Luminescence of stepped quantum wells in GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures

Luminescence spectra of doped and undoped GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures containing several tens of stepped quantum wells (QW) are investigated. The emission bands related to free and bound excitons and impurity states are observed in QW spectra. The luminescence excitation spectra indicate that the relaxation of free excitons to the e1hh1 state proceeds via the exciton mechanism, whereas an independent relaxation of electrons and holes is specific to bound excitons and impurity states. The energy levels for electrons and holes in stepped QWs, calculated in terms of Kane’s model, are compared with the data obtained from the luminescence excitation spectra. The analysis of the relative intensities of emission bands related to e1hh1 excitons and exciton states of higher energy shows that, as the optical excitation intensity increases, the e1hh1 transition is more readily saturated at higher temperature, because the lifetime of excitons increases. Under stronger excitation, the emission band of electron-hole plasma arises and increases in intensity superlinearly. At an excitation level of ～10⁵ W/cm², excitons are screened and the plasma emission band dominates in the QW emission. Nonequilibrium luminescence spectra obtained in a picosecond excitation and recording mode show that the e1hh1 and e2hh2 radiative transitions are 100% polarized in the plane of QWs.     

Conduction electrons bound to magneto-donors and magneto-acceptors in GaAs/GaAlAs quantum wells

Interband photoluminescence from asymmetric modulation-doped GaAs---GaAlAs quantum wells in the presence of a magnetic field has been investigated. The free-carrier Landau level excitations as well as transitions between magneto-donor and free hole states have been observed. Experiments with tilted magnetic field provide strong evidence for the existence of conduction electrons bound to ionized magneto-acceptors, characterized by discrete energies above the conduction band Landau levels. The observations are compared with the theory.     

High-speed electrooptic modulation in GaAs/GaAlAs waveguide devices

A figure of merit is constructed for broad-band electrooptic modulators which compares bandwidth with launched generator voltage standardized to a common wavelength. Comparison of various published results in terms of this figure shows that a lumped-element III-V semiconductor device performance may be no more than a factor of two below that of typical (i.e., LiNbO3) traveling wave devices and are probably easier to implement-especially in an integrated format. Accurate modeling, which incorporates all transit time and velocity match effects, is described and found to agree well with experimental results. Experimental GaAs/GaAlAs modulators have been made, using a Mach-Zehnder interferometer configuration. At a wavelength of 1.15 μm and with unterminated drive a bandwidth of 6.5 GHz was obtained with Vpiof 17.3 V. A shorter (34.6-v) device was ∼ 1.25-dB down at 8.4 GHz. The corresponding figures of merit are close to the maximum expected for the configurations used.     

Low-voltage phase modulation in GaAs/AlGaAs quantum well opticalwaveguides

Reports GaAs/AlGaAs quantum well waveguide phase modulators with high phase shift coefficients, as large as 520 degrees per V mm. By operating at wavelengths far below the bandedge and applying DC bias the authors achieve large electro-optic modulation with low absorption loss in device lengths on the order of 100 μm and drive voltages on the order of 1 V     

GaAlAs/GaAs single quantum well gain-coupled distributed feedbacklaser

A novel gain-coupled distributed feedback laser with a single quantum well (SQW) active layer is proposed and fabricated. The gain perturbation for the gain coupling is due to the periodically perturbed SQW active layer. The characteristics under continuous-wave (CW) operation are presented, included an excellent single-mode property and a very high yield in the single-mode oscillation. A CW threshold current of 31 mA and a high side-mode suppression ratio of 47 dB were achieved. The high single-mode-oscillation yield near 100% indicated the dominance of the gain-coupled optical feedback in the lasers     

Optical reflection and contactless electroreflection from GaAlAs layers with periodically arranged GaAs quantum wells

Optical reflection and electroreflection for the AlGaAs layers containing the periodically arranged GaAs quantum wells of different thickness are studied at photon energies ranging from 1 to 2 eV. It is established that the spectral dependence of the reflectance involves three different contributions made by (i) the reflection from the medium-air interface; (ii) the interference reflection due to the periodically modulated refractive index, since the materials of the wells and barriers have different refractive indices; and (iii) the reflection produced by the interaction of electromagnetic waves with the excition states in the quantum wells. Analysis of the reflection spectra shows that these contributions are characterized by different behavior with variations in temperature, angle of incidence of light, and polarization; however, quantitative separation of the spectra into individual contributions presents a rather difficult problem. To separate the contribution originating from the interaction of light with the exciton states from the optical spectra, a special approach based on contactless measurements of the optical electroreflectance over a certain spectral region is developed. It is shown that this method provides a means for determining the parameters of the exciton states in the quantum wells.     

