Anisotropic Effective Mass Research Articles

(Invited) Energy Efficient Transistors with 2D Materials

Emerging two-dimensional materials provide new opportunities to design post-Moore’s Law devices due to their atomically thin dimensions, layer dependent band gaps and effective masses, large density of states, and exotic band structures. Novel designs for energy efficient steep subthreshold swing transistors are investigated in 2D materials, highlighting the material and device properties that optimize the metrics needed to compete with CMOS. Based on atomistic non-equilibrium Green’s function simulations, the device characteristics of various 2D material tunnel field effect transistors (TFET) [1] are compared. Since chemical doping of 2D materials is still a challenge, we outline design considerations for generating electrostatic p-n junctions. Since low On-currents are a major challenge in TFETs, we show how On-currents and energy-delay products can be improved significantly in both chemically and electrically doped 2D TFETs. We also present a new design for an electrically doped 2D TFET, which uses a combination of high and low k dielectrics to achieve high On-currents [2]. We also highlight that the material properties of few-layer Phosphorene/Black Phosphorus make it an ideal candidate for low power TFETs. To replace CMOS transistors, TFETs need to achieve simultaneous supply voltage and channel length scaling. We show how the performance of a TFET degrades as the channel length is scaled. To solve this issue, effective masses and bandgaps need to be engineered according to some predicted conditions. We show that 2D materials such as Phosphorene help channel length scaling in TFETs compared to III-V TFETs. We also propose a novel design for a Phosphorene TFET using an L-shaped gate, which utilizes the large anisotropic effective mass of Phosphorene to improve On-currents and to suppress Off-state leakage [3]. Such a TFET also helps channel length scaling down to 2nm channels [4]. This research is supported by Semiconductor Research Corporation’s LEAST center. H. Ilatikhameneh et al, IEEE J-EDS Vol. 4, Issue 5, 260-265 (2016).H. Ilatikhameneh et al, IEEE EDL Vol. 36, No. 10, 1097-1100 (2015).T. A. Ameen et al, Scientific Reports 6, 28515 (2016).H. Ilatikhameneh et al, Scientific Reports 6, 31501 (2016). Figure 1 a) Design of an L-shaped gate in a Phosphorene TFET. b) The current-voltage characteristics of planar gates along armchair and zigzag directions compared to that of the L-shaped gate. Figure 1

Numerical investigation of triexciton stabilization in diamond with multiple valleys and bands

The existence of polyexcitons, the $N$-body complexes of excitons for $N > 2$ in 3D bulk systems, has been controversial for more than 40 years since its first theoretical suggestion. We investigated the stability of fundamental excitonic complexes in diamond numerically with the stochastic variational method (SVM) and an explicitly correlated Gaussian (ECG) basis. The electron-hole many-body system is described by an effective mass Hamiltonian. Our model includes the effective mass anisotropy and multiple valley and band degrees of freedom. We show that the excitons, trions, biexcitons, charged biexcitons, and triexcitons are stable in diamond. Numerical calculations reproduce from 81% to 86% of the experimentally reported binding energies for neutral bound states.

Anomalous anisotropy of critical currents in (Sr, K)Fe2As2 tapes

We measured critical currents and irreversibility fields of Ag-sheath (Sr, K)Fe2As2 tapes at various temperatures, magnetic fields and magnetic field directions. The sample tapes were made by a powder-in-tube with a hot-press process in the final heat treatments. The inverse anisotropy in Jc in comparison with the effective mass anisotropy can be found in the hot-pressed (Sr, K)Fe2As2 tapes. We found that the broad c-axis peak in angular dependence of Jc appears in a low field region and it decreases with increasing field and temperature. In addition, the crossover of Jc anisotropy appears around 10 T at 4.2 K and the crossover field decreases with increasing temperature. It is considered that these unusual anisotropic Jc behaviors may be originated by the strong random pinning center which is larger than the coherence length,

Ballistic Transport through a Strained Region on Monolayer Phosphorene**Supported by the National Natural Science Foundation of China under Grant No 11374002, the Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, and the Construct Program of the Key Discipline in Hunan Province.

