Showing papers on "Quantum well published in 1984"

Band-Edge Electroabsorption in Quantum Well Structures: The Quantum-Confined Stark Effect

Abstract: We present theory and extended experimental results for the large shift in optical absorption in GaAs-AlGaAs quantum well structures with electric field perpendicular to the layers. In contrast to the Stark effect on atoms or on excitons in bulk semiconductors, the exciton resonances remain resolved even for shifts much larger than the zero-field binding energy and fields g 50 times the classical ionization field. The model explains these results as a consequence of the quantum confinement of carriers.

Gain and intervalence band absorption in quantum-well lasers

Abstract: The linear gain and the intervalence band absorption are analyzed for quantum-well lasers. First, we analyze the electronic dipole moment in quantum-well structures. The dipole moment for the TE mode in quantum-well structures is found to be about 1.5 times larger at the subband edges than that of conventional double heterostructures. Also obtained is the difference of the dipole moment between TE and TM modes, which results in the gain difference between these modes. Then we derive the linear gain taking into account the intraband relaxation. As an example, we applied this analysis to GaInAs/InP quantum-well lasers. It is shown that the effects of the intraband relaxation are 1) shift of the gain peak toward shorter wavelength with increasing injected carrier density even in quantum-well structures, 2) increase of the gain-spectrum width due to the softening of the profile, and 3) reduction in the maximum gain by 30-40 percent. The intervalence band absorption analyzed for quantum-well lasers is nearly in the same order as that for conventional structures. However, its effect on the threshold is smaller because the gain is larger for quantum wells than conventional ones. The characteristic temperature T 0 of the threshold current of GaInAs/InP multiquantum-well lasers is calculated to be about 90 K at 300 K for well width and well number of 100 A and 10, respectively.

High‐speed optical modulation with GaAs/GaAlAs quantum wells in a p‐i‐n diode structure

Abstract: A new type of high‐speed optical modulator is proposed and demonstrated An electric field is applied perpendicular to GaAs/GaAlAs multiple quantum well layers using a ‘‘p‐i‐n’’ diode doping structure of 4‐μm total thickness The optical absorption edge, which is particularly abrupt because of exciton resonances, shifts to longer wavelengths with increasing field giving almost a factor of 2 reduction in transmission at 857 nm with an 8‐V reverse bias The shifts are ascribed to changes in carrier confinement energies in the wells The observed switching time of 28 ns is attributed to RC time constant and instrumental limitations only, and fundamental limits may be much faster

Energy levels of Wannier excitons in G a A s − Ga 1 − x Al x As quantum-well structures

Abstract: Energy levels of Wannier excitons in a quantum-well structure consisting of a single slab of GaAs sandwiched between two semi-infinite layers of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ are calculated with the use of a variational approach. Owing to lowering of symmetry along the axis of growth of this quantum-well structure and the presence of energy-band discontinuities at the interfaces, the degeneracy of the valence band of GaAs is removed, leading to two exciton systems, namely, the heavy-hole exciton and the light-hole exciton. The values of the binding energies of the ground state and of a few lowlying excited states of these two exciton systems are calculated as a function of the size of the GaAs quantum well for several values of the heights of the potential barriers and their behavior is discussed. The results thus obtained are also compared with the available experimental data. The reliability of the various approximations made in this calculation is discussed.

New mode of IR detection using quantum wells

Abstract: A new mode of IR detection using photoemission from a single quantum well is proposed and optimization of the device performance by the proper choice of parameters is discussed. Despite the very thin device structures, theoretical calculations show large absorption at wavelengths near cutoff. The largest photoemissive response is found by adjusting the well parameters so that an excited virtual state lies just above threshold.

Quantum noise and dynamics in quantum well and quantum wire lasers

Abstract: We calculate the relaxation oscillation corner frequency fr and the linewidth enhancement factor alpha for both a quantum well and a quantum wire semiconductor laser. A comparison of the results to those of a conventional double heterostructure device indicates that fr can be enhanced by 2× in the quantum well case and 3× in the quantum wire case while alpha is reduced in both cases.

