Session S44: Electrons, Phonons, Electron Phonon Scattering, and Phononics V

Characterization of Thermal Effects in Wide Bandgap Semiconductor Materials and Devices

Wide bandgap electronics made from nitrides (e.g., Gallium Nitride (GaN)) and oxides (e.g., Gallium Oxide (Ga₂O₃)) are currently under development due to their potential to create some of the most advanced RF and power electronic devices in the world. However, the thermal response of these devices under applied electric fields can create large power densities (RF and power electronics) that must be understood. For many of these devices, the electrothermal response plays a strong role in both the acceptable operation or long-term failure and reliability of the devices. Thus, tools that can help elucidate these responses and provide a method to help design better devices is of critical need for this field.
In this talk we will discuss advancements in thermal characterization techniques that have allowed new insights into electrothermal behavior in GaN and Ga₂O₃materials and devices. Specifically, a focus on the role of device architecture and material processing will be addressed. For GaN and Ga₂O₃ RF and power electronics, the role of thermal interfaces in the heat dissipation of both lateral and vertical device architectures will be discussed. We will cover aspects of semiconductor-semiconductor as well as metal-semiconductor interfaces and their roles in device thermal management. A highlight will be the use of plasma activated bonding to provide ultralow thermal resistance interfaces that allow for heterogenous integration of nitrides and oxides with high thermal conductivity substrates. Finally, we will discuss the implications of these effects on device behavior through experiments and modeling.   

Direct solution to the space-time dependent Peierls-Boltzmann transport equation using an eigendecomposition method

Nonlocal thermal transport is generally described by the Peierls-Boltzmann transport equation (PBE). However, solving the PBE for a general space-time dependent problem remains a challenging task due to the high dimensionality of the integro-differential equation. In this work, we present a direct solution to the space-time dependent PBE with a linearized collision matrix using an eigendecomposition method. Furthermore, we show that there exists a generalized Fourier type relation that links heat flux to the local temperature, and this constitutive relation is valid from ballistic to diffusive regimes. Combining this approach with ab initio calculations of phonon properties, we demonstrate that the derived solution gives a more accurate description of thermal transport in crystals that exhibit weak anharmonicity than the commonly-used single-mode relaxation time approximation and thus will lead to an improved understanding of phonon transport in solids.
  

Calculation of mode Gruneisen parameters made simple

Gruneisen parameters measure the degree of anharmonicicy of a material.
These parameters can be used to calculate the coefficient of thermal expansion,
as well as the thermal conductivity of a material. The existing method
to calculate mode Gruneisen parameters relies on the (quasi-)harmonic approximation,
and the numerical differentiation of phonon frequencies computed at different
volumes (through the use of energy schemes based on either classical force fields
or a first principles approach). Here we present a new method to calculate mode
Gruneisen parameters. This method involves two steps. First, the
calculation of the harmonic frequencies at only the equilibrium volume of
a material. Second, for each normal mode, a few self-consistent
field calculations are needed to calculate the corresponding
mode Gruneisen parameter. In this talk, we discuss the conceptual basis of
the new method, its technical implementation and validation, its advantages
and disadvantages, and selected applications. In particular, our method in combination
with a density functional theory approach is used to calculate
mode Gruneisen parameters and the coefficient of thermal expansion of
silicon and aluminum.   

Density functional perturbation theory for lattice dynamics with fully relativistic ultrasoft pseudopotentials: the magnetic case

We discuss the extension of density functional perturbation theory for lattice
dynamics with fully relativistic ultrasoft pseudopotentials to magnetic materials [1].
We avoid the computation of the response at -q by using explicitly the
time-reversal operator, which is applied to the Sternheimer linear system and to its
self-consistent solutions, an approach that has been introduced for the calculation
of magnons [2].
We validate our implementation by comparison with the frozen phonon method in fcc Ni
and in a monatomic ferromagnetic Pt wire, and present an application to MnBi.
We are working to extend the technique to the computation of the phonon dispersions
of magnetic polar insulators.

