We present experimental and numerical results demonstrating the drastic influence of attractive forces on the behaviour of the atomic force microscope when operated in the resonant tapping tip mode in an ambient environment. It is often assumed that tapping is related to repulsive interaction. In contrast, we find that in general the attractive forces are the most dominant interaction in this mode of operation. We show that attractive forces in combination with the repulsive elastic type of forces cause points of instability in the parameter space constituted by: the cantilever swing amplitude, the frequency bias point, and the distance between the fixed end of the cantilever and the sample. These points of instability can result in disturbances during image acquisition on hard elastic surfaces.

This letter presents a study of the nature of the periodic structure observed on the edge of laser-annealed spots on ion-implanted silicon. The direction of the periodic fringes was always found to be about perpendicular to the E vector of the light for linearly polarized beams. No fringe pattern was observed for circular polarization. We suggest that the pattern observed is due to heating by a standing wave resulting from the interference of the impinging wave and a radial (longitudinal) scattered wave.

By placing several Si δ-doped layers close to the surface of a GaAs molecular beam epitaxy–grown crystal, we achieve a compensation of the Schottky barrier and obtain a good Ohmic contact between an in situ deposited (without breaking the vacuum) Al metallization layer and a highly modulation doped (n++) conduction layer embedded below the δ-doped layers in the GaAs crystal. When cooled to below the critical temperature (≊1.2 K) of Al, superconductivity is induced in the conductive layer of the semiconductor. We have studied the current voltage (I–V) characteristics in a planar geometry where the Al has been removed in a thin stripe. We find a manifestation of the superconducting energy gap and a rich fine structure at injection energies both below and above the gap.

We demonstrate that GaAs grown by molecular beam epitaxy on silicon has ideal characteristics for THz receiver applications. The lattice mismatch between silicon and GaAs causes a disordered growth of GaAs, reducing the carrier lifetime to 1.8 ps. This is similar to the characteristics observed in low temperature grown GaAs. Furthermore, the high resistivity silicon substrate has a very low absorption and dispersion in the far infrared. This makes it an ideal material in THz system applications, and we show that a maximum frequency of 5 THz and a sixfold increase in sensitivity can be obtained using a GaAs-on-silicon based THz detector.

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