New Type of Transistor with Heterojunction of Tungsten Disulfide and Graphene

Recently, a new type of transistor with heterojunction of tungsten disulfide and graphene has been created by the research team of University of Groningen. Details of the research have been published in the October 16 issue of Physical Review B.

In order to make transistors that operate using the spin of electrons, rather than their charge, it is necessary to find a way of switching spin currents on and off. Furthermore, the lifetime of the spins should at least be equal to the time taken for these electrons to travel through a circuit. University of Groningen scientists have created a device that meets both of these requirements, based on a double layer of graphene on top of a layer of tungsten disulfide.

New Technique

Graphene, a two-dimensional form of carbon, is a special conductor of electron spins. However, it is difficult to manipulate spin currents in this material. Spin is a quantum mechanical property of electrons, which makes them behave like tiny magnets.

Previous researches have shown that it is possible to control spin currents if the graphene is placed on top of a layer of tungsten disulfide. However, this approach reduces the lifetime of the spins. Tungsten is a metal, and its atoms influence the electrons passing through the graphene, dissipating the spin currents.

A double layer of graphene on the tungsten disulfide was applied, based on the theory that electrons passing through the upper layer should 'feel' less of the metal atoms' influence.

Energy Level

As for the new type of transistor, as the electrons pass through the outer graphene layer, the in-plane spins are dissipated very quickly—in mere picoseconds. However, the lifetime of the out-of-plane spins is about one hundred times longer. This means that, even in the presence of tungsten disulfide, one component of spin currents (spins with an out-of-plane orientation) can travel far enough.

Moreover, the energy level of the spin currents causes them to pass through the upper layer of graphene. This energy level can be boosted by applying an electric field, pushing the spin currents into the lower layer. "Down there, the spins will feel the full effect of the metal atoms and the spin currents will quickly dissipate." This ability to switch the spin current off using an electric field is important, as it can be used to 'gate' transistors based on this technology.

Unfortunately, certain technical limitations of the substrate on which we built these devices prevent us from creating electric fields that are strong enough to produce this gating effect. “However, we have shown that it is possible to send spin currents through a heterostructure made of graphene and tungsten disulfide. That is an important step towards the creation of a spin transistor."

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