Photon-assisted tunneling in coupled double quantum wells

Using different techniques to individually contact two closely spaced electron gases, we study the tunneling characteristics between weakly coupled GaAs quantum wells, with and without resonant far-infrared excitation. We find that for barriers as thick as 300 Å, the alignment between the subbands in the wells can be observed as an increased tunneling conductivity. To study photon-assisted tunneling in our samples, we use the cyclotron resonance as a strong, tunable electronic excitation in the far-infrared. When the Landau-level spacing [hstrok]ω_c corresponds to the laser energy [hstrok]ω_L, the carriers are effectively pumped to higher Landau levels, which leads to a reduced resistance across the tunnel barrier. This photo-conductive signal is “doubly resonant” in that it is at maximum when ω_c coincides with ω_L, and at the same time the subbands of the two wells are aligned.     

Light-scattering determination of electron tunneling gaps in double quantum wells

Resonant inelastic light-scattering techniques have been used to measure directly the single-particle tunneling gap (Δ_SAS) in Al_xGa_1−xAs/GaAs double quantum wells. We have observed a systematic decrease in Δ_SAS with increasing height of the barrier in agreement with the Δ_SAS calculated self-consistently within a Hartree approximation.     

Growth of InAs-AlSb quantum wells having both high mobilities and high concentrations

Low-temperature mobilities in InAs-AlSb quantum wells depend sensitively on the buffer layer structures. Reflection high energy electron diffraction and x-ray diffraction show that the highest crystalline quality and best InAs transport properties are obtained by a buffer layer sequence GaAs → AlAs → AlSb → GaSb, with a final GaSb layer thickness of at least 1 μm. Using the improved buffer scheme, mobilities exceeding 600,000 cm²/Vs at 10 K are routinely obtained. Modulation δ-doping with tellurium has yielded electron sheet concentrations up to 8 × 10¹² cm⁻² while maintaining mobilities approaching 100,000 cm²/Vs at low temperatures.     

Resonant tunneling through rare earth arsenide, semimetal quantum wells

Resonant tunneling is observed in double barrier resonant tunneling diodes (RTDs) with semi-metallic ErAs quantum wells. Magnetic field dependence distinguishes two different resonant channels. From the thickness dependence of the voltage for resonant tunneling the dispersion of the channels is found to be hole-like. The dispersion agrees well with the theory that identifies the two channels with m_j = ±1/2 and m_j = ±3/2, hole states.     

Carrier capture competition between two different quantum wells in dual-wavelength semiconductor lasers

Based on a three-level model, we have numerically shown that a semiconductor gain medium with a structure consisting of two different quantum wells may lead to dual-wavelength laser operation if the balance of carrier capture competition between the two wells can be reached. The major controlling parameters for the operation are the ratio of the carrier quantum capture times of the two wells and the absorption constant of the short-wavelength photons by the long-wavelength quantum well. It is shown that there exists a large parameter space for dual-wavelength operation.     

Thermally activated resonance tunneling in asymmetric systems of CdSe/ZnSe double quantum wells with self-assembled quantum dots

