Peter R. Wilshaw

Initial in vitro interaction of osteoblasts with nano-porous alumina

In the present study we have used a characterised primary human cell culture model to investigate cellular interactions with nano-porous alumina. This material, prepared by anodisation, is being developed as a coating on titanium alloy implants. The structure of the alumina, as determined by X-ray diffraction and transmission electron microscopy, was amorphous. When studying cell/material interactions we used both biochemical and morphological parameters. Cell viability, proliferation and phenotype were assessed by measurement of redox reactions in the cells, cellular DNA, tritiated thymidine ([3H]-TdR) incorporation and alkaline phosphatase (ALP) production. Results showed a normal osteoblastic growth pattern with increasing cell numbers during the first 2 weeks. A peak in cell proliferation was seen on day 3, after which cell growth decreased, followed by an increase in ALP production, thus indicating that the osteoblastic phenotype was retained on the alumina. Cell adhesion was observed, the osteoblast-like cells having a flattened morphology with filipodia attached to the pores of the material. SDS-PAGE and western blot measurements showed that the nano-porous alumina was able to adsorb fibronectin. Trace amounts of aluminium ions were measured in the surrounding medium, but no adverse effect on cell activity was observed.

Oxygen-dislocation interactions in silicon at temperatures below 700 °C: Dislocation locking and oxygen diffusion

The locking of dislocations by oxygen atoms in Czochralski–silicon at temperatures between 350 and 700 °C has been studied. Both experimental and theoretical investigations were carried out for different oxygen concentrations, different annealing times (from 10 to 3×107 s), and different point defect concentrations. It was found that the unlocking stress of dislocations at low temperatures follows similar trends to those previously observed at higher temperatures and is determined by annealing temperature, time, and oxygen concentration. However, in the present temperature range, experimental results indicate an enhanced transport of oxygen to dislocations. Numerical simulations solving the diffusion equation for oxygen transport to the dislocations show that the effective diffusivity of oxygen at lower temperatures diverges from “normal” diffusivity of oxygen. We have shown that oxygen transport can be as much as three orders of magnitude higher than that which would be assumed by extrapolation of the “n...

A novel nano-porous alumina biomaterial with potential for loading with bioactive materials.

Nano-porous alumina, with the potential for being loaded with bioactive materials, has been proposed as a novel material for coating implants. In this study, the shear strength of the interface between such nano-porous anodic aluminium oxide (AAO) coatings and titanium substrates, their biocompatibility, and their potential for pore loading have been investigated. An interface shear strength in excess of 29 MPa was obtained which is comparable with that of conventional plasma sprayed hydroxyapatite implant coatings. The viability and differentiation of MG63 osteoblastic cells co-cultured on the coating was found to be broadly comparable to that of similar cells co-cultured on conventional bioinert implant materials such as titanium and fully dense alumina. Extensive pore loading with silica nano-particles of different sizes and in different combinations was demonstrated throughout the thickness of AAO layers 1 microm and 60 microm thick. This work has demonstrated, that with suitable choice of pore filling materials, this novel coating might simultaneously combat infection, encourage bone regeneration, and secure fixation of the implant to bone.

A study of the effects of post‐deposition treatments on CdS/CdTe thin film solar cells using high resolution optical beam induced current

The optical beam induced current (OBIC) technique was applied using a scanning optical microscope (SOM) to study n‐Cds/p‐CdTe thin film solar cells which had been subjected to different post‐deposition treatments. High spatial resolution maps were obtained of the current collection with and without an applied reverse bias. The quantum efficiency of the devices was also measured with high spatial resolution. The results both quantify and illustrate vividly the manner in which the well known CdCl2 treatment increases collection efficiency. The high uniformity in the best cells indicates that grain boundaries do not play a substantial role in limiting collection efficiency.

On the locking of dislocations by oxygen in silicon

Abstract Locking of dislocations by oxygen atoms in Czochralski silicon has been investigated both experimentally and theoretically. Experiments were performed at annealing temperatures between 700 and 850°C for different annealing times and different oxygen concentrations. These showed five distinct regimes for the unlocking stress as a function of annealing time. First the unlocking stress increases almost linearly with time and then saturates. The saturation stress, the time needed to reach saturation and the duration of saturation depend on the annealing conditions and oxygen content. Following the saturation a rapid increase and a second saturation of the unlocking stress with increasing annealing time were observed. Finally after long anneals the locking effect is much reduced. From the temperature dependence of the first saturation stress the interaction energy between an oxygen atom and a dislocation has been deduced and it is shown that the change in entropy when an oxygen atom is trapped at a dislocation is significant. The transport of oxygen to dislocations has also been investigated by solving the diffusion equation numerically. For these calculations both trapping and emission of oxygen atoms from the dislocation core have been considered.

