Semiconductor nanostructures have attracted a lot of interest recently due to their possible applications in optoelectronics, nanoelectronics, quantum information processing and chemical/biological sensing. There are many different experimental approaches to realize well controllable nanostructure devices. In the past few years we have concentrated our efforts on the fabrication of such devices based on two methods, cleaved edge overgrowth and self-assembly of quantum dots.

The focused electron beam of an electron microscope (SEM or TEM) decomposes tailored precursor molecules on substrates into functional deposits. Extremely high aspect ratios of more than 10 can easily be obtained. During the last years several materials could be deposited, most of them composed of metal nanoparticles embedded in amorphous matrices. The deposition process depends on the precursor supply, its surface adsorption behavior, and the beam induced chemical decomposition path of the molecule on one hand, and on the other hand on the electron beam properties like beam current, electron energy, and beam distribution. The limits in minimal size, growth rate, and chemical composition arise from all mentioned parameters, which are interdependent.

We study the heteroepitaxial growth of self-assembled quantum dots in strained semiconductors in the Stranski-Krastanov growth mode using kinetic Monte Carlo simulations. Optimization of growth parameters such as temperature, deposition rate, coverage, and growth interruption time is discussed. In particular, we investigate the crossover between kinetically controlled and thermodynamically limited growth, and thereby resolve the seemingly contradictory temperature dependence of the average dot size.

We have successfully observed room-temperature electroluminescence (EL) from InAs quantum dots (QDs) on (001) InP substrates. The InAs QDs were grown by droplet hetero-epitaxy using low-pressure organometallic vapor epitaxy (OMVPE). There were two kinds of InAs QDs; large and small QDs. The small InAs QDs with the average diameter and height of 40 nm and 7 nm were present at the density of 3 x 10¹⁰ cm^-2. In photoluminescence (PL) measurements at 77 K, a peak with full width of 84 meV at half maximum was observed around at 1506 nm. Room-temperature electroluminescence was successfully observed from InAs QDs embedded in InP matrix around at 1560 nm, together with InP emission at 980 nm. With increasing injection current density from 3 A/cm^-2 to 33 A/cm^-2, the shape of the spectra was unchanged, suggesting that the broadness of the luminescence is due to the size distribution of InAs QDs.

In recent years studies of II-group and rare-earth fluoride layers on semiconductors have been quite active. The fluorides have a number of features that make them attractive for epitaxial growth on the most important semiconductors for both practical applications and basic studies. Among other fluorides, ZnF₂ is known for its unusual features which are of special interest. The rutile structure of ZnF₂ is similar to the MnF₂ structure. Moderate (Δα/α = -3.34%, Δc/c = -5.38%) lattice mismatch allows MnF₂ epitaxial growth on ZnF₂. Zinc fluoride is diamagnetic, and magnetically ordered epitaxial films separated by thin diamagnetic layers are of interest in terms of low-dimensional effects. Polymorphous structural transitions at high pressures and temperatures (100 - 400°C, 30 - 160 KBar) have been found in ZnF₂. Electronic properties of ZnF₂ differ from those of II a-group metal fluorides. So, after doping by some trivalent impurities, zinc fluorite, being a wide band gap insulator, converses into an n-type semiconductor with the binding energy of shallow donors of about 0.07 - 0.25 eV. Other attractive feature is an effective electroluminescence in rare-earth doped bulk crystals and polycrystalline films. Depending on the impurity, the radiation wavelength changes from IR to UV. Efficient luminescence, in addition to high free electron concentrations, makes zinc fluorite a promising compound for electroluminescent device applications. In this work, we studied the epitaxial growth and structural properties of zinc fluoride films grown on silicon using fluorite buffer layer.

A porous GaAs (100) substrate was prepared with nanoscale (5 - 20 nm) surface roughness, 10⁸ cm^-2 density of nanoscale inlet holes at the surface, and a network of pores (20 - 100 nm) along [111] on the 50 - 100 nm depth. The lattice misfit of the heterostructure GaSb/GaAs with the porous substrate was 22% less than that for the same rigid substrate. The structure of pores is discussed in connection with possible electrochemical processes of pore formation in A₃B₅ materials. The initial stage of growth has been analyzed assuming a strain dependence of the lateral growth rate at the side film/pore interface. This dependence accounts for the formation of bridges over the pores followed by a smooth growth of continuous film. Using a strain distribution calculated for a model heterostructure of two mismatched layers with pores we estimated the dynamics of bridging and evolution of the crystallization front.

Using a kinetic Monte Carlo model atomic interlayer exchange influence on the Stranski-Krastsnov (SK) growth mode was investigated. With the increase of the deposited dose, transition from 2D to 3D growth mode within SK region without changing interlayer exchange and the growth parameters, was observed. Limit parameter values corresponding to 2D yields SK and SK yields 3D transitions were determined.

Using 3D Monte Carlo model evolution of Si(001) surface during epitaxy and high temperature annealing was investigated. The surface relief in the form of pyramidal pits is developed in the wide range of deposition rate. This relief prevents simultaneous creation of flat surface and thin solid layer formation over pores on Si(001) surface. Critical dose on (001) surface at the same deposition rates and temperatures was found to be an order of magnitude larger than on (111) surface.

We report on ultra-high catalytic activity of metallic structures consisting of densely packed assemblies of nano-size metallic (Cu) granules. These nanostructures are fabricated by a new method of laser electrodispersion. The major results on experimental studies of catalytic activity of Cu nanostructures are presented by the example of dichlorobutene isomerization. The observed high catalytic activity of granulated Cu nanostructure is explained by the presence of charges appeared at the granules due to thermally activated inter-granule electron tunneling. The charge state of the structure is analyzed by using newly developed Monte-Carlo model.

Optical properties of GaAsN/GaAs heterostructures with different N contents grown by molecular-beam epitaxy were investigated. We show that under the certain grows reigmes the optical properties of the GaAsN layers are determined by recombination via localized states which is due to composition fluctuation. An increase in the N concentration leads to increase in composition fluctuation and, correspondingly, to increase in energy of localized states. Thermal annealing reduces nonuniformity distribution of nitrogen atoms. In short-period GaAsN/GaAs superlattice the effects of phase separation can be enhanced.

The morphology and Cd distribution in CdSe/ZnSe quantum dot structures grown by molecular beam epitaxy is compared before and after the ZnSe cap layer deposition. Atomic force microscopy is applied to study the surface topography of uncapped samples without removing the samples from the ultra-high vacuum. The capped structures are analyzed by transmission electron microscopy. It is observed that only a highly specific growth procedure involving a thermal activation step leads to a Stranski-Krastanow morphology with three-dimensional CdSe islands on a thin wetting layer which are preserved after the overgrowth with ZnSe.

Variations in the lattice constant of Ge film were determined by RHEED in the course of the MBE film growth on the silicon surface. Oscillations of the in-plane atomic cell constant were observed for the Ge film growing according to the 2D mechanism. Variations in the two-dimensional lattice constant at the stage of 2D growth are caused by elastic deformation of edges of two-dimensional islands. It was shown that the germanium film growth on silica, unlike the growth on the pure silicon surface, proceeds without formation of the wetting layer.

High-mobility InAs single quantum well with symmetrical AlSb and asymmetrical AlSb and Al_0.8Ga_0.2Sb barriers were grown on GaAs (100) by MBE. Magneto-transport studies revealed enhancement of sufficient effective g-factor in a quantizing magnetic field. This enhancement is quite sensitive to the layer composition of the epitaxially-grown structures. The implications of these results for the implementation of InAs-based spintronics structures are discussed.

Optical and structural properties of self organized InGaAs quantum dots (QD), deposited in Al_0.3Ga_0.7As matrix, were investigated. Samples were grown by molecular-beam epitaxy (MBE). It is shown, that deposition of 1.7 - 4 monolayer of InAs on Al_0.3Ga_0.7As surface results in formation of nanoscale QDs on 1 - 2 monolayer thick wetting layer (Stranski-Krastanov growth mode). Large exciton localization energy of the InAs QDs in Al_0.3Ga_0.7As in compare with QDs in GaAs is demonstrated. This is due to increase in size of these QDs and significant bandgap offset in the case of InAs/AlGaAs system in compare with InAs/GaAs one.

The initial stage studies of CaF₂ molecular beam epitaxy (MBE) on Si(111) provide a lot of new information important for understanding basic mechanisms of the heteroepitaxy and interface formation between dissimilar materials. Unlike CaF₂ on Si(111) system, which has been thoroughly studied by many groups, the interaction of CaF₂ molecules with a technologically important Si(001) surface until recently remained almost unexplored except for the TEM and STM studies, where strong anisotropy at the initial stages of CaF₂ growth on Si(001) was observed. In our previous work, it was found that a so-called "wetting layer" forms during the initial stage of fluorite growth at above 650°C. This layer influences much the following stages of fluorite growth and determines unusual (110)-orientation of CaF₂ nanostructures, grown on Si(001) surface. The wetting layer formation was accompanied by transformation of the initially two-domain (2x1 and 1x2) silicon surface to single-domain. However, microscopic mechanism of this transformation remained unclear. In this work, we used atomic force microscopy to thoroughly investigate the evolution of the silicon substrate surface during the wetting layer formation. We have found the basic microscopic mechanism governing the transition between single- and two-domain surface morphology and revealed more details on nucleation and evolution of CaF₂ linear-ridge-like nanostructures described in our previous work as appearing at higher surface coverage.