In the luminescence study of double quantum wells formed by depositing two CdSe layers with different nominal thicknesses into a ZnSe matrix, a heavy dependence of the photoluminescence spectrum on the thickness of the ZnSe barrier separating the quantum wells, the excitation photon energy, and temperature is observed. The photoluminescence spectra are studied at barrier widths of 34, 50, and 63 monolayers, excitation photon energies of 3.06, 2.71, and 2.54 eV, and temperatures T in the range of 5–200 K. Upon above- (3.06 eV) and below-barrier (2.71 eV) excitation, the photoluminescence spectrum exhibits two bands, I ₁(T) and I ₂(T), corresponding to the annihilation of excitons localized in the quantum dots of the shallow and deep quantum wells. An increase in temperature to ～50 K yields only a slight decrease in the total integrated emission intensity of both bands I _PL(T) and the intensities of each of the two bands, I ₁(T) and I ₂(T). A further increase in temperature results in substantial redistribution of the photoluminescence intensity between the two wells, which is attributed to the tunneling of excitons from the QD (quantum-dot) states of the shallow well to states of the deep well. This process is of the activation character and manifests itself as a sharp decrease in the integrated emission intensity related to the shallow quantum well, I ₁(T), and a simultaneous increase in the integrated emission intensity of quantum dots of the deep quantum well, I ₂(T). The experimentally detected effect is most profound in the range of temperatures T = 110–130 K and in the samples with a barrier thickness of 50 monolayers. It is most likely that the tunneling is of a resonance nature. This inference follows from the fact that the barrier width is much larger than the well widths for both wells, which predetermines only slight penetration of the wave functions into the neighboring well, and the effect of tunneling itself is only slightly supressed, as the barrier thickness is increased. At the same time, the activation energy is at least three time higher that the optical phonon energy, which cannot be explained on the basis of existing theory.     

Influence of two-dimensional magnetic photonic quantum wells on resonant tunneling spectral character

The non-magnetic material closed photonic quantum well (PQW) and magnetic material PQW structures based on the non-magnetic material open PQW are proposed. The transmission spectra and the field distributions of these three PQW structures are calculated by finite-difference time-domain method, the quantized energy states are researched, and the feasibility of enhancing spectral intensity significantly by selfstructure is disclosed. It is found that the optical transmittance of the magnetic PQW is close to 1, and the energy loss is less compared to non-magnetic PQW.Compared with the closed PQW structures, the device's volume can be reduced, the degree of free regulation of the energy band project can be increased, and more photon bound states can be obtained. The results show that the open PQW is the traveling wave well, and its capability of capturing photons is weak. However, the closed PQW and the magnetic PQW are standing wavewells. Their capabilities for capturing photons are strong,while the light field gradient of the material PQW is bigger.     

A theory of shot noise in quantum wells and applications inresonant tunneling heterojunction bipolar transistors

Some semiclassical arguments are presented to show that the noise associated with the current through a resonant tunnel (RT) diode is reduced by the feedback through the modulation of the barriers by the space charge in the well, and at high frequencies by the reduction in the velocity spread. Theoretical calculations of shot noise are carried out on a double-barrier, one-well structure. The results show that the noise power is a function of the energy bandwidth of the transmitted electrons and that the noise may be significantly reduced by the quantum wells. These results can be applied to heterojunction bipolar transistors that contain a quantum well, and it is shown that these resonant tunneling heterojunction bipolar transistors (RTHBTs) should have a lower noise figure than homojunction transistors     

On the radiative recombination and tunneling of charge carriers in SiGe/Si heterostructures with double quantum wells

For SiGe/Si(001) epitaxial structures with two nonequivalent SiGe quantum wells separated by a thin Si barrier, the spectral and time characteristics of interband photoluminescence corresponding to the radiative recombination of excitons in quantum wells are studied. For a series of structures with two SiGe quantum wells different in width, the characteristic time of tunneling of charge carriers (holes) from the narrow quantum well, distinguished by a higher exciton recombination energy, to the wide quantum well is determined as a function of the Si barrier thickness. It is shown that the time of tunneling of holes between the Si_0.85Ge_0.15 layers with thicknesses of 3 and 9 nm steadily decreases from ~500 to <5 ns, as the Si barrier thickness is reduced from 16 to 8 nm. At intermediate Si barrier thicknesses, an increase in the photoluminescence signal from the wide quantum well is observed, with a characteristic time of the same order of magnitude as the luminescence decay time of the narrow quantum well. This supports the observation of the effect of the tunneling of holes from the narrow to the wide quantum well. A strong dependence of the tunneling time of holes on the Ge content in the SiGe layers at the same thickness of the Si barrier between quantum wells is observed, which is attributed to an increase in the effective Si barrier height.     

