Jonathan Painter

Structural dynamics in CdS–CdTe thin films

Abstract The performance of CdS–CdTe heterojunction solar cells depends critically upon the structures formed during thin film deposition and any subsequent processing. We have undertaken a detailed examination of solar cell materials (in particular CdTe and CdS) which has enabled some correlation between their fundamental properties and structural behaviour as thin films. In particular we have determined the Vegard coefficients and phase diagram for the CdS–CdTe system. We have also examined the diffusion characteristics of both single-crystal and polycrystalline CdTe and CdS with respect to Te and S in order to define the rate at which any intermixed region may grow. Thus we have determined several fundamental properties of CdTe and CdS which were either not available or apparently anomalous. These data have been used to underpin and interpret findings from studies of the structural and electronic changes that occur during the type conversion anneal of CdTe. In particular, we have shown how an intermixed region forms during the heat treatment and that this could be mediated by the initial, as-deposited structures. We have also been able to contrast the behaviour of CdTe films produced by PVD and electrodeposition. In order to characterise the structure of these thin films it has been essential to develop novel depth profiling methods based upon our primary analytical methods, i.e. X-ray diffraction and ion-beam analysis. These techniques, when used with the fundamental material properties, are shown to provide complementary information that has allowed us to build models of the CdTe and CdS layers that may allow the formation of the intermixed region to be controlled during the fabrication process.

Sulfur diffusion in cadmium telluride thin films: Part 1: the grain boundary diffusion coefficient

Previous data on sulfur diffusion in single crystal CdTe has been expanded to include the more practical example of diffusion in polycrystalline CdTe thin films. Samples of vacuum evaporated CdTe were sealed, with a source of sulfur vapour, in evacuated silica ampoules and annealed at temperatures of either 372, 450 or 550 °C for times up to 28 days. Sulfur diffusion profiles were measured by secondary ion mass spectrometry. A new method to correct for finite film thickness was used to determine the grain boundary diffusion coefficients. These show that films grown at high temperatures, such as those produced by close-space sublimation, may possess sulfur enriched grain boundaries that extend through the complete thickness of the CdTe film. However, extensive decoration of the grain boundary by hexagonal CdS x Te 1-x is believed unlikely.

Novel depth profiling in Cds-CdTe thin films.

As the material structures of CdS-CdTe heterojunction solar cells have a significant effect on cell efficiency, there is a requirement to investigate new methods for thin film, structural, depth profiling. In an attempt to characterise structural details such as stress, stoichiometry and texture, we have developed a novel method of depth profiling. This is based upon a chemical etch bevel followed by spatially resolved X-ray diffraction and Rutherford backscattering spectrometry. We show that the method provides a depth resolution of better than 0.1 μm. We have used this method to examine CdTe thin films (∼ 1.8 μm) produced by electrodeposition supported upon CdS (< 0.1 μm). We present the results of these studies and use the method to investigate the effect upon the structures of a type conversion anneal. The data is compared to previous studies using different depth profiling methods. The results indicate the formation of an spatially limited, intermixed CdTe (1-x) S x layer and the conversion of the whole CdS film into a CdS (1-y) Te y layer. The structural characteristics are correlated to optical and electrical properties of the films.

The structural features of TiO2 thin films formed by pyrosol methods

Photovoltaic cells employing an extremely thin absorber layer can be fabricated as p-i-n devices with TiO2 as the transparent, n-type component. We have examined in detail the structural features of such TiO2 films fabricated via an aerosol pyrolysis method. A precursor solution, produced by dissolving Ti powder in hydrogen peroxide and ammonium hydroxide, was diluted in water and atomized using a 2.4-MHz ultrasonic nebulizer. The resultant aerosol was transported with a carrier gas to heated, ITO-coated glass substrates. The effects on the film structures of different fabrication conditions were investigated. Samples were structurally characterized by X-ray powder diffraction and electron microscopy. Average properties were determined using synchrotron X-rays and a detailed mapping of structural features was provided by a laboratory diffraction system. The crystalline films were shown to consist of mostly anatase and to be compact. On average, the TiO2 crystallites possessed significant microstrain that remained constant with increasing precursor concentration. Mapping across the aerosol “footprint” showed that the thickest and most crystalline films were formed from the greatest precursor concentration at 350 °C. Furthermore, it was also shown that the film crystallinity was significantly lower in a “penumbra” region, where a greater degree of preferred orientation was demonstrated.

