Effect of interdiffusion on the band structure and absorption coefficient of a one-dimensional semiconductor superlattice

The effect of interdiffusion of Al and Ga atoms on the confining potential, band structure and absorption coefficient of electromagnetic radiation of a one-dimensional superlattice, composed of GaAs/Ga_1?x Al _x As quantum wells with the initially rectangular potential profile, is studied within the framework of the modified Wood-Saxon potential model. It is shown that the interdiffusion leads to the widening of the energy minibands and to the blueshift of the absorption spectrum observed in experiments.     

Effect of interdiffusion on nonlinear intraband light absorption in Gaussian-shaped double quantum rings

The effect of interdiffusion on electronic states and nonlinear light absorption in Gaussian-shaped double quantum rings is studied. The confining potential, electron energy spectrum, wave functions and absorption coefficient are obtained for different values of diffusion parameter. The effect of the variation of Gaussian parameters is considered as well. The selection rules for the intraband transitions in the cases of the light polarization parallel and perpendicular to the quantum rings' axis are obtained. It is shown that the interdiffusion can be used as an effective tool for the purposeful manipulation of the electric and optical properties of the considered structure.     

Effect of interdiffusion and magnetic field on two-electron states in Gaussian-shaped double quantum rings

The effects of interdiffusion and electrons' Coulomb interaction on the energy spectrum in Gaussian-shaped single and double quantum rings in the presence of magnetic field has been considered in the framework of exact diagonalization method. The one-electron energies as functions of magnetic field for different values of diffusion parameter have been obtained. The two-electron energies and electron probability density distributions are obtained as well. It is shown that the energy oscillations which are more pronounced for a single quantum ring, smooth out due to the interdiffusion. The Coulomb interaction transforms the crossings of the two-electron levels to anticrossings and can lead to the appearance of an additional level between the anticrossing levels.     

Effect of Al and Ga interdiffusion on the electronic states in GaAs/Ga<Subscript>1−x</Subscript>Al<Subscript>x</Subscript>As semiconductor superlattice

The effect of interdiffusion of Al and Ga atoms on the confining potential and band structure of a three-dimensional superlattice, composed of initially spherical GaAs/Ga_1?xAl_xAs quantum dots, is investigated in the framework of the modified Wood-Saxon potential model. It is shown that the interdiffusion leads to the disappearance of the quantum dots’ spherical symmetry and to the broadening of the superlattice energy minibands.     

Electronic spectrum and ballistic transport in a helical nanotube

A model of a helical nanotube permitting analytical calculation of the spectrum and wave functions is proposed and studied. It is shown that the specific character of the symmetry of the Hamiltonian results in an electronic spectrum having no band structure despite the periodicity of the potential along the system axis. A two-dimensional ribbon rolled into a helix and a quasi-one-dimensional helix in a magnetic field are considered as limiting cases of the model. It is shown that the presence of additional local minima in the subbands of the electronic spectrum leads to a nonmonotonic dependence of the ballistic conductance of the system on the chemical potential.     

Effect of interdiffusion on electronic states in one-layer quantum ring superlattice

The effect of interdiffusion on band structure and Bloch amplitudes of two dimensional superlattice composed of initially cylindrical quantum rings is investigated in the framework of adiabatic approach and using transformation to in-plane momentum space. It is shown that the adiabatic approximation is applicable even if the ring’s height is equal to its radius, because of weak localization of electron in the superlattice plane comparing with the localization in perpendicular direction. The consideration of the dependence of effective mass on spatial coordinate and time leads to the energy correction up to 10 meV. It is shown that energy minibands rise and become wider due to interdiffusion and the dependence of Bloch amplitudes on quasimomentum direction is more pronounced in the case of diffused potential profile.     

Rapid thermal processing of semimagnetic superstructures studied by magnetoreflectivity

Thermally induced interdiffusion in CdTe/CdMnTe quantum wells and a superlattice was investigated by magnetoreflectivity measurements. The structures were grown by the MBE technique and annealed by rapid thermal processing for 15s at 450 centigrade. A blue shift of the exciton states confined in quantum wells as well as a significant enhancement of the Zeeman effect were observed for the annealed samples. The manganese distribution profiles after diffusion process are discussed and it is shown that the simplest model of the interdiffusion of the Mn and Cd ions based on the Fick's law does not explain the observed enhancement of the Zeeman effect.     

