Growth mechanism and clustering phenomena: The Ge-on-Si system

Nanometer Resolution Studies of Microstructural Evolution During the Heteroepitaxy of Ge on Vicinal Si(100)

We have studied the initial stages of island formation and coarsening for epitaxial Ge on vicinal Si (100) using in-situ deposition and nanometer resolution biassed secondary electron imaging (b-SEI) in a UHV-STEM. Ge is deposited using MBE techniques on nominally flat Si(100) substrates as well as those misoriented 1° and 5° toward <110>. The temporal evolution of the islanded microstructure can be studied by analysis of computer generated island size distributions. Good statistics can be obtained for islands with radii between 2nm and lOOnm using high resolution b-SE imaging and a large magnification range. Both MBE and Solid phase MBE (SP-MBE) processes have been studied.

We explain the evolution of the islanded microstructure in terms of competition for Ge adatoms among the various available sinks. For the MBE case, control of diffusion distances by varying the substrate temperature has allowed us to identify effects related to coherently strained and highly dislocated Ge islands as well as contaminant particles. In all cases, coherently strained Ge islands appear to be the weakest sinks and contaminant particles the strongest. Metastable growth of the intermediate layer during interrupted depositions at 375°C may be a direct consequence of an energy cost for incorporating adatoms into coherently strained islands. For depositions at higher temperatures, strong adatom sinks influence nucleation densities and size distributions of Ge islands by reducing the effective supersaturation. Island size distributions analyzed for the case of room temperature deposition in the early stages of coarsening also show evidence of effects due to coherently strained islands. These size distributions evolve from an initial distribution to one with increasing number of large islands while the distribution of the smaller islands (< 10nm radius) remains constant.

Cluster Size Distributions in Different Temperature Regimes: The System Ga on GaAs(001)

In this paper we discuss results for the clustering of Ga on GaAs(001). The dominant dynamic process which drives this system toward this three dimensional equilibrium changes from Ostwald ripening at low deposition rates to coalescence at higher deposition rates. The experimental data allow us to test several theoretical predictions for cluster size distributions based on a detailed study of the microscopic processes. These include, at higher deposition rates, the observation of local ripening effects and diffusion limited growth.

Mechanism of silver particle formation during photoreduction using in situ time-resolved SAXS analysis

Formation mechanisms of silver (Ag) particles in an aqueous ethanol solution of poly(N-vinyl-2-pyrrolidone) (PVP) by the photoreduction of AgClO(4) were investigated by means of in situ small-angle X-ray scattering (SAXS) measurements. The kinetics of association process (nucleation, growth, and coalescence) of Ag(0) atoms to produce Ag particles was successfully revealed by the quantitative SAXS analysis for the number-average of radius (R(0)), number of particles (n(Ag)), reduced standard deviation (σ(R)/R(0)), and volume fraction (ϕ(Ag)) of Ag particles produced by the photoreduction. The rate of nucleation and growth process during Ag particle formation strongly depend on the initial metal concentration. The time evolution of radius and number of Ag particles indicates that a mechanism of Ag particle formation is composed of different three processes, that is, reduction-nucleation, Ostwald ripening, and particle coalescence. In a rapid reduction-nucleation process, small nuclei or particles (average radius ∼2.5 nm) are produced by an autocatalytic reduction. After the formation of small nuclei or particles proceeds, Ostwald ripening and particle coalescence, predicted by the Lifshitz-Slyozov-Wagner theory (LSW theory), subsequently occur, resulting in the particle growth (average radius ∼11.5 nm).

Thermodynamics of the size and shape of nanocrystals: epitaxial Ge on Si(001)

The growth and evolution of strained epitaxial Ge on a Si(001) surface provides a rich system for exploring the behavior of strongly interacting nanocrystals. In the temperature regime above 500 degrees C, there are two different (metastable) shapes of defect-free nanocrystals, termed pyramids and domes, that dominate the system depending on the temperature of the substrate during growth and the amount of Ge deposited. In contrast to the usual case considered in nucleation theory, the relaxation of the strain energy at the surface of the nanocrystals makes those surfaces stabilizing, i.e. the surface contribution to the free energy of the Ge nanocrystals is negative. Given that the edges of the nanocrystals are destabilizing (positive free energy), the interaction of the surfaces and edges of the nanocrystals in an ensemble renders an internal free energy for the system that has a local minimum with respect to the size (volume) of the nanocrystal. At finite temperatures, this free energy yields a size distribution with a characteristic centroid, width, and skewness for each nanocrystal shape. The smaller pyramids transform into domes when they grow to the point where they can surmount a kinetic energy barrier between the two structures. However, the Ge nanocrystals also effectively repel one another strongly via the strain fields that are produced in the Si substrate. This repulsive interaction makes the ensemble of Ge nanocrystals a highly nonideal thermodynamic system and, in turn, makes the free energies of the nanocrystals a function of their number density, or equivalently a function of the amount of Ge deposited. The interplay of the stabilizing effect of the nanocrystal surfaces and the destabilizing influence of their repulsive interactions yields a complex behavior for the nanocrystal-size distributions that can nonetheless be modeled using simple thermodynamic expressions.

Transition States Between Pyramids and Domes During Ge/Si Island Growth

Real-time observations were made of the shape change from pyramids to domes during the growth of germanium-silicon islands on silicon (001). Small islands are pyramidal in shape, whereas larger islands are dome-shaped. During growth, the transition from pyramids to domes occurs through a series of asymmetric transition states with increasing numbers of highly inclined facets. Postgrowth annealing of pyramids results in a similar shape change process. The transition shapes are temperature dependent and transform reversibly to the final dome shape during cooling. These results are consistent with an anomalous coarsening model for island growth.

In situ transmission electron microscopy observations of the formation of self-assembled Ge islands on Si

The in situ transmission electron microscope allows us to visualise processes occurring at surfaces and interfaces in real time and is therefore capable of providing detailed, quantitative information about reaction mechanisms. We have used a UHV TEM equipped with in situ growth capabilities to study the process of chemical vapour deposition of Ge on Si(100), with particular emphasis on the formation of self-assembled, nanosize Ge islands. Video-rate image acquisition enables us to track the development of individual islands from nucleation onwards and to observe the introduction of dislocations as the strained islands relax. For islands less than 80 nm in diameter, which are coherently strained, we observe an interesting coarsening process during growth. This coarsening results in a bimodal distribution of island sizes at certain times and a narrow size distribution at later times. We explain the phenomenon by a model in which coarsening occurs among a population of islands for which the equilibrium island shape depends on the size. Numerical simulations of coarsening in the presence of a shape transition are in good agreement with experiment. As the islands grow larger, dislocations form and we observe rapid shape changes associated with dislocation introduction. These changes can also be understood by considering a strain-dependent island shape. The insight that these results provide into the understanding of island growth and evolution can be used to develop arrays of uniformly sized islands ("quantum dots") for a variety of potential applications.