xAs Multiple Quantum Research Articles

Excitonic Effects and Impurity–Defect Emission in GaAs/AlGaAs Structures Used for the Production of Mid-IR Photodetectors

A series of undoped GaAs/AlxGa1 –xAs multiple quantum well heterostructures, whose doped analogs are used for the production of photodetectors operating in the spectral range 8–12 μm, is fabricated by molecular-beam epitaxy. For the heterostructures, the spectral position of absorption lines corresponding to the allowed transitions between quantum-confined electron and hole levels in GaAs layers is established. The influence of impurity–defect states on the luminescence and absorption spectra of quantum wells is studied. The excitonic corrections for the allowed transitions are determined in relation to the quantum-well width and the aluminum content in the barrier layers. The role of excitonic effects in restoring the structure of single-electron states from interband-absorption spectra (luminescence-excitation spectra) and the relationship between these states and the working region of IR photodetectors based on GaAs/AlxGa1 –xAs quantum wells are discussed.

Evidence for the light hole in GaAs/AlGaAs quantum wells from optically-pumped NMR and Hanle curve measurements

Optically-pumped 69Ga NMR (OPNMR) and optically-detected measurements of polarized photoluminescence (Hanle curves) show a characteristic feature at the light hole-to-conduction band transition in a GaAs/AlxGa1−xAs multiple quantum well sample. OPNMR data are often depicted as a “profile” of the OPNMR integrated signal intensity plotted versus optical pumping photon energy. What is notable is the inversion of the sign of the measured 69Ga OPNMR signals when optically pumping this light hole-to-conduction band energy in OPNMR profiles at multiple external magnetic fields (B0=4.7T and 3T) for both σ+ and σ− irradiation. Measurements of Hanle curves at B0=0.5T of the same sample exhibit similar phase inversion behavior of the Hanle curves at the photon energy for light hole excitation. The zero-field value of the light-hole state in the quantum well can be predicted for the quantum well structure using the positions of each of these signal-inversion features, and the spin splitting term in the equation for the transition energy yields consistent values at 3 magnetic fields for the excitonic g-factor (gex). This study demonstrates the application of OPNMR and optical measurements of the photoluminescence to detect the light hole transition in semiconductors.

Intrasubband and intersubband transitions in GaAs∕AlxGa1−xAs multiple quantum wells

Infrared absorption in GaAs∕AlxGa1−xAs multiple quantum wells is investigated using a polarizer. Two main peaks, with wave numbers 723 and 1092cm−1, are observed. The peak with wave number 1092cm−1 corresponds to the 0→1 intersubband transition, while the other one corresponds to the intrasubband transition. The polarized absorbance is one order of magnitude higher than the unpolarized one. The authors attribute the intrasubband transition to the plasma oscillation in the quantum wells.

Picosecond dynamics of spin-related optical nonlinearities in InxGa1−xAs multiple quantum wells at 1064 nm

Light-induced absorption and diffraction measurements of resonantly excited 10-nm-width InGaAs multiple quantum wells have been carried out, using circularly polarized beams at 1064 nm. Spin relaxation time τs≃280±15 ps has been determined by monitoring dynamics of light absorption bleaching at 0.04 mJ/cm2, while a nonlinearly compressed spin component in diffraction varied from 220±20 ps to 115±15 ps with excitation. The kinetics of carrier grating with randomized spins allowed the determination of the bipolar diffusion coefficient D=11.5 cm2/s, hole mobility of 230 cm2/V s, and carrier lifetime τR=0.73–1 ns.

Optimized energy separation for phonon scattering in three-level terahertz intersubband lasers

We examine the impact of the complex phonon spectrum on the design rules for three-level THz intersubband emitters that utilize resonant optical phonon scattering to obtain intersubband population inversion. In GaAs/AlxGa1−xAs multiple quantum wells, electron–optical phonon scattering occurs due to interaction with “GaAs-like” modes at ℏωLO∼36 meV and “AlAs-like” modes at ℏωLO∼47 meV. Scattering rates are calculated for interface and confined phonon modes to determine the optimal subband energy separations for enhancement of fast depopulation scattering and reduction of parasitic nonradiative scattering. While the results depend sensitively on details of the structure, we find that, for electrically pumped THz lasers, there is not an overwhelming advantage in increasing the subband separation to allow scattering by the higher energy modes. © 2001 American Institute of Physics.

Studies of far-infrared magneto-photoconductivity of D − centers in GaAs/Al xGa 1− xAs multiple quantum wells

We present magneto-photoconductivity studies on GaAs/Al 0.3Ga 0.7As multiple quantum wells selectively doped with Si donors in the center of the wells and in the barrier layers. Under extra illumination of an He–Ne laser, dramatic results of the D − centers were observed as a function of the electron density. A band-bending model associated with a “barrier D − center” configuration was used to analyze the evolution of the binding energy of the spin-singlet D − transition and magnetic vaporization.

