Ian Vickridge

Formation of modified Si/SiO2 interfaces with intrinsic low defect concentrations

The modification by postoxidation NO treatments of the Si/SiO2 interface in thermally grown Si(100)/SiO2 layers has been studied by nuclear reaction analysis and electron paramagnetic resonance spectroscopy. Our results demonstrate a selective incorporation of NO molecules at the Si/SiO2 interface and a drastic reduction in the interface defect density. In this new configuration, the Pb center density, which is typically 2×1012 cm−2 in the as oxidized samples, is reduced to below 1011 cm−2 without any hydrogen passivation. The thermal treatment in NO atmospheres opens the perspective for the formation of hydrogen free low defect Si(100)/SiOxNy interfaces conserving the qualities of the SiO2 dielectric.

Application of a nuclear microprobe to the study of calcified tissues

Abstract The mineral fraction of calcified tissue is largely calcium hydroxyapatite (bones and teeth) or calcium carbonate (shells and fish otoliths). Apatite has such a strong affinity for fluoride ions that the F/Ca ratio can vary markedly with position in a bone or tooth, depending on the amount of fluoride present at the time of calcification or partial recrystallization. New biological information can be obtained by introducing extra fluoride into the diet of an animal and using a microprobe later to scan sections of bones or teeth. In suitable burial sites extra fluoride is introduced after death, and the new distribution may have applications in forensic science and archaeology. Fish otoliths are also of interest since a new carbonate layer is formed each day and the distribution of trace elements may record some aspects of the fishs life history. Results from the following studies are presented: fluorine distributions in the teeth of sheep which ingested extra fluoride for known periods; distributions of calcium and fluorine in femurs of rats which drank water high in fluoride for periods from 2 to 15 weeks; calcium and fluorine distributions in artificially-prepared lesions in tooth enamel; diffusion profiles in archaeological human teeth and animal bones; patterns in the strontium/calcium ratio in sectioned otoliths of several species of fish.

High precision determination of 16O in high Tc superconductors by DIGME

Abstract A method is described for measuring the 16O content of high Tc superconductors with better than 1% precision by exploiting the detection of gamma rays emitted when they are irradiated by an MeV deuterium beam. The method is presently less accurate than the widely used titration and thermogravimetric methods, however it is rapid, and may be applied to materials such as Tl-containing high Tc superconductors which pose serious problems for the usual analytical methods.

Time dependence of the oxygen exchange O2↔SiO2 at the SiO2–Si interface during dry thermal oxidation of silicon

During dry thermal oxidation of silicon oxygen exchange reactions may occur between oxygen molecules (O2↔O2, catalyzed by the SiO2) or between oxygen from the gas phase and the oxygen in SiO2 (O2↔SiO2), both at the surface and at the Si–SiO2 interface. We found that the oxygen exchange rate at the Si–SiO2 interface is at least 25% of the oxygen uptake rate, requiring the movement of oxygen both from the surface to the interface, and from the interface to the surface. The oxygen exchange at the interface is further evidence in favor of the presence of a reactive interfacial layer between the growing oxide and the silicon substrate.

Oxygen transport during annealing of Pb(Zr,Ti)O 3 thin films in O 2 gas and its effect on their conductivity

Lead zirconate titanate (PZT) thin films were deposited in a reactive argon/oxygen gas mixture by radio-frequency-magnetron sputtering. The use of a metallic target allows us to control the oxygen incorporation in the PZT thin film and also, using oxygen 18 as a tracer, to study the oxygen diffusion in the thin films. Electrical properties and crystallization were optimized with a 90-nm PZT thin film grown on RuO 2 electrodes. These PZT films, annealed with a very modest thermal budget (550 °C) show very low leakage current densities (J = 2 × 10 −8 A/cm 2 at 1 V). In this article we show that a strong correlation exists between the oxygen composition in the PZT film and the leakage current density.

