Institute for Microstructure Research
General Overview
The Institut fur Mikrostrukturforschung (Institute for Microstructure Research) is working in a number of fields selected with an emphasis on the atomistic and microstructural understanding of materials properties and the possibility to contribute to the development of technology. In some of these fields the competence spans the whole range from materials preparation via basic research to technical devices. In others access to interesting materials and problems is provided by qualified collaborations. Besides this general-physics and technology part of the institute there is a second part of special competence. This is structure research by means of modern transmission electron microscopy and scanning tunnelling microscopy. This work is carried out within the Jiilich Centre for High-Resolution Electron Microscopy operated by the institute.
Research Fields
(1) Ceramic Superconductors: Thin-film and heterostructure production, Josephson effects, and their application in magnetometer systems and spectroscopic techniques.
(2) Semiconductors: Structural investigations, mainly by transmission electron microscopy, of thin films and heterostructures. In collaboration with various research groups we are studying growth-related problems, like the relaxation mechanisms in SiGe films or the influence of doping on the microstructure in as-grown and annealed low-temperature grown GaAs films. Another topic is the study of electronic states in compound semiconductors by scanning tunnelling microscopy employing a technique developed in our group. It permits, via the detection of the far-reaching Debye screening cloud at the surface, an investigation of charged doping or impurity atoms in the bulk.
(3) Metallic Alloys: This concerns two major fields, quasicrystalline alloys and structurally complex alloy phases (SCAP). We are growing large single quasicrystals and SCAP crystals for our own research but also for users world-wide. Our own work on quasicrystals and SCAP concentrates on phase-diagrams, plasticity and surface physics.
(4) Electroceramics: In the field of electroceramic materials we take advantage of our long-standing experience with respect to perovskitic materials both in preparation and in transmission electron microscopy. In collaboration with the Institut fur Elektrokeramische Materialien (Prof. Waser) we dedicate a large research capacity to the investigation of the structural aspects of the production and properties of electroceramic thin films.
(5) High-Resolution Electron Microscopy: The theoretical and technical aspects of atomic-resolution transmission electron microscopy are one of the central fields of interest of our group. Computer program packages for the exit wavefunction reconstruction developed in the institute are in use world-wide. The institute co-developed and houses the world's first aberration-corrected transmission electron microscope with a record resolution of 1.3 A at 200 kV.
Equipment
The institute has at its disposal sputtering deposition machines, some of them with three-target facilities, which were developed and built in the institute for the high-quality deposition of ceramic superconductor thin films and heterostructures. For device production local clean room, structuring and packaging facilities are available. The institute operates the Rilich Centre for High-Resolution Electron Microscopy with two 400 kV JEOL machines of the type 4000 EX/FX, a JEOL 2000 EX, a PHILIPS CM20 FEG, the spherical-aberration corrected PHILIPS CM200 FEG and a JEOL 840A scanning microscope. The priority in scanning tunnelling microscopy is on high-temperature investigations. Our instruments: Two microscopes with insitu cleaving facilities and ex-situ heating up to 750 °C. An in-situ heating STM (Omicron) was installed in 2000. For the work on alloy plasticity a Zwick mechanical testing system is available. The institute operates together with the Institut fur Streumethoden (Prof. Briickel) the IFF laboratory for crystal growth.
Special results and developments in 2001
Our dc-SQUIDs on the basis of ramp-type and bicrystal junction geometry continue their success with world record in sensitivity. As a supplier of TRISTAN (USA, formerly Conductus) our devices are used commercially. The market demands represent challenges to science and technology, and our work in this field will be continued as long as we can derive good science from it. We have a unique position with respect to the development of Hilbert-transform spectroscopy on the basis of the acJosephson effect. This technique provides an excellent tool for spectroscopy in the frequency range of 101° to 1013 Hertz. It is broad band and orders of magnitude faster than Fourier spectroscopy. Here we have an excellent collaboration with the Institute for Radioelectronics (IRE) in Moscow with which we operate a joint superconductivity group. After our successful demonstration of the applicability of Hilbert spectroscopy for the determination of the shape of electron bunches in the TESLA test facility in Hamburg the joint project with DESY was brought to an end. Experiments will be resumed as soon as the TESLA facility has reached a stable beam situation. The BMBF project of a fast gas spectrometer was finished successfully. Current developments are focussing on two aspects. (1) Still the relatively high noise level of HTc devices represents a principle obstacle for application. Our studies have shown that fabrication conditions of Josephson junctions can be still substantially improved with corresponding gains in performance. (2) The lack of high-intensity broad-band radiation sources in the
far infrared is one of the major obstacles on the way to a wider application of Hilbert spectroscopy (e.g. using the rotation bands of molecules). We are currently testing ultra-short pulse lasers as novel far-infrared sources.
