LEEM Investigations of bcc Metals Grown Heteroepitaxially on Sapphire

Abstract

AbstractWe describe studies of refractory metals Mo and Nb, grown heteroepitaxially in the (011) orientation on sapphire (1120), using low energy electron microscopy (LEEM). A wide variety of structural and dynamical phenomena are observed and recorded as video sequences. These are organized here in five categories as follows. First are surface impurity phases identified by LEED. It is an important point, established here for Mo, that almost ideally clean surfaces with mainly single stepped terraces, can be prepared on thin films for use in surface science. Second, surface reconstruction phenomena can also be identified by diffraction. This is illustrated here by a reconstruction of the Nb surface, with two equivalent domains that form a stripe phase, owing to competing long and short range interactions. Detailed studies of nucleation, fluctuations and the equilibrium of these variants are described. The third area is surface topography, illustrated here by almost ideal stepped surfaces of Mo and Nb at high temperatures, and by nanofaceting by step edge coalescence to form {110} facets on Nb at lower temperatures. The phase diagram for nanofaceting is discussed. Fourth are processes at the internal Al2O3-metal interface, as interfacial dislocations, misorientation steps and defects become visible in the LEEM image through the displacement fields they cause. The final category includes bulk process where the observed dynamics of threading screw and edge dislocations and accompanying slip traces reveal their release at high temperatures from Cottrell atmospheres. The kinetics are tracked as surface diffusion smooths dislocation slip traces, and the interactions among dislocations and surface steps are measured.