Nonparabolicity Effects Research Articles

Investigation of the effect of band-edge nonparabolicity on the carrier transport in ITO thin films

The impact of band-edge nonparabolicity on the charge carrier transport in degenerate n-type indium-tin-oxide (ITO) thin films has been investigated theoretically in order to understand the fundamental concepts of electron scattering in such materials regardless of the precise details of the preparation procedure. The Fermi energy level and the scattering parameter of the ITO thin films have been calculated as functions of the carrier concentration. The results indicate that the nonparabolicity has a drastic effect on the total mobility of carriers in the films. We have also discussed the role of grain boundaries in carrier transport. Additionally, to survey the dominant scattering mechanism in the grain bulk of ITO films, we have calculated the scattering parameter. The evaluated values lie near the trend expected for ionized impurity scattering.

Anharmonic effects in the thermoelectric properties of PbTe

In this work, we investigate the crystal anharmonic effects in the thermoelectric properties of n-type PbTe. The lattice thermal transport coefficient is computed by employing an isotropic continuum model for the dispersion relation for acoustic as well as optical phonon branches, an isotropic continuum model for crystal anharmonicity, and the single-mode relaxation time scheme. The electronic components of the transport coefficients in a wide temperature range are calculated using the isotropic-nearly-free-electron model, interaction of electrons with deformation potential of acoustic phonons, and the effect of the band non-parabolicity. It is found that the transverse optical branches play a major role in determining the phonon conductivity and the thermoelectric figure of merit of this material.

Self‐consistent analysis of InAsSb quantum‐well heterostructures

Based on our self‐consistent calculation of the Schrödinger–Poisson equation by taking into consideration both the strain effect and the nonparabolicity effect, we propose a material design for InAsSb‐based quantum‐well heterostructures. A two‐dimensional electron gas concentration exceeding 3 × 1012 cm−2 could be achieved within AlSb/InAs0.4Sb0.6 heterostructures without the occurrence of intersubband scattering, higher than that in AlSb/InAs heterostructures. Our InAsSb‐based quantum‐well heterostructure is proved to be advantageous for applications as high electron mobility transistors.

COOS: a wave-function based Schrödinger–Poisson solver for ballistic nanotube transistors

This paper gives an in depth overview on a wave-function based simulation framework (called coos) for modeling ballistic nanotube transistors by solving the effective-mass Schrodinger equation. The framework considers non-parabolic electronic band structure effects, band-to-band tunneling as well as a heterojunction-like model for extended contacts to describe the injection and reception of charge carriers into and from the channel. Special emphasis is put on an efficient and reliable numerical implementation. The applicability of the simulation framework and the necessity to include the aforementioned phenomena are shown by comparing simulation results with experimental data of a $$50\hbox { nm}$$ 50 nm long carbon nanotube transistor (cntfet). The intrinsic transit frequencies and the output characteristics for higher drain-source voltages are predicted and analyzed.

Simultaneous Effects of External Electric Field and Conduction Band Nonparabolicity on Optical Properties of a GaAs Quantum Dot Embedded at the Center of a GaAlAs Nano-Wire

An investigation of the optical properties of a GaAs spherical quantum dot which is located at the center of a Ga1−xAlxAs cylindrical nano-wire has been performed in the presence of an external electric held. The band nonparabolicity effect is also considered using the energy dependent effective mass approximation. The energy eigenvalues and corresponding wave functions are calculated by finite difference approximation and the reliability of calculated wave functions is checked by computing orthogonality. Using computed energy eigenvalues and wave functions, the linear, third-order nonlinear and total optical absorption coefficients and refractive index changes are examined in detail. It is found that (i) Presence of electric field causes both blue and red shifts in absorption spectrum; (ii) The absorption coefficients shift toward lower energies by taking into account the conduction band nonparabolicity; (iii) For large values of electric field the effect of conduction band nonparabolicity is less dominant and parabolic band is estimated correctly; (iv) In the presence of electric field and conduction band nonparabolicity the nonlinear term of absorption coefficient rapidly increases by increasing incident optical intensity. In other words, the saturation in optical spectrum occurs at lower incident optical intensities.

