Researchers from the Pierre Aigrain Laboratory in the ENS Physics department in Paris, France, have discovered a new cooling mechanism for electronic components made of graphene deposited on boron nitride. The efficiency of this mechanism reportedly allowed the team to reach electric intensities at the intrinsic limit of the laws of conduction.
Current-voltage (left) and temperature-voltage (right) characteristics of a graphene on boron nitride transistor. The transistor effect is visible by modulation of the current as a function of the gate voltage in the Zener-Klein tunnel transport regime.
Heat dissipation is vital in order to prevent deterioration or destruction of electronic components. The laws of physics dictate that increasing the density of components on a chipset implies increasing dissipation and thus heat. Nowadays, with the advances in 2D material devices, this question becomes particularly critical since components are required to be one atom thick. By producing a graphene-based transistor deposited on a boron nitride substrate, the team demonstrated a new cooling mechanism 10 times more efficient than basic heat diffusion. This new mechanism, which exploits the two-dimensional nature of the materials opens a "thermal bridge" between the graphene sheet and the substrate.
The researchers have demonstrated the effectiveness of this mechanism by imposing in graphene levels of electrical current still unexplored, up to the intrinsic limit of the material and without any degradation of the device. This result is an important step towards the development of graphene-based high-frequency electronic transistors.
To perform this experiment, the physicists first made a graphene-based transistor. To this end, they deposited the graphene on a large boron nitride crystal a few tens of nanometers thick itself deposited on a gold plate used as a thermostat. They then operated this transistor at increasing electrical intensities and measured both the temperature of the electrons in the transistor channel and that of the crystal. The electronic temperature has been deduced from the measurement of the high frequency fluctuations of the electric current. The temperature of the boron nitride crystal was measured by Raman spectroscopy.
Their first surprise was to observe that only the electrons heat up, thus sparing the crystalline structure of the material. The researchers then observed the ignition of an ultra-efficient electron cooling mechanism beyond a voltage threshold. They explained this phenomenon by the dielectric anisotropy of the boron nitride layer. This anisotropy gives this insulator the remarkable property of having mixed light-vibration modes called hyperbolic polaritons that propagate in the thickness of the material in a regime forbidden to most other insulators. These "hyperbolic" modes open a real thermal bridge between the graphene and the rear electrode guaranteeing a cooling 10 times more effective than the mere diffusion of heat.
The team has shown that the efficiency of this mechanism is increased tenfold when the transistor enters the Zener-Klein regime, obtained under a very strong electric field in high electron mobility graphene. In this new regime, of particular interest for high frequency amplification applications, the electrons are directly pumped from the valence band to the tunneling conduction band. Under these conditions, they couple optimally to hyperbolic modes, allowing heat to pass directly to the substrate without damaging the graphene network.
The team reports that results have been obtained for various thicknesses of hBN and mono, bi and trilayer graphene and that they are extremely relevant for high frequency power transistors and opto-electronics.