Phonon-limited mobility inn-type single-layer MoS2from first principles

Neither of the two typical two-dimensional materials, graphene and single layer MoS, are good enough for developing semiconductor logical devices. We calculated the electron mobility of 14 two-dimensional semiconductors with composition of MX, where M (=Mo, W, Sn, Hf, Zr and Pt) are transition metals, and Xs are S, Se and Te. We approximated the electron phonon scattering matrix by deformation potentials, within which long wave longitudinal acoustical and optical phonon scatterings were included. Piezoelectric scattering in the compounds without inversion symmetry is also taken into account. We found that out of the 14 compounds, WS, PtS and PtSe are promising for logical devices regarding the possible high electron mobility and finite band gap. Especially, the phonon limited electron mobility in PtSe reaches about 4000 cm·V·s at room temperature, which is the highest among the compounds with an indirect bandgap of about 1.25 eV under the local density approximation. Our results can be the first guide for experiments to synthesize better two-dimensional materials for future semiconductor devices.

We perform ab initio calculations of the coupling between electrons and small-momentum polar-optical phonons in monolayer transition metal dichalcogenides of the 2H type: MoS2, MoSe2, MoTe2, WS2, and WSe2. The so-called Fr\"ohlich interaction is fundamentally affected by the dimensionality of the system. In a plane-wave framework with periodic boundary conditions, this coupling is affected by the spurious interaction between the 2D material and its periodic images. To overcome this, we perform density functional perturbation theory calculations with a truncated Coulomb interaction in the out-of-plane direction. We show that the 2D Fr\"ohlich interaction is much stronger than assumed in previous ab initio studies. We provide analytical models depending on the effective charges and dielectric properties of the materials to interpret our ab initio calculations. Screening is shown to play a fundamental role in the phonon-momentum dependency of the polar-optical coupling, with a crossover between two regimes depending on the dielectric properties of the material relative to its environment. The Fr\"ohlich interaction is screened by the dielectric environment in the limit of small phonon momenta and sharply decreases due to stronger screening by the monolayer at finite momenta. The small-momentum regime of the ab initio Fr\"ohlich interaction is reproduced by a simple analytical model, for which we provide the necessary parameters. At larger momenta, however, direct ab initio calculations of electron-phonon interactions are necessary to capture band-specific effects. We compute and compare the carrier relaxation times associated to the scattering by both LO and A1 phonon modes. While both modes are capable of relaxing carriers on timescales under the picosecond at room temperature, their absolute and relative importance vary strongly depending on the material, the band, and the substrate.

We calculate the phonon-limited carrier mobility in (001) Si films with a fully atomistic framework based on a tight-binding (TB) model for the electronic structure, a valence-force-field model for the phonons, and the Boltzmann transport equation. This framework reproduces the electron and phonon bands over the whole first Brillouin zone and accounts for all possible carrier-phonon scattering processes. It can also handle one-dimensional (wires) and three-dimensional (bulk) structures and therefore provides a consistent description of the effects of dimensionality on the phonon-limited mobilities. We first discuss the dependence of the electron and hole mobilities on the film thickness and carrier density. The mobility tends to decrease with decreasing film thickness and increasing carrier density, as the structural and electric confinement enhances the electron-phonon interactions. We then compare hydrogen-passivated and oxidized films in order to understand the impact of surface passivation on the mobility and discuss the transition from nanowires to films and bulk. Finally, we compare the semi-classical TB mobilities with quantum Non-Equilibrium Green's Function calculations based on k ⋅ p band structures and on deformation potentials for the electron-phonon interactions (KP-NEGF). The TB mobilities show a stronger dependence on carrier density than the KP-NEGF mobilities, yet weaker than the experimental data on Fully Depleted-Silicon-on-Insulator devices. We discuss the implications of these results on the nature of the apparent increase of the electron-phonon deformation potentials in silicon thin films.