Onset of slip in silicon containing oxide precipitates

We report a study of the behavior of dislocations at oxide precipitates in (001) Czochralski silicon wafers for different oxide-precipitate sizes (100–600 nm), densities (108−1011 cm−3), and background oxygen concentrations (7.7×1017−10.35×1017 cm−3) using a bending technique with annular knife edges causing a biaxial stress distribution in the samples. The main advantage of the method we use is the possibility of detecting single slip events that may be caused by precipitates with special properties. We found that the stress level at which dislocation movement can be detected around precipitates depends mainly on the mean-precipitate diameter. The stress threshold at which dislocations begin to move can be increased by a thermal treatment prior to application of an external stress. This effect is due to the diffusion of oxygen to the dislocations causing a locking effect and shows that the dislocations are associated with the oxide precipitates prior to any external stress being applied. It has been show...

Oxygen and Nitrogen Transport in Silicon Investigated by Dislocation Locking Experiments

The behavior of oxygen and nitrogen impurities in silicon has been investigated using a novel dislocation locking technique. The locking effect of oxygen in Czochralski silicon (CZ-Si) was investigated in the 350-850°C temperature range and was found to display five well-defined regimes as a function of annealing time. Results indicate that enhanced transport of oxygen to dislocations takes place for temperatures below ∼700°C . Numerical simulations of the enhanced oxygen transport indicate that the effective diffusivity becomes dependent on oxygen concentration with an activation energy of approximately 1.5eV . The same technique has been used to investigate nitrogen transport in nitrogen-doped float-zone silicon in the 550-830°C temperature range and shows nitrogen to have a comparable locking effect to oxygen in CZ-Si, despite being present in a concentration that is 2 orders of magnitude lower. The stress required to unlock dislocations at 550°C which have previously been immobilized by nitrogen during an annealing step, initially increases approximately linearly with the duration of the anneal before saturating to a steady-state value of approximately 50MPa for all anneal temperatures investigated. An expression for the transport of nitrogen to the dislocations was deduced, which has an activation energy of 1.45eV

The effect of impurity-induced lattice strain and Fermi level position on low temperature oxygen diffusion in silicon

Oxygen diffusion in silicon is known to be affected by high concentrations of impurities, although the mechanism underpinning this is poorly understood. We have studied oxygen transport in Czochralski silicon by analyzing data on the locking of dislocations by oxygen as a function of time and temperature. In this paper, we present new data from crystals grown to contain high levels of germanium and arsenic. We analyze these new data, together with our previous data for silicon with a high boron concentration, to further the understanding of the mechanism by which high impurity concentrations affect oxygen transport at temperatures at which the oxygen dimer dominates transport (up to 550 °C). Our results show that a high level of boron doping (∼3 × 1018cm−3) enhances the effective diffusivity of oxygen by a factor of ∼8 to ∼25 relative to low doped material with the same oxygen concentration. High levels of germanium doping (∼8 × 1019cm−3) and arsenic doping (∼2 × 1019cm−3) can both have a slight retardati...

Enhanced oxygen diffusion in highly doped p-type Czochralski silicon

The locking of dislocations by oxygen has been investigated experimentally in Czochralski silicon (Cz-Si) with different concentrations of shallow dopants. Specimens containing well-defined arrays of dislocation half-loops were subjected to isothermal anneals in the 350–550°C temperature range, and the stress required to bring about dislocation motion at 550°C was then measured. This dislocation unlocking stress was found to increase with annealing time due to oxygen diffusion to the dislocation core. The dislocation unlocking stress was measured in n-type Cz-Si with a high antimony doping level (∼3.4×1018cm−3) and p-type Cz-Si with a low boron doping level (∼1.3×1015cm−3). An analysis of the data taking the different oxygen concentrations into account showed that the rate of increase in dislocation unlocking stress was unaffected by the high level of antimony doping. This indicates that a high antimony doping level has no significant effect on oxygen transport for the conditions used in this experiment. ...

Nitrogen diffusion and interaction with dislocations in single-crystal silicon

The results of dislocation unlocking experiments are reported. The stress required to unpin a dislocation from nitrogen impurities in nitrogen-doped float-zone silicon (NFZ-Si) and from oxygen impurities in Czochralski silicon (Cz-Si) is measured, as a function of the unlocking duration. It is found that unlocking stress drops with increasing unlocking time in all materials tested. Analysis of these results indicates that dislocation locking by nitrogen in NFZ-Si is by an atomic species, with a similar locking strength per atom to that previously deduced for oxygen atoms in Cz-Si. Other experiments measure dislocation unlocking stress at 550 °C in NFZ-Si annealed at 500–1050 °C. The results allow an effective diffusivity of nitrogen in silicon at 500–750 °C to be inferred, with an activation energy of 3.24 eV and a diffusivity prefactor of approximately 200 000 cm2 s−1. This effective diffusivity is consistent with previous measurements made at higher temperatures using secondary ion mass spectrometry. Wh...