Showing papers on "Quantum well published in 1988"

Observation of discrete electronic states in a zero-dimensional semiconductor nanostructure.

Abstract: Electronic transport through a three-dimensionally confined semiconductor quantum well (``quantum dot'') has been investigated. Fine structure observed in resonant tunneling through the quantum dot corresponds to the discrete density of states of a zero-dimensional system.

Quantum capacitance devices

Abstract: Two‐dimensional electron gas (2DEG) in a quantum well or inversion layer, unlike an ordinary grounded metallic plane, does not completely screen an applied transverse electric field. Owing to its Fermi degeneracy energy, a 2DEG manifests itself as a capacitor in series, whose capacitance per unit area equals CQ=me2/πℏ2, where m is the effective electron mass in the direction transverse to the quantum well. Partial penetration of an external field through a highly conducting 2DEG allows the implementation of several novel high‐speed devices, including a three‐terminal resonant‐tunneling transistor and a gate‐controlled thermionic emission transistor.

Atom diffusion and impurity‐induced layer disordering in quantum well III‐V semiconductor heterostructures

Abstract: The process of impurity‐induced layer disordering (IILD) or layer intermixing, in AlxGa1−xAs‐GaAs quantum well heterostructures (QWHs) and superlattices (SLs), and in related III‐V quantum well heterostructures, has developed extensively and is reviewed. A large variety of experimental data on IILD are discussed and provide newer information and further perspective on crystal self‐diffusion, impurity diffusion, and also the important defect mechanisms that control diffusion in AlxGa1−xAs‐GaAs, and in related III‐V semiconductors. Based on the behavior of Column III vacancies and Column III interstitials, models for the crystal self‐diffusion and impurity diffusion that describe IILD are reviewed and discussed. Because impurity‐induced layer disordering has proved to be an important method for III‐V quantum well heterostructure device fabrication, we also review the application of IILD to several different laser diode structures, as well as to passive waveguides. We mention that it may be possible to reali...

Rapid radiative decay and enhanced optical nonlinearity of excitons in a quantum well.

Abstract: An exciton has a macroscopic transition dipole moment because it is a coherent excitation over the whole crystal. The interaction of this exciton with a radiation field, which results in a polariton in a bulk crystal, brings about the rapid radiative decay of the exciton in low-dimensional systems due to breakdown of the translational symmetry. This large decay constant at the same time makes the excitons deviate from ideal bosons so that we have a large third-order optical susceptibility enhanced by the macroscopic transition dipole moment under near-resonant excitation. The nonlinear optical phenomena are expected to have a fast response time of a picosecond in GaAs quantum wells and a subpicosecond in CdS quantum wells through the short lifetime of excitons.

Extremely wide modulation bandwidth in a low threshold current strained quantum well laser

Abstract: Lasing characteristics of strained quantum well (QW) structures such as InGaAs/AlGaAs on GaAs and InGaAs/InAlAs on InP were analyzed by taking into account the band mixing effect in the valence band. A relaxation oscillation frequency fr, which gives a measure of the upper modulation frequency limit, was found increased three times in a 50 A In0.9Ga0.1As/In0.52Al0.48As QW structure compared with that in a 50 A GaAs/Al0.4Ga0.6As QW structure for the undoped case. One of the main factors for this improved frequency bandwidth is attributed to the reduced subband nonparabolicity as well as the reduced valence‐band density of state in the strained QW structure. The corresponding lasing threshold current is one order of magnitude smaller than that of the GaAs/AlGaAs QW structure. With a p doping in the QW the fr value increases, and the 3 dB cutoff frequency of about 90 GHz will be expected with an acceptor concentration of 5×1018 cm−3 in the In0.9Ga0.1As/In0.52Al0.48As QW.

A model for GRIN-SCH-SQW diode lasers

Abstract: A comprehensive model for graded-index, separate-confinement-heterostructure, single-quantum-well (GRIN-SCH-SQW) Al/sub x/Ga/sub 1-x/As diode lasers is presented, and compared with experimental data. The model combines many individual features not heretofore included together, and gives good agreement with gain-vs.-current density data for different structure variations. In addition, the threshold temperature dependence agrees well with data for typical laser conditions, and the high-gain kink in T/sub 0/ versus temperature is qualitatively explained. >

Diode laser--pumped solid-state lasers.