References:

[1] A. Urru and A. Dal Corso, Phys. Rev. B 100, 045115 (2019).
[2] T. Gorni, I. Timrov, and S. Baroni, Eur. Phys. J. B 91, 249 (2018).   

Photothermal imaging as a new tool for the investigation of the temperature-dependent properties of the medium in nanoscale

Photothermal imaging is a powerful technique to detect small light-absorbing nanoparticles down to 1.4 nm with a high spatial resolution (1). The signal magnitude is proportional to the absorption cross-section of the nanoparticle as well as properties of the medium. Finite thermal diffusivity of the medium causes a phase delay in the response of the heat dissipation. The phase delay at each point integrated over the resolution disk is identified as the photothermal signal phase that depends on the material property (2). In this study, first, we demonstrate that the photothermal phase is capable of monitoring the local medium modifications, with high sensitivity. Second, we determine that, with the aid of simulation, information about the temperature-dependent properties of the medium, such as heat conductivity is attainable in nanoscale.
(1) Berciaud, S., Lasne, D., Blab, G. A., Cognet, L., & Lounis, B. (2006). Photothermal heterodyne imaging of individual metallic nanoparticles: Theory versus experiment. Physical Review B, 73(4), 045424.
(2) Heber, A., Selmke, M., & Cichos, F. (2015). Thermal diffusivity measured using a single plasmonic nanoparticle. Physical Chemistry Chemical Physics, 17(32), 20868-20872.   

Thermal Transport in Phosphorene

Phosphorene, a novel elemental 2D semiconductor, possesses fascinating chemical and physical properties which are distinctively different from other 2D materials. The rapidly growing applications of phosphorene in nano/optoelectronics and thermoelectrics call for comprehensive studies of the thermal transport properties. In this talk, based on the theoretical and experimental progresses, the thermal transport properties of single-layer phosphorene, multilayer phosphorene (nanofilms), and bulk black phosphorus are summarized to give a general view of the overall thermal conductivity trend from single-layer to bulk form. The mechanism underlying the discrepancy in the reported thermal conductivity of phosphorene is discussed by reviewing the effect of different functionals and cutoff distances on the thermal transport evaluations. This review then provides fundamental insight into the thermal transport in phosphorene by reviewing the role of resonant bonding in driving giant phonon anharmonicity and long-range interactions. In addition, the extrinsic thermal conductivity of phosphorene is reviewed by discussing the effects of strain and substrate, together with phosphorene based heterostructures and nanoribbons.   

Thermoelectric properties of half-Heusler TaFeSb from first-principles electron-phonon scattering calculations

We investigate thermoelectrics transport properties of half-Heusler TaFeSb within the Boltzmann transport formalism, using first principles calculations of the electronic relaxation times. Our goal is to explore the microscopic origin of exceptionally high ZT values (up to 1.52 at 973 K) recently reported from the experimental measurements of transport properties of TaFeSb-based compounds. In particular, we focus on the effects arising from the electron-phonon interaction. For that we calculate corresponding scattering rates using Wannier-Fourier interpolation of electron-phonon matrix elements as well as the recently developed electron-phonon averaged (EPA) approximation. We discuss the likely mechanisms of high ZT in TaFeSb-based systems and possible ways to further improve thermoelectric properties of half-Heusler compounds.   

Thermal phonons with micron-scale mean free paths in ultra-drawn polyethylene

Heat conduction in highly oriented polymers is of fundamental and practical interest. It is well-known that the thermal conductivity of certain polymers such as polyethylene (PE) increases by orders of magnitude with drawing, but the microscopic properties of phonons responsible for heat conduction have remained difficult to access experimentally. Here, we report the observation of thermal phonons with micron-scale mean free paths (MFPs) in ultradrawn polyethylene using transient grating spectroscopy. The MFPs are comparable to those in covalent single crystals such as Si despite the imperfect nature of the sample’s microstructure. Further, the sample exhibits a decrease in thermal conductivity with increasing temperature above 200 K, indicating that anharmonic scattering is dominant over reflections from domains between crystallites. Our work provides new insights into the microscopic origin of high uniaxial thermal conductivity of ultradrawn polymers.   