We investigate quantum transport of carriers through a strained region on monolayer phosphorene theoretically. The electron tunneling is forbidden when the incident angle exceeds a critical value. The critical angles for electrons tunneling through a strain region for different strengths and directions of the strains are different. Owing to the anisotropic effective masses, the conductance shows a strong anisotropic behavior. By tuning the Fermi energy and strain, the channels can be transited from opaque to transparent, which provides us with an efficient way to control the transport of monolayer phosphorene-based microstructures.

The transparency of Cd(0001) films grown on Si(111)-7×7: Imaging the interface at atomic scale

The interfacial structure plays a crucial role in controlling the growth of epitaxial film. However, it is still a formidable task to observe the interface structure at the atomic scale, because the detected signals from the interface become very weak due to the strong attenuation and decoherence when the incident wave passes through the films. With the advent of scanning tunneling microscopy (STM), significant progress has been made in observing the interface structure of thin films. Many studies have demonstrated the typical Si(111)-7×7 superstructure below the metal films in the past. Although the 7×7 superstructures are visible, the 12 Si adatoms in the unit cells can not be resolved. Hence, imaging the 7×7 superstructures below thin films at the atomic scale has not been achieved. In this work, we have grown the Cd epitaxial films with high quality on Si(111)-7×7 substrate, and realized the imaging of interface structure of Cd(0001) at atomic scale by using low temperature scanning tunneling microscopy. Under low bias voltage, we can clearly observe the atomic image of Si(111)-7×7, indicating the Si (111)-7×7 substrate remains intact during the growth process of Cd films. Moreover, due to the quantum size effect, the Cd films with even or odd monolayers exhibit distinct lateral resolution as well as surface roughness, even at the same bias voltage. With the bias variation, such kind of diversities can be reversed. We attribute the highly transparency of Cd(0001) films to the anisotropic effective mass of the electrons in Cd films, i.e. the transverse motion is much faster than the in-plane motion.

Temperature and angular dependence of the upper critical field in K2Cr3As3

We report measurements of the upper critical field $H_{\mathrm{c2}}$ as functions of temperature $T$, polar angle $\theta$ (of the field direction with respect to the crystallographic $c$ axis), and azimuthal angle $\phi$ (of the field direction relative to the $a$ axis within the $ab$ plane) for the Cr-based superconductor K$_{2}$Cr$_{3}$As$_{3}$ with a quasi-one-dimensional and non-centrosymmetric crystal structure. We confirm that the anisotropy in $H_{\mathrm{c2}}(T)$ becomes inverse with decreasing temperature. At low temperatures, $H_{\mathrm{c2}}(\theta)$ data are featured by two maxima at $\theta$ = 0 ($\mathbf{H}\parallel c$) and $\pi/2$ ($\mathbf{H}\bot c$), which can be quantitatively understood only if uniaxial effective-mass anisotropy and absence of Pauli paramagnetic effect for $\mathbf{H}\bot c$ are taken simultaneously into consideration. The in-plane $H_{\mathrm{c2}}(\phi)$ profile shows a unique threefold modulation especially at low temperatures. Overall, the characteristic of the $H_{\mathrm{c2}}(\theta, \phi, T)$ data mostly resemble those of the heavy-fermion superconductor UPt$_3$, and we argue in favor of a dominant spin-triplet superconductivity with odd parity in K$_{2}$Cr$_{3}$As$_{3}$.

Giant magnetoresistance, three-dimensional Fermi surface and origin of resistivity plateau in YSb semimetal

