Electron Effective Mass Research Articles

Investigation of size and barrier dependent efficiency in InAs quantum dot solar cells

Semiconductor Quantum Dots have unique properties, such as the quantum size effect that provide the breakthrough in optoelectronic devices. The small effective mass of electron and hole open-up an avenue of wide bandgap tunability in InAs Quantum Dots (QDs), making them an attractive material for solar cell application. Therein, 4x4 Luttinger Kohn Hamiltonian method is used to calculate the absorption co-efficient of epitaxially grown InAs QDs. Further, we studied the size-dependent absorption co-efficient and the barrier material model of InAs QDs capping with InP and GaAs. Our theoretical simulation predicts the efficiency of InAs/GaAs and InAs/InP system in the intrinsic layer of p-i-n structure solar cells.

Quantum Hall effect systems of electrons with anisotropic patterns

An almost ideal two-dimensional system of electrons can now be easily created in semiconductor heterojunctions. The quantum Hall effect state of the electrons is induced via the application of a strong perpendicular magnetic under specific quantum conditions. The most robust integer and/or fractional quantum Hall states already observed show the expected characteristic magnetoresistance for such systems. However, anisotropic patterns and features in transport properties have been seen for a few other peculiar cases. The origin of such anisotropic patterns may have various mechanisms or may also be due the specific details of the system and material such as the isotropic or anisotropic nature of the effective mass of electrons, the nature of the host substrate parameters, the nature of the interaction potentials, as well as other subtler effects. The interplay between all these factors can lead to many outcomes. In this work we consider small quantum Hall states of electrons at filling factor 1/6 and study the appearance of such anisotropic patterns as a result of some form of innate interaction anisotropy in the system.

Spin-polarized transport properties in diluted-magnetic-semiconductor/semiconductor superlattices under light-field assisted

Based on the single electron effective mass approximation theory and the transfer-matrix method, the spin polarized transport properties of electrons in a diluted-magnetic-semiconductor/semiconductor superlattice are studied. The influence of a light-field and a magnetic-field on spin polarized transport and the tunneling time in the superlattice structure are discussed in more detail. The results show that, due to the sp-d electron interaction between conduction band electrons and doped Mn ions, giant Zeeman splitting occurs. It is shown that a significant spin-dependent transmission and the position and width of the resonant-transmission-band of spin-dependent electron can be manipulated by adjusting the magnetic- and light-field. Considering the light field irradiation, the resonance band of electron is deformed and broadened with the increase of the light field intensity. For the case of a strong magnetic field, the transmission coefficient (TC) in the low-energy region is almost zero when the light field is not added, but with the increase of light intensity, the TC increased significantly in the zone increases significantly, that is, a quasi-bound band appears. These features are due to the energy exchange between electrons and the light field when electrons tunnel through the superlattice structure under light irradiation. In addition, light and magnetic fields can significantly change the spin polarization of electrons. Under a certain magnetic field intensity (<i>B</i> = 2 T), the light field significantly changes the spin polarization of electrons, the main effect is that the width of the spin polarization platform narrows and oscillatory peaks are accompanied on both sides of the platform. This effect is strengthened with the increase of the light field intensity. However, when the magnetic field is stronger (<i>B</i> = 5 T), the opposite is true. These show that the spin polarization can be modulated by the light field. Finally, the tunneling time of spin-up and spin-down electrons is studied by the evolution of Gaussian wave packets in the structure. The results show that the tunneling time depends on a spin of electrons, and it can be seen that the tunneling time of the spin-down electron is shorter than that of the spin-up electron in the superlattice structure. These remarkable properties of spin polarized transport may be beneficial for the devising tunable spin filtering devices based on diluted magnetic semiconductor/semiconductor superlattice structure.

Fishbone resonance structure in the attosecond transient absorption spectrum of graphene

We investigate the attosecond transient absorption spectroscopy (ATAS) of graphene by numerically solving four-band density-matrix equations, which demonstrates apparent fish bone resonance structures. To gain insight into these interesting structures, we exploit a simplified model that only considers the electrons of $\Gamma$ and M points in the Brillouin zone. With the help of this model, we can analytically express the ATAS spectrum as the sum of zeroth- and first-order Bessel functions in the variables of the strength and frequency of the infrared pump field as well as the effective mass of electrons at the $\Gamma$ and M points. Lorentzian and Fano line shapes in the absorption spectrum are addressed. The fish bone structure consists of periodic V-shaped structure that can be explained by first-order Bessel functions and its tilt angle is solely determined by the frequency of the pump laser. The periodicity of the V-shaped structure in the fish bone originates from the periodic dependence of the Lorentzian and Fano line shapes of the absorption spectrum on the time delay between the pump and probe lasers. Compared with the numerical results, our analytical theory can qualitatively or even quantitatively predict the zeroth- and first-order fringes in the fish bone structures of the ATAS spectrum. The gauge issues in the numerical simulations are also discussed.

