The thermal stability and interfacial reactions of nitrided Hf-silicate [(HfO2)(x)(SiO2)(1-x)] thin films grown on Ge(001) substrate by atomic layer deposition were investigated. Bandgap and valence band offset were evaluated as a function of the quantity of SiO2 in the silicate film and nitridation temperature. After nitridation, the conduction band offset was decreased by 0.12 eV (x=0.5) and 0.39 eV (x=0.75), while the valence band offset was decreased by 0.58 eV (x=0.5) and 0.01 eV (x=0.75). In addition, the bandgap change was significantly affected by the stoichiometry of the films. The band alignment was closely consistent with the amount of nitrogen incorporated into the interfacial region of the film.