Binding energies of excitons in symmetric and asymmetric quantum wells in a magnetic field

The binding energy of the exciton in the symmetric and asymmetric GaAs/Ga_1 − xAl_xAs quantum wells is calculated with the use of a variational approach. Results have been obtained as a function of the potential symmetry, and the size of the quantum well in the presence of an arbitrary magnetic field. The applied magnetic field is taken to be parallel to the axis of growth of the quantum well structure. The role of the asymmetric barriers, magnetic field, and well width in the excitonic binding is discussed as the tunability parameters of the GaAs/Ga_1 − xAl_xAs system.     

Band structure of a cylindrical GaAs/AlxGa1 − xAs superwire

We investigate the existence of a band structure in GaAs/Al_xGa_1 − xsuperlattices with cylindrical symmetry, namely GaAs/Al_xGa_1 − xAs cylindrical superwires. These systems consists of a large number of concentric GaAs and Al_xGa_1 − xAs alternate cylindrical shells around a central GaAs cylindrical wire. Despite the radial configuration (that breaks the translational symmetry) and the electron confinement in the central three-dimensional well, a band structure can emerge depending on the number and thickness of the cylindrical shells.     

Influence of the antireflection,window, and active layers on optical properties of exponential-doping transmission-mode GaAs photocathode modules

In order to well study the influence of the thickness and doping concentration on optical properties of transmission-mode GaAs photocathode, three exponential-doping and one uniform-doping photocathode modules were prepared by molecular beam epitaxy with a structure of glass/Si₃N₄/Ga_1 ? xAl_xAs/GaAs. By use of the spectrophotometer, the optical properties were separately measured including the reflectivity and transmissivity curves. Based on thin film optical principles, the optical properties and their integral values are calculated by matrix formula for the four-layer photocathode module. The result shows that the antireflection and window layers affect the peak and valley of the optical property curves and the active layer influences the absorptivity values of the transmission-mode cathode modules. The photocathode module has high absorptivity within the response waveband when the optimal module has the Si₃N₄ antireflection layer of 0.1 μm, the Ga_1 ? xAl_xAs window layer of more than 0.4 μm, and the GaAs active layer of 1.5 μm–2 μm and low average doping concentration.     

Raman scattering from confined phonons in GaAs/AlGaAs quantum wires

We report on photoluminescence and Raman scattering performed at low temperature (T =  10 K) on GaAs/Al_0.3Ga_0.7As quantum-well wires with effective wire widths ofL =  100.0 and 10.9 nm prepared by molecular beam epitaxial growth followed by holographic patterning, reactive ion etching, and anodic thinning. We find evidence for the existence of longitudinal optical phonon modes confined to the GaAs quantum wire. The observed frequency at ο_L10 =  285.6 cm⁻¹forL =  11.0 nm is in good agreement with that calculated on the basis of the dispersive dielectric continuum theory of Enderleinas applied to the GaAs/Al_0.3Ga_0.7As system. Our results indicate the high crystalline quality of the quantum-well wires fabricated using these techniques.     

Doping profile effects on modulation-doped single nonabrupt GaAs/AlxGa1 − xAs quantum wells

We study inhomogeneous doping effects on the confinement properties of modulation-doped single nonabrupt GaAs/Al_xGa_1 − xAs quantum wells. We describe the inhomogeneous doping using error function profiles, and we solve self-consistently the coupled Schrödinger (with a position dependent kinetic energy operator) and Poisson equations to obtain the electron energy levels. When the nonabrupt interfaces (spacer layer) are 10Å(100Å) wide and the presence of Si-dopant density in a 100 Å GaAs well region is only 10% of the Si-dopant density in the Al_0.3Ga_0.7As barriers, the lowest intersubband transition energy increases 37 meV in comparison with that calculated within the homogeneous doping-abrupt interface picture.     

