Regensburg 2007 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 59: Oxides and Insulators: Clean Surfaces
O 59.5: Vortrag
Donnerstag, 29. März 2007, 16:45–17:00, H41
Stabilization of thin ZnO films by surface depolarization — •Christian Tusche, Holger L. Meyerheim, and Jürgen Kirschner — MPI für Mikrostrukturphysik Halle, D-06120 Halle, Germany
Heterostructures of the oxide semiconductor ZnO attracted interest for application in ultra violet laser diodes and spintronic devices. Theoretical predictions rely on detailed information on the film and interface structures. Scanning tunneling microscopy (STM) and surface x-ray diffraction (SXRD) was used to study morphology and structure of ultra thin ZnO films on Ag(111).
ZnO films, 0.3 to 5 monolayers (ML) in thickness, were grown by pulsed laser deposition in 10−7 mbar O2 atmosphere on Ag(111) at 300 K, followed by annealing at 680 K. SXRD indicates that the film is (0001) oriented, and the hexagonal lattice vectors are aligned parallel with the Ag(111) surface. While single crystal (0001) or (0001) surfaces are polar, and extended flat regions (>10 nm) are not stable, the 2 ML thick ZnO films form a flat double-layer over the whole surface.
Stabilization is achieved by a transition of the Wurtzite- towards a graphite like structure with reduced polarity, i. e., O ions move to the plane of Zn ions: SXRD shows an in-plane expansion of the ZnO lattice by 1.6 % compared to bulk (3.30 Å vs. 3.25 Å), corresponding to a 7/8 coincidence with the Ag substrate (Ag: 2.89 Å). In parallel, Zn-O bonds (bulk: 1.97 Å) within the hexagonal plane shorten to 1.92 Å and intra-plane Zn-O bonds expand to 2.20 Å. The ions change their four-fold coordination (Wurtzite structure) to a three-fold one.