Raman studies of delta-doped semiconductors and quantum wells

Theoretical and experimental studies of the Raman spectra of delta-doped bulk GaAs and GaAs–AlxGa1−xAs multiple quantum wells are presented. Intersubband plasmon modes are calculated with an energy-dependent effective-mass theory, which takes into account the band nonparabolicity. The screened external potential due to impurity and electron charge distribution including the exchange and correlation effects are calculated self consistently within the local density approximation. The calculated Raman spectra are in agreement with experimental data with a reasonable assumption of doping profile.

Linewidth of the infrared absorption spectra due to bound-to-continuum transition in GaAs/AlxGa1−xAs multiple quantum well structures

We have observed an extremely narrow absorption spectrum due to bound-to-continuum transition in GaAs/AlxGa1−xAs multiple quantum wells (MQWs). Its linewidth is only about one tenth of the values reported previously. Our calculation indicates that the broadening of the excited state in the continuum has little contribution to the absorption linewidth. We have grown a sample whose MQW region contains two kinds of wells with a minor thickness inhomogeneity. Its resultant absorption linewidth is six times as large as that of homogeneous well sample, which is in good agreement with our theoretical analysis. Thus we can suggest that the wider absorption spectra reported by many authors may be due to the well width inhomogeneity.

Modeling and analysis of photomodulated reflectance and double crystal x-ray diffraction measurements of tensilely strained InGaAs/InGaAsP quantum well structures

Room temperature photomodulated reflectance (PR) and double crystal x-ray diffraction (DCXRD) measurements have been performed on a series of tensilely strained InxGa1−xAs multiple quantum well (QW) laser structures, with In0.80Ga0.20As0.43P0.57 barriers, which are lattice-matched to an InP substrate. Seven samples are studied, with nominal QW In composition varying between x=0.533 and 0.316, corresponding to biaxial tensile strains between 0% and 1.5%, respectively. The DCXRD measurements provide accurate information on composition, strain and layer thickness, while the PR yields the energies of both allowed and forbidden critical point interband QW transitions, and how these vary with strain, particularly the transitions between the ground-state conduction band and heavy/light hole valence band levels. A three-band effective mass formalism is used to model the QW transitions and very good agreement with the PR measurements is obtained once excitonic binding energies, and the quantum confined Stark effect are taken into account.

Normal incidence n-type GaAs/AlxGa1−xAs quantum well infrared photodetector

The infrared absorption and photoresponse characteristics of n-type GaAs/AlxGa1−xAs multiple quantum wells infrared photodetector structure have been studied. With the normal incidence light, a detectivity D (8.7 μm) = 1.4 × 10★cm Hz★/W via free-carrier absorption in the wells is achieved at 77 K. This shows a possible fabrication of normal incidence quantum wells infrared photodetector without a grating coupler.

Appropriate analytical description of the temperature dependence of exciton peak positions in GaAs/AlxGa1−xAs multiple quantum wells and the Γ8v−Γ6c gap of GaAs

We have performed a detailed numerical reinvestigation of the photoluminescence peak position data given by G. Oelgart et al. [J. Appl. Phys. 74, 2742 (1993)] for the ground state heavy-and light-hole excitons in a high quality molecular beam epitaxially grown GaAs/Al0.3Ga0.7As multiple quantum well structure. Appropriate fittings of the measured temperature dependencies of exciton peak positions from 4.2 up to 340 K are shown to be provided by a novel analytical four-parameter representation developed recently by one of the authors for the gap shrinkage effect in semiconductors. The magnitude of the limiting (T→∞) shrinkage coefficient, α=0.475 meV/K, and the associated average phonon temperature, Θ=222.4 K, have been determined. Characteristic qualitative differences and basic deficiencies of earlier three-parameter models are discussed and illustrated numerically by comparisons with various experimental observations from low to high temperatures.

Comment on "Many-body analysis of the effects of electron density and temperature on the intersubband transition in GaAs/AlxGa1-xAs multiple quantum wells"

Comment on "Many-body analysis of the effects of electron density and temperature on the intersubband transition in GaAs/AlxGa1-xAs multiple quantum wells"

Electron mobility in selectively doped GaAs/Inx Ga1−xAs multiple quantum well structures

The photoluminescence, the temperature dependence of the electrical conductivity (0.4<T<300 K), the magnetoresistance and the Hall effect were investigated in selectively doped GaAs/In xGa1−xAs multiple quantum well (MQW) structures. The dependencies of the electron mobility on the width of the quantum wells and on temperature were measured. It is shown that in narrow MQW structures the value of mobility is restricted by interface roughness scattering. In wider MQW structures, the alloy scattering has a detrimental effect on the low magnetic field transport properties. A negative magnetoresistance was observed in low magnetic fields. From a detailed comparison between theory of weak localisation and experiment the wave function relaxation time τϕ and its temperature dependence were evaluated. The quantum Hall effect was investigated in all samples in the temperature interval 0.4 K ÷ 4.2 K in magnetic fields up to 40 T.

