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Characterisation of the electrical and mechanical properties of defects in semiconductors particularly with reference to photovoltaics. Development of novel passivation and gettering techniques for application to photovoltaics. The development of silicon substrates with very high resistivity which are suitable for high frequency active and passive devices. Understanding oxygen and nitrogen diffusion in silicon - Fermi level and strain effects. High resolution 2D mapping of dopant distributions in semiconductors.

Determination of grain boundary orientation in multi-crystalline silicon by photon reflection
Dr. J.D. Murphy, Dr. S. Senkader*, Dr. P.R. Wilshaw
Silicon solar cells account for over 80% of the solar cell market and this market is consistently growing at up to 40% per annum. Most of the silicon used for solar cells comes in the form of multi-crystalline wafers, in which the grains are typically 1 to 20mm across. Wafer manufacturers are in urgent need of a low-cost technique which allows them quickly to identify the orientations of the grains. The project involves developing such a technique by analysing the patterns produced when a laser beam reflects from the wafer surface. The pattern analysis is expected to be similar to that carried out in electron backscattered diffraction (EBSD). The project is in collaboration with Dr S. Senkader of ScanWafer, Norway, who has also provided some project funding. (* REC Wafer, Norway)

Atom probe tomography study of dislocations in mc-Si for solar cells
S. Lotharukpong, Prof CRM Grovenor, Dr. P.R. Wilshaw
The efficiency of processed mc-Si solar cells is adversely affected by the presence of dislocations decorated with transition metal impurities. However, whilst the concentration of these impurities is often known in the starting material and sometimes in the finished devices there is, as yet, no information as to which types of impurities, in what concentration, remain at dislocations after device processing. Thus one of the most important factors determining the efficiency of one of the most common types of solar cells is not properly understood. In this project we are measuring the electrical activity of individual dislocations in mc-Si and then, using focussed ion beam techniques, making an arom probe tomography sample of the same dislocation to measure, on an atom by atom basis, what impurities are present. In this way the atoms present at a particular dislocation can be directly correlated with that dislocation's electrical activity. Samples are being selected from different stages of processing to determine, for example, what effect steps such as impurity gettering have on the impurities present at the defects.

Novel passivation processes for semiconductor surfaces
R.S. Bonilla, R. Hiratsuka, Dr. J.D. Murphy, Dr. S.C. Speller, Dr. P.R. Wilshaw
Carrier recombination at surfaces and interfaces in solar cells reduces their effciency. We are devloping new processes which can be applied in a laboratory environment and perhaps, in the future, in an industrial context to passivate these structures. Our work focusses on manipulating the electrical charge associated with surface dielectric layers to repel the carriers from the interface so that recombination is mainly eliminated. We are investigating SiO2, Al2O3 and TiO2.

Development of novel geometry silicon solar cells
G. Martins, Dr. J.D. Murphy, Dr. P.R. Wilshaw
Silicon based solar cells are already sufficiently efficient to form a viable renewable energy source in the coming decades. Some estimates predict they will deliver more than 30% of the world's energy needs in the foreseeable future. However, they are presently too expensive and research is needed to drive costs down. This project continues work in the group to develop a novel solar cell geometry that would allow the use of poor quality but very cheap silicon whilst maintaining high electrical efficiencies.

