International Journal of Nanoscience

Results of experimental studies of photoluminescence (PL) and PL excitation (PLE) spectra of MBE grown single quantum wells (QWs) formed by insertion of few CdSe monolayers in the ZnSe matrix are reviewed. PL spectra of such quantum objects originate from the luminescence of CdSe-rich nanoislands. Two types of island emitting states, namely ground and metastable ones, contribute to the low- and high-energy parts of the PL band, respectively. An interplay between these contributions is responsible for the anomalous temperature dependence of the maximum position of the PL band. The optical orientation and optical alignment experiments at resonant excitations allow to elucidate the nature of the two types of the emitting states. PLE spectra of ground and metastable states have strongly differing characters at excitation below some characteristic energy E_ME which is identified as the exciton percolation threshold. A theoretical model of the absorption spectra of emitting island states is presented, and practical applications of the model for the characterization of the island lateral concentration profiles are reported.

The recombination processes leading to tunneling afterglow and photostimulated luminescence were studied in systems based on ionic host crystals with low-dimensional structures (semiconductor quantum dots) formed as a result of the self-organized growth. The systems of AgBr nanocrystals embedded into the KBr crystal lattice and CsPbBr₃ nanocrystals embedded into the CsBr crystal lattice were investigated. The energy of electron–hole recombination in matrices was shown to transfer to quantum dots. To identify the structure of recombining electron and hole centers, magnetic resonance detected optically by monitoring the tunneling afterglow was used.

MBE growth of GaSb/InGaAsSb/GaSb heterostructures of high crystal quality is performed under continual RHEED control. Transmission spectra of the films forming multiple quantum wells in λ ≈ 2–3 μm region confirm possibility to control optical properties of the structures through quantum confinement and through the content of semiconductor elements. New design of saturable absorption semiconductor mirror (SESAM) for Cr²⁺:ZnSe laser is proposed and manufactured on the base of the single quantum well GaSb/InGaAsSb/GaSb placed between dielectric antireflection and broadband high reflection coatings.

Polarized photoluminescence from InAs/GaAs self-assembled quantum dots has been investigated under continuous wave excitation by circularly polarized light. Depolarization of the photoluminescence in a magnetic field perpendicular to the wavevector of the exciting light (Hanle effect) was studied. For n-doped, undoped and p-doped quantum dots the observed Hanle curves exhibit qualitatively different shapes: single-, bi-, and tri-Lorentzian, respectively. In the two latter cases the complicated shape of the Hanle curve is attributed to the different processes of radiative recombination which take place simultaneously. Recombination of uncharged excitons and recombination of positively charged exciton complexes are considered. Corresponding spin lifetimes are determined.

The results of excess carrier diffusion length investigations in GaN and in the light emitting structures (LES) based on MQW InGaN/GaN with different ordering of mosaic structures are presented. It is confirmed that the diffusion length values in these structures indeed are essentially lower than 1 μm. The Electron Beam Induced Current investigations have shown that the effective diffusion length measured on GaN structures could be determined by the mosaic domain boundary effect on the excess carrier transport. Such nanosize domains could also determine the lateral localization in the MQW light emitting diodes. The mosaic structure ordering effect on the quantum efficiency of LES and high values of the effective carrier lifetime in well-ordered LES are discussed.

InGaN quantum dot (QD) formation in a wide pressure range MOCVD reactor was studied. The existence of QDs and their lateral size (2–5 nm) were demonstrated using transmission electron microscopy and high spatial resolution (~ 100 nm) near-field magneto-photoluminescence spectroscopy. We found that an increase of the reactor pressure from 400 to 1000 mbar leads to an order of magnitude increase in light emission efficiency of the InGaN/GaN QDs accompanied by ~ 100 meV redshift of the emission wavelength. We explored stimulated phase separation (SPS) to control carrier localization and emission wavelength. The SPS was achieved by adding In in the matrix material. This leads to formation of extremely deep InGaN/InGaN QDs having energy localization up to ~ 0.8 eV, which was revealed from selectively excited far-field photoluminescence (PL) spectra. Without SPS the QD activation energy is found to be below 0.2 eV. A nonequilibrium carrier population strongly suppresses the temperature-induced shift of the PL emission in deep InGaN QDs.