Theory of band warping and its effects on thermoelectronic transport properties

Optical and transport properties of materials depend heavily upon features of electronic band structures in proximity to energy extrema in the Brillouin zone (BZ). Such features are generally described in terms of multi-dimensional quadratic expansions and corresponding definitions of effective masses. Multi-dimensional expansions, however, are permissible only under strict conditions that are typically violated by degenerate bands and even some non-degenerate bands. Suggestive terms such as "band warping" or "corrugated energy surfaces" have been used to refer to such situations and ad hoc methods have been developed to treat them. While numerical calculations may reflect such features, a complete theory of band warping has not been developed. We develop a generally applicable theory, based on radial expansions, and a corresponding definition of angular effective mass. Our theory also accounts for effects of band non-parabolicity and anisotropy, which hitherto have not been precisely distinguished from, if not utterly confused with, band warping. Based on our theory, we develop precise procedures to evaluate band warping quantitatively. As a benchmark demonstration, we analyze the warping features of valence bands in silicon using first-principles calculations and we compare those with previous semi-empirical models. We use our theory and angular effective masses to generalize derivations of tensorial transport coefficients for cases of either single or multiple electronic bands, with either quadratically expansible or warped energy surfaces. From that theory we discover the formal existence at critical points of transport-equivalent ellipsoidal bands that yield identical results from the standpoint of any transport property. Additionally, we illustrate the drastic effects that band warping can induce on thermoelectric properties using multi-band models.

Investigation on the electrical properties of superlattice FETs using a non-parabolic band model

The effect of non-parabolic energy-bands on the electrical properties of an In0.53Ga0.47As/In0.52Al0.48As superlattice FET has been investigated. An energy dependent effective mass was fitted on k·p simulation results and the new band model was implemented into a self-consistent Schrödinger–Poisson solver. This analysis has shown that non-parabolicity effects lead to noticeable changes of the device characteristics with respect the parabolic band model, namely: an increase of the on-state current and a steeper transition from the off- to the on-state sustained across several decades of current, at the expense of an increased off-state leakage. Moreover, the larger density of states in the non-parabolic model causes a 47% growth of the output conductance at low VDS, as well as an increased drain conductance in saturation.

Strain and nonparabolicity effects in Mid-infrared GaxIn1-xAsySb1-y/AlxGayIn1-x-yAszSb1-z MQW QCL

In this paper, we present a theoretical study of strain and nonparabolicity effects in Mid-infrared (MIR) GaxIn1-xAsySb1-y/AlxGayIn1-x-yAszSb1-z Multiple Quantum Well (MQW) Quantum Cascade Lasers (QCL). We use an envelope function k.P theory model to show the strong effects of bi-axial compressive strain and nonparabolicity on conduction band states of the QCL. These effects are shown to result more particularly in an increase of the electron effective mass near the fundamental laser emission energy transition of the QCL. This in turn is found to have a significant influence on several optical properties of the QCL as a reduction of laser transition energy i.e. an increase of the laser emission wavelength, a decrease of the transit time in the injection region of the QCL, a reduction of the optical dipole matrix element, a reduction of the optical gain of the laser.

Conduction Band-Edge Non-Parabolicity Effects on Impurity States in (In,Ga)N/GaN Cylindrical QWWs

In this paper, the conduction band-edge non-parabolicity (NP) and the circular cross-section radius effects on hydrogenic shallow-donor impurity ground-state binding energy in zinc-blende (ZB) InGaN/GaN cylindrical QWWs are reported. The finite potential barrier between (In,Ga)N well and GaN environment is considered. Two models of the conduction band-edge non-parabolicity are taking into account. The variational approach is used within the framework of single band effective-mass approximation with one-parametric 1S-hydrogenic trial wave-function. It is found that NP effect is more pronounced in the wire of radius equal to effective Bohr radius than in large and narrow wires. Moreover, the binding energy peak shifts to narrow wire under NP effect. A good agreement is shown compared to the findings results.

A Theoretical study of the Thermoelectric Transport Coefficients of n-type PbTe

ABSTRACTIn this work we present a theoretical study of the transport coefficients of n-type PbTe. The electronic transport coefficients are calculated using the isotropic-nearly-free-electron approximation, including the effect of band non-parabolicity on electron-phonon scattering. The lattice thermal transport coefficient is computed by employing the isotropic continuum model for the dispersion relation for acoustic as well as optical phonon branches, an isotropic anharmonic continuum model for crystal anharmonicity, and the single-mode relaxation time scheme. The role of transverse optical (TO) phonon modes in anharmonic interactions will be discussed in detail.

