Electron Subbands Research Articles

Electronic Properties of Pyramidal CdTe/ZnTe Quantum Dotsand Comparisons with Experiment

The electronic properties of pyramidal CdTe/ZnTe quantum dots were studied using an eightband strain-dependent k·p Hamiltonian. A model based on the finite element method and the theory of elasticity of solids were used to show that the temperature-dependent photoluminescence measurement of the interband transition energies from the ground electronic subband to the ground heavy-hole subband reasonably explains the experimental data. We find that the interband transition energy decreases with increasing wetting layer thickness, regardless of the temperature of the QD, while the interband transition energies decrease with increasing temperature. We conclude that the wetting layer is an important structural element for predicting the electronic properties of CdTe/ZnTe QDs.

Strain distributions and interband transitions of CdxZn1−xTe/ZnTe asymmetric double quantum dots with different degree of coupling

Strain distributions and interband transitions of CdxZn1−xTe/ZnTe asymmetric double quantum dots (DQDs) with different degree of coupling were calculated by using a three-dimensional finite difference method (FDM) taking into account strain and nonparabolicity effects. Bird’s-eye views of the truncated contour plots of the ground state wave functions at the conduction band of the Cd0.6Zn0.4Te/ZnTe DQDs showed the transition behavior from the coupling to the decoupling behaviors with increasing ZnTe spacer layer thickness. The interband transition energies from the ground electronic subband to the ground heavy-hole band (E1-HH1) in the CdxZn1−xTe/ZnTe DQDs, as determined from the FDM calculations, were in reasonable agreement with the experimental peaks of the temperature-dependent photoluminescence spectra corresponding to the (E1-HH1) interband transition energies in the temperature range from 32 to 130 K.

Spin dynamics of electrons in the first excited subband of a high-mobility low-density two-dimensional electron system

We report on time-resolved Kerr rotation measurements of spin coherence of electrons in the first excited subband of a high-mobility low-density two-dimensional electron system in a GaAs/Al0.35Ga0.65As heterostructure. While the transverse spin lifetime (T2*) of electrons decreases monotonically with increasing magnetic field, it has a non-monotonic dependence on the temperature, with a peak value of 596 ps at 36 K, indicating the effect of inter-subband electron-electron scattering on the electron spin relaxation. The spin lifetime may be long enough for potential device application with electrons in excited subbands.

Application of band theory to experimental eigen-state energies of In0.53Ga0.47As quantum wells lattice-matched to InP

Nonparabolic subband structure of InGaAs/InAlAs quantum wells (QWs) was studied theoretically and experimentally. In this paper, nonparabolic effective masses of electrons and band offset of InAlAs barriers were experimentally deduced from eigen-states of conduction subbands confined within the two-dimensional InGaAs QWs. As the dispersion relations for electron, light hole and hole subbands were obtained using the envelope function approximation taking into account band nonparabolicity, a simple way to design was proposed.

Temporal behavior of entanglement between electronic spin and subband states in aRashba nanoloop

In this paper we study the entanglement dynamics of spin–subband states for an electronin a quasi-one-dimensional Rashba quantum loop, in a strong perpendicular magnetic field,explicitly including the confining potential and the Rashba spin–orbit couplinginto the entropy, which is a measure of entanglement, as a function of time. Ourresults indicate that the entanglement between the spin states and the structuralsubbands undergoes periodic oscillations. Furthermore, it is shown that the period ofoscillations strongly depends upon the Rashba coupling, while the amplitudes maybe optimized by making proper choices of the external magnetic field. It is alsoshown that the maxima of entanglement, in the long run, oscillate under periodicenvelopes, whose characteristics depend upon the magnetic field. Our results, thereby,provide means of controlling the degree of entanglement by adjusting these agents.

