exploit one of the fundamental strengths of the spectroscopic methodology, which is the ability to compare theoretical work with practical experiments. “Generally, this can prove very difficult,” says Tuomisto. “But, in the case of positron annihilation
spectroscopy, direct
comparisons can be made between the two, which is very useful.” To improve the validity of their research,
the Positron Physics and Defect Spectroscopy Group is keen to develop lab experiments in contexts that accurately recreate the world in which semiconductors operate. “Very little work has been done on transient semiconductor experiments, where you look at changing ambient conditions which are ultimately reversible,” explains Tuomisto. “These might include modifying the temperature of a material, or the illumination it is subjected to – either of which may alter its conductive properties, or change it from a basic to an excited state. Experimentally, conducting dynamic experiments is unprecedented in the field of positron annihilation research.”
- is also being addressed by the Aalto collective.
“Positron
AT A GLANCE annihilation
experiments can be carried out in two ways,” says Tuomisto. “They can either use ‘fast’ or ‘slow’ positrons. Fast positrons are employed to look at thick materials, in the range of tenths of millimetres. However,
they can’t be used to analyse
exceptionally thin subjects that are perhaps one or two micrometers thick, as they would pass through the material. Instead, slow positrons are used, as they travel at lesser velocities and will remain within the material.” Because efficient production of slow
positrons often requires a nuclear reactor or particle accelerator, this presents logistical challenges for scientists without requisite facilities or adequate radiation shielding. Indeed, as positrons are produced in facilities that are, in several territories, tightly regulated, scientists may in fact be unable to acquire them. To mitigate
these difficulties, Project Information
Project Title: Atomic, molecular and nanoscale spectroscopy with positrons
Project Objective: The development of new experimental and theoretical tools opens new horizons in condensed matter research with positron annihilation spectroscopy. Developments such as transient positron spectroscopy enable major breakthroughs in, e.g. understanding the physics of defects in semiconductors and devices, and provide new opportunities to the science & technology of (opto) electronics and photovoltaics.
Project Funding: Academy of Finland, European Commission, Aalto University
the Aalto
group intends to improve the moderation efficiency of the processes used to create slow positrons through the application of semiconductor
technology, which could
significantly improve outputs. “Combining these developments should
enable us to study atomic and electronic structural interfaces, particularly those with lower periodicity than crystalline systems,” says Tuomisto. “All of these are becoming more technologically significant. Most technology developed in the latter half of the twentieth century is actually based on crystalline silicon. Semiconductors play an important part.” “For example” says Tuomisto, “the PS3
A positron trapped in a divacancy in germanium
Other important challenges remain for
scientists in this area, such as understanding interfaces between crystalline solids, and establishing the significance of surrounding elements, like air, on the crystalline surface. To understand new generations of semiconductors
(including two-
dimensional, nanostructured materials), new analytical methodologies must be developed. Different approaches are also required to scrutinise biological and polymer-based materials like plastic, whose structure is more complex. Another
researchers - the availability of positrons
www.projectsmagazine.eu.com
uses a nitride-based semiconductor laser, which emits blue light. But certain atomic defects which occur in these devices may cause malfunctions over longer times, and are not yet fully understood.” To assess the reliability of the latest semiconductors,
and understand the logistical obstacle facing
limits to which they can be pushed, further research is needed. Similarly, opportunities to exploit novel materials with great potential for niche applications exist, although several challenges must be overcome to industrialise them. “Some materials such as zinc oxide are experimentally promising, and cheap due to their abundance,” explains Tuomisto. “However, progress in this area has stalled, creating a clear need for research which can help to develop future generations of semiconductors that meet modern technological requirements.”
★ 35
Contact: Tel: +358-50-3841799 Email:
filip.tuomisto@aalto.fi Web:
http://physics.aalto.fi/groups/ positron
MAIN CONTACT
Filip Tuomisto Filip Tuomisto obtained his PhD in engineering physics from the Helsinki University of Technology, Finland, in 2005. He has led the positron research group since 2006. At present he holds an Associate Professor position at the Department of Applied Physics, Aalto University, Finland. Prof. Tuomisto has published over 150 scientific articles.
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