John Colligon

Influence of mechanical properties on the nanoscratch behaviour of hard nanocomposite TiN/Si3N4 coatings on Si

A dual ion beam system has been used to produce hard nanocomposite TiN/Si3N4 coatings on silicon substrate. Mechanical properties have been determined by nanoindentation and tribological properties have been measured by nanoscratch testing. Nanoindentation showed that harder nanocomposites exhibited higher ratios of hardness to modulus (H/E). The dependence of the resistance to plastic deformation (H3/E2) on hardness was approximately linear. The H/E value influenced the nanoscratch behaviour. Coatings with higher H/E showed higher critical loads for elastic–plastic transition and also the total coating failure occurring in front of the probe. However, coatings with higher H/E also exhibited an unloading failure, occurring behind the probe at much lower load than the loading failure. Optimizing this stress-related unloading failure could be more important for tribological applications.

Nano-impact testing of TiFeN and TiFeMoN films for dynamic toughness evaluation

TiFeN and TiFeMoN films were deposited on silicon wafers by ion-beam-assisted deposition. Their mechanical properties were measured by nanoindentation (quasi-static) and nano-impact (dynamic) techniques. Nano-impact testing enabled assessment of their toughness and resistance to fatigue fracture under repetitive loading. At low impact forces, films with a higher resistance to plastic deformation (H3/E2) were much more resistant to the formation of cracks throughout the test. At higher impact forces, these films initially show impact resistance but with continued impacts they are unable to protect the Si substrate, performing as poorly as films with lower H3/E2 and suffer delamination from the Si substrate over a large area.

Effect of element concentration on nickel release from dental alloys using a novel ion beam method

OBJECTIVESnNickel chromium is widely used as a restorative material in dentistry but its biocompatibility is of concern as there are reports of patients suffering adverse effects caused by exposure to nickel-based restorations. The aim of this work was to quantify the amount of nickel released into solution from commercially available nickel-based alloys with varying compositions and to identify the potential use of thin films in further understanding the role of chromium in reducing nickel release.nnnMETHODSnSix commercially available nickel-based alloys were cast using the lost wax technique. Nickel chromium thin films were deposited onto silicon substrates by ion beam assisted physical vapor deposition. Both types of alloys were immersed into solution representative of saliva at pH 5 for 7 days. The amount of nickel released into solution was quantified by graphite-furnace atomic absorption spectrophotometry.nnnRESULTSnThe amount of nickel released from commercially available cast alloys did not correspond to the amount of nickel within the alloy. The total amount of chromium and molybdenum within the alloys proved to be of greater influence. Increasing the amount of chromium in the thin film alloys reduced the amount of nickel released but this was not linear.nnnSIGNIFICANCEnDifferences in the composition of commercial cast alloys highlighted the importance of other elements within the alloy on reducing the amount of nickel released. The use of thin film alloys can be useful in further understanding how the composition of an alloy can affect the amount of nickel released.

Transition from amorphous to crystalline beta phase in co-sputtered FeSi2 films as a function of temperature

A study of the stability of amorphous FeSi2 films and their transition to a crystalline phase as a function of deposition or annealing temperature is presented. Stoichiometric FeSi2 films, 300–400 u2004nm thick, were deposited on (100) Si substrates by co-sputtering of Fe and Si. It was found that the films grow in an amorphous form for the substrate temperature ranging from room temperature to 200u2009°C, while from 300–700u2009°C, they grow in form of a crystalline β‐FeSi2 phase. In a postdeposition 30u2004min heat treatments, the layers retain the amorphous structure up to 400u2009°C, transforming to the crystalline β phase at 500–700u2009 °C. The results are discussed in the frame of the existing models, and compared to those found in the literature. It is shown that in as-deposited films, the growth is controlled by surface diffusion, the crystalline layers growing in a columnar structure strongly correlated to the Si substrate. Postdeposition treatments induce a random crystallization controlled by bulk diffusion, the resulting structure not being influenced by the substrate. The results of this work contribute to a better understanding of the processes involved in a transition of amorphous FeSi2 films to a crystalline phase, and provide a basis to determine the processing parameters in potential applications of this promising semiconducting material.

In Situ Synchrotron X-Ray Microprobe Studies of Passivation Thresholds in Fe-Cr Alloys

The dissolution of metal during the passivation of Fe-Cr alloys has been studied in situ as a function of alloy composition in order to investigate the critical Cr content required for corrosion protection. This is achieved using synchrotron X-ray fluorescence microprobe analysis on a sputter-deposited Fe-Cr thin film with a continuous lateral variation in composition. The technique can resolve the dissolution of less than one layer of atoms during electrochemical treatment of the graded alloy surface. The results show that there is a sharp change in behavior in the region of 17-18% Cr. Above 18% Cr, almost no metal dissolution occurs during passive film development, while below this composition there is continual dissolution. This result is consistent with the gelation model of passivation that uses an approach based on percolation theory.

