Personal Homepages

• Defect engineering to improve the cost-effectiveness of silicon-based photovoltaics
• Diffusion of light elements (e.g. nitrogen and oxygen) in silicon
• The electrical properties of oxide precipitates in silicon
• The behaviour of iron in silicon, including its solubility, interaction with extended defects and gettering
• The mechanical properties of multi-crystalline silicon, including fracture and residual stress
• Silicon carbide and silicon nitride precipitates in multi-crystalline silicon
• Lifetime spectroscopy by quasi-steady-state photoconductance (QSS-PC)

 

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.).

Oxide precipitates in silicon: recombination and luminescence
Dr. J.D. Murphy, Dr. K. Bothe*, Professor R.J. Falster**
Oxide precipitates and associated extended defects (dislocations and stacking faults) play a crucial role in Czochralski silicon for integrated circuits, as they are energetically favourable sites for unwanted transition metal impurities. They can also form during cooling of multicrystalline silicon ingots used for solar cells. In this project, the recombination of electrons and holes at these precipitates and associated defects (dislocations and impurities) is studied as a function of precipitate morphology (unstrained or strained), size and density. Quasi-steady state photoconductance (QSS-PC) is used to study minority carrier lifetime as a function of injection level in a large matrix of carefully produced specimens and parameters pertaining to recombination at these defects are determined. We are also studying room temperature photoluminescence of these defects. (*Institut fur Solarenergieforschung in Hameln/Emmerthal (ISFH), Germany; **also of MEMC Electronic Materials Inc.)

The behaviour of iron in silicon at low temperatures
Dr. J.D. Murphy, Prof. R.J. Falster*
The efficency of silicon photovoltaics is partly limited by recombination of charge carriers at impurities in the material and iron is known to have particularly deleterious effects. To measure iron concentrations to remarkably low levels (less than parts in a million million) we measure changes in lifetime that occur due to photodissociation of iron-boron pairs. We aim to study the behaviour of iron in silicon at low temperatures, including its solubility, bulk precipitation and surface precipitation. The ultimate aim of the research is improving the gettering of iron which will allow dirtier (hence cheaper) material to be used for silicon-based photovoltaics. (* also of MEMC Electronic Materials).

Studying silicon solar cells by electrically-detected magnetic resonance
V. Lang, Dr J.D. Murphy, Dr. J.J.L. Morton
Silicon solar cells account for more than 80% of the world market. Their efficiency is partly limited by recombination of charge carriers at defects in the material. We are using the technique of electrically-detected magnetic resonance (EDMR) to study recombination at oxide precipitates, which can form during cooling of multicrystalline silicon ingots. EDMR allows us to identify the specific types of defect responsible for spin-dependent recombination, such as Pb0 or Pb1 dangling bonds in the case of oxide precipitates. This information cannot be found from conventional techniques for solar cell characterisation such as quasi-steady-state photoconductance measurements. (* also of MEMC Electronic Materials Inc.)

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.

Mechanical properties of multicrystalline silicon for solar cell applications
B.R. Mansfield, Dr. T.B. Britton, Dr. D.E.J. Armstrong, Dr. S. Senkader*, Dr. A.J. Wilkinson, Dr. J.D. Murphy
The thickness of multi-crystalline silicon (mc-Si) wafers used for photovoltaic applications is predicted to halve in the next five to ten years to just 100microns. The mechanical properties of such wafers are therefore of increasing importance and this project aims to further the understanding of these properties. The research will focus on effects pertaining to silicon carbide and silicon nitride precipitates - defects which are abundant in cast mc-Si. The project exploits novel methods of testing materials at the micron scale which have been developed in Oxford. These use a combination of focussed ion beam machining (to produce specimens) and atomic force microscopy / nanoindentation (to test them). The fracture toughness of individual grain boundaries decorated with nitride and carbide precipitates will be measured. Further experiments will be performed using high-resolution electron backscatter diffraction strain mapping to understand strains around these particles. The research is being performed in collaboration with leading suppliers of silicon to the photovoltaic industry. (* REC Wafer, Norway).

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.).

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.

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).