Photoluminescence spectra of MOCVD-grown P-doped GaAS/Al x Ga1−x As MQW

PhotoLuminescence (PL) measurement techniques have been used to investigate on MOCVD grown P-doped GaAs/AlxGa1−xAs(x = 0.3) Multiple Quantum Wells (MQW). The spectra reveal extrinsic luminescence characteristics of e-A0 transitions for interface and centre of well acceptors in addition to both bound and free exciton emissions.

Excitation-induced phase shifts of heavy- and light-hole quantum beats in GaAs/AlxGa1-xAs multiple quantum wells.

Excitation-induced phase shifts of heavy- and light-hole quantum beats in GaAs/AlxGa1-xAs multiple quantum wells.

Index of refraction of GaAs/AlxGa1−xAs multiple quantum wells with an applied electric field using the grating coupling technique

The grating coupling technique is used to determine the index of refraction under an applied electric field. Light is coupled into a GaAs/AlxGa1−xAs multiple quantum well slab waveguide using a conducting grating which has been etched onto the waveguide. The coupling angle is measured with high precision and the effective index of the mode is calculated with the mode coupling equation. The technique is very sensitive, allowing the index to be determined to within ±2×10−5. The strong absorption of the quantum wells prevents measurements at photon energies near the quantum well absorption peaks. This is not a serious limitation as the measurable range is the most technologically important region for electro-optic devices which utilize refractive index changes. The linear and quadratic electro-optic coefficients, evaluated from the change in index with electric field, agree well with previous measurements by other methods.

Investigation of excitonic saturation by time-resolved circular dichroism in GaAs-AlxGa1-xAs multiple quantum wells.

Investigation of excitonic saturation by time-resolved circular dichroism in GaAs-AlxGa1-xAs multiple quantum wells.

Thermal strain effects on the excitonic states in GaAs/AlxGa1−xAs multiple quantum wells

Temperature-dependent absorption experiments have been performed to investigate the effect of thermal strain in GaAs/AlxGa1−xAs multiple quantum wells clad by thick AlGaAs layers. A temperature-dependent splitting of the heavy-hole and light-hole excitons is observed between 10 and 300 K. This is ascribed to the different thermal expansion coefficient of GaAs and AlxGa1−xAs, causing the constituent GaAs layers to be under weak tensile strain during the optical measurements at different temperatures.

Femtosecond studies of excitonic optical non-linearities in GaAs/Al xGa 1− xAs multiple quantum wells under in-plane uniaxial strain

The pump-probe technique was employed in the study of excitonic optical non-linearities at 40 K in a GaAs/Al 0.1Ga 0.9As multiple quantum well under thermally induced in-plane uniaxial strain. We report the first observation of an ultrafast large-angle polarization rotation of the probe light effected by the bleaching of the anisotropic absorption near the heavy hole exciton peak in this material. Dynamic rotations as large as 42° have been measured for a compressive strain of ~ 0.2%. In addition, a dependence of the phase space filling produced by resonantly excited excitons on the orientation of the pump polarization vector has been observed which decays with a time constant of 0.5 ps. This decay is attributed to the ionization of excitons into free electron-hole pairs. The subsequent decline of the probe rotation is governed by the removal of these carriers from the wells.

Hot electron energy and momentum relaxation in GaAs/AlAs and GaAs/Ga 1-xAl xAs multiple quantum wells

Experimental results on high electric field longitudinal transport in GaAs/AlAs and GaAs/Ga 1-xAl xAs multiple quantum wells (MQW) are presented and compared with the prediction of a dielectric continuum model. We draw from our experiments the following four conclusions. (i) In GaAs/Ga 1-xAl xAs systems the dominant energy and momentum relaxation mechanism is through scattering with GaAs -modes. (ii) However, in GaAs/AlAs systems the AlAs interface mode is dominant in relaxing the energy and momentum of the quantum well electrons. (iii) The hot electron momentum relaxation as obtained from the high-field drift velocity experiments is strongly affected by the production of hot phonons as expected from a model involving a non-drifting hot phonon distribution. (iv) The importance of the AlAs interface mode in GaAs/Ga 1-xAl xAs MQW is not the result of the intrinsic scattering rate but related to its shorter lifetime, compared to GaAs modes.