GaAs/GaAlAs量子限制Stark效应及自电光双稳现象的实验研究

我们研制了GaAs/GaAlAs多量子阱pin结构的SEED器件。分析了器件的光电流光谱、光电流-电压特性。对于如何实现器件光学双稳态工作的有关问题进行了讨论。     

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     

Birefringence of disordered AlGaAs/GaAs quantum wells

Birefringence at room temperature is analysed for interdiffusion induced (disordered) Al/sub 0.3/Ga/sub 0.7/As/GaAs single quantum well structures in the wavelength range 0.5 mu m to 1.0 mu m. The confinement profile for the disordered QW is modelled by an error function and the refractive index model includes excitonic effects and contributions from the Gamma , X and L Brillouin zones. Results show that at longer wavelengths the birefringence is small and varies from positive to negative before reducing to zero as interdiffusion proceeds. For wavelengths between the QW and barrier band-edges, the birefringence is large and reduces with increasing interdiffusion.<>     

Optically pumped four-level infrared laser based on intersubbandtransitions in multiple quantum wells: feasibility study

The feasibility of optically pumped infrared laser based on the intersubband transitions in multiple quantum wells is studied theoretically. The criteria for population inversion and efficient lasing operations are established. A system consisting of a superlattice with four quantum wells per period is proposed and shown to satisfy the criteria. The relaxation processes are investigated, giving the electron-phonon interaction a rigorous theoretical treatment. It is shown that in the proposed structure, optical gain in excess of 300 cm ^-1 can be realized under practical pumping conditions. Modification and optimization of the proposed laser are discussed     

Intervalley scattering in GaAs/AlGaAs quantum wells and quantum cascade lasers

The results of simulations of Γ−X scattering in GaAs/AlGaAs quantum wells are presented, discussing the importance of the mole fraction, doping density, and lattice and electron temperatures in determining the scattering rates. A systematic study of Γ−X scattering in GaAs/Al_xGa_1−xAs heterostructures, using a single quantum well to determine the importance of well width, molar concentration x, lattice temperature, and doping density, has been performed. After this we consider a double quantum well to determine the role of intervalley scattering in the transport through single-layer heterostructures, i.e. Γ−X−Γ scattering compared with Γ−Γ scattering. Finally, we estimate the relative importance of intervalley scattering in a GaAs-based quantum-cascade laser device and compare it with other relevant scattering mechanisms important to describe carrier dynamics in the structure. Our simulations suggest that Γ−X scattering can be significant at room temperature but falls off rapidly at lower temperatures.     

GaAs/GaAlAs active-passive-interference laser

A GaAs/GaAlAs interference laser with an active and a passive section is reported. A very clean single longitudinal mode is obtained and maintained without mode hopping over about 10°C, which results from the combined effect of interference and real index waveguiding in the lateral direction.     

High bit rate modulation of narrow-stripe proton-implanted GaAs/GaAlAs injection lasers

High bit rate modulation up to 2 Gbit/s of a narrow -stripe (3 ?m) proton-implanted injection laser using resonance oscillation has been achieved. These results indicate that the narrow-stripe laser has possibilities of high bit rate modulation comparable to the broad-stripe laser. To obtain optimum modulation conditions, pulse height distortion was studied for various bias and pulse currents. Optimum was found for a bias current 5% above threshold and a pulse current of 42 mA in a laser with a threshold current of 111 mA.     

Shallow impurity levels in AlGaAs/GaAs semiconductor quantum wells

This paper reviews current knowledge of shallow impurity states (donors and acceptors) in AlGaAs/GaAs multiple-quantum-well structures. Calculations of these levels have been performed by a number of groups, most of which solved an effective-mass Schrödinger equation using the variational method. These results are generally consistent with each other, any differences being related to different approximations made in each calculation. The ground states of some of these shallow impurity levels have also been measured in several laboratories using low-temperature photoluminescence, Raman and far-infrared absorption techniques. These methods yield similar results which are consistent with the calculations. Both theoretical and experimental methods and results are discussed.     