Semi-insulating Czochralski silicon substrates for microwave devices
D. Jordan, Dr. K. Mallik, Jian Yang, Professor R.J. Falster**, Dr. C.H. de Groot*, Professor P. Ashburn*, Dr. P.R. Wilshaw, Dr. K. Strickland***, Dr. P. Osborne***
Silicon (Si) and silicon-germanium (Si-Ge) technology has now reached a point where silicon-based group IV-IV semiconductor devices are capable of operating at frequencies up to 100 GHz, approaching the frequencies of many III-V compound semiconductor devices. However, at these frequencies standard Si substrates grown by the Czochralski (Cz) technique become very difficult to use because of the high absorption of microwave power by background free carriers present in the substrate. This results in an unacceptable degradation of the circuit performance. Thus, the performance of silicon radio frequency (RF) technology is limited to a large extent by the properties of the Si substrate used. Furthermore, this substrate-dependent restriction in the potential uses of Si will become increasingly important in coming years as even faster Si-based devices are developed. The availability of semi-insulating Cz-Si substrates would remove this limitation, and hence lead to a paradigm shift in the RF technology by extending the reach of Si-based technologies through to higher frequencies. The aim of this project is to search for suitable deep level compensating impurities and determine the processing conditions required to produce semi-insulating handle wafers for application in RF SOI. It brings together the expertise of the University of Oxford in characterisation and diffusion of deep level impurities and that of the University of Southampton in high frequency measurements and device fabrication. The project is actively supported by close involvement of a multinational wafer manufacturer, MEMC, who will deliver the starting material and of a UK/EU RF device manufacturer, Plessey Electronics, who will fabricate passive and active microwave devices on the novel high-resistivity substrates. Potentially useful initial results obtained by us include observation of resistivity ca. 500kohm-cm in Au-doped Cz-Si wafers where there has been at least a ten-fold increase in the resistivity, and effectiveness of the SiO2 diffusion barrier to Au diffusion at 1000 C. (*University of Southampton, **MEMC Electronic Materials, ***Plessey Semiconductors.) (Funded by the EPSRC.)

New Detectors for Transmission Electron Microscopy
Professor A.I. Kirkland, Dr. G. Moldovan, Dr. P.R. Wilshaw, Dr. C. Lin, Dr. L. Cervera
Current generation imaging detectors for Transmission Electron Microscopy rely on a complex electron-photon conversion chain with the photons being detected by Charge Coupled Devices. As a result the overall sensitivity of these systems is poor and they are limited in their frame rate. We aim to construct the next generation of direct electron detector and this project will involve both computation and ultimately fabrication of a prototype device. (Funded by Leverhulme Trust)

Secondary electron mapping of doped regions in semiconductors
Dr. A. Chee*, Dr. P.R. Wilshaw
The secondary electron (SE) signal in an SEM is used to produce 2-dimensional maps of doped regions in silicon and III-V semiconductors. SE images of cross-sections of doped silicon devices reveal the type and extent of doping. Quantitative information about the observed contrast has been obtained experimentally. A model has been proposed and is being developed to account for the effect and new measurements are being made using energy filtering of the secondary electrons. (*Dept of Materials Science and Metallurgy, University of Cambridge)

Dislocation control in silicon using nitrogen implantation
Dr. J.D. Murphy, Professor R.J. Falster*, Dr. A. Jain**, Dr. P.R. Wilshaw
With the advent of advanced device technologies often involving heterostructures and strained layers there is an increasing problem with the generation and movement of dislocations in the near surface regions of Si wafers. This project is investigating the possibility of using ion implanted nitrogen as a high concentration source of electrically inactive nitrogen which can subsequently diffuse to and hence "lock" near surface dislocations so preventing the damaging effects that would be caused by their movement. In collaboration with Texas Instruments and MEMC Electronic Materials Inc.. (*also of MEMC Electronic Materials Inc. **Texas Instruments, USA).

The properties of nitrogen and oxygen in silicon
Dr. J.D. Murphy, Professor R.J. Falster*, Dr. P.R. Wilshaw
Nitrogen doped silicon is an attractive candidate for improved substrates for the next generation of high-performance electronic devices. As well as pinning dislocations (hence increasing wafer strength) and reducing the formation of void regions, nitrogen is known to affect oxygen precipitation. Since device manufacturers use oxygen precipitates in "internal gettering" processes to trap harmful impurities (such as iron, copper and nickel) in regions of the material in which they are least detrimental to device performance, the addition of nitrogen provides them with more flexibility in these gettering processes. Despite its undoubted promise for improved wafer performance, the properties of nitrogen in silicon are still largely unknown. This project will aim to further the understanding of nitrogen in silicon, and its interaction with oxygen, using a novel dislocation locking technique developed in Oxford over the past few years. The mechanism of nitrogen transport will be studied, elucidating the species responsible and the role of oxygen, together with parameters that describe its interaction with dislocations and hence the mechanical strength of wafers. With support from MEMC Electronic Materials Inc.. (*also of MEMC Electronic Materials Inc.).

