Creating room temperature Ohmic contacts to 4H-SiC: studied by specific contact resistance measurements and X-ray photoelectron spectroscopy

The use of a silicon interface pre-treatment to produce low resistance Ohmic nickel contacts to 4H-SiC, circumventing the need for contact post annealing, is reported. The effects of two different SiC pre-metal deposition surface preparation techniques: RCA cleaning (control sample) and a silicon interlayer pre-treatment (SIP), are discussed. Electrical characterization of contacts on treated surfaces, using circular transfer length measurements (CTLM), revealed that contacts to RCA cleaned samples were Schottky in nature, unless annealed at temperatures greater than 700 °C. In contrast, contacts formed on SIP SiC surfaces exhibited Ohmic behaviour directly after fabrication, without the need for post metallisation annealing. Average contact resistances as low as 1.3E−05 Ω cm² have been recorded for SIP samples. This fabrication process has distinct technological advantages compared to standard techniques for forming Ohmic contacts to SiC. To consolidate our findings the chemical and electrical nature of the SIP nickel-SiC interface, as it was sequentially formed and annealed, was examined using X-ray photoelectron spectroscopy (XPS). Based on these results, a model is proposed to explain the as-deposited Ohmic contact nature of the SIP sample.     

Electron-pair densities of group 2 atoms in their and terms

Electron-pair intracule (relative motion) and extracule (center-of-mass motion) densities are studied in both position and momentum spaces for the ¹ P and ³ P terms of the group 2 atoms Be (atomic number Z =4), Mg (Z =12), Ca (Z =20), Sr (Z =38), Ba (Z =56), and Ra (Z =88). In position space, the ¹ P - ³ P difference in the intracule densities shows that the probability of a small interelectronic distance is larger in the triplet for all the six atoms, as reported for the lightest Be atom in the literature. The position-space extracule density clarifies that the triplet electrons are more likely to be at opposite positions with respect to the nucleus than the singlet electrons for all the atoms. In momentum space, the singlet generally has a larger probability of a small relative momentum between two electrons as a na?ve manifestation of the Fermi hole in the triplet. The extracule density in momentum space shows that the ¹ P term has a distribution larger in a large center-of-mass momentum region than the ³ P term. Received: 26 August 1998 / Received in final form: 1 February 1999     

Ethanol and methanol induced changes in phospholipid monolayer

The main components of cell membranes are phospholipids and proteins. The aim of our study was to examine structural changes of dipalmitoyl-phosphatidylcholine (DPPC) monolayer as a simple model system of a cell membrane in different environments. Pure water, ethanol and methanol solutions were used as subphases of Langmuir films as a membrane models. For detection of changes in charge states of the molecules as well as relation with structural and conformational changes, a contactless method Maxwell's displacement currents (MDC) was used. Behaviour of DPPC molecules on two different subphases is substantialy different. In DPPC monolayer on the subphase of methanol-water, a gradual absorption (incorporation, penetration) of methanol molecules into the layer can appear. In DPPC monolayer on the subphase of ethanol-water adsorption of ethanol molecules on the layer can be observed. The membrane permeability might change. At both subphases (ethanol-water and methanol-water) the elasticity modulus of the monolayer decreases leading to the loss of membrane elasticity.     

Peeling of polydimethylsiloxane adhesives at low velocities: Cohesive failure

This work presents results obtained in 90‐degree peeling tests at low velocities in the case of Newtonian adhesives, when failure is cohesive. Peeling experiments are described and consider the influence of the thickness, viscosity, and surface tension of the adhesive, as well as the backing rigidity. A simple model, based on lubrication effects in thin films, is considered and compared with the measurements, when peeling is a two‐dimensional phenomenon. Furthermore, a criterion for predicting the transition between the two‐dimensional regime and the three‐dimensional regime at higher velocities is proposed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 145–157, 2005     

New surface phases for potassium adatoms on cleaved Si(111)

In a previous study (B. Reihl and K.O. Magnusson, Phys. Rev. B 42 (1990) 11 839) no change was observed in the LEED pattern from the 2 × 1 pattern of the clean Si(111) surface upon potassium adsorption. In contrast to this paper, we observe six surface phases during room temperature dosing of K on the cleaved Si(111) surface. In addition, we observe a 3 × 1 pattern on the cleaved K/Si(111) surface upon annealing. This paper will provide photographs of the new phases observed with LEED. It also will set limits on the K coverage required for these phases using results from photoemission and secondary electron cutoff measurements. Tentative models for these surfaces will be proposed. These new LEED results show a more complex interaction between K and the cleaved Si(111) surface than previously thought and hopefully will encourage further exploration of adsorption on the cleaved Si(111) surface.     