SrTiO3 (STO) is a versatile substrate with a high dielectric constant, which may be used in heterostructures with 2D materials, such as MoS2, to induce interesting changes to the electronic structure. STO single crystal substrates have previously been shown to support the growth of well-defined epitaxial single-layer (SL) MoS2 crystals. The STO substrate is already known to renormalize the electronic bandgap of SL MoS2, but the electronic nature of the interface and its dependence on epitaxy are still unclear. Herein, we have investigated an in-situ physical vapor deposition (PVD) method, which could eliminate the need for ambient transfer between substrate preparation, subsequent MoS2 growth and surface characterization. Based on this, we then investigate the structure and epitaxial alignment of pristine SL MoS2 in various surface coverages grown on two STO substrates with a different initial surface lattice, the STO(001)(4 × 2) and STO(111)-(9/5 × 9/5) reconstructed surfaces, respectively. Scanning tunneling microscopy shows that epitaxial alignment of the SL MoS2 is present for both systems, reflected by orientation of MoS2 edges and a distinct moiré pattern visible on the MoS2(0001) basal place. Upon increasing the SL MoS2 coverage, the presence of four distinct rotational domains on the STO(001) substrate, whilst only two on STO(111), is seen to control the possibilities for the formation of coherent MoS2 domains with the same orientation. The presented methodology relies on standard PVD in ultra-high vacuum and it may be extended to other systems to help explore pristine two-dimensional transition metal dichalcogenide/STO systems in general.

We study hot-electron cooling by acoustic and optical phonons in monolayer ${\mathrm{MoS}}_{2}$. The cooling power $P$ (${P}_{e}=P/n$) is investigated as a function of electron temperature ${T}_{e}$ (0--500 K) and carrier density $n\phantom{\rule{0.28em}{0ex}}({10}^{10}--{10}^{13} {\mathrm{cm}}^{\ensuremath{-}2}$) taking into account all relevant electron-phonon (el-ph) couplings. We find that the crossover from acoustic phonon dominated cooling at low ${T}_{e}$ to optical phonon dominated cooling at higher ${T}_{e}$ takes place at ${T}_{e}\ensuremath{\sim}50--75$ K. The unscreened deformation potential (DP) coupling to the TA phonon is shown to dominate $P$ due to acoustic phonon scattering over the entire temperature and density range considered. The cooling power due to screened DP coupling to the LA phonon and screened piezoelectric (PE) coupling to the TA and LA phonons is orders of magnitude lower. In the Bloch-Gr\"uneisen (BG) regime, $P\ensuremath{\sim}{T}_{e}^{4}\phantom{\rule{4pt}{0ex}}({T}_{e}^{6})$ is predicted for unscreened (screened) el-ph interaction and $P\ensuremath{\sim}{n}^{\ensuremath{-}1/2}\phantom{\rule{4pt}{0ex}}({P}_{e}\ensuremath{\sim}{n}^{\ensuremath{-}3/2})$ for both unscreened and screened el-ph interaction. The cooling power due to optical phonons is dominated by zero-order DP couplings and the Fr\"ohlich interaction, and is found to be significantly reduced by the hot-phonon effect when the phonon relaxation time due to phonon-phonon scattering is large compared to the relaxation time due to el-ph scattering. The ${T}_{e}$ and $n$ dependence of the hot-phonon distribution function is also studied. Our results for monolayer ${\mathrm{MoS}}_{2}$ are compared with those in conventional two-dimensional electron gases (2DEGs) as well as monolayer and bilayer graphene.

