The annealing of two different film stacks, i.e., HfOx/Si and Hf/SiO2/Si, was investigated in situ in an ultrahigh vacuum by using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Although a kinetic metastability in the incongruently melting compounds such as HfO2 was reported to form SiO2 and HfO2 separately at the interface with Si, interfacial silicates (Hf-O-Si bonding units) were grown irrespective of film stacks. Ternary phase consideration in the Hf-Si-O system suggests that many nonstoichiometric silicates can be formed from the solid solutions of various compositions. The presence of nonstoichiometric silicates is ascertained by the STM results that show vagueness between oxygen-poor silicates and oxygen-containing silicides. (C) 2004 Elsevier B.V. All rights reserved.