Low-temperature exciton trapping on interface defects in semiconductor quantum wells

Abstract: We present the results of low-temperature photoluminescence experiments performed on GaAs single quantum wells grown by metal organic chemical-vapor deposition. The luminescence line is down shifted by a few milli-electron-volts below the $n=1$ heavy-hole exciton absorption peak. This behavior is interpreted in terms of exciton trapping on interface defects. A simple model provides reasonable values for the exciton binding energy on these defects as well as insights on the lack of thermalization which characterizes the trapped exciton photoluminescence.

Magneto-optical determination of exciton binding energy in GaAs- Ga 1 − x Al x As quantum wells

Abstract: The binding energy of the exciton in GaAs quantum wells confined within Ga-Al-As is determined by the observation of the different behavior of the ground state and the excited states of excitonic transitions of different subbands with excitation spectroscopy in magnetic fields. An increase in the binding energy with decreasing well thickness is found with values higher than theoretically expected. This discrepancy is explained by an experimentally determined higher reduced mass than that used in the theoretical calculations.

Comment on Polarization Dependent Momentum Matrix Elements in Quantum Well Lasers

Abstract: The polarization dependent gain in quantum well lasers reported previously is explained theoretically in terms of the polarization dependence of momentum matrix elements related to conduction-to-heavy hole and conduction-to-light hole band transitions.

Impurity trapping, interface structure, and luminescence of GaAs quantum wells grown by molecular beam epitaxy

Abstract: A correlation has been demonstrated between impurity trapping and the interface structure in GaAs quantum well (QW) superlattices and single quantum well structures grown by molecular beam epitaxy (MBE) on (100) GaAs. Using low‐temperature cathodoluminescence, photoluminescence, and transmission electron miscroscopy, we have shown that interface roughness in QW superlattices is related to trapped impurities at interfaces. We have observed that impurities originate from either the substrate or from the GaAlAs MBE layers. A new getter smoothing effect associated with QW structures is shown to produce efficient impurity trapping and yield higher quality GaAs QW with atomically smooth interfaces.

Theory of photoluminescence line shape due to interfacial quality in quantum well structures

Abstract: We have developed a simple theory to understand the role of interfacial quality in the line shape of photoluminescence spectra in quantum wells. The interface is described in terms of microscopic fluctuations δ1 and δ2, where δ1 is the local fluctuation in the well width and δ2 is the lateral correlated extent of the fluctuation. We make use of Lifshitz theory of disordered alloys to determine the probability of distribution of fluctuations in the well size over the extent of the optical probe, i.e., the exciton. The line shape is then calculated from this probability distribution. Both δ1 and δ2 are found to be important in controlling the linewidths in quantum wells. The use of this quantitative theory to characterize the microscopic nature of interfaces is discussed.

Effect of Inversion Symmetry on the Band Structure of Semiconductor Heterostructures

Abstract: Two classes of artificial semiconductor heterostructures, differing only in the inversion symmetry of their internal quantum wells, are studied via magnetotransport. The samples consist of GaAs/(AlGa) As layered structures containing two-dimensional hole systems. The results reveal a lifting of the spin degeneracy of the lowest hole subband in the samples with inversion asymmetric quantum wells. In those structures with symmetric wells the subband remains doubly degenerate.

Strained‐layer quantum‐well injection laser

Abstract: Data are presented demonstrating room‐temperature operation of a strained‐layer quantum‐well injection laser. The laser structure, grown by molecular beam epitaxy, consists of an active region with three InxGa1−xAs (x∼0.35) quantum wells (LZ ∼40 A) separated by two GaAs barriers (LB ∼30 A). These layers are centered in a larger GaAs collection/confinement region (LZ ∼1600 A) bounded by AlyGa1−yAs ( y∼0.45) cladding layers. The lasers operate at λ∼1.0 μm with greater than 4‐mW optical power output/facet under pulsed conditions at 300 K. Threshold current densities between 1000 and 2000 A/cm2 are obtained.