Growth and thermal evolution of submonolayer Pt films on Ru(0 0 0 1) studied by STM

The growth of submonolayer Pt on Ru(0 0 0 1) has been studied with scanning tunneling microscopy. We focus on the island evolution depending on Pt coverage θ_Pt, growth temperature T_G and post-growth annealing temperature T_A. Dendritic trigonal Pt islands with atomically rough borders are observed at room temperature and moderate deposition rates of about 5 × 10⁻⁴ ML/s. Two types of orientation, rotated by 180° and strongly influenced by minute amounts of oxygen are observed which is ascribed to nucleation starting at either hcp or fcc hollow sites. The preference for fcc sites changes to hcp in the presence of about one percent of oxygen. At lower growth temperatures Pt islands show a more fractal shape. Generally, atomically rough island borders smooth down at elevated growth temperatures higher than 300 K, or equivalent annealing temperatures. Dendritic Pt islands, for example, transform into compact, almost hexagonal islands, indicating similar step energies of A- and B-type of steps. Depending on the Pt coverage the thermal evolution differs somewhat: While regular islands on Ru(0 0 0 1) are formed at low coverages, vacancy islands are observed close to completion of the Pt layer.     

Reactive Solid State Dewetting: Interfacial Cavitation in the System Ag-Ni-O

Micron thick silver films, vapour deposited onto high purity polycrystalline nickel substrates, dewet the substrate after high temperature annealing in oxygen rich atmospheres, while the films remain stable after annealing at the same temperature in a nitrogen atmosphere. Dewetting occurs when a nickel oxide layer is formed at the silver-nickel interface as a consequence of oxygen diffusion through the silver film.The sensitivity of the dewetting process on various parameters such as the annealing: temperature, time and oxygen partial pressure has been determined.Scanning Electron Microscopy (SEM) of cross-sections reveal that the main mechanism of dewetting at short annealing time is the nucleation of cavities at the Ag-NiO interface which grow towards the free surface of the Ag film. They are formed not only at Ag grain boundaries and triple junctions but also in the core of Ag grains. Such cavities do not occur when the Ag film is deposited onto a NiO single crystal. We propose a simple model for the cavitation: a vacancy supersaturation is sustained in Ag, at the Ag-NiO interface, as a result of oxygen consumption by the oxidation reaction. In regions of fast oxidation, the vacancy supersaturation is large enough to promote the nucleation and growth of interfacial cavities. The model qualitatively accounts for all the observed trends; quantitatively, on top of the vacancy supersaturation, extra-contributions to the driving force for cavitation must be invoked.     

Anomalous adhesion in adsorbed polymer layers

Anomalous adhesion behaviour observed in the polydimethylsiloxane/heptane/silica system is reported. This behaviour is characterised by the infrequent appearance of one or more elastic minima which occur in addition to the ever-present primary adhesion. The resulting elastic force is attributed to loops connecting the tip to the substrate and can be described by two models; a worm-like chain, and a freely jointed chain. Both models give similar values for the characteristic segment length of around 0.24 nm and indicate that the chains have been extended by between 90 to 95% of their chain contour length before desorption. In some cases multiple adhesions were observed between tip and substrate and have been used to suggest a method for determining the distribution of adsorbed polymer loops. Received: 12 November 1997 / Accepted: 6 March 1998     

Hydrogen adsorption on Gd(0001)

The electronic structure of hydrogen adsorbate-induced states on Gd(0001) was investigated by means of photoelectron spectroscopy with linearly polarized radiation. The E vector of the incoming photon beam is rotatable. Clean and well-ordered rare-earth (0001) surfaces exhibit a highly localized surface state near the Fermi edge. After the adsorption of hydrogen, the surface state disappears and an additional sharp feature at about 4 eV binding energy is observed. For this latter state, the ratio of the radial matrix elements as well as the relative phase shifts were determined to be R=R_p/R_f=2.4±0.3 and δ_f−δ_p=310±10°, respectively. The removal of the Gd surface state by hydrogen adsorption was investigated by means of scanning tunneling microscopy (STM) and spectroscopy (STS). The removal of the surface state exhibits domain-like behavior, with surface steps acting as domain boundaries. The tunneling spectra reveal that hydrogen adsorption causes a dramatic reduction in the differential conductivity near the Fermi level.     

Efficient Sensing of Explosives by Using Fluorescent Nonporous Films of Oligophenyleneethynylene Derivatives Thanks to Optimal Structure Orientation and Exciton Migration

The fluorescence of thin films of a diimine‐substituted phenyleneethynylene compound can be efficiently quenched by nitroaromatic vapors, which is not the case for the unsubstituted parent compound. Thin‐film porosity is usually considered to be an essential factor for efficient quenching, but in the present case the origin of the quenching is completely different, as both films are nonporous and hermetic to 2,4‐dinitrotoluene (DNT) molecules. The molecular organization in the two crystallized thin films offers a low level of π stacking for both compounds, but the orientation of the phenylenethynylene fluorophore differs markedly with respect to the surface of the films. For the substituted compound, the fluorophore is almost parallel to the surface, thus making it readily available to molecules of a nitroaromatic quencher. This rationale is also observed in the case of a related compound bearing methoxy side chains instead of the long octyloxy moieties. Fluorescence‐lifetime experiments show that the efficient quenching process in the nonporous crystallized films of the substituted compound is due to a fast (<70 ps) diffusion of excitons from the bulk of the film toward the surface where they are quenched, thus providing evidence of antenna effects.