Accelerated Screening of Electron-Phonon Transport in 2D Materials

Numerous experimental and theoretical studies have identified promising 2D materials for thermoelectric applications. These materials can exhibit an advantageous combination of electrical and thermal properties, sometimes resulting in large thermoelectric figure-of-merit values. Using an in-house method to perform accelerated screening of transport properties through first-principles electron-phonon calculations, we consider thermoelectric effects in a broader set of 2D materials. From these calculations, we consider how specific electronic and vibrational properties relate to thermoelectric potential, and identify the best candidate materials for use in application. Additionally, we consider the practical importance of substrate effects on the electron-phonon interaction and transport properties of these materials.   

Simulations of thermoelectric coefficients using DFT bandstructures and energy dependent scattering rates

Performance prediction for thermoelectric (TE) materials requires extracting DFT bandstructures and computation of TE coefficients using Boltzmann transport equation (BTE). The constant relaxation time approximation is commonly employed due to complexities in accurately computing scattering rates.
In this work, we describe the construction of an advanced simulator, which couples generic bandstructures (e.g. from DFT) with BTE, utilizing the full numerical energy/momentum/valley dependences of all states in the extraction of the relaxation times. The method provides more predictive capabilities and accuracy, but also considers all scattering mechanisms (acoustic, non polar optic and polar phonons, ionized impurities) independently, as well as intra- and inter- band transitions.
We show that according to the scattering physics under consideration, the performance ranking between different materials and their doping and temperature dependence varies significantly compared to constant relaxation time consideration.   

Zero-Point renormalization of the band structure within Quasiparticle Self-consistent GW

The reliability of Density Functional Theory (DFT) for the electron-phonon coupling in many materials, including even simple sp-bonded compounds, has been questioned in recent years. Hybrid functionals and quasiparticle GW corrections suggest that nonlocal exchange-correlation enhances the electron-phonon interaction as a consequence of an improved description of the electronic screening. This has highlighted the need to move beyond local exchange-correlation functionals within DFT, but complete field-theoretic investigations are still missing in literature. In this talk I will introduce the development of a field-theoretic methodology which is able to predict on an equal footing electronic quasiparticles and phonons as well as their interaction. Such an approach has been implemented within the Quasiparticle Self-consistent GW (QSGW) formalism which describes well the electronic properties for a wide range of materials, including many where standard DFT fails. The reliability of a such field-theoretic methodology will be discussed with applications ranging from the renormalization of the optical gap of nonpolar semiconductors to the renormalization of the Fermi surface of correlated materials such as FeSe.   

Phonon-Phonon Quantum Coherent Coupling in GaAs/AlAs Superlattice

Quantum coherent coupling between a zone-center phonon and two acoustic phonons was observed in two GaAs/AlAs superlattices (8 nm/8 nm and 5.4 nm/5.4 nm) at ambient temperature. Using degenerate coherent phonon spectroscopy, a multi-cycle oscillation feature appears in the time-resolved phonon amplitudes of both samples, as a result of the coherent energy exchange between a driving phonon mode near first Brillouin zone center and two target acoustic phonon modes. This feature resembles the photon resonant parametric down/up-conversion processes, as well as the reversible coherent energy exchange between the optical field and a mechanical oscillator, suggesting quantum coherent coupling between the driving and target phonon modes. In the 8nm/8 nmsuperlattice, the coupling strength increases nonlinearly at high pump fluences, which may eventually reach an extreme state where all three phonon modes share the same coherent state, as predicted by Orbach in the 1960’s.