Mass Of Valence Band Research Articles

One‐Particle and Excitonic Band Structure in Cubic Boron Arsenide

Cubic BAs has received recent attention for its large electron and hole mobilities and large thermal conductivity. This is a rare and much desired combination in semiconductor industry: commercial semiconductors typically have high electron mobilities, or hole mobilities, or large thermal conductivities, but not all of them together. Herein, predictions from an advanced self‐consistent many‐body perturbative theory are reported and it is shown that with respect to one‐particle properties, BAs is strikingly similar to Si. There are some important differences, notably there is an unusually small variation in the valence band masses. With respect to two‐particle properties, significant differences with Si appear. The excitonic spectrum for both q = 0 and finite q is reported, and it is shown that while the direct gap in cubic BAs is about 4 eV, dark excitons can be observed down to about ≈1.5 eV, which may play a crucial role in application of BAs in optoelectronics.

Important role of Cu in suppressing bipolar conduction in Bi-rich (Bi,Sb)2Te3

Cu doping has been known to enhance thermoelectric performance of p-type (Bi,Sb)2Te3 alloys by suppressing bipolar conduction at higher temperatures. However, detailed mechanisms are not provided yet. Herein, we investigate the unchallenged role of Cu doping in (Bi,Sb)2Te3 alloys by means of Bi-rich compositions of Bi0.6Sb1.4Te3 and Bi0.7Sb1.3Te3, which were chosen since they exhibit much inferior thermoelectric performance owing to severe bipolar conduction. It was found that Cu doping increases non-degenerate mobility and density-of-states effective mass of valence band as well as hole concentration. Small amount of Cu doping enhanced maximum thermoelectric figure-of-merit of Bi0.6Sb1.4Te3 composition from 0.25 to 0.98.

Structural and Optical Characteristics of Nanoscale Semiconductor Lasers for Telecommunication and Biomedical Applications: A Review

This paper presents the structural and optical characteristics of nanoscale semiconductor lasers for telecommunication and biomedical applications. Improved fabrication techniques, new materials and nano-scale heterostructures have led to improvement in the device performance. The material selection and their bandgap have an important role in the heterostructure to generate a lasing wavelength for particular applications. The bandgap modification can be done by the use of alloy semiconductor, quantum well structure, and strain layer epitaxy without changing the material itself. Semiconductor multilayers on the substrate are generally grown by using the metal-organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) process. Many researchers have provided different designs of heterostructures for the lasers. Generally, lasers are manufactured by using different semiconductor layers grown on GaAs, InP or GaSb substrate at the nanoscale. But controlling the thickness of the layer grown on the substrate at the nanoscale is the major problem in the fabrication. In a study, it has been found that for the proper functioning of semiconductor lasers it is beneficial to have light conduction and valence band masses. By using band structure engineering theory of quantum confinement and incorporation of strain on the active layer, this problem can be resolved. Red lasers are currently used in biomedical applications for treatment of superficial skin diseases like psoriasis, vitiligo etc. The manufacturing of red laser was earlier done by using nitrides material but they are harmful to skin and are expensive as well. An alternative for designing of red laser is manufacturing red lasers by using phosphides. The red laser is manufactured by using GaInP and AlGaInP ternary and quaternary compounds are widely used in the biomedical industry currently. This paper is the outcomes of the papers presented by many researchers in the field of optoelectronics.

Boltzmann transport calculation of thermoelectric properties in Ag2Se1−xTex (x = 0.0 and 0.5)

We calculated the thermoelectric properties of the Ag-based chalcogenides Ag2Se and Ag2Se0.5Te0.5 (AST) at room temperature using the Boltzmann transport equations within the density functional theory. We confirmed that the power factor S2σ and electronic figure-of-merit ZTe of the Ag2Se0.5Te0.5 compound can be enhanced by hole doping as compared with Ag2Se due to enhancement of the Seebeck coefficient and electrical conductivity. The localized electrons introduced by Te doping give rise to heavy valence bands and a small band gap near the Fermi energy. We also investigated the thermoelectric properties of the compounds in terms of the two-band model within the assumption of arbitrary isotropic band dispersions. We suggest that the hole doping in the AST compound can produce a good candidate for p-type thermoelectric material by controlling the effective mass of valence band and small band gap opening. Using large atomic size elements for chemical potential tuning and isostructural substitution in Ag2Se related chalcogenides can be a good route to increase the thermoelectric performance.

