Double Quantum Well Structure Research Articles

Measurements of far-infrared intersubband absorption linewidths in GaAs/AlGaAs quantum wells as a function of temperature and charge density

Intersubband transitions in doped quantum wells are of fundamental scientific interest, as well as technological interest for potential devices. One of the important characteristics of the transitions is their linewidth. We present the results of linear absorption spectroscopy on a coupled double asymmetric QW structure. Using a backgated sample, we can independently vary the charge density and DC field at the well. The absorption lines appear to be homogeneously broadened. The lines appear to become lifetime broadened for temperatures above 70 K.

Interaction effects on cyclotron resonance in Semiconductor double quantum well structures

We have investigated the cyclotron resonance (CR) in the asymmetric double quantum well (DQW) structures by far infrared transmission spectroscopy and discussed the effects of interlayer interactions on the CR. It is found that when the two quantum wells (QWs) are widely separated, two CR peaks due to two different cyclotron effective masses are observed. However, when the two QWs are strongly coupled, only a single CR peak is observed, even when both QWs are populated. Furthermore, the observed CR frequency gradually shifts as the electron population in the DQW structure is varied. This anomalous behavior of the CR in the strongly coupled DQW has been attributed to the interlayer mode coupling effect of the CR.

Thermal Stability Of SiGe/Si Quantum Well Structures Grown By APCVD

AbstractSingle and double Si1-xGexx/Si quantum well (QW) structures, which were grown by atmospheric pressure chemical vapor deposition (APCVD), are characterized by photoluminescence and secondary ion mass spectrometry. Systematic post-growth annealing treatments were carried out at temperatures between 600°C and 1100°C in pure N2 ambient. The interdiffusion between the Si layer and the Si1-xGex, well layers occurs at the annealing temperature around 900°C. The diffusion coefficient is deduced at different temperatures from SIMS measurements for single QW structures. The activation energy is about 3.9 eV in the temperature range between 950°C and 1100°C. The double QW structures show a similar value, but the accurate value is more difficult to obtain because it is more complicated to analyze the SIMS profile of the double QW structures. The intensity of the exciton recombination related to carriers confined in the double QW structures decreases with increasing annealing temperatures and becomes strongly suppressed at 750°C. When the annealing temperature is increased further, the intensity of the QW emission recovers. The results indicate that nonradiative centers were generated at annealing temperature of about 750°C

Asymmetric double-quantum-well phase modulator using surface acoustic waves

An AlGaAs-GaAs asymmetric double-quantum-well (DQW) optical phase modulator using surface acoustic waves is investigated theoretically. The optimization steps of the DQW structure, which so far have not been reported in detail, are discussed here. The optimized phase modulator structure is found to contain a five-period QDW active region. A surface acoustic wave induces a potential field which provides the phase modulation. Analysis of the modulation characteristics show that by using the asymmetric DQW, the large change of the induced potential at the surface and thus large modification of the quantum-well (QW) structure can be utilized. The modification of each QW structure is consistent, although this consistency is not always preserved in typical surface acoustic wave devices. Consequently, the change of refractive index in each of the five DQW's is almost identical. Besides, the change of effective refractive index is ten times larger here in comparison to a modulator with a five-period single QW as the active region and thus produces a larger phase modulation. In addition, a long wavelength and a low surface acoustic wave power required here simplify the fabrication of surface acoustic wave transducer and the acoustooptic phase modulator.

Optical Nonlinearities of Hybrid Direct–Indirect Wannier Excitons in Polytype Double Quantum Well Structures

We have calculated the nonlinear optical properties of a double QW structure, where it is possible to achieve a resonance between a real space direct exciton |dir〉 (electron and hole both confined in the same well) and a real space indirect exciton |ind〉 (electron and hole confined in separated wells). |dir〉 and |ind〉 are coupled by electron tunneling between the two QWs, so that the true excitonic eigenstates are the hybrid direct–indirect excitons. Because of the reduced electron–hole overlap, the |ind〉 oscillator strength is negligible compared to |dir〉, but owing to the electron–hole charge separation, it does have large nonlinear properties. Apart from the above electro-optic effect, many-body nonlinearities (due to large two-dimensional Bohr radii) are significant. We have applied our model to the realistic polytype AlSb/InAs/AlSb/GaSb/AlSb structure.