Calculation of the conduction band discontinuity for Ga(0.47)In(0.53)As/Al(0.48)In(0.52)As heterojunction

Abstract: Photoluminescence studies at 4 K on Ga0.47In0.53As/ Al0.48In0.52As single quantum wells exhibit emission ranging from 1.318 eV for a 15‐A well to 0.82 eV for thick wells. The emission energy of each single quantum well is compared to theoretical curves which are generated from a finite potential square well model. The closest agreement between the experimental curves and the theoretical curves occurs when the conduction band discontinuity is taken to be 70% of the band‐gap discontinuity or 0.52 eV.

Photoluminescence of AlGaAs/GaAs quantum wells grown by metalorganic chemical vapor deposition

Abstract: Photoluminescence of AlxGa1−xAs/GaAs(x=0.54) single quantum wells grown by metal organic chemical vapor deposition has been investigated at both 75 and 4.2 K. AlGaAs/GaAs heterojunction abruptness was estimated to be within a few atomic layers by comparing the peak energy of the quantum well photoluminescence with the values calculated on the assumption that the radiative transition takes place between the n=1 electron subband and the n=1 heavy‐hole subband in the finite square potential well at 75 K. At 4.2 K, however, the peak energy shifts by several meV below the calculated energy, the cause of which may be formation of a two‐dimensional free exciton. The sharp photoluminescence line of the narrower well indicates that the fluctuation in thickness is less than one half the lattice constant. The emission peak shift to lower energy with the increase of excitation intensity may be the result of exchange interaction among carriers.

Localization induced electron‐hole transition rate enhancement in GaAs quantum wells

Abstract: The transition rate of excitonic e‐h recombination in GaAs quantum wells is found to increase by almost one order of magnitude upon a decrease of the well width from infinity to 52 A due to increased excitonic localization. Competing capture of free carriers by impurities is increasingly suppressed. Time delayed cathodoluminescence spectra and decay times taken at 5–300 K show this unambiguously. Transfer of carriers from 176‐A Ga0.6Al0.4As cladding wells is found to occur ballistically in 10−13 s.

Threshold currents for AlGaAs quantum well lasers

Abstract: The threshold currents for AlGaAs quantum well lasers are studied theoretically. The structure dependent gain coefficient is obtained by taking into account the electron distribution in L valleys. Theoretical threshold current densities calculated using the gain coefficient agree well with reported experimental results for separate-confinement heterostructure lasers. A design procedure for low threshold current laser is elucidated. The lowest threshold currents are 570 and 53 μA per 1 μm stripe width for modified multiple quantum well lasers with 32 percent and 90 percent reflectivity facet mirrors, respectively.

Luminescence and transport in a-Si:H/a-Si1−xNx:H quantum well structures

Abstract: Multiple-quantum-well (MQW) heterostructures consisting of twenty layers of a-Si:H (30∼200 A thick) and a-Si1−xNx:H (30∼200 A thick, x ≅ 0.2) have been studied by photoluminescence and current transport. The luminescence from the MQW structure exhibits a single peak at 1.37 eV originating in the a-Si:H well layers, and there is no emission at ∼1.48 eV arising from the a-Si1−xNx:H barrier layers. This is because the photocarriers generated in the barrier layers flow into the a-Si:H wells. This confinement of photocarriers in the quantum well has been demonstrated by analysing the luminescence quenching in the electric field applied perpendicularly to the MQW heterojunctions. The current transport parallel to the quantum well under light illumination has revealed the remarkable increase of photo-conductance with decreasing the well layer width. This is tentatively interpreted in terms of a significant increase in the mobility and lifetime of quasi-two-dimensional electrons in the a-Si:H well layers.

