Browsing by Author "Saito, Mitsuhiro"

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HRTEM investigations of structure and composition of polar Pd/ZnO heterophase interfaces
(2005) Saito, Mitsuhiro; Rühle, Manfred (Prof. Dr. Dr. h.c.)
The present work is a fundamental, quantitative and systematic study of the structure and bonding of Pd/{0001}ZnO interfaces. The interface was studied experimentally via high-resolution transmission electron microscopy (HRTEM). These studies were completed by semi-quantitative first principle calculations and crystal truncation rod measurements. The ZnO substrate surfaces were prepared under well-defined oxygen atmospheres and in ultra-high vacuum (UHV). Flat, stepped, contamination free, and unreconstructed surfaces were obtained during heat treatment at 600 °C. The steps on the surfaces were half a unit cell in height, resulting in uniform terminating atomic species. Both polar ZnO surfaces, the (0001)ZnO (+ZnO) and the (000-1)ZnO (-ZnO) surface, were analyzed in detail with surface XRD. These investigations showed that the coverage of the surface depends very sensitively on the surface preparation process. The oxygen prepared +ZnO surface was Zn-terminated with a mean reduced coverage of 77 % (±5 %). This coverage is in good agreement with that calculated by Noguera as 75 %. Following the explanation of Noguera, a partial coverage of the surface leads to an electrostatic charging of the surface which compensates the electrostatic field caused by the Zn-O dipoles, stabilizing the surface structure. Single crystalline Pd films were grown on both ZnO surfaces via molecular-beam-epitaxy. Pd forms three-dimensional clusters (islands) on the ZnO surfaces. With XRD, RHEED, and TEM investigations the following well-defined epitaxial orientation relationship between Pd islands and ZnO crystal was observed (deposition temperature 600 °C): {111}Pd//{0001}ZnO und <110> Pd //<11-20> ZnO HRTEM investigations revealed that the Pd/ZnO interfaces are atomically abrupt and, as expected due to the low reactivity of Pd, free of reaction phases. The interfaces are atomically flat over regions of more than 100 nm. Structural defects were detected in the Pd/+ZnO and the Pd/-ZnO interfaces. These defects are misfit dislocations which are formed by the relaxation of the lattice misfit between Pd and ZnO. The experimentally determined distance between the cores of the dislocations is 1.5 nm and corresponds to the theoretical misfit dislocation distance. The structurally matching regions between the dislocations were used to perform a quantitative analysis of the interfacial translation state via quantitative image processing. It could be shown that the Pd/+ZnO interface is Zn-terminated and the Pd atoms are positioned on Zn lattice sites. The distance between the interfacial Pd layer and the terminating Zn layer increased by 0.02 nm compared to the theoretically expected distance between the {111}Pd lattice planes. Pd coverage resulted in a segregation of Zn atoms to the interface until 100 % coverage was reached. The periodic relaxations at the Pd/+ZnO interface were located only in the terminating Zn layer. Quantitative image analysis of the HRTEM micrographs from the Pd/-ZnO interface shows O termination of this interface. The interfacial Pd and O atoms are located top-on-top and the distance between them is about 0.01 nm smaller than the distances in the Pd lattice. The coverage of the interfacial O layer is 100 %. In contrast to the Pd/+ZnO interface the lattice relaxations at the Pd/-ZnO interface were located in the Pd. The charge distribution and bonding at both Pd/ZnO interfaces were investigated by semi-quantitative first principle calculations. The calculations revealed that the terminating Zn layer at the Pd/+ZnO interface was charged negative and the terminating O layer at the Pd/-ZnO positive. This is a change in interfacial charge distribution compared to the clean ZnO surfaces. In the case of the Zn-terminated Pd/+ZnO interface, the structural defects (e.g. Zn vacancies at the interface) which are compensating the electrostatic field of the Zn-O dipoles are now not necessary because electrons are supplied from Pd to Zn. The electrostatic field of the Zn-O dipoles is now compensated via the charged interfacial Zn layer. Thus, segregation of Zn to the Pd/+ZnO interface takes places. In the case of the O-terminated Pd/-ZnO interface, electrons are supplied from O to Pd leading to a segregation of O to the interface. The following inequalities could be deduced from the calculations of the bonding at the different interfaces: 1.Pd-1.Zn (Zn-terminated surface)> 1.Pd-1.O (O-terminated surface) („1.“ means 1. layer at the interface or interfacial layer). These differences in interfacial bonding result also in a different growth behavior of the Pd islands. In the case of the O-terminated interface large islands are formed, liquid like coalescence is observed more early which indicates that the Pd atoms on this surface are more mobile (less strongly bonded). Furthermore, a qualitative comparison of the first principle calculations with HRTEM results (relaxation behavior, termination) allows deducing the following inequalities for the interatomic bonding near the differently terminated interfaces: [1.O-Zn ; 1.Pd-1.Zn] > 1.Pd-1.O > 1.Pd-Pd > 1.Zn-O or 1.Pd-1.Zn > 1.Pd-1.O > 1.O-Zn > 1.Pd-Pd > 1.Zn-O From these inequalities one obtains that the 1.Zn-O bond is the weakest bond in the region near the interface. The inequality 1.O-Zn > 1.Zn-O indicates why the Zn terminated surface is mechanically softer than the O terminated surface. This result is an impressive example demonstrating how different terminated oxide surfaces strongly influence the formation of interfacial defects. These differences will also be reflected in different physical properties of the interfaces. In the case of the more strongly bonded Pd/+ZnO interface, epitaxial strain is transferred into the ZnO crystal. This is due to the relatively weak 1.Zn-O bond in ZnO. The more weekly bonded Pd/-ZnO interface behaves different. The relatively weak 1.Pd-Pd bond favors relaxations in the Pd. This shows how the growth behavior can be tuned by selecting the termination of the ZnO surface. Depending on surface polarity the terminating atomic species are adjusted in such a manner that the total energy of the system is minimized. This also results in the establishment of an interfacial local structure that preserves high geometrical coherency and a chemically interactive geometry as a function of interfacial termination.