Spatial reproduction and multiplication effects arising from the interference of electron waves in 2D semiconductor nanostructures based on rectangular quantum wells

The effects of spatial reproduction and multiplication of the probability current density that arise in a system composed of narrow and wide rectangular quantum wells (QWs) joined in the direction of propagation of the electron wave are investigated theoretically. It is found that the transverse distribution existing at the input of the wide QW is reproduced with a certain accuracy in its repetitive cross sections and that the initial peak splits into several identical peaks of lower intensity within each interval between these cross sections (the multiplication effect). It is shown that these effects arise from the interference of electron waves in the case when the waves propagate simultaneously in several quantum-dimensional subbands of the wide QW.     

Effects of spatial reproduction as a result of the interference of electron waves in two-dimensional semiconductor nanostructures with parabolic quantum wells

Effects of spatial nonuniformity for the probability flux density j_x(x, z) or for the density of the quantum-mechanical current ej_x(x, z), where e is the elementary charge) are studied. These effects arise in two-dimensional semiconductor nanostructures that consist of thin rectangular and wide parabolic quantum wells that are consecutively arranged in the direction of the electron-wave propagation (the x axis) and are oriented along the dimensional-quantization axis z. A nonuniform distribution of j_x(x, z) arises as a result of interference of electron waves that propagate simultaneously in a wide quantum well over different quantum-dimensional subbands. Particular attention is paid to the effects of the spatial reproduction of electron waves in the nanostructures under consideration. It is shown that the transverse distribution j_x(0, z) existing at the entry to the wide quantum well is reproduced to a certain accuracy at a distance of X₁ from the entry. In addition, the initial distribution j_x(0, z) is reproduced periodically in the sections X_q = qX₁ (coefficients q are integers). The results of numerical calculations of magnitudes of these effects in the structures that are symmetric with respect to the z axis are reported; a modification of the effects under consideration in asymmetric nanostructures is considered.     

Under-barrier leakage of the quantum-mechanical current density owing to the interference of electron waves in the 2D semiconductor nanostructures

The theoretical analysis of the under-barrier leakage of the local quantum-mechanical current density in the 2D semiconductor nanostructures that represent narrow and wide rectangular quantum wells sequentially located along the propagation direction of electron wave is presented. The wave arrives from the narrow quantum well at a semi-infinite rectangular potential barrier with height V ₀ in the wide quantum well. Under certain conditions, the exponentially decaying and coordinate-dependent leakage of the local quantum-mechanical current density under barrier is allowed for the waves with energies of less than V ₀ due to the interference of electron waves in such a nanostructure.     

Effect of transverse constant electric field on the electron interference effects related to the under-barrier penetration of quantum-mechanical current density in 2D semiconductor nanostructures

The effect of transverse constant electric field F on the under-barrier penetration of the local quantum-mechanical current density in a 2D semiconductor structure is theoretically studied. The structure represents two quantum wells with identical widths that are sequentially located along the propagation direction of electron wave: the first well has the rectangular cross section, and the second well exhibits a semi-infinite rectangular potential barrier with height V ₀ that is modified by the transverse electric field. When an electron wave whose energy is less than resulting height of the potential barrier V _eff is incident from the first well on the barrier under certain conditions, the coordinate-dependent exponentially decaying penetration of the local quantum-mechanical current density may take place under the barrier due to the interference of electron waves in the nanostructure. It is demonstrated that the penetration parameters depend on field strength F.     

Dielectric function of quasi-2D semiconductor nanostructures

The spatial and temporal dispersion of the dielectric function of the electron gas in quasi-2D quantum nanostructures has been studied. Analytical expressions for the dielectric function for a quantum well in the form of a δ function and a rectangular well of finite depth are derived for the first time. A criterion for transition to strictly 2D and strictly 3D cases was obtained.     

Studies of physical phenomena in semiconductor nanostructures using samples with laterally nonuniform layers: Photoluminescence of tunneling-coupled quantum wells

The previously suggested spectral-correlative method for studying nanostructures is applied to an analysis of photoluminescence of tunneling-coupled and isolated quantum wells in structures with laterally nonuniform layers. This method made it possible to use a single wafer to study the dependences of intensities of photoluminescence lines and their energy positions on the tunneling-barrier width for a system of tunneling-coupled GaAs-InGaAs-GaAs wells and on the quantum-well widths in a system of isolated AlGaAs-GaAs-AlGaAs quantum wells. Good agreement between the results of calculations and experimental data can be attained if it is assumed that a constant transverse electric field affecting the processes of trapping of charge carriers by quantum wells exists in a structure with tunneling-coupled quantum wells. The dependence of photoluminescence parameters on the width of isolated quantum wells is sensitive to the profile of heterointerfaces and to the processes of trapping the charge carriers by quantum wells.     