Infrared phase modulators with multiple quantum wells

The intraband transition of a graded Ga_1-xAl_xAs/AlAs quantum well is analyzed for its suitability as a phase modulator at infrared wavelengths. Using the effective mass model and numerically solving Schrodinger's equation for a graded energy well with an external electric field, it is found that significant phase changes are possible for comparatively small voltages even for operation far from the absorption resonance. In an example, the calculated value for V_π at 10.6 μm is 150 V, an improvement by over two orders of magnitude over conventional bulk electrooptic material for that wavelength regime     

Luminescence in ZnMnTe/ZnMgTe and CdMnTe/CdMgTe structures with different parameters of quantum wells

The low-temperature luminescence of ZnMnTe/ZnMgTe and CdMnTe/CdMgTe quantum well structures with different quantum well widths and different Mn proportions is studied at optical-excitation power densities ranging from 10⁴ to 10⁶ W cm^?2. Because of saturation of the lowest excited state ⁴ T ₁ of the 3d shell of Mn²⁺ ions, transitions to higher states start to play an important role. As a result, the intracenter luminescence of Mn²⁺ ions deteriorates at high excitation levels. Simultaneously, the temperature-dependent saturation of the main exciton-emission band e1hh1 of the quantum wells occurs, and the band e2hh2 emerges. As the optical excitation is increased, the intracenter luminescence band of Mn²⁺ ions changes its shape. This effect is attributed to the faster saturation of the excited states of interface ions. For CdMnTe/CdMgTe structures, the effect of the quantum well width and Mn content on the relation between the emission intensities corresponding to excitons in quantum wells, excitons in barriers, and the 3d shell of Mn²⁺ ions is established.     

Organic quantum wells with multiple negative differential resistance peaks and its photoswitch effect

Organic optoelectronic devices have experienced great progress in recent decades. However, quantum well has long been a challenge for organic semiconductors due to the weak intermolecular interactions, the difficulty of realizing high quality alternate crystalline films. Here, we construct a type II organic crystalline quantum well, in which both electron quantum well and hole quantum well show symmetrical and multiple negative differential resistance peaks, respectively. The blueshifts of absorption spectra and the peak voltage decreasing under illumination conditions are observed. Due to high absorption coefficient of organic semiconductors, photoswitch effect by taking advantage of negative differential resistance of organic quantum well, which owns very thin device structure is demonstrated. Our results prove the promising applications of organic quantum wells, which broaden the types of organic optoelectronic devices.     

Enhancement of the Stark effect in coupled quantum wells foroptical switching devices

Significant enhancement of the Stark effect on the electronic state and the optical dipole moments of coupled quantum wells is shown theoretically. The multiband effective mass theory (k&oarr;-p&oarr;), which takes into account coupling between heavy- and light-hole states of the coupled quantum wells is used. Mixing of states in the coupled quantum wells leads to the splitting of subband energy levels. An applied electric field causes repulsion between the split levels as well as the spin-splitting of the valence-subband structure. Comparison with the single quantum well shows that the optical dipole moment is substantially more reduced for the coupled quantum wells at the same electric field because of enhanced charge separation in this structure. A variational method is used to solve the exciton problem in coupled quantum wells. Calculated exciton peak positions versus electric field show very good agreement with recent experiments. Calculated exciton absorption spectra for the ground state show the quenching of the exciton peak at F=30 kV/cm at 5 K. These results may have interesting applications to low-voltage optoelectronic switching devices based on the quantum-confined Stark effect     

Well-width dependence of the phase coherent photorefractive effect in ZnSe quantum wells

We report on the well-width dependence of the phase coherent photorefractive (PCP) effect in ZnSe/ZnMgSe single quantum wells (QWs) using 90 fs light pulses. The experiments are performed in a four-wave-mixing configuration at temperatures between 25 and 65 K. The ZnMgSe/ZnSe QWs with 10, 5 and 3 nm well width were grown on GaAs substrate using molecular beam epitaxy. With decreasing QW thickness we observe a reduction of the PCP diffraction efficiency. This is attributed to the higher electron energy in small QWs due to the quantum size effect, which leads to a reduced equilibrium density of captured electrons in the QW. With increasing temperature the PCP signal further decreases which is attributed to thermal activation of the QW electrons back to the GaAs substrate.     