Effect of finite quantum-well width on the compressibility of a two-dimensional electron gas

Double quantum-well structures have been used to measure the inverse compressibility (${\mathrm{\ensuremath{\kappa}}}^{\mathrm{\ensuremath{-}}1}$) of a two-dimensional electron gas as a function of the confining quantum-well width. The lower layer is contacted independently and carrier density changes within this layer are used to measure ${\mathrm{\ensuremath{\kappa}}}^{\mathrm{\ensuremath{-}}1}$ of the upper layer. As the well width increases we observe a decrease in ${\mathrm{\ensuremath{\kappa}}}^{\mathrm{\ensuremath{-}}1}$ which may be attributed to the effect of the Hartree term. The inclusion of Hartree effects produces the correct trend of ${\mathrm{\ensuremath{\kappa}}}^{\mathrm{\ensuremath{-}}1}$ with well width, although discrepancies still exist between the model and experiment.

Delayed luminescence induced by intersubband optical excitation in a charge transfer double quantum well structure

Photoluminescence processes in a novel GaAs double quantum well (DQW) structure were studied and found to be controlled by a 10 μm mid-infrared (MIR) light. In this DQW structure, photogenerated electron-hole pairs are normally separated into different wells and their radiative recombination is inhibited. However, when a MIR light pulse is supplied to induce an electron intersubband transition, electrons are efficiently transferred to the hole-rich well, resulting in a significant enhancement of luminescence. By time shifting the MIR pulse with respect to the light pulse for interband excitation, we demonstrated the generation of a delayed photoluminescence. Device potentials of this MIR to near-infrared conversion are discussed.

High Resolution Electron Microscopy of InGaAs/InAIAs Interfaces

Recently, quantum wells (QW) have been constructed using the (In0.532Ga0.468)As/ (In0.522Al0.478)As system (hereafter InGaAs/In Al As), which is lattice matched to InP (lattice constant of 5.869Å). In order to understand the properties of such QWs, it is important to have knowledge of the structure and composition of interfaces. For III-V materials, compositional changes affect the <200> frequency component of the high resolution electron microscopy (HREM) image intensity (I200). This underlies the “chemical imaging” approach. Simulations for InGaAs/InAlAs interfaces suggest optimum conditions of microscope defocus df=-50nm and sample thickness t=14nm in the <100> orientation. A double-QW structure, consisting of InGaAs/ InAlAs/ InGaAs layers with nominal thicknesses of 4nm/ 2nm/ 4nm (respectively), and embedded in InAlAs layers, has been studied. All the layers were grown by molecular beam epitaxy at 500°C on a <100> InP substrate, with 60s growth interrupts between layers in the QW region.

Bipolar quantum-transport modeling of carrier injection into a SCH-quantum-well laser

A quantum mechanical model of carrier transport in separate confinement heterostructure (SCH) quantum-well lasers based on the Wigner function is presented for the first time. In the simulation, the three quantum Liouville equations with respect to the Wigner functions defined for electron, heavy-hole, and light-hole are solved simultaneously with the Poisson's equation to consider self-consistency in potential, As a simulation model, InGaAsP-InGaAs SCH quantum-well lasers are considered. The carrier injection into single and double quantum-well lasers is simulated. It is demonstrated that the amount of electrons and holes injected into the single quantum-well active layer is not equal in general if the quantum transport is considered. In the double quantum-well structure, the bottleneck phenomenon of heavy-hole injection into the second well and the quite different shape of heavy-hole density profiles in the two wells are simulated.

High-frequency conductivity of electrons in double quantum wells under an in-plane magnetic field

The high-frequency response of electrons in double quantum wells (DQWs) under an in-plane magnetic field is considered. The absorption, the Voigt effect and the transverse dipole moment due to electron transitions between tunnel-coupled levels are calculated for excitation by an electric field parallel to the 2D plane; the depolarization field, which is perpendicular to the 2D plane, is also taken into account. Strong modification of the intersubband absorption spectrum (the appearance of two absorption peaks, which demonstrate different collisionless broadening and shift) is found under increasing magnetic field. The spectral dependencies of the Voigt effect and induced dipole moment are calculated. Such dependencies are different in essence for small magnetic fields, for which both tunnel-coupled states are occupied; for intermediate magnetic fields, for which the Fermi level intersects the anticrossing gap; and for large fields, for which the electrons are localized in two valleys of the lowest level. Numerical estimates of these effects are presented for typical DQW structures.

High continuous wave output power InGaAs/InGaAsP/InGaP diode lasers: Effect of substrate misorientation

3 W cw output power has been obtained from aluminum-free, strained-layer double-quantum well (DQW) InGaAs/InGaAsP/InGaP uncoated, 100-μm-wide stripe diode lasers (λ=0.945 μm) grown by low-pressure MOCVD on exact (100) GaAs substrates. The combination of high-band-gap (1.62 eV) InGaAsP confinement layers and the DQW structure provides relatively weak temperature dependence for both the threshold current Ith as well as the external differential quantum efficiency ηd. Furthermore, the series electrical resistance for 100 μm×600 μm stripe-contact devices is as low as 0.12 Ω. As a result, the power conversion efficiency reaches a maximum of 40% at 8×Ith, and decreases to only 33% at the maximum power (i.e., 3 W) at 28×Ith. Low-temperature (12 K) photoluminescence measurements of InGaAs/InGaAsP quantum-well structures exhibit narrow linewidths (<10 meV) for material grown on exact (100) GaAs substrates, while growths on misoriented substrates exhibit linewidth broadening, as a result of ‘‘step bunching.’’ Laser structures grown on misoriented substrates exhibit increased temperature sensitivity of both Ith and ηd, compared with structures grown on exact (100) substrates.

