In conventional semiconductors used in integrated circuits, electrons travel as independent particles. As they travel, they are scattered by phonons (quanta of the crystal lattice vibrations), a phenomenon that produces electrical resistance in the material and leads to power loss in a finished device. The charge density wave (CDW) is a quantum state in which electrons and phonons become closely coupled and propagate together through the material, giving rise to a collective current and thus greatly reduced electrical resistance.

The problem is that the transition to this collective quantum state usually occurs at relatively low temperatures – of around 200K in CDW materials. A few years ago, Alexander Balandin's team discovered that this can be increased by about 40K in titanium diselenide (TiSe2) if its thickness is reduced to below about 100 nm. TiSe2 belongs to a family of layered transition-metal dichalcogenides, and several others of these have now been found to undergo above-room-temperature transitions to different CDWs. These transitions can be triggered by changing the thickness of the material or by applying electric fields, for example.

Exploiting collective states

As integrated circuits and electronic devices decrease in size, energy dissipation is one of the main limiting factors to continued miniaturization, explains Balandin. This energy, which is lost each time a transistor switches, for example, is proportional to the number of electrons in the device material and temperature.

"This fundamental condition arises from the laws of thermodynamics and cannot be changed, but the assumption underlying this limit is that the electrons act as an ensemble of independent particles. If the electrons were in a collective state instead, like a CDW, then the minimum dissipation limit for one switching cycle would be greatly reduced. We would thus like to exploit such collective states.

CDW phenomena at room temperature

“Researchers are greatly interested in 2D TMDCs that exhibit CDW and other correlated phenomena,” he continues, “but to date, they have not been able to make a useful, let alone room temperature, device that exploits CDWs in a 2D material. At the same time, we are still looking for ‘killer electronic applications’ for graphene. Our work achieves both goals: it demonstrates CDW phenomena at room temperature and finds applications for graphene that make use of its excellent conduction properties, while avoiding the drawbacks associated with the intrinsic absence of an energy gap in the carbon material.”

In their new work, the researchers studied the 1T polytype of tantalum disulphide (TaS2), a dichalcogenide that undergoes a transition from a normal metallic phase to an incommensurate CDW phase at 545K, to a nearly commensurate CDW phase at 350K and finally to a commensurate CDW phase at 180K. Each phase transition reconstructs the lattice, which in turn strongly modifies the material's electrical properties.

Simple and compact VCO device

“In our work we showed that an abrupt change in the electrical conductivity at the transition point between two different CDW phases in this material can be used to construct a voltage-controlled oscillator (VCO) that operates at room temperature,” explains Balandin. “We are able to control the voltage applied to the material by using an integrated graphene field-effect transistor that provides a linearly tuneable, low-resistance load and a high current drive that matches the low-resistance, semi-metallic 1T-TaS2. The third material, boron nitride (BN), protects the TaS2 from oxidizing while preserving the CDW phase, and serves as the gate electrode for the graphene FET.“

Integrating three very different 2D materials, in a way that exploits the unique properties of each, yields a simple, compact, room-temperature VCO device that could find use in a myriad of practical applications, he tells

“Our work is the first demonstration of room-temperature switching exploiting a CDW transition in a 2D material,” he adds. “Previous demonstrations were either below room temperature or on 1D materials, which are considerably more difficult to work with than 2D ones and are less suitable for integrating onto chips. The fact that TaS2 is a layered material is also important since its thickness can be scaled down to sub-10 nm dimensions, while still maintaining an above room-temperature phase transition temperature.”

And that is not all: the new research also proves that very different 2D materials can be combined to make an all-2D integrated circuit, he adds.

Towards the GHz regime

Another important point is that the transition temperature of 1T-TaS2 to a CDW phase is very high – above room temperature, as mentioned. “Moreover, it has several different CDW phases that have different electrical conductivities,” explains Balandin. “By applying a voltage to the TaS2 channel we were able to trigger a transition between two of these phases, something that was accompanied by an abrupt change in electrical resistance and hysteresis.”

The VCO made by the Riverside team works in the MHz frequency range and might be used in FM radios, computers and portable electronic gadgets to name but three. The researchers are also trying to adapt the device so that it works in the GHz range too.

The work is detailed in Nature Nanotechnology doi:10.1038/nnano.2016.108.