GaAs Multiple Quantum Wells Research Articles

Photomodulated transmission spectroscopy of [formula omitted] MQWs under hydrostatic pressure

We report the results of a comprehensive investigation of the dependence of various intersubband transitions within In xGa 1 − xAs GaAs multiple quantum wells (MQWs) on hydrostatic pressure. Several well-defined spectral features associated with the transitions of either heavy hole or light hole valence bands to the confined conduction subbands and the GaAs fundamental band gap transition are observed for all the samples under different pressures. It is demonstrated experimentally that the pressure coefficients for the intersubband transitions differ from that of the bulk counterparts in In x Ga 1 − x As, and are both composition and well-width dependent. The physical reasons which may be responsible for the abnormal behavior of the pressure coefficients of the intersubband transitions in the MQWs are discussed.

Dynamics of localized exciton recombination and migration in GaAs multiple quantum wells

A theory on the dynamics of localized exciton recombination and their intralayer migration in GaAs quantum wells is developed. Non-exponential photoluminescence decay is successfully interpreted by exciton recombination and its influence on the quasi-equilibrium localized exciton population. The recombination rate and the effective downmigration rate can be obtained.

Ultrafast nonlinear optics in GaAs/AlGaAs quantum wells

By degenerate four-wave mixing experiments in a two-beam geometry, we have investigated the initial coherence and dephasing of quasi two-dimensional excitions and biexcitons in GaAs multiple quantum wells. The dephasing has contributions from phonon scattering, interface-roughness scattering and exciton-exciton scattering. Inhomogeneous broadening and generation of coherent wavepackets play a significant role in the coherent exciton dynamics. The incoherent exciton dynamics, diffusion and recombination, is studied by three-beam transient grating experiments. A significant difference in the interface roughness scattering of coherent and incoherent (thermalized) excitons is observed.

Strain effects on carrier lifetimes in InGaAs/(Al)GaAs multiple quantum wells

Lifetimes were measured in several InGaAs/(Al)GaAs samples with various levels of strain and strain relief, with thick barriers and with GaAs barriers. Lifetimes of the order of 0.5 ns were measured. We have found that the introduction of indium, to obtain InGaAs wells, causes a dramatic decrease in the carrier lifetime by an order of magnitude, as compared with similar GaAs wells. Once indium is present in the wells then the dominant carrier loss-recombination process in the InGaAs multiple quantum well materials is unaffected by the strain, strain relief or barrier design.

Carrier lifetimes in strained InGaAs/(Al)GaAs multiple quantum wells

The effect of strain, strain relief, and barrier design on the carrier lifetime in InGaAs/(Al)GaAs multiple-quantum-well samples is investigated. Carriers lifetimes are measured in samples with varying amount of strain, due to increasing indium concentration in the wells, as well as in samples subject to strain relief, with thick barriers and GaAs barriers. Lifetimes of the order of 0.5 ns are measured. The lifetime is sensitive to the presence of indium in the wells but remarkably insensitive to the indium concentration, the strain in the samples, and the barrier composition.

Photorefractive quantum wells: transverse Franz–Keldysh geometry

The photorefractive properties of semi-insulating AlGaAs–GaAs multiple quantum wells are described for the transverse Franz–Keldysh geometry with the electric field in the plane of the quantum wells. Combining the strong electroabsorption of quantum-confined excitons with the high resistivity of semi-insulating quantum wells yields large nonlinear optical sensitivities. The photorefractive quantum wells have effective nonlinear optical sensitivities of n2 ≈ 103 cm2/W and α2/α0 ≈ 104 cm2/W for applied fields of 10 kV/cm. Photorefractive gains approaching 1000 cm−1 have been observed in two-wave mixing under dc electric fields and stationary fringes. The transverse Franz–Keldysh geometry retains the general transport properties and behavior of conventional bulk photorefractive materials. The resonant excitation of free electrons and holes in the quantum wells leads to novel behavior associated with electron–hole competition. We demonstrate that under resonant excitation of electrons and holes the device resolution is fundamentally limited by diffusion lengths but is insensitive to long drift lengths.