Accumulation layer and interface effects in doped nonabrupt GaAs/AlxGa1 − xAs single quantum wells

The electron energy levels in doped nonabrupt GaAs/Al_xGa_1 − xAs single quantum wells 100 Å wide are calculated. Interface widths varying from zero to four GaAs unit cells are taken into account, as well as band bendings of 0–90 meV. It is shown that interface effects on the energy levels are important and sensitive to the level of doping. When interfaces of only two GaAs unit cells and a band bending of 40 meV are considered, the ground-state (first excited state) energy level shifts toward energies as high as 4 meV (20 meV).     

Conductance fluctuations of chiral metals

We study conductance fluctuations of the edge-state sheath that forms in the integer quantum Hall effect from the coupled edge states of a GaAs/Al_xGa_1 − xAs multilayer. Comparison of the measured variance in the vertical conductance to recent theoretical predictions for mesoscopic fluctuations suggests dephasing lengths of 1–10 μm around the sheath perimeter at low temperatures. However, inconsistencies in the estimated inelastic lengths indicate that present understanding of dephasing on the sheath is incomplete.     

Functional heterostructure-emitter bipolar transistor (HEBT) with graded-confinement and pseudomorphic-base structure

In this paper, a new functional heterostructure-emitter bipolar transistor (HEBT) with a graded Al_xGa_1 − xAs confinement layer and a pseudomorphic InGaAs/GaAs base structure is fabricated and demonstrated. Due to the insertion of an InGaAs quantum well (QW) between the emitter–base (E–B) junction, the valence band discontinuity can be enhanced and a high emitter injection efficiency may be achieved. Furthermore, the potential spike of the graded Al_xGa_1 − xAs/GaAs heterojunction is expected to be smoothed out which results in a lower offset voltage. The excellent transistor characteristics include a high current gain of 120 and a low offset voltage of 100 mV. In addition, an interesting S-shaped multiple negative differential resistance (MNDR) phenomenon is observed in the inverted operation mode. This may be attributed to an avalanche multiplication and a sequential two-stage barrier lowering effect resulting from the accumulation of holes at the base and electrons at the InGaAs QW, respectively. Consequently, owing to the remarkable transistor performance and MNDR characteristics, the studied HEBT shows good promise for applications in amplifiers and multiple-valued logic circuits.     

Donor binding energies in GaAs quantum wells considering the band nonparabolicity effects and the wavefunction elongation

The donor binding energies in finite GaAs/Ga_xAl_1 − xAs quantum wells have been calculated by considering the confinement of electrons, which increases as the well width increases. The variational solutions have been improved by using a two-parameter trial wavefunction, and by including the conduction band nonparabolicity. It is shown that the method used gives results in agreement with those obtained in the experiments on the effective mass and the donor binding energy, both of which are strongly dependent on the well width.     

Optical nutation in a magnetized semiconductor quantum well structure

Using the semiclassical coherent radiation—semiconductor interaction model, optical nutation has been analysed in aGaAs / Al_xGa_1 − xAs quantum well structure (QWS) assumed to be immersed in a moderately strong magnetic field and irradiated by a not-too-strong near band gap resonant femtosecond pulsed Ti–sapphire laser. The finite potential well depth of the QWS and the Wannier–Mott excitonic structure of the crystal absorption edge is taken into account. The excitation intensity is assumed to be below the Mott transition where the various many-body effects have been neglected with adequate reasoning. Numerical analysis made for a GaAs quantum well of thickness  ∼  100 Åand the confining layers ofAl_xGa_1 − xAs withx =  0.3 at intensity I ≤  5  ×  10⁶Wcm ^− 2reveals that the real and imaginary parts of the transient complex-induced polarization are enhanced with an increase in the magnetic field and their ringing behaviour confirms the occurrence of optical nutation in the QWS.     

Exciton relaxation in Ga1 − xInxAs/GaAs self-organized quantum dots

The exciton dynamics in Ga_1 − xIn_xAs/GaAs self-organized quantum dots grown on GaAs (111)B substrates are studied by the time-resolved photoluminescence (PL). We have found the intra-dot exciton relaxation by the reduction of the linewidth and peak energy and also by the energy-dependent PL rise time in the transient PL spectra. Compared with the energy relaxation in the reference quantum wells, we have confirmed that the exciton relaxation in three-dimensionally confined quantum dots is slower than in the quantum wells.     

