Table of contents

This special issue is formed from a collection of selected papers that were presented at the 8th International Symposium on Nanostructures: Physics and Technology in 2000 (www.ioffe.rssi.ru/NANO2000/). The annual symposia are chaired by two Nobel Prize winners, Professor Zh Alferov and Professor L Esaki, and are traditionally organized by the Ioffe Institute in Repino - a suburb of St Petersburg. The Symposium was launched in 1993 by its Chairs as one of the first in an area that is becoming one of the leading and more important directions within modern solid state physics and technology.

Perhaps it would be appropriate to recall the history of the prefixnano in the title of the Symposium. In the late 1950s, Dr J Kilby at Texas Instruments and Dr R Noyce at Fairchild Semiconductor proposed the fabrication of integrated electronic circuits on germanium and silicon crystals, and successfully realized this idea. This created, after the invention of the transistor by J Bardeen, W Brattain and W Shockley in 1947, the next great revolution in information technology. The integrated circuits were formed with elements on the micrometre scale; this is why this area of science and technology became known as microelectronics. However, this development led to a decrease of the characteristic scale to a submicron level. Actually, the typical thickness of the oxide film on Si-based integrated circuits is now about 10 nm and the gate width is a little more than 100 nm.

In the middle of the 1960s, a new direction in solid state electronics and optoelectronics was pushed forward. This direction was based on applications of semiconductor heterostructures. The foundations of this area were laid by Zh Alferov and H Kroemer.

Heterostructure applications make real the exploitation of quantum mechanical behaviour of electrons and holes in solids. The basic concepts in this area were given by L Keldysh (1962), L Ioganson (1963), R Davis and H Hosack (1963), L Esaki and R Tsu (1969), R Kazarinov and R Suris (1971) and R Dingle and C Henry (1974). Naturally, to provide quantum mechanical behaviour and avoid carrier scattering by phonons and lattice defects, it was necessary to fabricate structures on the nanometre scale. This had become possible because of the pioneering work by A Cho on molecular beam epitaxy. Following L Esaki, the fabrication of structures and devices with quantized electrons and holes can be called `carrier wavefunction engineering'.

The development of nanoscale semiconductor heterostructures led to the low threshold injection lasers with quantum wells and the high-speed transistors that revolutionized information transmission and storage technologies.

Recently, the team headed by Zh Alferov demonstrated the possibility of further radical improvement in semiconductor laser performance characteristics with the use of nanostructures with quantum dots. New remarkable achievements in the mid- and far-infrared techniques are in progress. First of all, there is a quantum cascade laser that was created by the team of F Capasso and quantum well infrared photodetectors utilizing intrasubband electron (hole) transitions. New structures and technologies based on applications of nanoclusters such as carbon nanotubes are under consideration.

So now the terms nanoelectronics and nanostructures are as widely in use as the terms microelectronics and microstructures.

It is natural that the Symposium is traditionally organized by the Ioffe Institute and held in St Petersburg. Here, N Goryunova and A Regel, at the same time as H Walker, discovered the semiconducting properties of the group III-V compounds that became the basic materials for semiconductor nanostructures. Here, the first `ideal' GaAlAs heterostructures and the heterostructure lasers were proposed and developed, and here also, many of the basic concepts in semiconductor electronics and optoelectronics were formulated. The Ioffe Institute is still pioneering in many directions of this impetuously developing area.

The 9th International Symposium on Nanostructures: Physics and Technology will be held on 18-22 June, 2001, see www.ioffe.rssi.ru/NANO2001.

We report on the growth and properties of InGaAsN/GaAs heterostructures and on their applications for lasers emitting at λ≈1.3 µm. Material growth was performed by molecular beam epitaxy using an RF plasma source.

Broad area and ridge waveguide (RWG) laser structures based on such quantum wells (QWs) exhibit performances that can compete with those of 1.3 µm InGaAsP lasers. In particular, we have achieved 300 K operation of broad area lasers at 1.3 µm with threshold current densities down to 500 and 650 A cm^-2 for 800 µm long, single and triple QW structures. Similar structures with heat-sinking at 10 °C yield a maximum CW output power of up to 8 W.

RWG lasers have thresholds down to 11 mA and show CW operation up to 100 °C with a T₀ of up to 110 K.

