The interface passivation of the HfO₂/Si_0.7Ge_0.3 stack is systematically investigated based on low-temperature ozone oxidation and Si-cap methods. Compared with the Al₂O₃/Si_0.7Ge_0.3 stack, the dispersive feature and interface state density (D_it) of the HfO₂/Si_0.7Ge_0.3 stack MOS (Metal-Oxide-Semiconductor) capacitor under ozone direct oxidation (pre-O sample) increases obviously. This is because the tiny amounts of GeO_x in the formed interlayer (IL) oxide layer are more likely to diffuse into HfO₂ and cause the HfO₂/Si_0.7Ge_0.3 interface to deteriorate. Moreover, a post-HfO₂-deposition (post-O) ozone indirect oxidation is proposed for the HfO₂/Si_0.7Ge_0.3 stack; it is found that compared with pre-O sample, the D_it of the post-O sample decreases by about 50% due to less GeO_x available in the IL layer. This is because the amount of oxygen atoms reaching the interface of HfO₂/Si_0.7Ge_0.3 decreases and the thickness of IL in the post-O sample also decreases. To further reduce the D_it of the HfO₂/Si_0.7Ge_0.3 interface, a Si-cap passivation with the optimal thickness of 1 nm is developed and an excellent HfO₂/Si_0.7Ge_0.3 interface with D_it of 1.53 × 10¹¹ eV⁻¹cm⁻² @ E−E_v = 0.36 eV is attained. After detailed analysis of the chemical structure of the HfO₂/IL/Si-cap/Si_0.7Ge_0.3 using X-ray photoelectron spectroscopy (XPS), it is confirmed that the excellent HfO₂/Si_0.7Ge_0.3 interface is realized by preventing the formation of Hf-silicate/Hf-germanate and Si oxide originating from the reaction between HfO₂ and Si_0.7Ge_0.3 substrate.