Electroluminescent properties of heterostructures with GaInNas quantum wells

GaInNAs quantum wells were grown by metal-organic vapor-phase epitaxy. In order to improve the optical properties, the GaNAs barriers were incorporated on both sides of the quantum well; these barriers compensated the elastic stresses. Characteristics of the optical transitions were assessed from the measurements of photoluminescence and photocurrent. In order to fabricate light-emitting diodes, nonalloyed ohmic contacts based on heavily δ-doped layers were used. Electroluminescence was observed at a wavelength of ～1.2 μm at temperatures of 77 and 300 K; the electroluminescence intensity depended linearly on the injection current if the latter exceeded a certain threshold value.     

Electron transport in coupled quantum wells with double-Sided doping

The conductivity and Hall mobility have been measured in heterostructures with coupled quantum wells (QW) as functions of temperature and the QW width. If a tunnel-transparent barrier is inserted in the middle of a QW, the mobility increases in narrow wells and decreases in wide wells. The experimental data have been compared with the calculated dependences. It has been shown that the number of filled quantum-well subbands depends on the well width and the presence of a barrier. The magnetoresistance and Hall resistance were measured at a temperature of 4.2 K in the range of magnetic fields of 1–40 T. The filling of subbands was determined from a Fourier analysis of the Shubnikov-de Haas oscillations, and good agreement with the calculated data was obtained.     

Piezoelectric effect in micromachined [001] quantum wells: Theoretical aspects

In this work, we demonstrate the ability to take adBANtage of the piezoelectric effect in systems primarily grown on [001] GaAs substrates. Such an effect can be achieved by making use of elastic relaxation of micromachined strained quantum well structures, the etching direction being carefully chosen. Shear defbrmations present at the edge of released cantilevers can produce a significant piezoelectric field.     

Electrons and phonons in quantum wells

The modulation of electron and polar optical phonon states in an AlGaAs/GaAs/AlGaAs quantum well (QW) with an inserted thin AlAs barrier is considered. The OW width dependence of electron-phonon scattering rates are estimated. The large contribution to the change of the electron subband population, the photovoltaic effect, and the electron mobility in the QW accounts for the resonant intersubband scattering of electrons by interface phonons. The decrease of electron mobility limited by polar optical phonon scattering with increasing carrier concentration in the QW is established. The conditions for the increase of mobility in the QW by inserting the AlAs barrier are found. Fiz. Tekh. Poluprovodn. 33, 1049–1053 (September 1999) This article was published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Resonance donor states in quantum wells

The energies and wave functions of the resonance states of shallow donors in quantum wells (QWs) are calculated. The calculations are performed in a model of an isolated impurity center using the example of a GaAs/AlGaAs heterostructure. A formula for the probability of a spontaneous emission of polar optical (LO) phonons is derived. It is shown that, in the vicinity of the resonance-state energies, polar electron-phonon interaction is modified. This modification is produced by a hybridization of confinement subbands. Generally, due to hybridization, an electron interacts with phonons simultaneously in two channels (subbands). The absorption cross section for infrared radiation is calculated, taking into account both homogeneous (in the mid-infrared range) and inhomogeneous broadening (in the far-infrared range). The absorption of radiation whose electric field is normal to the heterointerfaces is related to optical transitions to the states near the resonances. Homogeneous broadening of the absorption lines, as well as the LO-phonon scattering rate, depends on the width of the resonance states (the degree of subband hybridization).