L. V. Sokolov

Large–scale geographical variation confirms that climate change causes birds to lay earlier

Advances in the phenology of organisms are often attributed to climate change, but alternatively, may reflect a publication bias towards advances and may be caused by environmental factors unrelated to climate change. Both factors are investigated using the breeding dates of 25 long–term studied populations of Ficedula flycatchers across Europe. Trends in spring temperature varied markedly between study sites, and across populations the advancement of laying date was stronger in areas where the spring temperatures increased more, giving support to the theory that climate change causally affects breeding date advancement.

Ge-on-Si films obtained by epitaxial growing: edge dislocations and their participation in plastic relaxation

Pure edge 90° misfit dislocations (MDs) are the most effective linear defects that combine the substrate and the film with different lattice parameters. A system consisting of a nonstressed film and a substrate approaches the perfect case in terms of the structural transition from one lattice parameter to the other if imperfections in the form of an ordered network of edge MDs are located exclusively at the interface, while threading dislocations are practically absent. The path to this perfect case goes through studying the possibilities of creating such an ordered network of edge MDs. The mechanism of formation of edge MDs proposed previously by Kvam et al (1990 J. Mater. Res. 5 1900) is discussed. This mechanism involves induced formation of a complementary pair of 60° MDs whose coalescence at the interface creates an edge MD. Some publications are presented, which demonstrate on the basis of experimental data that this mechanism under certain conditions can be the basic mechanism responsible for plastic relaxation of Ge-on-Si films. A cardinal method for decreasing the number of defects at the initial stages of growth of Ge/Si heterosystems is a set of procedures that allow a specified number of MDs to be inserted into the stressed film earlier than conditions of spontaneous nucleation of MDs from the film surface in the 2D–3D transition occur. When the low-temperature/high-temperature strategy of growth is used, the low-temperature GeSi seed layer tuned with respect to the growth temperature, composition and thickness can serve as a source of 60° dislocations, which facilitate earlier formation of edge MDs at the initial stage of plastic relaxation of the GeSi or Ge main layer. Results of some recent publications that report reaching high structural perfection of thin (∼1 µm and less) Ge-on-Si films are discussed. The proposed explanation of these results is based on postulates of controlled insertion of MDs and formation of edge MDs by the model of induced nucleation.

Surface processes and phase diagrams in MBE growth of SiGe heterostuctures

Abstract The recent results of development and application of MBE methods for direct growth (self-organization or spontaneous formation) of the heterostructures Ge x Si 1−x Si are discussed. The heterostructures involve nanoobjects confined in two- or three-dimensional quantum-sized boxes or wires. The influence of substrate temperature, film composition and thickness on the structure and the surface morphology of the growing film were studied. Particular attention was paid to the surface structure evolution of Ge x Si 1− x heteroepitaxial films growing on Si(111)7 × 7 and Si(001)2 × 1 substrates over a wide range of concentrations x and growth temperatures. The quantum-sized islands can be self-organized after achievement of the critical thickness of the pseudomorphic layer; the critical thickness can be calculated in terms of the Frank-van der Merwe theory (when the Stranski-Krastanov growth mechanism is achieved). It was shown that with respect to the electronic properties, such films demonstrate the behavior of quantum boxes in which a zero-dimensional electron (hole) gas is localized. A number of heteroepitaxial structures with Ge quantum boxes in a tunnel-thin Si layers are grown by MBE and investigated.

GeSi films with reduced dislocation density grown by molecular-beam epitaxy on compliant substrates based on porous silicon

To grow high-quality heteroepitaxial layers, we propose a compliant silicon substrate consisting of a thin epitaxial silicon film on a high-density porous layer as a membrane and an expansive low-density porous layer as a mechanical damper which shields the overlying layers from the massive wafer. GeSi films over the critical thickness have been grown by molecular-beam epitaxy on these substrates. Transmission electron microscopy analysis shows that Ge0.2Si0.8 films have no dislocations owing to just elastic strain relaxation whereas plastic flow in the pseudomorphic films that are being grown on conventional Si substrates occurs with generation of dislocations in a regular manner. The experimental data on porous silicon structure are presented in some detail and are briefly discussed in connection with substrate compliance.

Plastic relaxation of solid GeSi solutions grown by molecular-beam epitaxy on the low temperature Si(100) buffer layer

The role of a low temperature Si buffer layer (LT-Si) in the process of plastic relaxation of molecular-beam epitaxy grown GeSi/Si(001) is studied. Probable sources and mechanisms of generation of misfit dislocations (MD) are discussed. Transmission electron microscopic and x-ray diffraction techniques are used for studying 100 nm GexSi1−x films with LT-Si and those free of such a buffer layer. The MD density is found to be much lower in the former than in the latter, and the level of the film plastic relaxation is not higher than 20% in both as-grown and annealed films with LT-Si. As the thickness of the solid solution layer reaches 300-400 nm, the plastic relaxation of the films increases to almost 100%. Therefore, the determining role of the MD multiplication is supposed. We assume the double role of the LT-Si buffer layer. First, the diffusion flux of vacancies from the LT-Si layer to the GeSi/Si interface may cause erosion of the interface and, as a result, a decrease in the rate of MD generation at ...

