975 once every two years – as predicted by Gordon Moore, who was Fairchild’s director for R&D. Moore’s Law has since then become a self-fulfilling prophecy that has driven continued innovation in the silicon industry, with the latest designs using nanoscale fabrication techniques to cram more than a billion transistors onto a single chip.

The manufacturing capability and low cost of silicon microelectronics has spawned other applications too. Solar cells made of crystalline silicon dominate the photovoltaics market, accounting for more than 90% of installed devices, even though silicon’s less-than-perfect optical properties limit conversion efficiencies to around 20%. And researchers are pushing the boundaries of what’s possible with silicon photonics, developing all-optical chips that would boost transmission speeds for datacoms and on-chip interconnects, and recent work has exploited silicon to demonstrate the building blocks of quantum processors.

Purity and abundance drives applications

Critical to these high-tech applications is the ability to create ultrapure silicon wafers. Enormous cylindrical ingots, or “boules”, of almost defect-free single-crystal silicon can be formed by pulling a seed crystal from molten silicon, and these boules are then sliced and polished to form wafers up to 450 mm in diameter. Dopants can also be added to the melt, providing engineers with precise control over the wafers’ electronic properties.

But only a small percentage of elemental silicon is produced with such high purity. Other applications, which include metals manufacturing and the production of chemicals such as silicones, can tolerate higher impurity levels and can be produced using cheaper industrial processes.

Meanwhile, the vast majority of silicon used in everyday life exploits its natural form, with crystalline silicon dioxide – in the form of sand and quartz – making up about 12% of the Earth’s crust. This silica is chemically inert and has a high melting point, making it a popular ingredient in construction materials, ceramics, food, and cosmetics – and even the little sachets that remove moisture from packaged goods.

Silica can also be transformed into glass, which is made by heating sand to around 1600 °C and then cooling it quickly to form an amorphous solid. Pure silica glass is mainly used for demanding applications that demand high thermal resistance and chemical stability, such crucibles and furnace tubes, while the glassware found in science labs typically contains about 80% silicon dioxide.

Into the Internet

More recently, silica glass has found new importance as the material of choice for most of the optical fibre used in today’s communications networks. Silica offers good optical transmission at the telecoms wavelengths of 1.55 µm, but it wasn’t until 1986 – when David Payne of the University of Southampton and Emmanuel Desurvire at Bell Labs invented the erbium-doped fibre amplifier – that all-optical systems became a viable option for inter-continental links. Combined with the invention of wavelength-division multiplexing, which allows multiple optical signals to be transmitted through a single fibre, these all-optical systems enabled telecoms companies to double the capacity every six months between 1992 and 2001, by which time the data rate had reached 10 Tb/s.

Silicon, then, has not only allowed device engineers to pack more computer power into ever smaller devices, but has also delivered the high-speed data links that enable us to stay connected, to collect and process vast amounts of scientific data, and to stream videos on our phones. And even more astounding is that this high-tech world is, quite literally, based on sand. For this reason, silicon gets my vote.

What’s your favourite element? Contact us at pwld@ioppublishing.org with your pick – and the reason why – or via Twitter using the hashtag #battleofelements.

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