A high-quality HfGeO x interfacial layer (IL) was formed in a HfO2/Al2O3/HfO2/GeO x gate stack through thermal annealing. The diffusing of GeO into the HfO2 layer led to the mixing of the GeO x and HfO2 layers, as identified through energy-dispersive X-ray Spectroscopy (EDX). X-ray photo-electron spectroscopy (XPS) data for HfGeO x 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 HfGeO x 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 HfGeO x IL. Moreover, the Ge p-metal-oxide-semiconductor field-effect transistor HfGeO x IL exhibited a high effective hole mobility (approximately 704 cm2/V s).