Janus V2AsP monolayer : a ferromagnetic semiconductor with a narrow band gap, a high Curie temperature and controllable magnetic anisotropy

The search and design of two-dimensional (2D) magnetic semiconductors for spintronics applications are particularly significant. In this work, we investigated the electronic and magnetic properties of Janus structure based on Dirac half-metallic vanadium phosphide (VP) monolayer (ML) by first-principles calculations. Due to the vertical symmetry breaking, Janus V2AsP ML becomes an intrinsic ferromagnetic semiconductor with a narrow band gap of 0.21 eV. We analyzed the electronic structure and origin of the in-plane easy axis in Janus V2AsP. The electron effective mass is anisotropic and only 0.129 m0 along the x-direction. The Curie temperature and magnetic anisotropy energy (MAE) of Janus V2AsP reach 490 K and 178 µeV per V atom, respectively. A uniaxial tensile stain ɛ x of 5% can increase its band gap and MAE to 0.39 eV and 210.6 µeV per V atom while maintaining its being above room temperature. Moreover, the direction of the easy axis can be changed between the in-plane x- and y-direction by a small uniaxial tensile strain ɛ x of 2%. Our study can motivate further research on the design the magnetic semiconductors in Janus structures based on 2D Dirac half-metals for spintronics applications.

Dispersion-correction density functional theory (DFT+D) and spin-orbit coupling (SOC) method into the structural, electronic, optical and mechanical properties of CH3NH3PbI3

DFT simulations are used to determine the most appropriate approach to reproduce the experimental electronic structure and optical properties besides providing the reliable structural stability of CH3NH3PbI3. In this work, DFT calculations are performed using the generalized gradient approximation (GGA) that includes spin-orbit coupling (SOC) and empirical pairwise dispersion of the DFT + D method to investigate the structural, electronic, optical and mechanical properties of the material. Our results reveal that SOC effects reduced the band gap compared to GGA functional alone. Meanwhile, using the DFT + D method, an improvement in the calculation accuracy of the band gap obtained (1.689eV) is in excellent agreement with the experimental (1.630eV). Further analysis of the electronic properties demonstrates that including SOC reduces the effective masses of electrons due to the creation of splitting at the bottom of the conduction band. We have presented the absorption coefficient to describe the optical properties. It is found that CH3NH3PbI3 exhibit stronger optical absorption in the UV light region (300–400 nm). The mechanical properties of Young's modulus, bulk modulus, shear modulus and Poisson's ratio were calculated using DFT + D. It was discovered that the ratio (B/G) achieved was greater than 1.75, indicating that CH3NH3PbI3 is a ductile material.

Electron effective masses of ScxAl1−xN and AlxGa1−xN from first-principles calculations of unfolded band structure

The electron effective masses of ScxAl1−xN and AlxGa1−xN, two of the most promising wide bandgap materials for power and RF electronic applications, have been calculated using the predictions of the density functional theory (DFT). More specifically, the unfolding technique has been adopted to extract the effective band structure of the two alloys under investigation. It has been found that the AlGaN effective masses m∗ approximately follow the Vegard law. On the contrary, due to the larger amount of disorder inside the crystal, the ScAlN shows a non-monotonic change of m∗ as a function of the Sc concentration, which requires the DFT calculations to be consistently performed for an accurate prediction. The ScAlN effective masses as a function of Sc content have been reported in the range 0≤x≤0.25 for the first time.

Influence of biaxial strain on the electronic and thermoelectric properties of Sb2Te3 monolayer

We investigate the influence of biaxial strain on the electronic and thermoelectric properties of Sb2Te3 monolayer by first-principles calculation and semiclassical Boltzmann transport theory. The calculated results of formation energy and phonon spectrum suggest there is a limited strain for Sb2Te3 monolayer. The effective mass of hole increases with the increase of tensile strain, but decreases with the increase of compressive strain. However, the change trend of electron effective mass with strain is opposite to that of hole effective mass. The results of energy band structures show that the Sb2Te3 monolayer is an indirect energy gap semiconductor. Three conduction band valleys almost degenerate together at 5% compressive strain. At this strain, Seebeck coefficient and power factor are improved for n-type doped Sb2Te3 monolayer, suggesting that strain engineering is an effective way to improve the thermoelectric properties of Sb2Te3 monolayer.