We propose an original design of the active element of a quantum unipolar semiconductor laser for optical pumping. The peculiarities of the proposed design are strongly asymmetric barriers surrounding a double-well active element. The suppression of intersubband transitions to the lower working subband can be readily achieved if the transformation point of electronic state dimensionality for the lower subband occurs at small momentum. By this means the population inversion conditions in this system can be easily realized. The results of photoluminescence studies of the individual elements of the proposed structure are presented.

The possibility of intersubband population inversion and the necessary conditions for its achievement in wide quantum well structures with the energy space between the lowest states being lower than the optical phonon energy are considered. Two possible designs are proposed, both using resonant tunnelling for selective depopulation of the lowest subband. The analysis of resonance tunnelling times and intersubband relaxation rates due to various scattering processes is carried out, demonstrating the possibility, at sufficiently low carrier concentrations, of inverted carrier distributions over the lowest subbands under optical pumping or electrical injecting conditions.

Spin relaxation in-plane anisotropy is predicted for heterostructures based on zinc-blende semiconductors. It is shown that it manifests itself especially clearly if the two contributions to the D'yakonov-Perel' spin relaxation mechanism (due to the bulk spin-orbit interaction term and the Rashba term) are comparable in efficiency. Concentration dependences of spin relaxation rates for a heterostructure grown along the [001] direction are plotted for different electron spin orientations.

Optical absorption and birefringence due to intersubband electron transitions were investigated in GaAs/AlAs MQW structures. These structures are intended for creation of a mid-infrared laser of a new type. Experimental results on electron redistribution between size-quantization levels under electron heating were obtained up to an electric field of 3500 V cm^-1.

We report on the role of bismuth as a surfactant in the growth of InAs quantum dots (QDs) on GaAs (001) by metal organic vapour phase epitaxy. Atomic force microscopy investigations have shown that bismuth suppresses coalescence of the InAs QDs and advances a more uniform size distribution. The photoluminescence spectra of the Bi-assisted grown QDs present several narrow peaks from the ground and the excited state transitions with full width at half maximum (FWHM) as narrow as 25 meV (both at 77 and 300 K). Due to such low values of the FWHM we were able to observe up to two well resolved excited state transitions in the photovoltage spectra measured by an electrolyte cell technique. The lowest ground transition energies observed were 0.93 eV at 77 K and 0.875 eV at 300 K (emission wavelength 1.46 µm). So using Bi-assisted growth it is possible to cover the 1.3 µm band, which is important for optoelectronic applications in the InAs/GaAs material system. Formation of such `deep' QDs without misfit dislocations was explained by the formation of a graded-composition transient InGaAs alloy layer at the GaAs/InAs hetero-interface as a result of diffusion intermixing of the components. The proposed mechanism for the effect of Bi on the QDs' morphology is that Bi decreases the surface mobility of the In atoms on the growing surface, preventing the coalescence of the QDs. Because of its rather large covalent radius (compared with that of As), Bi is not incorporated into the QDs' material segregating on the growing surface.

We present a systematic investigation of the dependence of the hot-electron-optical-phonon interactions on Al composition and barrier width in GaAs/Al_xGa_1-xAs MQW structures. Raman scattering measurements at 15 K are presented for samples with different barrier widths and Al composition. The optical phonon energies emitted by the photoexcited electrons in quantum wells were also determined by using hot-electron-neutral acceptor luminescence techniques. It is shown that the relaxation of hot electrons in the quantum wells is dominated by the GaAs LO phonon emission for small x, but by AlAs-like LO phonons for larger Al composition. For samples with larger barriers, the electrons in the GaAs layer relax mostly through the AlAs-like optical phonon emission. However, in samples with smaller barriers, the relaxation of hot electrons is dominated by the GaAs optical phonon emission.

It is shown that the effective mass approximation together with interface short-range corrections can be used to calculate short-period abrupt (GaAs)_N/(AlAs)_N(001) superlattices. The results obtained are in excellent agreement with the results of pseudopotential calculations in a wide range of superlattice periods (from N = 1 to 20).

Starting from an equation of motion analysis of the density matrix, kinetic equations are derived that allow a study of semiclassical intraband and quantum-mechanical tunnelling contributions to the field-induced carrier redistribution in multiband superlattices. An analytical expression is obtained which describes the field-dependent carrier redistribution between the upper and lower miniband. We show that there is no global population inversion for superlattices where only two minibands are relevant. Numerical results for the current density are discussed.