Rearrangement of electron density-of-states in magnetic semiconductors

It is shown that in the presence of a strong intratomic correlation the hybridization scattering in magnetic f-semiconductors leads to a cardinal rearrangement of the bare electron spectrum. This arrangement results in the redistribution of electron density into the region far away from the chemical potential and is totally determined by the multi-site scattering processes. The results are compared with the available experimental data on light absorption in europium chalcogenides.     

Exciton optical absorption in disordered,strained InGaAs/GaAs single quantum wells

The absorption coefficient spectrum of undoped, disordered InGaAs/GaAs single quantum wells is calculated within the parabolic band approximation in this compressively strained structure. Results are presented for light propagating normal to and along the plane of the quantum well, taking into consideration the 1S-like exciton and all bound states, including the 2D enhancement Sommerfeld factor. The results presented here show that the exciton peaks in TE polarization remain constant with disordering and exhibit a larger wavelength shift than in TM polarization. The absorption edge in disordered InGaAs/GaAs, which is a function of the strain and interdiffusion, can be tailored to the desired wavelength around 1.0 μm. These results can be of interest in the design of photonic devices on a single substrate.     

Absorption and emission of far-IR radiation by hot holes in GaAs/AlGaAs quantum wells

It is found that when holes in GaAs/Al_0.5Ga_0.5As quantum wells are heated by a longitudinal electric field, the absorption in the far-IR region of the spectrum changes. The spontaneous emission spectrum in the far-IR range is measured. It is shown that the absorption and emission are due to direct intersubband transitions of holes near the peaks in the reduced density of states. The experimental data are in agreement with a theoretical calculation. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 928–932 (25 June 1996)     

Intersubband optical absorption of holes in quantum wells

The problem of hole energy spectrum and interlevel optical absorption in p -type quantum wells is considered theoretically. To obtain analytical results, terms in the Luttinger Hamiltonian containing in-plane momentum are treated as a perturbation. In this approximation energy spectrum, wave functions, hole statistics and interlevel optical matrix elements can be found for an arbitrary shape of quantum well. Finally, optical absorption spectra are calculated for different interlevel transitions.     

Plasma effects in magnetoabsorption of D<Superscript>−</Superscript> centers in quantum wells

The spectrum of magnetoabsorption of D^? centers in quantum wells is calculated with allowance for their interaction with a magnetic plasma under resonance conditions, when the distance between the Landau levels is of the same order of magnitude as the quantum energy of magnetoplasma oscillations. It is shown that splitting of Landau levels results in the fine structure of absorption lines and that the relative peak heights depend on the detuning from the resonance. The calculated frequency dependence of the absorption coefficient agrees with the experimental results.     

Exciton-polariton absorption in periodic and disordered quantum-well chains

The exciton-polariton transfer and absorption in regular and disordered structures with a finite number of quantum wells are studied theoretically. The transfer matrix method is invoked in the exciton resonance region to calculate the reflectivity, transmissivity, and absorptivity spectra, as well as the integrated absorptivity as a function of the γ/Γ₀ ratio of the parameters of nonradiative and radiative damping of quasi-two-dimensional excitons. It is shown that the integrated absorptivity as a function of γ (temperature) follows a universal pattern, more specifically, it increases monotonically from zero at γ = 0 to saturate at γ/Γ₀ ? 1. Because the exciton-polariton absorption being single mode, the integrated absorptivity in Bragg quantum-well structures is substantially lower than that in short-period structures, in which absorption involves the whole spectral multitude of modes. The intrawell disorder associated with fluctuations in the frequencies of exciton excitation in quantum wells enhances the integrated absorptivity to the level typical of light absorption with no resonance among excitons of different quantum wells. The interwell disorder originating from fluctuations in quantum-well separation likewise leads to an increase in the integrated absorptivity.     

Effect of heteroboundary spreading on the properties of exciton states in Zn(Cd)Se/ZnMgSSe quantum wells

Exciton states in Zn(Cd)Se/ZnMgSSe quantum wells with different diffusion spreading of interfaces are studied by optical spectroscopy methods. It is shown that the emission spectrum of quantum wells at low temperatures is determined by free excitons and bound excitons on neutral donors. The nonlinear dependence of the stationary photoluminescence intensity on the excitation power density and the biexponential luminescence decay are explained by the neutralization of charged defects upon photoexcitation of heterostructures. With the stationary illumination on, durable (about 40 min) reversible changes in the reflection coefficient near the exciton resonances of quantum wells are observed. It is shown that, along with the shift of exciton levels, the spreading of heteroboundaries leads to three effects: an increase in the excitonphonon interaction, an increase in the energy shift between the emission lines of free and bound excitons, and a decrease in the decay time of exciton luminescence in a broad temperature range. The main reasons for these effects are discussed.     