Effects of B and In on the band structure of BGa(In)As alloys

Highly mismatched semiconductor alloys (HMAs) offer unusual combinations of bandgap and lattice constant, which are attractive for myriad applications. Dilute borides, such as BGa(In)As, are typically assumed to be HMAs. BGa(In)As can be grown in higher alloy compositions than Ga(In)NAs with comparable bandgaps, potentially enabling routes to lattice-matched telecom lasers on Si or GaAs. However, BGa(In)As remains relatively unexplored, especially with large fractions of indium. Density functional theory with HSE06 hybrid functionals was employed to study BGaInAs with 4%–44% In and 0%–11% B, including atomic rearrangement effects. All compositions showed a direct bandgap, and the character of the lowest conduction band was nearly unperturbed with the addition of B. Surprisingly, although the bandgap remained almost constant and the lattice constant followed Vegard's law with the addition of boron, the electron effective mass increased. The increase in electron effective mass was higher than in conventional alloys, though smaller than those characteristics of HMAs. This illustrates a particularly striking finding, specifically that the compositional space of BGa(In)As appears to span conventional alloy and HMA behavior, so it is not well-described by either limit. For example, adding B to GaAs introduces additional states within the conduction band, but further addition of In removes them, regardless of the atomic arrangement.

Wigner molecules in phosphorene quantum dots

We study Wigner crystallization of electron systems in phosphorene quantum dots with confinement of an electrostatic origin with both circular and elongated geometry. The anisotropy of the effective mass allows for the formation of Wigner molecules in the laboratory frame with a confined charge density that has lower symmetry than the confinement potential. We find that in circular quantum dots separate single-electron islands are formed for two and four confined electrons but not for three trapped carriers. The spectral signatures of the Wigner crystallization to be resolved by transport spectroscopy are discussed. Systems with Wigner molecule states are characterized by a nearly degenerate ground state at $B=0$ and are easily spin-polarized by the external magnetic field. In electron systems for which the single-electron islands are not formed, a more even distribution of excited states at $B=0$ is observed, and the confined system undergoes ground state symmetry transitions at magnetic fields of the order of 1 Tesla. The system of five electrons in a circular quantum dot is indicated as a special case with two charge configurations that appear in the ground-state as the magnetic field is changed: one with the single electron islands formed in the laboratory frame and the other where only the pair-correlation function in the inner coordinates of the system has a molecular form as for three electrons. The formation of Wigner molecules of quasi-1D form is easier for the orientation of elongated quantum dots along the zigzag direction with heavier electron mass. The smaller electron effective mass along the armchair direction allows for freezing out the transverse degree of freedom in the electron motion.

Transient THz Emission and Effective Mass Determination in Highly Resistive GaAs Crystals Excited by Femtosecond Optical Pulses

We present comprehensive studies on the emission of broadband, free-space THz transients from several highly resistive GaAs samples excited by femtosecond optical pulses. Our test samples are characterized by different degrees of disorder, ranging from nitrogen-implanted to semi-insulating and annealed semi-insulating GaAs crystals. In our samples, we clearly observed transient THz emissions due to the optical rectification effect, as well as due to the presence of the surface depletion electrical field. Next, we arranged our experimental setup in such way that we could observe directly how the amplitude of surface-emitted THz optical pulses is affected by an applied, in-plane magnetic field. We ascribe this effect to the Lorentz force that additionally accelerates optically excited carriers. The magnetic-field factor η is a linear function of the applied magnetic field and is the largest for an annealed GaAs sample, while it is the lowest for an N-implanted GaAs annealed at the lowest (300 °C) temperature. The latter is directly related to the longest and shortest trapping times, respectively, measured using a femtosecond optical pump-probe spectroscopy technique. The linear dependence of the factor η on the trapping time enabled us to establish that, for all samples, regardless of their crystalline structure, the electron effective mass was equal to 0.059 of the electron mass m0, i.e., it was only about 6% smaller than the generally accepted 0.063m0 value for GaAs with a perfect crystalline structure.