Heterostructures with submonolayer insertions attract interest due to optical properties and to possibility of the studying of the self-organization effect. Besides, investigation of SML heterostructures is retarded as consequence of problems of the structural and composition characterization. The situation is changed on account of development of quantitative methods for HREM image analysis. There formation of superlattice heterostructures with InAs submonolayer insertions in GaAs matrix and them optical properties are investigated in the work.

We report on low temperature photoluminescence (PL) in In_xGa_1-xN multiple qunatum wells (MQWs) with x in the range 0.1 and highly Si doped barriers of In_0.01Ga_0.99N. One sample with 3 QWs of width 3.5 nm and barriers of width 10.5 nm had the MQW in the depletion region of the outer surface. Two PL peaks were observed, one QW exciton from the QW closest to the GaN buffer, one lower energy peak related to a 2DEG at the interface to the GaN buffer layer. In a second similar sample 5 QWs of width 3 nm and with 6 nm highly Si doped In_0.01Ga_0.99N barriers the MQW was placed in the n-side depletion region of a pn-junction. At low temperatures the PL and electroluminescence (EL) spectra are quite different at no, low, or reverse bias, the PL appearing at higher energy. At high forward bias a spectral component at the EL position appears. This proves a strong influence of the depletion field on the optical spectra. Preliminary results are also reported for n-doped Al_0.07Ga_0.93N/GaN structures, with near surface MQWs.

We present results of photoluminescence studies of the band gap of non-intentionally doped single-crystalline hexagona InN layers and In-rich In_xGa_1-xN alloy layers (0.36 < x < 1). The band gap of InN is found to be close to 0.7 eV. This is much smaller than the values of 1.8 eV to 2.1 eV cited in the current literature. A bowing parameter of b ≈ 2.5 eV allows one to reconcile our and the literature data for the band gap values of In_xGa_1-xN alloys in the entire composition region.

In this letter a number of latest results from the process development on AIXTRON production scale MOVPE reactors will be reported. Growth of GaN on alternative substrates has been examined. Up to 900 nm crack free GaN layer were deposited on Si using a double nucleation interfacing technique. Low yellow band vs. band-edge related photoluminescence emission ratios have been observed and sheet resistances of up to 3500 Ω have been achieved on 2" Si substrates. Also, first results are reported of the up-scaling of the Planetary Reactor to 24 x 2". First results from fully loaded runs show an average 2" on wafer peak wavelength standard deviation of 3.8 nm, average wafer to wafer standard deviation across all wafers of 2.0 nm and average 3 x 2" disk to disk standard deviation of 1.6 nm at an average wavelength of 477.9 nm across all wafers (evaluated with peak integration and 2 mm edge exclusion for the 2" wafers). Photoluminescence peak intensity (area) varied with 13.6% standard deviation wafer to wafer and 12.8% disk to disk. On wafer intensity deviation was 11.3%.

The intensive up-conversion photoluminescence (UPL) was observed at low temperatures in CdSe/ZnSe structures with single CdSe inserts of a nominal thickness of 1.5 and 0.6 ML. The quadratic-like dependence of UPL intensity on the excitation power was obtained. UPL mechanism was interpreted on the basis of non-linear process of two-step two-photon absorption (TS-TPA) through deep defect states including cation vacancies localized at the barier-nanoisland heterointerface.

CdTe ultra-thin quantum wells (UTQWs) within ZnTe barriers were grown by pulsed beam epitaxy (PBE) on GaAs(001) substrates. In-situ reflection high energy electron diffraction (RHEED) patterns and real-time spot intensity measurements indicated a high structural quality of the QWs. Low temperature photoluminescence (PL) experiments indicated a clear influence of the growth temperature on the structural properties of the samples. The 2 monolayer (ML) thick UTQW grown at T_s = 270°C exhibited an intense and sharp peak at 2.26 eV whereas the 4 ML thick UTQW (T_s = 290°C) presented an intense peak at 2.13 eV and a weak one around 2.04 eV. This behavior is discussed in terms of Cd re-evaporation at the higher T_s.

Photoluminescence spectra of modulation-doped quantum well structures based on II-VI semiconductors (CdTe/CdMgTe and ZnSe/ZnBeMgSe) were studied in high magnetic fields it the range of 2D electron concentrations of (1-5)x10¹¹ cm^-2. The following peculiarities were found at low mangetic fields: (1) linear increase of the photoluminescence energy with increasing magnetic fields, (2) jumps in this dependence at integer filling-factors, (3) periodical changing of Zeeman splitting. The observed behavior are interpreted in a frame of a model which takes into account combined exciton electron recombination processes in the presence of magnetic fields.

Photoluminescence of GaN/AlGaN quantum well structures was studied under high intensity of excitation. The blue shift of photoluminescence peak energy was observed when excitation intensity increased. The blue shift was most prominent for the wide wells, becoming smaller with decreasing the well width. We attribute the observed effect to the screening of the built-in electric field by photoexcited carriers, which leads to reducing of the initial red shift caused by the quantum confined Stark effect. The variation of exciton binding energy with carrier concentration also makes the contribution to the blue shift. The theoretical calculations of the blue shift were performed and compared with experimental data.

We have studied interface properties of Si₃N₄/GaN structures by capacitance-voltage methods, paying attention to semiconductor surface treatments before insulator deposition. ECR plasma deposition of Si₃N₄ and ECR plasma treatments of semiconductor surfaces have been used. The interface state density depends on the hydrogen incorporation in ECR silicon nitride and its composition, which are the function of ECR deposition parameters. Optical properties and H-content of the films were characterized by ellipsometry and Fourier transform infrared (FTIR) spectroscopy, respectively. Minimum interface state density (1x10¹¹ cm^-2 eV^-1) has been observed for optimized composition Si₃N₄ films and ECR O₂ and CF₄ plasma treatment of GaN. After passivation by optimized Si₃N₄ films, there were observed improvement in breakdown voltage, increase in saturation current, output power, and power added efficiency for AlGaN/GaN HEMTs.

The theory of diffraction in the system consisting from the left-handed and the right-handed materials is constructed. The theory is based upon the Huygens principle and the Kirchhoff integral and it is valid if the wavelength is smaller than any relevant length of the system. The theory is applied to the calculation of the smearing of the foci of the Veselago lens due to the finite wavelength. We show that the Veselago lens is a unique optical instrument for the 3D imaging, but it is not a "superlens" as it has been claimed recently. We analyze the possibility of obtaining a left-handed materials on the basis of the metallic photonic crystals. The recent experimental results on the LHM are discussed.

Three-dimensional opal-VO₂ photonic crystals were synthesized by the chemical bath deposition technique. The Bragg reflection spectra from the (111) planes of the crystals were measured as a function of the temperature in the range between 15 and 100°C. The thermal hysteresis loop of the reflection peak position due to the phase transition in VO₂ filling the opal voids was observed. A theoretical model of the periodic layered medium was proposed to describe quantitatively the reflection spectra of opal-like structures. The values of the dielectric constants of the VO₂ below and above the phase transition temperature have been estimated which give the best fit within the model considered.

Using Glancing Angle Deposition, a novel thin film deposition technique, it is possible to fabricate complex, periodic structures suited for applications in photonic band gap crystals. In comparison to complex lithography processes used to produce conventional structures on the scale of several nanometers, GLAD is ideally suited to a virtual single step deposition, producing a novel tetragonal square spiral crystal structure and having a large predicted band gap of up to 15%.

