GXSM is a full featured and modern scanning probe microscopy (SPM) software. It can be used both in stand alone mode for powerful image processing and analysis and connected to an instrument operating many different flavors of SPM, e.g., scanning tunneling microscopy and atomic force microscopy or in general two-dimensional multichannel data acquisition instruments. The GXSM core can handle different data types, e.g., integer and floating point numbers. An easily extendable plug-in architecture provides many image analysis and manipulation functions. A digital signal processor subsystem runs the feedback loop, generates the scanning signals, and acquires the data during SPM measurements. In addition it performs sophisticated spectroscopy tasks such as scanning tunneling spectroscopy. The GXSM software is released under the GNU general public license and can be obtained via the Internet.

X-ray photoelectrons excited by x-ray standing waves ͑XSW͒ are used to study the atomic structure of the low-formation-temperature (370°C) interface between CaF 2 and Si͑111͒. The core-level shift of the photoemission spectra of the Ca atoms at the CaF 2 /Si interface is used to distinguish interface Ca atoms from atoms in other ͑bulk͒ layers in the XSW measurements. Therefore, we obtained quantitative structure information specific to the buried CaF 2 /Si interface avoiding some of the ambiguities of XSW. Even at the low growth temperatures used here, the interface is well ordered, with interface Ca atoms exclusively adsorbed on T 4 sites. The majority of the interface layer has CaF stoichiometry. The CaF 2 films consist of domains with type-A and type-B orientation.

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The structure and morphology of ultrathin lattice matched CaF 2 films of very few monolayers thickness, which were deposited on Si͑111͒ substrates by molecular-beam epitaxy, have been studied in situ by synchrotron based grazing incidence x-ray diffraction. Even for the thinnest investigated film of three monolayers thickness, the in-plane structure of the CaF 2 film is determined by a lateral separation in two domains: a pseudomorphic phase assuming the lateral lattice constant of the Si͑111͒ substrate and a completely relaxed phase. Analysis of the crystal truncation rods verifies that both phases adopt the entire homogeneous CaF 2 film thickness. Therefore, we propose that atomic steps of the substrate bypass the nucleation barrier for the formation of ͑Shockley partial͒ dislocations so that the film starts to relax below the classical critical film thickness. While the relaxed phase assumes also the CaF 2 bulk lattice constant for the vertical direction, the vertical lattice constant of the pseudomorphic phase increases due to the compressive lateral strain at the interface. This vertical expansion of the pseudomorphic phase, however, is larger than expected from the elastic constants of the CaF 2 bulk. The fraction of the pseudomorphic CaF 2 phase decreases with increasing film thickness. The interface between the pseudomorphic CaF 2 phase and the Si͑111͒ substrate is characterized by Ca on T 4 sites, a smaller distance between the Si͑111͒ substrate and the CaF interface layer and an expanded layer distance between CaF interface layer and the completely stoichiometric CaF 2 film.

A special solid-phase epitaxy technique utilizing the surfactant B for the growth of crystalline Si-QWs on CaF2∕Si(111) enabled us to grow CaF2∕Si∕CaF2 double-barrier diodes exhibiting resonant tunneling effects from 77K up to room temperature with peak voltages at 0.2eV, which is very close to simple resonant tunneling model predictions. The peak voltages and currents were virtually independent of temperature. No trapping or hysteresis effects were found in the I–V characteristics which exhibited 2–7 orders of magnitude larger peak current densities than previously reported CaF2∕Si∕CaF2 RTDs.

Boron surfactant enhanced solid-phase epitaxy (SPE) of thin Si films on CaF2∕Si(111) substrates has been studied. Two different growth processes were investigated. In the first process, 1ML of boron atoms was predeposited on the CaF2 surface at room temperature before Si evaporation followed by an anneal to 635°C. This resulted in Si cluster formation. The surface morphology of the Si layer did not show any improvement compared to SPE without surfactant. In the second process, boron atoms were deposited directly on top of the amorphous Si film. This resulted in continuous and smooth epitaxial Si layers on CaF2 with a sharp B-induced (3×3)R30° surface reconstruction after annealing.

The contrast in scanning probe microscopy images of ultrathin CaF2 films epitaxially grown on Si(111) is studied using scanning tunneling microscopy (STM) and scanning force microscopy (SFM). Material contrast between CaF2 and the underlying Si–CaF interface layer can be achieved by multiple scan modes. STM images of CaF2∕Si(111) exhibit a distinct contrast depending on the bias voltage. Using SFM, CaF2 and CaF can be distinguished by the contact potential difference (CPD) determined by measuring the bias voltage dependence of the cantilever resonance frequency. The CPD also causes a material contrast in the dissipation signal.

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