The wealth of information existing on the general principle of S-layers has revealed a broad application potential. The most relevant features exploited in applied S-layer research are: (i) pores passing through S-layers show identical size and morphology and are in the range of ultrafiltration membranes; (ii) functional groups on the surface and in the pores are aligned in well-defined positions and orientations and accessible for binding functional molecules in very precise fashion; (iii) isolated S-layer subunits from many organisms are capable of recrystallizing as closed monolayers onto solid supports at the air-water interface, on lipid monolayers or onto the surface of liposomes. Particularly their repetitive physicochemical properties down to the subnanometer scale make S-layers unique structures for functionalization of surfaces and interfaces down to the ultimate resolution limit. The following review focuses on selected applications in biotechnology, diagnostics, vaccine development, biomimetic membranes, supramolecular engineering and nanotechnology. Despite progress in the characterization of S-layers and the exploitation of S-layers for the applications described in this chapter, it is clear that the field lags behind others (e.g. enzyme engineering) in applying recent advances in protein engineering. Genetic modification and targeted chemical modification would allow several possibilities including the manipulation of pore permeation properties, the introduction of switches to open and close the pores, and the covalent attachment to surfaces or other macromolecules through defined sites on the S-layer protein. The application of protein engineering to S-layers will require the development of straightforward expression systems, the development of simple assays for assembly and function that are suitable for the rapid screening of numerous mutants and the acquisition of structural information at atomic resolution. Attention should be given to these areas in the coming years.

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Fabricating device structures from the III-N semiconductors requires dry-etching processes that leave smooth surfaces with stoichiometric composition after transferring patterns with vertical sidewalls. Results obtained by standard methods are summarized, and the extent of concomitant ionbombardment damage is assessed. A new low-damage technique-law-energy electron-enhanced etching-that avoids ion bombardment altogether is described, and early results for III-N materials are summarized. Etching issues critical in forming contacts and fabricating laser facets and mirrors are highlighted, and some prospects for future work are also identified.

Low energy electron-enhanced etching of Si(100) has been achieved by placing the sample on the anode of a dc discharge in hydrogen/helium mixtures. Over a broad range of gas composition, gas pressure, and discharge current, nonpatterned samples gave etch yields of 0.01–0.02 atoms/electron, and average etch rates of 2000–3000 Å/min. Postetch examination by atomic force microscopy revealed surface roughness of 2–3 nm. These results are related to incident flux of H atoms and electrons through a simple model of the anode sheath layer above the sample.

Low energy electron-enhanced etching of GaAs(100) has been achieved by placing the sample on the anode of a low-pressure hydrogen/chlorine dc discharge. Samples etched at room temperature reveal good anisotropy (≳20), good selectivity (≳200 against SiO2 masks at room temperature), and smooth surfaces at etch rates of 250 Å/min; etch rates up to 4.5 μm/min were achieved at 150 °C. The dependence of the etch characteristics on gas composition, pressure, and temperature is described.