Two-dimensional Quantum Wells Research Articles

Cyclotron resonance study of the two-dimensional electron layers and double layers in tilted magnetic fields

The far-infrared absorption in two-dimensional electron layers subject to magnetic field of general orientation was studied theoretically. The Kubo formula is employed to derive diagonal components of the magneto-conductivity tensor of two-dimensional electron single- and double layers. The parabolic quantum well is used to model a simple single-layer system. Both single- and double-layer systems can be realized in a pair of tunnel-coupled, strictly two-dimensional quantum wells. Obtained results are compared to experimental data.

Nanolasers: lasing from nanoscale quantum wires

Semiconductor lasers are in many ways second only to transistors as to their impact on today's high-tech industries. The unique characteristics, such as narrow emission wavelength, high frequency modulation, and device integratibility, make semiconductor lasers ideal photon sources for applications as diverse as telecommunication, signal processing, material characterisation, and medical diagnostics. Advances in material growth technologies, particularly molecular beam epitaxy, metal-organic chemical vapor deposition, and a suite of chemical synthesis techniques, make the fabrication of high quality nanometer scale semiconductor structures possible. Thanks to the quantum size effects that drastically modify the energy spectra of confined electrons in reduced dimensions, the population inversion necessary for lasing action occurs more efficiently as the active semiconductor gain medium is scaled down from the bulk to the nanometer scale. Consequently, semiconductor lasers built with nanoscale active media are expected to exhibit extraordinary features such as great color range, high optical gain, and low lasing threshold. Indeed, miniaturised lasers using nanoscale semiconductor gain media - two-dimensional quantum wells, one-dimensional quantum wires, and zero-dimensional quantum dots - have shown significant improvements in device performance. This article provides an overview of the physics and technologies behind the rapid progress of miniaturisation of semiconductor lasers, in particular the quantum wire lasers based on one-dimensional nanoscale optical gain media. Since the first report of lasing in quantum wires by Kapon and his co-workers, quantum wire lasers have evolved from "microlasers" in which the one-dimensional nanostructure is embedded in a micron size optical cavity, to "nanolasers" in which, as we recently demonstrated, the material gain and optical feedback are simultaneously achieved by individual nanoscale quantum wires. One-dimensional semiconductor fabrication technologies based on nanoscale lithography, self-organisation, selective growth, and chemical synthesis will be reviewed along with recent advances of quantum wire lasers built upon each of these fabrication technologies.

Parametrized equations for excitons in two-dimensional semiconductor quantum wells with arbitrary potential profiles

We derive parametrized equations using variational calculation for energy levels and the spatial extension of Wannier excitons in two-dimensional quantum wells with an arbitrary shape of the confining potential. The formulation is performed in the framework of the effective mass approximation, two-band model and taking into account both mass mismatch and dielectric mismatch. According to the new formulation, qualitative and general features of excitons are deduced. Illustrations are given for GaAlAs–GaAs–GaAlAs and CdMnTe–CdTe–CdMnTe systems for both rectangular and parabolic quantum wells as well as for the 2s exciton state. The comparison with existing data displays good agreement. An application of the model is given for an exciton in a parabolic quantum well under a constant electric field.

Feature of the low-lying states of a positronium molecule in a two-dimensionalquantum well

Making use of hyperspherical coordinates, the feature of the low-lyingspectrum of a positronium molecule in a two-dimensional quantum wellis investigated. The second bound state of a positronium molecule ina two-dimensional quantum well, with orbital angular momentumL = 1, isreported.

Practical formulations of the electron capture rate in quantum wells by phonon emission at low carrier density

In this article the Green’s functions formalism is exploited to describe the coupled phonon-plasmon system in the plasmon pole approximation for the longitudinal dielectric constant, for two-dimensional quantum wells. Electron capture time in quantum wells is obtained in a closed form, separating contributions coming from different two-dimensional plasmon modes. Moreover, useful criteria are pointed out, in order to decide under which conditions a simpler description in the static limit of the plasmon pole approximation may be enough accurate. In the limit of low carrier density, an analytical expression for the capture rate by LO-phonon emission has been given, in the two-dimensional static limit of the longitudinal dielectric constant. By means of this analytic description, effects of screening, Fermi filling, and electrons momentum distribution at finite temperature on capture time may be separately investigated in detail.

Comparison of Optical Properties of In0.4Ga0.6As/GaAs(311)B Two-Dimensional Quantum Dot Superlattices and Quantum Wells

We present a detailed comparison of the optical properties of a quantum well (QW) and a two-dimensional (2D) quantum dot superlattice (QDSL) made of In0.4Ga0.6As on GaAs(311)B substrates under the same growth conditions. The lowest electron to hole transition energy in strained In0.4Ga0.6As/GaAs(311)B QWs is calculated. While good agreement between the calculated transition energy and the measured photoluminescence (PL) peak energy is found in the QW, the PL peak energy of the 2D QDSL significantly shifts to a lower energy with respect to the calculated transition energy for a QW with the same nominal thickness. Based on the linewidth broadening induced by increasing excitation density and temperature, the exciton–exciton and exciton–phonon interaction strengths in the 2D QDSL are found to be much larger than those in the QW. As the exciton–phonon scattering is sufficiently strong, a transition of the exciton motion from coherent to incoherent occurs.

