Yu. N. Drozdov

Using laser sputtering to obtain semiconductor nanoheterostructures

Laser sputtering of solid-state targets in a hydrogen atmosphere has been used to form semiconductor nanoheterostructures. Impurities (Te or Mn) in the form of delta-doped layers obtained by laser sputtering of the corresponding targets in the process of vapor-phase epitaxy using organometallic compounds can be introduced into light-emitting structures based on the InGaAs/GaAs system to make it possible to control the spectrum and the electroluminescence intensity. Reducing the hydrogen pressure in the reactor to 25-50Torr allows laser deposition to be carried out at reduced temperatures of the epitaxial layers of the base material and makes it possible to obtain GaAs and InAs semiconductors with a high manganese-doping level that demonstrate ferromagnetic properties at room temperature.

The special features of the Hall effect in GaMnSb layers deposited from a laser plasma

Epitaxial GaMnSb films with Mn contents up to about 10 at. % were obtained by deposition from a laser plasma in vacuum. The growth temperature Ts during deposition was varied from 440 to 200°C, which changed the concentration of holes from 3 × 1019 to 5 × 1020 cm−3, respectively. Structure studies showed that, apart from Mn ions substituting Ga, the GaMnSb layers contained ferromagnetic clusters with Mn and shallow acceptor defects of the GaSb type controlled by the Ts value. Unlike single-phase GaMnSb systems studied earlier with negative anomalous Hall effect values and Curie temperatures not exceeding 30 K, the films obtained in this work exhibited a positive anomalous Hall effect, whose hysteresis character manifested itself up to room temperature and was the more substantial the higher the concentration of holes. The unusual behavior of this effect was interpreted in terms of the interaction of charge carriers with ferromagnetic clusters, which was to a substantial extent determined by the presence of Schottky barriers at the boundary between the clusters and the semiconducting matrix; this interaction increased as the concentration of holes grew. The absence of this effect in semiconducting compounds based on III–V Group elements with MnSb or MnAs ferromagnetic clusters was discussed in the literature; we showed that this absence was most likely related to the low hole concentrations in these objects.

Critical thickness for the Stranski-Krastanov transition treated with the effect of segregation

A new estimation of the critical thickness for the 2D–3D transition during epitaxial growth of the Ge x Si1 − x /Si(001) and In x Ga1 − x As/GaAs(001) systems is suggested. In the estimation, the segregation of atoms in the process of growth is taken into account. For the criterion of such a Stranski-Krastanov transition, the balance between the gain in the energy of elastic strains in the system due to relaxation of the island and the loss in surface energy because of the increase in the surface area is used. In contrast to calculations for previously known criteria, the energy of elastic strains is calculated taking into consideration all deposited layers rather than only one upper layer. The segregation is described in the model of thermally activated exchange of atoms between the surface and the upper layer. A comparison of the critical thickness, calculated for different compositions of the deposited alloy at different temperatures and rates of growth, with the experimental data shows rather good agreement for both systems. The transition mechanisms corresponding to the criterion suggested in the study are discussed.

Features of two-dimensional to three-dimensional growth mode transition of Ge in SiGe/Si(001) heterostructures with strained layers

The results of the study of the effect of strained SiGe layers on the critical thickness of two-dimensional growth of Ge layer in different SiGe/Si(001) structures are presented. A significant influence of buried strained SiGe layer on the growth of Ge has been found out, which remains considerable even for SiGe layers capped by unstrained Si layer of thickness up to 3.5 nm. The experimental results are well described by the proposed model, where obtained features are explained by means of introducing a phenomenological parameter called “effective decay length” of the strain energy accumulated in the structure.

Segregation of indium in InGaAs/GaAs quantum wells grown by vapor-phase epitaxy

Distribution of indium atoms in structures which contained double InGaAs/GaAs quantum wells and were grown by vapor-phase epitaxy from metal-organic compounds was studied. Experimental indium-concentration profiles were obtained using Auger electron spectroscopy. A model of growth with allowance made for indium segregation and a model for the Auger profiling were used in the calculations of profiles. Fitting calculated profiles to experimental ones made it possible to estimate the activation energies for In-Ga exchange in the context of a kinetic model for segregation. These energies are found to be somewhat higher than those that are well known for molecular-beam epitaxy, which is related to stabilization of the growth surface by hydrogen atoms in a vapor-phase reactor.

