Structural and electronic properties of oxygen deficient HfO2-x thin films: from insulator to metal

Abstract Hafnium oxide (HfO₂) and its oxygen-deficient variants (HfO₂₋ₓ) are key candidates for next-generation electronics, ranging from photonics and photocatalysis to both resistive and ferroelectric memories. Sub-stoichiometric HfO₂₋ₓ reduces the forming voltage in ReRAM devices and stabilizes the orthorhombic ferroelectric phase crucial for non-volatile FeRAM, yet a systematic understanding of its electronic evolution remains elusive. Here, we synthesize a complete series of HfO₂₋ₓ thin films spanning from stoichiometric oxide to pure metal and map their structural and electronic transformations. By combining X-ray photoelectron spectroscopy, grazing-incidence X-ray diffraction, resistivity profiling, and Hf L₃-edge X-ray absorption near-edge structure, we reveal clear stoichiometry-driven trends in bonding, phase formation, and electronic structure, providing a definitive reference for tailoring HfO₂₋ₓ in advanced memory and photonic applications.