Tuning of a photonic band gap (PBG) in a composite one-dimensional photonic crystal infiltrated with nematic liquid crystal (LC) E7 has been evaluated by simulations. It has been found that a sufficient shift of the short wave edge can be obtained only in structures with a pronounced initial alignment of a LC director in the grooves of a single crystalline Si matrix. The largest effect Δλ_edge = 0.52 μm is predicted for the reorientation of LC molecules from planar homogeneous alignment along the grooves to homeotropic alignment with respect to the Si walls due to an electro-optical effect. The shift in PBG due to thermal tuning resulting from the transition of LC from mesophase to the isotropic phase (Δλ_edge = 0.4 μm) is less than that due to electric tuning. Thermo-tuning has been demonstrated experimentally, and PBG shift as high as Δλ_edge = 0.25 μm has been achieved.

Formation of semiconductor quantum dots from a metastable wetting layer with the effective thickness well below the critical thickness is observed experimentally in the InAs/GaAs(100) and in the Ge/Si(100) systems. The observed effect is explained within the frame of the kinetic theory of quantum dot formation in mismatched heteroepitaxial systems.

In order to create suitable nanoholes for quantum dot (QD) localization on InP and GaAs surfaces, we used atomic force microscopy in an intermittent contact mode coupled with a modulated voltage to realized local anodization at a nanometer scale. This method leads, after a few tens of milliseconds of oxidation, to an oxide height saturation and a low lateral growth rate for both surfaces. These specific results were used to control separately both the depth and the diameter of holes and to obtain compatible pattern for QD growth. We also demonstrated the thermal stability of these patterns at compatible temperatures with the InAs QD growth. First, results of QD growth on these patterns are presented.

We report on a theoretical study of photoluminescence (PL) of the resonant one-dimensional photonic crystals, the near-Bragg quantum-well (QW) structures. The PL spectral intensity is found by including random sources in the equations for the exciton dielectric polarization and introducing the discrete Green function. The position and the width of peaks in the calculated PL spectra are in agreement with the real and imaginary parts of the exciton-polariton eigenfrequencies.

Space-charge spectroscopy was employed to study electronic structure in a stack of four layers of Ge quantum dots coherently embedded in an n-type Si(001) matrix. Evidence for an electron confinement in the vicinity of Ge dots was found. From the frequency-dependent measurements the electron binding energy was determined to be ~50 meV, which is consistent with the results of numerical analysis. The data are explained by a modification of the conduction band alignment induced by inhomogeneous tensile strain in Si around the buried Ge dots.

Exciton recombination in asymmetric CdMgTe/CdTe/CdMgTe/CdTe/CdMnTe double quantum wells (ADQW) with different barrier widths is studied in magnetic field up to 10 T. As grown structures were subjected to temperature annealing to introduce Mn and Mg atoms from the barriers into the CdTe quantum wells (QW). At low fields exciton transition in QW with magnetic impurity (Mn) is higher in energy than that in QW with nonmagnetic impurity (Mg), and interwell exciton relaxation is fast independently on the spin state. In contrast, at high fields, when the energy order reverses, an unexpectedly low relaxation rate of σ^- polarized excitons from nonmagnetic QW to the ground σ⁺ polarized state in magnetic QW has been observed. Effect strongly correlates with hh–lh splitting value Δ_hh–lh and discussed in terms of valence band mixing. Such a slowing down of relaxation allows separation of oppositely polarized excitons in different QWs.

We consider theoretically a current flowing perpendicular to interfaces of a spin-valve type ferromagnetic metallic junction. For the first time a simultaneous action of the two current effects is investigated, namely, the nonequilibrium longitudinal spin injection and the transverse spin surface torque. Dispersion relations for fluctuations are derived and solved under the proper boundary conditions. Joint action of the two effects mentioned lowers the instability threshold, its typical value being 1 × 10⁶–3 × 10⁷ A/cm². Spin wave excitations may soften near the threshold. A nonlinear problem is solved about steady state arising due to instability development.

