Origin of phonon-limited mobility in two-dimensional metal dichalcogenides

Metal dichalcogenides are novel two-dimensional (2D) semiconductors after the discovery of graphene. In this article, phonon-limited mobility for six kinds of 2D semiconductors with the composition of MX2 is reviewed, in which M (Cr, Mo and W) is the transition metal, and X (S and Se) is the chalcogen element. The review is divided into three parts. In the first part, we briefly introduce the calculation method of mobility, including the empirical model and Boltzmann transport theory (BTE). The application scope, merits and limitations of these methods are summarized. In the second part, we explore empirical models to calculate the mobility of MX2, including longitudinal acoustic phonon, optical phonon (OP) and polar optical phonon (POP) models. The contribution of multi-valley to mobility is reviewed in the calculation. The differences between static and high-frequency dielectric constants (Δϵ) are only 0.13 and 0.03 for MoS2 and WS2. Such a low value indicates that the polarization hardly changes in the external field. So, their mobility is not determined by POP, but by deformation potential models. Different from GaAs, POP scattering plays a decisive role in its mobility. Our investigations also reveal that the scattering from POP cannot be ignored in CrSe2, MoSe2 and WSe2. In the third parts, we investigate the mobility of MX2 using electron–phonon coupling matrix element, which is based on BTE from the framework of a many-body quantum-field theory. Valence band splitting of MoS2 and WS2 is induced by spin–orbit coupling effect, which leads to the increase of hole mobility. In particular, we review in detail the theoretical and experimental results of MoS2 mobility in recent ten years, and its mobility is also compared with other materials to deepen the understanding.

Impact of the polar optical phonon and alloy scattering on the charge-carrier mobilities of FA0.83Cs0.17Pb(I1-xBrx)3 hybrid perovskites

Lead mixed-halide perovskites are promising absorption materials that are suitable for applications in tandem solar cells using existing silicon technology. Charge-carrier mobility is an important factor that affects the performance...

Strain Engineering of Polar Optical Phonon Scattering Mechanism – An Effective Way to Optimize the Power-factor and Lattice Thermal Conductivity of ScN

The tug-of-war between the thermoelectric power factor and the figure-of-merit complicates thermoelectric material selection, particularly for mid-to-high temperature thermoelectric materials. Approaches to reduce lattice thermal conductivity while maintaining a high-power...

Prediction of higher thermoelectric performance in BiOCuSe by weakening electron-polar optical phonon scattering

BiOCuSe is a promising thermoelectric material, but its applications are hindered by low carrier mobility. We use first principles calculations to analyse electron-phonon scattering mechanisms and evaluate their contributions to the thermoelectric figure of merit ZT. The combined scattering of carriers by polar optical (PO) and longitudinal acoustic (LA) phonons yields an intrinsic hole mobility of 32 cm2 V-1 s-1 at room temperature and a temperature power law of T-1.5, which agree well with experiments. We demonstrate that electron phonon scattering in the Cu-Se layer dominates at low T, while contributions from the Bi-O layer become increasingly significant at higher T. At room temperature, ZT is calculated to be 0.48 and can be improved by 30% through weakening PO phonon scattering in the Cu-Se layer. This finding agrees with the experimental observation that weakening the carrier-phonon interaction by Te substitution in the Cu-Se layer improves mobility and ZT. At high T, the figure of merit is improved by weakening phonon scattering in the Bi-O layer instead. The theoretical ZT limit of BiOCuSe is calculated to be 2.5 at 875 K.

Prediction of higher thermoelectric performance in BiCuSeO by weakening electron–polar optical phonon scattering

Targeting electron–polar optical (PO) phonon scattering for higher thermoelectric performance in BiCuSeO.

Crystallographic Orientation Effect On Hole Polar Optical Phonon Scattering Rates in Thin Gaas/Alxga1-Xas Quantum Wells

We report on hole polar optical phonon scattering processes in thin GaAs/AlxGa1-xAs quantum wells grown in various crystallographic directions, such as [001], [110]. Using the dielectric continuum model we focus on how the different scattering processes of holes with interface phonon modes depend on the initial hole energy. In our work, we use the Luttinger-Kohn (LK) 6×6 k.p Hamiltonian with the envelope function approximation, from which we compute numerically the electronic structure of holes for a thin quantum well sustaining only one bound state for each type of hole. Due to mixing between the heavy, light, and split off bands, hole subbands exhibit strong nonparabolicity and important warping that have their word to say on physical properties. Detailed and extensive calculations that the rates for intra-subband scattering processes differ significantly from those of bulk GaAs because of quantization and reduced dimensionality. Moreover, the study of scattering as a function of hole energy shows that the trend of the scattering rates is governed mostly by i) overlap integrals and ii) the density of the final states to which the hole scatters. The influence of warping, in the hole energy dispersion, on the phonon scattering rates is also explored and found to be important when the initial hole energy is high. Our calculations show evidence of strong anisotropy in the scattering rates especially for processes involving the heavy hole subband, which anisotropy is in fact quite important and far from being negligible. However, strain effect can reduce scattering rates.