Our project of a SQUID microscope is continuing. There is extraordinary strong interest in such instruments in semiconductor industry. The first prototypic parts have been built and tested successfully. We expect, after delays due to loss in personnel and difficulties with suppliers, that a prototype microscope can start operation during 2002.
The successful project of the spherical-aberration corrected transmission electron microscope in which Rilich was a partner has triggered new activities in electron optics world wide. In Germany this concerns the SATEM (our institute) and the SESAM (MPI Stuttgart) projects in collaboration with ZEISS-LEO, Oberkochen, and CEOS, Heidelberg. SATEM will be the world's first Subangstrom-Instrument. The delivery had to be postponed by LEO by 1 year to summer 2003. SATEM will maintain the institute's position as a pioneer in advanced instrumentation. However, this success has to been seen in connexion with the problem that the abundant technical problems with these prototypes require an extraordinary commitment in personnel and the need to accept excessive down times. On the other hand, the institute's basic microscope equipment is after 15 years of service definitely out of date. The result is a rather problematic situation, in particular with respect to modern materials applications. This is at the origin of plans to apply for financial support for the replacement of the JEOL 2000EX TEM and the 860A SEM.
Another field in which our institute is respected as an international leader is exit wave-function reconstruction for atomic resolution imaging in transmission electron microscopy. End of 2001 FEI has purchased our complete software package for implementation in the Tecnai series of advanced transmission electron microscopes.
The alloy physics group has successfully started an extended research program in a new field which we termed structurally complex alloy phases (SCAP). This concerns intermetallics with giant unit cells containing hundreds to thousands of atoms on which essentially nothing is known with respect to physical properties. A special Symposium is organized by us at the DPG Fruhjahrstagung 2002 in Regensburg. This field will also play a role in a Chinese-German research program on Modern Metallic Materials Design organized on behalf of the DFG and the NSFC by Profs. Herlach, Koln, and Urban, Rilich, whose kick-off meeting was held at Beijing in November 2001. We are, together with Prof. Dubois, Nancy, in the process of organizing an EU network on this subject.
In recent years great efforts went into joint doctor student programs with foreign universities. Formal contracts were signed with the Russian Academy of Sciences, the University of Kiev, the Tsinghua University and the Institute of Physics, Beijing, the Dalian University and Wuhan University, China. In the framework of this special program the doctor students are working up to three years in Rilich on a grant supervised by the Rilich Doktorandenausschul3, but they will pass their examina in their home university.
Outstanding results of the year 2001:
• The principal possibility to measure broadband spectra in the sub-teraherz range by Hilbert spectroscopy has been successfully demonstrated. The transmission spectra of CO-gas and mesh filters could be measured by Hilbert spectroscopy using a Hg-lamp as a broadband source of radiation.
• The interface structure, epitaxial strain and interfacial defect formation in the STO/SRO/LAO and BTO/MgO film systems could be elucidated by means of high-resolution TEM.
• By means of quantitative contrast analysis of electron micrographs the interdiffusion coefficient in AISb/GaSb heterostructures could be measured for the first time. Under Sb rich conditions interdiffusion is significantly faster than under Ga rich conditions. This can be explained by different diffusion mechanisms.
• By He implantation followed by subsequent temperature treatment it was possible to produce nearly entirely stress-free Sio,7Geo.3 layers on Si (100). This technologically relevant result can be explained on the basis of our TEM analyses which demonstrate that He bubbles are acting as sources for dislocations involved in stress relaxation.
• By extension of the exit wave-function reconstruction technique a method was developed by which all relevant optical parameters of atomically resolving TEM images can be determined directly in the area in which the structure is studied. These parameters are required as input in quantum-mechanical calculations of the images or the wave function.
• By cross sectional STM the roughness of the electronic interfaces of p-n GaAs multilayers was investigated. The high roughness level can be explained by dopant atom clustering induced by many-body interactions. This effect is limiting the precision of the spatial and energetic positioning of the Fermi energy in nanoscale semiconductor structures. Thus dopant atom clustering may induce an ultimate limit to miniaturization in semiconductor devices.
Prof. Dr. Knut Urban