Dynamics of carrier capture in AlGaAs / GaAs multiple quantum wells

Dynamics of carrier relaxation and capture in AlGaAs/GaAs multiple quantum wells (MQW) at 80 K is studied using picosecond luminescence and femtosecond absorption saturation measurements. Carriers generated in the wells and in the barriers scatter initially out of the excited states to a quasi-equilibrium state in 35 and 400 femtoseconds, respectively, before they are captured into the bound states of the quantum wells. Carrier capture occurs during carrier cooling and recombination. A carrier capture time of 25 ps has been deduced from time-resolved luminescence.

Tunable (Al)GaAs lasers using impurity-free partial interdiffusion

Impurity-free interdiffusion has been used to fabricate single mode quantum well ridge lasers from the same (Al)GaAs epitaxial material which differ in emission wavelength by as much as 11.7 nm. This represents a shift of approximately 80 laser linewidths, as measured under pulsed conditions. Threshold currents for the interdiffused and nondiffused lasers are nearly identical. However, the differential quantum efficiencies of the most interdiffused lasers are a factor of 2 lower than nondiffused lasers. The interdiffusion coefficients for the quantum well laser structure are approximately a factor of 6 smaller than those observed for intrinsic GaAs multiple quantum wells.

Growth and Stability of Strained-Layer Multiple Quantum Wells

ABSTRACTThirty-period In0.28Ga0.72As(100Å)/GaAs (100Å) multiple quantum wells (MQWs) have been grown by molecular beam epitaxy on (lOO)-oriented GaAs substrate with a 0.5μm-thick InyGa1-yAs (0≤y≤0.28) buffer layer interposed between the QW layer and the GaAs substrate. The MQWs with y close to the average InAs mole fraction of the QW layer exhibited good crystalline quality. It indicates that the strain is well-balanced between the GaAs and In0.28Ga0.72As layer. A significant change in their photoluminescence spectra has been observed when annealed above 750δC by means of rapid thermal annealing, implying a structural disorder in the QW“region.

Stimulated visible light emission from ultrathin GaAs single and multiple quantum wells sandwiched between indirect-gap (Al0.49Ga0.51As) confining layers

Spontaneous and stimulated emission from a series of Al0.7Ga0.3As -Al0.49Ga0.51As-GaAs ultrathin single [1,2, and 3 monolayer (ML), 1 ML=2.83 Å] and multiple (2 ML) quantum well separate confinement heterostructures are demonstrated and compared to a control sample (0 ML). Spectra from sample to sample are very different and depend on the alignment of the n=1, Γ electron bound state in the quantum well with the Xminima (lowest band edge) in the indirect-gap (Al0.49Ga0.51As) confining layers. Some samples (3 ML single and 2 ML multiple quantum well) can support stimulated emission despite the fact that the quantum well is undoped, unstrained, and very thin (Lz ≪scattering path length) and that most of the wave function is in the indirect-gap confining layers. These experimental results can be explained using a simple model based on the spatial extent of the wave function (rather than the well width) under the special condition of band alignment between the Γ and X states.

Photocurrent spectroscopy of strained-layer InGaAsGaAs multiple quantum wells

The band configuration of the undoped strained-layer In x Ga 1− x As(8 or 15 nm)-GaAs(15 nm) MQW with x=0.1, 0.15 and 0.2 have been investigated by photocurrent measurements at temperatures ranging from 10 to 300 K. The intersubband excitonic transitions 11H, 11L and 22H are observed. It is found that both electrons and heavy holes are confined to the InGaAs layers while light holes are confined to the GaAs layers. The photocurrent peak related to 2s or other excited states of a heavy-hole exciton is also observed and the binding energy thus obtained is about 8 meV. In addition the transitions between the confined subbands and continuum are also observed. The band offset Q v is derived in two different ways, which give Q v = 0.39 ± 0.03.

Enhancement of nonradiative interface recombination in GaAs coupled quantum wells

The recombination dynamics of GaAs multiple quantum wells as a function of barrier widths LB are studied in a semiconductor in the range between LB=0.87 nm (superlattice) and LB=18.1 nm (uncoupled wells) by means of cathodo- and photoluminescence. With decreasing LB the nonradiative recombination rate is found to be drastically enhanced, where-as the radiative recombination probability decreases. Thus a pronounced decrease of the quantum efficiency results. The controlled variation of the barrier width is found to be decisive for an unambiguous identification of the origin of the traps which are responsible for the nonradiative processes: Comparison with a theoretical calculation shows that they are localized at the heterointerfaces and not in the barriers.

