Percolation threshold and mean grain size in AlxSi1-x thin films

Reiss G, Vancea J, Hoffmann H. Percolation threshold and mean grain size in AlxSi1-x thin films. Journal of Physics, C: Solid State Physics. 1985;18(21):L657-L660.We determined the critical composition xc of the percolation threshold in polycrystalline AIxSi1-x thin films with a new method, using structural arguments only. The result xc = (0.55 + - 0.05) agrees with the results of the commonly used methods. Moreover, our model explains the wide spread of experimental values of xc as reported in the literature

Growth and structure of polycrystalline Cr/Au multilayered thin films

Brückl H, Vancea J, Lecheler R, Reiss G, Hoffmann H. Growth and structure of polycrystalline Cr/Au multilayered thin films. Thin solid films. 1994;250(1-2):56-60.Metallic multilayered thin films have recently been investigated due to their new magnetic and transport properties. The interest here is focussed on the characterization of the interfaces between the layers. The analysis of growth and structure of polycrystalline Cr/Au multilayers is accomplished by two complementary techniques: in situ ultrahigh vacuum scanning tunnelling microscopy and ex situ transmission electron microscopy. The combination of these powerful methods provides detailed information about structural characteristics such as crystallite size, surface roughness and crystallographic orientation. Moreover, conclusions can be drawn on the atomic arrangement and growth mechanism at the Cr-Au interface. The results are supported by semiempirical and theoretical expectations

Resistivity and the Hall effect in polycrystalline Ni-Cu and Ta-Cu multi-layered thin films

Reiss G, Vancea J, Kapfberger K, Meier G, Hoffmann H. Resistivity and the Hall effect in polycrystalline Ni-Cu and Ta-Cu multi-layered thin films. Journal of Physics, Condensed Matter. 1989;1(7):1275-1283.In the present paper the dependences of the resistivity p and the Hall voltage UH of polycrystalline Ni-Cu and Ta-Cu multi-layered thin films on the layer thickness dr are discussed. The thickness dependence of p and UH can be well understood using a simple model in which the layers are considered as parallel resistors, whereby the resistivity of a single layer is enhanced via surface scattering described by the well known Fuchs-Namba size theory. The Hall coefficients are independent of the layer thickness, although the measured Hall voltage varies with dr owing to the enhancement of the individual layer resistivities. For very thin layers, i.e. if the layer thickness becomes smaller than the layer roughness, the experimental data on both p and UH indicate a breakdown of the multi-layered structure to an island-like clustered film structure. For Ni-Cu a crossover from ferroparamagnetic to superparamagnetic behaviour was observed at this critical thickness

Substrate effects on the surface topography of evaporated gold films: a scanning tunnelling microscopy investigation

Vancea J, Reiss G, Schneider F, Bauer K, Hoffmann H. Substrate effects on the surface topography of evaporated gold films: a scanning tunnelling microscopy investigation. Surface science. 1989;218(1):108-126.Direct observation of surface roughness on metal films is a longstanding problem in thin film characterization. In this work the high quality of scanning tunnelling microscopy (STM) was used for investigation of evaporated gold films. A scanning tunnelling microscope able to scan areas up to 0.8 × 0.8 micro m with high reproducibility is presented. The topography of 80 nm thick gold films grown under identical evaporation conditions was investigated as a function of the selected substrate material (Corning glass, silicon, NaCl, mica and highly oriented pyrolitic graphite (HOPG)). The incipient growth mechanism on the substrate is the primary reason for the surface roughness. The real space images of the surface topography correlate very well with knowledge achieved from former growth experiments given in the literature. Moreover, very flat gold surfaces on HOPG allowed the observation of atomic corrugations in air environments

Grain boundary resistance in polycrystalline metals

Reiss G, Vancea J, Hoffmann H. Grain boundary resistance in polycrystalline metals. Physical review letters. 1986;56(19):2100-2103.Grain boundaries are known to reduce significantly the electrical dc conductivity of polycrystalline metallic materials. In this paper, we give a quantum mechanical calculation of the grain-boundary resistance based on the transfer-matrix approach. The results show an exponential decrease of the conductivity with respect to the number of grain boundaries per mean free path in accord with an empirical model proposed recently

