Conduction Band Effective Mass Research Articles

Structure-dependent optical and electrical transport properties of nanostructured Al-doped ZnO

The structure–property relation of nanostructured Al-doped ZnO thin films has been investigated in detail through a systematic variation of structure and morphology, with particular emphasis on how they affect optical and electrical properties. A variety of structures, ranging from compact polycrystalline films to mesoporous, hierarchically organized cluster assemblies, are grown by pulsed laser deposition at room temperature at different oxygen pressures. We investigate the dependence of functional properties on structure and morphology and show how the correlation between electrical and optical properties can be studied to evaluate energy gap, conduction band effective mass and transport mechanisms. Understanding these properties opens up opportunities for specific applications in photovoltaic devices, where optimized combinations of conductivity, transparency and light scattering are required.

Electronic structure and thermoelectric performance of Zintl compound Ca5Ga2As6

Ca5Ga2As6 gives us a chance to achieve a high thermoelectric performance with a normal bulk system. This is mainly due to the fact that its complex chemical structure can not only supply an ultra-low lattice thermal conductivity but also show a high Seebeck coefficient. In this paper, we have discussed the lattice, electronic, and transport properties of Ca5Ga2As6 for elucidating its high thermoelectric performance. In our calculations, density-functional theory and Boltzmann transport theory were used. For intrinsic Ca5Ga2As6, the valence band effective mass is larger than the conduction band effective mass, which leads to its positive sign of S over the whole temperature range. By studying the anisotropy of its transport properties, we find that the transport properties of p-type Ca5Ga2As6 are better than that of the n-type one, which mainly comes from the large anisotropy of its band structure. Moreover its transport properties along the z direction are much better than those along the x and y directions, which can be attributed to its anisotropic one-dimensional lattice structure. At different temperatures, the peak value of ZeT appears at different carrier concentrations, which makes it possible to obtain a highest efficiency under large temperature gradients. The carrier concentrations corresponding to large ZeT are in the range of 0.033–0.5 e per uc, which is equal to 5.19 × 1019 cm−3 to 7.87 × 1020 cm−3. The largest value of ZeT is equal to 0.95.

Electronic Tuning of Intersubband Transition of Inverted Core-Shell Cylindrical Quantum Wire for Novel Lasing Performance

A novel quantum wire laser structure is presented by designing inverted core-shell cylindrical quantum wire using AlxGa1-xAs/GaAs type-I heterostructure. The energy eigenvalues with corresponding density of states are numerically computed using finite difference technique. Intersubband transition energies are computed considering the lowest three energy states only where material composition of core material is changed to observe the effect on it. Dimensional effect is also studied for suitability of application in case of optical tuning. This analysis is carried out by taking into consideration the conduction band discontinuity and effective mass mismatch at boundaries following BenDaniel duke boundary condition. Results reflect that accuracy of control can be achieved on lasing performance by combining the dimensional variation effect with material parameter adjustment.

Effect of Europium Substitution on Thermoelectric Properties of Noble-Metal Silicon Clathrates with Ba<sub>8−</sub><i><sub>x</sub></i>Eu<i><sub>x</sub></i>Cu<i><sub>y</sub></i>Si<sub>46−</sub><i><sub>y</sub></i> Nominal Compositions

The effect of the substitution of a rare-earth element for a guest element on the electronic and thermoelectric properties is investigated for clathrates with the nominal composition Ba8−xEuxCuySi46−y (x = 0, 1, 2; y = 4, 5, 6). The Seebeck coefficient and the electrical conductivity of the rare-earth-substituted Ba8−xEuxCuySi46−y samples are consistent with n-type conduction and metal-like behavior. The Hall carrier mobility of Ba8−xEuxCu6Si40 at room temperature decreases with increasing Eu substitution. The conduction band effective mass of rare-earth-substituted Ba8−xEuxCu6Si40 (x = 1, 2) is experimentally estimated to be larger than that of Ba8Cu6Si40. On the basis of ab initio electronic structure calculations for Ba6Eu2Cu6Si40 and Ba8Cu6Si40, it is deduced that the conduction band edges of Ba8Cu6Si40 are modified by the substitution of Eu for the guest atom Ba, resulting in an increase in the density-of-states effective mass, which is consistent with the experimental result.