Defect engineering to improve the cost-effectiveness of multicrystalline silicon solar cells
Dr. J.D. Murphy, M. Wu, G. Martins, T. Burton, Professor R.J. Falster*, Dr. P.R. Wilshaw
Silicon solar cells account for over 80% of the solar cell market and this market is consistently growing at up to 40% per annum. Most of the silicon used for solar cells comes in the form of multi-crystalline wafers sliced from ingots of cast silicon. This material contains relatively high concentrations of metallic impurities, dislocations and grain boundaries, all of which enhance electron-hole recombination and hence reduce solar cell efficiency. This project aims to develop a range of novel ideas originating from the Semiconductor Group in Oxford. Ideas being developed include the use of low temperature processes to clean-up the material ("gettering") and removing defects from the material in other novel ways. The research relies on a wide range of experimental techniques, including basic electrical characterisation (IV, CV etc), electron beam induced current (EBIC), quasi-steady state photoconductance decay (QSS-PCD), deep-level transient spectroscopy (DLTS), cathodoluminescence (CL) and transmission electron microscopy (TEM). The research is performed in collaboration with leading suppliers of silicon to the photovoltaic industry (*also of MEMC Electronic Materials Inc.).

10 public active projects

Determination of grain orientations in multi-crystalline silicon by reflectometry
Y. Wang, J.D. Murphy, P.R. Wilshaw
Journal of the Electrochemical Society, 157 H884 (2010)

The development of semi-insulating silicon substrates for microwave devices
D. M. Jordan, R. H. Haslam, K. Mallik, R. J. Falster, P. R. Wilshaw
Journal of the Electrochemical Society, 157 H540 (2010)

An investigation into fracture of multi-crystalline silicon
B.R. Mansfield, D.E.J. Armstrong, P.R. Wilshaw, J.D. Murphy
Solid State Phenomena, 156-158 55 (2010)

Semi-insulating silicon for microwave devices
D.M. Jordan, K. Mallik, R.J. Falster, P.R. Wilshaw
Solid State Phenomena, 156-158 101 (2010)

A novel nano-porous alumina biomaterial with potential for loading with bioactive materials
A.R. Walpole, Z. Xia, C.W. Wilson, J.T. Triffitt, P.R. Wilshaw
Journal of Biomedical Materials Research - Part A, 90 46 (2009)

Secondary electron emission contrast of quantum wells in GaAs p-i-n junctions
E. Grunbaum, Z. Barkay, Y. Shapira, K.W.J. Barnham, D.B. Bushnell, N.J. Ekins-Daukes, M. Mazzer, P. Wilshaw
Microscopy and Microanalysis, 15 125 (2009)

Cathodoluminescence assessment of annealed silicon and a novel technique for estimating minority carrier lifetime in silicon
K.J. Fraser, R.J. Falster, P.R. Wilshaw
Materials Science and Engineering B, 159-160 194 (2009)

Nitrogen in silicon: diffusion at 500 to 700°C and interaction with dislocations
C.R. Alpass, J.D. Murphy, R.J. Falster, P.R. Wilshaw
Materials Science and Engineering B, 159-160 95 (2009)

Measurements of dislocation locking by near-surface ion-implanted nitrogen in Czochralski silicon
C.R. Alpass, A. Jain, J.D. Murphy, P.R. Wilshaw
Journal of the Electrochemical Society, 156 H669 (2009)

Nitrogen diffusion and interaction with dislocations in single-crystal silicon
C.R. Alpass, J.D. Murphy, R.J. Falster, P.R. Wilshaw
Journal of Applied Physics, 105 013519 (2009)

The development of semi-insulating silicon substrates for microwave devices
D.M. Jordan, R.H. Haslam, K. Mallik, P.R. Wilshaw
ECS Transactions, 16 41 (2008)

Measurements of dislocation locking by near-surface ion-implanted nitrogen in Czochralski silicon
C.R. Alpass, J.D. Murphy, A. Jain, P.R. Wilshaw
ECS Transactions, 16 249 (2008)

Cathodoluminescence assessment of low temperature gettering in silicon and a novel technique for estimating bulk minority carrier lifetime in silicon
K.J. Fraser, R.J. Falster, P.R. Wilshaw
ECS Transactions, 16 273 (2008)