Optimization of cubic GaN/AlGaN quantum well-based structures for intersubband absorption in the infrared spectral range

A method is proposed for the optimization of structural parameters of GaN/AlGaN quantum wells and Bragg-confined structures, with respect to peak intersubband absorption from the ground to the first excited electronic state in the mid and near infrared spectral range. It is based on the application of the Genetic Algorithm and delivers globally optimal structures with a preset number of embedded layers. Simple rectangular quantum well profile is investigated for the maximal Stark effect and applications to tunable mid-infrared photodetectors. In case of Bragg-confined structures, an above the barrier bound state is used to extend the range of transition energies above the values available in conventional quantum wells. The effects of band nonparabolicity are taken into account.

Importance of non-parabolic band effects in the thermoelectric properties of semiconductors

We present an analysis of the thermoelectric properties of of n-type GeTe and SnTe in relation to the lead chalcogenides PbTe and PbSe. We find that the singly degenerate conduction bands of semiconducting GeTe and SnTe are highly non-ellipsoidal, even very close to the band edges. This leads to isoenergy surfaces with a strongly corrugated shape that is clearly evident at carrier concentrations well below 0.005 e per formula unit (7–9 × 1019 cm−3 depending on material). Analysis within Boltzmann theory suggests that this corrugation may be favorable for the thermoelectric transport. Our calculations also indicate that values of the power factor for these two materials may well exceed those of PbTe and PbSe. As a result these materials may exhibit n-type performance exceeding that of the lead chalcogenides.

Theoretical study of transport property in InAsSb quantum well heterostructures

Theoretical studies of transport properties in InAsSb-based quantum well heterostructures are presented. The concentration of two-dimensional electron gas is calculated self-consistently from our improved Schrödinger-Poisson model by taking into account of nonparabolicity effect and strain effect, and the results are used to obtain the electron mobility. All major scattering mechanisms by acoustic phonon, polar optical phonon, remote ionized impurity, background impurity, interface roughness, and alloy disorder have been included in our calculation. Particularly, dislocation scattering, intrasubband scattering, and intersubband scattering, which are always neglected in heterostructure systems, are considered. The calculated electron mobility is proved to be in good agreement with the experimental data for modulation-doped AlSb/InAsSb quantum well heterostructures. With a view to optimize the transport property, quantum well width, spacer thickness, barrier thickness, and remote doping concentration for AlSb/InAsSb heterostructures are discussed to examine their effect on total electron mobility. Intrinsic electron mobility exceeding 40 000 cm2/Vs is predicted at 300 K for AlSb/InAs0.4Sb0.6 heterostructures which is dominated by a combination of polar optical phonon scattering, remote ionized impurity scattering, and interface roughness scattering.

Modification of structure and properties of AZO thin film by introducing H2 in sputtering atmosphere at low substrate temperature

Al-doped ZnO (AZO) thin films were prepared on soda-lime glass at 100°C by RF magnetron sputtering with different H2 fluxes. The influences of H2 flux on structural, electrical, and optical properties were investigated by XRD, Hall Effect measurement, and transmittance spectra. The results show that hydrogen introduction significantly modifies both structure and properties of AZO films. As H2 flux increases, the increase of unit-cell volume of the films implies that hydrogen is incorporated into ZnO lattice; the obvious decrease of crystallite size indicates that the crystallinity of the films degrades. The resistivity of the films can be continuously decreased with increasing H2 flux, accompanying with increase of both carrier concentration and Hall mobility. The main factor of increasing carrier concentration and mobility is found to be related to hydrogen incorporation and effective substitution of Zn2+ sites by Al3+. The films deposited in Ar+H2 atmosphere show improved conductive stability in air due to the passivation of inter-crystallite by hydrogen. The average transmittance in visible range of the films is hardly dependent on H2 flux. The Eg of the films increases with increasing H2 flux, and the blueshift values are close to the theoretical one according to the nonparabolic BM effect.

