High phonon-limited mobility of BAs under pressure

Recent experiment reports that high thermal conductivity of ~1000 W/mK is observed in cubic boron arsenide crystal (BAs). In order to expand the scope of future applications, we use first-principles calculations to investigate the phonon-limited electronic transport in BAs family and modulation effect of pressure. In the case of electron doping, BAs, AlAs and AlSb exhibit the coupling between high frequency optical phonons and electron as well as the low frequency acoustical phonons in BSb. And BAs has the weakest electron-phonon coupling thus has a high N-type carrier mobility of 1740 cm^2/Vs. After the introduction of pressure, phonon spectra has more obvious change than the electronic structure. The phonon hardening under the pressure gives rise to the weakening of electron-phonon coupling. It is obtained that the pressure of 50 GPa can improve the mobility of BAs up to 4300 cm^2/Vs, which is much high and great significance to the current semiconductor industry.Comment: 7 page

The Doping effect of Chalcogen on the Two-Dimensional Ferromagnetic Material Chromium Tribromide

Recently the discovery of magnetic order in two-dimensional monolayer chromium trihalides opens the new research field in two-dimensional materials. We use first-principles calculations to systematically examine the doping effect of chalcogen on CrBr3. In the case of S-doping, four stable configurations, Cr2Br5S, Cr2Br4S2-A, Cr2Br4S2-B and Cr2Br3S3-A, are predicted to be ferromagnetic semiconductors. It is found that the new bands appearing in the original bandgap are made up of S-p and Cr-d-egorbits, lead to the obvious reduce of bandgap and the enhanced optical absorption in the visible range. Due to the decrease of valence electron after chalcogen doping, the magnetic moment also decreases with the increase of S atoms, and the character of ferromagnetic semiconductor is always hold in a wide range of strain. The results shown that monolayer CrBr3with chalcogen doping supply a effectual way to control the magnetism and extend the optoelectronic applications.Comment: 7 pages, 10 figure

Promising Ferroelectricity in 2D Group IV Tellurides: a First-Principles Study

Based on the first-principles calculations, we investigated the ferroelectric properties of two-dimensional (2D) Group-IV tellurides XTe (X=Si, Ge and Sn), with a focus on GeTe. 2D Group-IV tellurides energetically prefer an orthorhombic phase with a hinge-like structure and an in-plane spontaneous polarization. The intrinsic Curie temperature Tc of monolayer GeTe is as high as 570 K and can be raised quickly by applying a tensile strain. An out-of-plane electric field can effectively decrease the coercive field for the reversal of polarization, extending its potential for regulating the polarization switching kinetics. Moreover, for bilayer GeTe the ferroelectric phase is still the ground state. Combined with these advantages, 2D GeTe is a promising candidate material for practical integrated ferroelectric applications.Comment: 5 pages, 5 figure

The direct and indirect optical absorptions of cubic BAs and BSb

Recently, boron arsenide (BAs) has been measured high thermal conductivity in the experiments, great encouraging for the low-power photoelectric devices. Therefore, in the present work, we have systematically investigated the direct and indirect optical absorptions of BAs and BSb and the doping effect of congeners by using first-principles calculations. We obtain the absorption onset corresponding to the value of indirect bandgap by considering the phonon-assisted second-order optical absorptions. And the redshift of absorption onset, enhancement and smoothness of optical absorptions spectra are also captured in the temperature-dependent calculations. In order to introduce one-order absorptions into the visible range, the doping effect of congeners on optical absorptions is studied without the assists of phonon. It is found that the decrease of local direct bandgap after doping derives from either the small bandgap in the prototypical III-V semiconductors or CBM locating at R$_c$ point. Thus, doping of congeners can improve the direct optical absorptions in visible range.Comment: 6 pages, 5 figure

Effect of modulations of doping and strain on the electron transport in monolayer MoS_2

The doping and strain effects on the electron transport of monolayer MoS_2 are systematically investigated using the first-principles calculations with Boltzmann transport theory. We estimate the mobility has a maximum 275 cm^2/(Vs) in the low doping level under the strain-free condition. The applying a small strain (3%) can improve the maximum mobility to 1150 cm^2/(Vs) and the strain effect is more significant in the high doping level. We demonstrate that the electric resistance mainly due to the electron transition between K and Q valleys scattered by the M momentum phonons. However, the strain can effectively suppress this type of electron-phonon coupling by changing the energy difference between the K and Q valleys. This sensitivity of mobility to the external strain may direct the improving electron transport of MoS_2.Comment: publised versio

Strain-tunable magnetic order and electronic structure in 2D CrAsS4_4

The effect of strain on the magnetic order and band structure of single-layer CrAsS$_4$ has been investigated by first-principles calculations based on density functional theory. We found that single-layer CrAsS$_4$ was an antiferromagnetic (AFM) semiconductor, and would have a phase transition from AFM state to ferromagnetic (FM) state by applying a uniaxial tensile strain of 2.99\% along the y-direction or compressive strain of 1.76\% along the x-direction. The underlying physical mechanism of strain-dependent magnetic stability was further elucidated as the result of the competition between the direct exchange and indirect superexchange interactions. Moreover, band gap exhibit a abrupt change along with phase transition of magnetic order. Our study provides an intuitional approach to design strain-modulated spintronic devices.Comment: 7 pages, 6 figure