Upconversion of photoluminescence due to subband resonances in a GaAs/AlAs multiple quantum well structure

We have investigated the upconversion of photoluminescence (PL) due to subband resonances in a simple GaAs(15.3 nm)/AlAs(4.5 nm) multiple quantum well embedded in a p–i–n diode structure. The systematic measurements of the PL spectra and the calculated results of the interband transition energies as a function of electric field strength reveal that the PL bands from the electron subbands with n=3 (E3) and n=4 (E4) sharply appear under the first-nearest-neighbor resonance conditions between the E1 and E3 subbands and the E1 and E4 subbands, respectively, owing to the carrier injection to the E3 and E4 subbands from the E1 subband. This result indicates that the resonant tunneling due to the subband resonance is a dominant mechanism for the carrier population in the higher lying subbands. Utilizing these subband resonances, we have demonstrated the upconversion of PL from the E3 and E4 subbands under the excitation condition of the fundamental interband transition between the E1 and the n=1 heavy-hole subbands.

Transport properties of a spin-split two-dimensional electron gas in an In0.53Ga0.47As∕InP quantum well structure

We study the magnetotransport properties of a gated In0.53Ga0.47As∕InP quantum well structure in the presence of spin splitting when only one electronic subband is occupied. We develop an analytical method to extract the quantum mobilities for the two spin subbands. Ionized impurity scattering and alloy disorder scattering are determined to be important in this system. Larger quantum mobility is found for the higher-energy spin subband. We also demonstrate that the difference between the quantum mobilities for the two spin subbands can be altered with the gate.

Simulation of electron transport in (0001) and (112¯0) 4H-SiC inversion layers

Monte Carlo simulations are used to investigate electron transport in the inversion layer of a 4H silicon carbide metal-oxide-semiconductor field-effect transistor (MOSFET). The electronic subband structure is solved self-consistently along with the perpendicular field at the semiconductor-oxide interface. Inversion channel scattering rates due to acoustic and polar optical phonons, ionized dopants, trapped charge, and interface roughness are considered. Transport within (0001) and (112¯0) oriented inversion layers are compared. Simulations of the MOSFET low-field mobility, incorporating previously published experimental results for threshold voltages and charge densities, are found to agree well with experimental results. The mobility of the (112¯0) channel is much larger (90 cm2/V s) than that of the (0001) channel (<40 cm2/V s) due to a reduction in interface states. Furthermore, the mobility has a temperature coefficient of approximately −3/2 for (112¯0) layers due to dominant phonon scattering and +1 for (0001) layers, where interface trap scattering dominates. Since the band structure is very similar, transport variations among the two crystal orientations are found to result largely from the enhanced interface trap density in the (0001)-oriented interfaces.

Electronic structure and optical properties of quantum-confined lead salt nanowires

In the framework of four-band envelope-function formalism, developed earlier for spherical semiconductor nanocrystals, we study the electronic structure and optical properties of quantum-confined lead-salt (PbSe and PbS) nanowires (NWs) with a strong coupling between the conduction and the valence bands. We derive spatial quantization equations, and calculate numerically energy levels of spatially quantized states of a transverse electron motion in the plane perpendicular to the NW axis, and electronic subbands developed due to a free longitudinal motion along the NW axis. Using explicit expressions for eigenfunctions of the electronic states, we also derive analytical expressions for matrix elements of optical transitions and study selection rules for interband absorption. Next we study a two-particle problem with a conventional long-range Coulomb interaction and an interparticle coupling via medium polarization. The obtained results show that due to a large magnitude of the high-frequency dielectric permittivity of PbSe material, and hence, a high dielectric NW/vacuum contrast, the effective coupling via medium polarization significantly exceeds the effective direct Coulomb coupling at all interparticle separations along the NW axis. Furthermore, the strong coupling via medium polarization results in a bound state of the longitudinal motion of the lowest-energy electron-hole pair (a longitudinal exciton), while fast transverse motions of charge carriers remain independent of each other.

Multi-interface roughness effects on electron mobility in a Ga0.5In0.5P/GaAs multisubband coupled quantum well structure

We analyse the effect of interface roughness scattering on low temperature electron mobility μn mediated by intersubband interactions in a multisubband coupled Ga0.5In0.5P/GaAs quantum well structure. We consider a barrier δ-doped double quantum well system in which the subband electron mobility is limited by the interface roughness scattering μIRn and ionized impurity scattering μimpn. We analyse the effect of the intersubband interaction and coupling of subband wavefunctions through the barrier on the intrasubband and intersubband transport scattering rates. We show that the intersubband interaction controls the roughness potential of different interfaces through the dielectric screening matrix. In the case of lowest subband occupancy, the mobility is mainly governed by the interface roughness of the central barrier. Whereas when two subbands are occupied, the interface roughness of the outer barrier predominates due to intersubband effects. The influence of the intersubband interaction also exhibits interesting results on the well width up to which the interface roughness dominates in a double quantum well structure.

