We propose a new HfGeOx interfacial layer (IL) for the high-κ gate-stacks on p-Type germanium substrate with improved thermal stability as compared with that of conventional GeOx IL. We inserted an additional HfO 2 layer after the formation of GeO x in the HfO 2 /Al 2 O 3 /GeOx/Ge gate-stack by using plasma-enhanced atomic layer deposition. Through the use of post-deposition annealing and post-metal annealing, the new system exhibited greater thermal immunity and was stable up to 600 °C. We speculate that the improvement originates from the formation of HfGeO x through the combination of HfO 2 and GeO x , according to the thermodynamic data. By incorporating Hf into interfacial layer, the fabricated high-κ gate-stack with an equivalent oxide thickness of 1.2 nm, a low interface states density (Dit) of approximately 3.3×10 11 eV -1 cm -2 , and an impressive gate leakage current of approximately 2.2×10 -6 A/cm 2 at V FBs -1V.