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Materials Science |
Though EME research is focussed on electronic materials, a broad range of studies are active. Many include the extensive usage of the departmental processing and characterisation facilities to probe fundamental questions in materials science and solid-state physics. Examples are listed below.
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Charge transport and trapping in high-k dielectric thin-films
Robert G Elliman, Nawaz Saleh Muhammad
Dielectric materials with high dielectric constant (high-k dielectrics) are needed as replacements for SiO2 in future integrated circuits, including low-power, high-density non-volatile memory (NVM) devices. Critical to such applications is an understanding of charge transport and trapping in such materials. This includes an understanding of transport through extremely thin films, such as direct and field-induced (Fowler-Nordheim (FN)) tunnelling and a range of trap-assisted tunnelling processes, as well as field-induced breakdown of such films. This project will examine such processes in an important class of high-k dielectrics based on hafnium oxide its alloys.
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Defects and Diffusion in Si Nanostructures
, Patrick Kluth, Mark Ridgway
Ion implantation into crystalline Si induces point defects. During subsequent annealing, these result in transient enhanced diffusion of dopant atoms and the formation of extended defects. In bulk Si both effects are reduced by surface proximity. For nanostructured Si the availability of free surfaces is greatly increased, modifying dopant diffusion and defect evolution. This is being investigated using a variety of techniques including transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS) and simulations.
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EXAFS characterisation of GaMnAs layers
, Mark Ridgway
GaMnAs ferromagnetic semiconductor alloys are promising materials due to their potential application in spintronic devices. In general, the GaMnAs lattice constant obeys a Vegard-type law and varies linearly with Mn concerntration. However, the exact nature of this lattice expansion is both unknown and unexpected given the Mn-As bondlength is predicted to be shorter than that of Ga-As. EXAFS at the Ga, Mn and As absorption edges is able to provide unique insight regarding the local strain and structure of both MBE grown, and ion implanted and laser annealed GaMnAs layers.
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Formation and Relaxation of Amorphous Phase Structure in Compound Semiconductors
Mark Ridgway, , Zohair Hussain,
Disorder in amorphised compound semiconductors can be both structural (distributions in bond length, bond angle, ring size and coordination number) and chemical (homopolar bonding). We utilise extended x-ray absorption fine structure (EXAFS) spectroscopy in conjunction with conventional analytical technques to determine the atomic-scale structure of the amorphous phase and identify the amorphistion and relaxation mechanisms. For example, the latter proceeds via a reduction in homopolar bonding. Binary materials investigated thus far include GaAs, GaP, InP and InAs while current interest is focussed on ternary alloys and the antimonides.
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Formation and Structural Properties of Metallic Nanocrystals
Patrick Kluth, , , Mark Ridgway
The structural properties of nanocrystals can vary significantly depending on their size, formation technique, host matrix and thermal history. We utilise extended x-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) in combination with other techniques for a detailed analysis of formation and structure of metallic nanocrystals formed by ion implantation into an insulating matrix. Furthermore the influence of subsequent ion irradiation on size and structure of the nanocrystals is investigated. EXAFS is capable of providing very accurate information about the local atomic environment and as such is very sensitive to structural disorder. Currently investigated systems are Au and Cu in a silica matrix.
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Hydrogen transport, trapping in semiconductors and its application for wafer cleaving
Robert G Elliman, Daniel Pyke
Smart-cut© is a commercial process based on hydrogen cleavage of silicon and is applied routinely in the production of silicon-on-insulator (SOI) wafers. It is however largely limited to (100) Si at this stage. Extension to (110) and (111) silicon, Ge and III-V semiconductors is highly desirable but an understanding of the operative/limiting processes is still lacking. Significantly, it has recently been shown that for (100) Si the implanted hydrogen accumulates in regions of high compressive stress rather than at the peak of the implant distribution. This has led to the use of thin, epitaxial strained layers, such as GexSi1-x, to initiate cleavage. This opens up several possibilities such as extension to III-V semiconductors, e.g. InP using InGaP strained layer, or the use of separate sources of hydrogen such as PECVD-deposited silicon-rich-oxides. This project examines hydrogen diffusion and trapping in semiconductors including the influence of controlled strain distributions.
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Ion Beam Modification of the Mechanical Properties of Nanoscale Silicon
Robert G Elliman
The mechanical properties of nanoscale amorphous and crystalline silicon can be determined by measuring the force-deflection characterisatics of self supporting silicon cantilevers with an atomic force microscope (AFM) before and after the structures have been ion-implantated. Such measurements show that the Young's modulus of amorphous silicon is around 20% less than that of crystalline silicon. Ion implantation can therefore be used to tailor the mechanical properties of such systems.
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Ion implantation processing in SiC for microelectronic applications
Jennifer Wong-Leung
Silicon carbide is a promising wide bandgap for high power, high voltage and high frequency devices. Ion implantation is the only viable way for achieving control over both in-depth and lateral dopant incorporation in devices. The project addresses some key issues in the implantation of both p and n-type dopants in silicon carbide with emphasis on the implantation related effects such as channeling, defect formation and annealing. A variety of characterisation techniques are used, namely TEM, RBS, SIMS and DLTS. The main aim of the project is to understand ion implantation effects and defect formation mechanisms and recommend optimum implantation processing for high electrical activation of dopants.
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Nanocrystalline Materials Synthesized By High-Energy Ball Milling
Ying Chen
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Palladium Gettering in Silicon
, Mark Ridgway
Gettering is a technique which is in widespread application within the semiconductor industry for the removal of detrimental transition metal impurities. However, the mechanisms by which gettering functions are not well understood. Using palladium as an impurity, we are investigating the gettering process via Rutherford Backscattering (RBS), Perturbed Angular Correlation Spectroscopy (PAC), Extended X-Ray Absorption Fine Structure (EXAFS) and Transmission Electron Microscopy (TEM), allowing us to examine the efficiency of the process as well as the local environments in which the palladium atoms are found.
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Rare-earth doping and waveguiding in HfO2 and its alloys
Robert G Elliman
Hafnium oxide is of great interest as a replacement for silicon dioxide in microelectronic devices due to its high dielectric constant (high-k). It is also of interest for optical and photonic applications due to its high refractive index (n > 2) and low optical loss. These properties make hafnium oxide and its alloys potential candidates for the fabrication of integrated planar waveguide devices or structures that combine electronic and photonic functionality on a single chip. This study will examine rare-earth doping of HfO2 films as a means of making optically active photonic structures, including waveguides.
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Semiconductor nanocrystal characterisation with synchrotron-radiation-based methods
, Mark Ridgway
The formation mechanisms and detailed local structure of semiconductor nanocrystals, formed within an insulating matrix by ion implantation and subsequent thermal annealing, are studied using techniques with an emphasis
on synchrotron radiation, such as XRD, SAXS and EXAFS, and also
TEM. EXAFS has been able to provide unique information of the nanocrystal
structure, as it probes only the local structural environment of the
elements within the nanocrystal. The local electronic structure (ie. the
chemical bonding and local, partial density of states) has been determined
using soft X-ray absorption and emission spectroscopies. Systems of
interest include: Si, Ge, GaAs and InP.
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