Step heights and terrace terminations of a vicinal (0 0 0 1) α-alumina surface annealed in UHV

A stepped α-alumina (0 0 0 1) surface annealed under UHV has been studied by in situ AFM in contact mode and LEED. A statistical analysis of the step heights derived from AFM images for surfaces displaying various degrees of reduction characterized by LEED brings direct evidences of difference in the chemical terminations of neighboring terraces. Contrasting with the steps observed after air annealing which mostly present 2n·c/6 heights (c∼1.3 nm, n integer) i.e. a 2n structure, in UHV the step heights favor a 3n structure. Indeed, for an alumina surface characterized by a (2√3×2√3) r30° LEED pattern, the step heights are found equal to 3n·c/6 or very close to (3n±0.5)c/6. The 2n–3n transition is attributed to the difference in the electric charge screening process in the first layers of the terraces. For the subsequent states of reduction, (√31×√31) r±9° structure, a stronger deviance from the 3n rule is observed. These deviations from the 3n rule are discussed in term of the chemical termination of the terraces.