Quantum size effects on exciton states in indirect-gap quantum dots

We investigate exciton ground states in Si and 3C-SiC quantum dots by using the effective mass theory, taking account of the conduction- and valence-band mass anisotropy as well as the small spin-orbit splitting energy. The degenerate hole and exciton states are partly split by the mass anisotropy. The anisotropy splitting energies in quantum dots are different dramatically from their bulk value due to quantum size effects. The assumed changeable spin-orbit splitting energy may change the ordering of the anisotropy-split energy levels. Taking account of the exchange interaction, the degeneracy of the exciton states is further lifted. Due to the anisotropy and exchange splitting, the 48-fold exciton ground state will be split into two 18-fold triplets and two 6-fold singlets. The lowest three states are optically forbidden for Si quantum dots, which leads to a Stokes shift of luminescence. The theroretical shift agrees well with the experimental data. Furthermore, the exciton band gap and binding energy as a function of dot radius are presented both for Si and for 3C-SiC quantum dots. The band gap of Si quantum dots agrees well with the recent photoluminescence results of size-separated quantum dots by Ledoux et al. and absorption data of Furukawa et al.

A tight-binding study of acceptor levels in semiconductors

Acceptor binding energies in zinc-blende semiconductors are determined within the tight-binding formalism. The importance of fitting the valence-band masses in the (100) as well as (111) directions is discussed, and parametrizations that specifically fit the valence-band anisotropy are used to calculate Ge acceptor levels in AlxGa1-x As alloys. The sensitivity of the calculated energies to the parameters that determine bulk masses is investigated, as well as the effect of varying the on-site energy of the impurity. A comparison is made between first-neighbor and second-neighbor hopping models. For shallow levels, both approaches give the same results. For deeper levels, however, important differences arise. Experimental evidence suggests that first-neighbor models are better suited for describing intermediate to deep levels.

Optical properties and valence-band masses in spontaneously ordered semiconductor alloys

Analytical results are obtained on the valence-band structure in zinc-blende semiconductor alloys that exhibit spontaneous CuPt ordering. The case of pure ordering is considered, as well as the case of coexistance of ordering and lattice mismatch. Ordering is entered through the chemical deformations, and lattice mismatch is introduced by the epitaxial strain. The interband transition probabilities are calculated as a function of the degree of ordering and the polarization angle. It is shown that the optical response in the two possible variants of ordering is different when light is polarized along a nonsymmetrical direction. A similar situation occurs in the case of circularly polarized light. The difference is less significant in complex structures where the two variants of ordering coexist or form overlapping regions. The optical anisotropy in the plane perpendicular to the growth direction is also examined. We find that as the ordering parameter increases, the polarization dependence of the transitions involving the upper valence band becomes more significant. The polarization degree for the light-hole related transitions is a nonmonotonical function of the ordering parameter. The hole masses are calculated analytically as a function of the degree of ordering. It is shown that in the presence of lattice mismatch all hole masses, as well as the orientation of the mass ellipsoid axes, change with the ordering parameter. For weak ordering the largest axis is close to the [001] cubic direction. The anisotropy of the hole masses is governed by the epitaxial strain and is presented for finite band parameter ${\ensuremath{\gamma}}_{2},$ if the growth direction is along the [001] axis. At large-ordering parameter the largest axis of the mass ellipsoid is close to the ordering direction. The mass anisotropy in this case is mostly due to ordering and is presented for finite ${\ensuremath{\gamma}}_{3}.$

Impurity states in semiconductors calculated via tight binding: A parameter sensitivity study

Acceptor binding energies in zinc-blende semiconductors are determined within the tight-binding formalism. The sensitivity of the calculated energies to the parameters that determine bulk masses and those that describe the coupling of the impurity to the host is investigated. Binding energies for Ge acceptors in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ alloys are calculated using two parametrizations recently proposed in the literature, and the results reveal the importance of fitting the valence-band masses in the (100) as well as (111) directions. New parametrizations are presented for GaAs, AlAs, and InAs that specifically fit the valence-band anisotropy, and the acceptor problem in these materials is investigated. A comparison is made between first-neighbor and second-neighbor hopping models. For shallow levels, both approaches give the same results. For deeper levels, however, important differences arise. Experimental evidence suggests that first-neighbor models are better suited for describing intermediate to deep levels.

