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Ge/Si(001) layers are grown by gas-source molecular beam epitaxy at 600 °C to probe island self-ordering phenomena. We vary the Ge growth rate by a factor of 40, 1.2-47 monolayers (ML) min^-1, and adjust the Ge coverage, 5.9-8.9 ML, to produce films consisting primarily of dome-shaped Ge islands. Measurements of the radial and nearest-neighbor distributions are compared to calculated distributions for random arrangements of circular islands. At low growth rates, island formation is inhibited at small separation. At high growth rates, the angular distributions of nearest-neighbor islands show pronounced island ordering along <100> directions.

A geometrical model of a porous silicon structure is proposed. The resulting size distribution spectra are analyzed by their relation to photoluminescence and Raman scattering. Both experimental data are investigated and compared to those results. The model well describes the presence of a low-energy part of the photoluminescence spectra with wavelength compatible to bulk crystalline silicon. Shapes and positions of Raman and photoluminescence lines are within the frames of model flexibility and indicate similar values of parameters, especially the minimal size of crystallites where Raman scattering and radiative recombination reveal the activity.

Metallic surface states on semiconducting substrates provide an opportunity to study low-dimensional electrons decoupled from the bulk. Angle resolved photoemission is used to determine the Fermi surface, group velocity, and effective mass for surface states on Si(111)√3×√3-Ag, Si(111)×√3-√3-Au, and Si(111)√21×√21-(Ag + Au). For Si(111)√3×√3-Ag the Fermi surface consists of small electron pockets populated by electrons from a few % excess Ag. For Si(111)√21×√21-(Ag + Au) the pockets increase their size corresponding to a filling by three electrons per unit cell. The √21×√21 superlattice leads to an intricate surface umklapp pattern and to minigaps of 110 meV, giving an interaction potential of 55 meV for the √21×√21 superlattice

We have extended the modified formalism of Sheng, Xing, and Wang [J. Phys.: Condens. Matter 11 L299 (1999)] to allow the calculation of the conductivity of a thin metallic film bounded by a rough fractal surface. We utilized the so-called k-correlation model proposed by Palasantzas and Barnas [Phys. Rev. B 48, 14 472 (1993); 56, 7726 (1997)], to describe the height-height autocorrelation function corresponding to a self-affine roughness. This extension permits the calculation of the conductivity of the film as a function of the r.m.s. roughness amplitude δ, of the lateral correlation length ξ, of the mean free path in the bulk l, and of the roughness exponent H. We found that the degree of surface irregularity, represented by the roughness exponent H characterizing the surface, does influence the conductivity of the film, as first discovered by Palasantzas and Barnas. However, this influence manifests itself for large bulk mean free paths l〜1000 nm and for large correlation lengths ξ〜5 nm, in which case the conductivity of the film for H = 1 exceeds by about 30% the conductivity for H = 0.2, an effect which is smaller than that reported by Palasantzas and Barnas. For correlation lengths ξ below 1 nm and mean free paths l〜100 nm, the influence of the roughness exponent H on the conductivity is reduced to below 10%, and for smaller mean free paths and correlation lengths the conductivity becomes insensitive to H. We also found that Mathiessen's rule is severily violated in the case of thin metallic films. The resistivity of the film coincides roughly with the surface-limited resistivity only in the case of ultrathin films t<5 nm. For thicker films 100 nm>t>5 nm, the resistivity of the film exceeds by some 20 to 30 % the value dictated by Mathiessen's rule. And conversely, the apparent surface-induced resistivity estimated assuming the validity of Mathiessen's rule, exceeds by nearly one order of magnitude the true surface-induced resistivity, except in the case of ultrathin films t<5 nm.

The reflectivity of short pitch metal gratings consisting of a series of narrow Gaussian ridges in the classical mount has been modeled as a function of frequency and in-plane wave vector (the plane of incidence containing the grating vector) for various ridge heights. Surface plasmon polaritons (SPP's) are found to be excited even in the zero-order region of the spectrum. These may result in strong absorption of radiation polarized with its electric field in the plane of incidence (transverse magnetic). For zero in-plane wave vector the SPP modes consist of a symmetric charge distribution on either side of the grating ridges, a family of these modes existing with different numbers of field maxima per grating period. Because of the charge symmetry these modes may only be coupled to at angles away from normal incidence where strong resonant absorption may then occur. The dispersion of these SPP modes as a function of the in-plane wave vector is found to be complex arising from the formation of very large band gaps due to the harmonic content of the grating profile, the creation of a pseudo high-energy mode, and through strong interactions between different SPP bands.

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