Probing the thermal decomposition behaviors of ultrathin HfO2 films by an in situ high temperature scanning tunneling microscope

The thermal decomposition of ultrathin HfO2 films (∼0.6-1.2 nm) on Si by ultrahigh vacuum annealing (25-800 deg. C) is investigated in situ in real time by scanning tunneling microscopy. Two distinct thickness-dependent decomposition behaviors are observed. When the HfO2 thickness is ∼ 0.6 nm, no discernible morphological changes are found below ∼ 700 deg. C. Then an abrupt reaction occurs at 750 deg. C with crystalline hafnium silicide nanostructures formed instantaneously. However, when the thickness is about 1.2 nm, the decomposition proceeds gradually with the creation and growth of two-dimensional voids at 800 deg. C. The observed thickness-dependent behavior is closely related to the SiO desorption, which is believed to be the rate-limiting step of the decomposition process.