The indium atom distribution in InGaAs/(Al)GaAs quantum wells (QWs) grown by metal-organic chemical vapour deposition was systematically studied. High-resolution grazing-sputter-angle Auger electron spectroscopy was used as a method of indium depth profile investigation. A broadening and shift to the surface of the indium concentration profile in a single QW and an increase of indium content in the upper QW for closely spaced QWs were found. These results were confirmed by photoluminescence measurements. It was observed that the use of AlGaAs barriers between QWs and growth interruptions at QW interfaces during the growth process reduce indium surface segregation.
Different types of InGaAs/GaAs deep-etched quantum wire (QWI) structure were successfully fabricated by high-energy electron beam lithography on GaAs(100) surfaces. A selective wet-chemical-etching technique, preceded by chemically assisted ion-beam etching, reduced the controlled lateral dimensions of the wires to similar to 10 nm due to strong under-etching. Various types of wire in the [01 (1) over bar] and  crystallographic directions were prepared by the combined etching method. The side-walls of the wires were defined by the selectively etched low-index crystallographic planes. A molecular-beam-epitaxy-grown graded InGaAs/GaAs quantum well was realized at the narrow 'neck' region of the wires, thus providing the strongest possible lateral confinement of the QWI structure. Consequently, similarly to the selective growth of self-narrowing ridge structures, selective wet-chemical etching induced a controlled self-narrowing of the wire structures. Scanning electron microscopy images of the QWI nanostructures showed smooth side-walls defined by the crystallographic planes. Low-excitation photoluminescence spectroscopy of the structures revealed extremely high quantum efficiency and a size-dependent blue shift as a result of the strong lateral confinement.
Fourier analysis is used to formulize and characterize grating-based displacement transducers. The analysis is carried out for linear and radial binary grating sensors, and for both single and grating-pair sensors. The influence of the grating (pair) structure and the receiving window on sensing sensitivity, linearity of response and dynamic range is investigated. Several types of photoresist grating are fabricated and are used to experimentally characterize the grating sensor. The tested result is found to be in general agreement with the analysis result.
Optically detected magnetic resonance (ODMR) and level anticrossing spectroscopy were applied to study g-factors and exchange splittings of localized excitons and separately localized electrons and holes at the X-z-Gamma crossover of the conduction band states in a GaAs/AlAs superlattice with a composition gradient. g-factors, exchange splittings and the order of the exciton radiative levels were determined. In the transition region we clearly observed the disappearance of type II excitons created by the X-z electron in AlAs and a heavy hole in GaAs layers. Appearance of type I excitons with an order of magnitude larger exchange was verified by LAC spectroscopy. Intermediate 'type II-like' and 'type I-like' excitons were found in the transition region by ODMR and LAC with different exchange splittings and luminescence decay times. Besides ODMR of excitons with a definite value of exchange splitting, ODMR ascribed to separately localized electrons and holes with a distribution of exchange splittings was detected.
The optical and structural properties of the Ge submonolayer nano-inclusions in a Si matrix grown by molecular beam epitaxy are investigated. It is shown that at relatively high growth temperatures >600 degreesC new features appear in the photoluminescence spectra. It is found that these features correspond to formation of the germanium nano-inclusions in a silicon matrix.
The fine structure and intensity of resonant exciton-phonon (REP) spectra in a Zn0.87Cd0.13Se/ZnSe single quantum well (QW) have been investigated in detail at tunable laser excitation in the energy region of ground ZnCdSe exciton state at 8 K. Observed REP lines have been shown to go through two different mechanisms. The most intensive component corresponds to Raman scattering through extended exciton states. Other less intensive REP components correspond to hot luminescence of localized excitons. An analysis of optical phonon modes, which can be involved in the formation or the REP spectra in biaxially strained Zn0.87Cd0.13Se QWs, has been carried out.
Optical phenomena in the mid-infrared range connected with interlevel and intersubband charge-carrier transitions in quantum dot and quantum well (QW) heterostructures under optical and electrical pumping were investigated. Spectra of interband photoluminescence are also presented. The existence of a metastable level in funnel-shaped QWs is experimentally confirmed. The intersubband transition dynamics in asymmetrical pairs of tunnel-coupled QWs was studied by means of pump-and-probe tunnel-resolved spectroscopy.
It is shown that the strong enhancement of electron scattering by polar optical phonon absorption with increasing sheet electron concentration n(s) is responsible for the negative change of conductivity in the Al0.25Ga0.75As/GaAs/Al0.25Ga0.75As quantum well (QW). The insertion of a thin AlAs barrier into the QW increases electron mobility by 1.6 times at n(s) = 1 x 10(16) m(-2) and extends the range of n(s) at which the negative conductivity changes. A modulation-doped field-effect transistor based on the high-doped Al0.25Ga0.75As/GaAs/Al0.25Ga0.75As QW as a channel acquires new specific properties. The transistor transconductance, dependent on applied gate and drain voltages, can change sign, and its value at high doping level can be comparable to that at low doping level, in spite of the great decrease of channel electron mobility.
The characteristic time of the far-infrared response of a quantum Hall effect detector in GaA/AlGaAs has been investigated versus the magnetic field. The response time is shown to increase exponentially with the field due to the spatial separation of photoexcited electrons and holes captured by localized states formed by the disordered potential. The results obtained allow us to estimate the length scale of long-range potential fluctuations.
