Conduction Band Effective Mass Research Articles

Electron transport properties in degenerate magnesium tin oxynitride (Mg1−xSn1+xN2−2yO2y) with average wurtzite structure

MgSnN2 with an average wurtzite structure (wz-MgSnN2) has recently emerged as a pseudo-III-nitride semiconductor, studied for applications in tandem solar cells, green light-emitting diodes, and other optoelectronic devices. This compound has only been researched recently, and, therefore, its charge-carrier transport properties are poorly understood. Understanding these properties is essential for optoelectronic applications. In this study, we grew wz-Mg1−xSn1+xN2 biaxially oriented polycrystalline films with x = −0.08 to 0.29 by reactive sputtering and investigated the charge-carrier transport properties using both direct current and optical techniques. We regarded the wz-Mg1−xSn1+xN2 films as magnesium tin oxynitride films (wz-MTNO) because a certain amount of oxygen was unintentionally incorporated into the sputtered wz-Mg1−xSn1+xN2 films. The wz-MTNO layers were n-type degenerate semiconductors with an electron density (ne) of the order of 1020 cm−3. In films with ne > 8 × 1020 cm−3, optically extracted resistivities (ρopt) obtained via a Drude-fit analysis of the infrared transmittance and reflectance spectra were almost identical to the direct-current resistivities (ρdc), indicating that the contribution of grain boundary scattering to the electron transport was negligible. However, the contribution of grain boundary scattering became unignorable with decreasing ne. The Drude-fit analysis also allowed the determination of the conduction-band effective mass (mc*) for the first time. A band edge mass of mc*/m0 ≈ 0.2 (m0 denotes the free-electron mass) was obtained in the wz-MTNO layers with |x| < 0.1. As x was increased from −0.18 to 0.29, mc*/m0 substantially increased from 0.18 to 0.56, indicating that the conduction-band dispersion decreased. That is, the conduction-band dispersion may be affected by the cation composition x. The findings of this study will provide important information to establish this material as a practical nitride semiconductor.

Electronic structure, effective masses and g-factors and strain induced crystal field splitting of WZ-InP by method

In this work, we study the electronic structure of wurtzite (WZ) InP, following a procedure developed earlier by us for WZ-GaN [(2018) J. Phys. Chem. Solids 112, 280] and WZ-ZnO [(2016) Semicond. Sci. Technol. 31, 035018], where the Hamiltonian was diagonalised by taking the conduction band and one valence band at a time. We consider the effects of other valence bands on a given valence band through second order perturbation theory of the Hamiltonian. Valence band dispersions are compared with previous calculations. We notice satisfactory agreement between our results and the previous one. Valence band effective masses are calculated from the valence band dispersions and conduction band effective masses were calculated from standard effective mass formulas. While valence band effective masses compare well with earlier calculations the transverse electronic effective mass is slightly higher compared to the experimental value and other reported values. However, when we use a different set of momentum matrix elements the transverse effective mass agrees well with the experimental value but the longitudinal value becomes slightly less than the experimental value. Thus the calculations are quite sensitive to the change in momentum matrix elements. Additionally, we calculate the effective masses as functions of the wave vector along and Our results for the g-factors are the first of its kind for WZ-InP. These results are quite sensitive to the change of sign of the spin–orbit parameters. The changes are not significant in case of electronic g-factors. But these are quite significant for the transverse valence band effective g-factors. Even change of sign is noticed. Spin–orbit mixing is found to be important and responsible for the observed anisotropy. Electronic g-factors agree well with reported experimental values. We also study the effect of strain on the crystal field splitting energy at the point. Trends obtained as functions of strain match with a previous calculation.

LO-Phonons and dielectric polarization effects on the electronic properties of doped GaN/InN spherical core/shell quantum dots in a nonparabolic band model

The electron energy spectrum of a core/shell spherical quantum dot made of zincblende GaN/InN compounds is investigated taking into account the presence of an off-center donor atom and the influence of band nonparabolicity. The interaction of both the charge carrier and the Coulombic core with longitudinal optical phonons is included through Fro hlich and Aldrich-Bajaj theories, respectively. The ground state energy is determined by solving the resulting conduction band effective mass equation via the variational Ritz principle. A detailed analysis of the features of electron and hole spectra as functions of the core and shell sizes is presented, highlighting the possibility of transitioning between type-I and type-II structures. A detailed discussion about the effects of conduction band nonparabolicity, dielectric mismatch and electron-phonon interaction onto the impurity binding energy is provided. It was found that, in general, nonparabolicity of the conduction band leads to larger impurity binding energy, and that LO-phonon and dielectric mismatch effects tend to reduce the value of the latter quantity.

