Solid-solid dewetting of scandium thin films on the W(100) surface observed using emission microscopy

Several operating modes of cathode lens emission microscopy are used to observe scandium on W(100), including mirror microscopy, thermionic emission microscopy, and low energy electron diffraction. Scandium thin films are deposited by thermal evaporation from a metal source onto W(100). The scandium films on W(100) are observed to dewet from the W(100) surface and to form droplets when the surface is heated: the dewetting temperature is thickness dependent and spans a range from 500 to 900 °C, which is far below the melting temperature (1541 °C). Thinner films produce smaller droplets; a range of film thicknesses from 5 to 30 nm was tested. X-ray photoelectron spectroscopy of the scandium films exposed to oxygen was also measured in order to understand the function of scandium in thermionic cathodes. The shift of the W 4f peaks upon oxygen absorption, scandium adsorption, and subsequent heating indicates that oxygen preferentially bonds to scandium. The shift in the scandium 3p level indicates formation of Sc2O3.