Quasiparticle band structure of Zn-IV-N2compounds

Electronic energy-band structures of the Zn-IV-N${}_{2}$ compounds, with IV equal to Si, Ge, and Sn calculated in the quasiparticle self-consistent GW approximation and using the full-potential linearized muffin-tin orbital approach, are presented. A comparison is made with local-density approximation results. The bands near the gap are fitted to an effective Kohn-Luttinger-type Hamiltonian appropriate for the orthorhombic symmetry, and conduction-band effective masses are presented. Exciton binding energies and zero-point motion corrections to the gaps are estimated. While ZnSiN${}_{2}$ is found to be an indirect gap semiconductor, ZnGeN${}_{2}$ and ZnSnN${}_{2}$ are direct gap semiconductors. The gaps range from the orange-red to deep UV. The valence-band maximum is split in three levels of different symmetry, even in the absence of spin-orbit coupling, and should show transitions to the conduction band, each for a separate polarization. Spin-orbit effects are found to be surprisingly small, indicating almost exact compensation of the N-$2p$ and Zn-$3d$ contributions.

Design strategies for mitigating the influence of polarization effects on GaN-based multiple quantum well light-emitting diodes

The optoelectronic properties of GaN-based multiple quantum well (MQW) light-emitting diodes (LEDs) are investigated using a detailed theoretical model, in which the effects of strain, well coupling, valence band mixing, and polarization effects are fully considered. By solving the conduction and valence band effective mass equations together with Poisson’s equation self-consistently, the influence of various major design parameters, such as the well width, the barrier components, and the barrier thickness, on the electronic and optical properties of GaN-based MQW LEDs is studied. Numerical results show that the emission spectra of the LEDs are very sensitive to the above design parameters due to the polarization effect that is unique for GaN-based devices. Further analysis and simulations reveal that this sensitivity can be obviously suppressed by choosing InGaN as the barrier material.

Electronic structure, vibrational spectrum, and thermal properties of yttrium nitride: A first-principles study

Yttrium nitride (YN) is a promising semiconductor for use in metal/semiconductor superlattices for thermoelectric applications. We determine its electronic structure, vibrational spectrum, and thermal properties using first-principles density functional theory (DFT) based simulations with a generalized gradient approximation (GGA) of the exchange correlation energy. We employ GGA+U and GW approximations in our calculations to (a) improve the accuracy of the calculation of bandgaps and (b) determine specific features of its electronic structure relevant to transport properties, such as transverse (mt*) and longitudinal (m1*) conduction band effective mass. To evaluate consequences of forming alloys of YN with other materials to its electronic properties, we have determined the volume deformation potentials. Our results for phonons show a large longitudinal optical (LO) and transverse optical (TO) splitting at the Γ point in the vibrational spectrum with a gap of 325 cm−1 arising from long- range dipole-dip...

Intersubband gain without global inversion through dilute nitride band engineering

We investigate the possibility of interconduction band gain without global inversion by engineering the conduction band effective masses so that the upper lasing subband has an effective mass considerably smaller than the lower lasing subband that could not be obtained in conventional III-V materials. We recover the expected dispersive gain shape for similar masses and contrasting results if the effective masses characterizing the relevant subbands are very different.

Laser-dressing and magnetic-field effects on shallow-donor impurity states in semiconductor GaAs–Ga1−xAlxAs cylindrical quantum-well wires

The influence of an intense laser field on shallow-donor states in cylindrical GaAs–Ga1−xAlxAs quantum-well wires under an external magnetic field applied along the wire axis is theoreticallystudied. Numerical calculations are performed in the framework of the effective-massapproximation, and the impurity energies corresponding to the ground state and2p ± excited states are obtained via a variational procedure. The laser-fieldeffects on the shallow-donor states are considered within the extendeddressed-atom approach, which allows one to treat the problem ‘impurity+ heterostructure+ laser field+ magnetic field’ as arenormalized ‘impurity + heterostructure + magnetic field’ problem, in which the laser effects are taken into account through arenormalization of both the conduction-band effective mass and fundamentalsemiconductor gap.

Analytical Model for Conduction Band Nonparabolicity to Transport Analysis of Nano-Scale SOI MOSFET

This paper reconsiders the mathematical formulation for the conventional nonparabolic band model, and discusses how the nonparabolicity of the conduction band impacts the effective masses of electrons that are confined within the barriers. To support this consideration, this paper proposes an analytical expression for the conduction-band effective mass of electrons including the term of band nonparabolicity.