In semiconductor microcavities (MCs) with embedded quantum wells (QW) a strong two-dimensional (2D) confinement of the light in the growth direction leads to an enhanced exciton-photon interaction, which results in the formation of mixed exciton-photon states described in terms of quasi 2D-polaritons. The density of these states is strongly reduced compared to exciton one, due to a very small in-plane mass. As a result, one can hope that the high filling of the polariton states near the polariton band bottom can be achieved at relatively small total density without destroying the strong coupling regime. However such a filling never was reached at low excitation density range where the polariton relaxation to the polariton branch bottom is determined by the emission of acoustic phonons. The reason is in the fact that the polariton lifetime is comparable with phonon scattering time. As a result, the energy distribution of polaritons clearly demonstrates 'bottleneck effect' both under the above band gap excitation and the resonant excitation below free exciton level. However the high occupation of polariton states near the band bottom is relatively easy reached under conditions of a strong resonant excitation into the lower polariton (LP) branch at particular wavenumbers close to the inflection point of the LP dispersion. It is explained as the result of stimulated hyper-Raman scattering (or four-wave mixing) of pumped polaritons with (E_p, k_p) to states with the energy and momentum [E₁, k₁ approximately 0] and [E₂ = 2E_p - E₁, k₂ = 2k_p], which referred to as the 'signal' and 'idler,' respectively. Here we investigate the influence of a temperature and an additional above band gap excitation on the stimulated scattering in MCs. A GaAs/AlAs MC containing 6 InGaAs quantum wells in the active layer (Rabi splitting Ω of 6 - 7 meV) has been investigated in a wide range of detunings between free exciton level and bottom of the photonic mode from δ = 0 to -3 meV. The resonance excitation into an LP branch was carried out with a tunable Ti-sapphire laser. The HeNe laser was used for the additional above GaAs band gap excitation. The sample was mounted in a helium thermostat with the temperature control at T = 5 - 30 K.

After artificial opals as well as opal-based infilled and inverted composites are considered to be promising representatives of photonic crystal materials. Earlier, photonic stop gaps in opals were studied mainly in transmission or specular reflection geometries corresponding to "one-dimensional" Bragg diffraction. On the contrary, this work was aimed at observing the typical patterns of optical Bragg diffraction in which phenomenon opal crystal structure acts as a three-dimensional diffraction grating. Although our experiments were performed for artificial opals possessing unavoidable imperfections a well-pronounced diffraction peaks were observed characteristic of a crystal structure. Each of the diffraction maxima reveals a photonic stop gap in the specified direction, while the spectral width of the peak is a measure of the photonic stop gap width.

We report on a new coherent phenomenon in semiconductor microcavities at polariton selective resonance excitation by two femtosecond pulses, propagating along k₂ and k₁, associated with exciton gratings, travelling in lateral direction ± (k₂ - k₁). Diffracted polaritons experience a frequency shift as observed in nondegenerate spectrally resolved transient four-wave mixing experiments.

An array of quantum dots in a microcavity is considered. It is shown that a vacancy in the array may give rise to one or two quasi-local states with a long life-time depending upon the structure of the array. One state always splits off the top of the polariton gap, and the other one may appear closer to the bottom of the gap. The life-time of the state is mostly determined by the exciton non-radiative relaxation and depends only weakly upon the properties of the cavity mirrors.

A simple technique to form glass microresonators is proposed and used to make microresonators of samarium-doped phosphate glass. Measured in the vicinity of ⁴G_5/2 - ⁶H_7/2 transition of Sm³⁺ ion photoluminescence spectra of the microresonators gave the value of Q-factor approximately 6000. This exceeds the Q-factor of glass microspeherts made with conventional flyway technique.

Three-dimensional (3D) photonic crystals entirely consisting of GaN have been fabricated for the first time. Detailed investigations of optical Bragg diffraction spectra have shown a high quality of the prepared photonic crystals.

In this contribution a short review is given of spectroscopic experiments on single molecules embedded in solid host materials at low temperatures. First it is shown how an individual pentacene molecule can be selected optically and how the magnetic-resonance transitions of its metastable triplet state can be observed. In addition it is shown that the magnetic-resonance transitions of a single nuclear spin, connected via hyperfine interactions to the single triplet spin can be observed. Single-molecule techniques can be applied with success to biologically relevant systems as is demonstrated by the first observation of the fluorescence-excitation spectra of individual light-harvesting complexes of photosynthetic bacteria. These results demonstrate unambiguously that the excited states of this complex must be described as Frenkel excitons, i.e., as excitations that are delocalized over the pigment molecules present in the complex.

We have studied new features of semiconductor nano-structures in cyclotron resonance and magneto-optical spectroscopy under very high magnetic fields up to a few megagauss. The new features are attributed to the shrinkage of wave-functions of electrons, holes and excitons. In PbSe/PbEuTe quantum dots that are regularly arranged to form an fcc-like lattice, sharp cyclotron resonance peaks from PbSe quantum dots were observed. It was found that the peaks show a number of anomalous features such as splitting, a remarkable dependence of the intensity on the wavelength, or a peculiar angular dependence of the resonance field. In the photoluminescence spectra of excitons for CdSe/ZnSe quantum dots, GaP/AlP and GaAs/AlAs short period superlattices, a remarkable decrease of the peak intensity and a red shift of the exciton peak were observed with increasing magnetic field; these effects are attributed to the general nature of excitons consisting of spatially separated electrons and holes.

The influence of many-particle interaction on tunneling characteristics in low dimensional structures is analyzed theoretically and investigated experimentally by means of STM/STS methods. It is shown that indirect interaction (trough band states of semiconductors) can often lead to increased tunneling conductivity for bias range where the direct interaction between impurity states is not significant. New method for preparation of electronic states with definite spin configuration on neighboring atoms is suggested. The energy splitting of opposite spin electrons can be about 0.5 - 1 eV. The different types of tunneling conductivity behavior in ultra small junction with superconductors is analyzed.

One of the important factors that restricts the power limit of semiconductor lasers is a catastrophic optical mirror damage. This process is significantly suppressed through decreasing the optical power density due to its redistribution over the broad transverse waveguide (BW). Recently it was shown that record-breaking values of the quasicontinuous and continuous-wave (QWC and CW) output power for 100-μm-wide-aperture devices can be achieved by incorporating a broad transverse waveguide into 0.97 μm emitting Al-free InGaAs(P)/InGaP/GaAs and Al-containing InGaAs/AlGaAs/GaAs separate confinement heterostructure quantum-well lasers (SCH-QWL). Another important factor limiting the CW output power is the Joule overheating of a laser diode due to an extra serial resistance. Traditionally, a decrease in the resistance is achieved by development of the contacts, whereas a voltage distribution across the device structure is not analyzed properly. At high operating currents the applied voltage can drop not only across the n-p-junction, but also at certain additional regions of the laser structure depending on a particular design of the device. Electrostatic force microscopy (EFM) provides a very promising method to study the voltage distribution across an operating device with a nanometer space resolution. An application of EFM for diagnostics of III-V laser diodes without and under applied biases have been recently demonstrated. However, the most interesting range of the biases, the lazing regime, has not been studied yet.

It is shown that such parameters of GaN/Al_xGa_1-xN superlattice as the period, build-in strain, composition of the alloy, and individual layer thicknesses can be extracted from the energy positions, intensities, and line shapes of various optical and acoustical modes detected in Raman scattering.

Mixed Crystals of Si_1-xGe_x (x < 0.05) grown by the method of zone melting have been studied by Electron Energy Loss Spectroscopy (EELS) in recoil. The contribution of Ge-bulk plasmon spectrum was revealed in the plasmon spectra of Mixed Crystals, and the conclusion has been made that an essential part of Ge atoms are included into Ge clusters. Plasma diagnostics of clusters in SiGe alloys is suggested on the basis of EEL spectroscopy.

A new surface-sensitive method of time-resolved optical studies is proposed. The method consists in the independent excitation of several surface electromagnetic waves (SEW) by two laser femtosecond pulse beams with varied time delay Δτ and distance Δr between corresponding excitation regions on surface. To fulfill phase matching condition for plasmon-photon coupling metal grating is used. Due to nonlinear plasmon interaction, the optical radiation with ω₁ + ω₂ and 2ω₁ - ω₂, (where ω₁, ω₂ are correspondent laser beam frequency) is generated. The intensity of this nonlinear response versus Δτ and Δr are studied. The direct measurements of the SEW temporal properties are presented. Experiments of this type are important for the development of the femtosecond surface plasmon optics for study of ultrafast phenomena in nanostructures.

A series of narrow emission lines (halfwidth 0.5 - 2 meV) corresponding to quantum-dot-like compositional fluctuations have been observed in low temperature near-field photoluminescence spectra of GaAsN and InGaAsN alloys. The estimation of the size, density, and nitrogen excess of individual compositional fluctuations (clusters) using scanning near-field magneto-spectroscopy reveals phase-separation effects in the distribution of nitrogen. We found a strong effect of In on the exciton g-factor in InGaAsN alloys.

Many attempts to create Si/Ge heterostructures with enhanced opto-electronic properties have been made since beginning of 90-th. Previously these hopes were connected with theoretical assumption that stressed ultra-thin Ge/Si superlattices (SLs) should have direct band gap structure. Recently, the great excitement of researchers was generated by promising optical properties of self-organizing Ge quantum dots (QDs) embedded in a Si matrix. Traditionally, Ge QDs are formed using relatively thick Ge deposition as a result of structural reconstruction of stressed system in order to minimize elastic energy (Stranski-Krastanow mechanism). It should be noted, that in this case there is always the wetting layer and the lateral sizes of Stranski-Krastanow QDs usually relatively high (tens of nanometers), which defines week vanishing of the wave-vector selection rules in the lateral direction. More recently, the mechanism of QDs formation using submonolayer (SML) insertions of a narrow-band semiconductor in a wide-band semiconductor matrix was proposed. In this case the average sizes of QDs are usually very small and its densities are very high. According to electron diffraction, electron microscopy and photoluminescence (PL) data the Ge QDs were formed in Ge/Si SLs with SML Ge insertions. Such Ge/Si heterostructures containing dense arrays of small Ge islands looks more promising for opto-electronic applications.

