Wide Parabolic Quantum Research Articles

Stark shift and exciton binding energy in parabolic quantum dots: hydrostatic pressure, temperature, and electric field effects

ABSTRACT The temperature, hydrostatic pressure, and external electric field effects on the confined exciton in cylindrical quantum dots by considering a parabolic confining potential are investigated. The effects of these external perturbations on the binding energy and interband emission energy are calculated numerically by adopting the variational method within the effective mass approximation. Our findings indicate that the exciton binding energy and interband emission energy depend significantly on decreasing electric field, diminishing temperature, and enhancing the hydrostatic pressure. The contribution of the electric field on the binding energy becomes more important for wide axial parabolic quantum well. We have also shown that the Stark effect of exciton diminishes almost linearly with increasing hydrostatic pressure. Furthermore, the electric field and the quantum dot height lead to enhancing the Stark shift. The behaviour of the excitonic Stark shift as a function of the applied electric field proves the existence of a dipole moment. This physical parameter becomes important for the weak confinement regime.

Dynamics of photogenerated carriers near magnetic field driven quantum phase transition in aperiodic multiple quantum wells

Time-resolved magneto-photoluminescence was employed to study the magnetic field induced quantum phase transition separating two phases with different distributions of electrons over quantum wells in an aperiodic multiple quantum well, embedded in a wide AlGaAs parabolic quantum well. Intensities, broadenings and recombination times attributed to the photoluminescence lines emitted from individual quantum wells of the multiple quantum well structure were measured as a function of the magnetic field near the transition. The presented data manifest themselves to the magnetic field driven migration of the free electrons between the quantum wells of the studied multiple quantum well structure. The observed charge transfer was found to influence the screening of the multiple quantum well and disorder potentials. Evidence of the localization of the electrons in the peripheral quantum wells in strong magnetic field is presented.

Excitonic spin-splitting in quantum wells with a tilted magnetic field

This work aims to investigate the effects of magnetic field strength and direction on the electronic properties and optical response of GaAs/AlGaAs-based heterostructures. An investigation of the excitonic spin-splitting of a disordered multiple quantum well embedded in a wide parabolic quantum well is presented. The results for polarization-resolved photoluminescence show that the magnetic field dependencies of the excitonic spin-splitting and photoluminescence linewidth are crucially sensitive to magnetic field orientation. Our experimental results are in good agreement with the calculated Zeeman splitting obtained by the Luttinger model, which predicts a hybridization of the spin character of states in the valence band under tilted magnetic fields.

Effect of quantum well shape and width on deep ultraviolet emission in AlGaN nanowire LEDs

The effect of quantum well shape and width on the emission properties of nanowire ultraviolet LEDs is experimentally investigated. Nanowire LEDs are grown by plasma-assisted molecular beam epitaxy and their electroluminescence is compared. A two- to three-fold increase in optical output is observed from 5 nm wide parabolic quantum wells compared with square quantum wells. This increased output is attributed to the both increased electron-hole overlap in agreement with band edge modeling, and supports previous theoretical predictions of decreased Auger recombination due to softening of the potential well in parabolic quantum wells. It is experimentally observed that when the quantum well width is decreased to 2 nm, the parabolic quantum wells exhibit a twofold output improvement over the square wells at low injection. However, at high current injection, the parabolic and square quantum wells perform similarly. Peak emission wavelengths for the nanowire LEDs is pushed into the deep ultraviolet by decreasing the AlGaN quantum well width. The emission wavelength shifts from 315 to 290 nm when the width is decreased from nominally 5 to 0.5 nm.

Quantum confined stark effect in wide parabolic quantum wells: real density matrix approach

We show how to compute the optical functions of wide parabolic quantum wells (WPQWs) exposed to uniform electric F applied in the growth direction, in the excitonic energy region. The effect of the coherence between the electron-hole pair and the electromagnetic field of the propagating wave including the electron-hole screened Coulomb potential is adopted, and the valence band structure is taken into account in the cylindrical approximation. The role of the interaction potential and of the applied electric field, which mix the energy states according to different quantum numbers and create symmetry forbidden transitions, is stressed. We use the real density matrix approach (RDMA) and an effective e-h potential, which enable to derive analytical expressions for the WPQWs electrooptical functions. Choosing the susceptibility, we performed numerical calculations appropriate to a GaAs/GaAlAs WPQWs. We have obtained a red shift of the absorption maxima (quantum confined Stark effect), asymmetric upon the change of the direction of the applied field (F → −F), parabolic for the ground state and strongly dependent on the confinement parameters (the QWs sizes), changes in the oscillator strengths, and new peaks related to the states with different parity for electron and hole.

