GaAs-AlAs Quantum Research Articles

Influence of interface modes compared to confined modes on phonon-assisted cyclotron resonance effect in quantum wells

Abstract We study the influence of interface phonon modes on the phonon-assisted cyclotron resonance (PACR) effect in GaAs-AlAs quantum well (QW) structures by calculating the absorption power (AP) based on the operator projection technique in comparison with bulk and confined phonon modes in previous study [N. D. Hien, Superlattices Microstruct. 131 (2019) 86]. The full width at half maximum (FWHM) of the PACR absorption peak is obtained by utilizing the profile technique. On the other hand, the FWHM is described as functions of the QW width and temperature for different kinds of phonon modes (interface, bulk, confined modes) presented to compare. The calculations clearly demonstrate important roles of interface-mode as the QW width is small enough. Also, the result indicates that the interface optical modes also gives a considerable contribution to the electron–phonon scattering as compared with that of bulk and confined optical phonon modes.

Spatial electric and axial magnetic fields effect in GaAs–AlAs quantum wires

In this work, we investigated the effect of both spatial electric and axial magnetic fields on the impurity ground state energy and the polarization of a donor impurity in square GaAs/AlAs QWWs. We show that the impurity ground state energy and the polarization depends strongly not only on the width of QWW, but also on the applied spatial component of the electric field and the impurity position.

Spatial electric field effect in GaAs–AlAs quantum wires

The response of an electron to a three-dimensional electric field in an infinite quantum well wire of square cross-section is investigated within a variational scheme. The ionization energy and the polarization are calculated for different locations of impurity ion. It is found that the results for the spatial electric field differ from the previous results found for the electric field applied in the direction perpendicular to the wire axis. The ionization energy weakens rapidly with the axial component of the field as the polarization of the carrier distribution intensifies.

Stability of electron-hole plasma in type-I and type-II GaAs-GaAlAs single quantum wells

We calculate and compare energies of the electron-hole plasmas and ground-state excitons in type-I and type-II GaAs-GaAlAs single quantum wells with various structural conditions. Calculations are performed with a flexible and numerically stable method using realistic material parameters. We confirm that the exciton states are stable in type-I quantum wells for well widths up to 250 \AA{}, as described in previous work [G. E. W. Bauer and T. Ando, Phys. Rev. B 34, 1386 (1986)]. We find that the electron-hole plasma becomes stable, particularly in narrow type-II quantum wells, e.g., about 2.5 meV below the ground state exciton in GaAs-AlAs quantum wells whose widths are 6 and 4 monolayers, respectively. Moreover, calculations including electric fields reveal two different conditions of the stable electron-hole plasma: one is that electrons and holes are separated and the other is that they are confined in the narrow regions. The results suggest the possibility of observing the electron-hole plasma as the lowest excited state in the GaAs-GaAlAs quantum well.

Strategies for reducing the emission wavelength of GaAs–AlAs quantum cascade lasers

We report two novel methods for reducing the emission wavelength of GaAs–AlAs quantum cascade lasers. We demonstrate that for lasing to occur electron injection into the upper laser level must proceed via Γ states confined below the lowest X state in the injection barrier. The limit this places on the minimum operating wavelength (λ≈8 μm) is overcome by utilising a novel injection barrier design to achieve lasing at λ=7.2 μm . In addition, we have deposited InAs monolayers in the active regions of a GaAs–AlAs quantum cascade laser to decrease the lasing transition wavelength. The InAs monolayers have a minimal effect on the upper laser level but decrease the confinement energy of the lower laser level. Thus a significantly reduced emission wavelength ( 8.3 μm compared with 11.2 μm ) is achieved whilst maintaining very similar laser performance.

Binding energies of ground and excited donor states bound toXvalleys in GaAs/AlAs type-II quantum wells

We calculate the binding energies of $1s,$ $2s,$ ${2p}_{x},$ ${2p}_{y},$ and ${2p}_{z}$ donor states bound to X valleys in type-II GaAs-AlAs quantum well structures using an anisotropic variational method that enables us to take into account the effective-mass anisotropy and the quantum confinement. These binding energies present a strong dependence on the effective mass and the valley symmetry. Intradonor $1\stackrel{\ensuremath{\rightarrow}}{s}2p$ transitions energies are shown to be up to 1000 ${\mathrm{cm}}^{\ensuremath{-}1},$ which is well above the energies usually observed for donors in type-I quantum structures.

