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Alison Harrison

  • Development of 3D electron holography for mapping of electrostatic fields with nanometre resolution
  • Development of in situ electrical biasing experiments in the transmission electron microscope (TEM) to investigate current and voltage driven materials properties on the nanoscale
  • Characterisation of novel semiconducting, ferroelectric and multiferroic device structures using transmission and scanning electron microscopy techniques
 Contact details:
Office: Bessemer 323
Tel: +44 (0)20 7594 9693
Email: a.harrisonimperial.ac.uk

Research Interests
Alison Harrison’s (neé Twitchett) research interests include the development of electron microscopy techniques for the examination of novel nanostructures including semiconductor and ferroelectric devices, and the development of equipment for the application of electrical bias to specimens in situ in the transmission and scanning electron microscopes. In particular, her research focuses on the development of electron holography as a quantitative tool for the characterisation of dopant potentials in semiconductor device structures, and its combination with electron tomography to allow 3D mapping of nanostructures.

Recent Publications

  • Twitchett-Harrison AC, Yates TJV, Newcomb SB, Dunin-Borkowski RE, Midgley PA, Nano Letters 7 (7) 2020-2023 (2007)

Three dimensional mapping of dopant distributions in nanoscale semiconductor devices is essential to characterise device properties and needs to be performed under “working conditions” ie under a bias field in situ in the microscope. This paper demonstrates the first application of combined electron holography/electron tomography in the TEM illustrating how this novel technique can be used to determine quantitatively the three-dimensional electrostatic potential in an electrically biased semiconductor device with nanometre spatial resolution. This novel technique promises to satisfy the need defined by ITRS (www.itrs.net) for a 3D quantitative dopant profiling technique with sub-nm resolution.

  • Twitchett AC, Dunin-Borkowski RE, Hallifax RJ, Broom RF, Midgley PA Microsc. Microanal 11 (1): 66-78 (2005)

Off-axis electron holography can be used to determine the electrostatic potential in a semiconductor device. However, the experimental noise present in the original data limits quantification of the charge density. This paper describes simulations of the phase profiles which were fitted to off-axis electron holographic phase profiles of an electrically biased silicon p-n junction device. The best-fitting simulated profiles were differentiated, thereby providing access to the quantitative charge density profiles. This approach revealed that in-situ biasing of the device increases the measured active dopant concentration of the device structure, highlighting the importance of characterizing semiconductor device structures under ‘working conditions’.

  • Twitchett AC, Dunin-Borkowski RE, Hallifax RJ, Broom RF, Midgley PA, J. Microsc 214 (Pt3): 287-296 (2004)

This paper presents the design and construction of a novel holder for in situ electrical biasing experiments using a new FIB specimen geometry in the TEM. This work led to the development of commercial biasing holders (Fischione Instruments). The effects of FIB preparation on the electrostatic potential in the specimen were assessed using off-axis electron holography, revealing that no external electrostatic fields are present. This result allows the phase change measured using electron holography to be interpreted quantitatively without the need for complex modeling as it arises only from the internal electrostatic potential within the specimen.

Biography

  • Lecturer in Nanometrology at the Department of Materials at Imperial College and the London Centre for Nanotechnology (2007-present)
  • Senior Research Associate, University of Cambridge (2006-2007)
  • University Lectureship (unestablished), University of Cambridge (2005-2006)
  • Junior Research Fellowship, Newnham College Cambridge (2002-2005)
  • Ph.D. on ‘Electron Holography of Semiconductor Devices,’ University of Cambridge (2002)
  • M.Sci. in Natural Sciences, Trinity Hall, University of Cambridge (1999)

Other Activities

Teaching:

  • Lecturing at undergraduate and postgraduate levels.
  • Undergraduate tutorials

Membership of professional bodies:

  • Institute of Physics, Member

Research
This image shows the 3D electrostatic potential in a 380 nm-thick cross-section of a silicon device structure containing a p-type to n-type semiconductor junction.  This data was acquired using the techniques of off-axis electron holography and tomography in a transmission electron microscope, with a 3V reverse electrical bias applied to place the device structure under ‘working’ conditions.  This data reveals that substantial modification of the potential is apparent near the top and bottom surfaces of the reconstruction arising from a combination of sample preparation effects and properties of semiconductors (such as surface depletion) near surfaces. The amorphous and crystalline surface layers have been represented schematically and were not reconstructed tomographically. Equipotential contours spaced every 0.2 V have been superimposed onto the reconstructed tomogram, highlighting the electrostatic potential distribution in the y-z cross-section of the specimen. 

This is the first 3-D visualisation of the electrostatic potential in a semiconductor device structure under applied electrical bias.  These results indicate that this new technique of 3D holography is a very powerful tool for the 3D imaging of many novel device structures with nanometre resolution.