Lattice strain influence on conduction band nonparabolicity in GaAs and InAs: Application to intraband optical absorption in InGaAs-GaAs asymmetric step quantum wells

We use empirical sps*d5 tight-binding calculations to determine the effects of compressive biaxial lattice strain, perpendicular to the [001] crystal direction, in zinc blende GaAs and InAs. Under that approach, we have been able to compute the behavior of quantities such as the average valence band energy, the energy band gap, the conduction band effective mass, and the spin-orbit split-off energy, as functions of the biaxial strain, within a range from 0 to −7%. Expressions governing these dependencies are reported for both materials. With such information at hand it is possible to calculate the variation of the coefficient of conduction band nonparabolicity due to the presence of strain. Also, the outcome for such quantities allows to evaluate the valence band offset in GaAs/InGaAs heterointerfaces as a consequence of the strain appearing from the difference between the lattice constants of the involved materials. Taking advantage of the above mentioned results, we have performed the calculation of confined conduction band states in step-like asymmetric quantum wells of the GaAs/Inx1Ga1-x1As/Inx2Ga1-x2As/GaAs prototype, using a k→⋅p→ formalism that solves the effective mass equation arising from a bi-cuadratic (nonparabolic) dispersion law. We report the calculation of the optical absorption coefficient related with intraband transitions that involve the ground and first excited energy levels. For that purpose, the study takes into account the variation of the layer widths and compositions.

Fermi surface complexity, effective mass, and conduction band alignment in n-type thermoelectric Mg3Sb2 – xBix from first principles calculations

Using first principles calculations, we study the conduction band alignment, effective mass, and Fermi surface complexity factor of n-type Mg3Sb2 – xBix (x = 0, 1, and 2) from the full ab initio band structure. We find that with an increase in the Bi content, the K and M band minima move away from the conduction band minimum CB1 while the singly-degenerate Г band minimum shifts rapidly downward and approaches the conduction band minimum. However, the favorable sixfold degenerate CB1 band minimum keeps dominating the conduction band minimum and there is no band crossing between the Г and CB1 band minima. In addition, we show that the connection of the CB1 carrier pockets with the energy level close to the band minimum M can strongly enhance the carrier pocket anisotropy and Fermi surface complexity factor, which is likely the electronic origin for the local maximum in the theoretical power factor. Our calculations also show that the density of states effective mass, Seebeck coefficient, and Fermi surface complexity factor decrease with an increase in the Bi content, which is unfavorable to the electrical transport. In contrast, reducing the conductivity effective mass with an increase in the Bi content is beneficial to the electrical transport by improving carrier mobility and weighted mobility as long as the detrimental bipolar effect is insignificant. As a result, in comparison with n-type Mg3Sb2, n-type Mg3SbBi shows higher power factors and a much lower optimal carrier concentration for the theoretical power factor at 300 K, which can be easily achieved by the experiment.

Dominant role of the shear strain induced admixture in spin-flip processes in self-assembled quantum dots

We study theoretically the spin-flip relaxation processes for a single electron in a self-assembled InAs/GaAs quantum dot, using an 8-band kp theory in the envelope function approximation. We show that the dominating channel of spin relaxation is spin admixture induced by symmetry-breaking shear strain, which can be mapped onto two effective spin-phonon terms in a conduction band (effective mass) Hamiltonian that have a similar structure and interfere constructively. Unlike the Dresselhaus coupling that dominates spin relaxation in larger, unstrained dots, the shear strain contribution cannot be modeled by a unique standard term in the Hamiltonian but rather relies on the actual strain distribution in the quantum dot.

Effect of electric field, dielectric screening and conduction band nonparabolicity of donor in a quantum dot

The effect of electric field, dielectric screening, conduction band nonparabolicity and effective mass mismatch of a hydrogenic donor in a GaAs/Ga[Formula: see text]Al[Formula: see text]As spherical quantum dot is investigated by assuming parabolic confinement using variational method. In the present work we obtain the increase of binding energy by decreasing the dot size for certain dot radii (50 Å) and the screening function gives uniformly larger values for smaller dot size. The effect of electric field and temperature decrease the donor binding energy whereas the conduction band nonparabolicity leads to increased binding energy.

