Designer optoelectronics – quantum mechanics for new materials

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European researchers have combined computer modelling of quantum mechanics and precision fabrication processes to create novel transparent conductive oxides made to order for a wide range of scientific and consumer applications.

TCOs – materials that combine transparency and conductivity, qualities that are not usually found together – have multiple applications. As sensors, photovoltaics, light emitting devices and electronically controllable films, they are found in scientific instruments, DVDs, digital cameras, mobile phones, computer displays and hundreds of other products.

Until recently, most TCOs relied on a material called ITO, an oxide of indium which is doped – slightly modified – by the addition of a small quantity of tin. ITOs have proved useful, but, Dr Garry says, suffer from two drawbacks. Their transparency is not very good, especially in the near-infrared range, and indium is in short supply and very expensive.

The NATCO team decided to explore a completely different material, strontium cuprate doped with varying amounts of barium. Copper, barium and strontium are far more abundant and much less expensive than indium.

Extensive calculations applying quantum mechanics predicted that, by doping strontium cuprate with a few percent by weight of barium, the researchers could create precisely the materials they wanted, combining good electrical conductivity and optical transparency.

Fabricating the new materials was a challenge. At first the materials were fabricated in the form of bulk ceramics and then, for actual applications, thin layers were deposited on suitable substrates.

In the end, the researchers settled on two deposition techniques – pulsed laser deposition (PLD) and metal organic chemical deposition (MOCVD).

In PLD, a burst of laser light vaporises the material to be deposited, creating a thin film on a glass or silicon surface. It allows precise control, but can’t be used on large surfaces.

MOCVD uses organic chemistry to create gasses that deposit the desired material onto a surface. It is a more complicated procedure, but has the advantage of being able to be scaled up to coat large surfaces.

Once they had fabricated the materials, the researchers could test how well their electrical and optical properties matched the predicted values. “This was the first time that this kind of work was done on TCOs,” says Dr Garry.

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