We need them for photovoltaics, transistors for computer chips, LEDs, image sensors and radiation detectors. But what exactly are semiconductors?

Semiconductors are called seminconductors because their electrical conductivity lies between that of insulators, such as glass, and conductors, such as metals. It is particularly important that the conductivity of semiconductors can be specifically controlled – a crucial advantage for applications in photovoltaics and microelectronics. Physicists can influence the properties of semiconductor materials by making targeted changes to their atomic crystal lattice; we call this doping.

In the doping process for semiconductor materials, electrons are introduced or ‘removed’. This is because only a free electron, which is not bound within the crystal structure, can carry an electric current. For the semiconductor silicon, such conventional dopants can be, for example, phosphorus, arsenic or boron.

For new applications – particularly in quantum computers – microchips now need to become ever smaller, capable of operating at low temperatures and more efficient, don’t they?

Exactly. The requirement is for a smaller volume of transistors on the microchips, whilst at the same time delivering better performance and lower power consumption! For about ten years now, however, transistors manufactured using conventional doping have reached a limit in terms of further miniaturisation. Their characteristic physical dimensions have plateaued at 15–20 nanometres.

Another challenge is posed by quantum computers, which usually have to be operated close to absolute zero (–273.15 degrees Celsius) in order to keep the qubits stable. Qubits are the fundamental unit of information used to encode data in quantum computing. However, to exchange data with a quantum computer, we will continue to rely on tried-and-tested electronics – which, in turn, are not designed to operate at such low temperatures. As the free charge carriers resulting from the conventional doping of the semiconductor freeze at these temperatures, control over conductivity is lost.

What solutions does quantum mechanics offer for these new requirements?

This is where the modulation doping we are researching comes into play. Put simply, the dopant atoms are not incorporated into the silicon itself, but into ultra-thin silicon dioxide layers on its surface. To achieve charge transfer despite their insulating properties, we use nanowires and what is known as quantum mechanical tunnelling. As the crystal structure of the semiconductor remains unchanged, the mobility of the charge carriers is preserved, which is beneficial for the energy efficiency of the devices. Furthermore, conductivity is maintained even at extremely low temperatures.

Together with various collaborative partners, we have already successfully tested the first modulation-doped transistors. However, there remains a great need for research and development to realise ultra-small transistors with optimised modulation doping.

How small is ‘ultra-small’ for this purpose?

If transistor lengths of less than 10 nanometres can be achieved, this will open up entirely new possibilities for energy efficiency in computing. By way of comparison: at 0.1 millimetres, an average human hair is around 10,000 times thicker.

The new processes for modulation doping are being carried out in the Central Cleanroom Laboratory at TU Bergakademie Freiberg. What makes this work so exciting?

I work in the cleanroom myself at least once a week on modulation doping using various thin-film materials, and I supervise undergraduates and PhD students. Whilst, together with a colleague in Canberra, Australia, we are able to calculate and model the properties of the new semiconductor surface functionalisations very reliably using computer simulations, we need to carry out experiments to verify the predictions. In the cleanroom, we build up the materials according to the calculated ‘recipe’ and then measure conductivity or charge carrier concentrations. We then draw conclusions about what has actually happened in the material – and use these insights to further develop the processes.

Actually, the Department of Applied Physics is soon to receive special equipment for the production of ultra-small nanotransistors: an electron beam lithography system. My work on the new materials is sure to become even more exciting!