P. Kury

Compact and transferable threefold evaporator for molecular beam epitaxy in ultrahigh vacuum

A very compact molecular beam epitaxy (MBE) source is presented. It contains three Knudsen-type crucibles, two resistively and one electron bombardment heated. An efficient water cooling is implemented to allow MBE deposition under ultrahigh-vacuum conditions. Due to its small size, the evaporator fits inside a DN 36 CF T-piece which makes a separate bakeout possible. Refills of the crucibles as well as changes of the source materials and even repairs of the source can be carried out within a few hours without breaking the vacuum of the main chamber. The design and the usage of the MBE source are described in detail and its functionality is demonstrated exemplarily with data of silver deposition on the silicon (111) surface.

Surfactant-mediated epitaxy of Ge on Si(111): Beyond the surface

For a characterization of interface and “bulk” properties of Ge films grown on Si(111) by Sb surfactant-mediated epitaxy, grazing incidence x-ray diffraction and transmission electron microscopy have been used. The interface roughness, defect structure, and strain state have been investigated in dependence of film thickness and growth temperature. For all growth parameters, atomically smooth interfaces are observed. For thin Ge layers, about 75% of the strain induced by the lattice mismatch is relaxed by misfit dislocations at the Ge∕Si interface. Only a slight increase of the degree of relaxation is found for thicker films. At growth temperatures below about 600°C, the formation of twins is observed, which can be avoided at higher temperatures.

SSIOD: The next generation

Surface stress induced optical deflection (SSIOD) is a bending sample method for the in situ determination of the surface stress with a typical resolution of about 0.15N∕m. Here we present the latest version of SSIOD with major improvements concerning the sample shape and clamping, the laser system and the position detectors. With these modifications SSIOD becomes an easily applicable method to most UHV systems and can be combined with other surface analytical methods like SPA-LEED, Auger-CMA, XPS, UPS or maybe even with microscopy such as the flange-on LEEM. The presented modifications also improve the resolution of the method to below 0.01N∕m.

Absence of surface stress change during pentacene thin film growth on the Si(111)-(7 ◊ 7) surface: a buried reconstruction interface

We use high-resolution surface stress measurements to monitor the surface stress during the growth of pentacene (C22H14) on the (7◊ 7) reconstructed silicon (111) surface. No significant change in the surface stress is observed during the pentacene growth. Compared to the changes in the surface stress observed for Si and Ge deposition on the Si(111)-(7 ◊ 7) surface, the insignificant change in the surface stress observed for the pentacene growth suggests that the pentacene molecules of the first adsorbate layer, although forming strong covalent bonds with the Si adatoms, do not alter the structure of the (7◊ 7) reconstruction. The (7 ◊ 7) reconstruction remains intact and, with subsequent deposition of pentacene, eventually becomes buried under the growing film. This failure of the pentacene to affect the structure of the reconstruction may represent a fundamental difference between the growth of organic thin films and that of inorganic thin films on semiconductor surfaces.

Impact of thermal dependence of elastic constants on surface stress measurements

The accuracy of surface stress data obtained by means of bending sample methods depends on the precise knowledge of the biaxial Youngs modulus E/1−ν which enters as a prefactor in the stress calculation formula of Stoney and describes the stiffness of the sample’s material against biaxial deformation. Room temperature values are commonly used for E/1−ν. However, E/1−ν is significantly temperature dependent. A second order polynomial fit of this dependence is presented for the benefit of future measurements as well as a correction curve for old data.

Precise calibration for surface stress induced optical deflection measurements

Bending sample methods, like surface stress induced optical deflection, are powerful tools for the in situ determination of thin film and surface stress: the bending radius of a sample is measured via the deflection of reflected laser beams. In most setups split-segment position sensitive detectors are used. A precise method for calibrating those detectors is presented.

Chopped sample heating for quantitative profile analysis of low energy electron diffraction spots at high temperatures

Spot profile analysis low energy electron diffraction (SPA-LEED) is one of the most versatile and powerful methods for the determination of the structure and morphology of surfaces even at elevated temperatures. In setups where the sample is heated directly by an electric current, the resolution of the diffraction images at higher temperatures can be heavily degraded due to the inhomogeneous electric and magnetic fields around the sample. Here we present an easily applicable modification of the common data acquisition hardware of the SPA-LEED, which enables the system to work in a pulsed heating mode: Instead of heating the sample with a constant current, a square wave is used and electron counting is only performed when the current through the sample vanishes. Thus, undistorted diffration images can be acquired at high temperatures.