Highly regular arrays of steps are produced on vicinal Si͑111͒7ϫ7. The step edges are atomically straight for up to 2ϫ10 4 lattice sites. The terraces are single domain, which produces a minimum kink width of 2.3 nm ͑half a 7ϫ7 unit cell͒ and thus a high barrier for creating kinks. Criteria for obtaining optimum step arrays are established, such as the miscut ͓Ϸ1°towards ͑112͔͒ and an annealing sequence which passes through step bunching regions quickly.
Highly regular arrays of steps are produced on vicinal Si(111)7ϫ7 surfaces. A tilt of the surface normal from ͑111͒ toward ͑1 1 2͒ produces single steps ͑0.3 nm high and typically 15 nm apart͒. The opposite tilt toward ͑1 1 2͒ produces bunched steps with adjustable height ͑1-5 nm͒ and a spacing of 70 nm. Preparation criteria for straight edges and regular spacings are determined, taking into account the miscut angle ͑azimuthal and polar͒, annealing sequence, current direction, and applied stress.
Nanostructures of CaF 2 and CaF 1 on Si͑111͒ are used to demonstrate a chemical imaging method for insulators. Chemical sensitivity is achieved in scanning tunneling microscopy via a sharp drop of the tunneling current for bias voltages below the conduction-band minimum. This imaging method has a spatial resolution of better than 1 nm and distinguishes different oxidation states. A resonance is found in (dI/dV)/(I/V) at the conduction-band minimum that enables an accurate determination of its position. We observe enhancements by up to a factor of 5 and absolute values in the range of 20-50, compared to 1 for an Ohmic metal. A minimal model is given, explaining the resonance in terms of tunneling across a thin insulator film. These methods should be generally applicable for determining local Schottky barriers and band offsets in nanostructures and for chemically selective imaging of insulators and wide-gap semiconductors. ͓S0163-1829͑99͒07215-X͔ I. CHEMICAL IMAGING IN SCANNING TUNNELING MICROSCOPY "STM…
Linear arrays of CaF2 stripes and dots, about 7 nm wide, are fabricated by self-assembly on stepped Si(111). Stripes are grown on a CaF1 passivation layer, dots directly on Si. The stripes have a precision of ±1 nm, are continuous, do not touch each other, and are attached to the top of the step edges. The stripe repulsion and their counter-intuitive attachment are explained via a reversal of the stacking at the CaF2/Si(111) interface. The dot density is 3×1011 cm−2=2 Teradots/in.2. These arrays may serve as masks in nanolithography.
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