We have applied the Raman spectroscopy technique to the study of phonon-plasmon coupled modes in Si-doped GaAsn/AlAsm superlattices (SLs), grown on (001), (311)A and (311)B substrates. We have observed the shift and broadening of the Raman peaks in doped SLs, compared with undoped SLs. When the GaAs layers were thinned from 10 to 1 monolayer, the coupled phonon-plasmon modes became three-dimensional-like, and the interaction of plasmons with LO-phonons of AlAs-type took place. The phonon-plasmon interaction of the SLs was numerically simulated. There was a qualitative agreement between the experiment and the calculations. We observed a difference in dispersion between the coupled phonon-plasmon modes for SLs grown on facet surface (311)A and surface (311)B. This is probably due to differences in the electron spectra of such SLs caused by the difference in shape of GaAs quantum objects formed on AlAs reconstructed surfaces.
We investigate resonant tunnelling in GaAs/(AlGa)As double-barrier resonant-tunnelling diodes in which a single layer of InAs self-assembled quantum dots is embedded in the centre of the GaAs quantum well. The dots provide a well-defined and controllable source of disorder in the well and we use resonant tunnelling to study the effect of this disorder on the electronic properties of the well.
We study the generation of femtosecond superradiance pulses due to collective recombination of electron-hole (eh) pairs in quantum wells placed in a strong magnetic field oriented perpendicular to the growth plane. Such a superradiant laser can work under the conditions of continuous pumping and at room temperature due to the complete quantization of particle motion, maximum possible spectral density of states, high volume density of cyclotron quantum dots and partial suppression of the intraband scattering. It is shown that two-colour superradiant generation is possible at frequencies resonant to frequencies of the neighbouring interband transitions defined by the electron and hole Landau levels.
A new approach for calculation of resonant state parameters is developed. The method proposed allows us to solve different scattering problems, such as scattering and capture probability as well as calculations of shifts and widths of energy levels. It has been applied to the problem of resonant states induced by impurities in the barrier of quantum wells and by strain in uniaxially stressed germanium.
Ullrathin SiO2 films (3-6 nm) have been electrically characterized with a conductive atomic force microscope. This technique allows the electrical characterization of areas of similar to 100 nm(2), which are comparable to the area of breakdown spots. Sequences of voltage ramps on fixed oxide locations have been applied to induce the oxide degradation and its conduction properties have been analysed within the nanometre scale range. Ln particular, on-off fluctuations before and after breakdown are reported on single breakdown spots.
We have theoretically studied the operation principle of a memory cell using the parallel coupled arrays. This makes use of the unique property of the coupled arrays that is the Coulomb blockade by electron-hole pairs, rather than the Coulomb blockade by electrons in other single-electron memory cells. The main feature, that the binding strength of the electron-hole pairs increases with the coupling strength between the two arrays, can lead to the enhanced stability of stored charges in the memory device.
Magneto-optical phenomena associated with cyclotron and magnetoplasma oscillations in a homogeneous two-dimensional electron system are theoretically studied. First, the electromagnetic wave (EW) polarization conversion under uniform cyclotron resonance (CR) conditions is discussed. A shift of the polarization conversion resonance with respect to conventional CR is calculated. This shift is more pronounced for a two-dimensional system with high electron density. Particular attention is given to the EW polarization conversion under non-uniform cyclotron and magnetoplasma resonances in two-dimensional electron plasma in the attenuated total reflection (ATR) arrangement. A crossover from magnetoplasma to a non-uniform cyclotron type of resonance, which takes place when the coupling between the ATR prism fields and the two-dimensional electron system grows, is studied. At any coupling strength, the polarization conversion at the resonance reaches 100% if the electron scattering in the two-dimensional system is neglected. The conclusion is drawn that the regime of non-uniform CR is more profitable from a practical point of view since the actual electron scattering in a two-dimensional system does not greatly deteriorate a high polarization conversion at resonance. The studied phenomena can be observed on gallium arsenide heterostructures at terahertz.
Illumination of a double p-Al0.5Ga0.5As/GaAs/Al0.5Ga0.5As heterostructure by a red light-emitting diode results in a negative photoconductivity that, after the diode is switched off, slowly relaxes to a positive persistent photoconductivity (PPPC), characterized by about a 1.5 times increase of two-dimensional hole concentration and a 1.7 times increase of mobility in comparison with the initial state in the dark. This metastable state may be explained in the framework of the model in which deep electron traps are supposed to be located above the Fermi level on the inverted heterointerface. The calculated concentration of interface charges reveals a 2.4 times decrease between the dark and PPPC states that is in qualitative agreement with the assumed model.
In this paper two ways of measuring current maps on gold micro-stripes on a nanometric scale are shown. In the first one, an alternating current is applied to the stripe and the thermal expansion of the tip-sample system is registered by the cantilever. In the second one, maps of adhesion of the surface are acquired simultaneously with the topography. The adhesion force map varies with the temperature because it depends mainly on the water meniscus between tip and sample, and the size of the meniscus depends on the temperature.
We demonstrate the nonlinear optical activity of large Na and K clusters, grown on Au metal film-supported alkane thiol monolayers of different chain lengths. Both the total optical second-harmonic signal intensity as well as its dependence on the angle of incidence depend on the surface coverage with the alkali metal, thus demonstrating the sensitivity of second-harmonic generation to morphological changes in these complex systems. Simultaneously performed extinction and scanning force microscopy measurements under ambient air conditions allow us to obtain independent information on the surface coverage and cluster morphology. The latter measurements also show that the optical activity of sodium clusters grown on alkane thiols at 150 K and annealed to 300 K is conserved on dosing with oxygen.