The optical properties of β-FeSi2 fabricated by ion beam assisted sputtering

β-FeSi2 has been shown to have a minimum direct band gap of 0.87 eV [T.D. Hunt, K.J. Reeson, K.P. Homewood, S.W. Teon, R.M. Gwilliam, B.J. Sealy, Nucl. Instr. and Meth. B 84 (1994) 168–171] which leads to the opportunity for Si based opto-electronics, optical communications and optical interconnects. Electroluminescence has been reported from structures containing β-FeSi2, which were produced by high dose ion implantation and annealing [D. Leong, M.A. Harry, K.J. Reeson, K.P. Homewood, Nature 387 (12 June 1987) 686]. In this paper we report the formation of β-FeSi2 by ion beam assisted co-sputtering of Fe and Si in varying percentages. n nThe layers were deposited with a varying Fe/Si ratio, with a Si capping layer applied to prevent oxidation. Separate regions of the sample were investigated at room temperature using optical absorption, to measure the band gap values. Absorption under the fundamental edge was also analysed at room temperature. Further investigations looked at the temperature dependence of the band gap and the absorption under the fundamental edge. n nThe results showed that a variety of Fe/Si ratios produced β-FeSi2, the formation of which was ascertained by the presence of a suitable band gap value [0.83–0.88 eV]. Absorption under the fundamental edge was shown to follow an exponential Urbach tail [C.H. Grein, S. John, Phys. Rev. B 39 (1989) 1140]. The temperature measurements are in good agreement with the Einstein model.

Correlation of Structural and Optical Properties of Sputtered FeSi2 Thin Films

Iron-disilicide films were sputter deposited on Si(100) wafers to 300–400 nm, at substrate temperatures ranging from room temperature to 700 °C. As-deposited films were amorphous at deposition temperatures up to 200 °C, and crystalline β-FeSi2 at 300–700 °C. Amorphous films were heat-treated after deposition at 300–700 °C. They remained amorphous up to 400 °C, and transformed to crystalline β-FeSi2 at 500–700 °C. Optical absorption measurements showed that the band gap of all films is direct in nature, ranging from 0.88 to 0.93 eV. The deposition temperature was seen to affect the crystallinity of the as-deposited films and to vary their optical properties significantly. The photoabsorption coefficient, measured at 1 eV, increased from 5.6 ×104 cm-1 for amorphous films to 1.2 ×105 cm-1 for the samples deposited at 700 °C. The films crystallized by heat-treatment had a markedly different and irregular structure, resulting in their lower optical absorption.

Annealing and deposition temperature dependence of the bandgap of amorphous FeSi2 fabricated by co-sputter deposition

We report here on the synthesis, optical and structural analysis of amorphous and polycrystalline FeSi2 fabricated by co-sputter deposition. For comparisons, a range of deposition temperatures from room temperature up to 700 °C and post-anneals between 300 and 700 °C were performed. Optical absorption measurements were taken on all samples and results reveal that the bandgap remains direct in nature ranging from 0.897 to 0.949 eV. It was found that for amorphous thin films, annealing at low temperatures below 500 °C had little effect on the optical properties. The bandgap value and absorption coefficient only significantly increased upon annealing above 500 °C. This was found to be in good agreement with the transformation of the silicide from its amorphous phase to its crystalline β-phase. In comparison, the deposition temperature was seen to affect the crystallinity of the as-deposited thin films and to vary both the optical and structural properties of the layers significantly. An increase in the deposition temperature not only decreased the bandgap energies but also significantly increased the photo-absorption by an order of magnitude.

The properties of β-FeSi2 fabricated by ion beam assisted deposition as a function of annealing conditions for use in solar cell applications

In this paper we investigate the formation of b-FeSi2 (from co-deposited layers of Fe and Si produced by ion beam assisted deposition) under a number of annealing regimes (annealing temperatures between 100 and 900 C and times up to 18 h) by optical characterisation of the band edge parameters. The results have indicated that both annealing temperatures and times have a strong effect on the number of defects underneath the fundamental edge of absorption. The measurement temperature dependency of the band gap is also found to be dependent on the annealing conditions. 2002 Published by Elsevier Science B.V.

Change of Auger-electron emission from Ni-Pd alloys under magnetic phase transition

The change of Auger-electron emission from polycrystals of disordered ferromagnetic NiPd3 and Ni3Pd alloys, under ferro- to paramagnetic transition, has been studied experimentally. It has been shown that the intensity of the Auger-lines, which are formed because of transition of valent zone 3d3/2 and 3d5/2 electrons, has local maxima near the Curie point T C for the alloys. Thus, the sensitivity of Auger-electron emission to a magnetic state of the alloy has been established.