M. Cannaerts

Field induced local oxidation of Ti and Ti/Au structures by an atomic force microscope with diamond coated tips

We have investigated field induced local oxidation of thin Ti films with the tip of an atomic force microscope. Tips, which have been coated with a diamond layer to improve their wear resistance, are shown to have a much longer lifetime than conventional uncoated Si tips. We have studied the oxidation characteristics as a function of the applied tip-sample voltage and scanning speed for both diamond coated and uncoated tips. We find that the diamond coated tips result in a thinner oxide layer for the same voltage and scanning speed. The dependence of the oxidation process on the film thickness was studied for diamond coated tips. Thin films can be completely transformed into an oxide layer for a thickness up to 15 nm. Moreover, for these sufficiently thin films the measured ratio between the oxide height and the Ti film thickness is a constant. It is also possible to completely oxidize Ti films which cover Au islands, opening the way to fabricate more complicated structures.

Reversible Switching of the Surface Conductance of Hydrogenated CVD Diamond Films

Chemical vapor deposited (CVD) diamond films have a controversial history regarding their surface electronic properties. Hydrogenation is known to induce a p-type conductive surface layer, which is not present on non-hydrogenated samples. The enhanced surface conductance can decrease significantly after annealing under high vacuum conditions at as low as 200 °C (a temperature which is sufficiently low to ensure that the hydrogen termination remains intact). Although the hydrogen is necessary for the surface conductance, the surface can be made poorly conductive without removing the hydrogen termination. We have performed scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) to better understand the origin of the enhanced surface conductance. Our STS experiments confirm that hydrogenation induces the appearance of a conductive surface layer, which can change considerably after high vacuum annealing. In this paper, we will discuss the conditions under which the surface conductivity can be restored. In particular we study the conductance changes during plasma hydrogenation and after exposing it to atmospheric conditions. Topographical STM scans confirm that the surface structure is not altered at low annealing temperatures.

Mapping nanometre-scale temperature gradients in patterned cobalt-nickel silicide films

We have implemented a thermal mapping technique, scanning Joule expansion microscopy, with a high spatial resolution (about 20 nm) in order to investigate heat generation and dissipation in mesoscopic thin-film structures. In particular, we investigated narrow cobalt-nickel silicide lines having a granular morphology. We are able to directly visualize heating of the silicide in between the grains due to a local increase of the resistance, resulting in current crowding. The increase in resistance can be associated with a local reduction of the silicide thickness in between the larger grains. On the other hand, an enhanced silicide resistivity at grain boundaries may also contribute to the resistance changes.

The influence of (1102) sapphire substrate on structural perfection of CeO2 thin films

Abstract Thin CeO2 layers were prepared by on-axis radio frequency sputtering on R-plane sapphire substrates that came from two different sources, A (A substrates) and B (B substrates). We have observed reproducible XRD rocking curves (Δω, shape) depending on the source of substrates and their pre-annealing. AFM measurements reveal a clear difference between the surface topography of A and B, pre-annealed or non-pre-annealed substrate. CeO2 layers deposited on pre-annealed A substrate show reproducibly two-component rocking curves, a higher concentration of holes, and increased roughness. On the other hand, CeO2 layers grown on pre-annealed B substrate give singular rocking curves, and almost no holes in the layers were observed. YBa2Cu3O7 thin films, deposited on the examined CeO2 by two different methods––high pressure dc sputtering, pulsed laser deposition, show different zero magnetic field microwave absorption on temperature, depending on the shape of rocking curve. The results obtained clearly show that the substrate itself can generate CeO2 layers with two-component (supercrystallinity) rocking curves.

Mesoscopic electronic devices made by local oxidation of a titanium film covering gold islands

The local oxidation produced by the tip of an atomic force microscope scanning on a thin metallic film allows to define narrow oxide lines, thus providing a method to fabricate lateral tunnel junctions. In such devices, with rather thick tunnel junction barriers, the electrical transport is governed by thermally activated hopping rather than by direct electron tunneling. In this letter we show that tunneling barriers can also be produced with Ti films covering small gold islands. The gold islands significantly shorten the effective tunneling distance, allowing to observe temperature-independent electron tunneling across the lateral barriers. The mixed Ti/Au tunnel barriers reveal Coulomb blockade effects which may be used for single-electron devices consisting of a single oxide line.

