Hot electrons and phonons in quantum-well AlxGa1-xAs-GaAs heterostructures

Formulas are presented to calculate the heating of charge carriers and the generation of nonequilibrium phonons by intense photoexcitation of quantum-well heterostructures. It is shown that the deviations from thermal equilibrium are more pronounced for quasi-two-dimensional structures than for bulk material. Supporting experimental data are given on an MO-CVD Al_xGa_1-xAs-GaAs heterostructure consisting of a large GaAs quantum well (L_z ～ 200 Å) coupled to a phonon-generating array of seven small GaAs quantum wells (L_z ～ 50 Å).     

Photoluminescence of AlxGa1−xAs/GaAs quantum well heterostructures grown by molecular beam epitaxy

Low-temperature photoluminescence measurements on nominally undoped Al_xGa_1–xAs/GaAs quantum well heterostructures (QWHs) grown by molecular beam epitaxy (MBE) exemplified the exclusivelyintrinsic free-exciton nature of the luminescence under moderate excitation conditions. Neither any spectroscopic evidence for alloy clustering in the Al_xGa_1–xAs barriers nor any extrinsic luminescence due to recombination with residual acceptors has been detected in single and double QWHs when grown at 670 °C under optimized MBE growth conditions. Carrier confinement in Al_xGa_1–xAs/GaAs QWHs starts at a well width ofL _z30 nm when x0.25. The minor average well thickness fluctuation ofL _z=4×10^–2nm as determined from the excitonic halfwidth allowed the realization of well widths as low asL _z=1 nm and thus a shift of the free-exciton line as high as 2.01 eV which is close to the conduction band edge of the employed Al_0.43Ga_0.57As confinement layer. The measurements further revealed a strongly enhanced luminescence efficiency of the quantum wells as compared to bulk material which is caused by the modified exciton transition probabilities due to carrier localization.     

Tilted magnetic field effect on the subbands of a GaAlAs diode with a GaAs quantum well

We have calculated the subbands in the GaAs quantum well at the n-side of the junction in a Ga_1−xAl_xAs diode. We show that the density of carriers confined in the quantum well increases by the increasing magnetic field strength but also decreases depending on the magnetic length. We have observed oscillatory behavior of the density of carriers due to the in-plane magnetic field.     

Electron capture probability by quantum wells

The electron capture probability in a quantum well is studied by using the theory of generalized random walks. A periodically oscillating capture probability versus quantum well width, L_z, is obtained. Our stochastic model has new aspects compared with both the classical and quantum mechanical models.     

Scattering wave vector dependence of Raman intensity of the v1 internal mode in KH2PO4

The v₁ internal breathing mode of PO₄ tetrahedrons in KH₂PO₄ is observed with the (yx) Raman spectra. It has been found that its intensity depends upon the direction of the scattering wave vector by means of the observations with the z-Δx(yx)y and x-Δz(yx)y scattering geometries. While this mode is very weak in intensity in the x(yx)y spectrum, it becomes very strong as the scattering geometry approaches the z(yx)y one.     

Specific features in the behavior of the effective mass and mobility in n-(Bi, Sb)2(Te, Se, S)3 solid solutions

The thermoelectric properties of the multicomponent solid solutions Bi_2?x Sb_xTe_3?y?z Se_yS_z with substitutions of atoms in both sublattices of Bi₂Te₃ were studied. The data obtained in studies of the galvanomagnetic effects in weak magnetic fields were used to properly take into account the change in the carrier scattering mechanisms due to the substitutions Sb → Bi, Se, and S → Te in the solid solutions. The mobility μ₀ with inclusion of the degeneracy, the effective density-of-states mass m/m ₀, and the lattice thermal conductivity κ_L were calculated. An analysis was carried out for the quantities μ₀, m/m ₀, and κ_L in the solid solutions under study as functions of the composition, carrier concentration, and temperature.     

Spin polarization and spin-dependent transmittance in II–VI diluted magnetic semiconductorheterostructure

In this work, we carried out detailed investigation of a Cd_1?xMn_xTe/CdTe/Cd_1?xMn_xTe diluted magnetic semiconductor based quantum well. Our theoretical results are based on an accurate self-consistent resolution of the one-dimension Schrödinger and Poisson’s equations in the framework of the mean-field approximation for spin-up and spin-down orientations of carriers coupled via the sp–d exchange interaction. From the calculation of spin-dependent carrier densities for ferromagnetic and anti-ferromagnetic coupling, we evidence the spin-up and spin-down space separation for holes in quantum well for different values of band offsets. From deduced spin polarizations, we show that the CdTe region acts as a layer of spin rearrangement and spin reversal, respectively, in the ferromagnetic and anti-ferromagnetic coupling. The transmittance coefficients T₊ and T_? of injected spin-up and spin-down carriers are evaluated as a preliminary work to assess the spin-dependent currents in devices consisting of alternatively layers of non-magnetic and diluted magnetic semiconductors.     

