IFF
Scientific Report 1999/2000

The research areas of the institute comprise (1) technologies for the integration of electroceramic materials into microelectronics and microsystems, (2) dielectric and ferroelectric properties of oxide ceramics, and (3) the defect structure in the vicinity of internal and external interfaces in oxides. These areas are complementary to the research areas of the Institute for Electronic Materials 2 (IWE 2) at the Aachen Technical University (RWTH). Project groups often comprise staff members and students from both institutes.

In 1999, the major event in the area of the integration technologies has been the completion of the clean room and the installation of the equipment. An official opening celebration has been on September, 23th, 1999. The deposition of oxide thin films is performed by means of MOCVD (MOCVD = Metal Organic Chemical Vapor Deposition) systems. In cooperation with AIXTRON AG, a multiwafer planetary reactor has been installed for the development and optimization of (Ba,Sr)TiO₃ films. In a first step, systematic parameter studies have been performed. Equipment for the electrical characterization of MOCVD processed BST films has been built and tested as part of an ESPRIT project (HECTOR 300). Our horizontal research MOCVD reactor has been tested by the deposition of SrTiO₃, BaTiO₃, and PbTiO₃ thin films and it is now employed for new model systems, such as the (Ba,Pb)TiO₃ solid solution which allows the materials tuning in a broad range of tetragonal ferroelectric distortions. Thin films made by CSD (Chemical Solution Deposition) at the IWE 2 in Aachen were prepared to provide high-precision compositional standards for the XRF analysis of the MOCVD films for both, the multiwafer planetory reactor and the horizontal reactor. In addition, alternative vaporizer systems have been tested. For the patterning of the ceramic films and electrode / ceramic film stacks, Reactive Ion Beam Etching (RIBE) and Reactive Ion Etching (RIE) techniques will be employed. In contrast to the situation in the standard Si and compound semiconductor technologies, dry etching processes of oxide ceramics have hardly been investigated as yet and, hence, represent a research area in which basic studies and industrially funded applied research can be linked in a beneficial manner. The RIBE system has been installed in the clean room is currently tested for a variety of reactive gases. The integration processes are complemented by metallization methods based on electron beam and sputter techniques. Within this area, our studies aim at a better understanding of the processes and material parameters which govern the adhesion, the mechanical stress, and the microstructure.

The second research area focuses on the dielectric and ferroelectric properties of oxide thin films and bulk materials, which are being investigated in Jülich as well as in Aachen. The material systems are based on compositions used for practical devices and model systems, e.g. SrTiO₃, BaTiO₃, SrBi₂Ta₂O₉, Pb(Zr,Ti)O₃ and (Ba_1-xPb_x)TiO₃. One of the research topics is the microscopic understanding of ferroelectric hysteresis including new approaches for the separation of reversible and irreversible contributions to polarisation based on the analysis of frequency-dependent small and large signals. These studies are linked to the aging (imprint) phenomenon, i.e. the polarisation-dependent shift of the hysteresis curve with time and to the ferroelectric fatigue process, i. e. the reduction of the remanent polarisation by cycling. Both aging and fatigue processes play an important role in the operation of the novel non-volatile memories (Ferroelectric Random Access Memories, FeRAM). Impedance spectroscopy in the lower GHz regime is employed to determine the relaxation of the ferroelectric domain wall motion and to separate this contribution from the contribution of the crystal lattice. By varying the microstructure of the ceramics and by comparison between bulk ceramics and thin films, the model of Arlt will be extended with respect to the impact on 2D constraints imposed by mechanical stress due to the presence of substrates. For dielectric ceramics, impedance spectroscopy is used to elucidate the interrelation of extrinsic losses and lattice defects. This activity includes the development and characterization of new microwave ceramics and is embedded into a cooperation with the Institute for Microstructural Research (Urban) and Norbert Klein’s group within the framework of a BMBF-Leitprojekt. In the case of ferroelectric materials, existing theories are further developed and extended towards a more quantitative description of the dielectric, piezoelectric, and elastic properties. Numerical finite-element-methods are used to describe the mutually coupled mechanical, thermal, and electrical properties of ceramic components such as multilayer capacitors and actuators.

Our third research area comprises the lattice disorder in the vicinity of internal interfaces (grain boundaries) and external interfaces (surfaces and electrode interfaces) and their impact on electronic and ionic (oxygen ions and protons) charge transport. In the case of acceptor and donor doped titanate ceramics, the studies are focused on the formation of space charge depletion layers at grain boundaries as well as the related potential barriers and the transport of charge carriers along and across the grain boundary barrier. A hot-pressing technology has been developed to decorate the grain boundary area with additional dopants and to study the influence of these artifical grain boundary states. In some material systems, such as titanate-zirconate solid solutions, it is necessary to determine the equilibrium constants of the defect reactions and the diffusion constants of the system in order to create the basis for the research on interfaces. In this respect, the comparison of bulk ceramics and thin films of the same composition is of vital interest. In thin film systems, the significant influence of the electrode metals, the unexpectedly high stability under conditions of dc-voltage-induced resistance degradation, as well as the tolerance of the lattice concerning the incorporation of non-stoichiometries represent current research topics. Research on electronically conducting perovskites can be divided into studies on semiconducting and on metallically conducting oxides. In semiconducting, donor-doped SrTiO₃, the emphasis is placed on the interrelationship between point defects and extended defects involved in oxidation and reduction processes using single crystals and ceramics. The metallically conducting manganates and cobaltates are investigated with respect to their magnetoresistive and ferromagnetic properties. By comparing epitaxial and polycrystalline thin films, the influence of the grain boundaries on charge transport and magnetic properties is studied. Artificial grain boundaries have been built and charge transport, including tunneling, across the barriers is investigated.

Rainer Waser