New semiconductor structure involving exciton pairs has implications for microchip technology

A new semiconductor structure created by researchers at the Australian National University could pave the way for next generation technologies.

Semiconductors are among the most important pieces of technical hardware in modern life, being the key ingredient in producing microchips used in computers, smartphones and tablets, and have been critical in making these devices smaller and more mobile.

Semiconductors are materials which conduct better than an insulator (like glass), but not as well as a conductor (usually metals).

By adding “impurities” into a non-conducting material like silicon, you can make it conduct. This process, known as “doping,” produces two types of semiconductors: n-type and p-type.

In n-type semiconductors, an atom is added into a crystalline structure with more electrons than the other atoms in the lattice. For example, silicon (which has four outer-shell electrons) can be doped with phosphorous or arsenic (both with five outer-shell electrons). Because the fifth electron in the phosphorous or arsenic atoms have nothing to bond to, they are free to move around and conduct their negative charge through the material – hence these are called n-type semiconductors, conducting negative charge.

P-type semiconductors involve the doping of atoms with fewer electrons. Again, taking silicon as an example, boron or gallium (both with three outer-shell electrons) can be added to create what is called a “hole” in the lattice where a silicon electron has nothing to which to bond. In this case, when an electron moves to “fill” the hole, the hole acts like a moving positive charge – hence p­-type semiconductors conduct positive charge.

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The fun happens when you put n-type and p-type semiconductors together, getting interesting effects at the “junction” between them.

Electrical engineers at ANU have come up with a new way of putting semiconductors together which has yielded some exciting results. They believe their research could lay the foundations for a new generation of faster and more energy efficient smartphones and computers. Their results are published in Nature.

Sandwiching two sheets of semiconductor material, the researchers have found that they have produced an “exciton” pair between the layers with some useful properties.

Excitons are a type of “quasiparticle” – a microscopic physical phenomenon, first theorised by Soviet physicist Lev Landau, which is not really a particle like an electron or proton but has the properties of a particle. Excitons are formed when an electron (negative charge) and an electron hole (positive charge) bind together to form a “bound state” – they are bound by the Coulomb force of attraction which governs electrostatics.

When light is absorbed by the double-layered semiconductor, these bound states are formed and the exciton layer produced.

“Interlayer exciton pairs were predicted by theory decades ago, but we are the first to observe them in experiment,” says lead author of the new study, ANU’s Professor Yuerui (Larry) Lu.

The discovery could help the researchers achieve room-temperature superfluidity where electrical currents can travel without any loss of energy. Superfluids are presently restricted to super low-temperature experiments (approaching absolute zero). Superfluids have no viscosity or friction and, by virtue of their remarkable properties, can flow without energy loss – even penetrating non-porous materials and defying gravity by flowing upward.

PhD researcher at ANU and first author of the paper Xueqian Sun says superfluidity is best visualised as a “super highway” that allows excitons to travel at incredibly fast speeds. Current semiconductor technologies cause electron traffic jams.

“The current generation of semiconductor technology used in our smartphones and laptops limits the speed that excitons can travel, stopping them from reaching their full potential,” she explains. “A good way to visualise this is to think of a car that is bumper-to-bumper on a highway full of traffic. A car can only travel so fast in these conditions, and the same is true for excitons.”

“The incredibly small, lightweight and versatile nature of this new semiconductor structure, which isn’t visible to the naked eye, means it can be incorporated into a range of miniature technologies, with promising implications for the space sector, quantum lasers and other quantum light sources,” adds Lu.

Currently, the experiments have shown the formation of exciton pairs in interlayer semiconductors at room temperature, but they aren’t functionally useful except at very low temperatures. Professor Lu says the next challenge is to figure out a way to make an exciton super highway at room temperature to be able to integrate the new discovery into our smart devices.