Frédéric Houzé

Imaging the local electrical properties of metal surfaces by atomic force microscopy with conducting probes

A promising technique capable of performing localized resistance measurements over a surface is presented using a modified commercial atomic force microscope with a conducting probe. Its overall purpose is to obtain simultaneous cartographies of surface roughness and local resistance within a given microscopic area of a sample with nanometer scale resolution. Although an elaboration of suitable probes remains an ongoing problem, convincing images of some metal surfaces that reveal occasionally surprising features have already been obtained. Calculations performed from measurements have allowed us to clarify the mechanical nature of the tip/surface nanocontact and hence to determine the most probable transport process according to the range of resistance considered.

Electronic and topographic properties of amorphous and microcrystalline silicon thin films

Electronic properties of microcrystalline silicon (μc-Si) thin films prepared by different techniques are presented and compared to that of device-grade, undoped, hydrogenated amorphous silicon (a-Si:H). It is found that whatever the preparation technique, the conductivity of μc-Si is significantly larger and the mobility-lifetime products and ambipolar diffusion lengths of optimised layers can be higher than in a-Si:H. In addition, no light-induced degradation of electronic properties is observed. Local topographic and electrical probing results on μc-Si films are also shown. The surface roughness of μc-Si samples depends on the preparation technique, but a common aging phenomenon in the local electrical probing is found and described.

Field emission and material transfer in microswitches electrical contacts

Material transfer from one electrical contact part to the other has already been reported in microswitches operating under hot switching conditions. By using an atomic force microscope with a conductive cantilever, we highlighted that electrons are emitted from the cathode when electrode separation becomes less than a few tens of nanometers. This electronic emission proves to follow Fowler–Nordheim theory and leads to the damage of the opposite contact member (anode) by impact heating. Anode material evaporates under this extreme heating and deposits on the opposite contact member (cathode), leading to a material transfer from anode to cathode.

Self-assembled monolayers of alcanethiols on nickel surfaces for low level electrical contact applications

Nickel can be used as final coating for separable electrical contacts in various types of applications: batteries, automotive connectors, etc. The possible growth of a poorly conducting layer on the metal due to environmental conditions has limited up to now the use of such coatings to common applications. However durable quality performances can be expected with a proper protective material avoiding corrosion. The new approach we have been working on, is to find compounds that bind to nickel and form well-defined layers. We report here the first results describing the behaviour of self-assembled monolayers of thiol molecules deposited on bare nickel substrates. We briefly describe the deposition method, the type of molecules and of substrates used and summarize the main physicochemical characterisations available. The electrical and tribological properties of the monolayers formed on nickel slabs are investigated in a ball/plane configuration simulating a real contact element. The influence of an electrochemical pretreatment of the nickel surface prior to the layer assembly is shown. Excellent tribological behaviours can already be obtained with corresponding values of the contact resistance varying between 1 /spl Omega/ and 10 m/spl Omega/. The results show that building organised monolayers acting as protective coatings is of high interest for electrical contacts.

Study of the local electrical properties of metal surfaces using an AFM with a conducting probe

The performances of coating materials for electrical contact elements are more and more often investigated through various means. We report here a new method we have developed for a few years in our lab, which consists in performing localized resistance measurements over a surface by means of an AFM with a conducting probe. This technique enables us to simultaneously obtain a cartography of the surface roughness and of the local conductance within a given microscopic area of a sample with nanometer scale resolution. Although the elaboration of suitable probes remains an open problem, some convincing images of metal surfaces have already been obtained, revealing occasionally surprising features. It can be observed for instance that the local resistance values can vary of several orders of magnitude between two adjacent grains. Calculations performed from the measurements allow to clarify the mechanical nature of the tip/surface nanocontact and hence to determine the most probable transport process according to the range of resistance considered.