The resonance of the first- and second-order Raman spectra of ZnTe has been measured at room temperature in the region of the ${E}_{0}$ edge using tunable cw dye lasers and ion lasers. The allowed first-order TO scattering is always weaker than the corresponding component of the allowed LO scattering, a fact which is interpreted in terms of the electro-optic coefficient. The forbidden LO scattering for the parallel-parallel polarization (Fr\"ohlich interaction) becomes stronger than the allowed scattering near resonance. The second-order spectra were separated into irreducible components. Their most strongly resonant parts are a $2\mathrm{LO}(\ensuremath{\Gamma})$ resonance (${\ensuremath{\Gamma}}_{1}$ component) and an LO-$\mathrm{TO}(\ensuremath{\Gamma})$ resonance (${\ensuremath{\Gamma}}_{15}$ component). These resonances are interpreted as iterated first-order processes involving the Fr\"ohlich interaction. The rest of the second-order spectra resonates in a manner similar to the allowed first-order spectra. It is therefore attributed to electron-two-phonon interaction vertices. Its strongest feature corresponds to 2TA overtones (${\ensuremath{\Gamma}}_{1}$ component). 2TO and 2LO overtone scattering is negligible. From the ratio of first- to second-order scattered intensities and the deformation potential ${d}_{0}$ for the electron-one-phonon interaction, values of several electron-two-phonon deformation potentials are determined.

Atomic-Scale Studies of Overlapping Grain Boundaries between Parallel and Quasi-Parallel Grains in Low-Symmetry Monolayer ReS2

Single-layer MoS2, with its ultimate atomic thickness, has shown promise to scale down transistors for modern integrated circuitry. On the way to implementing two-dimensional (2D) electronic devices, controlled wafer-scale synthesis of single-layer MoS2, single-layer MoS2 metal-oxide-semiconductor field-effect transistors, ohmic contact of single-layer MoS2 for low contact resistance, etc, have been extensively studied. However, the most commonly used two-terminal electronic component, a diode, which conducts current primarily in one direction, has rarely been reported based on single-layer MoS2. Here, a two-terminal high rectification ratio metal-insulator-semiconductor (MIS) tunnel diode was reported based on single-layer MoS2. The In/Au (10/70 nm) electrode via thermal evaporation was used to form a good ohmic contact with the single-layer MoS2. The Si3N4/Pd/Au (5/10/70 nm) electrode via electron beam evaporation was used to form an MIS tunneling structure with the MoS2, showing a current rectification ratio of up to 107 at room temperature. The high current rectification ratio is realized by controlling the quantum tunneling carrier density and the tunneling barrier width. The single-layer MoS2 MIS tunnel diode fabricated via the silicon technology compatible evaporation method has potential application as a fundamental electronic building block for future 2D electronics.

Room-temperature superparamagnetism due to a large magnetic anisotropy energy (MAE) of a single atom magnet has always been a prerequisite for nanoscale magnetic devices. Realization of two dimensional (2D) materials such as single-layer (SL) MoS2, has provided new platforms for exploring magnetic effects, which is important for both fundamental research and for industrial applications. Here, we use density functional theory (DFT) to show that the antisite defect (MoS) in SL MoS2 is magnetic in nature with a magnetic moment μ of ∼2 and, remarkably, exhibits an exceptionally large atomic scale MAE of ∼500 meV. Our calculations reveal that this giant anisotropy is the joint effect of strong crystal field and significant spin–orbit coupling (SOC). In addition, the magnetic moment μ can be tuned between 1 and 3 by varying the Fermi energy , which can be achieved either by changing the gate voltage or by chemical doping. We also show that MAE can be raised to ∼1 eV with n-type doping of the MoS2:MoS sample. Our systematic investigations deepen our understanding of spin-related phenomena in SL MoS2 and could provide a route to nanoscale spintronic devices.