Graphene Base Transistors: A Simulation Study of DC and Small-Signal Operation

A simulation study aimed at investigating the main features in dc and small-signal operating conditions of the hot-electron graphene base transistor (GBT) for analog terahertz operation is presented. Intrinsic silicon is used as reference material. The numerical model is based on a self-consistent Schrodinger-Poisson solution, using a 1-D transport approximation and accounting for multiple-valley and nonparabolicity band effects. Some limitations in the extension of the saturation region and in the output conductance related to the finite quantum capacitance of graphene and to space charge effects are discussed. A small-signal model is developed that catches the essential physics behind the voltage gain and the cutoff frequency, which shows that the graphene quantum capacitance severely limits the former but not the latter. According to simulations carried out within the ballistic transport approximation, a 20-nm-long GBT can achieve at the same time a voltage gain larger than 10 and a cutoff frequency largely above 1 THz within a reasonably wide bias range.

Two-band finite difference method for the bandstructure calculation with nonparabolicity effects in quantum cascade lasers

We present a two-band finite difference method for the bandstructure calculation of quantum cascade lasers (QCLs) based on the equivalent two-band model of the nonparabolic Schrödinger equation. Particular backward and forward difference forms are employed in the discretization procedure instead of the common central difference form. In comparison with the linearization approach of the nonparabolic Schrödinger equation, the method is as accurate and reliable as the linearization approach, while the velocity of the method is faster and the matrix elements are more concise, therefore making the method more practical for QCLs simulations.

A computational study of nonparabolic conduction band effect on quantum wire transport (e.g. GaN)

We studied the physics insight the GaN (example) quantum wire FET transistors. The model is based on the four \(\mathbf{k}{\cdot } \mathbf{p}\) Kane band model. We have introduced closed compact model for the Einstein relation of the diffusivity to mobility ratio (DMR) in quantum wires. The model can be applied for both wide and narrow band gaps of nonparabolic conduction band dispersion. The model is related to the optical matrix elements between conduction and valence bands. We have used 1D electrostatic to model the electron density over the maximum energy point. We have studied the effects of gate-to-source and drain-to-source voltages on the DMR by calculating the electron density using flux theory. We observed that above the threshold the non-parabolic dispersion increases the DMR. Additionally, we have studied the nonparabolic effects on the Fermi level and found that for low doping concentrations, the nonparabolic effect must be considered and an accurate calculation for the optical matrix elements is needed.

The possibility of semiconductor–metal transition in a spherical quantum dot

We observe an abrupt change in diamagnetic susceptibility at critical donor concentration for an \(\text{ Al }_\mathrm{x}\text{ Ga }_\mathrm{1-x}\text{ As/GaAs }\) quantum dot system in the effective mass approximation indicating a possible semiconductor metal transition. The effect of confining potential and the laser intensity on the abrupt change in diamagnetic susceptibility has also been studied. The effect of nonparabolicity of the conduction band has been included in our calculations. Results are presented and discussed.

Effects of band non-parabolicity on cavity modes in photonic crystals

We include the effect of band non-parabolicity on photonic defect states within the Wannier theory, which improves the quadratic approximation adopted by Painter et al. [Phys. Rev. B 68, 035214 (2003)] for large-size defects, as well as extends the theory to a wider range of defect size. A 2D hexagonal photonic crystal is considered, and analyzed for the origin and degree of non-parabolicity in the 1st TE band around J-point, and for the effect of non-parabolicity on acceptor type cavity modes. The non-parabolicity is shown to derive primarily from the inherent anisotropy of band dispersion around the point. Overall, with the inclusion of non-parabolicity, (i) mode degeneracy is lowered, (ii) the “binding energy” of a cavity mode is increased, and (iii) the cut-off of defect size for a given cavity mode is reduced, by as much as 35% in certain cases, in comparison with that calculated without the non-parabolicity. A simple “overall effective mass” picture is provided for the understanding of non-parabolicity effects.

Numerical Investigation of the Effects of Conduction Band Nonparabolicity on the Conduction Sub-band Dispersion Relationships in a Al0.88In0.12N/Ga0.9

Numerical Investigation of the Effects of Conduction Band Nonparabolicity on the Conduction Sub-band Dispersion Relationships in a Al0.88In0.12N/Ga0.9