Conducting probe atomic force microscopy applied to organic conducting blends

Atomic force microscopy (AFM) is used in contact mode with a conducting tip to probe the conducting network of the conductive polymer polyaniline blended in an insulating polymer matrix. The high resistance contrast and sharp boundaries between conductive and insulating phases is observed down to scales in the 10 nm range. The very low scale electric dispersion corresponds to the morphologic phase segregation known from conventional AFM or transmission electron microscopy measurements, which is responsible for the ultralow electrical percolation threshold previously demonstrated in this system.

Surface modifications of nickel substrates with self-assembled monolayers of alkanethiols for electrical contact applications

Abstract Electrical and tribological properties of alkanethiol monolayers chemisorbed on nickel slabs, bare or electrochemically reduced, are investigated in a ball/plane configuration simulating a real contact. In the case of the electrochemically reduced nickel surfaces, friction coefficients of the order of 0.2 and contact resistances varying between 1 Ω and 10 mΩ are obtained.

Electrical properties of very thin heat-treated polyacrylonitrile layers electropolymerized on nickel for contact application

Abstract Recent investigations concerning weakly pyrolysed thin films of polyacrylonitrile (PAN) grafted and grown by electropolymerization on a nickel underplate are reported, which clearly attest the potential interest of the obtained material for low-level connector coating application. The electrical characterization of these organic layers in a contact situation requires various precautions, principally the use of surfaces as smooth as possible in order to avoid piercing the film and therefore metal/metal junctions. An original device for static contact resistance measurements is presented, simulating the real case of industrial connector parts by a sphere/plane contact with an adjustable normal load. The tested samples consist of Ni-electroplated plane brass strips covered by PAN layers about 550 A thick and heated at different temperatures in the range 350–500 °C for 10 or 30 s; the pressing sphere is a 3.2 mm diameter smooth golden ball. IRRAS and XPS spectra of raw and thermally treated samples reveal a series of drastic changes in the molecular structure of the polymer and the correlative evolution of the electrical properties of the films is unambiguously observed. The most promising results concern the 400 °C/30 s heated coating, for which stable and reproducible contact resistance values lower than 10 mΩ are found under a 50 gf normal load.

Constriction resistance of a multispot contact: an improved analytical expression

When dealing with a multispot metallic contact two different problems are raised: whether the calculation of the constriction resistance R/sub c/ is required from geometrical considerations and whether one calculates the real area of contact from electrical measurements. Attention is focused on the second case, and it is shown that when the total area built up by the elementary spots represents a large fraction of the apparent area of contact, then the usual analytical expressions of the resistance can be imprecise. Considering the case of n elementary spots of radius r regularly spread in a disc of radius R representing the interface of contact of two metals of equal resistivity rho , they give an improved expression of the resistance. When the total area of the spots is equal to the disc area, this expression gives for any value of n the usual R/sub c/= rho /2R. Conversely, when n=1, it gives, for any value of R, the correct expression: R/sub c/= rho /2r. It is shown that this improved expression is quite useful when the number of spots is small and the real area of contact close to the apparent one, i.e. in many practical cases. >

From single III-nitride nanowires to piezoelectric generators: New route for powering nomad electronics

Ambient energy harvesting using piezoelectric nanomaterials is today considered as a promising way to supply microelectronic devices. Since the first demonstration of electrical energy generation from piezoelectric semiconductor nanowires in 2006, the piezoelectric response of 1D-nanostructures and the development of nanowire-based piezogenerators have become a hot topic in nanoscience. After several years of intense research on ZnO nanowires, III-nitride nanomaterials have started to be explored thanks to their high piezoelectric coefficients and their strong piezo-generation response. This review describes the present status of the field of piezoelectric energy generation with nitride nanowires. After presenting the main motivations and a general overview of the domain, a short description of the main properties of III-nitride nanomaterials is given. Then we review the piezoelectric responses of III-N nanowires and the specificities of the piezo-generation mechanism in these nanostructures. Finally, the design and performance of the macroscopic piezogenerators based on nitride nanowire arrays are described, showing the promise of III-nitride nanowires for ultra-compact and efficient piezoelectric generators.