Direct observations of dislocation half-loops inserted from the surface of the gesi heteroepitaxial film

The initial stage of relaxation of mechanical stresses in the Ge0.32Si0.68∕Si(001) heterostructure grown by low-temperature (300°C) molecular-beam epitaxy is studied by means of transmission electron microscopy. Dislocation half-loops propagating from the film surface and generating misfit dislocations during expansion are visualized.

Features of formation and propagation of 60° and 90° misfit dislocations in GexSi1-x/Si (x∼0.4-0.5) films caused by Si substrate misorientation from (001)

We have studied the dislocation structure at the initial stage of relaxation of GexSi1−x films (x∼0.4–0.5) grown on Si substrates tilted 6° about the ⟨011⟩ axis. It is demonstrated that edge misfit dislocations (MDs) in the miscut direction arise in the form of short segments on intersections of 60° MDs. Substrate misorientation from the singular plane made it possible to discover the MD configurations consisting of a short segment of an edge MD and only two 60° MDs diverging from this segment in the miscut direction.

Mechanisms of edge-dislocation formation in strained films of zinc blende and diamond cubic semiconductors epitaxially grown on (001)-oriented substrates

Ninety degree edge misfit dislocations (MDs) are «sessile» dislocations; such dislocations, however, were found in large amounts in relaxed films. The commonly accepted formation mechanism of such dislocations is an interaction of two complementary 60° dislocations with appropriate Burger’s vectors, for example: a/2[101−] + a/2 [011] = a/2 [110]. In the present study, four possible types of interaction were analyzed: (i) random meeting of two complementary MDs; (ii) crossing of two complementary 60° MDs in the vicinity of film-substrate interface in systems grown on substrates misoriented from exact (001) orientation; (iii) formation of edge MDs during cross-slipping of a secondary MD; and (iv) induced nucleation of a secondary complementary 60° MD. Examples of discussed interactions are given. Contrary to the widespread opinion that edge MDs in GeSi and InGaAs films grown by MBE on Si and GaAs substrates predominantly form under elastic strains greater than 2% and at the final stage of plastic relaxation...

Solid solutions GeSi grown by MBE on a low temperature Si (001) buffer layer: specific features of plastic relaxation

Abstract The role of a low temperature Si buffer layer (LT-Si) in the process of plastic relaxation of MBE grown GeSi/Si (001) is studied. Probable sources and mechanisms of a generation of misfit dislocations (MD) are discussed. Transmission electron microscopic and X-ray diffraction techniques are used for studying 100 nm GexSi1−x films with LT-Si and those free of such a buffer layer. The MD density is found to be much lower in the former than in the latter and the level of the film plastic relaxation is not higher than 20% in both as-grown and annealed films with LT-Si. As the thickness of the solid solution layer reaches 400 nm, the plastic relaxation of the films increases to almost 100%. Therefore, the determining role of the MD multiplication is supposed. We assume the double role of the LT-Si buffer layer. Firstly, the diffusion flux of vacancies from the LT-Si layer to the GeSi/Si interface may cause erosion of the interface and as a result a decrease in the rate of MD generation at the early stages of epitaxy. Secondly, the generation of intrinsic defect clusters in the LT-Si, which are potential sources of MDs, occurs in the field of mechanical stresses of the growing pseudomorphic layer. This process is thought to be the key feature of plastic relaxation of GeSi/LT-Si/Si (100) films which promotes MD self-organization.

Potentialities and basic principles of controlling the plastic relaxation of GeSi/Si and Ge/Si films with stepwise variation in the composition

GeSi/Si heterostructures consisting of a plastically relaxed layer that includes various fractions of Ge and which is grown on Si (001) span the values of the lattice parameter from equal to that in silicon to equal to that in germanium. The corresponding substrates are conventionally referred to as artificial. A number of methods exist for growing high-quality GeSi layers with as large as 100% of Ge on Si (001) substrates through an intermediate GeSi layer with a varying composition. However, it is desirable in a number of cases to have ultrathin (<1 μm) GeSi and Ge layers directly on the Si (001) substrate for practical applications. The results of new methods such as the use of a buffer Si layer grown at a comparatively low temperature (300–400°C) in plastic relaxation of the GeSi/Si(001) heterostructures and also the use of surfactants (antimony and hydrogen) are analyzed. The examples of artificial introduction of centers for origination of misfit dislocations as an alternative to their introduction from the rough surface are considered. It can be concluded that, in order to expand the range of potentialities of growing perfect plastically relaxed GeSi (001) films, it is necessary to (i) make it possible to form in a controlled manner the centers for origination of the misfit dislocations and (ii) retard or completely suppress the transition of the growth mechanism from two-to three-dimensional in order to prevent the formation of additional misfit dislocations from the surface of the stressed film and, correspondingly, additional threading dislocations.