Hyperbolic Phonon Polariton Electroluminescence as an Electronic Cooling Pathway

Abstract

Engineering of the cooling mechanism is of primary importance for the development of nanoelectronics. While radiation cooling is rather inefficient in current electronic devices, the strong anisotropy of 2D materials allows for enhanced efficiency because their hyperbolic electromagnetic dispersion near phonon resonances allows them to sustain much larger (≈105) number of radiating channels. In this review, radiation cooling in 2D materials is addressed. The hyperbolic dispersion of electromagnetic waves is presented, and how the spontaneous fluctuations in current in a 2D electronic channel can radiate thermal energy in its hyperbolic surrounding medium is described. Both the regime of thermal current fluctuations and out‐of‐equilibrium current fluctuations can be described within the same framework leading to superPlanckian thermal emission and electroluminescent cooling. A recent experimental investigation on graphene‐on‐hBN transistors using electronic noise thermometry is discussed. In high mobility semimetal‐like graphene at large bias, a steady‐state out‐of‐equilibrium situation is caused by Zener tunneling of electrons, opening a route for electroluminescence of hyperbolic electromagnetic modes. Electroluminescent cooling is particularly prominent once the Zener tunneling regime is reached: observed cooling powers are nine orders of magnitude larger than in conventional LEDs.

Ultra-long carrier lifetime in neutral graphene-hBN van der Waals heterostructures under mid-infrared illumination

“…However, in this regime of low power, low bias and low temperature, scatterings via optical phonon in graphene and via coupling with hyperbolic phonon in hBN are strongly suppressed by Pauli blocking, as ℏω is well below their energy band 170-200 meV. With ℏω = 117 meV, one cannot rule out scattering by the lower 95-100 meV HPhP branch, a mechanism which is expected to be weak 31 . Acoustic phonon scattering cannot cause recombination because energy and momentum would not be conserved for interband transitions 32 .…”

High-Frequency Limits of Graphene Field-Effect Transistors with Velocity Saturation

“…In Refs. [11,16] exfoliated graphene was used and the mobility was high enough (µ > 3 m 2 V −1 s −1 ) to access interband transport at very high bias, the so-called Zener-Klein regime, which is not reached in the present study due to limited mobility.…”

“…That is how we can identify thehΩ I 0.1 ± 0.01 eV energy of h-BN HPhPs as the main source of scattering and rule out the scattering by intrinsic graphene optical phonons or the Ω I I -HPhP band, ashΩ OP ∼hΩ I I ∼0.2 eV. Note that the role of intrinsic OPs or Ω I I -HPhPs in energy relaxation can be identified using noise thermometry respectively in suspended [26] and h-BN supported graphene [11,16]. This illustrates the difference between momentum relaxation (the resistance) and energy relaxation (the temperature or electronic distribution) mechanisms (Another example of such a difference, is acoustic phonon scattering which gives rise to a linear temperature dependence for resistivity but a cubic one for relaxation, reflecting single phonon and inelastic supercollision scattering respectively [27]).…”

“…The saturation current being proportional to the OP energy, power applications favor non-oxide dielectrics such as SiC (hΩ SiC ≈ 116 meV), or hexagonal boron nitride (h-BN), which has two Reststrahlen bands,hΩ I ≈ 95-100 meV and hΩ I I ≈ 170-200 meV, associated respectively with out-of-plane and in-plane optical phonons. Uniaxial dielectrics, such as h-BN, actually sustain hyperbolic phonon polariton modes (HPhPs) that differ from SPPs (and intrinsic OPs), insofar as they propagate deep in the dielectric bulk, providing a unique radiative cooling pathway [11,16]. This paper discusses velocity saturation GFETs, their optimal design and operating conditions, their intrinsic frequency limit, and envisions the possibility to overcome this limit using plasma resonance devices [17,18].…”

Dielectric permittivity, conductivity and breakdown field of hexagonal boron nitride

“…After annealing (1h at 240 Celsius under N 2 ) the capacitor's hBN thickness is determined by AFM and falls in the range d = 6-98 nm. This range exceeds the minimal thickness (10-20 nm) for mobilitypreserving encapsulation, is relevant for gate dielectric applications, and reaches the value (∼ 100 nm) for fully-developed radiative cooling [16,17]. It is however not relevant for tunnelbarrier applications which are described elsewhere [3][4][5].…”