An Exciton Bound to a Neutral Donor in Quantum Dots

The binding energies for an exciton (X) trapped in a two-dimensionalquantum dot by a neutral donor have been calculated using the method offew-body physics for the heavy hole (σ = 0.196) and the light hole(σ = 0.707). We find that the (D0, X) complex confined in a quantumdot has in general a larger binding energy than those in a two-dimensionalquantum well and a three-dimensional bulk semiconductor, and the bindingenergy increases with the decrease of the dot radius. At dot radius R→∞, we compare our calculated result with the previous results.

Singletand triplet states of X+ and X- trions in two-dimensional quantumwells

A new simple variational wavefunction with a fewvariational parameters for two-dimensional X+ and X- trions issuggested. The function gives accurate results for the singletand triplet state energies of X+ and X- trions in thewhole range of electron-to-hole mass ratio. The mass ratio range where the triplet state exists has been found, and thebehaviour of the triplet state energy has been examined near thecritical mass ratio. The other excited states of the trion areanalysed in the vicinity of their critical mass ratios.

Dimensionality of photoluminescence spectrum of GaAs/AlGaAs system

We have theoretically studied the radiative recombination process between a conduction-band electron and a valence-band hole to analyze the photoluminescence (PL) spectrum. It is shown that due to the characteristics of the energy density of states, the line shape of the PL peak depends strongly on the dimensionality of the system under investigation, thus indicating the importance of the PL line shape in the identification of the corresponding optical transition process. Increasing the quantum confinement of a system from three-dimensional (3D bulk material) to two-dimensional (quantum well), one-dimensional (quantum wire) and zero-dimensional (quantum dot) results in a transition from a highly nonsymmetric PL peak to a symmetric Lorentzian one centered at the optical transition energy.

Nonparabolic effective masses of conduction subbands in InGaAs/InAlAs quantum wells in normal and parallel directions

Nonparabolic effective masses of conduction electrons were comprehensively studied in two-dimensional InGaAs quantum wells (QWs) deeply confined within InAlAs barriers of the 0.52-eV band offset. Cause of nonparabolicity was attributed not to the penetration of wavefunctions into barriers but to the InGaAs bulk band structure of bandgap energy of 0.74 eV. Band calculations by a Kane's three-level model for narrow-gap semiconductors and a Zawadzki's model under Landau quantization modified for QW confinement were fairly compared with in-plane apparent cyclotron masses of electrons measured in a 10-nm-wide InGaAs QW. Simulation of optical transmittance through complex epitaxial wafer structures fit quite well with cyclotron resonance experiments. Masses normal to the QW plane were also determined from a series of eigen-energies observed in 20-nm-wide InGaAs QW. In-plane nonparabolicity was found to be several times larger than normal nonparabolicity.

Fabrication and optical study of quantum dots, quantum wires and quantum wells of II–VI diluted magnetic semiconductors

Nanometer-scale structures of II–VI diluted magnetic semiconductors were synthesized by the microfabrication techniques. Quantum dots of Cd 1− x Mn x Se ( x=0.03) with diameters of 9.0–9.4 nm were grown by the self-organized mode in the molecular beam epitaxy. Quantum wires of Cd 1− x Mn x Se ( x=0.08) were synthesized with the width of 60–125 nm from the two-dimensional quantum wells by the electron beam lithography and chemical etching. The optical properties and the exciton dynamics of these quantum structures were studied by the transient photoluminescence spectroscopy. The magneto-optical properties of the confined excitons in the quantum dots, the wires and the wells are discussed.

Organic–inorganic system of two-dimensional quantum wells prepared by the dye incorporation into sol–gel glass

Molecules of the organic colorants were encapsulated in SiO 2 glass derived by the solution-gelatinization process; as the result, the colored glass was obtained containing the colorant embedded within the glass in a molecular or molecular aggregates form. The system thus prepared manifests an efficient photoluminescence; the optical absorption as well as the photoluminescence excitation and emission spectra reveal the existence of sets of the discrete energy levels. The structure of the colorant’s molecule and the percentage of doping determine the level separation; the dependence upon the environment is relatively small. The quantum mechanical description of the system is given considering the organic molecules as the two-dimensional quantum wells, with the boundary conditions taking account of the finite depth of the wells and the existence of aggregation. The results of calculations agree with the experiment.

Wavelength shifting of adjacent quantum wells in V-groove quantum wire structure by selective implantation and annealing

Intermixing induced by selective implantation was used to modify the two-dimensional (2D) quantum wells in the V-grooved quantum wire structure. Photoluminescence measurement of the implanted samples shows the obvious blueshift of the interband transition energy while quantum wire is not influenced by implantation. So the selective implantation method has been demonstrated in this article as a useful technique to isolate the energy levels of quantum wire structure from its neighbor 2D structures, which is preferred for the optoelectronic device application of quantum wire.