Narrow resonance profiling study of the oxidation of reactively sputtered Ti1−xAlxN thin films

Abstract The bottom electrode structure used with ferroelectric (FE) and high dielectric constant (HDC) materials requires a material to promote FE or HDC cristallisation (Pt or IrO2) and a material with diffusion barrier properties; this last material being between Pt (or IrO2) film and Si substrate. TiN, TiAlN and TaSiN have been proposed for diffusion applications. Ti1−xAlxN films have drawn much attention as alternatives to TiN diffusion barriers. In this paper we have investigated the effect of Al content on the oxidation resistance of Ti1−xAlxN films prepared by radio frequency reactive sputtering in a mixed Ar+N2 discharge. The concentration depth profiles of both 18O and 27Al were measured before and after the rapid thermal annealing of samples at 750°C for 30 s in 18O2, via the narrow resonances of 18O(p,α)15N at 151 keV (fwhm=100 eV) and 27Al(p,γ)28Si at 992 keV (fwhm=100 eV). It was found that Al incorporation in the films reduces oxide growth. The Al excitation curves indicate a uniform Al content for as deposited Ti1−xAlxN, and reveal Al diffusion to the surface during oxidation, which indicates the formation of an Al rich oxide layer at the Ti1−xAlxN surface. The results suggest that Ti1−xAlxN films with x>0.39 are promising candidates as electrically conductive diffusion barrier layers.

Calculation of energy loss spectra through very thin layers: Application to tandem accelerator gas strippers

The extreme asymmetry of energy loss spectra through very thin layers means that their shape and behaviour with thickness are not reflected solely by the knowledge of their two first moments. We show that for protons traversing very thin films or gas strippers the FWHM of the energy loss spectrum varies as the thickness x, even though the standard deviation varies as x, and the dependence of the most probable energy loss on x is far from linear. This peculiar behaviour is similar to predictions of the well-known Landau theory for electron energy loss. The consequences of these observations are discussed, in particular with respect to the energy spread of beams produced by tandem accelerators.

Fluorine depth profiles as a relative dating method of chipped flints

Abstract Archaeologists have an increasing need for nondestructive dating methods (even relative) applicable to chipped flints in order to better understand prehistoric stone workshop practices. We have started a systematic study on the use of the fluorine uptake by lithic material for dating, by application of the 19F(p, αγ) resonance at 872 keV to Grand Pressigny flint. The application in archaeology of this technique has given us strong evidence that the working of flint continued during the Early Bronze Age at Fort Harrouard, a prehistoric site near Paris, France. In parallel we have started a series of laboratory-based experiments in order to better understand the penetration of fluorine from groundwater into Grand Pressigny flint and we present the first results from these experiments.

The precise determination by NRA of the relative stoichiometry of silicon nitride thin films on heavy substrates

Abstract A method has been developed for measuring the relative stoichiometry of silicon nitride films less than 3000 A thick on heavy substrates. By using (d, p) reactions and a single particle detector, a precision of ± 2% in the relative stoichiometry can be obtained without standards, charge integration or knowledge of the detector solid angle. Measurement of the absolute stoichiometry is dependent on the availability of a suitable standard. An application of this method to a study of the properties of magnetron sputtered Si3N4 thin films on GaAs is described.

Modification of the Oxide/Semiconductor Interface by High Temperature NO Treatments: A Combined EPR, NRA and XPS Study on Oxidized Porous and Bulk n-Type 4H-SiC

The effect of thermal treatments in nitric oxide (NO) on the paramagnetic defects at the 4H-SiC/SiO2 interface are analyzed by EPR in oxidized porous samples. The results on ultrathin thermal oxides show that the NO treatment at 1000°C is insufficient for an efficient reduction of the two dominant paramagnetic interface defects: PbC centers and carbon clusters. From the NRA and XPS analysis of bulk samples treated under the same conditions we attribute the weak effect to the low nitrogen concentration of only 1% at the interface.