Some characteristics of the GaAs/GaAlAs graded‐index separate‐confinement heterostructure quantum well laser structure

Abstract: We describe the characteristics of graded‐index separate‐confinement heterostructure (GRIN‐SCH) quantum well laser structures for a wide range of quantum well thickness and graded layer composition. It was deduced that the ‘‘GRIN’’ region enhances carrier confinement and assists the thermalization of carriers into the quantum well. A maximum value of T0 of 190 K was measured for these single quantum well lasers.

Hydrogenic impurity states in a quantum well wire

Abstract: A variational calculation of hydrogenic impurity binding energies in quantum well wires has been performed. The binding of the hydrogenic impurity has been calculated as a function of the transverse dimensions of the wire. It is found that the binding energy of the hydrogenic impurity increases as the ratio of the Bohr radius of the impurity in a bulk semiconductor to the transverse dimension of the wire increases. To test the sensitivity of the binding energies to the trial wave function we have used in our calculations, we use a wave function of the same type to calculate the binding energy of hydrogenic impurities confined in a quasi‐two‐dimensional quantum well as a function of well width and compare our results for the binding energies to the results obtained by Bastard [Phys. Rev. B 24, 4714 (1981)].

Picosecond time-resolved study of excitons in GaAs-A1As multi-quantum-well structures

Abstract: Energy- and time-resolved measurements of luminescence of $1s$ excitons ($n=1,e\ensuremath{-}hh$) in GaAs-A1As multi-quantum-well structures have been carried out for the first time in the picosecond time domain. Dynamical population changes of excitons are directly visualized in the energy-time coordinates. Results indicate that excitons lose their energy in the exciton band at a rate of 1.0\ifmmode\times\else\texttimes\fi{}${10}^{6}$ eV/s. This rate is much slower than the calculated kinetic-energy-loss rate and is ascribed to the random nature of the well.

On the properties of InSb quantum wells

Abstract: A brief survey of the properties of InSb quantum wells is reported. The energy level scheme is described on the basis of a square-well model taking into account the pronounced non-parabolicity of the conduction band. A simple model for electron transport with polar optical mode scattering is presented. The temperature dependence of the low-field electron mobility is calculated for various well-widths. Threshold fields for negative differential resistance and for energy runaway are estimated. Threshold energies for impact ionization, including resonance effects involving the electronic sub-bands are also obtained. Novel features expected of InSb quantum wells are summarized and discussed.

Screened Coulombic impurity bound states in semiconductor quantum wells

Abstract: The ground bound state of Coulombic impurity screened by free carriers in a quantum well versus the free-electron concentration ${n}_{e}$ is calculated for several well thicknesses. Zero temperature and electric quantum limit are assumed. The random-phase-approximation dielectric function is used to describe the screening effect. For a given well thickness the binding energy decreases with increasing electron concentration until a saturation is reached at large ${n}_{e}$. The remaining binding is not negligible. At low temperature this may give rise to a freeze-out effect.

Heterostructure semiconductor lasers prepared by molecular beam epitaxy

Abstract: 0.72-0.88 \mu m (AlGa)As, 1.3-1.65 \mu m GaInAsP and AlGa-InAs, and 1.78 μm AlGaSb double-heterostructure (DH) lasers were prepared by molecular beam epitaxy. For AlGaAs DH lasers very low 300 K threshold current densities and long operating life (mean time to failure >106h at 300 K) were achieved and optical transmitters containing MBE-grown lasers have been field-tested. For lasers with lasing wavelength >1\mu m, MBE is in the development stage. The unique capabilities of MBE as an epitaxial growth technique and its important contributions to the field of optoelectronics are illustrated by a discussion of a new class of laser structures including quantum well heterostructure of GaAs/ Al x Ga 1-x AS and Ga 0.47 In 0.53 As/InP, double-barrier double-heterostructure, and graded-index waveguide separate-confinement-heretostructure lasers. These new lasers, made possible by MBE, have characteristics unmatched by conventional liquid phase epitaxial growth techniques.