Modulation of Fano Resonance by a Perpendicular Electric Field in GaAs-AlAs Quantum Well

We study the formation of Fano resonance in excitonic states in GaAs–AlAs quantum wells. Our calculation indicates that an electric field applied perpendicularly to the layers modulates the Fano resonance. We have found that for a given excitonic state there exists a field-range for which Fano resonance is well discernible. The electric field effects are more pronounced for thick wells.

Excited states of donors bound toXvalleys in GaAs–AlAs type II structures

We calculate the binding energies of 2s donor states bound tovalleys in type II GaAs–AlAs quantum well structures using an anisotropic variational method which enables us to take into account the effective mass anisotropy and quantum confinement. For a comparative study, we use two sets of effective masses obtained from different measurements [B. Rheinländeret al., Phys. Stat. Sol. (b)49, K167 (1972) and M. Goiranet al., Physica B177, 465 (1992)].

Effects of localized perturbations on Fano resonance in GaAsAlAs quantum well

We study the Fano resonance in the E 2-HH 2 absorption line in GaAsGaAlAs quantum wells. Our calculation indicates that this resonance is stronger in wells with larger width and lower barrier height. On the other hand, the PLE measurements and the calculation show that a repulsive and localized perturbation, consisting of an AlAs monolayer inserted at the well center, makes this resonance more discernable and an attractive and localized perturbation, consisting of an InAs monolayer inserted at the same location has an opposite effect.

Exciton localization and interisland exciton diffusion in interrupted-growth GaAs–AlAs quantum well

We present a study of the exciton localization and the interisland exciton diffusion in interrupted-growth thin GaAs–AlAs quantum wells (QW), by photoluminescence (PL). These quantum wells include extended monolayer flat regions. We have shown that, for a given position on the sample, the exciton localization is stronger in the wide well regions than in the narrow well regions. We have also developed a quantitative model, of independently distributed cylindrical holes at the AlAs/GaAs interface, to explain the evolution of the interisland exciton diffusion as a function of temperature and have obtained the average value of the narrow regions extension and the ratio of the area of the wide regions to the area of the narrow regions.

Saturation of Intersubband Absorption by Real-Space Transfer in Modulation Doped Single GaAs–AlAs Quantum Well

Intersubband absorption spectrum was investigated with intense infrared light in a modulation doped GaAs quantum well (QW) of 10 nm thickness with 6 nm AlAs barriers. When the excitation intensity was 9.8 kW/cm, the absorption was bleached to half of its weak field value. The corresponding effective intersubband relaxation time t was 14 ps, which is much larger than the intersubband LO phonon emission time. We also observed that the peak position energy Ep shifts toward lower photon energy as the excitation intensity is increased, however, the amount of the shift was about a factor four smaller than the value predicted from the depolarization effect. The observed large t and the small shift of Ep indicates an efficient real-space transfer of the excited electrons. The escape of electrons was directly detected as a photocurrent.

Magnetic field induced Mott transition of a one-component electron plasma in GaAs quantum wells

We have measured the time-resolved magneto-photoluminescence of mixed type GaAs AlAs quantum wells, in which the electron density can be varied from ∼10 9 to ∼10 11 cm −2. We confirm that as the electron density is increased the precursor of the Mott transition is the trion (negatively charged exciton) and that the transition occurs when the Landau level filling factor v = 2. In zero field the transition is broad and ill-defined, but in a field it is sharp and is accompanied by an abrupt change in linewidth and radiative lifetime.

High quality interfaces in GaAs–AlAs quantum wells determined from high resolution photoluminescence

High resolution photoluminescence (PL) measurements performed on several GaAs–AlAs quantum well structures revealed sharp excitonic transitions separated in energies corresponding to roughly half-monolayer fluctuations in well size. The narrow linewidths correlate with interface island structure whose lateral extent is either much larger or much smaller than the exciton diameter. The half-monolayer separation results from a sharply peaked PL intensity response occurring around those areas of the laterally nonuniform interface which have roughly 50% island coverage, with the average island size much smaller than the exciton diameter, about 225 Å.