Counting electrons in transmission electron microscopes
G. Moldovan, X. Li, P. Wilshaw, A.I. Kirkland
Microscopy and Microanalysis, 14 912 (2008)

Thin silicon strip devices for direct electron detection in transmission electron microscopy
G. Moldovan, X. Li, P. Wilshaw, A.I. Kirkland
Nuclear Instruments and Methods in Physics Research A, 591 134 (2008)

Piezospectroscopic measurement of the stress field around an indentation crack tip in ruby using SEM cathodoluminescence
R.I. Todd, D. Stowe, S. Galloway, D. Barnes, P.R. Wilshaw
Journal of the European Ceramics Society, 28 2049 (2008)

The role of dislocations in producing efficient near-bandgap luminescence from silicon
K. Fraser, D. Stowe, S. Galloway, S. Senkader, R. Falster, P. Wilshaw
Physica Status Solidi C, 4 2977 (2007)

Out-diffusion of nitrogen from float-zone silicon measured by dislocation locking
C. R. Alpass, J.D. Murphy, A. Giannattasio, S. Senkader, R.J. Falster, P.R. Wilshaw
Physica Status Solidi A, 204 2256 (2007)

Semi-insulating Czochralski-silicon for radio frequency applications
K. Mallik, C.H. de Groot, P. Ashburn, P.R. Wilshaw
Proceedings of the 36th European Solid-State Device Research Conference, 435 (2006)

Enhanced oxygen diffusion in highly-doped p-type Czochralski silicon
J.D. Murphy, P.R. Wilshaw, B.C. Pygall, S. Senkader, R.J. Falster
Journal of Applied Physics, 100 103531 (2006)

Nitrogen-doped silicon: mechanical, transport and electrical properties
J.D. Murphy, C.R. Alpass, A. Giannattasio, S. Senkader, D. Emiroglu, J.H. Evans-Freeman, R.J. Falster, P.R. Wilshaw
ECS Transactions, 3 239 (2006)

Nitrogen in silicon: transport and mechanical properties
J.D. Murphy, C.R. Alpass, A. Giannattasio, S. Senkader, R.J. Falster, P.R. Wilshaw
Nuclear Instruments and Methods in Physics Research B, 253 113 (2006)

Oxygen transport in Czochralski silicon investigated by dislocation locking experiments
J.D. Murphy, S. Senkader, R.J. Falster, P.R. Wilshaw
Materials Science and Engineering B, 134 176 (2006) Wilshaw, P.R., Mallik, K. and Falster, R.J. (2006). 'Substrate for High Frequency Integrated Circuits UK Patent 0618202.6 15th Sept 2006'.

Mallik, K., de Groot, C.H., Ashburn, P. and Wilshaw, P.R. (2006). 'Enhancement of resistivity of Czochralski silicon by deep level manganese doping' Applied Physics Letters 89(11).

Breese, M.B.H., Huang, L., Teo, E.J., King, P.J.C. and Wilshaw, P.R. (2005). 'Transmission ion channeling analysis of isolated 60 degrees misfit dislocations' Applied Physics Letters 87(21).

Stowe, D.J., Fraser, K.J., Galloway, S.A., Senkader, S., Falster, R.J. and Wilshaw, P.R. (2005). 'Efficient, room-temperature, near-band gap luminescence by gettering in ion implanted silicon'. "Microscopy of Semiconducting Materials". Cullis, A.G.,Hutchison, J.L. 107 355-358.

Murphy, J.D., Giannattasio, A., Alpass, C.R., Senkader, S., Falster, R.J. and Wilshaw, P.R. (2005). 'The influence of nitrogen on dislocation locking in float-zone silicon'. "Gettering and Defect Engineering in Semiconductor Technology Xi". 108-109 139-144.

Evans-Freeman, J.H., Emiroglu, D., Vernon-Parry, K.D., Murphy, J.D. and Wilshaw, P.R. (2005). 'High resolution deep level transient spectroscopy applied to extended defects in silicon' Journal of Physics-Condensed Matter 17(22) S2219-S2227.

