What happens when electrons connect with gaps in the crystal lattice? This unusual liaison, called exciton, is intended to enable new types of semiconductor components. The prerequisites have been created.

Will electron-hole pairs soon flow in the conductor tracks of a computer chip instead of electrons (shown here as blue and red balls)?  That would have many advantages.  This would produce less heat.

WIf physicists describe the complex phenomena in solids and other many-particle systems, they often resort to unconventional approaches. In doing so, they sometimes pretend that matter contains atomic nuclei and electrons as well as other fictitious particles, which they call quasiparticles. In accordance with their real counterparts, they are assigned physical properties such as mass, energy or momentum. With quasiparticles, complex processes can suddenly be seen through – such as those that take place in semiconductors. The fictitious particles are not only used for theoretical considerations, but also for the development of new types of electrical and optical components for the electronics of tomorrow. Three years ago, for example, scientists from the École polytechnique fédérale de Lausanne presented a semiconductor transistor in which, instead of electrons, a special type of quasiparticle, so-called excitons, flowed for the first time.

Excitons consist of a negatively charged electron and a positively charged electron vacancy in a crystal lattice, also called a hole. These electron-hole pairs are created in a semiconductor when an electron from an atom absorbs a photon (such as that of a laser beam) and changes to a higher excited energy state. Excitons usually do not last long. Electrons and holes created usually separate from one another immediately, or they “fuse” into one another, whereupon a light quantum is released. Because of their fragility, for a long time it was only possible at extremely low temperatures to maintain a significant flow of excitons in a semiconductor. Under these conditions, the electron and hole move slowly and have a chance to form a bonded state for a long time.

The fact that the exciton transistor developed by Andras Kis’s researchers also works at room temperature is based on the use of two extremely thin layers of tungsten diselenide (WSe2) and molybdenum disulfide (MoS2) stacked on top of each other. Electrons and holes are created – through the irradiation of light – in a different semiconducting layer. “The excitons in these materials have a particularly strong bond and, more importantly, they don’t dissolve as quickly at room temperature,” explains Kis.

The researchers around Andras Kis (right) from EPFL: Alberto Ciarrocchi, Ahmet Avsar, Dmitrii Unuchek (from left)
The researchers around Andras Kis (right) from EPFL: Alberto Ciarrocchi, Ahmet Avsar, Dmitrii Unuchek (from left): Image: A.Herzog, EPFL

Because excitons are electrically uncharged, they do not migrate towards a positive or positive electrode like electrons or holes. In order to be able to manipulate the quasiparticles, the researchers make use of their electrical dipole moment. This quantity, also known as polarization, arises from the spatial separation of the positive and negative charge. It causes excitons to always move to where the electric field strength is greatest. Because there the energy of the excitons is lowest.

Kis compares the behavior with balls that always strive for the depressions in a landscape with hills and hollows. “In our first transistor, however, the excitons tended to fall into the ‘holes’ and not come out,” says Kis. To prevent this and to increase the life of the quasiparticles, the researchers inserted a middle layer of hexagonal boron nitride (h-BN) between two layers of tungsten and molybdenum diselenide. This structure effectively reduces the depth of the holes. In this way, the Swiss researchers were able to increase the conductivity for excitons in their system.