CA SEGREGATION AND STEP MODIFICATIONS ON CLEAVED AND ANNEALED MGO(100) SURFACES

Cleaved MgO(100) surfaces were annealed in situ, in a UHV electron microscope, and ex situ, in alumina furnaces, to study changes in surface topography and composition. Steps on surfaces cleaved in air were imaged directly by reflection electron microscopy. They were shown to remain unchanged by in situ annealing procedures used for surface cleaning in contrast to previous studies with indirect decoration imaging which had not revealed steps on cleaned air-cleaved MgO(100) surfaces. The surface steps were found to be predominantly parallel to the MgO[010] crystallographic directions with average terrace lengths of about 80 nm, in agreement with previous studies of in situ cleaved surfaces. Segregation of Ca to the surface was confirmed in annealing experiments between 1000-1700-degrees-C and was found to change the step directions and distribution. Straight steps in [011] directions and curved steps separated by larger terraces were observed. Ca-rich line structures with typical lengths of about 100 and periodicity of 0.3 nm were found commensurately aligned with the MgO[011] direction on the terraces and around the steps of crystals with 270 ppm Ca.