Aidan Cowley

The luminescent properties of CuAlO2

In this work we examine the room temperature photoluminescence, Raman and low temperature photoluminescence properties of CuAlO2 prepared using different precursors. At room temperatures the luminescence associated with bulk CuAlO2 occurs at 355 nm and is associated with strong resonant Raman effects. At low temperatures we find that the UV emission is dominated by strong electron–phonon coupling leading to a Franck–Condon type emission band. A second strongly phonon coupled band is also observed in the blue spectral region. In addition we also show that at low temperatures the luminescent properties of CuAlO2 are meta-stable, with anomalous temperature dependence. The possible origins of the blue band, the meta-stability and anomalous temperature dependence are discussed.

Dellafossite CuAlO2 film growth and conversion to Cu–Al2O3 metal ceramic composite via control of annealing atmospheres

In this work we demonstrate simple techniques to form well crystallised CuAlO2 powders and thick films from CuO and boehmite or alumina, using a novel molten salt painting process. We examine the formation mechanism using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and in situ high temperature X-ray diffraction and find that the annealing atmosphere plays a critical role. From this we develop a method to create Cu–Al2O3 conductive metal–ceramic composite materials with novel morphologies via the thermal decomposition of CuAlO2 precursor films.

Enhanced Seebeck coefficient in silicon nanowires containing dislocations

In recent years, research on thermoelectric (TE) materials has intensified—thanks to the exciting potential of low-dimensional structures such as nanowires. Experiments have shown that nano-structuring materials can greatly reduce their thermal transport properties, significantly enhancing thermoelectric performance. With reduced thermal conductivity, nano-structured silicon—which is plentiful and low-cost—becomes a competitive TE material, but still trails traditional TE materials in overall performance. In this study, we show that the creation of extended defects within the crystal structure of silicon nanowires can create an additional enhancement. Relative to regular silicon nanowires, extended defects lead to an increased Seebeck coefficient. The effect is a consequence of the creation of dislocations and dislocation-loops, intentionally introduced in the nanowires. These defects create nano-scale potential barriers which theoretical studies have predicted can enhance silicons thermopower by energy ...

Influence of substrate metal alloy type on the properties of hydroxyapatite coatings deposited using a novel ambient temperature deposition technique.

Hydroxyapatite (HA) coatings are applied widely to enhance the level of osteointegration onto orthopedic implants. Atmospheric plasma spray (APS) is typically used for the deposition of these coatings; however, HA crystalline changes regularly occur during this high-thermal process. This article reports on the evaluation of a novel low-temperature (<47°C) HA deposition technique, called CoBlast, for the application of crystalline HA coatings. To-date, reports on the CoBlast technique have been limited to titanium alloy substrates. This study addresses the suitability of the CoBlast technique for the deposition of HA coatings on a number of alternative metal alloys utilized in the fabrication of orthopedic devices. In addition to titanium grade 5, both cobalt chromium and stainless steel 316 were investigated. In this study, HA coatings were deposited using both the CoBlast and the plasma sprayed techniques, and the resultant HA coating and substrate properties were evaluated and compared. The CoBlast-deposited HA coatings were found to present similar surface morphologies, interfacial properties, and composition irrespective of the substrate alloy type. Coating thickness however displayed some variation with the substrate alloy, ranging from 2.0 to 3.0 μm. This perhaps is associated with the electronegativity of the metal alloys. The APS-treated samples exhibited evidence of both coating, and significantly, substrate phase alterations for two metal alloys; titanium grade 5 and cobalt chrome. Conversely, the CoBlast-processed samples exhibited no phase changes in the substrates after depositions. The APS alterations were attributed to the brief, but high-intensity temperatures experienced during processing.

Fabrication and characterisation of GaAs nanopillars using nanosphere lithography and metal assisted chemical etching

We present a low-cost fabrication procedure for the production of nanoscale periodic GaAs nanopillar arrays, using the nanosphere lithography technique as a templating mechanism and the electrochemical metal assisted etch process (MacEtch). The room-temperature photoluminescence (PL) and Raman spectroscopic properties of the fabricated pillars are detailed, as are the structural properties (scanning electron microscopy) and fabrication process. From our PL measurements, we observe a singular GaAs emission at 1.43 eV with no indications of any blue or green emissions, but with a slight redshift due to porosity induced by the MacEtch process and characteristic of porous GaAs (π-GaAs). This is further confirmed via Raman spectroscopy, where additionally we observe the formation of an external cladding of elemental As around our nanopillar features. The optical emission is enhanced by an order magnitude (∼300%) for our nanopillar sample relative to the planar unprocessed GaAs reference.

Development of B-spline X-ray Diffraction Imaging techniques for die warpage and stress monitoring inside fully encapsulated packaged chips