Formula omitted] electronic structure and strain-dependence of the crystal field splitting and inter-band transition energy of W–InN

In this paper, we study the electronic structure of wurtzite Indium Nitride (W–InN) using a k→∙π→ theory. Here π→ is the relativistic momentum operator. It is based on an eight-band model; three valence bands and one conduction band, each Kramers’ split by spin-orbit interaction. We diagonalize the k→∙π→ Hamiltonian following a two-band model, where each valence band is treated separately with the conduction band by using a suitable basis set in the presence of spin-orbit interaction. Effects of other valence bands on the dispersion of a given valence band are incorporated using second order perturbation theory. Valence band dispersions are plotted and compared with a previous calculation and our own results obtained by using a different method. Agreement is found to be satisfactory. Electronic and hole effective masses are calculated and compared with previously reported values, which are substantially different from each other. Fair agreement is noticed. Discrepancies are discussed. Effect of strain is calculated on the electronic structure by calculating the crystal field energy and transition energy for small variations of strain. Although results could not be compared with any previous calculation, due presumably to their absence in literature, trends and systematics are found to be in accord with the same found in case of other wurtzite semiconductors.

Optical properties of InI3: Density functional theory calculations and experimental results

The electronic and optical properties of InI3 have been investigated by both experimental and theoretical approaches. The First Principle electronic structure and optical properties were calculated by Density Functional Theory (DFT) formalism using Full Potential Linearized Augmented Plane Wave plus local orbitals (FP-LAPW + lo) implementing different appropriate exchange-correlation functionals in WIEN2k code. The calculations were also carried out implementing other functionals and only TB-mBJ results tally the closest with experimental results. The optical measurements of a well characterized InI3 films indicate the band gap as of direct type and residual strain dependant. The stress free intrinsic band gap Eg = 2.8 eV (average of Eg⊥c and EgIIc) has been determined from the residual strain dependence of the band gap in closest agreement with theoretically calculated value of 2.9 eV. Both theory and experiment show that the band gap is anisotropic along and perpendicular to c-axis. The electron effective mass me*=0.79m0 and the hole effective mass mh*=0.85m0 have been determined from the band structure. The experimental and theoretical optical properties match reasonably well.

Trends in opto-electronic properties of MgxZn1-xSnN2 using first principles methods

We have computationally studied MgxZn1-xSnN2 alloy system in wurtzite structure using density functional theory and beyond. Cationic disorder of the random limit is chosen for modeling the crystal structure. Lattice mismatch of less than 1 % in the alloy system helps in the miscibility of Mg and Zn, thus making it widely useful in tandem solar cells. Electronic band gaps increase linearly with higher Mg concentration. These materials have hole effective masses significantly lower than electron effective masses. Considerable absorption together with a lower reflectivity can render this alloy an effective absorber. All the alloys are mechanically stable, but (x=0.25) and (x=0.50) are dynamically unstable. These alloys also exhibit large hardness.

Magnetoplasmonics beyond Metals: Ultrahigh Sensing Performance in Transparent Conductive Oxide Nanocrystals.

Active modulation of the plasmonic response is at the forefront of today's research in nano-optics. For a fast and reversible modulation, external magnetic fields are among the most promising approaches. However, fundamental limitations of metals hamper the applicability of magnetoplasmonics in real-life active devices. While improved magnetic modulation is achievable using ferromagnetic or ferromagnetic-noble metal hybrid nanostructures, these suffer from severely broadened plasmonic response, ultimately decreasing their performance. Here we propose a paradigm shift in the choice of materials, demonstrating for the first time the outstanding magnetoplasmonic performance of transparent conductive oxide nanocrystals with plasmon resonance in the near-infrared. We report the highest magneto-optical response for a nonmagnetic plasmonic material employing F- and In-codoped CdO nanocrystals, due to the low carrier effective mass and the reduced plasmon line width. The performance of state-of-the-art ferromagnetic nanostructures in magnetoplasmonic refractometric sensing experiments are exceeded, challenging current best-in-class localized plasmon-based approaches.

Evaluation of Effective Mass in InGaAsN/GaAs Quantum Wells Using Transient Spectroscopy.

Transient spectroscopies are sensitive to charge carriers released from trapping centres in semiconducting devices. Even though these spectroscopies are mostly applied to reveal defects causing states that are localised in the energy gap, these methods also sense-charge from quantum wells in heterostructures. However, proper evaluation of material response to external stimuli requires knowledge of material properties such as electron effective mass in complex structures. Here we propose a method for precise evaluation of effective mass in quantum well heterostructures. The infinite well model is successfully applied to the InGaAsN/GaAs quantum well structure and used to evaluate electron effective mass in the conduction and valence bands. The effective mass m/m0 of charges from the conduction band was 0.093 ± 0.006, while the charges from the valence band exhibited an effective mass of 0.122 ± 0.018.