The dependence of photoluminescence spectra of SiGe/Si(001) structures with self-assembled islands on Ge deposition temperature was investigated. Due to inhibition of SiGe alloying and an increase of the Ge content in islands the photoluminescence peak from the islands significantly shifted to low energy with a lowering temperature. The maximum of the peak from the island grown at 600°C was observed at energies less than the energy of the bandgap for bulk Ge. As a result of holes localization in islands the photoluminescence peak from the islands was observed up to room temperature. Sufficient enhancement of the room-temperature intensity of the photoluminescence signal at 1.55 μm was obtained for structures with islands grown on pre-deposited Si_1-xGe_x layer. It is associated with a more effective capturing of holes by densely packed islands in structures with a pre-deposited Si_1-xGe_x layer.

The photogalvanic effects, which require a system lacking inversion symmetry, become possible in SiGe based quantum well (QW) structures due to their built-in asymmetry. We report on the removal of spin degeneracy in the k-space of SiGe nanostructures. This is concluded from the observations of the circular photogalvanic effect induced by infrared radiation in asymmetric p-type QWs. We discuss possible mechanisms that give rise to spin-splitting of the electronic subband states.

We report on the transport properties of Si field-effect transistors with an array of approximately 10³ 10-nm-diameter Ge self-assembled quantum dots embedded into the active channel. The drain current versus gate voltage characteristics show oscillations caused by Coulomb interaction of holes in the fourfold-degenerate excited state of the dots even at room temperature. A dot charging energy of approximately 43 meV (i.e., > kT = 26 meV at T = 300 K) and disorder energy of approximately 20 meV are determined from the oscillation period ad the temperature dependence sudy of current maxima, respectively.

Heterostructure with In_xGa_1-xAs quantum dots on Si (001) substrate was grown by molecular beam epitaxy (MBE). Step graded Si-Si_1-xGe_x-Ge buffer layers and In_xGa_1-xAs quantum dots (QDs) in GaAs matrix were deposited consecutively in two different MBE systems. Optical and structural characterizations of heterostructure were performed by photoluminescence (PL) at 77K and 300 K and transmission electron microscopy (TEM), respectively. Si-Si_1-xGe_x-GaAs heterostructure with InGaAs QDs exhibited intense photoluminescence in range 1.3 μm at room temperature.

This paper deals with the effect of the electromagnetic field enhancement in three-layered system due to the propagation of a polariton wave at the interface of nanoporous carbon film -- silicon substrate.

The new theoretical method for calculation of acceptor spectra in Si/SiGe heterostructures using 6x6 Luttinger Hamiltonian, taking into acount the anisotropy effects has been developed.

The question about the mechanisms of the photoluminescence enhancement of the system of silicon nanocrystals embedded into silica matrix at phosphorus doping is described. Both the experimental and theoretical arguments are presented.

A negative magnetoresistance (NMR) reaching maximum 30-40% of its zero-field value is observed under in-plane magnetic field for the hole gases confined in wide p-Ge_1-xSi_x/Ge/p-Ge_1-xSi_x quantum wells (QW), while in an analogous narrow QW the magnetoresistance doesn't exceed 1%. In the QWs of intermediate widths and hole densities, the NMR is explained as being caused by suppression of the intersubband scattering due to the upper subband depopulation. In the widest QWs with the highest hole densities the hole gas is self-divided into two 2D sublayers. A similar NMR observed in these samples is interpreted as also been due to suppression of the intersubband scattering, but subbands are the lowest symmetric and antisymmetric states of the formed double quantum well. The main effect of the in-plane magnetic field in this case is a relative shift of subbands along the wave vector, rather than the shift in energy.

The new effect -- stimulated light backscattering and anomalous light transmission in Bose-condensed exciton system are considered. The effects are connected with photoinduced coherent recombination of two excitons from the condensate with production of two photons with opposite momenta. The effect of two-exciton coherent recombination leads also to the appearance of the second order coherence in exciton luminescence connected with squeezing between photon states with opposite momenta. Coherent 3- and 4-photon emission processes stimulated by respectively two and three laser beams are also considered.

A new method to measure the coherence of inhomogeneously broadened optical excitations in semiconductor nanostructures is presented. The secondary emission of excitons in semiconductor quantum wells is investigated. The spectrally-resolved coherence degree of resonantly-excited light emission is deduced from the intensity fluctuations over the emission directions (speckles). The spectral correlations of the speckles give direct access to the homogeneous line width as function of spectral position within the inhomogeneously broadened ensemble. The combination of static disorder and phonon scattering leads to a partially coherent emission. The temperature dependence of the homogeneous line width is well explained by phonon scattering.

The photocurrent spectra in 27 - 37 μm wavelength range of the InGaAs/GaAs heterostructure with carbon δ-doping on the edge of the quantum wells has been investigated using free electron laser (FELIX). The resonant response has been revealed at the wavelength 34.3 μm. The energy of observed photocurrent peak (approximately 36.2 meV) is in a good agreement with the calculated energy of the hole transition from the ground state to the first excited resonant state of the impurity. The hole relaxation time to the acceptor ground state is estimated.

Based on the dielectric continuum model, we have studied the dependence of electron-optical phonon scattering rates in GaAs/Al_xGa_1-xAs quantum wells with different structure parameters. It was found that the dependence of scattering rates of symmetric interface mode on Al composition in the barrier was stronger than that of the confined mode. The average phonon energy emitted by hot electrons in GaAs/Al_xGa_1-xAs quantum wells with various Al composition was estimated and the calculated value agrees with the experimental results qualitatively. For the dependence on the well width, scattering rates of the S+ mode dropped considerably as the well width is increased. The hot electron-neutral acceptor luminescence spectrum of the strained In_xGa_1-xAs/GaAs quantum well sample shows an oscillation period of about 22 meV which indicates that the hot electrons relaxed mostly through emissions of the InAs confined phonons.

The results of optical phenomena investigations in quantum dot and quantum well structures under interband optical pumping are presented. Interband and intraband light absorption in nanostructures with quantum dots has been studied experimentally and theoretically. Photoluminescence and interband light absorption in stepped quantum wells have been investigated including PL studies under picosecond optical pumping. Experimental results have been compared with results of calculation of energy spectrum and transition probabilities. It is shown that inversion of population exists between the third and second excited levels of stepped quantum well.

We study the optics and transport in p-type heterojunctions with lateral surface quantum dot (antidot) superlattice and in the presence of perpendicular magnetic field. The Azbel'-Hofstadter problem is solved for holes in valence band described by 4 x 4 Luttinger Hamiltonian. The probabilities of transitions between quantum states in magnetic subbands and monolayer of donors located inside heterojunction are calculated. The investigation of Hall conductance in the Integer Quantum Hall Effect (IQHE) mode in p-type heterojunctions with lateral surface superlattice has been performed. The mutual influence of heavy and light holes, and of the spin-orbit coupling on the quantization of Hall conductance was elucidated.

The magnetic field, applied to a size-quantized system produces equilibrium persistent current non-uniformly distributed across the system. The distributions of dia- and paramagnetic currents and magnetic field in a quantum well is found. We discuss the possibility of observation of field distribution by means of NMR.

We consider a semiconductor superlattice biased into the regime of negative differential conductivity and driven by an additional GHz ac voltage, and find frequency-locked or quasiperiodic propagating field domains. With increasing driving frequency, the complex impedance exhibits strong variations of its amplitude and phase. An anomalous phase shift appears as a result of phase synchronization of the traveling domains.

Composite double qunatum wells made from materials with a type-II band line-up have been grown to realize separate confinement in real space for electrons and holes. We have observed a substantial blue shift of the lowest energy transition in such composite double quantum wells. The photocurrent measurements demonstrate a linear Stark shift due to the separate confinement in real space for electrons and holes. The charge separation is up to 45 Å in the strain balanced InAs_0.42P_0.58/Ga_0.67In_0.33As samples. The experimental results agree very well with calculations in the framework of Bir-Pikus strain Hamiltonian.

Auger coefficients have been measured in five light emitting diodes (LEDs) with strained quantum wells in active region covered a wide range of wavelength. LED were fabricated from identical high efficient laser heterostructures. The obtained Auger coefficients increase from 3 x 10^-30 to 1.8 x 10^-28 cm⁶/c as wavelength changes from 0.78 to 1.8 μm.