Interdiffusion and relaxation in metalorganic vapor phase epitaxy grown InGaAs/GaAs strained layer quantum wells

Low pressure metalorganic vapor phase epitaxy grown strained InGaAs/GaAs quantum well structures have been characterized by photoluminescence and x-ray diffraction. It is shown that beyond the pseudomorphic limit, these structures show considerable gallium/indium interdiffusion at the interfaces and partial strain relaxation in the quantum well layers.     

Quantum interference, Stark, and carrier density infraredelectrooptical modulation based on intersubband transitions inasymmetrical quantum wells

A new approach toward studying electrooptic modulation utilizing intersubband transitions in quantum wells is presented. Using first-order perturbation theory for analyzing the effect of a dc electric field on the linear susceptibility, an understanding of the mechanisms which give rise to intersubband electrooptic susceptibility is presented. This includes modulation due to the dc Stark effect, modulation due to coherent interference of the envelope states, and modulation of the carrier densities in populated subbands. We study several structures that maximize the electrooptic susceptibility of a particular origin and discuss the suitability of the various schemes for practical realizations. Finally, we derive a figure of merit for each type of modulator, taking into account the linear intersubband absorption, and show that highly efficient near-infrared modulators that operate at a wavelength of 1.5 μm can be realized     

Optical nonlinearities in a passive GaAs/GaAlAs multiple quantum well strain-induced waveguide

Nonlinear Fabry-Perot switching effects have been observed in a 400 μm long GaAs/GaAlAs MQW strain-induced waveguide resonator. Experimental evidence showing that the mechanism responsible for the switch has a negative coefficient in the refractive index nonlinearity is described; this rules out the possibility that heating could be the cause of the observed switching. However, thermal effects were always present in the background and became more prominent at slow input-power sweep rates. Bistability due to increased absorption has also been observed in a similar nonresonant waveguide structure, at slightly longer wavelengths.     

Waveguide effect of GaAsSb quantum wells in a laser structure based on GaAs

The waveguide effect of GaAsSb quantum wells in a semiconductor-laser structure based on GaAs is studied theoretically and experimentally. It is shown that quantum wells themselves can be used as waveguide layers in the laser structure. As the excitation-power density attains a value of 2 kW/cm² at liquid-nitrogen temperature, superluminescence at the wavelength corresponding to the optical transition in bulk GaAs (at 835 nm) is observed.     

High-speed mid-IR modulator using Stark shift in step quantum wells

We show in calculations that there is a capability for high speeds with a low applied voltage in modulators based on intersubband transitions in step quantum wells (QWs). A waveguide based on surface plasmons is assumed to achieve the necessary tight confinement of the optical field. In a structure with 8 GaInAs-AlGaInAs-AlInAs step QWs, we obtain a device capacitance of 14 fF corresponding to a RC limitation of electrical f_{3 dB}=190 GHz. The extinction ratio of 6.6-μm light is 10 dB at an applied voltage of 0.9 V and T=300 K. By simple reasoning, we find that the device capacitance is approximately proportional to the absorption linewidth cubed when the linewidth is considered in the device design. Thus, the linewidth is very decisive for the modulation speed. We propose to place the dopants asymmetrically in the barriers in order to reduce broadening caused by doping induced potential fluctuations. In addition, the doping levels in the outermost barriers of the multi-QW structure are proposed to be reduced and asymmetrical, in order to achieve a uniform electric field over the step QWs, which is shown to increase the achievable f_{3 dB} very markedly     

Photoluminescence in n and p modulation-doped GaInNAs/GaAs quantum wells

Experimental results concerning the steady-state photoluminescence (PL) studies in n and p modulation doped and undoped GaInNAs/GaAs quantum wells are presented. The effects of modulation, type of doping and nitrogen concentration on the PL and the temperature dependence of the band gap, carrier localization and non-radiative recombination are investigated. Increasing the nitrogen composition decreases energy band gap as expected. The n-type modulation doping eliminates most of the defect-related effects and blue shifts the energy band gap. However, the p-type doping gives rise to additional features in the PL spectra and red shifts energy band gap further compared to the n-type-doped material.