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Ma J.,Huazhong University of Science and Technology | Li W.,Scientific Computing and Modelling NV | Luo X.,Huazhong University of Science and Technology
Applied Physics Letters | Year: 2014

Despite wurtzite InN being a widely used semiconductor, its intrinsic thermal conductivity (κ) is still little known. In this work, the κ of wurtzite InN is studied from first principles. The calculated room temperature κ is 130 Wm-1K-1 and 145 Wm -1K-1 for the in-plane and out-of-plane direction, respectively, showing an anisotropy of about 11%. The anisotropy increases with decreasing temperature, and it reaches 20% at 100 K. The evident anisotropy is contrast to the conventionally used isotropic assumption, and is explained by performing comprehensive velocity analysis. We also calculate the cumulative κ as a function of mean free path, which can help understand the size dependence of κ in the non-bulk forms. The obtained cumulative κ is in good agreement with the experimental κ of InN films with thicknesses between 0.5 and 2.1 μm, and shows the size effect can persist up to 10 μm thickness at room temperature. © 2014 AIP Publishing LLC. Source

Ma J.,Huazhong University of Science and Technology | Huang B.,Hong Kong University of Science and Technology | Li W.,Scientific Computing and Modelling NV | Luo X.,Huazhong University of Science and Technology
ASME 2015 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems, InterPACK 2015, collocated with the ASME 2015 13th International Conference on Nanochannels, Microchannels, and Minichannels | Year: 2015

The thermal conductivities of the alloys of wurtzite AlN, GaN and InN are usually analyzed with the virtual crystal mod- el based on the values of the constituent compounds. However, latest experiments and calculations reveal that the thermal con- ductivity of wurtzite InN is about three times larger than the pre- viously used value. Thus it is necessary to reanalyze the thermal conductivities of these alloys. In this work, the intrinsic ther- mal conductivities of AlxGa1-xN, InxGa1-xN and InxAl1-xN are calculated with first-principles calculations along with the vir- Tual crystal treatment. It is found that the thermal conductiv- ities of these alloys are strongly suppressed even after a small amount of alloying. For instance, the in-plane and out-of-plane thermal conductivities of In0:99Ga0:01N are 66 Wm-1K-1 and 76 Wm-1K-1 respectively, while they are 40 Wm-1K-1 and 48 Wm-1K-1 for In0:99Al0:01N, compared with the corresponding values of 130 Wm-1K-1 and 145 Wm-1K-1 for bulk wurtzite InN. When the fraction x varies from 0.2 to 0.8, the thermal con- ductivities of the alloys do not change much. Additionally, the distribution of mean free path indicates that the size effect can persist up to 10 mm for both pure compounds and their alloys at room temperature. © Copyright 2015 by ASME. Source

Li W.,Scientific Computing and Modelling NV | Mingo N.,CEA Grenoble
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

We study the lattice thermal conductivity (κ) of fully filled skutterudites YbFe4Sb12 and BaFe4Sb12 from first principles. The calculated κ of YbFe4Sb12 is ten times lower than that calculated for BaFe4Sb12, and much lower than any values ever reported for other skutterudites. The ultralow κ of YbFe4Sb12 is closely related to the flat Yb-dominated modes appearing in the frequency range from 5 to 7.2 rad/ps. The flat modes significantly increase the three-phonon scattering channels, which is reflected by the unique characteristics of weighted phase space and leads to much stronger anharmonic scattering of intermediate-frequency optical phonons. Although those flat modes are dominated by the Yb atoms, the filler-related anharmonic interaction does not play a role in the increased phonon scattering. This underlines the importance of the hybridization of the filler and the host matrix in those flat modes, which can be guaranteed by the avoided crossing with acoustic phonons. The depressed phonon spectrum mechanism, common to other skutterudites, also plays a role in the ultralow κ. Our work presents a reduction mechanism of κ by the filler in cage-like structures such as skutterudites and clathrates, and demonstrates the nonuniqueness of the reduction mechanism in these materials. © 2015 American Physical Society. Source

Li W.,Scientific Computing and Modelling NV
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

We demonstrate the ab initio electrical transport calculation limited by electron-phonon coupling by using the full solution of the Boltzmann transport equation (BTE), which applies equally to metals and semiconductors. Numerical issues are emphasized in this work. We show that the simple linear interpolation of the electron-phonon coupling matrix elements from a relatively coarse grid to an extremely fine grid can ease the calculational burden, which makes the calculation feasible in practice. For the Brillouin zone (BZ) integration of the transition probabilities involving one δ function, the Gaussian smearing method with a physical choice of locally adaptive broadening parameters is employed. We validate the calculation in the cases of n-type Si and Al. The calculated conductivity and mobility are in good agreement with experiments. In the metal case we also demonstrate that the Gaussian smearing method with locally adaptive broadening parameters works excellently for the BZ integration with double δ functions involved in the Eliashberg spectral function and its transport variant. The simpler implementation is the advantage of the Gaussian smearing method over the tetrahedron method. The accuracy of the relaxation time approximation and the approximation made by Allen [Phys. Rev. B 17, 3725 (1978)PRBMDO0163-182910.1103/PhysRevB.17.3725] has been examined by comparing with the exact solution of BTE. We also apply our method to n-type monolayer MoS2, for which a mobility of 150 cm2 v-1 s-1 is obtained at room temperature. Moreover, the mean free paths are less than 9 nm, indicating that in the presence of grain boundaries the mobilities should not be effectively affected if the grain boundary size is tens of nanometers or larger. The ab initio approach demonstrated in this paper can be directly applied to other materials without the need for any a priori knowledge about the electron-phonon scattering processes, and can be straightforwardly extended to study cases with electron-impurity scattering. © 2015 American Physical Society. Source

Zibouche N.,Jacobs University Bremen | Philipsen P.,Scientific Computing and Modelling NV | Heine T.,Jacobs University Bremen | Kuc A.,Jacobs University Bremen
Physical Chemistry Chemical Physics | Year: 2014

The influence of an external electric field on single-layer transition-metal dichalcogenides TX2 with T = Mo, W and X = S, Se (MoWSeS) has been investigated by means of density-functional theory within two-dimensional periodic boundary conditions under consideration of relativistic effects including the spin-orbit interactions. Our results show that the external field modifies the band structure of the monolayers, in particular, the conduction band. This modification has, however, very little influence on the band gap and effective masses of holes and electrons at the K point, and also the spin-orbit splitting of these monolayers is almost unaffected. Our results indicate a remarkable stability of the electronic properties of TX2 monolayers with respect to gate voltages. A reduction of the electronic band gap is observed starting only from field strengths of 2.0 V Å-1 (3.5 V Å-1) for selenides (sulphides), and the transition to a metallic phase would occur at fields of 4.5 V Å-1 (6.5 V Å-1). © 2014 the Partner Organisations. Source

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