High CW output power and ‘wallplug’efficiency Al-free InGaAs/InGaAsP/InGaPdouble quantum well diode lasers

2W CW output power has been obtained (driver-limited) from aluminum-free, strained-layer double quantum well (DQW) InGaAs/InGaAsP/InGaP uncoated ‘broad area’ diode lasers (λ = 0.96 µm) grown by low pressure MOCVD. Power conversion (‘wallplug’) efficiencies of 43.3% are achieved at 1.6 W CW output power. The combination of high bandgap (1.62 eV) InGaAsP confinement layers and the DQW structure provides relatively weak temperature dependence of both the threshold current and the external differential quantum efficiency.

Gigantic negative transconductance and mobility modulation in a double-quantum-well structure via gate-controlled resonant coupling

Transport of two-dimensional electrons in a novel double-quantum-well (DQW) field-effect transistor was systematically studied with emphasis on the effect of resonant interaction. By introducing ionized impurities appropriately into one of the QWs, wave-function-dependent scattering process was sensitively controlled by the gate voltage Vg. A prominent valley structure was observed in the channel conductance −Vg characteristics at resonance with the peak-to-valley ratio of 3 at 4.2 K. This nonlinear characteristic is caused by the deformation of electron wave functions in the DQW and is found to be well explained by the theoretical calculation. The DQW structure can be utilized for both negative transconductance and velocity modulation devices.

Characterization of GaAs and InGaAs double-quantum well heterostructure FETs

It is pointed out that among the large variety of heterostructure field effect transistors, multichannel devices present a particular originality: their transconductance profile is very flexible and depends on the structure parameters. They are therefore suited for high signal nonlinear applications. The specific case of double quantum-well structures is studied. Conventional and pseudomorphic devices are characterized under DC and RF conditions. Very high current densities (up to 1.2 A/mm) are demonstrated. The effect of different structural parameters on the transconductance and cutoff frequency is discussed. The results are analyzed in order to give a full understanding of these devices and to demonstrate their performances. >

Dissipative dynamics of an electronic wavepacket in a semiconductor double well potential

The authors present a detailed study of coherent tunneling oscillations of an optically excited electronic wavepacket in a semiconductor double quantum-well structure. The oscillation of the wavepacket between the two wells is traced by time-resolved pump-probe spectroscopy as well as time-resolved degenerate four-wave mixing. The experimental observations are compared with a simple theoretical model. The motion of the wave packet is studied as a function of both an external electric field and the lattice temperature. While the oscillation frequency shows the expected dependence on electric field, it increases with temperature at fixed electric field when perfectly delocalized electronic states exist at low temperature. >

Modeling of electron tunneling times in asymmetric double quantum wells

A scattering process modeled by an imaginary potential VI in the wide well of an asymmetric double quantum well structure (DQWS) is used to model the electron tunneling from the narrow well. Taking VI ∼ −5 meV, the ground resonant level lifetimes of the narrow well in the DQWS are in quantitative agreement with the experimental resonance and non-resonance tunneling times. The corresponding scattering time 66 fs is much faster than the intersubband scattering time of LO-photon emission.

Magneto-optical transitions in GaAs-AlGaAs coupled double quantum wells

Magnetophotoluminescence excitation spectroscopy has been employed to study GaAs-AlGaAs coupled double quantum wells. The investigations were undertaken at liquid helium temperatures and in magnetic fields up to 15 T applied parallel to the growth direction. In zero magnetic field, under flat-band conditions, the spectrum is dominated by ground-state excitonic transitions between electron and hole states with envelope functions of the same symmetry. In a magnetic field, the oscillator strengths of excited excitonic states are enhanced and four series of transitions are observed. The spectral features corresponding to the ground state and the excited states have been analyzed and these results were utilized to determine the exciton binding energies for several excitonic transitions of different subbands in this coupled double quantum-well structure. A number of additional features are visible at higher magnetic fields; some of these peaks are believed to be associated with 2p exciton transitions between hole and electron states with envelope functions of opposite parity due to exciton mixing. A comparison is made between the experimental results and theoretical calculations which include these effects.