The scanning tunneling microscope has been used to desorb hydrogen from hydrogen-terminated silicon (100) surfaces. As a result of control of the dose of incident electrons, a countable number of desorption sites can be created and the yield and cross section are thereby obtained. Two distinct desorption mechanisms are observed: (i) direct electronic excitation of the Si-H bond by field-emitted electrons and (ii) an atomic resolution mechanism that involves multiple-vibrational excitation by tunneling electrons at low applied voltages. This vibrational heating effect offers significant potential for controlling surface reactions involving adsorbed individual atoms and molecules.

Photon-assisted tunneling of electrons through an insulating barrier may be used to detect long-wavelength radiation with a sensitivity approaching the limit imposed by the Heisenberg uncertainty principle. A new generation of ultra-low-noise millimeter-wave receivers, currently being developed for astronomical observation, utilizes the extremely sharp nonlinearity produced by single-electron quasiparticle tunneling between two superconductors in a superconductor-insulator-superconductor (SIS) tunnel junction. At millimeter wavelengths, the quantum energy fico/e may be larger than the voltage width for onset of quasiparticle tunneling in a SIS junction; and under these conditions the absorption of a single photon can cause one additional electron to tunnel through the barrier. Several newly discovered quantum effects become possible in this regime, including power amplification of an incoming signal during the process of frequency downconversion in a heterodyne receiver. The experimental development of SIS millimeter-wave receivers is reviewed, along with the quantum theory of mixing which predicts their performance. CONTENTS

Nanoscale patterning of the hydrogen terminated Si(100)-2×1 surface has been achieved with an ultrahigh vacuum scanning tunneling microscope. Patterning occurs when electrons field emitted from the probe locally desorb hydrogen, converting the surface into clean silicon. Linewidths of 1 nm on a 3 nm pitch are achieved by this technique. Local chemistry is also demonstrated by the selective oxidation of the patterned areas. During oxidation, the linewidth is preserved and the surrounding H-passivated regions remain unaffected, indicating the potential use of this technique in multistep lithography processes.

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A finite charging energy, e2/2C′, is required in order to place a single electron onto a small isolated electrode lying between two tunnel junctions and having a total capacitance C′ to its external environment. Under suitable conditions, this elemental charging energy can effectively block all tunnel events near zero bias voltage in series arrays of ultrasmall junctions, an effect that has come to be known as the ‘‘Coulomb blockade.’’ This article outlines a new approach to the design of digital logic circuits utilizing the Coulomb blockade in capacitively biased double-junction series arrays. A simple ‘‘on’’/‘‘off ’’ switch is described and complementary versions of this switch are then employed to design individual logic gates in precise correspondence with standard complementary metal–oxide semiconductor architecture. A planar nanofabrication technique is also described that may eventually allow the integration of Coulomb blockade logic onto conventional semiconductor chips, thereby realizing hybrid integrated circuits having device densities and operating speeds far in excess of present technology.

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PtSi source/drain p-type metal–oxide–semiconductor field-effect transistors (MOSFETs) have been fabricated at sub-40 nm channel lengths with 19 Å gate oxide. These devices employ gate-induced field emission through the PtSi ∼0.2 eV hole barrier to achieve current drives of ∼350 μA/μm at 1.2 V supply. Delay times estimated by the CV/I metric extend scaling trends of conventional p-MOSFETs to ∼2 ps. Thermal emission limits on/off current ratios to ∼20–50 in undoped devices at 300 K, while ratios of ∼107 are measured at 77 K. Off-state leakage can be reduced by implanting a thin layer of fully depleted donors beneath the active region to augment the Schottky barrier height or by use of ultrathin silicon-on-insulator substrates.