Exciton states in semiconductor spherical nanostructures

The results of theoretical studies of the energy spectra of excitons moving in semiconductor spherical quantum dots are described. The contributions of the kinetic electron and hole energies, the energy of the Coulomb interaction between an electron and hole, and the energy of the polarization interaction between them to the energy spectrum of an exciton in a quantum dot with a spherical (quantum dot)-(insulator medium) interface is analyzed.     

Spectral interferometry of semiconductor nanostructures

Fourier transform spectral interferometry is applied to measure both amplitude and phase of the electric field in different types of semiconductor nanostructures, thus determining the real and imaginary parts of the dielectric function. The importance of measuring the phase is shown and discussed in three studies. First, the phase measurement is used to access directly the refractive index across excitonic resonances in bulk GaAs and AlGaAs-GaAs quantum wells, with unprecedented resolution. Second, we measure the density dependence of the full dielectric function across a Fano resonance in bulk GaAs and show that this allows us to obtain some information on the collisional broadening of the usually hidden linewidth of the coupled exciton/continuum. Third, the phase is studied in a complex heterostructure, a semiconductor microcavity. We investigate and discuss the effect of the cavity detuning and of the excitation density     

Recombination lifetime in a semiconductor laser diode

The effect of minority carrier lifetime on the impedance of the laser within a diffusion or a drift length from the junction is analyzed. Lifetime shorting due to stimulated recombination (which is a function of the injection current) is considered and the effect of cavity size on the ability to modulate the laser at high frequencies is pointed out.     

Interband absorption of light in semiconductor nanostructures

Interband absorption of light in a small semiconductor microcrystal was investigated theoretically within the dipole approximation. An expression for the light-absorption coefficient was derived for the situation where the polarization-related interaction of an electron and hole with the microcrystal surface is dominant. It is shown that the edge of absorption in a microcrystal shifts to shorter wavelengths if the polarization-related interaction of electrons and holes with the microcrystal surface is taken into account. It is ascertained that the absorption edge for small microcrystals is formed by two transitions comparable in intensity; these transitions occur from different levels of size-related quantization for a hole to the lower level of size-related quantization for an electron.     

Surface-enhanced Raman scattering from semiconductor nanostructures

正Developing the surface-enhanced Raman scattering(SERS)substrate from noble metals to semiconductors would bring the SERS more promising applications in biomedicine detection.Based on the photo-induced charge transfer(PICT)mechanism the SERS model in the semiconductor-molecule system was built up.The analytical solution of the contribution from PICT to the molecular polarizability tensor was presented according to calculation of quantum mechanics,and the results suggested that arising SERS from the nanostructured Si substrate is possible.Experimental observation revealed that the numerous hydrogen atoms on the surface of the Hterminated Si nanowire(H-SiNW)arrays may serve as electron sink,and play a critical role in promoting efficient PICT and enable the SERS effect.The evident Raman enhancement of the standard probe Rodamine 6G(R6G),dye(Bu4N)2[Ru(dcbpyH)2-(NCS)2](N719),and the 4-aminothiophenol(PATP)from such substrates were detected.1 A significant enhancement of b2 modes could be observed from PATP chemisorbed on the H-SiNWs substrate,which indicated that a strong PICT process had taken place in the system.     

Formation of three-dimensional ZnSe-based semiconductor nanostructures

Nanostructures consisting of a 10-nm-thick sacrificial layer of ZnMgSSe and a 20-nm-thick stressed bilayer of ZnSSe/ZnSe were grown by molecular-beam epitaxy on GaAs substrates. Upon removal of the sacrificial layer by selective etching, multiwall ZnSSe/ZnSe microtubes were formed.     

Variable optical buffer using slow light in semiconductor nanostructures

A compact variable all-optical buffer using semiconductor quantum dot (QD) structures is proposed and analyzed. The buffering effect is achieved by slowing down the optical signal using an external control light source to vary the dispersion characteristic of the medium via an electromagnetically induced transparency effect. We present a theoretical investigation of the criteria for achieving slow light in semiconductor QDs. A QD structure in the presence of strain is analyzed with the inclusion of polarization-dependent intersubband dipole selection rules. Experimental methods to synthesize and the measurements of coherent properties in state-of-the-art QDs are surveyed. Slow-light effects in uniform and nonuniform QDs are compared. Finally, optical signal propagation through the semiconductor optical buffer is presented to demonstrate the feasibility for practical applications.     

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.     

Power dropout statistics of nearly-single-longitudinal-mode semiconductor lasers

An experimental study of power dropouts in the main mode of a nearly-single-longitudinal-mode GaAlAs semiconductor laser is presented. Knowledge of the statistics of main-mode power dropout interarrival times, depths, and widths is necessary to estimate the performance of a communication system using such laser diodes. We found that at a typical laser operating bias (I/I_{th} = 2.1), the mean dropout width was 1.8 ns, dropout depths were exponentially distributed, and dropouts falling below 90 percent of the average main-mode power occurred at a rate of1.5 times 10^{3}s^-1. Also, the rate of dropouts falling below 90 percent of the average main-mode power was found to increase substantially near mode hops, sometimes by several orders of magnitude.     

