E. P. Pokatilov

Extremely high thermal conductivity of graphene: Prospects for thermal management applications in nanoelectronic circuits

The authors reported on investigation of the thermal conductivity of graphene suspended across trenches in Si∕SiO2 wafer. The measurements were performed using a noncontact technique based on micro-Raman spectroscopy. The amount of power dissipated in graphene and corresponding temperature rise were determined from the spectral position and integrated intensity of graphene’s G mode. The extremely high thermal conductivity in the range of ∼3080–5150W∕mK and phonon mean free path of ∼775nm near room temperature were extracted for a set of graphene flakes. The obtained results suggest graphene’s applications as thermal management material in future nanoelectronic circuits.

Dimensional crossover of thermal transport in few-layer graphene

Graphene, in addition to its unique electronic and optical properties, reveals unusually high thermal conductivity. The fact that the thermal conductivity of large enough graphene sheets should be higher than that of basal planes of bulk graphite was predicted theoretically by Klemens. However, the exact mechanisms behind the drastic alteration of a materials intrinsic ability to conduct heat as its dimensionality changes from two to three dimensions remain elusive. The recent availability of high-quality few-layer graphene (FLG) materials allowed us to study dimensional crossover experimentally. Here we show that the room-temperature thermal conductivity changes from approximately 2,800 to approximately 1,300 W m(-1) K(-1) as the number of atomic planes in FLG increases from 2 to 4. We explained the observed evolution from two dimensions to bulk by the cross-plane coupling of the low-energy phonons and changes in the phonon Umklapp scattering. The obtained results shed light on heat conduction in low-dimensional materials and may open up FLG applications in thermal management of nanoelectronics.

Lattice thermal conductivity of graphene flakes: Comparison with bulk graphite

The authors proposed a simple model for the lattice thermal conductivity of graphene in the framework of Klemens approximation. The Gruneisen parameters were introduced separately for the longitudinal and transverse phonon branches through averaging over phonon modes obtained from the first principles. The calculations show that Umklapp-limited thermal conductivity of graphene grows with the increasing linear dimensions of graphene flakes and can exceed that of the basal planes of bulk graphite when the flake size is on the order of a few micrometers. The obtained results are in agreement with experimental data and reflect the two-dimensional nature of phonon transport in graphene.

Photoluminescence of spherical quantum dots

In order to interpret the phonon-assisted optical transitions in semiconductor quantum dots, a theory is developed comprising the exciton interaction with both adiabatic and Jahn-Teller phonons and also the external nonadiabaticity (pseudo-Jahn-Teller effect). The effects of nonadiabaticity of the exciton-phonon system are shown to lead to a significant enhancement of phonon-assisted transition probabilities and to multiphonon optical spectra that are considerably different from the Franck-Condon progression. The calculated relative intensity of the phonon satellites and its temperature dependence compare well with the experimental data on the photoluminescence of CdSe quantum dots, both colloidal and embedded in glass.

Phonon spectrum and group velocities in AlN/GaN/AlN and related heterostructures

We theoretically investigated acoustic phonon spectrum and group velocities in an ultra-thin layer of wurtzite GaN embedded within two AlN cladding layers. The core GaN layer thickness has been chosen on the order of the room-temperature dominant phonon wavelength so that the phonon spectrum in such a structure is strongly modified compared to bulk. We derived equations of motion for different phonon polarizations in the anisotropic medium approximation, which allowed us to include specifics of the wurtzite lattice. Based on our model we calculated phonon density of states and phonon group velocity. Using several other example material systems, it has also been demonstrated that the phonon group velocity in the core layer can be made higher or lower than that in corresponding bulk material by a proper selection of the cladding material and its thickness. Presented results shed new light on the effect of barrier material on the phonon spectrum of heterostructures and can be used for modeling the thermal and electrical properties of such structures.

Built-in field effect on the electron mobility in AlN/GaN/AlN quantum wells

The authors demonstrated theoretically that compensation of the built-in electric field in AlN∕GaN∕AlN heterostructures with the externally applied perpendicular electric field may lead to the increase of the in-plane electron drift mobility. It has been shown that two- to fourfold increase of the room temperature mobility can be achieved for both nondegenerate and degenerate electron densities. Their calculations clarified the role of the intersubband electron transitions mediated by optical phonons in limiting the carrier mobility in GaN-based heterostructures. The tuning of the electron mobility with the perpendicular electric field may impact design of the high-power GaN∕AlGaN heterostructure field-effect transistors.

Photoluminescence of Tetrahedral Quantum-Dot Quantum Wells

Taking into account the tetrahedral shape of a quantum-dot quantum well (QDQW) when describing excitonic states, phonon modes, and the exciton-phonon interaction in the structure, we obtain within a nonadiabatic approach a quantitative interpretation of the photoluminescence spectrum of a single CdS/HgS/CdS QDQW. We find that the exciton ground state in a tetrahedral QDQW is bright, in contrast to the dark ground state for a spherical QDQW. The position of the phonon peaks in the photoluminescence spectrum is attributed to interface optical phonons. We also show that the experimental value of the Huang-Rhys parameter can be obtained only within the nonadiabatic theory of phonon-assisted transitions.

Multiphonon Raman scattering in semiconductor nanocrystals: importance of nonadiabatic transitions

Multi-phonon Raman scattering in semiconductor nanocrystals is treated taking into account both adiabatic and non-adiabatic phonon-assisted optical transitions. Because phonons of various symmetries are involved in scattering processes, there is a considerable enhancement of intensities of multi-phonon peaks in nanocrystal Raman spectra. Cases of strong and weak band mix- ing are considered in detail. In the first case, fundamental scattering takes place via internal electron-hole states and is participated by s- and d-phonons, while in the second case, when the intensity of the one-phonon Raman peak is strongly influenced by the interaction of an electron and of a hole with in- terface imperfections (e. g., with trapped charge), p-phonons are most active. Calculations of Raman scattering spectra for CdSe and PbS nanocrystals give a good quantitative agreement with recent experimental results.

Electron mobility enhancement in AlN∕GaN∕AlN heterostructures with InGaN nanogrooves

The authors show that the electron mobility can be strongly enhanced in AlN∕GaN∕AlN heterostructures with the shallow InxGa1−xN channel—nanogroove—in the middle of the potential well. The modified heterostructure has the room-temperature electron mobility, which is five times larger than that in conventional quantum wells. The maximum mobility enhancement is achieved for In content x≈0.05, which is sufficient to weaken the intersubband electron scattering without leading to the substantial electron—interface-phonon scattering. The mobility enhancement is pronounced for a wide range of the carrier densities (1011–1013cm−2), which is important for GaN technology.

Phonon-engineered mobility enhancement in the acoustically mismatched silicon/diamond transistor channels

The authors have shown that the low-field electron drift mobility in the ultrathin silicon films can be enhanced if they are embedded within acoustically hard materials such as diamond. The increase results from phonon spectrum modification in the acoustically mismatched silicon/diamond heterostructure and suppression of the deformation-potential electron-phonon scattering. The room temperature mobility in silicon films with 2 nm thickness can be increased by a factor of 2–3 depending on the hardness and thickness of the barrier layers. The obtained results suggest a new phonon-engineering approach for increasing the speed and drive current of downscaled electronic devices.