Abstract: Diode laser-pumped solid-state lasers are efficient, compact, all solid-state sources of coherent optical radiation. Major advances in solid-state laser technology have historically been preceded by advances in pumping technology. The helical flash lamps used to pump early ruby lasers were superseded by the linear flash lamp and arc lamp now used to pump neodymium-doped yttrium-aluminum-garnet lasers. The latest advance in pumping technology is the diode laser. Diode laser-pumped neodymium lasers have operated at greater than 10 percent electrical to optical efficiency in a single spatial mode and with linewidths of less than 10 kilohertz. The high spectral power brightness of these lasers has allowed frequency extension by harmonic generation in nonlinear crystals, which has led to green and blue sources of coherent radiation. Diode laser pumping has also been used with ions other than neodymium to produce wavelengths from 946 to 2010 nanometers. In addition, Q-switched operation with kilowatt peak powers and mode-locked operation with 10-picosecond pulse widths have been demonstrated. Progress in diode lasers and diode laser arrays promises all solid-state lasers in which the flash lamp is replaced by diode lasers for average power levels in excess of tens of watts and at a price that is competitive with flash lamp-pumped laser systems. Power levels exceeding 1 kilowatt appear possible within the next 5 years. Potential applications of diode laser-pumped solid-state lasers include coherent radar, global sensing from satellites, medical uses, micromachining, and miniature visible sources for digital optical storage.

Temperature dependence of photoluminescence from GaAs single and multiple quantum‐well heterostructures grown by molecular‐beam epitaxy

Abstract: The photoluminescence (PL) from GaAs/AlxGa1−xAs single and multiple quantum well (QW) heterostructures grown by molecular‐beam epitaxy (MBE) has been studied in the temperature range 10

Scaling laws and minimum threshold currents for quantum-confined semiconductor lasers

Abstract: Basic scaling laws are derived for bulk, two‐dimensional (quantum well) and one‐dimensional (quantum wire) semiconductor lasers. Starting from quantum derivation of the optical properties of confined carriers, the dimensional dependencies of the scaling laws are made explicit. Threshold currents of ∼100 and 2–3 μA are predicted for single quantum well and quantum wire lasers, respectively. The basic considerations of this analysis were used recently to obtain ultralow threshold quantum well lasers [P. L. Derry, A. Yariv, K. Lau, N. Bar‐Chaim, K. Lee, and J. Rosenberg, Appl. Phys. Lett. 50, 1773 (1987)].

Free carrier and many-body effects in absorption spectra of modulation-doped quantum wells

Abstract: The temperature-dependent optical absorption and luminescence spectra of GaAs/AlGaAs and InGaAs/InAlAs n-doped modulation-doped quantum wells is discussed with emphasis on the peak seen at the edge of the absorption spectra of these samples A many-body calculation of the electron-hole correlation enhancement is presented, which identifies this peak with the Mahan exciton-the result of the Coulomb interaction between the photoexcited hole in the valence band and the sea of electrons in the conduction band This calculation accounts for the strong dependence of the absorption edge peak on both the temperature and carrier concentration, in good qualitative agreement with experimental data and with previously published results The changes induced by the carriers on the subband structure through self-consistent calculations are also analyzed, and it is concluded that in these symmetric structures, the changes are small for achievable carrier densities >

Quantum box fabrication tolerance and size limits in semiconductors and their effect on optical gain

Abstract: Lower and upper limits on size are established for quantum boxes. The lower limit is shown to result from a critical size below which electronic states no longer exist. This critical size is different for electrons and holes. The optical gain of arrays of quantum boxes is computed, taking into account the inhomogeneous broadening of the gain spectrum resulting from fabricational variations in quantum box size and shape. >

Electroabsorption of highly confined systems: Theory of the quantum‐confined Franz–Keldysh effect in semiconductor quantum wires and dots

Abstract: Semiconductor quantum wells are known to show large electroabsorption (e.g., the quantum‐confined Stark effect) that results in low‐energy optical modulating and switching devices. We show theoretically that the electroabsorption and associated electrorefraction in lower dimensional structures could be much larger, suggesting very low energy devices. We illustrate the theory with specific calculations for hypothetical GaAs‐like quantum wires and dots.

