As means to control interface reactions between HfO₂ and Ge(100), chemical vapor deposition (CVD) of ultrathin Ta-rich oxide using Tri (tert-butoxy) (tert-butylimido) tantalum (Ta-TTT) on chemically-cleaned Ge(100) has been conducted prior to atomic-layer controlled CVD of HfO₂ using tetrakis (ethylmethylamino) hafnium (TEMA-Hf) and O₃. The XPS analysis of chemical bonding features of the samples after the post deposition N₂ annealing at 300°C confirms the formation of TaGe_xO_y and the suppression of the interfacial GeO₂ layer growth. The energy band structure of HfO₂/TaGe_xO_y/Ge was determined by the combination of the energy bandgaps of HfO₂ and TaGe_xO_y measured from energy loss signals of O 1s photoelectrons and from optical absorption spectra and the valence band offsets at each interface measured from valence band spectra. From the capacitance-voltage (C-V) curves of Pt-gate MIS capacitors with different HfO₂ thicknesses, the thickness reduction of TaGe_xO_y with a relative dielectric constant of 9 is a key to obtain an equivalent SiO₂ thickness (EOT) below 0.7nm.