We use resonant magnetotunneling spectroscopy, with the magnetic field applied parallel to the interfaces, to investigate the local band structure in the quantum well (QW) of a resonant tunneling diode. By rotating the magnetic field in the plane of the interfaces, we investigate the energy surface at constant k_||. Using this technique, we have studied two different types of double barrier structures. We obtain different results depending on whether or not the QW contains a narrow InAs layer.

Dephasing of optical excitations in semiconductor quantum dots (QDs) has recently received much attention. A common model used for understanding such processes is a two-level electronic system interacting with phonons. In our work we construct a consistent non-perturbative theory of the ZPL homogeneous broadening and resolve the contradictions by pointing out the limits of validity of the theoretical papers mentioned.

For memory structures based on optically induced charge in self-organized quantum dots, the concept of wavelength-domain multiplexing in the quantum dot ensemble is an essential prerequisite. The electric properties of quantum dots in various material systems as studied by time-resolved capacitance spectroscopy are summarized, and candidates suitable for future memory applications are discussed. By combining optical excitation and capacitance spectroscopy, direct evidence is obtained for energy-selective hole charge generation and storage in InAs/GaAs quantum dots. A clear dependence of the activation energy of the emitted holes on the energy of the excitation is observed.

Photocurrent and capacitance spectroscopy are used to investigate a Schottky barrier structure containing a single layer of self-organized InAs/GaAs quantum dots. We show that the temperature dependence of the photocurrent signal from the quantum dots is governed by thermal escape of electrons as the faster carriers.

Unusual coupling of heterostructure optical phonons and electronic excitations in quantum dots (QD's) was observed by photoluminescence spectroscopy in strain-induced InGaAs/GaAs QD's (SIQD's) and self-assembled InAs/GaAs QD's (SAQD's). Phonon-assisted interband transitions in SIQD's were found to be governed by zone center bulk GaAs TO and LO phonons via deformation potential interaction, whereas polar interaction was inessential. For SAQD's, a n-doped GaAs substrate was found to effect on QD intraband relaxation of carriers via coupling between them and substrate LO-phonon-plasmon modes at spacing between QD's and substrate as long as 100 nm.

We obtain inelastic electronic light scattering for interband transitions between valence-band states of GaAs layer embedded by self-assembled InAs quantum dots (QDs). Under a low-power selective cw excitation above the InAs band gap but below that of GaAs at a lattice temperature T_l = 5.1 K we find anomalous photoexcitation of carriers in the InAs QDs. Unusual photoinjection of the carriers to the GaAs barrier via strong Coulomb interactions results in creation of the nonequilibrium electron-hole plasma in the GaAs layer with density of n = p = 2.5 x 10¹⁸ cm^-3 and an electron temperature T_e = 25 K. Observed spectra reflects the band anisotropy and extends from zero to rather large frequency shifts with a long tail with a peak at about 300 - 400 cm^-1 in good agreement with theoretical prediction.

Time-resolved inhomogeneously broadened spectra of CdSe/ZnSe self-assembled QDs excited by weak picosecond and powerful nanosecond laser pulses have been measured. The peculiarities of the kinetics of the CdSe/ZnSe self-assembled QDs' spectra and their changes at high excitation have been explained by different recombination times of QDs (it shortens for QDs of smaller size) and by saturation (state filling) of QDs that arises first of all for QDs of greater size.

The temperature dependencies of luminescence spectra of the InAs/GaAs quantum dots L₀ (E_L0 = 1.235 eV), L₁ (E_L1 = 1.290 eV) and I₁ (E_I1 = 1.343 eV) and wettings layer (WL) (E_WL = 1.408 eV) have been investigated at P = 0 and P = 15 kbar. The InAs quantum dots on vicinal substrates GaAs at misorientation angle 7° [001] have been grown in submonolayer migration enhanced epitaxy mode (SMEE). The activation energies have been determined from the temperature quenching of luminescence. Their dependence on a value of hydrostatic pressure have been studied. The available scheme of energy levels of quantum dots has been proposed.

We have investigated an Auger type mechanism of the relaxation of the carriers onto the deep levels of the self-assembled quantum dots (QD). Our analysis shows that resonant electron-hole collisions can lead to unexpectedly fast energy relaxation. Basic idea of the proposed mechanism is the Breit-Wigner type scattering of a delocalized particle with a particle trapped inside a dot. If energy of an incoming particle is close to the energy of a virtual two-particle state scattering cross-sections resonantly enhanced. We show that the number of resonances is sufficiently large leading to large average cross-sections. Realistic estimations for the In dots in the GaAs matrix shows that energy relaxation takes about 5 - 50 picosecond, which is comparable with experimental findings.

A new property of the charge collective mode in a 1D electron system is found. In the wave number region close to 2k_F (k_F being the Fermi wave number) the mode frequency goes to zero, showing the soft mode behavior. In addition, the common 1D plasmon spectrum exists in the long-wave region. The soft mode is related to the short-range dynamic electron correlations in 1D and is absent in the systems of higher dimensionality. The dynamic correlations are described adequately in the frame of the Luttinger model.

We have investigated the conductance of long quantum wires formed in GaAs/Al_xGa_1-xAs heterostructures. Using realistic fluctuation potentials from donor layers we have simulated numerically the conductance of four different kinds of wires. While ideal wires show perfect quantization, potential fluctuations from random donors may give rise to strong conductance oscillations and degradation of the quantization plateaux. Statistically there is always the possibility of having large fluctuations in a sample that may effectively act as a microconstriction. We therefore introduce microconstrictions in the wires by occasional clustering of donors. These microconstrictions are found to restore the quantized plateaux. A similar effect is found for accidental lithographic inaccuracies.

The one-particle spin splitting of Landau levels in 2D electron systems is governed both by Zeeman mechanism and spin-orbit interaction. For III-V heterostructures the latter has two contributions: bulk (Dresselhaus contribution) and interface (Rashba contribution). It is analyzed the combined effect of all three mechanisms on the spectrum of Landau levels in a quantum well (001) III-V in perpendicular to plane or near to perpendicular magnetic field. The goal of the work is to find conditions when the bare spin splitting, Δ_sN, of Nth Landau level disappears. It is shown that (1) the Rashba and Dresselhaus mechanisms are not equivalent to each other; (2) the result depends on the sign of the Lande effective g-factor. Zero of Δ_sN is induced by the Dresselhaus mechanism if g_zz < 0, or Rashba mechanism if g_zz > 0. The results are qualitatively applicable to other low-dimensional systems (quantum wires and dots).

Persistent high-frequency (hf) hopping conductivity near the middles of well developed quantum Hall plateaus has been studied in the Si modulated heterostructures GaAs/Al_0.3Ga_0.7As, both δ- and modulation doped, under successive infrared irradiation in the 0.8 - 1.44 micron region. The conductivity has been determined by simultaneous measurement of attenuation and velocity of surface acoustic waves. With the increase of the radiation dose the conductivity decreases, whereas the carrier density in the 2D channel grows. There is a threshold of the persistent conductivity from the low energy side located between 0.86 and 0.48 eV. The observed behavior can be attributed to the so-called DX^- centers which are localized two-electron states bounded by local lattice distortion. Such states, as known, exist in Si-doped GaAs/Al_xGa_1-xAs with x ≈ 0.3. We believe that hf hopping is due to tunneling of electron pairs between a two-electron center and its empty neighbor.

Effect of spin splittings on magnetooscillation phenomena is studied theoretically. It is shown that interference takes place between two types of spin splittings linear in the wave vector. The simultaneous presence of these spin terms in the Hamiltonian of the two-dimensional electron gas causes a suppression of beats in the magnetooscillation phenomena. Zeeman splitting in tilted magnetic fields is shown to be able to lead to the appearance of the oscillations with the double frequency.

We study the electric field induced current instability in two-dimensional semiconductor structures with respect to formation of charge density wave (CDW). We calculate the increment of the instability and demonstrate that spatial period of the CDW is inversely proportional to the electric field.

A remarkable change of LO-phonon line for tunneling from metal to 2DEG, in comparison with 3D-case, was observed. It was shown that the line shapes and the amplitudes of LO-phonon and zero-bias anomaly in tunneling spectra of Al/δ-GaAs junctions depended on magnetic field in-plane of the 2DEG. The parameters of the lines were changed when threshold for resonant intersubband polaron in δ-layer was exceeded.

In samples used to maintain the US resistance standard the breakdown of the dissipationless integer quantum Hall effect occurs as a series of dissipative voltage steps. A mechanism for this type of breakdown is proposed and a parameter-free microscopic model is presented. In this model we calculate the rate of generation of electron-hole pairs due to charged impurity scattering and develop a simple, self-consistent, model for their motion. We thereby determine the rate of pair production and the size of the voltage steps. We then draw an analogy between this type of breakdown and vortex anti-vortex pairs formed behind a stationary obstacle in a flowing fluid.