Implementation and optimization of a scanning Joule expansion microscope for the study of small conducting gold wires

In order to study heating phenomena with submicrometer spatial resolution, we have implemented a scanning Joule expansion microscope (SJEM). When compared to scanning thermal microscopy (SThM), which requires the microfabrication of thermal probes, SJEM has superior spatial resolution and can be implemented more easily since no special tips or cantilevers have to be fabricated. Our SJEM measurements on small gold wires reveal that electrostatic force interactions between sample and tip can strongly affect the SJEM images. Therefore, we have identified the relevant parameters, in particular the cantilever stiffness, which have to be optimized to obtain reliable results. The temperature profile in the gold wires can be fitted to theoretical model calculations.

Influence of annealing on the electronic properties of chemical vapor deposited diamond films studied by high vacuum scanning tunneling microscopy and spectroscopy

Abstract Scanning tunneling spectroscopy (STS) under high vacuum conditions (2×10 −8 mbar), combined with high-resolution topographical imaging with the scanning tunneling microscope (STM), enabled us to investigate local variations in the electronic structure of the surface of chemical vapor deposited diamond films. We studied the variations in the current–voltage I(V) characteristics of hydrogen terminated films when varying the distance between tip and surface. Our STS measurements confirm the surface p-type conductance. We also studied the influence of changes in the hydrogen termination by annealing under high vacuum conditions. Annealing at relatively low temperatures is shown to dramatically influence the local I(V) characteristics measured with STM, confirming that hydrogen termination alone is not sufficient for explaining the enhanced surface conductance.

Influence of the morphology on the magneto-transport properties of laser-ablated ultrathin La0.7Ba0.3MnO3 films

We investigate the thickness dependent properties of manganite films characterized by colossal negative magnetoresistance. Ultrathin, wedge-type films (0–120 A) of La0.7Ba0.3MnO3 were deposited by laser ablation onto SrTiO3 and LaAlO3 substrates. The films were patterned into strips of different thickness and magneto-transport measurements were performed at temperatures between 5 and 290 K and in magnetic fields up to 5 T. Atomic force- and transmission electron microscopy were done to correlate the microstructure with the transport data. The resistivity of the films increases slightly with decreasing thickness due to substrate-induced compressive strain. Below 50 A, the resistivity rises abruptly indicating a crossover to discontinuous and finally island-like film growth as confirmed by the microstructural techniques. At thicknesses slightly above the threshold for percolative conduction (≈30 A), an enhanced low-field magnetoresistance was observed as a signature of spin-dependent tunneling.

In-situ sem observation of electromigration in thin metal films at accelerated stress conditions

Abstract With so-called in-situ SEM experiments, electromigration experiments are performed in a SEM (scanning electron microscope) equipped with a heating stage. BSE (back scattered electron) images are taken continuously over the entire length of a metal line submitted to high current and temperature stress, monitoring in detail the microstructure. Comparing the electrical resistance curves with the corresponding SEM micrographs and with ex-situ AFM measurements leads to detailed qualitative and quantitative information about the occurring electromigration and precipitation / dissolution effects in the metal lines.

Direct observation of preferential heating near grain boundaries in patterned silicide films

Heat generation in silicide stripes is studied using scanning thermal expansion microscopy. Local hot spots in the lithographically patterned silicide stripes can be imaged with high spatial resolution (around 20 nm). On the micrometer scale the enhancement of temperature can be linked to pronounced depressions appearing in the film topography, resulting in current crowding. On the submicrometer scale hot spots appear that do not correlate with large thickness variations and can be accounted for by crystallographic silicide grain boundaries with a locally enhanced resistivity. Our experimental approach, which enables to monitor local heating with submicrometer spatial resolution, provides a sensitive method to monitor device reliability.