Fermi-edge singularity in heavily doped GaAs multiple quantum wells.

The absorption enhancement at the Fermi level of n-type modulation-doped multiple quantum wells is studied with photoluminescence excitation spectroscopy. We show that the limit of the pure Fermi-edge singularity (i.e., correlation between the sea of electrons and single holes), where no remnants of the band-edge exciton (correlation between single electrons and holes) are left, is only reached for the case of heavy doping (n=1.2\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$). A direct comparison between experiments and available many-body theories is only valid in this limit. We find both quantitative as well as qualitative deviations from these theories. Previously neglected broadening mechanisms significantly reduce the enhancement effects. We report for the first time the dependence of the enhancement on the density of the photoexcited holes.

High-resolution nonlinear laser spectroscopy of room-temperature GaAs quantum-well structures: observation of interference effects

We describe frequency-domain four-wave mixing spectroscopy measurements near the fundamental band edge in room-temperature GaAs multiple quantum wells. The line-shape information gives a measure of the dynamical behavior of the material and what is to our knowledge the first observation of an interference line shape due to a dominant slow contribution to the nonlinear response.

Effect of temperature variations on optical waveguiding in GaAs multiple quantum wells

Photoluminescence and grating coupling techniques have been used to investigate the effect of temperature variations, about room temperature, on the optical waveguiding properties of GaAs/AlGaAs multiple-quantum-well structures. The modal properties are found to exhibit a large temperature dependence near the excitonic resonance and an equally strong dependence on detuning. From the simultaneous measurement of changes in refractive index and absorption over a 20 K temperature range, it is concluded that temperature variations should be kept to within several kelvin in quantum-well devices, if substantial detuning and dielectric shifts are to be avoided.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-resolution nonlinear laser spectroscopy of heavy-hole excitons in GaAs/AlGaAs quantum-well structures: A direct measure of the exciton line shape.

We report high-resolution nonlinear laser-spectroscopy studies of the n=1 heavy-hole exciton in GaAs multiple quantum wells. Using a new method of cw frequency-domain four-wave mixing, we first show that the excitation relaxation consists of both fast and slow components. We then show that the contribution of well-width fluctuations to inhomogeneous broadening can be elimiated enabling us to obtain a direct measure of the intrinsic exciton line shape. Under certain conditions, this line shape is non-Lorentzian and cannot be explained on the basis of simple excited-state relaxation dynamics.

Optical transmission spectroscopy of the two-dimensional electron gas in GaAs in the quantum Hall regime.

Optical transmission spectroscopy of a two-dimensional electron gas is shown to be a sensitive probe of the Fermi level and electronic density of states. Results are presented for interband transmission of modulation-doped GaAs multiple quantum wells in high magnetic fields. The appearance of new absorption transitions at even integral filling factors is the optical observation of quantum oscillations of the Fermi level of a two-dimensional electron gas. The broadening of the Landau level density of states is found to be $1.2\sqrt{B}$ meV ${\mathrm{T}}^{\frac{\ensuremath{-}1}{2}}$ by fitting the Fermi level oscillations.

Coupled GaAs multiple-quantum-well channel waveguides including quadratic electrooptic effect

Two coupled channel waveguides using GaAs multiple quantum wells are investigated using an improved coupled-mode theory for anisotropic waveguides and the effective index method. The quadratic electrooptic effect in the multiquantum-well structures is taken into consideration. It is shown that only a moderate electric field for multiquantum-well waveguides is necessary to achieve the optical power switching compared with that required for a bulk GaAs waveguide. >

Charge-carrier dynamics in GaAs multiple quantum wells determined by contactless photoconductivity measurements.

Charge-carrier dynamics in GaAs multiple quantum wells determined by contactless photoconductivity measurements.

Evidence for excitonic decay of excess charge carriers in high quality GaAs quantum wells at room temperature

A comparative study is presented of the room-temperature decay of laser excited microwave photoconductivity and of cathodoluminescence of high quality GaAs multiple quantum wells grown by molecular beam expitaxy and of high quality GaAs liquid phase epitaxial layers. The results from both experiments are in quantitative agreement and prove that carrier recombination in multiple quantum wells occurs via excitonic decay channels at excess carrier densities less than 1017 cm−3. In contrast, band-band recombination prevails in three-dimensional material.