Compositional and Interface Engineering



Interface engineering of nanostructured ferroelectrics:


M.-W. Chu, I. Szafraniak, R. Scholz, D. Hesse, and M. Alexe



Cross section transmission electron microscope image of an epitaxial lead zirconate titanate island ("PZT") grown by chemical solution deposition onto a strontium titanate substrate ("STO"), figure (a). The white "T" symbols designate misfit dislocations visualized in cross section. The high-resolution electron microscopy image figure (b) shows a section of the undistorted crystal lattice far from the misfit dislocations. Figure (c) shows a computer-processed representation of the deformations eyy which are located in a tube of 8 by 4 nm cross section around a misfit dislocation. The tube is seen in cross section. The peaks in yellow and red mark the highly deformed parts of the crystal lattice, whereas the green and blue sections represent the undistorted lattice. Ferroelectricity vanishes in the highly deformed parts, resulting in a modified size effect.


For details, see, e. g. Nature Materials  3 (2004) 87-90.


Ferroelectric ultrathin films and multilayers:


A. Visinoiu, R. Scholz, S. Chattopadhyay, M. Alexe, and D. Hesse


Since present efforts are concentrated on reducing the size of electronic devices, another challenge is to find new materials or to improve the dielectric properties of existing materials used in fabrication of ceramic capacitors and dynamic random access memories (DRAMs) which require high dielectric constant at small dimensions. For this purpose, multilayers of BaTiO3/SrTiO3 are deposited by PLD on vicinal SrTiO3 substrates. The microstructure of the grown films plays an important role in the dielectric behaviour. It is also important to take into account the influence of the interface of different multilayers on the dielectric properties.


High-resolution TEM picture of BaTiO3/SrTiO3 multilayers deposited on a SrTiO3 substrate. The thickness of the BaTiO3 film is increased from 0.5 nm to 15 nm.


For details, see, e. g. Jpn. J. Appl. Phys., Part 1  41 (2002) 6633-6638.


Direct wafer bonding of ferroelectrics:


M. Alexe


Ferroelectric thin films have to be deposited and/or processed at relatively high temperatures, causing various damages (interdiffusion, strain, presence of structural defects) at the interface with the substrate or underlying layer. If the ferroeletric films are deposited directly on top of silicon, the high temperature processing ruins the quality of the semiconductor directly in contact with the ferroelectric material. Deposition of ferroelectric films directly on top of a semiconductor material is, however, a requirement for ferroeletric field effect-based devices. On the other hand, it is known that  Direct Wafer Bonding (DBW) allows to intimately join dissimilar materials at relatively low temperature without any "glue" of any kind, provided that their surface is smooth and flat enough. Direct wafer bonding was therefore applied for the fabrication of ferroelectric layers in intimate contact with silicon, and the interface trap densitiy, for instance, has indeed been improved by a factor 500.


XTEM picture of the interface between a PZT film grown by CSD and a Si (100) 3-inch wafer bonded by direct wafer bonding.


For details, see, e. g.
M. Alexe, U. Gösele
"Wafer Bonding: Applications and Technology"
Springer, Berlin, Heidelberg, New York (2004).

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