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We report a boron δ-doping layer in crystalline silicon with an electrically active concentration of 1×1022 cm−3 and a mobility of ∼20 cm2/V s. This structure was fabricated by low-temperature molecular-beam epitaxy with boron confined to 3 monolayers in the silicon growth direction. Complete electrical activation is observed, showing metallic conduction down to 4 K. This two-dimensional doped layer, incorporated into the crystal lattice, represents a volume concentration exceeding the solid solubility of boron in silicon by two orders of magnitude. These high-concentration structures fill an unexplored region of the mobility versus concentration curve.

We have performed three different optical experiments to determine the mean size of the bacterial strain Streptococcus mutans 10449, a microorganism with dimensions comparable to the wavelength of the light used in our experiments. The three optical measurements give size values which are consistent with one another and favorably comparable to the consistency we found in identical measurements on a test system of polystyrene spheres of dimensions similar to the bacteria. Homodyne time correlation and power spectrum spectroscopy both depend on the coherence and monochromaticity of laser light for the determination of the mean diffusion coefficient of the scatterers. The Stokes-Einstein equation then relates the diffusion constant to the cell size. Differential light scattering relies for the interpretation of experimental data on the classical theoretical analysis of the angular distribution of scattered light from a scattering object. The three methods yield mean values for the radius of S. mutans 10499 of 0.324 +/- 0.006 mum, 0.0325 +/- 0.007 mum, and 0.315 +/- 0.009 mum, respectively. However, since intensity correlation spectroscopy provides a direct measure of polydispersity, it would appear to be the preferred single measurement technique for size determination.

We have fabricated and characterized a novel structure: an ordered B monolayer deposited onto Si(100) and capped with epitaxial Si. X-ray structural data, compared to a first-principles model, sug-

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