First-principles calculations of phosphorus-doped SnO2 transparent conducting oxide: Structural, electronic, and electrical properties

The structural and electronic properties of phosphorus-doped tin oxide (PTO) were investigated by density functional theory (DFT). The lattice parameters computed with the Perdew-Burke-Ernzerhof (PBE) functional were decreased as phosphorus (P) impurities were substituted for Sn cations. The band structure of PTO computed with the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional showed an optical energy bandgap widening effect, because of a large Moss-Burstein shift and a small exchange–correlation-induced bandgap narrowing. Also, the P impurities in SnO2 induced shallow donor P-3s states in the conduction band minimum near Fermi level. The electron effective mass of the systems was calculated to be 0.25 m0. DFT calculations also predicted a 5.9 × 1020 cm−3 electron density for PTO, which could increase the electrical conductivity of SnO2. These features make PTO a promising material for transparent conducting applications.

Determination of the Strain Influence on the InAs/InAsSb Type-II Superlattice Effective Masses.

A3B5 materials used for the superlattice (SL) fabrication have properties that enable the design of devices optimized for infrared (IR) detection. These devices are used in the military, industry, medicine and in other areas of science and technology. The paper presents the theoretical assessment and analysis of the InAs/InAs1-xSbx type-II superlattice (T2SL) (grown on GaSb buffer layer) strain impact on the bandgap energy and on the effective masses of electrons and holes at 150 K. The theoretical research was carried out with the use of the commercial program SimuApsys (Crosslight). The k·p method was adopted in T2SL modeling. Luttinger coefficients (γ1, γ2 and γ3) were assessed assuming the Kane coefficient F = 0. The bandgap energy of ternary materials (InAsxSb1-x) was determined assuming that the bowing parameter (bg) for the above-mentioned temperature is bg = 750 meV. The cutoff wavelength values were estimated based on the theoretically determined absorption coefficients (from approximation the quadratic absorption coefficient). The bandgap energy was calculated according to the following formula: Eg = 1.24/λcutoff. The theoretical simulations allowed us to conclude that the strain in T2SL causes the Eg shift, which also has an impact on the effective masses me and mh, playing an important role for the device's optical and electrical performance. The T2SLs-simulated results at 150 K are comparable to those measured experimentally.

Current–voltage characteristics of a Co/Ni bilayer nanofilm

Widespread interest in optical and electrical properties of nanostructured systems began more than 20 years ago. Here, optical and electrical properties of Co/Ni bilayer nanofilms are explored. These optical and electrical properties were described in terms of a common confined wavefunction spanning both Co and Ni layers. The effective relative electron mass parameter f estimated for this bilayer film was about 0.1489. Temperature dependence of the current–voltage characteristics of the Co/Ni bilayer nanofilms was described as that of a Schottky diode with the Schottky barrier height of US = 24 cm−1, very small compared to other semiconducting materials. Therefore, Co/Ni bilayer nanofilms apparently had semiconducting properties, which may approximately be described using Schottky diode theory. Such semiconducting properties of bilayer metal nanofilms could be used in special low-temperature low-consumption electronic devices.

Strain effect on the ground-state structure of Sr2SnO4 Ruddlesden-Popper oxides

Ruddlesden-Popper (RP) oxides (${A}_{n+1}{B}_{n}{\mathrm{O}}_{3n+1}$) comprised of perovskite ${(AB{\mathrm{O}}_{3})}_{n}$ slabs can host a wider variety of structural distortions than their perovskite counterparts. This makes an accurate structural determination of RP oxides more challenging. In this study, we investigate the structural phase diagram of $n=1$ RP ${\mathrm{Sr}}_{2}{\mathrm{SnO}}_{4}$, one of alkaline earth stannates that are promising for opto-electronic applications by using group theory based symmetry analysis and first-principles calculations. We explore the symmetry breaking effects of different dynamical instabilities, predict the energies of phases they lead to, and take into account different (biaxial strain and hydrostatic pressure) boundary conditions. We also address the effect of structural changes on the electronic structure and find that compressive biaxial strain drives ${\mathrm{Sr}}_{2}{\mathrm{SnO}}_{4}$ into a regime with wider band gap and lower electron effective mass.