Very strong magnetoresistance and a resistivity plateau impeding low temperature divergence due to insulating bulk are hallmarks of topological insulators and are also present in topological semimetals where the plateau is induced by magnetic field, when time-reversal symmetry (protecting surface states in topological insulators) is broken. Similar features were observed in a simple rock-salt-structure LaSb, leading to a suggestion of the possible non-trivial topology of 2D states in this compound. We show that its sister compound YSb is also characterized by giant magnetoresistance exceeding one thousand percent and low-temperature plateau of resistivity. We thus performed in-depth analysis of YSb Fermi surface by band calculations, magnetoresistance, and Shubnikov–de Haas effect measurements, which reveals only three-dimensional Fermi sheets. Kohler scaling applied to magnetoresistance data accounts very well for its low-temperature upturn behavior. The field-angle-dependent magnetoresistance demonstrates a 3D-scaling yielding effective mass anisotropy perfectly agreeing with electronic structure and quantum oscillations analysis, thus providing further support for 3D-Fermi surface scenario of magnetotransport, without necessity of invoking topologically non-trivial 2D states. We discuss data implying that analogous field-induced properties of LaSb can also be well understood in the framework of 3D multiband model.

Exchange effect and magneto–plasmon mode dispersion in an anisotropic two-dimensional electronic system**Project supported by the National Natural Science Foundation of China (Grant Nos. 61076092 and 61290303).

The exchange effect and the magneto–plasmon mode dispersion are studied theoretically for an anisotropic two-dimensional electronic system in the presence of a uniform perpendicular magnetic field. Employing an effective low-energy model with anisotropic effective masses, which is relevant for a monolayer of phosphorus, the exchange effect due to the electron–electron interaction is treated within the self-consistent Hartree–Fock approximation. The magneto–plasmon mode dispersion is obtained by solving a Bethe–Salpeter equation for the electron density–density correlation function within the ladder diagram approximation. It is found that the exchange effect is reduced in the anisotropic system in comparison with the isotropic one. The magneto–plasmon mode dispersion shows a clear dependence on the direction of the wave vector.

Anisotropy of Critical Current Density in LiFeAs Using Two-Band Ginzburg-Landau Theory

In this study, the temperature dependence of anisotropy parameter of critical current density for LiFeAs superconductor is calculated. The two-band Ginzburg-Landau theory, generalized for the anisotropic effective mass in different bands, is utilized. This paper reveals that in the absence of external magnetic field, two-band theory is able to give analytical expression for the temperature dependence of anisotropy parameter of critical current density. The results of microscopical simulations for Fermi surface of LiFeAs compound are taken into account during the calculations. It is shown that such a theory is in qualitative agreement with the experimental data.

First-principles hybrid functional study of the electronic structure and charge carrier mobility in perovskite CH3NH3SnI3**Project supported by the National Natural Science Foundation of China (Grant No. 51172067), the Hunan Provincial Natural Science Fund for Distinguished Young Scholars, China (Grant No. 13JJ1013), the Specialized Research Fund for the Doctoral Program of Higher Education, China (Grant No. 20130161110036), and the New Century

We calculate the electronic properties and carrier mobility of perovskite CH3NH3SnI3 as a solar cell absorber by using the hybrid functional method. The calculated result shows that the electron and hole mobilities have anisotropies with a large magnitude of 1.4 × 104 cm2·V−1·s−1 along the y direction. In view of the huge difference between hole and electron mobilities, the perovskite CH3NH3SnI3 can be considered as a p-type semiconductor. We also discover a relationship between the effective mass anisotropy and electronic occupation anisotropy. The above results can provide reliable guidance for its experimental applications in electronics and optoelectronics.

The tunable electronic structure and optic absorption properties of phosphorene by a normally applied electric field

We studied the electronic structure and optical absorption properties of phosphorene (a monolayer black phosphorus) under a normally applied electric field. The electric field enlarges the energy gap, weakens the effective mass anisotropy, and increases the effective mass component along the armchair direction (x-direction) for both conduction and valence bands but provides little change to the component along the zigzag direction (y-direction). The band edge optical absorption is completely polarized in the x-direction, and decreases when increasing the electric field. If the exciting frequency is beyond the energy gap, the absorption for the y-polarized light becomes nonzero, but the absorption is still highly polarized.

Saving Moore's Law Down To 1 nm Channels With Anisotropic Effective Mass.