Ternary effects on the optical interface phonons in onion-like GaN/AlxGa1 − xN quantum dots

The interface optical phonons and its ternary effects in onion-like quantum dots are studied by using dielectric continuum model and the modified random-element isodisplacement model. The dispersion relations, the electron–phonon interactions and ternary effects on the interface optical phonons are calculated in the GaN/Al_xGa_1 − xN onion-like quantum dots. The results show that aluminium concentration has important influence on the interface optical phonons and electron–phonon interactions in GaN/Al_xGa_1 − xN onion-like quantum dots. The frequencies of interface optical phonons and electron–phonon coupling strengths change linearly with increase of aluminium concentration in high frequency range, and do not change linearly with increasing aluminium concentration in low frequency range.     

The static and dynamic screening of power loss of a two-dimensional electron gas

Experimental results concerning the well-width dependence of the acoustic-phonon-assisted energy relaxation of a two-dimensional electron gas in GaAs/Ga_1 − xAl_xAs quantum-well structures are compared with theoretical models that involve piezoelectric and deformation-potential scattering and the effects of static and dynamic screening of the electron–acoustic phonon interaction. It is shown that screening only slightly modifies the predictions of the approximate calculations.     

An oxygen permeable membrane La0.8Sr0.2(Ga0.8Mg0.2)1 − xCrxO3 − δ for a reduced atmosphere

Mixed electron hole and oxide ion conducting perovskite-type oxides, La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ (0 ≤ x ≤ 1.0)_, were prepared by solid state reaction. The phase stability and the oxygen permeation properties of the oxides were examined as a function of the content of Cr. La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ has a perovskite related tetragonal phase with x = 0.1 to 0.8. The total electrical conductivity of La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ increases with increasing x. The oxygen permeation flux across the La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ membranes at higher temperatures increases with x up to x = 04. The maximum oxygen permeation flux of 1.6 × 10^? 7 mol^? 1 cm^? 2 at 1100 °C in a oxygen activity gradient of air/10^? 2 Pa is observed in La_0.8Sr_0.2(Ga_0.8Mg_0.2)_0.6Cr_0.4O_3 ? δ. This perovskite-type oxide is stable under an oxygen partial pressure of 7 × 10^? 10 Pa at 1000 °C.     

Electrical conductivity and thermal expansion of the oxy-cuspidine Gd4Al2O9 substituted with Ca and Sr

The series of Gd_4 ? xM_xAl₂O_9 ? x/2 (M = Ca, Sr) with x = 0, 0.01, 0.05, 0.10 and 0.25 was prepared by the citrate complexation method. Both Gd_4 ? xCa_xAl₂O_9 ? x/2 and Gd_4 ? xSr_xAl₂O_9 ? x/2 show the monoclinic cuspidine structure with space group of P2₁/c up to 0.05–0.1 and 0.01–0.05 mol for Ca and Sr, respectively. Beyond the substitution limit of Gd₄Al₂O₉, GdAlO₃ and SrGd₂Al₂O₇ appear as additional phases. The highest electrical conductivity obtained at 900 °C yielded σ = 1.49 × 10^? 4 S/cm for Gd_3.95Ca_0.05Al₂O_8.98. In comparison, the conductivity of pure Gd₄Al₂O₉ was σ = 1.73 × 10^? 5 S/cm. The conductivities determined are in a similar range as those of other cuspidine materials investigated previously. The thermal expansion coefficient of Gd₄Al₂O₉ at 1000 °C was 7.4 × 10^? 6 K^? 1. The phase transition between 1100 and 1200 °C reported earlier changes with increasing substitution of Ca and Sr.     