Thickness dependence of the work function in double-layer metallic films

Hornauer H, Vancea J, Reiss G, Hoffmann H. Thickness dependence of the work function in double-layer metallic films. Zeitschrift für Physik, B: Condensed Matter. 1989;77(3):399-407.The work function of metallic thin films limited by symmetric surfaces is expected to be thickness dependent at a level of 0.1 eV and a thickness range of about 5 nm. Recent experiments, however, demonstrated that Cu films on glass or Ni substrates show a long ranging (10–20 nm) increase of the work function with increasing film thickness [1]. This effect was attributed to a violation of local charge neutrality in films with unlike surfaces. In this paper we show that the barrier height of thin film diodes like metal-insulator-metal (MIM)-, metal-semiconductor (Schottky contacts)-and metal-vacuum-metal (Kelvin capacitors) structures decreases with increasing thickness of one metal electrode. This metal electrode consists of a double layer whose single layer thicknesses are of the order of few tens of nm. The observed effect can be attributed to a decrease of the work function at the counter limiting interface not exposed to the evaporation beam. A possible explanation can be found again in the violation of the local charge neutrality in films with unlike surfaces

Scanning tunneling microscopy on rough surfaces: tip-shape-limited resolution

Reiss G, Vancea J, Wittmann H, Zweck J, Hoffmann H. Scanning tunneling microscopy on rough surfaces: tip-shape-limited resolution. Journal of applied physics. 1990;67(3):1156-1159.This paper discusses the reliability of scanning tunneling microscopy (STM) images of mesoscopically rough surfaces. The specific structure of these images represents a convolution between the real surface topography and the shape of the tip. In order to interpret these images quantitatively, the line scans of steep and high steps can be used to obtain an image of the tip itself. This image shows tip radii ranging typically from 5 to 15 nm and cone angles of about 30° over a length of 80 nm, and can in turn be used to recognize the limits of STM resolution on a rough surface: High-resolution transmission electron microscopy cross-section images of Au island films on a Au-Nb double layer are convoluted with the experimentally observed tip shape; the resulting line scans correspond very well with STM graphs of the same samples. Finally an overall criterion for the resolution of the STM on such surfaces is proposed

Ballistic electron magnetic microscopy on epitaxial spin valves

The tip of a scanning tunnelling microscope has been used as an injector of hot electrons or hot holes into a spin-valve epitaxially grown on n-GaAs$_{67}$P$_{33}$. Spin dependent transport of injected- and hole excited electrons has been studied in an external magnetic field at room temperature. Significant variations in the collector current due to the spin dependent inelastic decay of the hot charge carriers have been measured for parallel and antiparallel configurations of the magnetization of the individual layers. We found magneto-current effects on the order of 600% and relative large transmission values compared to other BEMM-studies. In addition, we investigated the excitation of electron hole pairs with its subsequent electron transport in the spin-valve and found a magneto-current effect with positive sign

Determination of the intershell conductance in a multiwall carbon nanotube

We have measured the current induced voltage drop along an individual multiwall carbon nanotube as a function of the distance to the current injecting electrode. The measurements have been performed at room temperature using scanning probe potentiometry combined with scanning electron microscopy. For a nanotube with an incomplete outer shell, a sharp potential jump was observed at the end of the outermost shell. The electric potential variation along the carbon nanotube has been used to determine the contact resistance between metal electrodes and the tube, the intrashell resistance, and the intershell conductance

Hot-electron transport and magnetic anisotropy in epitaxial spin valves

We report on ballistic electron magnetic microscopy studies at room temperature using an epitaxially grown Fe$_{34}$Co$_{66}$/Au/Fe$_{34}$Co$_{66}$ trilayer. Local hysteresis loops are obtained as a function of the in-plane magnetic field angle. In order to understand the underlying local magnetization behavior, the magnetic anisotropies were determined by ferromagnetic resonance. These results served as input for simulations of the hysteresis loops, which are compared to magneto-optic Kerr effect and ballistic electron magnetic microscopy data of the spin valve. In doing so, the relative magnetization configuration of the spin valve can be calculated as a function of the external magnetic field, and the magnetization behavior during the reversal can be explained. Since different magnetization configurations of the spin valve are available, epitaxial spin valves allow multimagnetocurrent values, when the magnetic field is applied along different directions