Scaling transistors’ dimensions has been the thrust for the semiconductor industry in the last four decades. However, scaling channel lengths beyond 10 nm has become exceptionally challenging due to the direct tunneling between source and drain which degrades gate control, switching functionality, and worsens power dissipation. Fortunately, the emergence of novel classes of materials with exotic properties in recent times has opened up new avenues in device design. Here, we show that by using channel materials with an anisotropic effective mass, the channel can be scaled down to 1 nm and still provide an excellent switching performance in phosphorene nanoribbon MOSFETs. To solve power consumption challenge besides dimension scaling in conventional transistors, a novel tunnel transistor is proposed which takes advantage of anisotropic mass in both ON- and OFF-state of the operation. Full-band atomistic quantum transport simulations of phosphorene nanoribbon MOSFETs and TFETs based on the new design have been performed as a proof.

Suppressed Magnetic Circular Dichroism and Valley-Selective Magnetoabsorption due to the Effective Mass Anisotropy in Bismuth.

We measure the far-infrared reflectivity and Kerr angle spectra on a high-quality crystal of pure semimetallic bismuth as a function of magnetic field, from which we extract the conductivity for left- and right-handed circular polarizations. The high spectral resolution allows us to separate the intraband Landau level transitions for electrons and holes. The hole transition exhibits 100% magnetic circular dichroism; it appears only for one polarization as expected for a circular cyclotron orbit. However, the dichroism for electron transitions is reduced to only 13±1%, which is quantitatively explained by the large effective mass anisotropy of the electron pockets of the Fermi surface. This observation is a signature of the mismatch between the metric experienced by the photons and the electrons. It allows for a contactless measurement of the effective mass anisotropy and provides a direction towards valley polarized magnetooptical pumping with elliptically polarized light.

Rotational symmetry breaking in the topological superconductor SrxBi2Se3 probed by upper-critical field experiments.

Recently it was demonstrated that Sr intercalation provides a new route to induce superconductivity in the topological insulator Bi2Se3. Topological superconductors are predicted to be unconventional with an odd-parity pairing symmetry. An adequate probe to test for unconventional superconductivity is the upper critical field, Bc2. For a standard BCS layered superconductor Bc2 shows an anisotropy when the magnetic field is applied parallel and perpendicular to the layers, but is isotropic when the field is rotated in the plane of the layers. Here we report measurements of the upper critical field of superconducting SrxBi2Se3 crystals (Tc = 3.0 K). Surprisingly, field-angle dependent magnetotransport measurements reveal a large anisotropy of Bc2 when the magnet field is rotated in the basal plane. The large two-fold anisotropy, while six-fold is anticipated, cannot be explained with the Ginzburg-Landau anisotropic effective mass model or flux flow induced by the Lorentz force. The rotational symmetry breaking of Bc2 indicates unconventional superconductivity with odd-parity spin-triplet Cooper pairs (Δ4-pairing) recently proposed for rhombohedral topological superconductors, or might have a structural nature, such as self-organized stripe ordering of Sr atoms.

Effect of thermo-hydrodynamic instability on structure and characteristics of filtration combustion wave of solid fuel

Numerical modelling of flame front stability for the inverse wave (with trailing combustion front) of filtration combustion of solid fuel is performed. The problem is treated in terms of dimensionless variables and parameters. It is found that propagation of a plane combustion front becomes unstable under certain conditions. In this case the front spontaneously inclines. The thermo-hydrodynamic mechanism is supposed to be responsible for instability developing. Anisotropic effective mass diffusivity (dispersion) is also taken into account. It turns out that anisotropic diffusivity affects structure and conversion distribution of the inclined combustion front. It is shown that the key parameters determining stability of combustion wave are dimensionless gas flow rate and width of reactor. The range of these parameters corresponding to the stable plane front is determined. It is shown that stability occurs either for small reactor widths (dimensionless values <1), or low gas flow rate (below 0.2). The optimised values of considered dimensionless parameters for maximal productivity are determined.

A single-phase elastic hyperbolic metamaterial with anisotropic mass density.