Electronic properties of a strained ⟨100⟩ silicon nanowire

The effects of uniaxial strain on the electronic properties of silicon nanowires grown in ⟨100⟩ direction are studied using a tight binding sp3d5s∗ orbital basis quantum simulation. Calculations are performed using both Harrison and Boykin formalisms (discussed in Sec. II). The energy difference between the fourfold (Δ4) and the twofold (Δ2) degenerate valleys of conduction bands reduces with compressive strain and the nanowire becomes an indirect band gap material when the compressive strain exceeds a certain value. With tensile strain, this energy difference increases and the nanowire band structures remain direct. The conduction band edge is downshifted with compressive strain and is upshifted with tensile strain. However, the valence band edge is upshifted with both types of strain that results in band gap reduction with strain. The four-valley degeneracy of conduction band at the center of one dimensional wire Brillouin zone is slightly lifted with both types of strain. The energy difference between the top two valence bands is insensitive to tensile strain and is significantly changed with compressive strain. The strain has no effect on conduction band effective mass but changes the valence band effective mass significantly. A 1% strain can change the hole effective mass by ≈53%. Harrison and Boykin formalisms produce very similar valence band edge and hole and electron effective masses and significantly different conduction band edge and band gap. In Boykin formalism, strain affects the energy levels of both the Δ4 and Δ2 valleys of conduction band while the energy level of only Δ2 valleys is affected by strain in Harrison calculations. The direct to indirect transition occurs at a slightly higher compressive strain in Boykin formalism.

Extension of analytical model for conduction band nonparabolicity to transport analysis of nanoscale metal-oxide-semiconductor field-effect transistor

This paper reconsiders the mathematical formulation for the conventional nonparabolic band model and discusses how the nonparabolicity of the conduction band impacts the effective masses of electrons that are confined within the barriers. Since the conventional simplified model for band nonparabolicity does not include the external potential effect as a perturbation, it is examined whether this perturbation can be implemented into the conventional model for usefulness. When the perturbation energy is smaller than the unperturbed energy, the insertion of the perturbation energy term into the conventional expression for the nonparabolic band model is reasonably valid. It is demonstrated that this approximation is actually accurate given a sub-10-nm-thick Si layer surrounded by an SiO2 layer. A major part of this discussion is focused on the low-dimensionality electron system confined with insulator barriers so this study contributes to the analysis of future electron devices. To support this consideration, this paper proposes an analytical expression for the conduction-band effective mass of electrons including the term of band nonparabolicity.

Electronic structure and optical properties of freestanding [0001] oriented GaN nanowires and nanotubes

We report on a theoretical study of the electronic and optical properties of freestanding, [0001] oriented wurtzite GaN nanowires and nanotubes based on an atomistic tight binding approach. The developments of band structure, optical properties, and effective mass are studied as functions of nanowire size. It is shown that the valence band structure of the nanowire depends on the lateral size of the nanowire and that the order between the first two valence bands is reversed above a critical size. The fundamental optical transition is found to be strong for nanowire sizes below, and weak for nanowire sizes above, this critical size. The first strong optical transition is found to have a very large optical polarization anisotropy with the dominant component parallel to the nanowire axis. It is also shown that there is a simple functional relationship between the conduction band effective mass and the subband energy, while no such general relation can be found for the valence bands. For the nanotubes the change in energy compared to the solid nanowire is found to be strongly related to the distribution of the original nanowire state wave function. The incorporation of a hole in the nanowire will force a change in the ordering between the first two valence band states compared to a below critical size nanowire.

Quantum-well intermixing influence on GaInNAs/GaAs quantum-well laser gain: theoretical study

The effect of quantum-well intermixing on the material gain of a GaInNAs/GaAs quantum-well laser is investigated theoretically. The diffusion of gallium and indium atoms in the intermixed sample is assumed and their compositional profiles are modelled using Fick's law. The band-anti-crossing model is used to calculate the band structure of the GaInNAs quantum well, which is appropriate for this non-randomly-alloyed material system. The calculated results show good agreement with the observed photoluminescence excitation for both non-intermixed and intermixed samples, which confirms this model. It is found that the strain gradient, the variation of material band gap and the degeneracy between heavy and light holes are the main factors determining the quantized energy levels of the intermixed quantum well. With the increase of diffusion length, the material gain and differential gain decrease due to the increase of the conduction band effective mass and the rapid decrease of the dipole moments. These characteristics of quantum-well intermixing effects will be useful in the design of integrated photonic devices based on this material.

