Synchrotron X-ray Diffraction Studies on Oxide Surfaces and Interfaces.

[摘要] Surfaces and interfaces of oxide crystals have gained a burst of attention in recent years due to their importance in technological applications as well as fundamental interest in their exotic behavior. Zinc oxide (ZnO) is one of the oxide materials which has been intensively studied especially for its transport behavior at the Schottky interface, which leads to many electronic device applications. Bismuth ferrite (BiFeO3) is a unique material that exhibits stable magnetoelectric multiferroicity at room temperature, yielding new paradigms in the design of novel electromagnetic devices. Both the Schottky property of ZnO and the multiferroic behavior of BiFeO3 depend critically on the atomic structure of their surfaces and interfaces. Therefore, the accurate determination of their structure is a prerequisite for controlling and optimizing their properties for applications.The atomic surface and interface structures of uncoated and metal-coated ZnO (0001) Zn-polar and (000-1) O-polar wafers are measured with surface x-ray diffraction. All Zn-polar surfaces and Schottky interfaces show the presence of a fully occupied (1x1) overlayer of oxygen atoms on top of the terminating Zn layer, and no significant atomic relaxations are observed. O-polar surfaces are significantly rougher than Zn-polar surfaces, exhibiting Gaussian-shaped roughness profiles with a width of about 1.5 unit cells. They show a decreased layer distance between the topmost oxygen and zinc layers. These findings are important because they are the first results on ZnO Schottky interfaces prepared under typical ambient device processing conditions.The unit-cell symmetry of BiFeO3 thin films is determined via 3-dimensional reciprocal space mapping. The maps clearly show a phase transition from monoclinic to tetragonal symmetry when the film thickness decreases below a critical thickness, both for highly strained and moderately strained films. In the case of moderately strained films, this transition is accompanied by a change in the half-order diffraction peak pattern, which reflects an untilting of the oxygen octahedra. This establishes a definitive connection between the octahedral tilting and the symmetry changes occurring at the structural transition. These results are essential for device applications, since the ferroelectric and magnetic properties are strongly related to the unit-cell symmetry and oxygen octahedral structure.

[发布机构] University of Michigan

[效力级别] Oxide Surfaces and Interfaces

[关键词] Surface X-ray Diffraction;Oxide Surfaces and Interfaces;ZnO;Schottky Contact;BiFeO3;Multiferroic;Physics;Science;Physics