810-nm InGaA1As/A1GaAs double quantum well semiconductor lasers with asymmetric waveguide structures

The 810-nm InGaA1As/A1GaAs double quantum well (QW) semiconductor lasers with asymmetric waveguide structures, grown by molecular beam epitaxy, show high quantum efficiency and high-power conversion efficiency at continuous-wave (CW) power output. The threshold current density and slope efficiency of the device are 180 A/cm2 and 1.3 W/A, respectively. The internal loss and the internal quantum efficiency are 1.7 cm-1 and 93%, respectively. The 70% maximum power conversion efficiency is achieved with narrow far-field patterns.     

Graded doping low internal loss 1060-nm InGaAs/AlGaAs quantum well semiconductor lasers

Internal loss is a key internal parameter for high power 1060-nm In Ga As/Al Ga As semiconductor laser.In this paper,we discuss the origin of internal loss of 1060-nm In Ga As/Ga As quantum well(QW) Al Ga As separate confinement heterostructure semiconductor laser,and the method to reduce internal loss.By light doping the n-cladding layer,and stepwise doping the p-cladding layer combined with the expanded waveguide layer,a broad area laser with internal loss of 1/cm is designed and fabricated.Ridge waveguide laser with an output power of 350 m W is obtained.The threshold current and slope efficiency near the threshold current are 20 m A and 0.8 W/A,respectively.     

Improved thermal property of strained InGaAlAs/AlGaAs quantum wells for 808-nm vertical cavity surface emitting lasers

The 808-nm vertical cavity surface emitting laser (VCSEL) with strained In_0.13Ga_0.75Al_0.12As/Al_0.3Ga_0.7As quantum wells is designed and fabricated. Compared with the VCSELs with Al_0.05Ga_0.95As/Al_0.3Ga_0.7As quantum wells, the VCSEL with strained In_0.13Ga_0.75Al_0.12As/Al_0.3Ga_0.7As quantum wells is demonstrated to possess higher power conversion efficiency (PCE) and better temperature stability. The maximum PCE of 43.8% for 10-μm VCSEL is achieved at an ambient temperature of 30 ℃. The size-dependent thermal characteristics are also analyzed by characterizing the spectral power and output power. It demonstrates that small oxide-aperture VCSELs are advantageous for temperature-stable performance.     

Optical gain and threshold properties of strained InGaAlAs/AlGaAs quantum wells for 850-nm vertical-cavity surface-emitting lasers

The valence subband structures, optical gain spectra, transparency carrier densities, and transparency radiative current densities of different compressively strained InGaAlAs quantum wells with Al_0.3Ga_0.7As barriers are systematically investigated using a 6 × 6 k · p Hamiltonian including the heavy hole, light hole, and spin-orbit splitting bands. The results of numerical calculations show that the maximum optical gain, transparency carrier densities, transparency radiative current densities, and differential gain of InGaAlAs quantum wells can be enhanced by introducing more compressive strain in quantum wells. However, further improvement of the optical properties of InGaAlAs quantum wells becomes minimal when the compressive strain is higher than approximately 1.5%. The simulation results suggest that the compressively strained InGaAlAs quantum wells are of advantages for application in high-speed 850-nm vertical-cavity surface-emitting lasers.     

Plasmons in asymmetric double quantum well structures

We investigate the effects of spatial asymmetry, tunneling coupling, and exchange-correlation correction on the plasmon modes in asymmetric double quantum well (DQW) structures in a time-dependent local-density approximation. Special attention is paid to the properties of the ω _- mode which is always damped in symmetric DQW systems. In addition, the results on the spectral weight of the excitations are also presented. In general, all the modes carry finite spectral weights and should be observable in resonant inelastic light scattering experiments for the specified values of the parameters. Received 2 July 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: c412-1@aphy.iphy.ac.cn     

Single quantum well transistor with modulation doped AlGaAs/GaAs/AlGaAs structures

Self-consistent calculations have been performed to obtain the wave functions and energy subbands of the two-dimensional electrons confined in a single quantum well of a Al_xGa_1?xAs/GaAs/Al_xGa_1?xAs heterostructure. The wave functions of the two-dimensional electron gas are found to be easily controlled by an external gate voltage applied between the AlGaAs-barriers, indicating a capability of fabricating a novel quantum well device, a modulation-doped single quantum well transistor.     

High performance 1689-nm quantum well diode lasers

1689-nm diode lasers used in medical apparatus have been fabricated and characterized. The lasers had pnpn InP current confinement structure, and the active region consisted of 5 pairs of InGaAs quantum wells and InGaAsP barriers.     