Assessment of electronic properties of InN xSb 1− x for long-wavelength infrared detector applications

This work assesses theoretically the potential of dilute nitride alloys of InN x Sb 1− x for long-wavelength IR applications. A 10-band k.p approximation modified to account for conduction/valence band coupling is implemented to extract the bandgap as a function of the nitrogen concentration in the alloy and the temperature. The calculations show the possibility to obtain a band closure at ∼2% of nitrogen for InSbN at 300 K. The absorption coefficient, and its temperature dependence, is then determined using an Elliot-like formalism, predicting stronger absorption properties associated with the enhancement of conduction band effective masses. This enhancement yields over an order of magnitude increase in the non-radiative Auger recombination lifetimes suggesting the potential of InNSb for significantly enhancing detectivity limits and operation temperatures of long-wavelength IR detectors.

Electronic structure and effective masses of InN under pressure

The pressure dependence of the electronic band structure of InN is investigated with emphasis on the conduction band effective mass and its dependence on free-electron concentration. Transport measurements are performed under hydrostatic pressure on three n-type samples of InN with different electron concentrations. The effective mass extracted from the electron mobility is found to increase with pressure, however with a pressure coefficient, which is lower for the samples with higher electron concentration. Calculations of the InN band structure are performed within the density functional theory by means of the linear muffin-tin-orbital method, including an empirical adjustment to reproduce known band edges at ambient pressure. The calculated variations of the highest occupied conduction band energy and the electron effective mass with free-electron concentration are compared to available experimental information. The results show a pronounced deviation from parabolic behavior of the lowest conduction band, and for k>0 this induces large differences between the values of the optical and curvature masses. Both the fundamental band gap and the electron effective mass increase with pressure, but due to the nonparabolic character of the conduction band, the pressure coefficient of the effective mass decreases with electron concentration, in agreement with the experimental observation.

Evidence of nonparabolicity and size of wave function confined in In0.53Ga0.47As/In0.52Al0.48As multi quantum wells

Nonparabolicity of the conduction band in In0.53Ga0.47As was analyzed using nanoscale InGaAs/InAlAs multi quantum well structures. The nonparabolic effective mass was determined from a set of eigen-energies using a single finite-square-well model. Conduction-band eigen-energies were obtained by analyzing the interband optical transition spectra of photocurrent and photoreflectance. Regarding the conduction band effective mass, Kane’s theory was applicable over the wide range from 0 to 0.5 eV. Based on the theory, the size of the electron wave function was determined with the resolution of 0.1 nm. This method provides a direct means to determine the quantum well thickness.

Rashba spin splitting of the minibands of coupled InAs∕GaAs pyramid quantum dots

The Rashba spin splitting of the minibands of coupled InAs∕GaAs pyramid quantum dots is investigated using the k⋅p method and valence force field model. The Rashba splitting of the two dimensional miniband in the lateral directions is found due to the structure inversion asymmetry in the vertical direction while the miniband in the vertical direction has no Rashba spin splitting. As the space between dots increases, the Rashba coefficients decrease and the conduction-band effective mass increases. This Rashba spin splitting of the minibands will significantly affect the spin transport properties between quantum dots.

Carrier Concentrations in Degenerate Semiconductors Having Band Gap Narrowing

The density-of-states effective mass approximation and the conduction-band effective mass approximation are employed to formulate carrier concentrations and the diffusivity-mobility relationship (DMR) for heavily doped n-semiconductors exhibiting band gap narrowing. These are very suitable for the investigation of electrical transport also in heavily doped p-semiconductors. Numerical calculations indicate that the DMR depends on a host of parameters including the temperature, carrier degeneracy, and the non-parabolicity of the band structure.

Conduction-band dispersion relation and electron effective mass in III-V and II-VI zinc-blende semiconductors

Adjusting calculated spectra of the imaginary part ${\ensuremath{\epsilon}}_{2}$ of the dielectric function to experimental ones in a spectral region close to and above the fundamental absorption edge, we determined the conduction-band dispersion relation for 12 III-V and II-VI semiconductors with zinc-blende crystal structure and deduced the corresponding nonparabolicity coefficients. This yields an experimental determination of the conduction-band nonparabolicity and momentum electron effective mass as a function of the wave vector and free electron concentration. For most of the semiconductors, we present experimental data extended to electron energies between 0.6 and $2.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, which are significantly higher than those achievable by doping or in magnetic fields. In addition, examination of experimental dielectric functions reported by various authors showed that the magnitude of ${\ensuremath{\epsilon}}_{2}$ is overestimated in the existing literature data for GaSb, ZnTe, ZnS, and CdSe, probably due to the neglect or incorrect treatment of possible overlayers.