Search

We report on the electronic structure, efficient carrier injection, and quantitative lasing characteristics of T-shaped GaAs/AlGaAs quantum-wire-laser diodes with parallel p- and n-doping layers grown by a cleaved-edge-overgrowth method with molecular-beam epitaxy. Continuous single-mode lasing from the ground subband of the quantum wires was demonstrated between 30 and 70 K in laser diodes with high-reflectivity Au coating on both cavity facets. The lowest threshold of 0.27 mA and the highest differential quantum efficiency of 12% were achieved at 30 K. Micro-photoluminescence measurements demonstrated the high optical quality of the quantum wires with narrow linewidth of 0.9 meV and provided electronic structures of surrounding layers. Microscopic electroluminescence (EL) imaging measurements demonstrated the efficient carrier injection into the quantum wires at 30 K. These two factors, i.e., high material quality and efficient carrier injection, contribute to the low threshold current and high efficiency of the laser device. The result of EL imaging at 5 K indicates an inefficient carrier injection into the active region, which limits the operating temperature of the devices. [ABSTRACT FROM AUTHOR]

Temperature and surface-coverage dependence of the evolution of surface morphology during growth-interrupt in situ annealing on GaAs epitaxial layers grown on the singular (110) cleaved edges by the cleaved-edge overgrowth method with molecular-beam epitaxy has been studied by means of atomic force microscopy. Annealing at substrate temperatures below 630 °C produced atomically flat surfaces with characteristic islands or pits, depending on the surface coverage. The atomic flatness of the surfaces is enhanced with increasing annealing temperature owing to the enhanced adatom migration. At a higher annealing temperature of about 650 °C, however, 2-monolayer-deep triangular pits with well-defined step edges due to Ga-atom desorption from the crystal appeared in the atomically flat surface. The growth-interrupt annealing temperature optimal for the formation of atomically flat GaAs(110) surfaces is therefore about 630 °C. [ABSTRACT FROM AUTHOR]

We review recent studies by optics methods of emergent phases in the quantum Hall (QH) regimes of double layers with finite tunneling at Landau level filling factor ν=1. In measurements of spin excitations by inelastic light scattering and of elastically scattered Rayleigh light under the application of in-plane magnetic fields, we uncovered evidence of a quantum phase transition that occurs when a many-body tunneling gap collapses. The transformation can be regarded as a transition from an incompressible highly correlated QH state to a compressible composite-fermion bilayer system. The correlated QH state is characterized by the presence of populations of bound electron-hole pairs across the tunneling gap. Quantitative determinations of the density of such excitonic pairs are obtained from inelastic light scattering spectra of spin excitations. The correlated QH state displays resonant Rayleigh scattering with unusual temperature dependence. [ABSTRACT FROM AUTHOR]

Local electronic states due to characteristic surface islands and pits formed on an atomically flat (110) GaAs interface of a quantum well fabricated by a cleaved-edge-overgrowth method with high-temperature growth-interrupt annealing are characterized by micro-photoluminescence (PL) imaging and spectroscopy. With the overall observation of the 6-nm-thick (110) quantum well with the area of 6.8-μm width and 3-mm length, we found local PL signals due to isolated 2- or 3-monolayer (ML) islands, isolated 1-ML pits, and 1-ML islands and pits formed along the cleavage atomic-step lines, and gradual change of the states across the full range of the sample. [ABSTRACT FROM AUTHOR]

Intersubband electroluminescence is reported in a quantum-cascade structure based on asymmetric superlattice active regions and designed for emission in the THz range (λapprox. 80 μm). Comparison with a structure based on a “vertical transition” in a single quantum well shows an increased full width at half maximum (2.8 vs 0.9 meV) of the emission line. In both cases the dependence of the optical power on the injected current is linear or sublinear and remains in the pW range. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Engineered honeycomb lattices, called artificial graphene (AG), are tunable platforms for the study of novel electronic states related to Dirac physics. In this work, we report the achievement of electronic bands of the honeycomb topology with the period as low as 40 nm on the nano-patterned modulation-doped AlGaAs/GaAs quantum wells. Resonant inelastic light scattering spectra reveal peaks which are interpreted as combined electronic transitions between subbands of the quantum well confinement with a change in the AG band index. Spectra lineshapes are explained by joint density of states obtained from the calculated AG electron band structures. These results provide a basis for further advancements in AG physics. [ABSTRACT FROM AUTHOR]