Photonic valence/conduction subbands in optically generated photonic quantum wells: Tunable optical-delay line application

It has been shown that, in the absence of a control laser field, functional photonic superstructures can act as homogeneous photonic band gap structures formed via uniform corrugations of their background refractive indexes. When some sections of these structures are illuminated with such a laser, those sections become resonant (active) photonic band structures with higher refractive index contrast, forming photonic heterostructures. In this paper we study how by controlling the phase of quantum interference processes in such structures one can control alignment of such heterostructures, coherently generating photonic quantum wells with either conduction- or valencelike subbands (resonant transmission states). We show the energies of these subbands can be tuned by the control laser and demonstrate that their longitudinal mode profiles follow one-by-one correspondence with the envelope functions of the electron or hole subbands in electronic quantum well structures. In particular, we show how such photonic quantum wells can act as optically tunable time-delay lines, capable of increasing delay of a signal passing through them significantly via laser-induced control of optical confinements of the photonic subbands.

Spin susceptibility and polarization field in a dilute two-dimensional electron system in (111) silicon

We find that the polarization field, B_chi, obtained by scaling the weak-parallel-field magnetoresistance at different electron densities in a dilute two-dimensional electron system in (111) silicon, corresponds to the spin susceptibility that grows strongly at low densities. The polarization field, B_sat, determined by resistance saturation, turns out to deviate to lower values than B_chi with increasing electron density, which can be explained by filling of the upper electron subbands in the fully spin-polarized regime.

Temperature dependence of optical properties of InAs/GaAs self-organized quantum dots

Self-organized InAs/GaAs quantum dots (QDs) were grown by molecular beam epitaxy. The photoluminescence, its power, and temperature dependences have been studied for the ensembles of InAs QDs embedded in GaAs matrix to investigate the interband transition energies. Theoretical calculations of confined electron (heavy-hole) energy in the InAs/GaAs QDs have been performed by means of effective mass approximation, taking into account strain effects. The shape of the InAs QDs was modeled to be a convex-plane lens. The calculated interband transition energies were compared with the results of the photoluminescence spectra. The calculated interband transition energy from the ground electronic subband to the ground heavy-hole state was in reasonable agreement with the transition energy obtained by the photoluminescence measurement.

Electronic coupling in MgxZn1−xO/ZnO double quantum wells

The electronic coupling in ZnO/MgZnO double-quantum-well (DQW) structures grown with pulsed-laser deposition is investigated. The two ZnO quantum wells, separated by a thin MgxZn1−xO barrier (x≈0.14–0.16), have either an equal (SDQW) or a different thickness (ADQW). The authors report here a detailed investigation by spatially and spectrally resolved cathodoluminescence (CL). For the SDQW structures, the shift of the transition energy of the free exciton at room temperature to lower energies due to electron coupling, with decreasing barrier width, is in good agreement with effective mass theory. For a barrier thickness LB≤5 nm, the ADQWs show an additional recombination peak located spectrally between the luminescence maxima of the corresponding single quantum wells. It is tentatively attributed to a spatially indirect excitonic transition involving the electron subband of the narrow well and the heavy hole subband of the wide well. An extension of the barrier thickness to 300 nm allows an estimation of the exciton diffusion length via lateral CL scans. In the case here the diffusion length in the MgZnO barrier is about 135 nm at 10 K and below 75 nm at room temperature.

Emergent Lorentz Symmetry with Vanishing Velocity in a Critical Two-Subband Quantum Wire

We consider a quantum wire with two subbands of spin-polarized electrons in the presence of strong interactions. We focus on the quantum phase transition when the second subband starts to get filled as a function of gate voltage. Performing a one-loop renormalization group analysis of the effective Hamiltonian, we identify the critical fixed-point theory as a conformal field theory having an enhanced SU(2) symmetry and central charge 3/2. While the fixed point is Lorentz invariant, the effective "speed of light" nevertheless vanishes at low energies due to marginally irrelevant operators leading to a diverging critical specific heat coefficient.

