Stimulating Insights into the Nanoworld
Jülich scientists measure oxygen contents in the microscopic world of the atoms
[26. März 2004]
Nobody is perfect! This is not only true of humans, but also of many materials. Perovskites, for example, which play an important role in modern electronics, in practice never form the perfectly ordered crystals needed for their application. Instead, various defects always "creep in", which influence important properties of the material. On the road to novel devices, the defect problem is one of the decisive obstacles. In the perovskites, in particular, the oxygen content deviates locally from the ideal composition. By means of a high-resolution electron microscope scientists at Research Centre Jülich are now able, for the first time, to measure this oxygen content with atomic resolution and thus analyse defects quantitatively. This will make it possible in future not only to understand the properties of these defects, but also to largely avoid them by suitable fabrication methods. This will open up the path to new electronic memories andincreasingly smaller microelectronic devices. The findings of the Jülich scientists have been published in the latest edition of the leading journal "Science" (Science, 26 March 2004).
Ceramic materials based on oxides with perovskite structure – which include barium or strontium titanate – play a major role in modern electronics. They are also already widely applied today in telephone or money cards. Perovskites are also the base material for high-temperature superconductors and will be increasingly needed in microelectronics in the future, where they are used in ultrathin films of only some ten to a few hundred atomic layers. One of the most important problems on the road to such applications is the correct adjustment of the oxygen content of these oxides, which must then be maintained across a large number of process steps in device fabrication. The requirements to be fulfilled here are high. Even the lack of just a few oxygen atoms in the electrically active zones of the thin films can significantly impair their function.
"We can now for the first time measure how many a fewoxygen atoms are", explains Prof. Knut Urban from the Jülich Institute of Solid State Research (IFF). "With our transmission electron microscope we do not record the oxygen average contents of the entire sample, as has been done in the past. We can look at regions with a diameter of the order of magnitude of an atom – we thus measure with atomic resolution." As an example, the scientists have examined a frequently occurring type of defect in a thin film of barium titanate (BaTiO3), where the thin sample does not only contain one crystal orientation but several. Tiny, nanometre-sized regions with two additional orientations are embedded here in a dominant orientation and meet each other at so-called grain boundaries. Since the samples examined are only three nanometres in thickness, that is to say three millionths of a millimetre, only about ten sites for oxygen atoms are placed on top of each other like a column on one spot. The scientists have found out that one third of these sites are empty at the grain boundaries,which means that about three to four oxygen atoms are missing in each column. "Our findings are in excellent agreement with structure-chemical analyses of other working groups", reports Dr Chun Lin Jia, a co-author of the Science article now published. "They indirectly inferred the lack of one third of the oxygen atoms from chemical substitution experiments."
For their measurements the scientists, together with colleagues from the European Molecular Biology Laboratory (EMBL) in Heidelberg and from the Darmstadt University of Technology, have developed a special electron microscope, with which they were able, for the first time, to make oxygen atoms directly visible (Jia, Lentzen, Urban, Science, Vol. 299, p. 870, 7 February 2003). In the latest Science publication the scientists have taken an important step forward. "In the first step, we obtained images of oxygen atoms in perovskites with our microscope. They reveal the chemical structure of the material", explains Knut Urban. "It took us more than twelve months to exactly understand the quantum-physical background of these images. As a result, we can now not only see the oxygen atoms, but we can also measure their content – the chemical concentration." For the future, the Jülich scientists have thus pointed out a promising approach towards controlling the properties of perovskites. In fact, they can analyse defects resulting from certain preparation conditions withatomic resolution and correlate them with the observed properties of the perovskites. They can thus track down how such defects influence the material properties and can ultimately fabricate materials with tailor-made properties.
Electron microscope image of barium titanate (BaTiO3). Each bright spot is an atom or an atom column. The grain boundaries at which regions of different crystal orientations meet are visible. The arrow marks a column of oxygen atoms at one of these boundaries, where one third of the sites remain empty.
Photo: Research Centre Jülich
Same image as above, but digitally stained
Dr. Renée Dillinger
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