Giannattasio, A., Murphy, J.D., Senkader, S., Falster, R.J. and Wilshaw, P.R.: 'Oxygen and nitrogen transport in silicon investigated by dislocation locking experiments' Journal Of The Electrochemical Society 152 (6) (2005) G460-G467.

Giannattasio, A., Murphy, J.D., Senkader, S., Falster, R.J. and Wilshaw, P.R.: 'Oxygen and nitrogen transport in silicon investigated by dislocation locking experiments (vol 152, pg G460, 2005)' Journal Of The Electrochemical Society 152 (8) (2005) L12-L12.

Murphy, J.D., Giannattasio, A., Senkader, S., Falster, R.J. and Wilshaw, P.R.: 'Nitrogen transport in float-zone and Czochralski silicon investigated by dislocation locking experiments' Physica Status Solidi A-Applications And Materials Science 202 (5) (2005) 926-930.

Senkader, S., Giannattasio, A., Falster, R.J. and Wilshaw, P.R.: (2004). 'Dislocation locking in silicon by oxygen and oxygen transport at low temperatures'. Gettering And Defect Engineering In Semiconductor Technology. 95-96: 43-52.

Barkay, Z., Grunbaum, E., Shapira, Y., Wilshaw, P., Barnham, K., Bushnell, B., Ekins-Daukes, N.J. and Mazzer, M. (2004). High-resolution scanning electron microscopy of dopants in p-i- n junctions with quantum wells. Electron Microscopy And Analysis 2003: 143-146.

Briggs, E.P.K., M., Walpole, A.R., Palsgard, E. and Wilshaw, P.R.: 'Formation of highly adherent nano-porous alumina on Ti-based substrates: a novel bone implant coating.' Journal Of Materials Science: Materials in Medicine 15 (2004) 1-9.

Giannattasio, A., Senkader, S., Falster, R.J. and Wilshaw, P.R.: 'The role of prismatic dislocation loops in the generation of glide dislocations in Cz-silicon.' Computational Materials Science 30 (1-2) (2004) 131-136.

Mallik, K., Falster, R.J. and Wilshaw, P.R.: 'Schottky diode back contacts for high frequency capacitance studies on semiconductors.' Solid-State Electronics 48 (2) (2004) 231-238.

Giannattasio, A., Murphy, J.D., Senkader, S., Falster, R.J. and Wilshaw, P.R. (2004). Impurity Locking of Dislocations in Silicon. Electrochemical Society Proceedings "High Purity Silicon": 39-54.

Booth S.E., Marsh C.D., Mallik K., Baranauskas V., Sykes J.M. and Wilshaw P.R.: 'Fabrication of nanocrystalline aluminium islands using double- surface anodization' Journal of Vacuum Science & Technology B 21, 316-318 (2003).

Giannattasio A., Senkader S., Azam S., Falster R.J. and Wilshaw P.R.: 'The use of numerical simulation to predict the unlocking stress of dislocations in Cz-silicon wafers' Microelectronic Engineering 70, 125 (2003).

Karlsson M., Palsgard E., Wilshaw P.R. and Di Silvio L.: 'Initial in vitro interaction of osteoblasts with nano-porous alumina' Biomaterials 24, 3039-3046 (2003).

Mallik K., Falster R.J. and Wilshaw P.R.: ''Semi-insulating' silicon using deep level impurity doping: problems and potential' Semiconductor Science and Technology 18, 517-524 (2003).

Senkader S., Giannattasio A., Falster R.J. and Wilshaw P.R.: 'On the dislocation-oxygen interactions in Czochralski-grown Si: oxygen diffusion and binding at low temperatures' Journal of Physics-Condensed Matter 14, 13141-13145 (2002).

Senkader S., Giannattasio A., Falster R.J. and Wilshaw P.R.: 'Dislocation locking in silicon by oxygen and oxygen transport at low temperatures' Solid State Phenomena 95/96, 43-52 (2003).

Giannattasio, A., Senkader, S., Falster, R.J. and Wilshaw, P.R.: 'Dislocation locking by nitrogen impurities in FZ-silicon.' Physica B-Condensed Matter 340 (2003) 996-1000.

