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.     

Optical properties of GaNAs/GaAs triple quantum well structures

Optical properties of the GaNAs/GaAs triple quantum well structures were characterized by using photoreflectance and photoluminescence spectroscopy at different temperatures. The excitonic interband transitions of the triple quantum well systems were observed in the spectral range above hν=E_g(GaN_xAs_1−x). A matrix transfer algorithm was used to match the GaN_xAs_1−x/GaAs boundary conditions and calculate the triple quantum well subband energies numerically for theoretical comparison. The internal electric field in the system was extracted from Franz-Keldysh oscillations in the photoreflectance spectra. The influences of the annealing treatment on the transition energy and the internal electric field are also analyzed.     

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.     

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.     

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.     

Photoluminescence of InGaAs/GaAs strained-layer quantum well structures under high pressure

Abstract

Measurements of the photoluminescence (PL) of strained In_0.2Ga_0.8As/GaAs and In_0.15Ga_0.85As/GaAs quantum well structures together with the PL from bulk GaAs, in a diamond anvil cell show that the pressure coefficient of the ground confined state in the wells depends upon well width (L_Z). In the thinnest wells, the coefficient is closer to that of the bulk GaAs (10.7 meV/kbar), as expected. However, in the widest wells the coefficients tend to values (9.5meV/kbar for the 15% alloy and 9.1meV/kbar for the 20% alloy) that are significantly lower than the pressure coefficient of unstrained In_0.53Ga_0.47As (10.9meV/kbar). It is found that the low pressure coefficients can not be explained by the change in uniaxial stress with pressure due to a difference in bulk moduli between the barrier and well.     

High-strain InGaAs/GaAs quantum well grown by MOCVD

High-strain InGaAs/GaAs quantum wells （QWs） are grown by low-pressure metal-organic chemical vapor deposition （LP-MOCVD）. Photoluminescence （PL） at room temperature is applied for evaluation of the optical property. The influence of growth temperature, V/III ratio, and growth rate on PL characteristic are investigated. It is found that the growth temperature and V/III ratio have strong effects on the peak wavelength and PL intensity. The full-width at half-maximum （FWHM） of PL peak increases with higher growth rate of InGaAs layer. The FWHM of the PL peak located at 1039 nm is 20.1 meV, which grows at 600 ℃ with V/ III ratio of 42.7 and growth rate of 0.96 μm/h.     

Electro-optical probing of envelope wavefunctions in GaAs/AlGaAs parabolic quantum well structures

We report on a systematic experimental and theoretical study of the interband electroabsorption in GaAs/AlGaAs parabolic quantum well structures. Investigating a sample with an appropriately designed well width, we are able to observe a complex interplay of various electro-optical effects. The obtained results can be interpreted in terms of probing the electric-field dependent overlaps between the harmonic oscillator type envelope wavefunctions of electrons and holes.     

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.     

Post-growth tailoring of the optical properties of GaAs/AlGaAs quantum well structures

In this paper we describe a method for performing post-growth bandgap engineering in the GaAs/AlGaAs quantum well system. The method used is impurity-free vacancy diffusion. Using both single and multiple quantum well data we show that this allows blue shifts in the optical properties, while retaining both their distinctive excitonic and electrical characteristics. The electrical response is modelled and no comparative degradation of the quantum confined Stark effect is predicted, and this is confirmed experimentally. Possible applications of this technique are mentioned.     

Photoluminescence of n-i-n GaAs/AlAs single quantum well structures under electric field bias

GaAs/AlAs single quantum well structures designed with well thickness near the type-I/type-II crossover show distinctive photoluminescence peaks corresponding to both type-I and type-II recombinations. Photoluminescence measurements as a function of applied electric field and temperature ranging from 23 to 180 K and current–voltage measurements are presented for two MBE-grown structures clad with Si-doped Al_0.45Ga_0.55As layers onn ⁺  -GaAs [100] substrates. The large and field-dependent energy separation between type-I and type-II luminescence peaks is understood to arise from the build-up of electrons at the X point in the AlAs barrier.     

