Physics
Semiconductors are critical components in modern electronics. Interestingly, their useful properties are derived from defects in their atomic structure, and so improving semiconductor technology depends on understanding and manipulating the effects of these microscopic faults. To explore them in sufficient detail, a team of Finnish scientists has developed novel experimental methodologies using positrons, the antimatter counterparts of electrons
Improving semiconductors using positron spectroscopy
for example, help to develop computers which are more powerful, but use less electrical power.” The team’s work has been significantly
enhanced through an ongoing collaboration with Aalto’s Centre of Excellence for Computational Nanoscience (COMP). To date, the group’s activities include studies of group IV semiconductors and wide band gap semiconductors, including type III- nitrides and zinc oxide. Recently, it has begun to analyse more complex alloys, and to pursue an exciting new research theme: nanostructured, biological matter. Tuomisto and his colleagues are passionate
A positron trapped in an In vacancy in InN
Semiconductors are materials offering varying levels of electrical conductivity and resistance. This renders them suitable for numerous applications, including computer processors, solar cells and LEDs. But, rather than being naturally occurring, these attributes need to be carefully engineered. “These properties are often realised through the introduction of impurities at an atomic level,” explains Filip Tuomisto, a professor in the Department of Applied Physics at the Aalto University School of Science. “However, when you create them, you risk producing defects you didn’t intend, which might have unpredictable consequences.” Free electrons carry electrical current
across semiconductors. Structurally, these materials are typically solids, possessing an ordered, crystalline lattice composition through which electricity flows. Minute structural defects, such as irregularities or vacant lattice slices, help to determine its
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conductive properties by adjusting the potential landscape ‘felt’ by the electrons. Impurities can also influence certain aspects of a semiconductor’s behaviour, such as its mechanical properties and responses to light. Understanding how these defects
function, and reproducing them through the chemical ‘doping’ of materials, can help to engineer bespoke semiconductors. Tuomisto’s current research interests include the development of new spectroscopic tools for state-of-the-art materials research, which can be used to help develop novel semiconductors. As leader of the Positron Physics and Defect Spectroscopy Group at Aalto University, Tuomisto is keen to develop new theoretical and practical approaches that are applicable to current technologies. “Our work is relevant, as it’s
indirectly applicable to
many electrical systems,” he says. “Research in this area is very important, and could,
advocates of positron annihilation spectroscopy, a methodology that allows them to precisely determine the composition of matter at atomic level by measuring radiated energy and its interaction with matter. Emissions used by scientists include light, but may also involve other types of particle such as positrons. These are also known as ‘antielectrons’, as they are the antimatter equivalent of an electron, and can be synthesised for research purposes. In a common type of spectroscopic experiment, low-energy positrons are emitted and directed towards a sample. When the positrons interact with low energy electrons,
a phenomenon known as
annihilation occurs, producing gamma ray photons. By observing the time it takes for a positron to be annihilated (called positron lifetime) within a material, scientists can examine its atomic structure in detail, and discern features such as vacant missing atoms, atomic clusters and even quantum dots – a minute form of semiconductor crystal that exists at nanometre scale. “We can obtain additional details of the
atomic structures by looking at the Doppler broadening of the energy of the annihilation radiation,” says Tuomisto. “It provides information about the chemical identity of the atoms surrounding the site of annihilation.”
The Aalto scientists Insight Publishers | Projects
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