Molecular-beam-epitaxy-grown ZnTe/CdZnTe/ZnTe and ZnSe/ZnCdSe/ZnSe strained single-quantum-well structures with non-doping layers were investigated by cathodoluminescence (CL) and deep-level transient spectroscopy (DLTS). The activation energies for deep levels in ZnTe and ZnSe buffer layers grown on GaAs were determined by the DLTS spectra. Moreover, an additional DLTS peak that depends on the quantum well (QW) parameters and correlates with the QW emission line position in the CL spectra was observed. This peak is interpreted as an emission of electrons from a ground level in the QW. Obtained DLTS and CL results were used for the estimation of the conduction band offset parameter Q_C.

In this paper we present a low-temperature (T<30 K) photoluminescence (PL) study of space-indirect excitons in GaAs/AlGaAs double quantum wells at pumping density P and external electric field V_dc. At certain external parameter values (P, T and V_dc) part of the spectral profile of the indirect exciton (IX) line reveals an enormous increase (shot) of PL intensity (threefold), which fluctuates in time on the characteristic scale of tens of seconds. To explain this anomalous behaviour we use the theoretical model of the Bose-Einstein condensation effect which takes place in a system of 2D bosons which, in addition to extended states, have a discrete energy spectrum below the bottom of the free zone.

The photoluminescence and resonant Raman scattering spectra of thin films and multiple-quantum-well (MQW) structures were investigated at room temperature. The light emission spectra of MQWs consist of an intensive band of the quantum well luminescence and additional narrow bands. The energy shift of these additional bands was equal to a multiple of the longitudinal phonon value of the strained ZnSe barriers. The intensity of these bands is resonance amplified on approach of the excitation energy to the MQW luminescence. We make the supposition that this process consists of several stages: absorption of stimulating light by quantum wells; exchange of energy between hot electrons and barriers and relaxation of the system through LO-phonon generations in the barriers.

We present the results of a theoretical study of the electronic structure of self-organized GaN/AlN quantum dots (QDs) with a hexagonal truncated pyramid shape. Our analysis takes into account the effects of built-in elastic strain, and of the piezoelectric and spontaneous polarization, and is carried out in the framework of a multi-band k×P model. A novel Green function technique which we have developed previously is used to calculate the 3D distribution of the strain and of the built-in electric field in each QD structure. The QD carrier spectra and wavefunctions are calculated using a plane-wave expansion method. We show that due to the strong built-in electric field the electrons are localized near the QD top and the holes are localized in the wetting layer just below the pyramid. These electric fields also provide additional lateral localization of the carriers, so that the effective lateral size of the dot becomes smaller. The energy of the ground state optical transition decreases with increasing dot size, in good quantitative agreement with available experimental data.

It is shown that bulk MOVPE-grown GaN can exhibit dramatically increased near-band-edge and below-bandgap emission after argon plasma etching. The temperature dependence of the near-band-edge emission shows that donor-bound excitons are the dominant species in Ar-etched samples while free excitons dominate spectra from unetched samples. Under UV illumination and at low temperatures metastable behaviour is apparent from the defect-related blue and yellow luminescence bands. The clear linkage between the yellow and blue emission bands indicates that any microscopic model of the defect(s) associated with the yellow luminescence must also be able to explain the blue luminescence. At room temperature, UV illumination over a period of time enhances the yellow emission by a factor of 20 and it is shown that this effect can be employed for room-temperature optical data storage and retrieval.

A two-dimensional rectangular network has been fabricated for the first time based on fullerene films using electron lithography. Such a system shows the optical properties of a two-dimensional photonic crystal, which have been investigated by reflection and transmission methods. An enhancement of the nonlinear optical properties of the fullerene due to the formation of an open network structure has been found. The periodic structure formed from a fullerene film may be transferred easily into the substrate by plasma etching due to the resistive properties of solid state fullerenes. This method allows one to fabricate semiconductor networks and field-induced quantum dot structures.

