A2: Polarization interaction in Laser-MBE Wurzit-Perovskit heterostructures
Up to now, the interaction of the crystal lattice charges of a polar II-VI semiconductor (as for example ZnO) with the switchable ferroelectric polarization charges of an oxide ferroelectric (as BaTiO3) at hetero-interfaces is not widely investigated. In project A2, electrical methods will be exploited to develop new switching and memory functionalities and related models of such structures and to explore their application potential. Complementary electro-optical investigations will be performed in project B3. In addition, A2 prepares all thin film hetero-structures for B4, B6 and others.
Our electrical characterization of first polycrystalline BaTiO3-ZnO structures on silicon resulted in asymmetric current-voltage hysteresis and bistable capacitance-voltage characteristics. These effects open new potential functionalities as switchable metal-ferroelectric-semiconductor (MFS) thin film diode and capacitive memory cell. For realization of improved, lattice-matched hetero-interfaces the pulsed laser deposition (PLD) method which is particularly suitable for oxide growth will be developed into the so called laser molecular beam epitaxy (laser-MBE) with in-situ RHEED control of film growth. In the first funding period, special emphasis will be given to experimental investigation and modeling of electrical characteristics of BaTiO3-ZnO interfaces grown pseudomorphic either on SrTiO3 or ZnO single crystal substrates. By means of controlled strain, the spontaneous polarization of the films shall be varied.
The optical characterization of these structures will be done in B3. The development of demonstrators of new MFS devices and memory cells would be continued in the following period based on the coupling effects worked out in first period. In addition, the magnetic interaction will be included by combining either multiferroic materials and ZnO or ferroelectrics and diluted magnetic semiconductors (DMS). As DMS could serve for example ZnO, ZrO2 or HfO2 doped with 3d or 4f elements, as investigated in more detail in B6. Multiferroic thin films can show much higher ferroelectric polarization and magnetic moments as compared to the corresponding bulk materials.
Furthermore, A2 will grow the oxide film structures for field-effect transistors for B4 and magnetic tunnel junctions for B6. Here, using the laser-MBE method, more defined interfaces and improved gate and tunneling barriers shall be achieved.
Principal Investigator
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Prof. Dr. Michael Lorenz ⇒
phone: +49 (0) 341/97 32661 fax: +49 (0) 341/97 39286 |
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