Γ- X hybridization effects on the carrier subband transition times in type I GaAsAlAs quantum wells

We calculate the Γ → Γ carrier scattering times due to emission of confined and interface optic phonons taking into account the Γ- X hybridization of the electron states for type I GaAs/AlAs quantum well structures. We show that the intersubband transition times increase for well widths where the second GaAs electron subband becomes close in energy to the X point of the AlAs barrier and that this increase is strongly dependent on the amount of hybridization. These results suggests that the hybridization may affect carrier transport and relaxation and that intersubband scattering times could be used for measuring the hybridization of the second electron subband.

Voltage-tuneable Bragg reflector based on the quantum confined Stark effect

We consider the Bragg effect in an in-plane reflector realized with a field-tuneable superlattice. The latter is induced via an interdigitated gate on a waveguide formed by a GaAs–AlAs quantum well system. In such a system the Bragg resonance arises from the laterally periodic modulation of the quantum confined Stark effect. The reflection coefficient is shown to be voltage tuneable, and the angular and frequency dependences are analyzed theoretically. We find that acceptable values of reflection can be achieved for frequencies close to the interband resonance. Furthermore, we point out that the in-plane Bragg reflector can be used to study exciton–polariton coupling in semiconductor waveguides.

Spin splitting of excitons in GaAs quantum wells at zero magnetic field

We have observed an energy splitting between +1 and −1 excitons in GaAs AlAs quantum wells at zero magnetic field. The studies, which have been done by picosecond time-resolved photoluminescence spectroscopy, show that exciton spin splitting depends strongly on excitonic density and on the dynamics of spin relaxation: the spin splitting increases, raising the initial carrier density, and the time evolution of the spin splitting follows the exponential decay of the degree of polarization of the photoluminescence. For large carrier densities and short time delays the observed exciton spin splitting is as large as 4 meV. We present theoretical results which support the interpretation of experimental spin splittings in terms of inter-excitonic exchange interactions.

Elastic strains in [formula omitted] quantum dots studied by high resolution X-ray diffraction

We have studied a GaAs AlAs quantum dot array using reciprocal space mapping around the (004) and (II3) reciprocal lattice points. Both the coherently and the diffusely scattered X-ray intensities were analysed by performing two-dimensional model calculations. From the distribution of the diffracted intensities we deduced the average strain status in the dots. From the numerical simulations it is evident that random elastic strain fields are present, which extend through almost the whole volume of the quantum dot. The simulations of the X-ray measurements revealed that the crystalline part of the dots is considerably smaller as scanning electron micrographs would indicate, namely 50 nm instead of 65 nm, respectively.

Donors bound to X valleys in type-II GaAs–AlAs quantum well structures

We calculate the binding energies of donors bound to X valleys in type-II GaAs–AlAs quantum well structures using an anisotropic variational method which enables us to take into account the effective mass anisotropy and quantum confinement. For a comparative study, we use two sets of effective masses obtained from different measurements [B. Rheinänder et al. Phys. Status Solidi B 49, K167 (1972) and M. Goiran et al., Physica B 177, 465 (1992)]. We show that the binding energies have a pronounced dependence with the effective mass, AlAs layer thickness, and impurity position.

X-ray reciprocal space mapping of GaAs/AlAs quantum wires and quantum dots

Periodic arrays of 150 and 175 nm-wide GaAs–AlAs quantum wires and quantum dots were investigated, fabricated by electron beam lithography, and SiCl4/O2 reactive ion etching, by means of reciprocal space mapping using triple axis x-ray diffractometry. From the x-ray data the lateral periodicity of wires and dots, and the etch depth are extracted. The reciprocal space maps reveal that after the fabrication process the lattice constant along the growth direction slightly increases for the wires and even more so for the dots.

Finite Element Solutions to GaAs-AlAs Quantum Wells with Connection Matrices at Heterojunctions

In this paper, the irreducible finite element formulations for the solutions to the single-band effective-mass equation are developed to incorporate the connection conditions at the heterojunctions. The exact interface formulation utilizes the exact form of the connection matrix. The finite-difference interface formulation uses the finite difference approximation to the connection matrix. For the investigation of the GaAs-AlAs quantum well, the finite-difference interface formulation has to be employed, and the relative errors are shown to be less than 103. Then, this finite element formulation is utilized to compare the effects of the electric field on the GaAs-AlAs quantum well with the heterojunctions modeled by the conventional, the effective-mass-dependent, and the energy-dependent connection matrices. It is found that the eigenenergies predicted by the models with non-conventional connection matrices are larger in magnitude and less dependent on the electric field than the conventional model.