We report measurements of the diffraction pattern of a two-dimensional photonic quasicrystal structure and use the set of plane waves defined by the diffraction pattern as the basis of a theoretical approach to calculate the photonic band structure of the system. An important feature of the model is that it retains the essence of the rotational and inflational properties of the quasicrystal at all levels of approximation: properties lost in approximate models which artificially introduce elements of periodicity.

The calculated density of modes of the quasicrystals is found to display a weakly depleted region analogous to the bandgap that occurs in a periodic system. The calculated transmission spectra for different polarizations and directions of propagation show features that correlate with the behaviour of the density of modes.

We have studied the high magnetic field (⩽50 T) dependence of the negatively charged exciton properties in GaAs/Al_xGa_1-xAs (x = 0.33) quantum wells using photoluminescence (PL) spectroscopy. Observation of all the optically allowed transitions of the charged exciton allows us to experimentally verify a revised energy-level diagram of spin-split singlet and triplet states. The PL data obtained on all samples are completely consistent with this diagram, leading us to conclude that the negatively charged exciton is a model three-particle system. The binding energy of the negatively charged exciton is measured between 23 and 50 T, and found to be constant for both singlet and triplet states. Our results are compared with recent theory.

Under the conditions of 2-3D transformation of one-particle states, the dependence of exciton binding energy E_ex on the well width h has been theoretically studied for quantum structures with barriers of asymmetric height. It is shown that the contribution of one-particle continuous spectrum states to the exciton becomes considerable when the inverse exciton Bohr radius 1/a_b is comparable to the value of wavevector k_c critical for 2-3D transformation. The lack of coincidence between electron and hole k_c values leads to a complicated shape of the E_ex(h) dependence with a minimum at intermediate values of h. The minimum value of E_ex appears to be less than that of the 3D exciton binding energy. The behaviour of the exciton PLE peak position is investigated for an asymmetric structure under the influence of an external electric field.

In this paper, regular three-dimensional systems of GaN, InN and InGaN nanoclusters have been fabricated for the first time in a void sublattice of artificial opal. The opal consisted of 220 nm diameter close packed amorphous silica spheres and had a regular sublattice of voids accessible to filling by other substances. GaN, InN and InGaN were synthesized directly in the opal voids from precursors such as metal salts and nitrogen hydrides. The composites' structures have been characterized using x-ray diffraction, Raman spectroscopy, atomic force microscopy and optical measurements.

The Si⁺-dose dependence of the photoluminescence (PL) intensity at λ≈800 nm for SiO₂ with Si nanoinclusions (NIs) produced by ion implantation is studied. It is shown that this dependence is determined by the increase of the surface density of NIs, the mean size of NIs being constant until they overlap. Doping by phosphorus enhances the PL due to the joint action of two mechanisms: (i) the supply of additional electrons from the donors to the conduction band of quantum dots and (ii) the passivation of dangling bonds by phosphorus atoms.

Molecular beam epitaxy has been utilized to grow small C-induced Ge islands in silicon. The evolution of C-induced Ge dots from nucleation to the formation of larger dots has been studied in detail. The Ge grows in a Vollmer-Weber mode of growth in areas between the C-rich patches on the Si surface. Using in situ STM analysis as well as transmission electron microscopy (TEM) it is shown that an increase in the Ge coverage leads to the overgrowth of the C-rich patches and to island coalescence, reducing the island density. The strongest luminescence intensity is found in this region where the Ge has buried the C-rich areas. The amount of C deposited on the Si(100) surface prior to the growth of Ge permits the control of the lateral size and the height of Ge quantum dots. Accordingly, intense photoluminescence (PL), with a stronger confinement shift dependending on the amount of Ge deposited, is observed for samples prepared with large C concentrations. The impact of the Si spacer layer width on the size of the dots has been studied by TEM and compared with PL data. The data give new insights into the structural peculiarities of C-induced Ge dots and their consequences on the electron and hole confinement.

The photoresponsivity spectra of double-barrier resonant tunnelling diodes have been measured in a wide range of light wavelength as well as applied voltage. The complex behaviour of measured spectra is analysed, taking into account different channels for electron injection into the quantum well (QW). The results obtained yield evidence for modulated electron-hole (e-h) recombination in the QW provided by direct excitation of e-h pairs in the QW.