Advanced packaging is a key “More than Moore” (MtM) enabling technology [1]. In all of these advanced packaging processes the semiconductor die are becoming much thinner (e.g. 25-50 μm thick) and many packages include multiply stacked silicon die. This leads to very thin packages where there is a trade-off between the thickness of constituent package layers and their rigidity, thus leading to reliability problems. Currently there are no compelling metrologies that can non-destructively measure the stress and/or warpage of the semiconductor die inside these packaged chips. Furthermore, since the thermal processing of these packages leads to the generation of thermal/mechanical stresses a new metrology, which is capable of real-time, or near real-time, monitoring of the generation or amelioration of these stresses during the thermal processing, would be a major advantage. In this study, we report on recent advances in the development of a new technique, which we describe as B-Spline X-Ray Diffraction Imaging (B-XRDI), which produces a reconstruction of strain field and/or lattice misorientation data from x-ray diffraction data/images of the in situ semiconductor die inside a test wirebonded encapsulated BGA package. High-speed digital x-ray topography images are captured at a synchrotron source (ANKA, Germany and Diamond, UK) in times as short as 8 seconds for a full 8 mm × 8 mm semiconductor die inside the fully encapsulated packages. Using a laboratory-based source (Jordan Valley Bede D1 High Resolution X-Ray Diffractometer) and applying the B-Spline technique, maps are also produced of the entire silicon die, which reveal warpage via measurements of x-ray rocking curve full-widths-at-half-maximum (FWHM) as a function of position across the encapsulated packages. These maps are also correlated with warpage measurements performed by mechanical and interferometric profilometry and finite element modelling (FEM).

Nondestructive Monitoring of Die Warpage in Encapsulated Chip Packages

We describe an X-ray diffraction imaging technique for nondestructive, in situ measurement of die warpage in encapsulated chip packages at acquisition speeds approaching real time. The results were validated on a series of samples with known inbuilt convex die warpage, and the measurement of wafer bow was compared with the results obtained by optical profilometry. We use the technique to demonstrate the impact of elevated temperature on a commercially sourced micro quad flat nonlead chip package and show that the strain becomes locked in at a temperature between 94 °C and 120 °C. Using synchrotron radiation at the Diamond Light Source, warpage maps for the entire 2.2 mm × 2.4 mm × 150-μm Si die were acquired in 50 s, and individual line scans in times as short as 500 ms.

Study of exciton-polariton modes in nanocrystalline thin films of CuCl using reflectance spectroscopy

CuCl thin films grown on (100) Si by thermal evaporation are studied using reflectance spectroscopy. The reflectance spectra in the near UV spectral range close to the CuCl bandgap are modeled using a dielectric response function based on an exciton-polariton response with various models involving dead layers and reflected waves in the thin film. The exciton-polariton structure obtained is compared to other studies of bulk CuCl crystals. These different models are analyzed using a matrix-based approach and they yield theoretical spectra of reflected intensity. The fits provide parameter values which can be compared to bulk data known for CuCl and provide a non-destructive means of quantitative analysis of CuCl thin films. The best models are shown to match the experimental data quite well, with the closest fits produced when thin film front and rear interfaces are included. This model also accurately simulates the Fabry-Perot fringes present at energies lower than the Z3 free exciton position in CuCl (at 3.272 eV).

Electroluminescence of γ-CuBr thin films via vacuum evaporation depositon

γ-CuBr is a I–VII wide band gap mixed ionic–electronic semiconducting material with light emitting properties suitable for novel UV/blue light applications. Its structural and physical properties allow for vacuum deposition on a variety of substrates and herein we report on the deposition of γ-CuBr on Si and indium tin oxide coated glass substrates via vacuum evaporation with controllable film thickness from 100 to 500 nm. Temperature dependent photoluminescence characteristics of these γ-CuBr films on Si (1 0 0) reveal familiar Zf and I1 excitonic features. A thin film electroluminescent device using a γ-CuBr active layer was fabricated and room temperature electroluminescence was obtained for γ-CuBr for the first time. CuBr features relating to known excitonic (Zf, 3.1 eV) emissions were observed as well as a number of previously unknown emissions at 3.81, 3.02, 2.9, 2.75 and 2.1 eV. We speculate on the origins of these peaks and attribute them to the presence of monovalent Cu+ generated during ac excitation.

Evaluation of conduction mechanism and electronic parameters for Au/organic?inorganic CuCl hybrid film/ITO structures

Hybrid materials are capable of combing organic and inorganic compounds into a nano-composite with unique characteristics. An example of such organic–inorganic CuCl hybrid films is studied here using a combination of organic polysilsesquioxane and inorganic CuCl; γ-CuCl is an ionic I–VII compound semiconductor material with the zincblende structure at room temperature. It has excellent ultraviolet (UV) emission properties at room temperature and is a promising candidate material for optoelectronic applications. The CuCl hybrid films were deposited on indium tin oxide (ITO)-coated glass by simple spin-coating techniques. Au/CuCl hybrid film/ITO structures were fabricated, and field-dependent electrical studies were carried out at room temperature in the range 2.5 × 105–3.5 × 106 V m−1. We confirm that the organic–inorganic CuCl film structure behaves as an effective single semiconducting medium, possessing bandstructure and other related properties. For electric field magnitudes up to 1.25 × 106 V m−1, an ohmic conduction mechanism was observed, and for field magnitudes >1.5 × 106 V m−1, Schottky emission conduction prevails in these structures. The electronic parameters were evaluated and an effective barrier height, ideality factor and series resistance were found to be 0.84 ± 0.05 eV, 1.12 ± 0.08 and 50 ± 2 MΩ, respectively, whereas the effective barrier height obtained from the C–V measurement was 1.05 ± 0.05 eV. This value is somewhat higher than the value obtained from the I–V measurement. This difference is likely caused by the presence of a thin intervening insulating layer between the hybrid film surface and the Schottky metal. The density distribution of the interface states decreases with an increase of the energy of the interface states. This organic–inorganic CuCl hybrid film behaves as an effective single semiconductor material structure, and a schematic energy-level diagram for the device is proposed.