Electric-field enhancement and extinguishment of opticalsecond-harmonic generation in asymmetric coupled quantum wells

The second-harmonic susceptibilities of asymmetric coupled quantum wells (ACQW) and compositionally asymmetric coupled quantum wells (CACQW) under the influence of the applied electric field are investigated theoretically. Analytic forms of the second-harmonic susceptibility are derived by using the density matrix formalism. Coupled one-dimensional Schrodinger equation and Poisson's equation are solved self-consistently to find the subband eigenenergies and the envelope wavefunctions for the ACQW and CACQW structures. Dipole moment matrix elements for the ACQW and CACQW are evaluated from the resulting envelope wavefunctions and large nonlinear optical effects are predicted for these two structures. Based on the theoretical calculations, the second-harmonic susceptibilities of 50 nm/V and 35 nm/V can be achieved for the ACQW and the CACQW, respectively. This is more than two orders of magnitude enhancement as compared to that of the bulk GaAs. By a suitable choice of the composition of CACQW, a novel structure which the second-order nonlinear optical effect can be turned on or off by the applied electric field is proposed based on the results of the theoretical calculation. This phenomenon is attributed to the symmetry restoration of envelope wavefunctions of the CACQW under the quenching electric field F_off. A simple physical model to estimate the F_off has also been developed successfully     

Electric-field-controlled effects of spatial recurrence and multiplication of electron waves in two-dimensional semiconductor nanostructures

Effects of recurrence and multiplication in the spatial distribution of the probability-flux density j _x(x, z) (or the quantum-mechanical current density ej _x(x, z), where e is the elementary charge), which arise from electron-wave interference in two-dimensional semiconductor nanostructures, are analyzed, and the possibility of controlling these effects by the application of a dc transverse electric field is examined. A type of nanostructure represented by two rectangular quantum wells (a wide one and a narrow one) whose widths are measured in the direction of the z axis (the quantum-confinement axis) with the wells arranged sequentially in the direction of propagation of the electron wave (the x axis) is considered. It is shown that, for an electron wave entering the wide well from the narrow well, the initial transverse distribution peak j _x(0, z) is reproduced with some accuracy at distances X _p = pX ₁ (recurrence) and, in nanostructures symmetric along the z axis, splits at distances X ₁/q into q identical peaks of magnitude reduced by a factor of q (multiplication) (here, p and q are integers). It is demonstrated that these effects can be controlled by a dc electric field applied in the transverse direction (along the z axis) in the region of the wide quantum well. A reduction in the effective well width and appearance of asymmetry in the transverse potential profile upon application of the electric field cause a radical change in the j _x(x, z) distribution in this quantum well and make possible inverse population of the quantum-confinement subbands.     

Current-injection efficiency in semiconductor lasers with a waveguide based on quantum wells

A dynamic distributed diffusion-drift model of laser heterostructures, which takes into account carrier capture by quantum wells, is developed. The leakage currents in the lasing mode are calculated for different laser structures without wide-gap emitters: InGaAs/GaAs (lasing wavelength λ = 0.98 μm), InGaAsP/InP (λ = 1.3 μm), and InGaAs/InP (λ = 1.55 μm). It is shown that consideration of the finite carrier-capture time is of major importance for calculating structures with deep quantum wells. The ratio of the leakage currents to the total current in the structures with deep quantum wells (InGaAsP/InP and InGaAs/InP) increases with an increase in the injection current and may reach a few percent when the lasing threshold is multiply exceeded.     

Quantum interference and space charge effects in double barrier structures incorporating wide quantum wells

The current-voltage characteristics of resonant tunneling devices with well widths between 12 and 180 nm are studied. The voltage interval between the resonant peaks in the current is measured as a function of well width. For the wide wells the amplitude of the peaks in the differential conductance is modulated by an “over the barrier” interference effect involving the collector barrier. Space charge buildup and intrinsic bistability effects for a particular resonant state are found to depend critically on its energy difference from the top of the collector barrier and from lower lying standing wave states of the quantum well.     