Stowe, D.J., Galloway, S.A., Senkader, S., Mallik, K., Falster, R.J. and Wilshaw, P.R.: 'Near-band gap luminescence at room temperature from dislocations in silicon.' Physica B-Condensed Matter 340 (2003) 710-713.

Stowe, D.J., Galloway, S.A., Senkader, S., Mallik, K., Falster, R.J. and Wilshaw, P.R. (2004). A room temperature cathodoluminescence study of dislocations in silicon. Electron Microscopy And Analysis 2003: 67-70.

Walpole, A.R., Briggs, E.P., Karlsson, M., Palsgard, E. and Wilshaw, P.R.: 'Nano-porous alumina coatings for improved bone implant interfaces.' Materialwissenschaft Und Werkstofftechnik 34 (12) (2003) 1064-1068.

Giannattasio A., Senkader S., Falster R.J. and Wilshaw P.R.: 'Generation of dislocation glide loops in Czochralski silicon' Journal of Physics-Condensed Matter 14, 12981-12987 (2002).

Production of Semi-Insulating Silicon for High Frequency Devices
P R Wilshaw / Kanad Mallik

This project will provide an opportunity to research the making of semi-insulating (resistivity of 10-100kohm-cm) silicon-on-insulator (SOI) substrates grown by the Czochralski technique for microwave monolithic integrated circuits (MMIC). This has been identified as a novel substrate material in the ITRS Roadmap and has the potential to bring about a paradigm shift in the semiconductor industry.

The research involves enhancement of the resistivity of SOI handle wafers by doping with transition elements, which introduce deep impurity levels in the silicon energy band gap. Another challenging aspect of the research is to confine these dopants to the handle wafer by use of diffusion barriers. The work will provide an excellent opportunity to get training and gain experience in a wide range of experimental skills in modern semiconductor science and technology. For example:

• Transition element doping techniques, including ion implantation and diffusion.
• Characterization techniques like, deep level transient spectroscopy (DLTS), secondary ion mass spectroscopy (SIMS), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and microwave absorption measurements.
• Some of the characterisation experiments will involve training in high vacuum techniques and cryogenics.
• Semiconductor device fabrication using photolithography.
• Working in semiconductor-grade cleanrooms in Southampton and Oxford.
• There is also scope for numerical modelling work concerning the electronic properties of semiconductors and microwave device simulation.

The student will work in a team involving members of the Department of Materials, Oxford, the Electronic Engineering Department, Southampton, a multi-national Si wafer manufacturer, and a UK/EU semiconductor device manufacturer.

Also see homepages: Kanad Mallik Peter Wilshaw

New detectors for transmission electron microscopy
A Kirkland / P R Wilshaw / G. Moldovan

Current generation imaging detectors for Transmission Electron Microscopy rely on a complex electron-photon conversion chain with the photons being detected by Charge Coupled Devices. As a result the overall sensitivity of these systems is poor and they are limited in their frame rate. We aim to construct the next generation of direct electron detector and this project will involve both simulation and fabrication of prototype devices.

Also see homepages: Angus Kirkland Peter Wilshaw

Defect engineering to increase the efficiency of multi-crystalline silicon solar cells
Dr P.R. Wilshaw, Dr J.D. Murphy

Silicon solar cells account for over 80% of the solar cell market and this market is consistently growing at up to 40% per annum. Most of the silicon used for solar cells comes in the form of multi-crystalline wafers sliced from ingots of cast silicon. This material contains relatively high concentrations of metallic impurities, dislocations and grain boundaries, all of which enhance electron-hole recombination and hence reduce solar cell efficiency. This project aims to develop a range of novel ideas originating from the Semiconductor Group in Oxford. Ideas to be developed include the use of low temperature processes to clean-up the material ("gettering") and removing defects from the material in other novel ways. The research will rely on a wide range of experimental techniques, including basic electrical characterisation (IV, CV etc), electron beam induced current (EBIC), deep-level transient spectroscopy (DLTS), quasi-steady-state photoconductance decay (QSS-PCD), cathodoluminescence (CL) and transmission electron microscopy (TEM). The research will be performed in lose collaboration with leading suppliers of silicon to the photovoltaic industry.

Also see homepages: John Murphy Peter Wilshaw

Also see a full listing of New projects available within the Department of Materials.