Electronic states in magnetic nanostructures

The electronic states responsible for oscillatory magnetic coupling, giant magnetoresistance (GMR), and spin-polarized tunneling are explored. They occupy well-defined locations in ( E, k ) space. Their energy E has to be within a few kT of the Fermi level, a range that is now becoming accessible to high-resolution photoemission. Particular attention is paid to k -regions near the Fermi level crossings of the s, p-band, where a sizable group velocity is combined with a d-like magnetic splitting and spin-polarization. These electronic states can be tailored by quantization in structures with single-digit nanometer dimensions, such as two-dimensional quantum wells and one-dimensional arrays of stripes and dots. Such arrays can be produced by self-assembly on top of stepped silicon surfaces.

Quantum dot-like behavior of GaInNAs in GaInNAs/GaAs quantum wells grown by gas-source molecular-beam epitaxy

A quantum dot-like behavior of GaInNAs in Ga0.7In0.3NxAs1−x/GaAs quantum wells (QWs) grown by gas-source molecular-beam epitaxy has been studied using high-resolution x-ray rocking curves (XRC), cross-sectional transmission electron microscopy (XTEM), and photoluminescence (PL) spectroscopy. XRC determines the average alloy composition and thickness of the QWs to be In 0.30 and N 0–0.030, and 6.2 nm, respectively. XTEM images show that the wells of both Ga0.7In0.3As/GaAs and Ga0.7In0.3N0.02As0.98/GaAs are undulated with lateral variations in strain, but the latter is much rougher. For Ga0.7In0.3As/GaAs QWs, rapid thermal annealing results in the splitting of a broad excitonic emission into two peaks presumably due to In composition fluctuation. The separation between these two peaks increases with increasing N concentration. Increasing excitation intensity results in a significant blueshift for the low-energy peak, while little blueshift for the high-energy peak. The earlier PL behavior can be interpreted by a simple model where the low-energy peak originates from In and N-rich regions in the wells acting as quantum dots. The high-energy peak is likely due to the excitons of a more two-dimensional QW.

Giant magneto-optical effects in diluted magnetic semiconductor nanostructures

Nanostructures of diluted magnetic semiconductors show varieties of optical phenomena caused by the interaction of electrons and holes with magnetic ions in the confined electronic states. We present giant magneto-optical effects and ultrafast exciton dynamics in two-dimensional quantum wells of the Cd 1− x Mn x Te/ZnTe system and quantum dots of Cd 1− x Mn x Se.

Optical anisotropy of InAs submonolayer quantum wells in a (311) GaAs matrix

The steplike density of states obtained from reflectance-difference spectroscopy demonstrates that ultrathin InAs layers should be regarded as two-dimensional quantum wells rather than isolated clusters, even for the sample with only 1/3 monolayer InAs in (311)-oriented GaAs. The degree of anisotropy is within the intrinsic anisotropy of (311)-oriented ultrathin quantum wells, indicating that there is little structural or strain anisotropy in the InAs islands.

Electronic states in magnetic nanostructures

The electronic states responsible for oscillatory magnetic coupling, giant magnetoresistance (GMR), and spinpolarized tunneling are explored. They occupy well-dened locations in (E, k) space. Their energy E has to be within a few k„ of the Fermi level, a range that is now becoming accessible to high-resolution photoemission. Particular attention is paid to k-regions near the Fermi level crossings of the s, p-band, where a sizable group velocity is combined with a d-like magnetic splitting and spin-polarization. These electronic states can be tailored by quantization in structures with single-digit nanometer dimensions, such as two-dimensional quantum wells and one-dimensional arrays of stripes and dots. Such arrays can be produced by self-assembly on top of stepped silicon surfaces. ( 1999 Elsevier Science B.V. All rights reserved.

Photoluminescence fromSi/SiO2single quantum wells by selective excitation

We have studied the photoluminescence (PL) mechanism of crystalline Si single quantum wells sandwiched between ${\mathrm{SiO}}_{2}$ layers by site-selective excitation spectroscopy. The asymmetric PL spectrum is observed in the red spectral region at low temperatures and can be divided into two PL bands. The peak energy of the strong PL band is almost independent of the well thickness and appears near $\ensuremath{\sim}1.65\mathrm{eV}.$ Under selective excitation at energies within the weak PL band, TO-phonon structures are observed in both PL and PL polarization spectra. Steplike TO-phonon structure in the PL spectrum of two-dimensional Si quantum wells is similar to that of zero-dimensional Si nanocrystals. The PL properties of very thin Si quantum wells are discussed.

Optical anisotropy of InAs submonolayer quantum wells in a (311) GaAs matrix

The in-plane optical anisotropy which comes from the heavy-hole and the light-hole transitions in InAs submonolayers inserted in a (311)-oriented GaAs matrix is studied by reflectance-difference spectroscopy (RDS). The steplike density of states obtained from RDS demonstrates that the ultrathin InAs layers should be regarded as two-dimensional quantum wells rather than isolated clusters, even for the sample with only $$$\frac{1}{3}$-ML InAs. The degree of anisotropy is found to be independent of the layer coverage, and is within the intrinsic anisotropy of (311)-oriented ultrathin quantum wells, indicating that there is little structural or strain anisotropy in the InAs submonolayer grown on a (311) GaAs surface.