Photoluminescence of Ge(Si) self-assembled islands embedded in a tensile-strained Si layer

We report photoluminescence (PL) studies of Ge(Si) self-assembled islands embedded into a tensile-strained Si layer grown on smooth relaxed Si0.75Ge0.25∕Si(001) buffer layers subjected to chemical-mechanical polishing. The intense PL from Ge(Si) islands embedded into a strained Si layer compared to the PL from islands grown on unstrained Si(001) is associated with efficient confinement of electrons in a strained Si layer on the heterojunction with islands. The observed dependence of the island PL peak position on thickness of strained Si layer confirms the validity of the model for real-space indirect optical transition between electrons confined in the strained Si layer, and holes localized in islands.

Evidences for direct magnetic patterning via diffusive transformations using femtosecond laser interferometry

The application of femtosecond laser interferometry to direct patterning of thin-film magnetic alloys is demonstrated. The formation of stripe gratings with submicron periodicities is achieved in Fe1−xVx (x = 18–34 wt. %) layers, with a difference in magnetic moments up to Δμ/μ ∼ 20 between adjacent stripes but without any significant development of the topographical relief (<1% of the film thickness). The produced gratings exhibit a robust effect of their anisotropy shape on magnetization curves in the film plane. The obtained data witness ultrafast diffusive transformations associated with the process of spinodal decomposition and demonstrate an opportunity for producing magnetic nanostructures with engineered properties upon this basis.

Strain-driven alloying: effect on sizes, shape and photoluminescence of GeSi/Si(001) self-assembled islands

The effect of strain-driven alloying on sizes, shape and the photoluminescence (PL) properties of Ge(Si)/Si(001) self-assembled islands was investigated for temperatures of Ge deposition above 550 °C using atomic force microscopy (AFM), X-ray analysis, Raman spectroscopy. We found out that strain-driven Si diffusion into Ge/Si(001) self-assembled islands causes formation of an alloy in islands at these growth temperatures. An increase in the Ge content in the islands with a lower growth temperature results in a decrease of the pyramid-islands volume at which they transform to dome-islands. The energy of the optical transition in the free-standing islands was calculated using the observed values of the islands composition and elastic strain. The discrepancy between the calculated energy and the data observed from photoluminescence spectra is related to the changes of composition and height of the islands during Si overgrowth.

Low-energy photoluminescence of structures with GeSi/Si(001) self-assembled nanoislands

The photoluminescence spectra of structures with self-assembled GeSi/Si(001) islands are investigated as functions of the growth temperature. It is shown that the shift of the peak of photoluminescence from islands toward lower energies on decreasing the growth temperature is due to the suppression of Si diffusion into islands and an increase in the fraction of Ge in islands. A photoluminescence signal from the GeSi islands is found in the region of energies down to 0.6 eV, which is considerably smaller than the band-gap width in bulk Ge. The position of the peak of photoluminescence from islands is described well by the model of a real-space indirect optical transition with account of the real composition and elastic strains of the islands. Mono-and multilayer structures are obtained with self-assembled GeSi/Si(001) nanoislands exhibiting a photoluminescence signal in the region 1.3–2 μm at room temperature.

The elastic strain and composition of self-assembled GeSi islands on Si(001)

The paper presents the results of investigation of self-assembled GeSi islands growth on Si(001) at 700°C and the evolution of Ge islands parameters during annealing. The islands with the narrow (standerd deviation ∼6%) lateral size and height distributions were grown. Dissolution of Si in islands was revealed from the Raman scattering and the X-ray diffraction measurements. Both the alloy composition and the elastic strain in the islands were determined. It was found that the content of Si in islands increased during annealing. This increase was shown to result in changes of the shape and sizes of islands.