Electron tunneling between two dimensional electron systems of different electron concentration in magnetic field normal to the layers have been studied. Parallel 2DES were separated from each other by Al_0.3Ga_0.7As barrier and from highly doped n⁺-GaAs contact regions by undoped GaAs-layers. In our samples each of the 2DES and adjacent contact region were in thermodynamic equilibrium provided by free carrier exchange. The measured I-V dependencies demonstrated pronounced resonant features arising when ground states of the 2DES's were adjusted by external bias. The magnetic field shifted resonant peak position. The shift on the voltage scale was linear versus magnetic field, but exhibited discontinuity in particular magnetic fields. The Landau levels pinning in the 2DES's by chemical potential in the contact region explains the experimental findings.

A carbon nanotube is one of the best candidates for the nano-scale devices because of its small features. The problem of the carbon nanotube is the difficulty how to control the position and the direction of the carbon nanotube. So far, the carbon nanotube was dispersed in to the alcohol and distributed onto the substrate, and was connected to the metal electrode by moving the carbon nanotube by AFM cantilever to the electrode. This process took a lot of time and showed low yield. In order to solve this problem, we proposed the new technology that could control the position of the carbon nanotube by using the patterned chemical catalyst. The three terminal device which used the carbon nanotube as a channel was fabricated and its electrical properties were examined. The carbon nanotube three terminal device showed the single electron transistor characteristics and showed the room temperature Coulomb diamond characteristics. The Coulomb temperature of the device is as high as approximately 5000 K.

A new technique for formation of tunnel junctions from Ti stripe providing junction capacitance of about 10 aF (150 nm wide stripe) has been developed. The technique is based on through oxidizing thin sites that form when Ti stripe crosses a step previously etched in the dielectric substrate. Charge transfer through the single junctions was investigated. Inelastic tunneling via electron states localized in the barrier region was found. This results in the essential nonlinearity of junction I-V curves to a scale of bias voltage of 2 - 3 mV. The single electron transistor built on such junctions demonstrates the I-V curves peculiarities originating from strong nonlinearity of single junctions.

A mechanism of electric current-induced cooling of nanostructures is proposed and analyzed. The conditions are studied of electric current flow through a heterostructure with two quantum wells, with electrons from one quantum well passing into the other via phonon-assisted indirect tunneling. As a result, the system is cooled by the flowing current, with the temperature of the system depending on the current nonmonotonically. A universal law for the maximal cooling temperature is derived.

A dc voltage changed periodically with magnetic field is observed on segments of asymmetric aluminum loop without any external dc current at temperatures corresponded to superconducting transition. According to this experimental result a segment of the loop is a dc power source. A possibility of a persistent voltage on segments of an inhomogeneous normal metal mesoscopic loop follows from this result.

A possibility of fabricating all-perovskite field effect transistor is shown, which can provide the development of a nonvolatile memory cell with a nondestructive readout of information. A thin (approximately 5 - 10 nm) Sr-doped lantanate cuprate (LSCO) film was used as a transistor channel while a ferroelectric gate insulator was a lead zirconate titanate (PLZT) film of about 100 nm thickness. The modulation of a channel conduction was found in the studied transistors to be approximately 70%, which is an order of magnitude larger than that reported in the world literature.

Conditions for appearance of a self-consistent quantum well in both the near-surface region of a vacuum semi-conductor field-emitter and a case of metal-insulator-semiconductor heterostructures (Auger-transistor) under strong electric field have been studied.

The nature of the 1/f noise in GaN/GaAlN Heterostructure Field Effect Transistors (HFETs) has been discussed. New experimental results on the 1/f noise in GaN/GaAlN HFETs and different models on the flicker noise have been also described. It has been demonstrated that the 1/f noise in GaN/GaAlN HFETs might be linked to the electron tunneling from the 2D gas into the tail states in GaN or AlGaN layers.

InGaAs/InAlAs strained multiple quantum well structures for amplitude modulators based on the quantum confined Stark effect were studied in order to find the most efficient one for optical communication at 1.55 μm. Parameters such as contrast ratio, polarization sensitivity, operation voltage and chirp are determined for structures with different InGaAs composition and thickness. Simulations of the devices' performance are carried out and are in good agreement with experiment.

Gold nanoelectrodes with gaps of less than 10 nm were formed by conventional E-beam lithography on silicon substrates covered by Al₂O₃. Molecular films were deposited on the electrodes by Langmuir-Shaefer technique. The I-V curves of such systems show a suppressed conductance indicating a correlated electron tunnelling through the system. All measurements were made at room temperature.

Charge accumulation in the quantum-well of a double-barrier resonant-tunneling diode (DBRT) may result in bistability that provides a basis for formation of lateral current-density patterns. A typical pattern expected to appear is a current density front. Such fronts correspond to switching transitions and generally cannot be stabilized by a conventional external circuit. In this paper we discuss formation and stabilization of stationary current density patterns in bistable DBRT. We demonstrate that the intrinsic dynamics of the DBRT can lead to the onset of regular or chaotic spatio-temporal oscillations if an attempt to stabilize unstable current density patterns by means of a simple control loop is undertaken.

We present temperature-dependent measurements of the dephasing time in the ground-state transition of strongly-confined InGaAs quantum dots, using a highly sensitive four-wave mixing technique. At low temperature we measure a dephasing time of several hundred picoseconds. Between 7 and 100 K the polarization decay has two distinct components resulting in a non-Lorentzian lineshape with a sharp zero-phonon line and a broad band from elastic exciton-acoustic phonon interactions. We also explore the dephasing time beyond the one exciton occupation, by electrically injecting carriers. Electrical injection into the barrier region results in a dominantly pure dephasing of the excitonic ground-state transition. Once the injected carriers have filled the electronic ground state, additional filling of the excited states creates multiexcitons that show a fast dephasing due to population relaxation.

We have proposed a new physical approach for the design of mid-IR lasers operating at λ = 3.2 - 3.26 μm based on type II heterojunctions with a large asymmetric band-offset at the interface (ΔE_C > 0.6 eV and ΔE_V > 0.35 eV). These high potential barriers produce effective electron-hole confinement at the interface and results in a tunnel-injection radiative recombination mechanism within the device due to reduce leakage current from the active region. The creation of high barriers for carriers leads to their strong accumulation in the active region and increases quantum emission efficiency of the spatially separated electrons and holes across the heteroboundary. Our approach also leads to the suppression of non-radiative Auger-recombination and a corresponding increase in the operation temperature of the laser. The active region of the laser structure consists of the type II heterojunction formed by narrow-gap In_0.83Ga_0.17As_0.82Sb_0.18 (E_g = 0.393 eV at 77 K) and wide-gap Ga_0.84In_0.16As_0.22Sb_0.78 (E_g = 0.635 eV at 77 K) layers lattice-matched to InAs substrate.

This paper reports on transient switch-off of the emission of an electrically pumped quantum dot laser with perturbation by a short optical pulse. This laser response is explained in terms of hot carrier absorption on intraband optical transitions leading to the transient suppression of the laser mode and, hence, to the switch-off. The switching time constant is determined to be as fast as 2 ps.

The intersubband light emission of GaAs/GaAlAs quantum cascade lasers is measured under pulsed magnetic fields parallel to the current, up to 60 T. A giant modulation of the laser intensity is observed with complete suppression of the laser emission when the energy spacing between intersubband Landau quantized states matches the GaAs optical phonon energy. When the level separation is not equal to the phonon energy, the laser output increases as a result of quenched phonon emission from the upper subband electrons. In this situation, the laser threshold current was found reduced by a factor of two.

Optical characteristics are investigated and compared of nanostructure semiconductor lasers with quantum dots and quantum dashes. Spectra of optical gain and of linewidth enhancement factor are obtained. Optical anisotropy in quantum dash structures is investigated.

Development of submonolayer deposition technique can offer significant flexibility in creation of strained heterostructures of different types and material systems. It was found that under certain growth conditions the deposition of InAs insertions of less than 1 monolayer (ML) thickness in GaAs matrix forms so-called sub-monolayer quantum dots (SML QDs). The energy spectrum of these QDs can be varied over a wide range by tuning the InAs coverage and the thickness of GaAs spacers. Stranski-Krastanow (In,Ga)As QDs (SK QDs), which have been investigated in more details, have proved theoretically predicted lower threshold current density of 26 A/cm² in compare with QW lasers. However, strong size variation of SK QDs in combination with the relatively low sheet density leads to low peak gain achievable in the ground state. This problem is the reason of typically low efficiency of SK QD-based lasers. Due to higher gain, SML QDs have proved their potential for high power laser application. In this presentation we report on further progress in the technology of SML QD lasers demonstrating high output power (6W) from 100-μm-wide laser diode emitting at 0.94 μm. High power QW-based lasers of the state-of-the-art performance are also presented for comparison.

Recent results on molecular-beam epitaxy growth of the quantum dot InGaAs/GaAs heterostructures for long-wavelength lasers on GaAs substrates are presented. As a result of optimization of the growth procedure for active region and emitter layers low-threshold current density (45 - 80 A/cm²) long-wavelength (1.27 - 1.3 μm) laser diodes may be fabricated with high reproducibility.