The beam properties of high-power InGaAs/AlGaAs quantum well lasers

The vertical beam quality factor of the fundamental TE propagating mode for InGaAs/AlGaAs SCH DQW lasers emitting at 940 nm is investigated by using the transfer matrix method and the non-paraxial vectorial moment theory for non-paraxial beams. An experimental approach is given for the measurement of the equivalent vertical beam quality factor of an InGaAs/AlGaAs SCH DQW laser. It has been shown that the vertical beam quality factor M_x² is always larger than unity, whether the thickness of the active region of LDs is much smaller than the emission wavelength or not.     

Electro-optic effects in GaAs/AlGaAs parabolic quantum well structures

We report on a systematic experimental and theoretical investigation of the interband electroabsorption in GaAs/AlGaAs parabolic quantum well (PQW) structures. In our experiments, we performed transmission and reflectance measurements using a sensitive double modulation technique. The measurements were carried out on p–i–n diode type PQW samples with two different well widths. In the sample with the narrow PQW, the discrete structure of the interband subband transitions can be resolved in the electroabsorption spectra. In the sample with the wide PQW, and therefore a small subband spacing, these transitions can no longer be resolved individually due to the broadening. Their superposition, however, results in a red-shifting, quasi-linear pattern in the electroabsorption spectra, which is modulated by the blue-shifting, non-linear fan of the ‘ arabolic Franz–Keldysh effect’. The experimental results are in very good agreement with calculated single-particle absorption spectra based on a simple harmonic oscillator model.     

Aluminium implantation-induced disordering of AlGaAs/GaAs quantum well structures

The process of Al implantation-induced disordering of AlGaAs/GaAs quantum well structures has been studied for optical waveguide applications. A study of the implanted samples using photoluminescence demonstrates that disordering is primarily a damage-based process and that this process may be suitable for the fabrication of surface gratings.     

Photoluminescence studies of confined states in AlGaAs/GaAs asymmetric quantum well

In the recombination spectra of AlGaAs/GaAs heterostructures, a peculiar and asymmetric photoluminescence (PL) band F has previously been reported [Aloulou et al., Mater. Sci. Eng. B 96 (2002) 14] to be due to recombinations of confined electrons from the two-dimensional electron gas (2DEG) formed at AlGaAs/GaAs interface in asymmetric quantum well (AQW). Detailed experiments are reported here on GaAs/Al_0.31Ga_0.69As/GaAs:δSi/Al_0.31Ga_0.69As/GaAs samples with different spacer layer thicknesses. We show that the band F is the superposition of two PL bands F′ and F″ associated, respectively, to AQW and a symmetric quantum well (SQW). In the low excitation regime, the F′ band present a blue shift (4.4 meV) followed by important red shift (16.5 meV) when increasing optical excitation intensity. The blue shift in energy is interpreted in terms of optical control of the 2DEG density in the AQW while the red shift is due to the narrowing of the band gaps caused by the local heating of the sample and band bending modification for relatively high-optical excitation intensity. Calculation performed using self-consistent resolution of the coupled Schrödinger–Poisson equations are included to support the interpretation of the experimental data.     

Energy and effective mass of a polaron in asymmetric semiconductor quantum well structures

In this paper, the correct electron extended states wave functions and the density of states in asymmetric single quantum wells (QWs) are given for the first time, we put right mistakes from some previous papers of some other authors. Within the framework of the secondorder perturbation theory, the ground-state polaron binding energy and effective mass correction in asymmetric single QWs are studied including the full energy specturm, i.e., the discrete energy levels in the well and the continuum energy spectrum above the barrier, and all possible optical-phonon modes. The effects of the finite electronic confinement potential and the subband nonparabolicity are considered. The relative importance of the different phonon modes is investigated. Our results show that the polaron energy and effective mass are sensitive to the asymmetry of the structure and have a close relation to the interface phonon dispersion. When well width and one side barrier height of asymmetric QWs are fixed and identical with those of symmetric QW, the polaron binding energy and effective mass in asymmetric QWs are always less than those in symmetric QW. It is necessary to include the continuum energy spectrum as intermediate states in the study of polaron effects in QWs in order to obtain the correct results. The subband non-parabolicity has little influence on the polaron effects. The polaron energies given in this paper are excellent agreement with our variational results.     