Theoretical modeling for quantum-confined Stark effect due to internal piezoelectric fields in GaInN strained quantum wells

Recent experimental investigations revealed that the biaxial stress in thin InGaN layers grown on thick GaN layer induces a large piezoelectric field along [0001] orientation that causes red-shift in optical transitions and reduction in oscillator strengths because of spatial separation of the electron and hole wave functions. In this Letter based on theoretical modeling we determined the well width z-dependent effect on red-shifted quantum-confined Stark effect (QCSE) in GaN/In_xGa_1 − xN (x=0.13) strained quantum well structures. Analyses are based on the solution of Schrödinger equation in a finite well including the internal piezoelectric electric field (F) due to the strained polarization as the perturbation potential. Our theoretical results show: (1) the red-shift in optical transition has a quadratic well-width form as it is for infinite wells (Davies, 1998) [1], (2) assuming the model based on a carrier effective mass dependence on the width of quantum wells, m^∗(z), fits the experimental data (Takeuchi et al., 1997) [2] much more accurate compare to the model with constant effective mass, m^∗.     

Determination of the indirect band edge of GaAs by quantum-well bandfilling (Lz∼100Å)

Photoexcitation-bandfilling of a quantum-well Al_xGa_1?xAs-GaAs-Al_xGa_1?xAs (x～0.6, L_z～100Å) heterostructure is used to generate visible-red recombination radiation that cuts-off at high energy (1.864 eV, 4.2°K) due to electron transfer to indirect minima. This leads to an estimate of ΔE = E_L?E_Γ～310 meV.     

Resonant Raman scattering in nanostructures with InGaAs/AlAs quantum dots

Raman scattering by optical phonons in In_xGa_{1 ? x} As/AlAs nanostructures with quantum dots has been studied experimentally for compositions corresponding to x = 0.3?1 under out-resonance conditions. Features due to scattering by GaAs-and InAs-like optical phonons in quantum dots have been detected, and the phonon frequencies have been determined as a function of the dot composition. With increasing excitation energy, a red shift is observed in the frequency of the GaAs-like phonon in quantum dots, which testifies to Raman scattering selective by the size of quantum dots. Under resonant conditions, multiphonon light scattering by optical and interface phonons is observed up to the third order, including overtones of the first-order phonons of InGaAs and AlAs materials and their combinations.     

An analysis of the thermoelectric efficiency of n-(Bi,Sb)2(Te,Se,S)3 solid solutions within an isotropic scattering model

A study is reported on the thermoelectric properties of n-type solid solutions Bi₂Te_3?y Se_y (y=0.12, 0.3, 0.36), Bi_2?x Sb_xTe_3?y Se_y (x=0.08, 0.12; y=0.24, 0.36), and Bi₂Te_3?z S_z (z=0.12, 0.21) as functions of carrier concentration within the 80-to 300-K range. It has been established that the highest thermoelectric efficiency Z is observed in the Bi₂Te_3?y Se_y (y=0.3) solid solution containing excess Te at optimum carrier concentrations (0.35×10¹⁹ cm^?3) and at temperatures from 80 to 250 K. The increase in Z in the Bi₂Te_3?y Se_y solid solution compared with Bi_2?x Sb_xTe_3?y Se_y and Bi₂Te_3?z S_z is accounted for by the high mobility μ₀, an increase in the effective mass m/m ₀ with decreasing temperature, the low lattice heat conductivity κ_L, and the weak anisotropy of the constant-energy surface in a model assuming isotropic carrier scattering.     

Spacer layer thickness fluctuation scattering in a modulation-doped AlxGa1-xAs/GaAs/AlxGa1-xAs quantum well

We theoretically study the influence of spacer layer thickness fluctuation（SLTF） on the mobility of a twodimensional electron gas（2DEG） in the modulation-doped Al x Ga 1 x As/GaAs/Al x Ga 1 x As quantum well.The dependence of the mobility limited by SLTF scattering on spacer layer thickness and donor density are obtained.The results show that SLTF scattering is an important scattering mechanism for the quantum well structure with a thick well layer.     

Ferromagnetism in heterostructures based on a dilute magnetic semiconductor

Ferromagnetism of magnetic impurity atoms located in the barrier regions of various heterostructures (solitary heterojunction, single quantum well, double quantum well, or superlattice) is considered theoretically. The indirect magnetic interaction of impurities occurs via charge carriers localized in quasi-two-dimensional conducting channels of these structures due to “penetration” of the wavefunction of charge carriers into the barrier regions. The wavefunctions defined analytically in the triangular potential model are virtually the same as in “exact” numerical calculations (joint solution of the Poisson and Schrödinger equations). The corresponding Curie temperatures are determined, which may attain approximately 500 K in Ga_{1 ? x} Mn _x As-based structures according to calculations.     