We present a theoretical investigation of the spectral and modal properties of a near-threshold two-dimensional curved-grating distributed-feedback laser provided with high transverse confinement. All fields are described in the spectral domain in the frame of an extended (3x3) transfer matrix formalism which takes into account the internal sources, as well as saturation of the gain medium through amplified spontaneous emission.

Mechanisms causing non-linearity of power-current characteristics (PCC) of quantum-well lasers at high injection levels were studied both experimentally and theoretically. A critical injection current that switch on a nonlinear PCC mode was found to depend to a great extend on the resonator length (on the threshold concentration). The PCC non-linearity is well described within the framework of the gain saturation mechanism where a dependence of the gain coefficient on the radiation intensity at high injected carriers concentration is taken into account.

We present a novel concept of using quantum dot superlattices for coherent acoustic phonon emission. Coherent lattice wave generation is proposed by collective dot vibrations in a self-assembled quantum dot array, where superlattice structure acts as a phonon resonator. The dots vibrations are synchronized by an external electric field. The proposed scheme has advantage of low phonon scattering rate in quantum dots, that enables generating high intensity phonon beams at low amplitude of the pumping electric field. The advantage is illustrated by numerical simulations carried out for Si/Ge superlattice. The proposed scheme may find many phonon applications, such as for local temperature control, phonon assisted optical transition enhancement, and characterization of low-dimensional structures.

A set of high power single mode InGaAsP/InP ridge waveguide laser diodes emitting in 1440 - 1500 nm range on peak wavelengths used for optical fiber pumping were developed and fabricated. Room temperature continuous wave output power of 300 mW was reached. Stable operation on fundamental optical mode with only 1° increase of lateral far-field pattern FWHM was confirmed up to 180 mW output power.

The observation of optical gain at the trion transition of n-doped ZnSe quantum wells is reported. The specific optical coupling between the trion and electron band gives rise to stimulated emission on the low-energy wing of the trion photoluminescence band without degeneracy and inversion in the total particle numbers. Gain values as large as 10⁴ cm^-1 are found for excitation intensities of some kW/cm². A calculation of the absorption-gain crossover photon energy based on a kinetically determined equilibrium of excitons, trions and electrons with a common carrier gas temperature describes the experimental data well.

InGaAsP/InP SC DHS lasers with different waveguide design were fabricated and studied. Extremely high values of internal quantum efficiency of stimulated emission η_i^st about 97% was demonstrated experimentally in structures with step-like waveguide design which is related to lowest leakage currents above threshold and reduced threshold carriers concentration. Theoretically was shown, that it is possible to create lasers emitting at λ = 1.5 μm, with an internal quantum efficiency of stimulated emission close to 100%. η_i^st for structure with different waveguide design was calculated and prove to be in good agreement with experimental data.

We report short (30 - 35 ps), high energy (more than 100 pJ) optical pulses at 1.5 μm from Q-swithced laser diodes with multisection implantation-induced saturable absorbers. Dramatic improvement in pulse parameters over tandem lasers is due to suppression of spatial hole-burning and amplified spontaneous emission.

We report the first realization of novel tapered DBR laser diodes incorporating curved gratings. The devices exhibited single-longitudinal mode operation with a side-mode suppression ratio of over 30 dB and a laterally focused beam for focal lengths around 0.5 mm. These laser sources will be suitable for applications requiring both spectrally and spatially enhanced beam quality.

Properties of a new active optical medium, Er-doped SiO₂ with silicon nanocrystals, are discussed. We have considered in detail the mechanism of excitation of erbium ions by quantum confined electron-hole pairs in silicon nanocrystals, the diffusion of excitation over the erbium ions inserted into the silicon dioxide matrix and the lifetime of erbium in the excited state limited by de-excitation cneters (traps, "black holes") in SiO₂.

The effect of carrier-carrier relaxation on threshold and power-current characteristics of InAs and GaAs quantum well (QW) lasers is studied. Dependence of carrier relaxation time on temperature and carrier density is considered. It is shown that in this case the gain coefficient becomes a more pronounced function of temperature and carrier density, and threshold current density increases drastically.

Terahertz electroluminescence was produced by intersubband transitions in silicongermanium quantum wells. The devices were grown by solid-source molecular-beam epitaxy on high-resistivity silicon substrates, and were fabricated by standard photolithography and processing techniques. Using FTIR spectroscopy at at temperature of 5 K, electroluminescence was observed around 9 THz. The emission was attributed to heavy-hole-to-light-hole transitions and demonstrates the potential for SiGe technology as terahertz emitters.

An intense THz emission was observed from strained SiGe/Si quantum-well structures under strong pulsed electric field. The p-type structures were MBE-grown on n-type Si substrate and δ-doped with boron. Lines with wavelengths near 100 microns were observed in the emission spectrum. The modal structure in the spectrum gave evidence for the stimulated nature of the emission. The origin of the THz emission was attributed to intracenter optical transitions between resonant and localized boron levels.

Terahertz Bloch oscillator based on GaAs-GaAlAs superlattice with weak barriers is proposed. Here due to interminiband tunneling current is a rising function of electric field while the tunneling and the Bloch oscillations produce dynamic negative differential conductivity at the Terahertz frequencies. Monte-Carlo simulation demonstrates existence of the negative conductivity in 1 - 7 THz range in the superlattices with moderate mobility at 77 K. The oscillator should consist of the 200 - 600 superlattice periods sandwiched between two n⁺ contact regions which form also strip-line waveguide (the oscillator cavity) and could be operational at 77 K in CW mode.

A study of the formation of 2D resonant states in the conduction band, induced by impurities outside heterostructure quantum wells, is presented. General expressions for the capture and scattering amplitudes are derived, and the effect on the energy spectrum and the density of states in the quantum well is calculated. The theory is applied to the Coulomb potential of donor impurity.

Resonant acceptor states induced by internal strain in Si/SiGe/Si quantum wells delta-doped with boron have been investigated theoretically. Sample design of Si/SiGe/Si MQW structures for resonant state THz laser is suggested.

In photoluminescence kinetics of the InP quantum dots, we have found a long-lived component of the degree of circular polarization, showing practically no decay during the lifetime of excitation. It is shown that this effect is observed only in the charged quantum dots. The amplitude of the long-lived PL polarization can be controlled by an applied electric field and may be as high as 50% in the case of quasi resonance excitation. The existence of the long-lived component of the polarization is the clear evidence of large lifetime of the electron spin polarization in quantum dots.

The theory of spin orientation of two-dimensional (2D) electron gas has been developed for intrasubband indirect optical transitions. The monopolar optical orientation of electrons in the conduction band is caused by the indirect scattering with virtual intermediate states in the valence band and allowance for selection rules for interband transitions. The considered mechanism of optical orientation is shown to be in an inherent relation with the special Elliot-Yafet mechanism of electron spin relaxation induced by virtual interband scattering.

Time resolved photoemission of highly spin-polarized electrons from thin strained and unstrained GaAs_xP_1-x films of various thicknesses has been investigated. An upper limit for the response time of a photocathode has been found to be 1 ps for layer thicknesses less than 150 nm. We show that the electron depolarization during the electron extraction to the surface band bending region can be as low as 2% while the losses in the band bending region can contribute to 4% spin relaxation.

Circular photogalvanic effect under interband absorption in quantum wells is considered. The photocurrent is calculated as a function of light frequency for direct transitions between ground electron and hole size-quantized subbands. It is shown that the current spectrum near the absorption edge depends dramatically on the form of spin-orbit interaction: it is a linear function of photon energy in the case of structure inversion asymmetry induced spin-splitting, and it has a parabolic dependence when bulk inversion asymmetry dominates. At higher energies, the existence of a peak in the photocurrent spectrum is predicted.

The polarized luminescence spectra and kinetics of the GaAs quantum wells with excess free electrons are studied experimentally. Quantum beats between the exciton and electron spin sublevels, split by a magnetic field, are detected. The dynamics of the degree of polarization is found to contain a long component associated with slow relaxation of electron spins.

An overview is given of the quantum-dot cellular automata (QCA) architecture, along with a summary of experimental demonstrations of QCA devices. QCA is a transistorless computation paradigm that can provide a solution to such challenging issues as device and power density. The basic building blocks of the QCA architecture, such logic gates and clocked cells have been demonstrated. The experiments to date have used QCA cells composed of metallic islands, and operate only at low temperatures. For QCA to be practical, the operating temperature must be raised, and molecular implementations are being investigated that should yield room temperature operation.

We present an experimental demonstration of novel Quantum-dot Cellular Automata (QCA) devices based on clocked architecture -- a QCA latch and a two-bit QCA shift register. The operation of the devices is demonstrated, and sources of the digital errors occurring in clocked QCA devices are discussed.