The unusual electronic properties of graphene, which are a direct consequence of its two-dimensional honeycomb lattice, have attracted a great deal of attention in recent years. Creation of artificial lattices that re-create graphene's honeycomb topology, known as artificial graphene, can facilitate the investigation of graphenelike phenomena, such as the existence of massless Dirac fermions, in a tunable system. In this work, the authors present the fabrication of artificial graphene in an ultrahigh quality GaAs/AlGaAs quantum well, with lattice period as small as 50nm, the smallest reported so far for this type of system. Electron-beam lithography is used to define an etch mask with honeycomb geometry on the surface of the sample, and different methodologies are compared and discussed. An optimized anisotropic reactive ion etching process is developed to transfer the pattern into the AlGaAs layer and create the artificial graphene. The achievement of such high-resolution artificial graphene should allow the observation for the first time of massless Dirac fermions in an engineered semiconductor. [ABSTRACT FROM AUTHOR]

Quantum Hall measurements are performed for a two-dimensional hole system (2DHS) confined to a 20 nm quantum well in 〈100〉 GaAs. Quantum oscillations reveal a density of 4 - 5 ×1010 cm-2 with mobility μ = 1 ×106 cm²/V s. For temperatures less than ~350 mK, anomalous insulating peaks are observed between integer fillings 1-2, 2-3, and 3-4. A large out-of-phase signal appears at these peaks, which indicates a substantial inductance inherent to the charge carriers. [ABSTRACT FROM AUTHOR]

An imaging and loss evaluation method for semiconductor waveguides coupled with non-doped quantum wells is presented. Using the internal emission of the wells as a probe light source, the numbers and widths of the modes of waveguides with various ridge sizes were evaluated by CCD imaging, and the obtained values were consistent with effective index method calculation. Waveguide internal losses were obtained from analyses of the Fabry-Pérot fringes of waveguide emission spectra. We quantified the quality of 29 single-mode waveguide samples as an internal loss and variation of 10.2 ± 0.6 cm-1. [ABSTRACT FROM AUTHOR]

An absorption-spectroscopy method that utilizes internal emissions as the source of the probe light was used to measure the absorption spectra of quasi-one-dimensional (q-1D) excitons in T-shaped quantum wires embedded in an optical waveguide. The modal absorption area of the 1D ground-state excitons was estimated to be 0.39 eV cm-1 and was almost independent of temperature in the range 4-150 K. Quantitative evaluation using the absorption spectra revealed that the absorption cross-section per unit length at resonance peak and the spectrally integrated absorption cross-section area per unit length of the 1D ground-state excitons were 1.0 nm and 2.5 × 10-3 eV nm, respectively. [ABSTRACT FROM AUTHOR]

Quantum Hall measurement of two-dimensional high-mobility [μ ~ 2 s 106 cm²/(Vs] hole systems confined in a 20 nm wide (100)-GaAs quantum well have been performed for charge densities between 4 and 5s1010 cm-2 in a temperature range of 10-160 mK. The Fourier analysis of the Shubnikov-de Haas (SdH) oscillations of the magnetoresistance vs. the inverse of the magnetic field 1/B reveals a single peak, indicating a degenerate heavy hole (HH) band. The hole density p = (e/h)f agrees with the Hall measurement result within 3%. The HH band degeneracy is understood through the diminishing spin-orbit interaction due to the low charge density and the nearly symmetric confinement. SdH oscillations fitted for 0.08 T≤B≤0.24 T to the Dingle parameters yield an effective mass between 0.30 and 0.50me in good agreement with previous cyclotron resonance results. [ABSTRACT FROM AUTHOR]

