Wide Parabolic Quantum Wells Research Articles

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.

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.

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.

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.

Coexistence of a two- and three-dimensional Landau states in a wide parabolic quantum well

Shubnikov--de Haas oscillations are measured in wide parabolic quantum wells with five to eight subbands in a tilted magnetic field. We find two types of oscillations. The oscillations at low magnetic fields are shifted toward higher field with the tilt angle increasing and can be attributed to two-dimensional Landau states. The position of the oscillations of the second type does not change with increasing the tilt angle which points to a three-dimensional character of these Landau states. We calculate the level broadening due to the elastic scattering rate $\ensuremath{\Gamma}=\ensuremath{\Elzxh}/2\ensuremath{\tau},$ where $\ensuremath{\tau}$ is the quantum time, and the energy separation between two-dimensional subbands, ${\ensuremath{\Delta}}_{\mathrm{ij}}{=E}_{j}\ensuremath{-}{E}_{i},$ in a parabolic well. For all levels we obtain ${\ensuremath{\Gamma}}_{j}\ensuremath{\sim}{\ensuremath{\Delta}}_{\mathrm{ij}},$ which means that the levels overlap, supporting the observation of three-dimensional Landau states. Surprisingly, we find that the lowest subband, which has a smaller energy separation from the higher level, does not overlap with these subbands and forms a two-dimensional state.

Depolarization shift in coupled quantum wells with tunable level spectrum

The degeneracy of electronic subband levels in spike-inserted wide parabolic quantum wells is investigated by far-infrared spectroscopy and magnetotransport measurements. A narrow potential barrier within the parabolic well divides the electron system into two well-separated but strongly coupled layers, leading to a drastical change of the subband system and the collective intersubband excitations. In contrast to a single robust resonance, which, obeying Kohn's Theorem is usually seen in a purely parabolic quantum well, the collective intersubband transitions now recover the complex coupling and splitting of the single-particle states. By applying two different experimental methods being sensitive to either the single-particle spectrum or the collective excitation spectrum, we are able to confirm the theoretical predictions that the size of the observed splitting caused by resonant subband coupling is not altered by collective effects. Rather, the corresponding intersubband resonance lines are shifted to higher energies in a depolarization like manner.

Quantum hall effect in a wide parabolic quantum well

We investigate the activated conductivity for Landau levels at lling factors v =1,2 in a parabolic Al xGa1-x As well down to 30 mK. We obtain a very small activation energy ~ 0.02 meV, which is almost 1-2 orders of magnitude smaller than the expected values for energy gaps in a wide quantum well. The resistivity minima vanish completely when a parallel component of magnetic field is applied. The collapse of the energy gaps in the wide parabolic well occurs due to the new electron correlated state, probably the charge density wave state predicted for a uniform wide electron slab in high magnetic field.

Landau-level lifetimes in PbTe nipi superlattices, PbTe/PbEuTe and InAs/AlSb quantum wells

Landau-level lifetimes are determined from saturation cyclotron resonance (CR) in wide parabolic wells, quantum wells and bulk PbTe–Pb 1− x Eu x Te systems. These narrow gap structures exhibit strong band non-parabolicity necessary to terminate the normally equi-spaced Landau-level ladder. It was not possible to saturate the bulk sample, but short lifetimes, of between 1.5 and 8 ps, were obtained for the wide parabolic well and the quantum well, respectively, utilising a multi-level rate equation model. We also report the first pump–probe cyclotron resonance result in an InAs–AlSb quantum structure. The pump–probe experiment provides a direct determination of the lifetime, giving τ=40 ps in this InAs–AlSb sample. This shows good agreement with an 8×8 k · p calculation.

Anomalous carrier lifetime enhancement and effective mass discontinuity observed during magnetic-field-induced subband depopulation in a wide parabolic quantum well

GaAs/AlGaAs wide parabolic quantum wells (WPQWs) contain uniform, quasi-two-dimensional slabs of electrons, with thickness ∼1000 Å. Electronic subbands are depopulated by magnetic fields B in the well plane. Small additional perpendicular fields induce Shubnikov–de Haas (SdH) oscillations which we use to determine the carrier density, effective mass, and scattering lifetime, throughout the two subband to one subband transition. Densities in each subband are consistent with a model in which electron–electron interactions simply renormalize the parabolic confining potential. Discontinuities in carrier mass and lifetime at depopulation are best explained by a model in which interactions flatten the potential profile. The second subband SdH lifetime is enhanced near depopulation, even though the onset of two subband transport increases sample resistance. Electrons in the lowest subband prove unexpectedly effective in screening local alloy disorder; this does not agree with long-standing explanations for the limited mobility in WPQW structures.

Electro-optic effects in GaAs/AlGaAs parabolic quantum well structures

We report on a systematic experimental and theoretical investigation of the interband electroabsorption in GaAs/AlGaAs parabolic quantum well (PQW) structures. In our experiments, we performed transmission and reflectance measurements using a sensitive double modulation technique. The measurements were carried out on p–i–n diode type PQW samples with two different well widths. In the sample with the narrow PQW, the discrete structure of the interband subband transitions can be resolved in the electroabsorption spectra. In the sample with the wide PQW, and therefore a small subband spacing, these transitions can no longer be resolved individually due to the broadening. Their superposition, however, results in a red-shifting, quasi-linear pattern in the electroabsorption spectra, which is modulated by the blue-shifting, non-linear fan of the ‘ arabolic Franz–Keldysh effect’. The experimental results are in very good agreement with calculated single-particle absorption spectra based on a simple harmonic oscillator model.

Conductance fluctuations in PbTe wide parabolic quantum wells

We report on conductance fluctuations which are observed in local and non-local magnetotransport experiments. Although the Hall bar samples are of macroscopic size, the amplitude of the fluctuations from the local measurements is close to e 2/ h. It is shown that the fluctuations have to be attributed to edge channel effects.