Experimental study of Resonant Rayleigh Scattering (RRS) of light by a semiconductor microcavity was carried out on a double cavity sample. The shifts of RRS lines spectral positions with sample rotation demonstrate clearly polariton dispersion. The shape of the lines depends strongly on the position of the excitation spot on the structure surface and temperature. The treatment of experimental data in terms of the Fano quantum mechanical interference between a discreet state and continuum gives a direct method to study polariton scattering probabilities.

We report on a number of new effects of self-organization at nanoscale, leading to creation of new functional nanomaterials, including carbon and carbon–metal nanotoroids and nanodiscs and self-assembling of magnetic nanoparticles into helices and chains. We also extensively used a new approach of biopattern nanoengineering to create DNA-based complexes with metal or CdSe/ZnS core-shell nanorods (22 × 4.5 nm) which possess strong linearly polarized photoluminescence due to unidirectional orientation of nanorods along DNA filaments. Optical, electrical, and topological (geometrical) properties of such complexes were investigated. This work is a result of a coherent effort (since 1980s) of a consortium of Russian research groups in Nano-technology (INTC: Interdisciplinary Nanotechnology Consortium) aimed at creating molecular electronic devices based on individual and collective properties of specially designed and fabricated nanoclusters.

The wave-function of a hole localized on a deep acceptor is considered. The cubic symmetry of the crystal has been taken into account. The obtained pictures qualitatively agree with the STM image of Mn in GaAs.

This paper presents the photoluminescence study at 12 K and scanning photoluminescence spectroscopy investigation of the ground and excited states at 80 and 300 K on InAs QDs inserted in In_0.15Ga_0.85As/GaAs QW structures and created at different QD growth temperatures. It is shown that investigated structures are characterized by the long range variation of an average QD size in QD ensemble across the wafer. This long range QD size inhomogeneity was used for investigation of the multi-excited state energy trend versus ground state energy (or QD sizes).

Dissipative electron dynamics in a double quantum dot, influenced by external ac and dc electric fields, has been analyzed. It was shown that the dissipation caused by phonon environment disappears under certain relations between electric field parameters. In this case one may perform the dynamic localization and form stable electron states localized within one of the dots.

We consider the dynamics of electrons in semiconductor superlattices in intense multifrequency electric fields. We examine the conditions for dynamic localization and electromagnetic transparency. We investigate processes of formation, destruction, and stabilization of electromagnetic transparency in biharmonic field. The stable states with static fields are determined.

The results of investigation of the room temperature growth of thin Bi films on Si(111)-7 × 7 are present. In the initial stage of Bi film growth the rotationally disordered, pseudo-cubic, Bi{012} islands with a uniform height of 13 Å are formed. With increasing bismuth on the surface, islands interconnect, maintaining however their uniform height, and structural phase transition of the {012} film into a hexagonal Bi(001) film takes place.

MBE growth and photoluminescence study of thin films and bulk Cr²⁺:ZnSe are reported. We show that MBE provides optically active chromium in ZnSe and is viable for fabrication of optically and electrically pumped laser structures.

ZnSSe/ZnMgSSe MQW structures were grown by molecular beam epitaxy on GaAs substrates. The band gap of ZnMgSSe barriers was approximately 3 eV at room temperature. Cathodoluminescence, X-ray diffraction, optical, scanning electron beam, and atomic force microscopy were all used for structure characterization. Decay of the ZnMgSSe solid solution in at least two phases was observed. Improvement in the quality of the crystal lattice and surface morphology was achieved by mismatching the ZnMgSSe from the GaAs substrate by increasing the lattice period by 0.24%.

Tunneling through a system with two discrete electron levels coupled by electron–phonon interaction is considered. The interplay between elastic and inelastic tunneling channels is analyzed for a strong electron–phonon coupling in the resonant case. It is shown that the intensity and the width of peaks in tunneling conductivity are strongly influenced by nonequilibrium effects.