We proposed and analyzed a semiconductor multi-barrier tunneling structure, which is incorporated with a quantum dot layer for a cellular automata logic module. Both in-plane and cross plane directions of tunneling in the self-assembled quantum dot layer were taken into consideration. Nonlinear I-V characteristics as a result of tunneling of a multi-cell system were simulated and used for the modular-logic construction. Elemental units, "AND," "OR" and "EXCHANGE" gate operations where shown. In addition, we demonstrated a set of local transition rules for use in one logic module driven by the edge bias. The stability of the scheme with respect to material structure imperfections is discussed.

We propose an implementation of the spin waves for massively parallel quantum network. A spin wave based quantum network offers an advantage of random access to any qubit in the network and, consequently, the ability to recognize two qubit gates and performance gate operation between any two distant qubits. Using model simulations we illustrate the process of the two distant qubit entanglement via spin waves exchange. The utilization of spin waves allows us to avoid the most difficult single electron spin measurements procedure. Instead, qubit state recognition is accomplished by measurement of spin wave excited in a ferromagnetic layer. By estimate the proposed scheme has as high as 10⁴ ratio between quantum system coherence time and the time of a single computational step.

The physical processes in a two-dimensional electron gas (2DEG) driven into a nonequilibrium state by light illumination or increased microwave power are studied by combined optical and dimensional magnetoplasma resonance (DMPR) spectroscopies. A hysteretic DMPR, an optical bistability of the exciton and 2DEG Photo-luminescence, an evolution of the photoexcited electron-hole system from exciton to metal state in modulation doped and undoped heterostructures are investigated.

Magnetoresistance of long correlated nanowires of degenerate semiconductor InSb in asbestos matrix (wire diameter is around 50 Å, length 0.1 - 1 mm) is studied over temperature range 2.3 - 300 K. At zero magnetic field the electric conduction G and current-voltage characteristics of such wires obeys the power laws G varies direct as T^α, I varies direct as V^β, expected for one-dimensional electron systems. The effect of magnetic field corresponds to 20% growth of the exponents α, β at H = 10 T that may result from breaking of the spin-charge separation in the one-dimensional electron system.

We have investigated the temperature dependence of resistance in the temperature range T = 0.07 - 300 K, the quantum Hall effect (qHe) and the Shubnikov-de Haas (SdH) effect in InAs/GaAs quantum dot structures in magnetic field up to 35 T. Two-dimensional Mott variable range hopping conductivity (VRHC) has been observed at low temperatures in samples with low carrier concentration. The length of localization correlates very well with the quantum dot cluster size obtained by Atomic Force Microscope (AFM). In samples with relatively high carrier concentration the transition qHe-insulator was observed.

We report the suppression of the Hall effect in a mesoscopic Hall cross with a strong magnetic field only in the center and vanishingly small outside. The local magnetic field is produced by placing Dy pillar on top of a structure with high-mobility two-dimensional electron gas (2DEG). The effect is found to be due to a sharp increase of the number of back-scattered and quasi-localized electron orbits. The possibility of localizing electrons inside the magnetic inhomogeneity region is discussed.

Ferromagnetic Ga_1-xMn_xAs films containing up to 5.1 at%Mn were grown by low-temperature MBE. The structural, electrical, and magnetic properties of the layers are reported. At x > 0.01, the materials show a ferromagnetic behavior. The Curie temperature reaches 80 K at 5.1at% Mn. We propose the use of a n⁺-GaAs/p⁺-GaMnAs Esaki-diode (ferromagnetic Esaki-diode, FED) to provide injection of spin-polarized electrons via interband tunneling. Under reverse bias, spin-polarized electrons at the Fermi level in the valence band of GaMnAs tunnel to the conduction band of GaAs in contrast to the injection of spin-polarized holes used before.

The effects of different spin orientations on transport through carbon nanotubes are studied, using a simple tight-binding model within the mean-field approximation. It turns out that, in the absence of external magnetic field, the mean-field ground states of both semiconducting and metallic nanotubes are antiferromagnetic. As regards electronic/transport properties, it is observed that the conductance characteristics of spin-up and spin-down carriers are separated, and a negative differential resistance (NDR) feature in the I-V characteristics is detected, when the system is subjected to external magnetic field. NDR is particularly interesting for a wide range of applications.

We have observed that films of a polyimide precursor of poly[4,4'-bis(4"-N-phenoxy)diphenyl-sulfone] amid acid of 1,3-bis(3',4-dicarboxyphenoxy) benzene which is called type (1) polymer- or co-poly[4,4'-bis(4"-N-phenoxy)diphenyl-sulfone-α,ω-bis(η-amino propyl)oligodimethylsiloxane]imide of 1,3-bis(3',4-dicarboxyphenoxy)benzene type (2) polymer, placed between two metallic electrodes become highly conducting in a relatively small electric field (E<1 V/cm). If the metallic electrodes (Sn, Nb) in sandwich structures were in the superconducting state an effective resistance of zero was recorded. A typical current-voltage characteristic of an S-P-S structure looks like a Josephson type. We hve experimentally shown that for a S-P-S structure, a point contact between the superconductor and the polymer film plays the role of a weak link.

We found that a ballistic planar constriction exhibits different properties in angular dependence of the magnetoresistance in compare with that of the diffusive point contact. It shows unusual two-minima angular dependence. This behavior also differs much from that of ballistic four terminal bridges, where monotonic angular dependence of the magnetoresistance is observed.

Results of detailed investigations of the conductivity and Hall effect in gated single quantum well GaAs/InGaAs/GaAs heterostructures with two-dimensional electron gas are presented. A successive analysis of the data has shown that the conductivity is diffusive for k_Fl = 25 - 2.5. The absolute value of the quantum corrections for k_Fl = 2.5 at low temperature is not small, e.g., it is about 70% of the Drude conductivity at T = 0.46 K. For k_Fl < 2.5 the conductivity looks like diffusive one. The temperature and magnetic field dependences are qualitatively described within the framework of the self-consistent theory by Vollhardt and Wolfle. The interference correction is therewith close in magnitude to the Drude conductivity so that the conductivity σ becomes significantly less than e²/h. We conclude that the temperature and magnetic field dependences of conductivity in the whole studied k_Fl range are due to changes of quantum corrections and that transition to hopping conductivity occurs at lower conductivity value.

The observation of the Quantum Hall effect (QHE) in a semimetal channel with coexisting electrons and holes, simultaneously, at the type II broken-gap p-GaIn_0.16As_0.22Sb/p-InAs single heterointerface based on unintentionally doped quaternary solid solution obtained by liquid phase epitaxy (LPE) was reported for the first time elsewhere. In this report the quantum magnetotransport in the p-Ga_0.84In_0.16As_0.22Sb_0.78/p-InAs single heterostructure has been studied for a set of the samples with the both undoped and doped with Zn impurity quaternary layer at low temperatures in high magnetic fields up to 14 T.

The acoustoelectric current in the ballistic quantum channels is investigated. It is shown theoretically that the acoustoelectric current as a function of chemical potential (which is controlled by gate voltage itself) demonstrates either giant quantum oscillations or quantized plateaux. The conditions of such step-like behavior have been found and investigated in detail.

We discuss the mechanism of hopping conduction in nanocomposites and similar materials where the size of hopping sites is large compared to edge-to-edge separations between the sites. In this case the dominant mechanism involving transitions via virtual intermediate localized states is characterized by the increase in tunneling probabilities between distant sites; moreover the coherent tunneling path via virtual intermediate states is restricted to the fractal incipient percolation cluster. The application of percolation arguments to the calculation of the system conductivity is based on the concept of the generalized chemical distance. The resulting d.c. conductivity temperature dependence is of the form lnσ approximately -(T_o/T)^x, where the exponent x is expressed in terms of the critical exponent for the chemical distance (or superlocalization exponent) and fractal dimensionality of the backbone cluster.

We investigate the dynamics of a Bloch-oscillating wave packet in the presence of strong coupling to delocalized above barrier states, using time-resolved intraband polarization-sensitive measurements. At a threshold electric field, the resonance of localized and delocalized states causes a quantum beating which is observed as a revival in the interband polarization. Our numrerical simulation visualizes the spatial wave packet decomposition and reformation.

We have studied the near-equilibrium tunnelling between identical two-dimensional electron systems at 0.3 K over a wide range of magnetic field applied normal to the electron layers. The magnetic field suppresses the electron tunnelling. Our results are consistent with the co-existence of two types of tunneling gap. Moreover, at ν < 2 additional features arise in the tunneling spectra, which we interpret in terms of the emission of some as yet unidentified quasiparticle.

We have studied magneto-oscillations of the tunnelling current through a quantum well (QW) incorporating InAs self-assembled quantum dots in magnetic fields up to 28 T applied normal to the QW plane. We find evidence for the strong modification of the Landau levels in the host GaAs quantum well in the presence of dots embedded at the center of the well, which we attribute to electron-electron interactions.