Galvanomagnetic and thermoelectric properties of multicomponent n-type solid solutions based on Bi and Sb chalcogenides

The galvanomagnetic and thermoelectric properties of n-Bi_2-x Sb_xTe_3-y-z Se_yS_z multicomponent solid solutions with atomic substitutions (Sb → Bi; Se, S → Te) are studied. The principal components of the effective mass tensor (m ₁, m ₂, m ₃) for the isotropic mechanism of charge carrier scattering are determined within a many-valley model of the energy spectrum for compositions 0.08 ≤ x ≤ 0.4 and 0.06 ≤ y = z ≤ 0.15. The effect of a variation in the parameters of the constant-energy surface on the thermoelectric efficiency is analyzed for different compositions and carrier concentrations in solid solutions.     

Resonant-tunneling structure of quantum wells in the p-i-n photovoltaic element

The method for efficient separation of photoexcited carriers, based on the resonant tunneling phenomenon in the quantum well structure placed into the i-region of the p-i-n photovoltaic element, is proposed. The parameters of quantum well structures based on the GaAs/Ga_1?x In _x As system, implementing the mode of sequential resonant tunneling in the electric field of the GaAs p-i-n junction, is calculated. A microscopic model of resonant-tunneling transport in such structures is constructed, and the kinetic tunneling times are calculated depending on well and barrier parameters. The possibility of achieving sufficiently short (<～10 ps) tunneling times and, hence, quite efficient removal of photoelectrons and photoholes from quantum wells at a proper choice of barrier powers is shown.     

Two-magnon Raman scattering in dielectric and superconducting YBa2Cu3O6+x crystals

Two-magnon Raman scattering in dielectric, as well as superconducting, YBa₂Cu₃O_{6 + x} single crystals with mobile oxygen content x = 0.2–0.7 and superconducting transition temperature T _c = 0–74 K is studied in detail. Doping with oxygen in the range of x = 0.2–0.5 leads to two-magnon scattering peak broadening and a shift in the spectral position of the peak towards lower energies. The most significant qualitative changes in two-magnon scattering in YBa₂Cu₃O_{6 + x} crystals are observed in a narrow oxygen concentration range near x = 0.7. This is explained by a considerable decrease in the correlation length ξ_AF of antiferromagnetic (AF) correlations upon an increase in the concentration of free carriers. For instance, doping is accompanied with a reduction of ξ_AF to values of several lattice constants a for x ≈ 0.7, a transition to the regime of short-range AF order, and local scattering of light from a small AF cluster with a size of 3 × 4 lattice constants. An increase in the free charge carrier concentration destroys the short-range AF order in a narrow range of the stoichiometry index near x = 0.7. Experimental data also indicate heterogeneity of cuprate planes at microscopic level, which leads to coexistence of superconducting and AF regions in YBa₂Cu₃O_{6 + x} super-conducting crystals.     

Sub-picosecond Raman spectroscopy and dynamics of nonequilibrium electrons and phonons in semiconductors

Recent progress made in the use of time-resolved pump-probe Raman scattering technique to study the dynamics of photo-excited nonequilibrium carriers and LO-phonons in III–V semiconductors up to the sub-picosecond time scale will be discussed. It will be shown how this technique has allowed direct time-domain measurements of electron-LO-phonon scattering times for “hot” carriers and lifetimes for “hot” LO-phonons in semiconductors like GaAs. The presentation will include new experimental results of Kash, Jha and Tsang on picosecond Raman studies of the Fröhlich interaction in alloys like Al _x Ga _1?x As and In _x Ga _1?x As. The present theoretical understanding of the dynamics of the highly excited carriers, dominated by strong LO-phonon emission, will be examined along with the discussion of a recent calculation of transient electrical conductivity of such hot carriers, showing extremely interesting oscillations with respect to the pump laser frequency, on the scale of the long wavelength LO-phonon frequency, and a highly nonlinear behaviour as a function of time.     

Temperature dependent dynamic of excitons in Zn1 − xCdxSe/ZnSe multiple quantum wells

The exciton dynamics in Zn_1 − xCd_xSe/ZnSe multiple quantum wells have been investigated by temperature-dependent time-resolved photoluminescence experiments. A polariton effect (leading to a linearT-dependence of the decay time τ_PL) and thermal escape of carriers from the quantum well at relatively high temperature (resulting in a decrease of τ_PLwith the temperature) are observed in samples of different stoichiometry and well.