Characterization of the composition and extension of the SiO2/HfO2 interface in the model systems Si-sub./SiO2 (7.5 nm)/FE:HfO2 (9.5 nm)/TiN (9 nm) for standard impurity concentrations and annealing temperatures (Si 3.6%, 1000 °C; Al 5.5%, 850 °C) were carried out using X-ray photoelectron spectroscopy with high kinetic energies. In addition, the crystalline properties of HfO2 were characterized by transmission electron microscopy and grazing incidence X-ray diffraction depending on the type of impurity. Formation of HfxSi1–xO2 at the SiO2/HfO2 interface was revealed, and the dependence of its thickness dsilicate and stoichiometry X on the formation of the tetragonal phase P42/nmc of HfO2 was established. It is shown that doping with aluminum followed by annealing prevents the formation of a tetragonal phase and reduces both dsilicate and X by about 20% (with respect to undoped HfO2 annealed at the same temperature), in contrast to doping with Si followed by annealing, which practically does not affect dsilicate and X. The results obtained are important for the problem of imbalance in the voltage drop across the gate oxides stack in a ferroelectric field-effect transistor and improvement of its endurance and retention characteristics.