The transient optical gain was quantitatively determined from the rise time of output pulses of a gain-switched semiconductor laser via impulsive optical pumping. The optical gain for a high carrier density during gain switching was greatly reduced from that during steady-state operation because of hot carriers. [ABSTRACT FROM AUTHOR]

The authors report that anisotropic confining potentials in laterally coupled semiconductor quantum dots (QDs) have large impacts in optical transitions and energies of intershell collective electronic excitations. The observed anisotropies are revealed by inelastic light scattering as a function of the in-plane direction of light polarization and can be finely controlled by modifying the geometrical shape of the QDs. These experiments show that the tuning of the QD confinement potential offers a powerful method to manipulate electronic states and far-infrared intershell optical transitions in QDs. [ABSTRACT FROM AUTHOR]

Dilute arrays of GaAs/AlGaAs modulation-doped quantum dots with same sizes fabricated by electron beam lithography and low impact reactive ion etching exhibit highly uniform luminescence lines. Single quantum dots display spectral emission with peak energies and linewidths linked largely to the geometrical diameter of the dot and to the built-in electron population. Multicharged excitonic and biexcitonic emission intensities have activation energy of about 2 meV. These results highlight the potential of high quality nanofabricated quantum dots for applications in areas that require fine control of optical emission. [ABSTRACT FROM AUTHOR]

Tunneling excitations of electrons in dry-etched modulation-doped AlGaAs/GaAs coupled quantum dots (QDs) are probed by resonant inelastic light scattering. A sequence of intra- and intershell excitations are found at energies determined by the interplay between the QD confinement energy hω0 and the tunneling gap ΔSAS, the splitting between the symmetric and antisymmetric delocalized single particle molecular states. The narrow linewidths displayed by electronic excitations in these nanostructures indicate promising venues for the spectroscopic investigation of entanglement of electron states in these artificial molecules. [ABSTRACT FROM AUTHOR]

We measured the absorption spectra of T-shaped quantum wires at room temperature using waveguide-transmission spectroscopy. A strong and narrow room-temperature one-dimensional-exciton absorption peak was observed, which indicates a peak modal absorption coefficient of 160 cm-1 per 20 wires with a Γ-factor of 4.3×10-3, a width of 7.2 meV, and strong polarization anisotropy. [ABSTRACT FROM AUTHOR]

We measured the absorption spectrum of a single T-shaped 14×6 nm lateral-sized quantum wire embedded in an optical waveguide using waveguide-transmission spectroscopy at 5 K. In spite of its small volume, the one-dimensional-exciton ground state shows a large absorption coefficient of 80 cm-1, or a 98% absorption probability for a single pass of the 500 μm long waveguide. [ABSTRACT FROM AUTHOR]

An unbalanced Fabry–Pérot resonator is proposed as a miniature electro-optic probe. The resonator allows simultaneous enhancement of the electro-optic effect and the total reflectance at the resonance wavelength. We demonstrate a compact 3.31 μm long electro-optic probe based on the unbalanced AlGaAs/GaAs microresonator. [ABSTRACT FROM AUTHOR]

Spatially and spectrally resolved microscopic images of spontaneous and stimulated emissions are imaged at the mirror facets of a GaAs T-shaped quantum wire laser with high uniformity. Laser emission from the one-dimensional ground state reveals a circular image located at the core of a T-shaped optical waveguide but significantly smaller in area than the low power spontaneous emission from the same waveguide. These images unambiguously allow assignment of all spontaneous and laser emissions to the wire ground state and respective intersecting wells in the structure. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We measured polarization-dependent photoluminescence-excitation spectra of highly uniform T-shaped quantum wires at 5 K. We attribute one peak to the one-dimensional- (1D-) exciton ground state and the continuous absorption band to 1D continuum states. These had similar polarization dependences. We also observed some other peaks above the 1D-exciton ground state and attribute them to exciton states consisting of excited hole subbands. These results show good agreement with a model calculation of a single electron–hole pair in T-shaped geometry with exact diagonalizations of the Coulomb interaction. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Atomically flat (110) GaAs surfaces fabricated by the cleaved-edge overgrowth method and high-temperature growth-interrupt annealing are characterized at the atomic scale. We observe atomically flat (110) surfaces extending over areas more than 100 μm in size. Moreover, deposition of slightly less or more than integral monolayers (MLs) causes the appearance of characteristic step-edge shapes such as 1-ML-deep pits, or 2-to-3-ML-high isolated islands. Statistical analysis on the size and shape distribution of the 1-ML-deep pits supports a simple model based on the stability of Ga and As atoms on step edges with different bonding configurations, and reveals driving force to form an atomically flat (110) surface. [ABSTRACT FROM AUTHOR]