Lead salt quantum effect structures

Abstract: Lead salt (IV-VI) compounds have been grown epitaxially by a variety of growth techniques, such as molecular-beam epitaxy and hot-wall epitaxy. Recently, compositional superlattices and quantum-well heterostructures have been grown that exhibit strong quantum optical effects. These structures have been used to fabricate midinfrared diode lasers with greatly improved operating temperatures. Thus, it appears that these devices will continue to maintain a significant advantage over II-VI and III-V compound diode lasers. Doping superlattices have been made which possess enhanced minority carrier properties. Ferromagnetic ordering in PbSnTe-MnTe alloys suggests potential areas for future work in magnetic field sensitivity devices. Lead salt quantum-effect structures are included. >

Optical gain in a strained-layer quantum-well laser

Abstract: The optical gain and the refractive index change of a uniaxially stressed GaAs-Al/sub 2/Ga/sub 1-x/As quantum-well laser is studied theoretically using the multiband effective mass theory (k-p method) and density matrix formalism with intraband relaxations. It is found that uniaxial strain of the quantum well substantially alters the subband structures and the optical gain of the quantum-well laser. In particular, the gain of the TM mode increases while the gain of the TE mode decreases with increasing stress. Thus, the threshold current either decreases or increases with the stress, depending on whether the laser is operating in a TM or TE mode. >

Capture of electrons and holes in quantum wells

Abstract: The capture of electrons and holes by quantum wells in multiple quantum well samples of InGaAs/InP is investigated using subpicosecond luminescence spectroscopy. For samples with thin barriers, quantum capture or carrier thermalization dominates. For thicker barriers (>500 A), transport of carriers to the well dominates. We show that quantum capture time is <0.3 ps for holes and <1 ps for electrons. No significant dependence on well thickness is observed. Finally, Coulomb interaction between electrons and holes is shown to ‘‘trap’’ the electrons in unbound states in InGaAs before they are captured by the well.

Exchange interactions in quantum well subbands

Abstract: It is shown that exchange interactions in the two‐dimensional electron gas in quantum wells could cause observable effects on subband energies and intersubband transition energies. In the case of doped quantum wells, the intrasubband exchange interaction can produce an energy shift which is substantially larger than the direct Coulomb energy shift. Theoretical estimates of such shifts are compared with experimental measurements of the infrared photoconductivity of multiple quantum well AlGaAs/GaAs structures with wells doped at about 1018 cm−3.

Effect of doping on the optical gain and the spontaneous noise enhancement factor in quantum well amplifiers and lasers studied by simple analytical expressions

Abstract: The maximum optical gain and the spontaneous noise enhancement factor in quantum well structures are expressed as extremely simple functions that are accurate over a wide range of carrier densities. These expressions are used to study the effect of doping on the optical gain and the noise enhancement factor in a 100 A InGaAs/InP quantum well structure. n-type doping is most effective in reducing the transparency excitation level (laser threshold) and the noise enhancement factor (amplifier noise figure), whereas p-type doping enables increased gain at a given excitation level.

Resonant tunneling in Si/Si1−xGex double‐barrier structures

Abstract: Resonant tunneling of holes has been observed for the first time in double‐barrier diodes with strained Si1−xGex quantum wells formed between unstrained Si barriers. Negative differential resistance with a peak‐to‐valley ratio in current of 1.8 at 77 K and of 2.2 at 4.2 K has been exhibited by a sample with a 3.3‐nm‐wide Si0.79Ge0.21 well between 6.0 nm Si barriers. The positions of the current peaks are in reasonable agreement with calculations of the positions of heavy‐hole levels in the quantum well.

