Quantum-well Heterostructures Research Articles

Dynamics of the Accumulation of Excess Holes in n-AlGaAs/GaAs Heterostructure Quantum Wells

The effect of the excess hole capture efficiency on carrier accumulation with increasing photoexcitation in quantum wells of the n-AlGaAs/GaAs heterostructures is investigated. The oscillatory character of the dependence of the ratio between the intensities of photoluminescence from the quantum well and barrier layers on the well width is demonstrated. The hole capture times in the oscillation maximum and minimum are estimated as 3 and 370 ps. The shift of the energy transition in a resonant well under strong excitation is attributed to the charge buildup effect caused by the difference between the electron and hole capture rates.

Formation of low-dimensional structures in the InSb/AlAs heterosystem

Low-dimensional quantum-well and nanoisland heterostructures formed in the InSb/AlAs system by molecular-beam epitaxy are studied by transmission electron microscopy and steady-state photoluminescence spectroscopy. The structures are grown under conditions of alternate In and Sb deposition (the socalled atomic-layer epitaxy mode) and the simultaneous deposition of materials (the traditional molecularbeam epitaxy mode). In both modes of growth, at a nominal amount of the deposited material in a single layer, large-sized (200 nm–1 μm) imperfect islands arranged on the In x Al1 – x Sb y As1–y quantum-well layer are formed. In the heterostructures grown under conditions of atomic layer epitaxy, the islands are surrounded by ring-shaped arrays of much smaller (~10 nm), coherently strained islands consisting of the In x Al1 – x Sb y As1 – y alloy as well. The composition of the alloy is defined by the intermixing of Group-V materials in the stage of InSb deposition and by the intermixing of materials because of the segregation of In and Sb atoms during overgrowth of the InSb layer by an AlAs layer.

Laser diode arrays based on AlGaAs/GaAs quantum-well heterostructures with an efficiency up to 62%

The results of development of quasi-cw laser diode arrays operating at a wavelength of 808 nm with a high efficiency are demonstrated. The laser diodes are based on semiconductor AlGaAs/GaAs quantum-well heterostructures grown by MOCVD. The measured spectral, spatial, electric and power characteristics are presented. The output optical power of the array with an emitting area of 5 × 10 mm is 2.7 kW at a pump current of 100 A, and the maximum efficiency reaches 62%.

Uniaxial strain induced optical properties of complex type-II InGaAs/InAs/GaAsSb nano-scale heterostructure

Semiconducting heterostructures offer an extra degree of flexibility in terms of tuning of optical gain and transition energies or wavelengths. Modifications in the wave functions and alterations in optical transitions in binary and ternary QW (quantum well) heterostructures due to external uniaxial strain provide valuable insights on the optical characteristics of the heterostructure. In this paper, we have reported the effect of uniaxial strain along [001], [100] and [110] on the optical properties such as optical gain, and optical transition energies of W − shaped complex type-II InGaAs/InAs/GaAsSb nano-scale heterostructure consisting of two quantum wells of InAs material using the six band k.p theory. On the basis of results obtained in the work, it can be reported that the TE and TM optical gain can be enhanced by application of uniaxial strain along [110] only. It is also reported that application of strain along [001] is useful for improvement of TE optical gain while strain along [100] is useful for TM optical gain. Thus, for the type-II InGaAs/InAs/GaAsSb nano-heterostructure TE and TM gain and as well as transition energies can be controlled by the application of uniaxial strain along the choice of direction.

Optical gain tuning within IR region in type-II In0.5Ga0.5As0.8P0.2/GaAs0.5Sb0.5 nano-scale heterostructure under external uniaxial strain

Quaternary and ternary alloy semiconductors offer an extra degree of flexibility in terms of bandgap tuning. Modifications in the wave functions and alterations in optical transitions in quaternary and ternary QW (quantum well) heterostructures due to external uniaxial strain provide valuable insights on the characteristics of the heterostructure. This paper reports the optical gain in strained InGaAsP/GaAsSb type-II QW heterostructure (well width = 20 Å) under external uniaxial strain at room temperature (300 K). The entire heterostructure is supposed to be grown on InP substrate pseudomorphically. Band structure, wave functions, energy dispersion and momentum matrix elements of the heterostructure have been computed. 6 × 6 diagonalised k→·p→ Hamiltonian matrix of the system is evaluated and Luttinger-Kohn model has been applied for the band structure and wavefunction calculations. TE mode optical gain spectrum in the QW-heterostructure under uniaxial strain along [110] is calculated. Optical gain of the heterostructure as a function of 2D carrier density and temperature variation is investigated. The variation of the peak optical gain as a function of As and Sb fractions in InGaAsP as a barrier and GaAsSb as a well respectively is exhibited. For a charge carrier injection of 5×1012/cm2, the TE optical gain is 3952 cm−1 at room temperature under no external uniaxial strain. Significant increase in TE mode optical gain is observed under high external uniaxial strain (1, 5 and 10 GPa) along [110] within IR (Infrared region) region.