An experimental study of carrier relaxation mechanisms and carrier distribution in InAs-InGaAs-GaAs quantum dot (QD) arrays is presented. The structures were investigated by resonant photoluminescence (PL) at different temperatures. For a sample with low confinement energy for electrons and holes in a QD the carrier distribution within the QD array changes from non-Fermi (random) to Fermi (equilibrium) when temperature increases from 20 to 300 K. In contrast, for the sample with high QD confinement energy the carrier distribution remains random even at room temperature. Near-resonant PL at 20 K reveals a series of broad phonon replicas ascribed to multiphonon relaxation processes involving LO phonons in the QD, in the wetting layer, at the interface and in GaAs as well as acoustic phonons. The complex distribution of InGaAs in QDs formed by activated alloy phase separation is believed to cause a dispersion of QD phonon energies and to lead to a further broadening of phonon lines.

We present, to our knowledge, the first experimental evidence for the existence of acoustical plasma oscillations in photoexcited electron-hole plasma observed in resonant quasielastic electronic light scattering spectra in the near-infrared spectral range in a GaAs matrix embedded with self-assembled InAs quantum dots.

The effect of spontaneous spin polarization in quantum point contacts (QPCs) is investigated by self-consistent modelling within the Kohn-Sham local spin-density formalism. The existence of the spin-polarized state of the QPC channel in the low-density regime is shown to change the value and the shape of the effective potential barrier for up- and down-spin electrons. We suggest that this effect can be relevant to the explanation of the anomalous 0.7-structure in the QPC conductance and may assist spin injection in potential spintronic devices.

Optical and structural properties of multilayer structures with InAs/GaAs quantum dots are investigated. It is shown that under optimal growth conditions, 1.3-1.4 µm emission can be achieved. Possible scenarios of quantum dot behaviour evaluation are discussed in a frame of elastic theory to explain differences in optical properties of the grown structures.

We theoretically investigated electron and phonon transport in a quantum superlattice and evaluated a possible thermoelectric figure of merit increase. The presented model takes into account electron and phonon transport modifications due to the space confinement caused by the mismatch in electronic and thermal properties between dot and host materials. The numerical calculations were carried out for a structure that consists of multiple layers of Si with regimented quantum dots separated by wetting layers and spacers. Transport coefficients (electric conductivity, Seebeck coefficient and lattice thermal conductivity) were calculated as functions of quantum dot volume fraction for different dot sizes. It is shown that additional thermoelectric figure of merit enhancement due to the presence of quantum dots may be obtained. The predicted enhancement is mostly due to the significant drop in the lattice thermal conductivity caused by the acoustic phonon scattering by quantum dots.

A giant level shift, resulting from the interaction of an electron in a spherical quantum dot with zero-point oscillations of confined modes of the electric field, is divulged. This electron potential energy renormalization depends on the dot radius which has to show up in experiment. The size scaling of the depolarization effect is computed semiclassically. A change of the optical properties of the matrix surrounding the dot also provides a method with which to study the shift experimentally.

The existence of a predominantly symmetrical spin configuration (spin polarized phase) and `diluted' density of states (pseudo-gap) in a layer under the Fermi level in a quantum wire is predicted. The condition of cross-over from a non-polarized phase to a polarized one was derived. The effect essentially depends upon screening. The transition occurs for sufficiently low electron density in a wire and is accompanied by an acute decrease of the electron density of states near the Fermi level. It may result in a corresponding decrease in conductance.

We report our recent results of the effect of deuterium termination on the optical properties of porous Si nanostructures. The photoluminescence (PL) spectrum of deuterium-terminated porous Si (D-PS) was different from that of ordinary hydrogen-terminated porous Si (H-PS) although H-PS and D-PS show the same structure and the same absorption spectrum. We quantitatively discuss these results based on the trapping of conduction electrons at a surface Si-H or Si-D. In addition to these basic results, we also show that the replacement of hydrogen with deuterium reduces the PL degradation. Surface termination, especially photo-induced oxidation of the surface, was precisely studied by Fourier transform infrared absorption measurements.

A new nonvariational theoretical approach allowing us to calculate both the localized and the continuum states of charge carriers in quantum well heterostructures in the presence of the Coulomb potential has been developed. The method has been used to calculate the energy spectra of shallow acceptors in strained Ge/GeSi multiple-quantum-well heterostructures. Optical transitions from the acceptor ground states to the resonant states have been revealed in the measured far-infrared photoconductivity spectra of the heterostructures.