Time-resolved cathodoluminescence of InGaAs/AlGaAs tetrahedral pyramidal quantum structures

An original time resolved cathodoluminescence set up has been used to investigate the optical properties and the carrier transport in quantum structures located in InGaAs/AlGaAs tetrahedral pyramids. An InGaAs quantum dot formed just below the top of the pyramid is connected to four types of low-dimensional barriers: InGaAs quantum wires on the edges of the pyramid, InGaAs quantum wells on the (111)A facets and segregated AlGaAs vertical quantum wire and AlGaAs vertical quantum wells formed at the centre and at the pyramid edges. Experiments were performed at a temperature of 92 K, an accelerating voltage of 10 kV and a beam probe current of 10 pA. The cathodoluminescence spectrum shows five luminescence peaks. Rise and decay times for the different emission wavelengths provide a clear confirmation of the peak attribution (previously done with other techniques) to the different nanostructures grown in a pyramid. Moreover, experimental results suggest a scenario where carriers diffuse from the lateral quantum structures towards the central structures (the InGaAs quantum dot and the segregated AlGaAs vertical quantum wire) via the InGaAs quantum wires on the edges of the pyramid. According to this hypothesis, we have modeled the carrier diffusion along these quantum wires. An ambipolar carrier mobility of 1400 cm²/V s allows to obtain a good fit to all temporal dependences. PACS 78.67.-n     

Bound polaron in quantum dot quantum well structures

The problem of bound polarons in quantum dot quantum well (QDQW) structures is studied theoretically. The eigenfrequencies of bulk longitudinal optical (LO) and surface optical (SO) modes are derived in the framework of the dielectric continuum approximation. The electron--phonon interaction Hamiltonian for QDQW structures is obtained and the exchange interaction between impurity and LO-phonons is discussed. The binding energy and the trapping energy of the bound polaron in CdS/HgS QDQW structures are calculated. The numerical results reveal that there exist three branches of eigenfrequencies of surface optical vibration in the CdS/HgS QDQW structure. It is also shown that the binding energy and the trapping energy increase as the inner radius of the QDQW structure decreases, with the outer radius fixed, and the trapping energy takes a major part of the binding energy when the inner radius is very small.     

Effect of low-energy electron irradiation on the cathodoluminescence of multiple quantum well (MQW) InGaN/GaN structures

The effect of low-energy electron-beam (e-beam) irradiation on the InGaN-related cathodoluminescence in multiple quantum well (MQW) InGaN/GaN light-emitting diode (LED) structures has been studied. It is shown that the e-beam exposure leads to an increase of emission intensity and to a formation of new blue-shifted emission bands. The changes observed were explained by the enhancement of In diffusion stimulated by excess carrier recombination.     

Electroreflectance studies of thin GaAs/AlGaAs quantum well structures with tunable 2DEG densities

We report the results of low-temperature electroreflectance (ER) and photoluminescence (PL) investigations on thin n-type modulation-doped GaAs/AlGaAs single quantum well structures. The density of the two-dimensional electron gas (2DEG) is varied continuously from zero to N_s∼6*10¹¹ cm^-2, using a Schottky gate, and is detected optically via the Stokes shift between the ER and PL subband transitions. For the first time, the splitting between transitions involving the first electron subband and the first heavy and light hole subbands, respectively, is studied as a function of the 2DEG density using ER spectroscopy. The results are discussed in comparison with the calculated in-plane dispersion of the hole subbands. Clear evidence is given for the influence of exciton screening effects.     

Picosecond luminescence of AlxGa1-xAs and GaAs double quantum well structures under high pressure

Abstract

The time-resolved luminescence of an electron-hole plasma in Al_0.36Ga_0.64As was studied as a function of pressure. Application of hydrostatic pressure varies the energy separation between the conduction band minima at Г and X. The dependence of the intensity ratio of the zero-phonon line and the two phonon replicas on this energy separation shows that scattering from X to Г by alloy disorder is as effective as scattering by phonons. We have further studied the tunneling of electrons between two GaAs quantum wells (QW) of different thicknesses through an Al_0.35Ga_0.65As barrier. The lifetime of electrons in the narrow QW, which is limitd by electron tunneling into the wider QW, stays constant from 0 to 2.4 GPa, where it drops within 0.1 GPa from 140 ps to less than 7 ps. At this pressure the X-point energy of the barrier coincides with the electron level in the wider QW. We infer that tunneling occurs only via the Г states in the barrier and that X states become effective only when real-state transfer is possible.     

Reduction in relaxation time due to ionized impurities in GaAs/AlGaAs quantum well structures

Time-resolved photoluminescence measurements in δ -doped GaAs/AlGaAs on the quantum well structures are performed to study effects of ionized impurities relaxation process of photoexcited carriers. It is theoretically shown that a thin quantum well with a δ -doping layer inserted in the barrier layer of double quantum wells enhances the impurity scattering rate significantly. Photoluminescence decay time in the δ -doped samples is found to decrease compared with the undoped samples.