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)

10 public active projects

26. Chemical etching to dissolve dislocation cores in multicrystalline silicon
N.J. Gregori, J.D. Murphy, J.M. Sykes, P.R. Wilshaw
Physica B, in press (2012)

25. Spin-dependent recombination in Czochralski silicon containing oxide precipitates
V. Lang, J.D. Murphy, R.J. Falster, J.J.L. Morton
Journal of Applied Physics, 111 013710 (2012)

24. The effect of oxide precipitates on minority carrier lifetime in p-type silicon
J.D. Murphy, K. Bothe, M. Olmo, V.V. Voronkov, R.J. Falster
Journal of Applied Physics, 110 053713 (2011)

23. Contamination of silicon by iron at temperatures below 800°C
J.D. Murphy, R.J. Falster
Physica Status Solidi Rapid Research Letters, 5 370 (2011)

22. Recombination at oxide precipitates in silicon
J.D. Murphy, K. Bothe, R. Krain, M. Olmo, V.V. Voronkov, R.J. Falster
Solid State Phenomena, 178-179 205 (2011)

21. The effect of impurity-induced lattice strain and Fermi level position on low temperature oxygen diffusion in silicon
Z. Zeng, J.D. Murphy, R.J. Falster, X. Ma, D. Yang, P.R. Wilshaw
Journal of Applied Physics, 109 063532 (2011)

20. Minority carrier lifetime in Czochralski silicon containing oxide precipitates
J.D. Murphy, K. Bothe, M. Olmo, V.V. Voronkov, R.J. Falster
ECS Transactions, 33 121 (2010)

19. 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)

18. 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)

17. Characterisation of plastic zones around crack-tips in pure single-crystal tungsten using electron backscatter diffraction
J.D. Murphy, A.J. Wilkinson, S.G. Roberts
IOP Conference Series: Materials Science and Engineering, 3 012015 (2009)

16. 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)

15. 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)

14. The mechanical properties of tungsten grown by chemical vapour deposition
J.D. Murphy, A. Giannattasio, Z. Yao, C.J.D. Hetherington, P.D. Nellist, S.G. Roberts
Journal of Nuclear Materials, 386-388 583 (2009)

13. 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)

12. Nanoindentation and micromechanical testing of iron-chromium alloys implanted with iron ions
F.M. Halliday, D.E.J. Armstrong, J.D. Murphy, S.G. Roberts
Advanced Materials Research, 59 304 (2009)

11. 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)

10. 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)

9. 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)

8. 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)

7. 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)

6. 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)

5. The influence of nitrogen on dislocation locking in float-zone silicon
J.D. Murphy, A. Giannattasio, C.R. Alpass, S. Senkader, R.J. Falster, P.R. Wilshaw
Solid State Phenomena, 108-109 139 (2005)

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

3. Oxygen and nitrogen transport in silicon investigated by dislocation locking experiments
A. Giannattasio, J.D. Murphy, S. Senkader, R.J. Falster, P.R. Wilshaw
Journal of the Electrochemical Society, 152 G460 (2005)

2. Nitrogen transport in float-zone and Czochralski silicon investigated by dislocation locking experiments
J.D. Murphy, A. Giannattasio, S. Senkader, R.J. Falster, P.R. Wilshaw
Physica Status Solidi (a), 202 926 (2005)

1. Impurity locking of dislocations in silicon
A. Giannattasio, J.D. Murphy, S. Senkader, R.J. Falster, P.R. Wilshaw
Proceedings of the 206th Meeting of The Electrochemical Society, High Purity Silicon VIII, 2004-05 39 (2004)

 

Recombination at extended defects in silicon solar cells
Dr J.D. Murphy

Solar cells made from multicrystalline silicon (mc-Si) make up more than half the market for photovoltaics at present. Mc-Si contains a host of defects, including grain boundaries, precipitates, dislocations and transition metal point defects. All these defects act to reduce the efficiency of the final cell as they act as recombination centres for photo-generated charge carriers. In this project, the minority carrier lifetime is studied by quasi-steady-state photoconductance (QSS-PC). Model systems (e.g. oxide precipitates) are used in order to isolate and understand the effects of individual defects. Some samples are intentionally contaminated with transition metals, to understand the effect of transition metal decoration on the recombination activity of extended defects. The project is performed in collaboration with leading suppliers of silicon to the photovoltaic industry.

Also see homepages: John Murphy

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.