We report measurements of resonant first- and second-order Raman scattering in GaAs with exciting photon energies covering the entire visible spectrum. Two sets of energy gaps were investigated: the three-dimensional $\frac{{E}_{0}}{{E}_{0}}+{\ensuremath{\Delta}}_{0}$ and the two-dimensional $\frac{{E}_{1}}{{E}_{1}}+{\ensuremath{\Delta}}_{1}$ critical points. The symmetry components of the second-order spectrum were separated and observed structures were interpreted by comparison with neutron scattering data. For a theoretical description of the Raman cross section we used either experimental values or a model description of the electric susceptibility. The resonance behavior could be explained for nearly all observed scattering processes with the exception of $2\mathrm{LO}(\ensuremath{\Gamma})$ and forbidden LO scattering by assuming for first order the electron-one-phonon and for second order the renormalized electron-two-phonon deformation potential coupling. Second-order deformation potentials are given as well as a comparison of the theoretically and experimentally determined ratios of the electron-one-phonon deformation potential near $L$ to that near $\ensuremath{\Gamma}$. Forbidden LO scattering is explained well by the Fr\"ohlich coupling mechanism, not only its resonance shape near ${E}_{0}+{\ensuremath{\Delta}}_{0}$ and near ${E}_{1}$ but also the ratio of its strength to that of TO scattering. $2\mathrm{LO}(\ensuremath{\Gamma})$ scattering is attributed to an iterated electron-one-phonon scattering process caused also by the Fr\"ohlich interaction.

The resonance of several first-order [allowed $\mathrm{TO}(\ensuremath{\Gamma})$, forbidden $\mathrm{LO}(\ensuremath{\Gamma})$] and second-order [$2\mathrm{TA}(\ensuremath{\Delta}\ensuremath{\rightarrow}K)$, $2\mathrm{TO}(\ensuremath{\Delta}, \ensuremath{\Sigma}, \ensuremath{\Lambda})$, $2\mathrm{LO}(\ensuremath{\Gamma})$] structures of the Raman spectrum of InAs is investigated around the energy of the ${E}_{1}$ critical point. The deformation-potential coupling mechanism explains well the resonances of the $2\mathrm{TA}(\ensuremath{\Delta}\ensuremath{\rightarrow}K)$ and $2\mathrm{TO}(\ensuremath{\Delta}, \ensuremath{\Sigma}, \ensuremath{\Lambda})$ whereas those of the $1\mathrm{LO}(\ensuremath{\Gamma})$ and $2\mathrm{LO}(\ensuremath{\Gamma})$ are accounted for by the Fr\"ohlich interaction. Several electron-two-phonon deformation potentials are estimated. The sharp resonances of the $1\mathrm{LO}(\ensuremath{\Gamma})$ and $2\mathrm{LO}(\ensuremath{\Gamma})$ are followed over a wide range of temperatures (10-300 K). The results show clearly that the "Raman gap" differs from the optical one. It depends upon the phonons involved in contrast to its temperature dependence. This variation (- 5.0\ifmmode\pm\else\textpm\fi{}0.4) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ eV ${\mathrm{K}}^{\ensuremath{-}1}$ agrees with that of the optical gap.