Simulation of Temperature Dependences of Emission Characteristics of Nano-Layer Injection Lasers Based on Symmetric Heterostructures

The exact problem of electromagnetic wave propagation in a multilayer nanostructure with complex values of dielectric permittivity is solved. The contribution to the refractive index of an additive to the dielectric constant connected with the injected carriers is taken into account. Within the framework of this problem, it is shown that an anomalous temperature dependence of the threshold current of injection lasers based on nanoheterostructures is related to the anti-waveguide action of injected carriers. A quantum-well heterostructure based on InGaAs/AlGaAs/GaAs nanosystems used for fabrication of 0.94-1.14 μm lasers is considered. The applied techniques and approaches are also acceptable for the optimization of multilayer nanostructures based on other solid solutions. As an optical model of the active region of injection lasers based on nanostructures, a planar multilayer dielectric waveguide with complex values of dielectric permittivity in the layers is considered. It is shown that with decreasing thickness of the active region of injection lasers, the dependence of the mode gain on the local gain is essentially sublinear. The reason for this is the antiwaveguide action of electrons. The results of calculations of the temperature dependence of the threshold current of injection lasers indicate the presence of a critical point Tc, at which a sharp decrease in the characteristic temperature occurs. The performed calculations and optimization of the temperature dependence of emission characteristics of injection lasers based on nanostructures show that the anomalous behavior of the temperature dependence of the threshold current is also associated with the weakening of the waveguide properties of theirs active region.

Laser diode bars based on AlGaAs/GaAs quantum-well heterostructures with an efficiency up to 70%

The results of the development and fabrication of laser diode bars (λ = 800 – 810 nm) based on AlGaAs/GaAs quantum-well heterostructures with a high efficiency are presented. An increase in the internal quantum and external differential efficiencies together with a decrease in the working voltage and the series resistance allowed us to improve the output parameters of the semiconductor laser under quasi-cw pumping. The output power of the laser diode bars with a 5-mm transverse length reached 210 W, and the efficiency was ~70%.

Nanoscale visualization of electronic properties of AlxGa1-xN/AlyGa1-yN multiple quantum-well heterostructure by spreading resistance microscopy

Cross-sectional spreading resistance microscopy has been used to investigate nanoscale variations in electronic properties of an undoped Al0.75Ga0.25N/Al0.95Ga0.05N multiple quantum well (MQW) heterostructure grown by plasma-assisted molecular beam epitaxy on an AlN/c-sapphire template, prepared by metalorganic vapor phase epitaxy. It is found that a current signal from the MQWs can be detected only at a negative sample bias. Moreover, its value changes periodically from one quantum well (QW) to another. Analysis of the current-voltage characteristics of the contacts of a tip with the structure layers showed that periodic contrast of MQWs is the result of fluctuations of the chemical composition of the QWs and the concentration of electrons accumulated in them. Mathematical simulations indicate that this modulation is associated with the periodic fluctuations of an Al-mole fraction in the barrier layers of the structure due to counter gradients of the intensity of Al and Ga molecular fluxes across the surface of a substrate rotating slowly during growth. The nanoscale fluctuations of the current contrast observed along the QW layers are caused, most likely, by the presence of the areas of lateral carrier localization, which originate during the formation of QWs by sub-monolayer digital alloying technique.

Anisotropy and optical gain improvement in type-II In0.3Ga0.7As/GaAs0.4Sb0.6 nano-scale heterostructure under external uniaxial strain

Alterations in optical transitions and distortions in wave symmetry in nano-scale QW (quantum well) heterostructures are seen due to external uniaxial strain under different polarizations. This paper reports the anisotropy phenomena and optical gain improvement realized in In0.3Ga0.7As/GaAs0.4Sb0.6 type-II QW-heterostructure (well width = 20 Å) under uniaxial strain in the SWIR (short wave infra red) region. The detailed study of the band structure, wave functions associated with the charge carriers in the respective bands and optical gain under electromagnetic field perturbation is reported. The 6 × 6 diagonal k→·p→ Hamiltonian matrix is evaluated and Luttinger-Kohn model is used for the band structure calculation. Optical gain spectrum in the QW-heterostructure under uniaxial strain along [110] for different polarizations of light is calculated. For a charge carrier injection of 5 × 1012/cm2 the optical gain is ∼1600/cm under input z-polarization, ∼14500/cm under x-polarization and ∼15700/cm under y-polarization without external uniaxial strain applied. A significant improvement in optical gain is observed under uniaxial strain along [110] direction under different input polarizations. Keeping in views its utilization in optoelectronics due its very high optical gain in near-infra-red region in x- or y-polarization mode, such structure can be considered as a novel structure.