Wave propagation can be manipulated at a deep subwavelength scale through the locally resonant metamaterial that possesses unusual effective material properties. Hyperlens due to metamaterial's anomalous anisotropy can lead to superior-resolution imaging. In this paper, a single-phase elastic metamaterial with strongly anisotropic effective mass density has been designed. The proposed metamaterial utilizes the independently adjustable locally resonant motions of the subwavelength-scale microstructures along the two principal directions. High anisotropy in the effective mass densities obtained by the numerical-based effective medium theory can be found and even have opposite signs. For practical applications, shunted piezoelectric elements are introduced into the microstructure to tailor the effective mass density in a broad frequency range. Finally, to validate the design, an elastic hyperlens made of the single-phase hyperbolic metamaterial is proposed with subwavelength longitudinal wave imaging illustrated numerically. The proposed single-phase hyperbolic metamaterial has many promising applications for high resolution damage imaging in nondestructive evaluation and structural health monitoring.

Dipolar matter-wave solitons in two-dimensional anisotropic discrete lattices

We numerically demonstrate two-dimensional (2D) matter-wave solitons in the disk-shaped dipolar Bose-Einstein condensates (BECs) trapped in strongly anisotropic optical lattices (OLs) in a disk's plane. The considered OLs are square lattices which can be formed by interfering two pairs of plane waves with different intensities. The hopping rates of the condensates between two adjacent lattices in the orthogonal directions are different, which gives rise to a linearly anisotropic system. We find that when the polarized orientation of the dipoles is parallel to disk's plane with the same direction, the combined effects of the linearly anisotropy and the nonlocal nonlinear anisotropy strongly influence the formations, as well as the dynamics of the lattice solitons. Particularly, the isotropy-pattern solitons (IPSs) are found when these combined effects reach a balance. Motion, collision and rotation of the IPSs are also studied in detail by means of systematic simulations. We further find that these IPSs can move freely in the 2D anisotropic discrete system, hence giving rise to an anisotropic effective mass. Four types of collisions between the IPSs are identified. By rotating an external magnetic field up to a critical angular velocity, the IPSs can still remain localized and play as a breather. Finally, the influences from the combined effects between the linear and the nonlocal nonlinear anisotropy with consideration of the contact/local nonlinearity are discussed too.

Nonorthogonal tight-binding parameterization of single-layer phosphorene under biaxial strain and application to FETs

This paper presents a new set of tight-binding (TB) parameters for single-layer phosphorene within the Naval Research Laboratory (NRL) scheme. For this, we develop the numerical algorithm to find the NRL TB parameters fitted to ab initio results. It is shown that the proposed NRL TB parameters successfully reproduce the band structure of a single-layer phosphorene, and even under biaxial or uniaxial strain, they appropriately describe the effects, such as modification of anisotropic effective masses and band gap. Via the top-of-the-barrier model, we also investigate the performance of single-layer phosphorene FETs under biaxial strain with the NRL TB Hamiltonian and find that the results are well in accordance with those of previous studies.

Anisotropic Ballistic Transport through a Potential Barrier on Monolayer Phosphorene**Supported by the National Natural Science Foundation of China under Grant No 11374002, the Hunan Provincial Natural Science Foundation of China under Grant No 13JJ2026, the Scientific Research Fund of Hunan Provincial Education Department under Grant No 12B010, the Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan

We demonstrate theoretically the anisotropic quantum transport of electrons through a single barrier on monolayer phosphorene. Using an effective k·p Hamiltonian, we find that the transmission probability for transport through n–n–n (or n–p–n) junction is an oscillating function of the incident angle, the barrier height, as well as the incident energy of electrons. The conductance in such systems depends sensitively on the transport direction due to the anisotropic effective mass. By tuning the Fermi energy and gate voltage, the channels can be transited from opaque to transparent, which provides us with an efficient way to control the transport of monolayer phosphorene-based microstructures.

Excitonic terahertz absorption in semiconductors with effective-mass anisotropies

A microscopic approach is developed to compute the excitonic properties and the corresponding terahertz response for semiconductors characterized by anisotropic effective masses. The approach is illustrated for the example of germanium where it is shown that the anisotropic electron mass in the L-valley leads to two distinct terahertz absorption resonances separated by 0.8 meV.