Robust intrinsic ferromagnetism in 2D half-metallic material MnAsS4_4

Two-dimensional (2D) intrinsic half-metallic materials are of great interest to explore the exciting physics and applications of nanoscale spintronic devices, but no such materials have been experimentally realized. Using first-principles calculations based on density-functional theory (DFT), we predicted that single-layer MnAsS$_4$ was a 2D intrinsic ferromagnetic (FM) half-metal. The half-metallic spin gap for single-layer MnAsS$_4$ is about 1.46 eV, and it has a large spin splitting of about 0.49 eV in the conduction band. Monte Carlo simulations predicted the Curie temperature (\emph{T}$_c$) was about 740 K. Moreover, Within the biaxial strain ranging from -5\% to 5\%, the FM half-metallic properties remain unchanged. Its ground-state with 100\% spin-polarization ratio at Fermi level may be a promising candidate material for 2D spintronic applications.Comment: 12 pages, 6 figure

Large thermoelectric power factor of high-mobility 1T'' phase of transition-metal dichalcogenides

The experimental studies about monolayer transition metal dichalcogenides in the recent year reveal this kind of compounds have many metastable phases with unique physical properties, not just 1H phases. Here, we focus on the 1T'' phase and systematically investigate the electronic structures and transport properties of MX2 (M=Mo, W; X=S, Se, Te) using the first-principles calculations with Boltzmann transport theory. And among them, only three molybdenum compounds has small direct bandgap at K point, which derive from the distortion of octahedral-coordination [MoS6]. For these three cases, hole carrier mobility of MoSe2 is estimated as 690 cm^2/Vs at room temperature, far more high than that of other two MoX2. For the reason, the combination of the modest carrier effective mass and weak electron-phonon coupling lead to the outstanding transport performance of MoSe2. The Seebeck coefficient of MoSe2 is also evaluated as high as 300 10^-6 V/K at room temperature. Due to the temperature dependent mobility of T^-1.9 and higher Seebeck coefficient at low temperature, it is found that MoSe2 has a large thermoelectric power factor around 6 10^-3 W/mK^2 in the low to intermediate temperature range. The present results suggests 1T'' MoSe2 maybe a excellent candidate for thermoelectric material.Comment: 9 pages, 6 figure

Robust large-gap topological insulator phase in transition-metal chalcogenide ZrTe4_4Se

Based on density functional theory (DFT), we investigate the electronic properties of bulk and single-layer ZrTe$_4$Se. The band structure of bulk ZrTe$_4$Se can produce a semimetal-to-topological insulator (TI) phase transition under uniaxial strain. The maximum global band gap is 0.189 eV at the 7\% tensile strain. Meanwhile, the Z$_2$ invariants (0; 110) demonstrate conclusively it is a weak topological insulator (WTI). The two Dirac cones for the (001) surface further confirm the nontrivial topological nature. The single-layer ZrTe$_4$Se is a quantum spin Hall (QSH) insulator with a band gap 86.4 meV and Z$_2$=1, the nontrivial metallic edge states further confirm the nontrivial topological nature. The maximum global band gap is 0.211 eV at the tensile strain 8\%. When the compressive strain is more than 1\%, the band structure of single-layer ZrTe$_4$Se produces a TI-to-semimetal transition. These theoretical analysis may provide a method for searching large band gap TIs and platform for topological nanoelectronic device applications.Comment: 13 pages,8 figure

Hexagonal MASnI3_3 exhibiting strong absorption of ultraviolet photons

MASnI$_3$, an organometallic halide, has great potential in the field of lead-free perovskite solar cells. Ultraviolet photons have been shown to generate deep trapping electronic defects in mesoporous TiO$_2$-based perovskite, affecting its performance and stability. In this study, the structure, electronic properties, and optical properties of the cubic, tetragonal, and hexagonal phases of MASnI$_3$ were studied using first-principles calculations. The results indicate that the hexagonal phase of MASnI$_3$ possesses a larger indirect band gap and larger carrier effective mass along the \emph{c}-axis compared with the cubic and tetragonal phases. These findings were attributed to the enhanced electronic coupling and localization in the hexagonal phase. Moreover, the hexagonal phase exhibited high absorption of ultraviolet photons and high transmission of visible photons, particularly along the \emph{c}-axis. These characteristics demonstrate the potential of hexagonal MASnI$_3$ for application in multijunction perovskite tandem solar cells or as coatings in mesoporous TiO$_2$-based perovskite solar cells to enhance ultraviolet stability and photon utilization.Comment: 7 pages, 4 figure