Resonant magnetopolaron effects in GaAs/AlGaAs multiple quantum well structures

A detailed experimental study of electron cyclotron resonance (CR) has been carried out at 4.2 K in three modulation-doped GaAs/Al_0.3Ga_0.7As multiple quantum well samples in fields up to 30 T. A strong avoided-level-crossing splitting of the CR energies due to resonant magnetopolaron effects is observed for all samples near the GaAs reststrahlen region. Resonant splittings in the region of AlAs-like interface phonon modes of the barriers are observed in two samples with narrower well width and smaller doping concentration. The interaction between electrons and the AlAs interface optical phonon modes has been calculated for our specific sample structures in the framework of the memory-function formalism. The calculated results are in good agreement with the experimental results, which confirms our assignment of the observed splitting near the AlAs-like phonon region is due to the resonant magnetopolaron interaction of electrons in the wells with AlAs-like interface phonons.     

Quasibound states in semiconductor quantum well structures

We present a study on quasibound states in multiple quantum well structures using a finite element model (FEM). The FEM is implemented for solving the effective mass Schrödinger equation in arbitrary layered semiconductor nanostructures with an arbitrary applied potential. The model also includes nonparabolicity effects by using an energy dependent effective mass, where the resulting nonlinear eigenvalue problem was solved using an iterative approach. We focus on quasibound/continuum states above the barrier potential and show that such states can be determined using cyclic boundary conditions. This new method enables the determination of both bound and quasibound states simultaneously, making it more efficient than other methods where different boundary conditions have to be used in extracting the relevant states. Furthermore, the new method lifted the problem of quasibound state divergence commonly seen with many other methods of calculation. Hence enabling accurate determination of dipole matrix elements involving both bound and quasibound states. Such calculations are vital in the design of intersubband optoelectronic devices and reveal the interesting properties of quasibound states above the potential barriers.     

Electronic wavefunctions in asymmetric double quantum well structures studied via spin-flip Raman spectroscopy

Spin-flip Raman spectra have been obtained for two heterostructures, each containing a single CdTe quantum well and a single Cd_1-xMn_xTe quantum well, with barriers of Cd_1-yMn_yTe (y>x). The spectra show a clear sensitivity to the thickness of the central barrier between the two wells. The magnitude of the Raman shifts and the resonance energies help identify the origin of the signals in the structures. When the barrier is broad, two signals are observed, one from each separate well. When the width of the barrier is reduced, a single signal is seen, reflecting the coupling of the electronic states of the two wells. The size of the Raman shifts observed also suggest the presence of a low concentration of donors in the material, which modify the form of the electronic wavefunction in the quantum wells.     

Interface phonons in asymmetric quantum well structures

In semiconductor microstructures with many layers, the phonon modes change from their bulk form and split into ‘confined LO phonons’ (LC) and ‘interface phonons’ (IF), the number and variety of which depends on both the number of layers and the number of different materials in the structure. This affects the electron–phonon scattering rates. Because of the current interest in inter-subband THz emitters, we use these LC and IF modes to evaluate the inter-subband electron–phonon scattering rate in THz emitter prototypes that are based on four-subband stepped quantum wells. These scattering rates in turn affect the population inversion predicted for these devices, so we compare the predicted population inversions for the most promising prototypes against those obtained using bulk phonon scattering rates.     

Resonant tunneling GaAs/AlGaAs quantum well structures for p-i-n photovoltaic cells

This study is devoted to the development of resonant-tunneling structures of quantum wells implementing resonant matching of lower subbands of size quantization in an electric field of the p-i-n junction of photovoltaic elements. The method for controlling the lower subband position in quantum wells by introducing a series of the tunnel-transparent barriers into a quantum well is proposed. The possibility of varying the level position in deep quantum wells in a wide range up to the continuous spectrum is demonstrated on a grown model structure; in this case, agreement between calculated and experimental subband positions is achieved.     

Toward soliton emission in asymmetric GaAs/AlGaAs multiple-quantum-well waveguide structures below the half-bandgap

We report what is to our knowledge the first experimental evidence of nonlinear beam displacement in a strip-loaded GaAs/AlGaAs multiple-quantum-well waveguide with an asymmetric, nonlinear cladding. An intensity-dependent spatial displacement of ~2 mum was observed for the guided mode at a wavelength of 1.55 mum. Numerical simulations that correspond to the experiment are also presented. The device has the potential of providing a soliton-emission-based, ultrafast all-optical switch.     

Hanle effect in an asymmetric CdTe/CdMnTe double quantum well

The observation of optical orientation of excitons and the Hanle effect in an asymmetric CdTe/CdMnTe double quantum well is reported. The characteristics of the magnetic depolarization of radiation from CdTe/CdMnTe quantum wells are discussed. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 4, 241–245 (25 February 1996)