Fabrication of periodical nanostructures using electron interference fringes

A scanning interference electron microscope (SIEM) has been developed for periodical nanostructure fabrication. This system can produce electron interference fringes with a period from 2 nm to several ten microns. Fabrications of periodical nanostructures with 23 to 170 nm period have been demonstrated by transferring electron interference fringe onto semiconductor surfaces.     

Momentum relaxation time and temperature dependence of electron mobility in semiconductor superlattices consisting of weakly interacting quantum wells

Formulas are derived for the longitudinal and transverse electron momentum relaxation times in a superlattice consisting of weakly interacting quantum wells in the approximation of a bulk-semiconductor scattering potential. Scattering by impurity ions, neutral atoms, and volume-type longitudinal acoustic and polar optical phonons is studied. A numerical analysis is performed of the longitudinal and transverse momentum relaxation times for scattering by impurity ions and neutral atoms as a function of the electron energy, temperature, density of the electron gas, and quantum well width. Fiz. Tekh. Poluprovodn. 33, 1240–1245 (October 1999)     

Shallow impurity levels in AlGaAs/GaAs semiconductor quantum wells

This paper reviews current knowledge of shallow impurity states (donors and acceptors) in AlGaAs/GaAs multiple-quantum-well structures. Calculations of these levels have been performed by a number of groups, most of which solved an effective-mass Schrödinger equation using the variational method. These results are generally consistent with each other, any differences being related to different approximations made in each calculation. The ground states of some of these shallow impurity levels have also been measured in several laboratories using low-temperature photoluminescence, Raman and far-infrared absorption techniques. These methods yield similar results which are consistent with the calculations. Both theoretical and experimental methods and results are discussed.     

准一维半导体纳米结构的电子显微学研究

本文总结了近年来我们在功能准一维纳米结构材料研究方面所获得的一些有意义的结果。借助于现代电子显微镜技术，不仅研究了硅、氮化稼、氧化锌等一维纳米材料的形貌和显微结构，还研究了其一维择优生长机理及小尺度效应。尤其是利用高能量分辨电子能量损失谱、高角环形暗场探头等先进技术，解决了一个传统X－光等结构分析手段所不能解决的难题，分析了一种SiOx/SiC复合纳米电缆的成份与结构。     

Magnetooptical properties of quantum wells with D (−) centers

The D ^(?) states in a quantum well under the magnetic field longitudinal with respect to the axis of growth of the structure are considered. In the context of the model of the zero-radius potential, an equation that defines the dependence of the binding energy of the D ^(?) state on the parameters of the potential of the structure, the coordinates of the D ^(?) center, and the strength of the magnetic field is derived. The results are compared with the experimental data on the dependence of the binding energy of the D ^(?) state on the magnetic field. There is satisfactory agreement between the theoretical calculations and experimental data in the range of magnetic fields B < 10 T. The role of dimensionality in the modification of the coordinate dependence of the binding energy on the 2D → 1D → 0D transition is clarified. The impurity magnetooptical absorption coefficient in a multiple quantum well structure is calculated, and the spectral dependence of the coefficient is studied. It is shown that a substantial contribution to the broadening of the absorption lines is made by the dispersion of quantum well widths in the structure.     

On the optimization of resonant intersubband nonlinear opticalsusceptibilities in semiconductor quantum wells

A systematic procedure is proposed for the optimal design of quantum-well structures that provides maximal nonlinear optical susceptibility of second-order under conditions of double resonance. The method is based on the supersymmetric quantum mechanics. It starts from the linear harmonic oscillator potential, itself lacking any nonlinear susceptibility, to generate a family of isospectral asymmetric potentials-its supersymmetric partners-which are then varied to maximize the products of matrix elements relevant for nonlinear susceptibility. The best value of susceptibility obtained exceeds those reported in the current literature. Possibilities of practical realization of quantum-well structures having optimized potentials are also discussed     

Size effect of semiconductor quantum wells in excitonic spin generation under drift

The excitonic spin polarization in dependence of the size of semiconductor quantum well (QW) was investigated by observing the two different circular polarizations of photoluminescence (PL). From the measurements of PL in QWs, it was found that there is a difference between the two different polarization conditions, which is caused by spin-dependent phase-space filling. The PL spin polarization was estimated from the signals of the left and right circularly polarized PL and was found to depend on the size of the wells as well as on the strength of the bias field. The effects of the size of the well and applied electric field on the excitonic PL spin polarization were studied.