The thermal stability of the dielectric interfacial layer (IL) inside the metal-oxide-semiconductor field-effect transistor can affect the quality of the final products. One vital problem is whether the size and location of the band gap might be influenced by the annealing treatment. Here, we report the direct measurement of the band gap by scanning tunneling spectroscopy (STS) for ultrathin GeO x film (about 0.9 nm) and Ti-GeO x film (about 0.7 nm) grown on Ge substrates by atomic layer deposition as a dielectric IL. To examine the thermal stability, annealing treatment was performed for the films at 500 C. From the topography images, the root-mean-square roughness showed a difference of less than 0.03 nm. STS spectra show that the band gap is significantly modified after the annealing treatment for the case of GeO x film. Ultraviolet photoemission spectroscopy also confirms the shift of the valence band (VB) edge. For GeO x film doped with Ti for 30 s by sputtering, both the band gap and the VB edge remain unchanged after the annealing treatment. Our results show that Ti doping can improve the thermal stability of GeO x .