Limited accuracy of conduction band effective mass equations for semiconductor quantum dots

Effective mass equations are the simplest models of carrier states in a semiconductor structures that reduce the complexity of a solid-state system to Schrödinger- or Pauli-like equations resempling those well known from quantum mechanics textbooks. Here we present a systematic derivation of a conduction-band effective mass equation for a self-assembled semiconductor quantum dot in a magnetic field from the 8-band k · p theory. The derivation allows us to classify various forms of the effective mass equations in terms of a hierarchy of approximations. We assess the accuracy of the approximations in calculating selected spectral and spin-related characteristics. We indicate the importance of preserving the off-diagonal terms of the valence band Hamiltonian and argue that an effective mass theory cannot reach satisfactory accuracy without self-consistently including non-parabolicity corrections and renormalization of k · p parameters. Quantitative comparison with the 8-band k · p results supports the phenomenological Roth-Lax-Zwerdling formula for the g-factor in a nanostructure.

Conduction-band effective mass and bandgap of ZnSnN2 earth-abundant solar absorber

Pseudo III-V nitride ZnSnN2 is an earth-abundant semiconductor with a high optical absorption coefficient in the solar spectrum. Its bandgap can be tuned by controlling the cation sublattice disorder. Thus, it is a potential candidate for photovoltaic absorber materials. However, its important basic properties such as the intrinsic bandgap and effective mass have not yet been quantitatively determined. This paper presents a detailed optical absorption analysis of disordered ZnSnN2 degenerately doped with oxygen (ZnSnN2−xOx) in the ultraviolet to infrared region to determine the conduction-band effective mass (mc*) and intrinsic bandgap (Eg). ZnSnN2−xOx epilayers are n-type degenerate semiconductors, which exhibit clear free-electron absorption in the infrared region. By analysing the free-electron absorption using the Drude model, mc* was determined to be (0.37 ± 0.05)m0 (m0 denotes the free electron mass). The fundamental absorption edge in the visible to ultraviolet region shows a blue shift with increasing electron density. The analysis of the blue shift in the framework of the Burstein-Moss effect gives the Eg value of 0.94 ± 0.02 eV. We believe that the findings of this study will provide important information to establish this material as a photovoltaic absorber.

Quasiparticle self-consistentGWelectronic band structure ofCd-IV-N2compounds

Quasiparticle self-consistent $GW$ calculations of the electronic band structures of ${\text{Cd-IV-N}}_{2}$ with group-IV elements, Si, Ge, and Sn, are reported. The lattice parameters in the $Pna{2}_{1}$ crystal structure are obtained both in the local density approximation and generalized gradient approximation (GGA) and provide respectively an underestimate and overestimate of the lattice constants for these until now not synthesized materials. The Wyckoff positions are obtained by structural minimization. At the GGA lattice constant, which is expected to be the most reliable, ${\mathrm{CdSiN}}_{2}$ is found to have an indirect gap $(U\ensuremath{-}\mathrm{\ensuremath{\Gamma}})$ of 2.72 eV, which is 0.55 eV lower than the direct gap at $\mathrm{\ensuremath{\Gamma}}$ of 3.27 eV. In ${\mathrm{CdGeN}}_{2}$, the indirect gap of 2.01 eV is only 0.1 eV smaller than the direct gap of 2.11 eV, and in ${\mathrm{CdSnN}}_{2}$ the gap is direct and equals 0.64 eV close to that of InN, as expected. The direct/indirect nature of ${\mathrm{CdGeN}}_{2}$ is found to be sensitive to shear strain. The conduction band effective masses decrease from ${\mathrm{CdSiN}}_{2}$ to ${\mathrm{CdGeN}}_{2}$ to ${\mathrm{CdSnSn}}_{2}$. The energies of formation indicate these materials to be stable with the possible exception of ${\mathrm{CdSnN}}_{2}$.