We have measured the temperature (0.1⩽T⩽15 K) and magnetic field (0⩽B⩽32 T) dependences of longitudinal and Hall resistivities for the p-Ge_0.93Si_0.07/Ge multilayers with different Ge layer widths 10⩽d_w⩽38 nm and hole densities p_s = (1-5)×10¹⁵ m^-2. An extremely high sensitivity of the experimental data (the structure of magnetoresistance traces, relative values of the inter-Landau-level (LL) gaps deduced from the activation magnetotransport etc) to the quantum well (QW) characteristics has been revealed in the cases when the Fermi level reached the second confinement subband. The background density of states (5-10)×10¹⁴ m^-2 meV^-1 deduced from the activation behaviour of the magnetoresistance was too high to be attributed to the LL tails, but may be accounted for within a smooth random potential model. The hole gas in the Ge QW was found to separate into two sublayers for d_w>~35 nm and p_s≈5×10¹⁵ m^-2. Concomitantly the positive magnetoresistance emerged in the weakest fields, from which different mobilities in the sublayers were deduced. A model is suggested to explain the existence of the plateaux close to the fundamental values in a system of two parallel layers with different mobilities.

We have shown, firstly, that the response time (τ_resp) of the double-barrier resonant-tunnelling diode (RTD) can be much smaller as well as much larger than the quasibound-state lifetime in the quantum well (τ_dwell). Secondly, the real part of the RTD conductance can be negative and large at the frequencies higher than the reciprocal τ_dwell in the RTDs with a heavily doped collector without spacer layers. The Coulomb interaction of the electrons in the quantum well with emitter and collector is responsible for the effects. A simple analytical expression for the impedance of the RTD has been derived and an equivalent circuit has been proposed.

We discuss the results of experimental studies of coherent electron transport in open quantum dot arrays. These studies suggest that the details of this transport remain highly phase coherent over much of the array, allowing features associated with the quantum mechanical coupling of the dots to be observed. Studies of magneto-conductance oscillations in the arrays appear to show the influence of this coupling through a change in frequency content as their gate voltage is varied. We speculate that this change arises as the contribution to interference of long orbits is suppressed by reducing the strength of the inter-dot coupling. Temperature-dependent studies of these devices reveal evidence for an unexpected `metal-insulator' transition, which is manifest as a logarithmic variation of the conductance at low temperatures, whose sign can correspond to either metallic or insulating behaviour. It is speculated that this logarithmic term provides a novel signature of electron-interaction effects in confined nanostructure devices.

We investigate electron transport and shot noise in a single-barrier GaAs/AlGaAs heterostructure of nanometric size in the presence of ballistic and thermalized carriers. The coupling between space charge and the dependence of the transmission coefficient on energy is found to provide the positive feedback which enhances shot noise and ultimately leads to a current instability of S type. Theoretical results are in qualitative agreement with existing experiments and confirm recent Monte Carlo simulations evidencing shot-noise enhancement in GaAs/AlGaAs heterostructures.

We develop a theory of vertical hopping transport in doped superlattices with intentional uncorrelated vertical disorder introduced by controlled random variations of well widths. For structures with sufficiently large disorder, the vertical conductance (in the direction of the growth axis) is limited by phonon-assisted hopping between the wells. It is shown that, due to the quasi-equilibrium situation within the wells, the master rate equation for transitions between the electronic states of the structure can be reduced to a truncated rate equation for inter-well transitions only. At low bias, the solution of this rate equation is shown to be equivalent to finding the total resistance of a quasi-one-dimensional network of resistances expressed in terms of integral transition rates between the wells. Using this approach, we estimate the temperature dependence of the vertical resistance of superlattices with intentional disorder and show that it can be nonactivated for not too low doping levels and temperatures.

We have performed magnetotransport measurements in multi-terminal single-crystalline (001) molybdenum and tungsten nanostructures at 4.2 K. In such nanostructures with thicknesses of d = 90, 170, 290 and 340 nm and lead width w = 7d, the resistance is mainly due to electronic scattering by the boundaries of the device. An applied magnetic field changes the electronic trajectories and therefore modifies the surface scattering. In particular, in nanowires we have observed a strong reduction (respectively, increase) of the resistance when the magnetic field is directed parallel (respectively, perpendicular) to the current. When the mean free path of electrons is of the order of or larger than the lateral sample dimensions, the nanostructures exhibit strong geometric effects, such as longitudinal magnetic focusing and non-monotonic dependence of the bend resistance against the magnetic field. The latter is explained by crossover from the edge to the bulk state dominated electron transport in a moderate magnetic field, resulting in a re-entrance effect in the ballistic properties of cross-type nanostructures.