On differences in the equation-of-state for a selection of seven representative mammalian tissue analogue materials

Tissue analogues employed for ballistic purposes are often monolithic in nature, e.g. ballistic gelatin and soap, etc. However, such constructs are not representative of real-world biological systems. Further, ethical considerations limit the ability to test with real-world tissues. This means that availability and understanding of accurate tissue simulants is of key importance. Here, the shock response of a wide range of ballistic simulants (ranging from dermal (protective/bulk) through to skeletal simulant materials) determined via plate-impact experiments are discussed, with a particular focus on the classification of the behaviour of differing simulants into groups that exhibit a similar response under high strain-rate loading. Resultant Hugoniot equation-of-state data (Us-up; P-v) provides appropriate feedstock materials data for future hydrocode simulations of ballistic impact events.

On the dynamic tensile strength of Zirconium

Despite its fundamental nature, the process of dynamic tensile failure (spall) is poorly understood. Spall initiation via cracks, voids, etc, before subsequent coalesce, is known to be highly microstructure-dependant. In particular, the availability of slip planes and other methods of plastic deformation controls the onset (or lack thereof) of spall. While studies have been undertaken into the spall response of BCC and FCC materials, less attention has paid to the spall response of highly anisotropic HCP materials. Here the dynamic behaviour of zirconium is investigated via plate-impact experiments, with the aim of building on an ongoing in-house body of work investigating these highly complex materials. In particular, in this paper the effect of impact stress on spall in a commercially sourced Zr rod is considered, with apparent strain-rate softening highlighted.

Numerical classification of curvilinear structures for the identification of pistol barrels

This paper demonstrates a numerical pattern recognition method applied to curvilinear image structures. These structures are extracted from physical cross-sections of cast internal pistol barrel surfaces. Variations in structure arise from gun design and manufacturing method providing a basis for discrimination and identification. Binarised curvilinear land transition images are processed with fast Fourier transform on which principal component analysis is performed. One-way analysis of variance (95% confidence interval) concludes significant differentiation between 11 barrel manufacturers when calculating weighted Euclidean distance between any trio of land transitions and an average land transition for each barrel in the database. The proposed methodology is therefore a promising novel approach for the classification and identification of firearms.

On the influence of texture on spall evolution in the HCP materials Ti-6Al-4V and Zr

Dynamic tensile failure (spall) is known to be a highly microstructure-dependant phenomena. In particular, spall is greatly influenced by the availability of plastic deformation modes such as slip systems. Significant effort has been put into understanding spall in the common engineering BCC and FCC materials, however there is a relative paucity of data on such behaviour in the highly anisotropic HCP class of materials. Here, preliminary results pertaining to the dynamic behaviour of two important HCP materials, Ti-6Al-4V and Zr, are presented, with the aim of enhancing understanding of this complex class of materials.

On a novel graded areal density solution to facilitate ramp wave generation in plate-impact studies

Building on a substantial body of work on functionally graded materials in the literature, it has been previously shown that the use of graded areal density impactors, in conjunction with buffer materials, allows generation of ramp-wave loading profiles in impacted targets. Such off-principle-Hugoniot loading paths are of particular interest where control of one or more state variables (e.g. temperature) is desirable during the loading event. Previous attempts to produce suitable graded areal density impactors have focused on rapid prototyping techniques such as 3D printing. While suitable for small-scale production of impactors, such technologies are relatively immature. Instead, here a novel approach to creating graded areal density structures -- TWI Ltd.’s novel surface modification process, Surfi-Sculpt®, with a nominal surface spike distribution of 1.5 per mm2, has been employed to produce the required impactors. Initial experimental results are presented highlighting the potential of this experiment...

Tolerance of Artemia to static and shock pressure loading

Hydrostatic and hydrodynamic pressure loading has been applied to unicellular organisms for a number of years due to interest from food technology and extremophile communities. There is also an emerging interest in the response of multicellular organisms to high pressure conditions. Artemia salina is one such organism. Previous experiments have shown a marked difference in the hatching rate of these organisms after exposure to different magnitudes of pressure, with hydrostatic tests showing hatching rates at pressures up to several GPa, compared to dynamic loading that resulted in comparatively low survival rates at lower pressure magnitudes. In order to begin to investigate the origin of this difference, the work presented here has focussed on the response of Artemia salina to (quasi) one-dimensional shock loading. Such experiments were carried out using the plate-impact technique in order to create a planar shock front. Artemia cysts were investigated in this manner along with freshly hatched larvae (nauplii). The nauplii and cysts were observed post-shock using optical microscopy to detect motility or hatching, respectively. Hatching rates of 18% were recorded at pressures reaching 1.5 GPa, as determined with the aid of numerical models. Subjecting Artemia to quasi-one-dimensional shock loading offers a way to more thoroughly explore the shock pressure ranges these organisms can survive.