Excitonic spectra of wide parabolic quantum wells

The optical properties of wide Quantum Wells are considered, taking into account the screened electron-hole interaction potential and parabolic confinement potentials, different for the electrons and for the holes. The role of the interaction potential which mixes the energy states according to different quantum numbers is stressed. The results obtained by our method are in agreement with the observed spectra and give the possibility to the assessment of the resonances.

Excitons in undoped AlGaAs/GaAs wide parabolic quantum wells

In this work the electronic structure of undoped AlGaAs/GaAs wide parabolic quantum wells (PQWs) with different well widths (1000 Å and 3000 Å) were investigated by means of photoluminescence (PL) measurements. Due to the particular potential shape, the sample structure confines photocreated carriers with almost three-dimensional characteristics. Our data show that depending on the well width thickness it is possible to observe very narrow structures in the PL spectra, which were ascribed to emissions associated to the recombination of confined 1s-excitons of the parabolic potential wells. From our measurements, the exciton binding energies (of a few meV) were estimated. Besides the exciton emission, we have also observed PL emissions associated to electrons in the excited subbands of the PQWs.

Electric-field control of electron interference effects in 2D semiconductor nanostructures with parabolic quantum wells

The possibility of the influence of a constant electric field on spatial reproduction effects for quantum-mechanical current density, which arise during interference of electron waves in 2D semiconductor nanostructures, is studied theoretically. It is found that, in structures comprising a narrow rectangular quantum well and a wide parabolic quantum well placed successively in the direction of propagation of an electron wave, the transverse current density distribution existing at the input of the wide quantum well is reproduced in this well with a certain accuracy in periodically repeated cross sections. The inhomogeneous current density distribution arises because of the interference of electron waves propagating in the wide quantum well simultaneously in different quantum-dimensional subbands. It is shown that it is possible to control these effects via the use of a constant electric field perpendicular to the axis of the structure in the region of the wide quantum well.

Transport properties of AlGaAs/GaAs parabolic quantum wells

We report on the magnetotransport properties of 100 nm wide parabolic quantum wells and observe an enhancement of the Hall resistance in one sample but not the other. This phenomenon is likely related to the effective thickness of the electronic slab. We also observe a parabolic negative magnetoresistance originating from electron-electron interactions when only one subband is occupied in one of the samples. The interaction correction to the Drude conductivity is extracted using two methods. We find that the extracted interaction correction increases with increasing tilted angle, for which two possible explanations are given.

Many-body effects in wide parabolic AlGaAs quantum wells

Photoluminescence measurements at different temperatures have been performed to investigate the optical response of a two-dimensional electron gas in n-type wide parabolic quantum wells. A series of samples with different well widths in the range of 1000–3000Å was analyzed. Many-body effects, usually observed in the recombination process of a two-dimensional electron gas, appear as a strong enhancement in the photoluminescence spectra at the Fermi level at low temperature only in the thinnest parabolic quantum wells. The suppression of the many-body effect in the thicker quantum wells was attributed to the decrease of the overlap between the wavefunctions of the photocreated holes and the two-dimensional electrons belonging to the highest occupied electron subband.

Electron spin resonance in a wide parabolic quantum well

Electron spin resonance spectra were obtained by electrical detection in a remotely Si-doped 400 nm wide GaAs/AlAs digital parabolic quantum well, yielding $g=\ensuremath{-}0.423$ and $g=\ensuremath{-}0.405$ in perpendicular and parallel fields of 5 T, respectively. The tilt angle dependence of $g$ in the wide parabolic quantum well is qualitatively different from the previously reported $g$-factor anisotropy in narrow GaAs quantum wells. The $g$-factor angle dependence, together with the electrically detected electron-nuclear double resonance spectra, indicate that the spin resonance signals originate mainly from the center of the well. Furthermore, the nuclear spin relaxation in the three-dimentional electron gas (3DES) is found to be independent of temperature in the 1.5--5 K range, while the Korringa law is observed in the 2DES.

High mobility of a three-dimensional hole gas in parabolic quantum wells grown on GaAs(311)A substrates

The transport properties of a three-dimensional hole gas were investigated in wide parabolic quantum wells grown by molecular beam epitaxy on top of GaAs(311)A substrates. The p-type doping was performed using silicon and the parabolic potential was achieved with the digital-alloy technique. Hall-effect and Shubnikov–de Haas measurements carried out at low temperature revealed that the carrier mobility was more than twice higher than the one usually obtained from similar samples grown on GaAs(100) substrates using beryllium.