We propose an approach to realization of all-optical logical operations based on the nonlinear dynamics of an ensemble of two-level systems (TLSs) in a coherent photonic field. Using a theoretical analysis of the behaviour of a TLS ensemble in a resonant field, we formulate requirements for dynamic characteristics of a medium suitable for implementation of a photonic gate. Experimental studies of optical dynamics and excitonic state spectra of epitaxial GaAs/AlGaAs quantum wells and superlattices has allowed us to conclude that their physical parameters make it possible to build photonic-gate matrices exceeding the transistor-based electronic switches in terms of their productive capacity by five to six orders of magnitude.

We investigated the use of quantum bits (qubits) - semiconductor quantum dots containing one electron and each consisting of two tunnel-connected parts - as basic elements of the quantum computer. The numerical solution of a Schrödinger equation taking account of the Coulomb field of adjacent electrons shows that in such structures the realization of a full set of basic logic operations, which are necessary for fulfillment of quantum computations, is possible. Durations of one- and two-qubit operations versus qubit geometry are obtained. Decoherence rates due to spontaneous emission of phonons and acoustic phonons (both piezoelectric and deformation) are evaluated. Analysis of these rates shows the proposed qubit to be coherent enough to work for an unlimited time.

The influence of an electric field created by a gate potential of the silicon quantum computer on the hyperfine interaction constant (HIC) is obtained. The errors due to the technological inaccuracy of the location of donor atoms under a gate are evaluated. The energy spectra of an electron-nuclear spin system of two interacting donor atoms with various values of HIC are calculated. The presence of two pairs of anticrossing levels in the ground electronic state is shown. The parameters of the structure at which the error rate can be greatly minimized are found.

We study 1.3 µm diode lasers based on self-organized InAs quantum dots grown by molecular beam epitaxy on GaAs substrates. Overgrowing the InAs quantum dot array with a thin InGaAs layer allows us to achieve 1.3 µm emission and keep a sufficiently high surface density of quantum dots. Using transmission electron microscopy we show that the main reason for the long-wavelength PL shift in InAs/InGaAs quantum dots is non-uniform distribution of In in InGaAs leading to the increase in effective volume of a quantum dot. Long-stripe lasers showed low-threshold current density (<100 A cm^-2), high differential efficiency (>50%) and low internal loss (~1-2 cm^-1). The maximum output power for wide-stripe lasers was as high as 2.7 W and for single-mode devices 110 mW. The lasing wavelength for VCSELs was 1.3 µm. The threshold current for the device with the 8 µm aperture was 1.8 mA. The output power of 220 µW at a drive current of 2.4 mA was observed under pulsed mode.

The use of photovoltaic solar cells provides an elegant way of converting sunlight to electricity. The photovoltaic industry is currently growing very rapidly, at a compounded rate of about 30% each year. Energy conversion efficiency is a key parameter with this technology since it directly impacts both material and deployment costs. The performance of the traditional bulk semiconductor solar cell is limited to about 33% while thermodynamic limits on the conversion of sunlight to electricity are much higher, at 93%. Low-dimensional structures appear capable of allowing much of this gap to be bridged. These structures allow increased flexibility with traditional efficiency enhancement approaches such as those based on `stacked' or tandem cells, which double efficiency limits to 68%. Perhaps more interestingly, they offer scope for completely new device concepts such as those relying on excitations between multiple energy bands and improved `hot-carrier' cells, that offer scope for similarly high performance.

The weak-localization correction to the conductivity in coupled double-quantum-well structures is studied both experimentally and theoretically. The statistics of closed paths has been obtained from the analysis of magnetic field and temperature dependences of negative magnetoresistance for a magnetic field perpendicular and parallel to the structure plane. The comparison of experimental data with the results of computer simulation of carrier motion over two 2D layers with scattering shows that inter-layer transitions play a decisive role in the weak localization.