Nanoscale in situ investigation of ultrathin silicon oxide thermal decomposition by high temperature scanning tunneling microscopy

A surface chemical reaction-the thermal decomposition of ultrathin silicon oxide (similar to 1 nm) by ultrahigh vacuum (UHV) thermal annealing at 600-800 degrees C-is in situ investigated on a nanometer scale by high temperature scanning tunneling microscopy (STM). The reaction is initiated by the creation of circular voids which expose the underlying silicon substrate. Growth kinetics of the voids is scrutinized via time-lapse STM movies. It is verified that the void perimeters grow linearly with time before coalescence and the reaction occurs peripherally around the void perimeters. It is also demonstrated that the decomposition rate varies concomitantly with the local environment near the reaction fronts. The observed low-high-low rate evolution is qualitatively explained. Increased reaction activation energy is found in the final decomposition stage and the origin of the increase is proposed to be due to the local morphological evolution.