Role of the valence-band mass on the many-body optical properties of a two-dimensional electron gas

The Fermi-edge singularity and shake-up sidebands have been measured in the optical emission spectrum of a modulation doped single quantum well of InGaAs/InP with spatially variable Fermi energy and valence-band mass. Both magnitudes change across the sample due to a slight variation of the well width. The intensity of the optical Fermi-edge singularity at zero magnetic field increases with increasing hole mass, in excellent quantitative agreement with our many-body calculations. Emission spectra under a magnetic field at even filling factors show that, contrary to the Fermi-edge singularity, shake-up bands do not depend on the hole mass, in qualitative agreement with previous calculations.

Negatively charged exciton X − in quantum wells with strong magnetic field

In this paper we present a theoretical study of X − in an infinite rectangular quantum well with a strong magnetic field perpendicular to the well layer. Using a variational calculation, we have calculated the binding energies of X − (the energy required to remove one of the two electrons from X − to infinity), as functions of the ratio of the electron conduction band mass to the hole valence band mass, σ = m e m h . The results in the two-dimensional and three-dimensional limits agree with the theoretical results obtained by other authors.

Magnetoluminescence Studies in Ordered InGap2

AbstractPhotoluminescence measurements on ordered InGaP2 were studied as a function of temperature, laser power density, and magnetic field. The temperature varied between 1.4 and 300 K, the laser power densities ranged from 10 nW/cm2 to 20 W/cm2, and the maximum magnetic field was 13.6 T. The data show two spectra, excitonic and band-to-band transitions, depending upon the incident laser power density. A consistent interpretation of the band-to-band spectrum leads to a spatially separated electron-hole transitions between the ordered domains. Three different mechanisms for spatially indirect transitions are presented. An analysis of the linear data also allows a determination of the valence-band mass in the 100 direction of mv ∼0.25 m0.

Radiative performance of strained-layer lasers

It is shown that application of biaxial tension to the active region of a bulk-like III–V semiconductor laser can significantly enhance TM gain compared to TE gain and reduce the threshold current density, owing to improved suppression of spontaneous emission polarised in the growth plane of the laser structure. The differential gain is enhanced compared to unstrained structures, and a larger peak gain can be achieved than in comparable structures under biaxial compression. The authors include the spin-split-off band in their calculations and show that the strain-induced interaction with this band has a significant influence on the character of the valence states, particularly in tensile-strained structures. Using idealised quantum-well calculations, the authors investigate the effect of changing the valence band mass mv with respect to the conduction band mass and how this affects the radiative properties of a laser structure. It is shown that, although it is indeed the case that the transparency carrier density always increases with increasing mu, for the authors' ideal model, the radiative current density can decrease.

Envelope Equation and Wave Function Matching for Narrow‐Gap Semiconductors

AbstractIt is shown that Zawadzki's equation for the conduction band envelope in a narrow‐gap semiconductor can be derived from the Ben Daniel‐Duke equation containing a position dependent effective mass if the position dependence of the band edge is taken into account in the Kane type energy dependence of the effective mass. Contrary to the original derivation of Zawadzki generalizations to other types of band structures are easily possible. As examples envelope equations are derived for an anisotropic Kane band structure and for the case of different conduction and valence band masses. In addition, the procedure presented here leads also to the conclusion that the envelope matching condition used till now has to be replaced by a more general and energy dependent one.

Stability and electronic properties of InAs/InP strained superlattices.

Results of all-electron full-potential local-density electronic-structure calculations on strained 1\ifmmode\times\else\texttimes\fi{}1 (001) superlattices are presented. The in-plane lattice parameter and the anion-cation equilibrium distances are determined with use of the total-energy for lattice parameters to simulate (i) the case in which the InAs/InP system is considered to be grown in the ``free-standing'' mode and (ii) the case in which InAs is considered to be grown on an InP substrate. In both cases, the instability of the two superlattices to disproportionation is found to be rather small and mainly due to the deformation energy. Only a small amount of charge is transferred at the interface and the energy term related to this charge transfer tends to stabilize the structure. Studies on the effects of strain on the electronic properties (band gap and effective valence-band masses) of the superlattices show that the uniaxial distortion affects the shape and the relative position of the valence bands so that the transport properties might be adjusted by properly varying the strain.