Effects of Spatial Reproduction as a Result of the Interference of Electron Waves in Two-Dimensional Semiconductor Nanostructures with Parabolic Quantum Wells

Effects of spatial nonuniformity for the probability flux density jx(x, z) or for the density of the quantum-mechanical current ejx(x, z), where e is the elementary charge) are studied. These effects arise in two-dimensional semiconductor nanostructures that consist of thin rectangular and wide parabolic quantum wells that are consecutively arranged in the direction of the electron-wave propagation (the x axis) and are oriented along the dimensional-quantization axis z. A nonuniform distribution of jx(x, z) arises as a result of interference of electron waves that propagate simultaneously in a wide quantum well over different quantum-dimensional subbands. Particular attention is paid to the effects of the spatial reproduction of electron waves in the nanostructures under consideration. It is shown that the transverse distribution jx(0, z) existing at the entry to the wide quantum well is reproduced to a certain accuracy at a distance of X1 from the entry. In addition, the initial distribution jx(0, z) is reproduced periodically in the sections Xq = qX1 (coefficients q are integers). The results of numerical calculations of magnitudes of these effects in the structures that are symmetric with respect to the z axis are reported; a modification of the effects under consideration in asymmetric nanostructures is considered.

Transport of the quasi-three-dimensional hole gas in a magnetic field in the ultra-quantum limit

We observed anomalous oscillations in the magnetoresistance of quasi-three dimensional holes in a magnetic field in the ultra-quantum limit. For this study we used wide p-type AlGaAs parabolic quantum wells in the presence of the in-plane magnetic field. We attribute the anomalous oscillations to the formation of the correlated electronic states.

Charge density wave instability in a parabolic well in perpendicular magnetic field

We report magnetotransport measurements of a wide parabolic quantum well with quasi-two-dimensional holes in a tilted magnetic field. We find that in a perpendicular magnetic field the magnetoresistance minima at filling factor ν=1 are missed. This effect is attributed to the formation of the charge density wave (CDW) state proposed theoretically by Brey for quantum Hall system with several subbands (Phys. Rev. B 44 (1991) 3772). In the tilted magnetic field CDW state is destroyed and quantum Hall effect is recovered.

Evolution of the two-dimensional towards three-dimensional Landau states in wide parabolic quantum well

Shubnikov-de Haas oscillations are measured in wide parabolic quantum wells with 2 subbands occupied in tilted magnetic field. Experiments reveal anticrossing of the Landau level (LL) belonging to lowest subband and last LL belonging to the second subband. Such anticrossing occurs due to decrease of the energy of the LL with tilt angle Θ. This observation is supported by the measurements of the Θ-dependence of the activation energy in the quantum Hall effect regime.

Landau levels in two and three-dimensional electron gases in a wide parabolic quantum well

Shubnikov-de Haas oscillations are measured in a wide parabolic quantum well with 6 subbands in a tilted magnetic fi eld. We find two types of oscillations. The oscillations at low magnetic field are shifted towards higher fields with tilted angles, and can be attributed to the two-dimensional Landau state at the bottom subband. The position of the second type oscillations do not shift with tilted angles indicating a three-dimensional character of the Landau state formed by the highest subbands. The bottom level in the quantum well is not overlapped with the highest subbands due to the enhanced quantum scattering time of the lowest subbands.

Low-temperature electron mobility in parabolic quantum wells

We present a theoretical study on the electron mobility and scattering mechanism in a remotely doped AlGaAs wide parabolic quantum well. Electron mobilities in different subbands are calculated from the self-consistent results of the subband energy and wavefunction in the system. The scattering due to ionized impurities and alloy disorder is considered. We show theinterplay of the different scattering mechanisms.

Magnetotransport of a quasi-three-dimensional electron gas in the lowest Landau level

We have observed features in the magnetoresistance of a wide parabolic quantum well in the presence of the in-plane magnetic field at field three times larger than fundamental field corresponding to the depopulation of the last Landau level. The magnetoresistance structures shift with a specific sample parameter, such as potential width. We suggested the formation of correlated states of a three-dimensional electron gas at Landau filling factor 1/3, in analogy with a two-dimensional fractional quantum Hall effect.

Geometrical resonance in the resistivity of wide quantum wells

We studied size-effects in wide parabolic quantum wells by measuring the low field magnetoresistance in perpendicular and parallel magnetic field configurations. We have also studied the evolutions of the resistivity oscillations as a function of the electron sheet density N S, by in situ illumination of the samples at the temperature of 1.5 K . We observed the maximum in the resistivity in the perpendicular magnetic field when the cyclotron radius was equal to the width of the electronic slab R c= W e. It allows to determine the width W e as a function of the electron density. The oscillation amplitude decreases with N S, which can be attributed to the increase of the probability of electrons to be specularly reflected by the boundary with increase of the electron slab width. In parallel magnetic field we also found oscillations corresponding to R c≈ W e and R c/ W e≈2.2 in agreement with quantum-mechanical calculations of the size effects in narrow quantum wires.