From lattice Hamiltonians to tunable band structures by lithographic design

Recently, new materials exhibiting exotic band structures characterized by Dirac cones, nontrivial flat bands, and band crossing points have been proposed on the basis of effective two-dimensional lattice Hamiltonians. Here, we show using atomistic tight-binding calculations that these theoretical predictions could be experimentally realized in the conduction band of superlattices nanolithographed in III-V and II-VI semiconductor ultrathin films. The lithographed patterns consist of periodic lattices of etched cylindrical holes that form potential barriers for the electrons in the quantum well. In the case of honeycomb lattices, the conduction minibands of the resulting artificial graphene host several Dirac cones and nontrivial flat bands. Similar features, but organized in different ways, in energy or in $k$-space are found in kagome, distorted honeycomb, and Lieb superlattices. Dirac cones extending over tens of meV could be obtained in superlattices with reasonable sizes of the lithographic patterns, for instance in InAs/AlSb heterostructures. Bilayer artificial graphene could be also realized by lithography of a double quantum-well heterostructure. These new materials should be interesting for the experimental exploration of Dirac-based quantum systems, for both fundamental and applied physics.

Enhanced photoluminescence efficiency in AlGaN quantum wells with gradient-composition AlGaN barriers

We present photoluminescence studies of AIxGa1-xN/AlyGa1-yN (y = x+0.3) quantum well (QW) heterostructures with graded AI content in barrier layers, emitting in the range 285-315 nm. The best-established internal quantum efficiency of the QW emission is as high as 81% at 300 K, owing to enhanced activation energy of charge carriers and exciton binding energy in the QW heterostructure with optimized design.

Plasmonic enhancement of four-particle radiative recombination in SiGe quantum wells

The influence of gold nanoparticles deposited on the surface of a Si0.95Ge0.05/Si quantum-well heterostructure with a thin Si cap layer on the spectra of low-temperature recombination radiation of biexcitons and an electron–hole liquid confined in the quantum well is investigated. The spectra of both visible and near-infrared radiation are recorded from a region on the sample surface without nanoparticles and regions coated with nanoparticles of different areal densities. It is found that the presence of gold nanoparticles causes strong plasmonic enhancement of collective emission processes in which two holes simultaneously recombine with two electrons from opposite valleys of the conduction band, with the energy of the four particles being transferred to a single photon in the visible spectral range.

Highly reliable high-power superluminescent diodes with three single-mode active channels

We report superluminescent diodes (SLDs) with three ridged active channels, each having a width of , based on one ‘bulk’ and two quantum-well heterostructures. At a cw output power greater than , the emission spectra of these SLDs possess a quasi-Gaussian shape with centre wavelengths near 840, 860 and and widths about 15, 25 and , respectively. In the above operating conditions, the median service life of the SLDs amounted to approximately 50000, 25000 and more than , respectively.

Ion-sensitive field-effect transistor with sSi/Si0.5Ge0.5/sSOI quantum-well for high voltage sensitivity

An ion-sensitive field-effect transistor (ISFET) with improved sensing current and voltage sensitivity is realized by using a sSi/Si0.5Ge0.5/sSOI quantum-well (QW) heterostructure and an Al2O3 dielectric layer as sensing membrane coupling with the back-gate. The voltage sensitivity is much higher than the reference SOI ISFET, typically at low drain current due to the high hole mobility confined in the SiGe QW. A high voltage sensitivity 360mV/pH with a linearity of 99.89% was achieved for QW ISFET. The results of the planar QW ISFET show great potential for low cost, real-time monitoring of bio-chemicals due to its simplified process.