Reduction of lasing threshold current density by the lowering of valence band effective mass

In present day semiconductor lasers, there is a serious asymmetry between the very light conduction band mass and the very heavy valence band mass. Under laser threshold conditions, the hole occupation remains classical even while the electrons are degenerate. This results in a significant penalty in terms of threshold current density, carrier injection level, and excess Auger and other nonradiative recombination. We propose a combination of strain and quantum confinement to reduce the valence band effective mass and to lessen the laser threshold requirements.

Direct Observation of Valence-Band Mixing in GaAs/GaPAs Strained-Layer Superlattices

ABSTRACTLow-temperature magneto-luminescence studies are used to examine the in-plane valence-band dispersion in GaAs/GaPAs strained-layer-superlattice structures. The results provide a direct observation of valence-band mixing effects in strained-layer systems.Theoretical and experimental studies of the effect of valence-band mixing upon the magneto-transport properties and magneto-luminescence data for GaAs/AlGaAs latticed-matched superlattices have been recently reported.[1–] Due to the presence of the 2D-quantum well, the valence-band degeneracy between the light-hole and heavy-hole bands at is removed. These studies show, however, that the lowest energy valence-band mass is still strongly dependent upon due to valence-band mixing. On the other hand, because of the large biaxial compression in strained-layer-super lattices (SLS), valence-band mixing effects should be minimal.[8–9] Recent magneto-transport measurements[10] on p-type InGaAs/GaAs SLS structures have provided information regarding nonparabolic valence-bands in these structures.

Two-photon spectroscopy ofBexcitons in CdS

High-resolution spectra are obtained for the free $A$ and $B$ excitons in CdS by two-photon absorption photoconductivity techniques at 1.8 K. At zero applied magnetic field, spectra are shown which indicate for the first time the presence of anisotropy splitting of the $B(2P)$ exciton states. The first observation of the $B(3P)$ exciton state at $B=0$ T then allows the conclusion that both the effective Rydberg and the anisotropy parameter of the $A$ and $B$ excitons are the same. This further indicates that the $A$ and $B$ valence-band masses are equal. These conclusions are confirmed by the first observation of the Zeeman splitting of the $B(2{P}_{\ifmmode\pm\else\textpm\fi{}1})$ and $B(3{P}_{\ifmmode\pm\else\textpm\fi{}1})$ exciton states at fields up to 10 T. Fine-structure splitting of the $B(2{P}_{+1})$ and $B(2{P}_{\ensuremath{-}1})$ states results from an effective $g$ factor for the $B$ valence band of ${g}_{h}^{B}=0.7\ifmmode\pm\else\textpm\fi{}0.3$.

A calculation of valence band masses, exciton and acceptor energies and the ground state properties of the electron-hole liquid in cubic SiC

The Luttinger parameters of cubic SiC are determined by using a modified Lawaetz approach, including the hole-phonon coupling for small spin—orbit interaction: γ ∗ 1 = 2.817, γ ∗ 2 = 0.508, γ ∗ 3 = 0.860, κ ∗ = -0.41. Revised ground state calculations of the electron-hole liquid, including the nonparabolic valence band dispersion, for conflicting theories of the electron-phonon interaction, yield a good agreement between the ϵ ∗ 0-approximation and experiment. The calculated ground state binding energy of the free exciton and of the point-charge acceptor are 26.7 and 179 meV respectively. These calculations yield a revised value of the band gap energy E g = 2.416 eV.

Energy bands and effective masses of CuCl

We have made the first self-consistent energy-band calculation of CuCl with sufficient variational freedom to calculate the effective masses accurately. The conduction-band mass agrees with experiment whereas the valence-band mass is much less than any of the experimental values, indicating that the hole forms a small polaron. The calculation was repeated for a reduced lattice constant. The indirect gap proposed by Rusakov to explain the high-temperature "superconductivity" anomaly does not exist and the direct gap widens with pressure.

Electronic structure of PbTe near the band gap

Existing experimental data for the electronic band parameters of the conduction and valence bands of PbTe are analyzed and compared using a multiband model electron dispersion relation. The model permits the identification of inconsistencies among the data and the derivation of an improved set of band parameters. Among the principal conclusions are (i) the most commonly used value of the transverse-conduction-bandedge effective mass is too large; (ii) the conduction- and valence-band mass anisotropy ratios vary only weakly with carrier concentration, in contradiction to much of the early data for $p\ensuremath{-}\mathrm{PbTe}$; (iii) existing theoretical calculations of the far-band contributions to the conduction- and valence-band-edge effective masses are not in very good agreement with experiment.