Realization of band gap shrinkage to the spectral characteristics of high-luminous-efficiency 658 nm AlGaInP/GaInP multiple quantum well lasers at room temperatures

The temperature dependent spectral shifts in 658 nm AlGaInP multiple quantum well (MQW) red laser diodes due to band gap narrowing at room temperatures (5 °C45 °C) is reported. The density of states effective mass approximation and the conduction band effective mass approximation are employed to formulate the carrier concentrations. The spectral shift mechanism is explored with a threshold current density of 42.28 kA/cm2 and a good characteristic temperature of 149 K. The photoluminescence (PL) peak intensity shifts towards the higher wavelength(red shift) and the full width at half maximum (FWHM) increases with the increase in temperature. The band gap narrowing value determined by a simple formula amounts to 67.4 meV and displays N1/3 dependence at higher densities. The carrier density dependence conveys that the red shift of the spectral emission is due to band gap narrowing.

Conduction band edge effective mass of La-doped BaSnO3

BaSnO3 has attracted attention as a promising material for applications requiring wide band gap, high electron mobility semiconductors, and moreover possesses the same perovskite crystal structure as many functional oxides. A key parameter for these applications and for the interpretation of its properties is the conduction band effective mass. We measure the plasma frequency of La-doped BaSnO3 thin films by glancing incidence, parallel-polarized resonant reflectivity. Using the known optical dielectric constant and measured electron density, the resonant frequency determines the band edge electron mass to be 0.19 ± 0.01. The results allow for testing band structure calculations and transport models.

Play the heavy: An effective mass study for α-Fe2O3 and corundum oxides.

Iron(iii) oxide (α-Fe2O3) is a known water splitting catalyst commonly used in photoelectrochemical cells. These cells are severely impaired by poor conductivity in α-Fe2O3, and resolving the conductivity issue is therefore crucial. One of the most intrinsic properties of matter, which governs conductivity, is the carrier effective masses. In this work, we investigate the carrier effective masses in α-Fe2O3 and other corundum oxides, including Al2O3, Cr2O3, Ga2O3, and In2O3 with different theoretical constructs: density functional theory (DFT), DFT+U, hybrid DFT, and G0W0. We find DFT sufficiently describes the carrier masses and a quasi-particle theory is only required for accuracies better than 30% for the conduction band effective mass. Additionally, we compare the density of states (DOS) and band effective mass approximations and conclude the DOS effective mass provides poor results whenever the band structure is anisotropic. We find that the charge carriers in Fe2O3 "play the heavy" since they have large effective masses that reduce conductivity and device efficiency. Finally, we conclude that the less heavy electron effective masses of other corundum oxides studied relative to Fe2O3 could contribute to efficiency improvements in Fe2O3 upon Al2O3, Ga2O3, and In2O3 coverage.

Electronic Structure and Photocatalytic Water-Splitting Properties of Ag2ZnSn(S1–xSex)4

The silver-based quaternary chalcogenides have potential applications in solar cell absorbers and photocatalytic water splitting to produce hydrogen. In the present work, the electronic structures and photocatalytic properties of Ag2ZnSn(S1–xSex)4 (x = 0–1) were investigated by the combination of GGA+U and hybrid functional methods. The results indicate that Ag2ZnSnS4 (x = 0) and Ag2ZnSnSe4 (x = 1) have a dispersive conduction band and small electron effective masses, which are beneficial to the photogenerated carrier separation. The hole effective masses are remarkably direction dependent, and that along the [100] and [010] directions are sensitive to strain, which indicates easy modulation of photocatalytic properties. In addition, the band gaps of the Ag2ZnSn(S1–xSex)4 solid solution can be tuned continuously by controlling the x component, which indicates the easy manipulation of light response range. The transfer abilities of charge carriers can also be improved with increasing Se incorporation. In a...

Characterization of carrier transport properties in strained crystalline Si wall-like structures in the quasi-quantum regime

We report the transport characteristics of both electrons and holes through narrow constricted crystalline Si “wall-like” long-channels that were surrounded by a thermally grown SiO2 layer. The strained buffering depth inside the Si region (due to Si/SiO2 interfacial lattice mismatch) is where scattering is seen to enhance some modes of the carrier-lattice interaction, while suppressing others, thereby changing the relative value of the effective masses of both electrons and holes, as compared to bulk Si. In the narrowest wall devices, a considerable increase in conductivity was observed as a result of higher carrier mobilities due to lateral constriction and strain. The strain effects, which include the reversal splitting of light- and heavy-hole bands as well as the decrease of conduction-band effective mass by reduced Si bandgap energy, are formulated in our microscopic model for explaining the experimentally observed enhancements in both conduction- and valence-band mobilities with reduced Si wall thickness. Also, the enhancements of the valence-band and conduction-band mobilities are found to be associated with different aspects of theoretical model.