Resistance switching memory characteristics of CaF2/Si/CaF2 resonant-tunneling quantum-well heterostructures sandwiched by nanocrystalline Si secondary barrier layers

A novel resistance switching memory using CaF2/Si/CaF2 resonant-tunneling quantum well heterostructures sandwiched by nanocrystalline Si (nc-Si) as secondary barrier layers has been proposed and the room temperature current–voltage characteristics of the basic resistance switching memory operation have been demonstrated. A resistance switching voltage of 1.0 V, a peak current density of approximately 42 kA/cm2, and an ON/OFF ratio of 2.8 were observed. In particular, more than 28000 write-read-erase cyclic memory operations have been demonstrated by applying pulsed input voltage sequences, which suggests better endurance than the device using a CaF2/CdF2/CaF2 heterostructure.

Terahertz radiation generation process in the medium based on the array of the noninteracting nanotubes

Consideration of periodically alternating arrays of noninteracting carbon nanotubes with the metal type of conductivity as elements of the quantum layer heterostructures (quantum wells), which will play the role of the active field, is suggested. From the earlier obtained results based on the models of the elementary cell, it follows that the system in the neighborhood of the Dirac points at the presence of the longitudinal electric field will have equidistant spectrum. The eigenvalues of the energies do not depending on the longitudinal electric field. The two-component eigenfunctions do not depend on the longitudinal electric field. The amplification of the generation terahertz radiation by the analogy with the cascade lasers in the multilevel medium, based on the system of the noninteracting metallic carbon, is considered. If concentration of the nanotubes is 107 cm−2 then the output power per unit area will be about \(Q \sim n_{0} k_{0} \hbar \omega J_{k} /e \approx 8\;{\text {W}}/{\text {cm}}^{2}\). The wavelength of the terahertz radiation is 65 µm.

Radiative decay rate of excitons in square quantum wells: Microscopic modeling and experiment

The binding energy and the corresponding wave function of excitons in GaAs-based finite square quantum wells (QWs) are calculated by the direct numerical solution of the three-dimensional Schrödinger equation. The precise results for the lowest exciton state are obtained by the Hamiltonian discretization using the high-order finite-difference scheme. The microscopic calculations are compared with the results obtained by the standard variational approach. The exciton binding energies found by two methods coincide within 0.1 meV for the wide range of QW widths. The radiative decay rate is calculated for QWs of various widths using the exciton wave functions obtained by direct and variational methods. The radiative decay rates are confronted with the experimental data measured for high-quality GaAs/AlGaAs and InGaAs/GaAs QW heterostructures grown by molecular beam epitaxy. The calculated and measured values are in good agreement, though slight differences with earlier calculations of the radiative decay rate are observed.

Polarisation fields in III-nitrides: effects and control

III-Nitrides are materials that have revolutionised the lighting industry allowing for the development of high brightness and efficiency white light emitting diodes (LEDs), enabling cost and energy savings at an unprecedented scale. However, there remain several obstacles to the further enhancement of the efficiency of LEDs, particularly for emission at longer wavelengths. The existence of polarisation fields as an inherent property of wurtzite III-nitride materials severely hampers LED performance. The origin of these fields due to the deviation from an ideal tetrahedral bonding structure and their relation to strain has been addressed in this review. The effect of the polarisation fields on the band structure of heterostructure quantum wells, known as the quantum confined stark effect, and its implications for the efficiency and spectral stability of LEDs have also been reviewed. Finally, the effectiveness and viability of several proposed methods of mitigating the harmful effects of the polarisation fields, such as the growth of III-nitrides on alternative planes, doping, strain engineering and growth of cubic GaN, have been addressed.

Bendable III-N Visible Light-Emitting Diodes beyond Mechanical Flexibility: Theoretical Study on Quantum Efficiency Improvement and Color Tunability by External Strain

We show that bending of flexible light-emitting diodes based on polar group III–V nitride structures can function as more than mechanically flexible devices through numerical studies. Controlled external bending can improve internal quantum efficiencies and electron-to-photon conversion efficiencies. Moreover, emission wavelength and color can be changed using active polarization control with external bending strains. We show that applying external strain on InGaN/GaN quantum-well (QW) heterostructures can mitigate or enhance the quantum-confined Stark effect. Significant IQE improvement (≥10%) is estimated by introducing external strain with concave-side up bending of flexible heterostructures. We also show significant wavelength shifts in peak emission. Especially, the In0.35Ga0.65N QW can be tuned in the overall visible spectral range by choosing different curvatures and bending modes. We have suggested the concept of a photoelectromechanical device that can emit different colors depending on the appli...

Role of nitrogen in carrier confinement potential engineering and optical properties of GaAs-based quantum wells heterostructures

Role of nitrogen in carrier confinement potential engineering and optical properties of GaAs-based quantum wells heterostructures