A high-quality HfGeOx interfacial layer (IL) was formed in a HfO₂/Al₂O₃/HfO₂/GeOx gate stack through thermal annealing. The diffusing of GeO into the HfO₂ layer led to the mixing of the GeOx and HfO₂ layers, as identified through energy-dispersive X-ray Spectroscopy (EDX). X-ray photo-electron spectroscopy (XPS) data for HfGeOx IL confirmed the formation of Ge-O-Hf bonds owing to the induced shift of the Ge3dox spectra to lower binding energies. The electrical and reliability data indicated that the capacitor with HfGeOx IL presented not only lower interface states density (Dit, approximately 7 × 1011 eV-1cm-2) but also less Dit increment (approximately 3 × 1011 eV-1cm-2) after stressing than did the capacitor without the HfGeOx IL. Moreover, the Ge p-metal-oxide-semiconductor field-effect transistor HfGeOx IL exhibited a high effective hole mobility (approximately 704 cm2/V s).