J. A. Venables

Nucleation and growth of thin films

A review is given of the nucleation and growth processes occurring in thin film formation. Emphasis is given to quantitative nucleation theories and to the role of electron microscopy and surface techniques in providing data to test such theories. The relations between the thermodynamics of adsorption and the kinetics of crystal growth is stressed. Experimental examples are taken from the island growth, layer plus island (or Stranski-Krastanov) and layer growth modes. The shapes of growing crystallites are briefly discussed.

Rate equation approaches to thin film nucleation kinetics

Abstract The nucleation and growth of crystals on a substrate are discussed in terms of rate equations for the atom cluster concentrations as a function of time. Simple approximations allow this general set of equations to be reduced to three coupled equations. Many physical processes can be incorporated into these rate equations, including coalescence of clusters, and cluster mobility. The problem of increasing correlation between single atoms and stable clusters as growth proceeds is studied. It is shown that the problem can be solved self-consistently using an auxiliary diffusion equation and that approximations may be obtained which give upper and lower bounds for the cluster growth rates. These diffusion equations also give expressions which enable the cluster-cluster correlations and cluster size distributions to be discussed. With these approximations, expressions are derived for observable quantities and the expressions are compared with one experimental example. In this case, that of gold on alka...

Electron back-scattering patterns—A new technique for obtaining crystallographic information in the scanning electron microscope

Abstract It is shown that the angular distribution of back-scattered electrons can be observed in a scanning electron microscope, and that the patterns observed can be used to obtain crystallographic information about the specimen. The patterns are termed electron back-scattering patterns (E.B.S.P.). The use of these patterns as a crystallographic techniques is shown to have several significant advantages over two other techniques currently in use in scanning electron microscopes.

Atomic processes in crystal growth

Abstract The thermodynamic and kinetic processes which are involved in the early stages of crystal growth are discussed, with especial reference to vapor deposition of thin films. The atomic processes taking place during deposition are described in terms of rate and diffusion equations; the concept of “competitive capture” is outlined, where adatoms are forced to choose between competing sinks. The use of microscopy and surface physics techniques to study nucleation in films is emphasised. Examples of island (Volmer-Weber), layer (Frank-van der Merwe) and layer plus island (Stranski-Krastanov) growth in metal/insulator, metal/semi-conductor and semiconductor/semiconductor deposition systems are given.

Microstructural evolution during the heteroepitaxy of Ge on vicinal Si(100)

Microstructural evolution during the initial stages of islanding of Ge on vicinal Si(100) has been studied in situ with nanometer resolution in an ultrahigh‐vacuum scanning transmission electron microscope. Ge is deposited using molecular‐beam‐epitaxy (MBE) techniques on vicinal Si(100) misoriented 1° and 5° toward 〈110〉. For MBE‐type experiments, there is evidence for metastable growth of the Ge intermediate layer to much greater than the equilibrium critical thickness. The layer may grow up to seven monolayers thick before islanding in the Stranski–Krastanov growth mode. The presence of strong adatom sinks significantly alters the growth and size distribution of the islands when the spacing of these sinks is less than an adatom diffusion distance. Studies of the initial stages of islanding in solid‐phase MBE indicate that there is no long‐range adatom diffusion. There is an initial fast transformation from a disordered layer growth, followed by a sluggish growth of islands. We have studied the coarsenin...

Direct observation of the nucleation and growth modes of Ag/Si(111)

Abstract Direct observation in an ultrahigh vacuum scanning electron microscope (SEM) has revealed the growth modes of Ag on Si(111), previously deduced indirectly from analysis of Auger electron spectra, and the form and number of crystallites as a function of deposition time and temperature. At high temperatures, T s > 200°C, the silver grows definitively in a Stranski-Krastanov (layer plus island) growth mode, with a very strong dependence of the island density on the deposition temperature, varying from ≲ l0 6 to ∼ 10 10 cm −2 between T s = 500 and 200°C. The crystals have more or less regular hexagonal forms with (111) and {1 1 1} faces predominating with a height to width ratio which decreases with deposition time in the range 0.1–0.6. At temperatures below 200°C, the island density is too high to be observed directly by SEM, and silver appears to grow in a Frank-van der Merwe (layer by layer) mode. However, this uniform deposit is unstable on heating above 200°C, and annealed deposits recreate rather similar islands. An analysis of published Auger amplitude-time curves is made to estimate the initial island density produced by depositions below T s = 200° C, and it is shown that these densities constitute a very reasonable extrapolation of the higher temperature SEM values. A model of Stranski-Krastanov growth is given in outline and applied to Ag/Si(111). It is argued that the island density is determined largely by island instability at high temperatures, even though condensation is complete. At low temperatures it is argued that Stranski-Krastanov growth becomes essentially equivalent to Frank-van der Merwe growth for high enough island density, for kinetic rather than thermodynamic reasons, without any change in the basic Stranski-Krastanov mechanism.

Nucleation, growth and the intermediate layer in Ag/Si(100) and Ag/Si(111)

Abstract The growth of Ag on Si(100)2 × 1 and Si (111)7 × 7 surfaces has been examined as a function of deposition time and substrate temperature, using an ultra high vacuum-scanning electron microscope (UHV-SEM), equipped with micro-Auger electron spectroscopy and RHEED. Ag grows on both Si surface in the layer plus island or Stranski-Krastanov growth mode. The thickness of the two-dimensional layer has been measured as 0.27 ± 0.03 monolayer on Si(100) in comparison with 0.63 ± 0.04 monolayer on Si(111). The Ag island shapes and sizes, and density dependencies N ( T ) on substrate temperature were found to be very different in the two orientations. The N ( T ) data are interpreted in terms of nucleation theory, and values for the critical nucleus sizes, binding, diffusion and adsorption energies are estimated. Biassing the Si samples made it possible to visualize the intermediate Ag layer by SEM, and examples for the two systems are shown.

High spatial resolution surface potential measurements using secondary electrons

Abstract High spatial resolution surface potential and work function measurements have been made by observing the onset energy of secondary electron emission using a band-pass analyser (CMA) in an ultra high vacuum scanning electron microscope (UHV-SEM). Multiple secondary electron spectra can be displayed for closely spaced positions on the sample. Alternatively a feedback loop is used to maintain a constant surface potential as the SEM beam is scanned across the sample. Both methods have been shown to yield linear measures of surface potential, and of work function differences, with a sensitivity of

Accurate microcrystallography using electron back-scattering patterns

Abstract A method for precise determination of the orientation of mierocrystals is described. Electron back-scattering patterns are observed on a fluorescent screen in a scanning electron microscope, and high precision is obtained by casting geo metrical shadows onto the screen to determine the point from which the back-scattered electrons are emitted. Orientational accuracy around ±0.5° can be routinely obtained. The geometrical constructions used for evaluating the crystal orientation, and the errors in this orientation, are described. Some experimental examples are given.

Structural studies of xenon and krypton solid monolayers on graphite using transmission electron diffraction

Abstract Lattice parameter, epitaxial orientation and diffraction intensity measurements, obtained by transmission high energy electron diffraction (THEED), are reported for Xe and Kr monolayers on graphite. The results span the gas-solid and the commensurate solid-incommensurate solid (C-I) transitions in both cases, in the pressure range 10 −6 –10 −8 Torr, well below the two-dimensional triple points. The lattice parameter results are interpreted in terms of a misfit dislocation model of the (C-I) transition, and the intensity results are used to discuss the defect structure of the adsorbed solid monolayers. The onset of an incommensurate rotated phase is observed for Kr but not for Xe.