High-quality T-shaped quantum wires are fabricated by cleaved-edge overgrowth with molecular-beam epitaxy on the interface improved by a growth-interrupt high-temperature anneal. Characterization by micro-photoluminescence (PL) and PL excitation (PLE) spectroscopy at 5 K reveals high uniformity, a sharp spectral width, and a small Stokes shift of one-dimensional (1D) excitons. The PLE spectrum for 1D states shows a large peak of ground-state excitons and a small absorption band ascribed to 1D continuum states with an onset at 11 meV above the exciton peak. [ABSTRACT FROM AUTHOR]

A laser with an active volume consisting of only a single-quantum wire in the one-dimensional (1D) ground state is demonstrated. The single wire is formed quantum mechanically at the T-shaped intersection of a 14 nm A1[sub 0.07]Ga[sub 0.93] As quantum well and a 6 nm GaAs quantum well, and is embedded in a 1D single-mode optical waveguide. We observe single-mode lasing from the quantum-wire ground state by optical pumping. The laser operates from 5 to 60 K, and has a low threshold pumping power of 5 mW at 5 K. [ABSTRACT FROM AUTHOR]

By a cleaved-edge overgrowth method with molecular beam epitaxy and a (110) growth-interrupt anneal, we have fabricated a GaAs quantum well exactly 30 monolayers thick bounded by atomically smooth AlGaAs heterointerfaces without atomic roughness. Microphotoluminescence imaging of this quantum well indeed shows spatially uniform and spectrally sharp emission over areas of several tens of microns in extent. By adding a fractional GaAs monolayer to our quantum well we are able to study the details of the atomic step-edge kinetics responsible for flat interface formation. [ABSTRACT FROM AUTHOR]

Examines an array of GaAs/AlGaAAs cleaved edge overgrowth quantum wires. Use of near-field scanning optical microscope; Sources of the luminescence peaks; Link of the quantum wire emission linewidth with the (110)-oriented single quantum well roughness.

We report an on-chip solid-state Mach-Zehnder interferometer operating on two-dimensional (2D) plasmonic waves at microwave frequencies. Two plasmonic paths are defined with GaAs/AlGaAs 2D electron gas 80 nm below a metallic gate. The gated 2D plasmonic waves achieve a velocity of ∼c/300 (c: free-space light speed). Due to this ultra-subwavelength confinement, the resolution of the 2D plasmonic interferometer is two orders of magnitude higher than that of its electromagnetic counterpart at a given frequency. This gigahertz proof-of-concept at cryogenic temperatures can be scaled to the terahertz-infrared range for room temperature operation, while maintaining the benefits of ultra-subwavelength confinement. [ABSTRACT FROM AUTHOR]

An imaging and loss evaluation method for semiconductor waveguides coupled with non-doped quantum wells is presented. Using the internal emission of the wells as a probe light source, the numbers and widths of the modes of waveguides with various ridge sizes were evaluated by CCD imaging, and the obtained values were consistent with effective index method calculation. Waveguide internal losses were obtained from analyses of the Fabry-Pérot fringes of waveguide emission spectra. We quantified the quality of 29 single-mode waveguide samples as an internal loss and variation of 10.2 ± 0.6  cm(-1).