Spin splitting of upper electron subbands in a SiO2/Si(100)/SiO2 quantum well with in-plane magnetic field

We observe a lifting of the twofold spin degeneracy of conduction-band electrons in an upper-valley subband with in-plane magnetic field in a SiO2/Si(100)/SiO2 quantum well, which is manifest in a splitting of a feature in the conductivity accompanying the occupation of the upper-valley subband. The splitting increases in proportion to the in-plane magnetic field, allowing the product of the effective g-factor and effective mass g∗m∗ to be obtained. The value remains constant over wide ranges of valley splitting, total electron density, and potential bias.

Low temperature electron mobility in Ga0.5In0.5P/GaAs quantum well structures

We analyse the low temperature subband electron mobility in a Ga0.5In0.5P/GaAs quantum well structure where the side barriers are delta-doped with layers of Si. The electrons are transferred from both the sides into the well forming two dimensional electron gas (2DEG). We consider the interface roughness scattering in addition to ionised impurity scattering. The effect of screening of the scattering potentials by 2DEG on the electron mobility is analysed by changing well width. Although the ionized impurity scattering is a dominant mechanism, for small well width the interface roughness scattering happens to be appreciable. Our analysis can be utilized for low temperature device applications.

Strain effects, potential profiles, and electronic properties of InAs/GaAs coupled double quantum dots with a disk shape

AbstractStrain potentials, potential profiles, and electronic subband energies of InAs/GaAs coupled double quantum dots (QDs) were calculated by using a finite‐difference method (FDM) taking into account shape‐based strain effects. The shape of the InAs/GaAs coupled double QDs on the basis of the transmission electron microscopy image was modeled to be a truncated hemi‐ellipsoid. The interband transition energies from the ground electronic subband to the ground heavy‐hole band (E1‐HH1) in the InAs/GaAs coupled double QDs, as determined from the FDM calculations taking into account strain effects, were in qualitatively reasonable agreement with the excitonic peaks corresponding to the (E1‐HH1) interband transition energies at several temperatures, as determined from the temperature‐dependent photoluminescence spectra. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Interband transition energies and carrier distributions of CdxZn1−xTe/ZnTe quantum wires

Interband transition energies and carrier distributions of the CdxZn1−xTe/ZnTe quantum wires (QWRs) were calculated by using a finite-difference method (FDM) taking into account shape-based strain effects. The shape of the CdxZn1−xTe/ZnTe QWRs was modeled to be approximately a half-ellipsoidal cylinder on the basis of the atomic force microscopy image. The excitonic peak energies corresponding to the ground electronic subband and the ground heavy-hole band (E1-HH1) at several temperatures, as determined from the FDM calculations taking into account strain effects, were in qualitatively reasonable agreement with those corresponding to the (E1-HH1) excitonic transition, as determined from the temperature-dependent photoluminescence spectra.

Influence of changing the forbidden-band gap on radiation frequency sweeping of quantum-well heterolasers

The principal features of changing (sweeping) the "instantaneous" emission frequency of quantum-well heterolasers as functions of the modulation frequency and depth and the constant component of the pump current when tuning the lasing frequency within the gain band are established using numerical modeling. The active medium is described in the framework of a two-band model with similar distributions of levels in subbands of electrons and holes assuming transitions between ground subbands with no k-selection rule. Emission frequency sweeping occurs because of changes in both the index of refraction of the active medium due to a variation in the concentration of non-equilibrium charge carriers and the forbidden-band gap. Emission frequency sweeping does not occur at low frequencies of current modulation that correspond with quasi-stationary lasing regimes. The modulation depth of the output radiation and, therefore, the emission frequency sweeping, also approach zero for the other limiting case of relatively high current modulation frequencies. The sweeping is greatest at intermediate current modulation frequencies. The sweeping value is approximately halved in certain instances by taking into account the change of forbidden-band gap of the semiconductor. In general, the sweeping value is determined by the combined effect of the modulation frequency and depth, the constant component of the pump current, and the spectral position of the laser emission frequency within the gain band.