Exploring Oxygen Vacancies in Sputtered HfO 2 Films for Multi‐Bit RRAM Applications

Abstract Compute‐in‐memory architectures demand resistive random‐access memory (RRAM) devices with atomic‐level control over conductance states. Here, a dopant‐free HfO 2 RRAM platform is demonstrated by changing stoichiometry across the film by radio‐frequency magnetron sputtering. The secondary ion mass spectrometry reveals a gradual decrease in Hf concentration without much change in oxygen until the HfO 2 /ITO interface. This is consistent with depth dependent X‐ray photoelectron spectroscopy showing gradual reduction of Hf 4 d and 4 f peak intensities with a subsequent evolution of In and Sn intensities deep inside the film. Detailed analysis reveals a decrease in non‐lattice oxygen concentration toward the film surface, while X‐ray diffraction results reflect the formation of an orthorhombic phase. Conductive atomic force microscopy however elucidates the involvement of oxygen vacancies in the formation and dissolution of conductive filaments for RRAM operation. By fabricating Ag/HfO 2 /ITO devices, deterministic control over multi‐level resistive switching (RS) behavior is recorded with incremental RESET voltages, while the conductance states are modulated by varying compliance currents during SET operations. Impedance spectroscopy further deciphers the importance of interfacial mixing to engineer composition gradient in HfO 2 for achieving stable RS behavior. It is believed the current approach provides a simple route for fabricating energy‐efficient RRAM toward neuromorphic computing.