Systematic approach for simultaneously correcting the band-gap andp−dseparation errors of common cation III-V or II-VI binaries in density functional theory calculations within a local density approximation

We propose a systematic approach that can empirically correct three major errors typically found in a density functional theory (DFT) calculation within the local density approximation (LDA) simultaneously for a set of common cation binary semiconductors, such as III-V compounds, (Ga or In)$X$ with $X=\mathrm{N},\mathrm{P},\mathrm{As},\mathrm{Sb},$ and II-VI compounds, (Zn or Cd)$X$, with $X=\mathrm{O},\mathrm{S},\mathrm{Se},\mathrm{Te}$. By correcting (1) the binary band gaps at high-symmetry points $\mathrm{\ensuremath{\Gamma}}, L, X$, (2) the separation of $p$-and $d$-orbital-derived valence bands, and (3) conduction band effective masses to experimental values and doing so simultaneously for common cation binaries, the resulting DFT-LDA-based quasi-first-principles method can be used to predict the electronic structure of complex materials involving multiple binaries with comparable accuracy but much less computational cost than a GW level theory. This approach provides an efficient way to evaluate the electronic structures and other material properties of complex systems, much needed for material discovery and design.

Doping-dependent charge dynamics inCuxBi2Se3

Superconducting CuxBi2Se3 has attracted significant attention as a candidate topological superconductor. Besides inducing superconductivity, the introduction of Cu atoms to this material has also been observed to produce a number of unusual features in DC transport and magnetic susceptibility measurements. To clarify the effect of Cu doping, we have performed a systematic optical spectroscopic study of the electronic structure of CuxBi2Se3 as a function of Cu doping. Our measurements reveal an increase in the conduction band effective mass, while both the free carrier density and lifetime remain relatively constant for Cu content greater than x=0.15. The increased mass naturally explains trends in the superfluid density and residual resistivity as well as hints at the complex nature of Cu doping in Bi2Se3.

Band gap engineering of In2O3 by alloying with Tl2O3

Efficient modulation of the bandgap of In2O3 will open up a route to improved electronic properties. We demonstrate using ab initio calculations that Tl incorporation into In2O3 reduces the band gap and confirm that narrowing of the gap is observed by X-ray photoemission spectroscopy on ceramic surfaces. Incorporation of Tl does not break the symmetry of the allowed optical transitions, meaning that the doped thin films should retain optical transparency in the visible region, in combination with a lowering of the conduction band effective mass. We propose that Tl-doping may be an efficient way to increase the dopability and carrier mobility of In2O3.

Rapid diffusion of electrons in GaMnAs

We report ultrafast transient-grating measurements, above and below the Curie temperature, of the dilute ferromagnetic semiconductor (Ga,Mn)As containing 6% Mn. At 80 K (15 K), we observe that photoexcited electrons in the conduction band have a lifetime of 8 ps (5 ps) and diffuse at about 70 cm2/s (60 cm2/s). Such rapid diffusion requires either an electronic mobility exceeding 7,700 cm2/Vs or a conduction-band effective mass less than half the GaAs value. Our data suggest that neither the scattering rate nor the effective mass of the (Ga,Mn)As conduction band differs significantly from that of GaAs.

Band gap bowing and band offsets in relaxed and strained Si1−xGex alloys by employing a new nonlinear interpolation scheme

We present nonlocal empirical pseudopotential calculations for SiGe alloys employing a novel nonlinear interpolation scheme. Our interpolation scheme is able to correctly model for the first time the band gap bowing observed in relaxed SiGe alloys. The valence-band-edge and conduction-band-edge energies in relaxed Si1−xGex for arbitrary x, which are difficult to obtain by experimental techniques, have been evaluated using pseudopotential calculations. We have also calculated the band energies of pseudomorphic [100]-strained Si1−xGex alloys grown over unstrained Si1−yGey substrates. The energy gaps, valence and conduction band offsets, effective masses, and strain induced splittings in pseudomorphic SiGe layers are calculated for the whole range of alloy compositions x and y.