Spatially varying effective mass effects on energy level populations in semiconductor quantum devices

For the analysis and design of semiconductor intersubband devices, accurate values for the Fermi energy and the subband electron population are needed. The effect of the position-dependent electron effective massm ^*  (z) is commonly neglected in the determination of these two intersubband device parameters. This approach is nearly valid for single-well devices in the GaAs/Al _xGa_1 − x As material system. However, in multiple-coupled-well devices, the variable nature of the effective mass must be taken into account. In material systems other than the ones based on GaAs, failure to include the position-dependence of the electron mass may give rise to significant errors in the values of the Fermi energy and other device parameters such as the intersubband absorption coefficient. In this paper, the effects ofm ^*  (z) on the Fermi level and the intersubband charge distribution are explored and quantified.Theoretical formulation for the intersubband Fermi energy and the subband electron distribution, with the inclusion of position-dependent electron mass, is presented. The eigenenergies of the intersubband structures are obtained by solving the single-band effective-mass Schroedinger equation using the argument principle method. The electron distribution and the Fermi energy are calculated using both the approximate method ( m₀ ^* ) and the rigorous formulation [ m ^* (z)], and the relative differences in the corresponding values are presented. It is demonstrated that these differences are small in the GaAs/Al _xGa_1 − x As material system, but can become very significant in other materials. In addition, the variable nature of carrier effective mass plays an important role in other types of devices such as interband quantum well photodetectors and lasers that employ optical transitions between the valence and the conduction bands. Electronic devices such as the resonant tunneling diode are also affected by the position-dependence of carrier mass and thus the results are applicable to both optoelectronic and electronic quantum devices.     

Nonlinear properties in InxGa1 − xAs/GaAs multiple quantum well laser structures

In this paper we investigated nonlinear properties and lasing in In_xGa_1 − xAs/GaAs multiple quantum wells grown by metal-organic chemical vapour deposition. A systematic study, performed by high excitation photoluminescence measurements as a function of excitation intensity, allowed us to identify the minimum well width for observing stimulated emission from well states. We also determined the threshold for stimulated emission for well and barrier lasing. Radiative recombination energies are identified by using theoretical data obtained in the effective mass approximation, including boundary conditions and strain.     

Exciton binding energy in a double quantum well: effect of the barrier shift

We have calculated the exciton binding energy in an Al _xGa_1 − x As  / GaAs double quantum well by a variational envelope function procedure using a simple two-band model. The influence of the shift of the AlAs separating barrier, introducing an asymmetry into the system, on the value of the exciton binding energy has been analysed. It has been observed that this shift induces significant changes of the exciton binding energy—even several meVs in the case of very thin barriers.     

Exciton binding energies in GaN/AlxGa1 − xN pseudomorphic quantum wells

Interband transitions of pseudomorphic GaN/Al_xGa_1 − xN quantum wells are analysed theoretically with respect to the piezoelectric field utilizing a 6  ×  6 Rashba–Sheka–Pikus (RSP) Hamiltonian. Band structure modifications due to the built-in Stark effect explain a shift of the emission peak in GaN/Al_0.15Ga_0.85N of up to 400 meV. Quantum well exciton binding energies are calculated by the variational method and are discussed in terms of spatial separation of electrons and holes by the built-in electric field, as well as the interaction between valence subbands.     

Optical transitions in AlGaAs/GaAs quantum wires on GaAs(6 3 1) substrates studied by photoreflectance spectroscopy

We present the synthesis and characterization of a system of self-assembling GaAs quantum wires (QWRs) embedded in Al_x Ga_1−x As barriers grown by molecular beam epitaxy on GaAs(6 3 1)-oriented substrates. We studied the optical transitions in the QWRs as a function of temperature (T) by photoreflectance (PR) spectroscopy. The energy transitions were extracted from the PR spectra employing the third-derivative functional form, and they were compared with the transitions theoretically calculated from both, a model of QWRs with cylindrical geometry and a model of a conventional square quantum well. The results show a good agreement between experimental and theoretical data in the case of the QWR model, and from this comparison we were able to identify up to 12 different transitions in the PR spectra and to study their behavior dependent on temperature.