Some general recombination statistics for semiconductor surfaces

Trapping recombination by the usual phonon emission processes and by Auger processes can be combined to yield generalized Shockley-Read-Hall statistics. This has been generalized further to include a spectrum of trapping states, as well as the effect of "extra carriers." These are carriers which do not come directly from, or go to, the main bands of the semiconductor considered. The result should be useful in devices, for example, in studies of surface recombination.     

Self-organization of nanostructures on planar and patterned high-index semiconductor surfaces

In order to directly control the size and in particular the position of nanostructures naturally formed on high-index semiconductor surfaces during molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE), the growth on patterned high-index GaAs substrates is investigated. During MBE of (AlGa)As on patterned GaAs (311)A substrates, a new phenomenon in the selectivity of growth has been found to form a fast growing sidewall on one side of mesa stripes oriented along the [01-1] direction. Preferential migration of Ga adatoms from the mesa top as well as the mesa bottom toward the sidewall develops a smooth convex curved surface profile without facets. This unique growth mode that does not occur for the perpendicular stripe orientation nor on other patterned GaAs (n11)A and B substrates is stable for step heights down to the quantum size regime to produce lateral quantum wires on patterned GaAs (311)A substrates. Quantum confinement of excitons in the wires has been demonstrated by the transition from two-dimensional to magnetic confinement with increasing magnetic field. For device applications it is important that the wires can be stacked in the growth direction without any increase in interface roughness or wire size fluctuations, indicating a self-limiting lateral growth mechanism. Finally, in strained layer epitaxy, (InGa)As islands can be selectively placed on the mesa top and bottom leaving entirely free the curved part along the fast growing side-wall.     

Magnetic nanostructures and nanodevices with a semiconductor or dielectric spacer

Magnetic properties of nanoscale magnetic-layer systems using FeNi, Co, FeMn, Al₂O₃, and SiC layers are studied experimentally. The dependence of magnetic behavior on the system geometry and parameters is addressed. Ferromagnet-semiconductor exchange coupling subject to the amplitude of an applied magnetic field is revealed. It is found that the magnetic behavior of the nanostructures varies in a nonlinear manner with applied magnetic field. Spin-tunneling magnetoresistance is observed in magnetic-layer junctions containing an Al₂O₃ spacer. In-plane-conduction magnetic-field sensors using an Al₂O₃ or a SiC spacer are fabricated and tested.Translated from Mikroelektronika, Vol. 34, No. 1, 2005, pp. 56–64.Original Russian Text Copyright © 2005 by Kasatkin, Muravjev, Plotnikova, Pudonin, Azhaeva, Sergeeva, Khodzhaev.     

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。     

Recombination statistics for auger effects with applications to p-n junctions

A generalization is presented of the usual Shockley-Read recombination statistics to include the effect of Auger transitions. This yields a new expression for the steady-state recombination rate of electrons and holes. The current density through a p-n junction is then exhibited as essentially a sum of recombination currents. With these ideas it is shown how junction characteristics can be discussed, and a rough classification is given of these characteristics for various conditions. Only the case of moderate bias is considered, subject to the assumption of mass-action laws, with the mass-action constants independent of applied potential. A simple variation of quasi-Fermi and electrostatic potential through the junction is also assumed, and the limitations of the model are discussed. Suggestions are made concerning the identification of mass-action constants from experimental characteristics.     

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.     

Optical characterization of individual semiconductor nanostructures using a scanning tunneling microscope

By injecting low-energy minority carriers from the tip of a scanning tunneling microscope (STM) and analyzing the light emitted from the tip-sample gap of the STM, it is possible to study the optical and electronic properties of individual semiconductor nanostructures with an extremely high spatial resolution close to the atomic scale. This technique has been applied to investigate the transport properties of hot electrons injected into AlGaAs/GaAs quantum well structures and the optical properties of single self-assembled InAs/AlGaAs quantum dots. The physical principles, usefulness and future expectations of this novel technique are discussed.     

Recombination kinetics of the excitons in GaP:N electroluminescent diodes

The model of electronic transitions proposed by Dapkus et al.[3], is used for the calculation of the exciton recombination rate to investigate the transport phenomena in GaP p-n junctions. Similarity is found between the excitonic rate of recombination and that given by S.H.R.[1–2] for a trap having a single electronic level. A method allowing the determination of the nitrogen atom concentration responsible for every excitonic emission peak is proposed.     

Self-assembled metal/molecule/semiconductor nanostructures for electronic device and contact applications

We report a fabrication approach in which we combine self-assembled metal/molecule nanostructures with chemically stable semiconductor surface layers. The resulting structures have well controlled dimensions and geometries (∼4 nm Au nanoclusters) provided by the chemical self-assembly and have stable, low-resistance interfaces realized by the chemically stable semiconductor cap layer (low-temperature grown GaAs passivated by the organic tether molecules). Scanning tunneling microscope imaging and current-voltage spectroscopy of nanocontacts ton-GaAs fabricated using this approach indicate high quality, ohmic nanocontacts having a specific contact resistance of ∼1 × 10⁻⁷Ω·cm² and a maximum current density of ∼1×10⁷ A/cm², both comparable to those observed in large area contacts. Uniform 2-D arrays of these nanocontact structures have been fabricated and characterized as potential cells for nanoelectronic device applications.