Two-dimensional (2D) materials have been giving a chance to advance high performance electronic technology for next generation nanoelectronics devices. Among 2D materials, molybdenum disulfide (MoS2) is drawing attention due to its relative large bandgap (1.3-1.8 eV) and the 50~200 cm2/V·S range of high carrier mobility at room temperature. In the few early report, multilayer MoS2 could have been much better than single-layer MoS2 because it has larger density of state (DOS) and helps to create multiple conducting channels by field effect, which will lead to boost the current drive of TFTs.At realistic operating temperatures, however, the carrier transport mechanism of these 2D layered MoS2 FETs has not yet been intensively explored for use in electronics. Therefore, we investigate carrier transport and the impact of the operating ambient conditions on back-gated multilayer MoS2 field-effect transistors with a thickness of ~50 nm at their realistic working temperatures and under different ambient conditions (in air and in a vacuum of ~10-5 Torr). Moreover, we examine the quality of the device interface and an energy distribution of carrier trapping/scattering sites through the low-frequency noise (LFN) measurement.A Schottky barrier of ~65 meV was formed between the Ti/Au metal contact and the MoS2 semiconductor and the barrier partially hindered the flow of carriers. However, when the operating temperature increased, the carriers easily overcame the barrier by thermionic emission at defects. This caused Imin to rise as shown in Fig. 1(a) and (b). However, at high VGS (blue region), the drain current decreased slightly as the operating temperatures increased. This is consistent with phonon scattering which is inversely proportional to the operating temperature.Fig. 2(a) shows that VTH shifts toward the negative direction as the temperature increases. This behavior is observed due to an increase in thermally generated carriers, resulting in a shift in the Fermi level in the semiconductor. Additionally, VTH was higher in air than in a vacuum. One possible explanation is that the oxygen and water molecules on the surface of MoS2 semiconductors could be adsorbed from the ambient air, and they could trap the charge carriers from the conduction band. This could make depleted channels and induce a positive threshold voltage shift. Also, the chemisorption on the MoS2 flake created defects and associated increased scattering which causes a reduction in μeff , unlike in a vacuum (Fig. 2(b)). However, thick multilayer MoS2 was less affected by the ambient conditions, because the thickness of the MoS2 layers suppressed interactions with the underlying channel. This fact was also supported negligible effects of ambient conditions on interface quality as determined by sub-threshold slope values (Fig. 2(a)).In Fig. 3 (a), obtained exponent values (γ) from low-frequency noise analysis, 1/fγ, exhibited the increase of γ at low gate bias range due to active slow traps. And the slow traps were filled and thus γ decreased and saturated to near 1 (0.95) as VG increased. In addition, the Hooge’s parameter, αH , was extremely large in the subthreshold region and drastically decreased as the gate bias increased until it matched Vth . This result also supports that the amount of active traps which cause the carrier number fluctuation decreases more significantly below Vth since the increased gate bias fills traps; it agrees with the change of the exponent value aforementioned.It is also interesting fact that the Hooge’s parameter (measured in ambient conditions) during fully ON regime is on the order of 10-2. This value is comparable to that observed in single-layer MoS2 FETs measured under vacuum conditions. This result provides multilayer structures more robust than single-layer for the effect of surface adsorbates in air and is consistent with our previous statement mentioned above.

Hexagonal boron nitride (h-BN) has emerged as one of the most promising candidates for two-dimensional (2D) materials due to its exciting optoelectrical properties and a broad range of applications. In this work, we explore the potential applications of h-BN nanosheets and nanoribbons as wide band gap semiconductors in terms of carrier mobility. Based on the first-principles calculations and deformation potential (DP) theory, the phonon-limited carrier mobility of monolayer h-BN and nanoribbons at room temperature is predicted. We find that the hole mobility of armchair-edge h-BN nanoribbons (ABNNRs) oscillates regularly with the ribbon width Nac in 1–3 nm. The ABNNRs in the Nac = 3p + 1 family have larger hole mobility with the highest value of 1.9 × 104 cm2 V–1 s–1 in the narrow nanoribbons. Molecular orbital analyses reveal that the large hole mobility originates from the delocalization of the occupied orbitals of valence electrons in the transport direction. By studying the effect of ribbon width on mobility, we identify the role of quantum confinement in tuning the transport properties of h-BN nanoribbons. The potential technological application of h-BN nanostructures as a P-channel material in wide band gap 2D field effect transistors (FETs) is discussed.

Two-dimensional (2D) materials have promising applications in optoelectronics, photonics, and quantum technologies. However, their intrinsically low light absorption limits their performance, and potential devices must be accurately engineered for optimal operation. Here, we apply a transfer matrix-based source-term method to optimize light absorption and emission in 2D materials and related devices in weak and strong coupling regimes. The implemented analytical model accurately accounts for experimental results reported for representative 2D materials such as graphene and MoS2. The model has been extended to propose structures to optimize light emission by exciton recombination in MoS2 single layers, light extraction from arbitrarily oriented dipole monolayers, and single-photon emission in 2D materials. Also, it has been successfully applied to retrieve exciton-cavity interaction parameters from MoS2 microcavity experiments. The present model appears as a powerful and versatile tool for the design of new optoelectronic devices based on 2D semiconductors such as quantum light sources and polariton lasers.

Light-Enhanced Blue Energy Generation Using MoS2 Nanopores