Strained-layer InGaAs-GaAs-AlGaAs photopumped and current injection lasers

Abstract: The materials growth, materials characterization, device fabrication, device results, and modeling of strained-layer InGaAs-GaAs-AlGaAs photopumped and current injection quantum-well lasers are presented. Experimental and theoretical methods for determining the electronic energy states in pseudomorphic quantum wells are presented and discussed, and design curves for the emission energy of biaxially compressed InGaAs and GaAs are presented as a function of indium composition and quantum well width. Photopumped lasers with thresholds comparable to early lattice-matched AlGaAs-GaAs quantum-well lasers, as well as continuous-wave room temperature strained-layer injection lasers, are demonstrated. The temperature dependence of the current injection devices is good (T/sub 0/=147 K) in marked contrast to photopumped samples. Preliminary life test results indicate that long-lived strained-layer injection lasers may be possible. >

Electric field control of optical second-harmonic generation in a quantum well

Abstract: We present theoretical predictions of the second‐harmonic susceptibility due to an intersubband transition within the conduction band of a quantum well in an external applied electric field. The asymmetry of the quantum well due to the electric field accounts for the nonvanishing of the second‐order susceptibilities. It is shown that for moderate values of an applied electric field of 10–70 kV/cm, the second‐harmonic susceptibility is generally 10–100 times larger than that of bulk GaAs. Furthermore, this procedure of second‐harmonic generation can be controlled by an external modulating voltage.

Effects of two‐dimensional confinement on the optical properties of InGaAs/InP quantum wire structures

Abstract: We describe fabrication and photoluminescence excitation of InGaAs/InP quantum wires with a lateral dimension of ∼350 A. Transverse confinement results in the splitting of the n=1 heavy hole‐electron transition. Three of these levels are observed in the excitation spectrum. The exciton energies agree with the theoretical predictions based on a new method of solving the two‐dimensional effective mass Schrodinger equation.

Quantum wells with enhanced exciton effects and optical non-linearity

Abstract: Exciton effects are studied theoretically for a quantum well of a semiconductor sandwiched by barriers with a smaller dielectric constant and a larger energy gap. The exciton binding energy increases markedly so that the radiative decay rate of the exciton and the non-linear optical susceptibility are also shown to be enhanced.

Electro‐optic phase modulation in GaAs/AlGaAs quantum well waveguides

Abstract: We present the first absolute measurements of the electric‐field‐induced refractive index change in GaAs/AlGaAs quantum well waveguides. Phase and intensity modulation are characterized as a function of wavelength both above and below the n=1 exciton resonances.

Tunable superlattice p-i-n photodetectors: characteristics, theory, and application

Abstract: Extended measurements and theory on the recently developed monolithic wavelength demultiplexer consisting of voltage-tunable superlattice p-i-n photodetectors in a waveguide confirmation are discussed. It is shown that the device is able to demultiplex and detect two optical signals with a wavelength separation of 20 nm directly into different electrical channels at a data rate of 1 Gb/s and with a crosstalk attenuation varying between 20 and 28 dB, depending on the polarization. The minimum acceptable crosstalk attenuation at a data rate of 100 Mb/s is determined to be 10 dB. The feasibility of using the device as a polarization angle sensor for linearly polarized light is also demonstrated. A theory for the emission of photogenerated carriers out of the quantum wells is included, since this is potentially a speed limiting mechanism in these detectors. It is shown that a theory of thermally assisted tunneling by polar optical phonon interaction is able to predict emission times consistent with the observed temporal response. >

Band-mixing effects and excitonic optical properties in GaAs quantum wire structures-comparison with the quantum wells

Abstract: The excitonic properties in quantum-wire structures (QWS) are analyzed by taking into account the band-mixing effects in the valence band of the structure. The effective mass value in the wire direction at the zone center of the lowest heavy-hole-like subband is found to be as small as 0.027 m/sub 0/ for GaAs/AlGaAs QWS. This reduced effective mass and the related nonparabolicity of the subband structure play a significant role in determining the exciton properties. Using these results, the maximum excitonic contribution to the refractive index value is estimated to be 0.59, i.e. 17.4% of the bulk value for a GaAs/Al/sub 0.4/Ga/sub 0.6/As QWS with a 50 A*50 AA cross section. This value is six times larger than that in the 50-AA quantum well. With an electric field of 8*10/sup 4/ V/cm perpendicular to the heterointerface, a maximum refractive index change 30% larger than this value is estimated. >

Advances in diode laser pumps

Abstract: Diode laser pumps for solid-state lasers are reviewed with special emphasis on current and future capabilities. Included are discussions of pumps for CW (continuous-wave) and quasi-CW operation, and end- and side-pumping configurations. >