Enhanced/suppressed interdiffusion of lattice-matched and pseudomorphic III–V heterostructures by controlling Ga vacancies

Data are presented showing that the interdiffusion of Ga and Al (AlGaAs-GaAs) and of Ga and In (pseudomorphic InGaAs-GaAs) at a heterojunction can be significantly enhanced or suppressed by controlling group III vacancies and interstitials. High resistivity low-temperature GaAs which contains excess As is shown to accelerate the interdiffusion process. The emission wavelength from monolayer-thick quantum wells can remain relatively unchanged even after annealing for 200 h at 850° C.     

Absorption and emission of terahertz radiation in doped GaAs/AlGaAs quantum wells

Spontaneous emission of terahertz radiation from structures with GaAs/AlGaAs quantum wells in a longitudinal magnetic field has been studied. It is shown that some bands in the emission spectrum can be related to radiative electron transitions between resonant and localized impurity states, as well as to the transitions with participation of subband states. The temperature dependence of the equilibrium intraband absorption of terahertz radiation and its modulation in a longitudinal electric field in GaAs/AlGaAs quantum wells has been investigated.     

System of quantum wells in a parallel magnetic field

The energy spectrum and quantum states of electrons in a system of quantum wells in a strong magnetic field parallel to the heterogeneous boundaries are studied. The combined effect of the quantizing magnetic field and the potential of the system of quantum wells leads to a radical change in the electron dispersion relation owing to the appearance of one-dimensional Landau bands. The neighborhoods of the anticrossing points of the different bands correspond to an effective redistribution of the electron envelope functions, which becomes stronger as the magnetic field is raised. The character of the electron-state density in the size-quantization subbands is examined qualitatively in connection with the change in the system of isoenergy contours when a magnetic field is applied. Fiz. Tverd. Tela (St. Petersburg) 40, 1719–1723 (September 1998)     

Trapped and free electrons in the near-anode region of a striated discharge

Spatial potential profiles and electron energy distribution functions are measured in the near-anode region of a striated neon glow discharge. It is discovered that potential wells of small depth adjacent to the anode appear on the spatial potential profiles at certain moments in time. The distribution functions measured in the potential wells have a pronounced maximum of slow electrons, which sharply distinguishes them from the distribution functions in the striation phases where there are no wells. The mechanism which shapes the electron distribution function for electrons trapped in a potential well is analyzed. A perturbing effect of the anode on the electron distribution function as the anode is approached is discovered experimentally, and an interpretation of this effect is given. Zh. Tekh. Fiz. 68, 25–32 (March 1998)     

Electrostatic potential, energy spectrum, and Fano resonances in a ballistic ring interferometer based on an AlGaAs/GaAs heterojunction

For a ballistic ring interferometer based on high-mobility two-dimensional electron gas in an AlGaAs/GaAs heterojunction, the electrostatic potential and the energy spectrum are determined. It is shown that the splitting points in such an interferometer have the form of triangular potential wells. Calculation is performed for the two-dimensional electron transmission through the ring, and the Fano resonances caused by the coupling of the transmitted waves with the levels of higher transverse modes in triangular wells are predicted. These resonances are observed in the experiment.     

Study of the effect of irradiation with the SEM electron beam on cathodoluminescence and the induced current in InGaN/GaN structures with multiple quantum wells

The effect of irradiation with low-energy electrons on the optical and electric properties of InGaN/GaN-based LED structures with multiple quantum wells is studied using the methods of cathodoluminescence (CL) and electron-beam induced current (EBIC). It is shown that such irradiation leads to a change in both the electric and optical properties of these structures. The fitting of the dependences of the induced-current signal on the beam energy makes it possible to show that irradiation leads to the suppression of the recombination in the upper p-GaN layer, to a decrease in the probability of transition of charge carriers through the active region of the structure with quantum wells, and to a increase in the effective concentration of donors in the active region. These changes make it possible to explain the increase in the integral CL intensity, but it does not explain the change in its spectrum