Electronic states and thicknesses of GaAs/GaAlAs quantum wells as measured by electroreflectance and spectroscopic ellipsometry

Abstract: The use of electroreflectance and spectroscopic ellipsometry is demonstrated for the assessment of quantum well structures consisting of, respectively 4‐, 5‐, and 7‐nm‐thick GaAs layers deposited by organometallic chemical vapor deposition between undoped layers of Ga0.46Al0.54As. Electroreflectance provides a direct determination of all allowed optical transitions (Δn=0) in the GaAs well around the Γ point as well as their dependence versus an electric field. Spectroscopic ellipsometry gives both the thickness and the dielectric function of the GaAs well. This is a unique case where these two quantities are determined accurately at the same point under analysis. Furthermore, the two techniques have been used to analyze the optical transitions in the well around the L point (E1, E1+Δ1 type of transition), a usually unexplored region. This provides complementary information upon the electronic states in the quantum well.

Short wavelength (visible) GaAs quantum well lasers grown by molecular beam epitaxy

Abstract: GaAs‐AlGaAs multiple quantum well (MQW) injection lasers with well widths from 55 to 13 A have been grown by molecular beam epitaxy and operated at room temperature, showing emission at wavelengths down to 704 nm, the shortest reported for a MQW injection laser with GaAs wells. In a device with 25‐A wells some evidence of coupling was apparent when barrier widths were reduced to 40 A. For devices with 80‐A barriers there is a difference of about 20 nm between the calculated n=1 (e–hh) transition wavelength and the lasing wavelength, whereas the calculation agrees with photovoltage absorption measurements on the same structures.

Raman Scattering from Electrons Bound to Shallow Donors in GaAs-Al sub x Ga sub 1-x As Quantum-Well Structures,

Abstract: Transitions involving donor states were observed in resonant Raman scattering experiments on Si-doped $\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}\ensuremath{-}{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ multiple quantum wells. The electronic scattering gradually transforms into photoluminescence as the exciting energy is tuned across the resonance. The largest contribution in the spectra is associated with $1S\ensuremath{\rightarrow}2S$ transitions of donors near the center of the wells. The experimental results show good agreement with recent calculations.

Two-dimensional numerical analysis of the high electron mobility transistor

Abstract: We develop a two-dimensional model for the high electron mobility transistor (HEMT) including conduction outside the quantum well. The model uses the continuity and power balance moment equations for both inside and outside the well, with electron concentration and average energy as dependent variables, and with parameters determined by Monte Carlo simulation. We show that conduction outside the well is dominant in the "pinchoff" region and that consequently the speed advantage of the HEMT over conventional devices does not arise from high saturation velocities in the quantum well but rather from a lower access resistance as suggested by a velocity profile calculation. It is further demonstrated that several effects which are unimportant in conventional FET's are of significance in the HEMT. Among these effects are electronic heat conduction and to some extent real space transfer.

Single quantum well lead‐europium‐selenide‐telluride diode lasers

Abstract: It is desirable to increase the operating temperature of long wavelength lead‐chalcogenide diode lasers to simplify cooling system requirements. Recently, double heterostructure Pb1−xEuxSeyTe1−y diode lasers (grown by molecular beam epitaxy) operated up to 147 K cw (180 K pulsed). The output photon energy of these devices is linear with x in the wavelength range 6.6– 2.7 μm. The growth of single quantum well lead‐chalcogenide diode lasers is now reported for the first time. These devices had PbTe quantum wells with PbEuSeTe confinement layers. The width of the quantum wells, Lz, was varied from 300 to 250 A. Strong quantum effects are observed for Lz ≲ 1200 A because of the small carrier masses (me∼mh∼0.04mo ). The shift in laser emission energy is in approximate agreement with that calculated from a finite square well potential. At low temperatures (≲100 K), these lasers appear to operate on transitions between n=1 states in the conduction and valence bands at threshold. Transitions between the n=2 state...