We have investigated the resistive switching behavior in stoichiometric HfO2 and oxygen-deficient HfO2−x thin films grown on TiN electrodes using reactive molecular beam epitaxy. Oxygen defect states were controlled by the flow of oxygen radicals during thin film growth. Hard X-ray photoelectron spectroscopy confirmed the presence of sub-stoichiometric hafnium oxide and defect states near the Fermi level. The oxygen deficient HfO2−x thin films show bipolar switching with an electroforming occurring at low voltages and low operating currents, paving the way for almost forming-free devices for low-power applications.

Hafnium oxide (HfO x )-based memristive devices have tremendous potential as nonvolatile resistive random access memory (RRAM) and in neuromorphic electronics. Despite its seemingly simple two-terminal structure, a myriad of RRAM devices reported in the rapidly growing literature exhibit rather complex resistive switching behaviors. Using Pt/HfO x /TiN-based metal-insulator-metal structures as model systems, it is shown that a well-controlled oxygen stoichiometry governs the filament formation and the occurrence of multiple switching modes. The oxygen vacancy concentration is found to be the key factor in manipulating the balance between electric field and Joule heating during formation, rupture (reset), and reformation (set) of the conductive filaments in the dielectric. In addition, the engineering of oxygen vacancies stabilizes atomic size filament constrictions exhibiting integer and half-integer conductance quantization at room temperature during set and reset. Identifying the materials conditions of different switching modes and conductance quantization contributes to a unified switching model correlating structural and functional properties of RRAM materials. The possibility to engineer the oxygen stoichiometry in HfO x will allow creating quantum point contacts with multiple conductance quanta as a first step toward multilevel memristive quantum devices.

Filament-type HfO2-based RRAM has been considered as one of the most promising candidates for future non-volatile memories. Further improvement of the stability, particularly at the “OFF” state, of such devices is mainly hindered by resistance variation induced by the uncontrolled oxygen vacancies distribution and filament growth in HfO2 films. We report highly stable endurance of TiN/Ti/HfO2/Si-tip RRAM devices using a CMOS compatible nanotip method. Simulations indicate that the nanotip bottom electrode provides a local confinement for the electrical field and ionic current density; thus a nano-confinement for the oxygen vacancy distribution and nano-filament location is created by this approach. Conductive atomic force microscopy measurements confirm that the filaments form only on the nanotip region. Resistance switching by using pulses shows highly stable endurance for both ON and OFF modes, thanks to the geometric confinement of the conductive path and filament only above the nanotip. This nano-engineering approach opens a new pathway to realize forming-free RRAM devices with improved stability and reliability.

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Structural, chemical and electronic properties of electroforming in the TiN/HfO(2) system are investigated at the nanometre scale. Reversible resistive switching is achieved by biasing the metal oxide using conductive atomic force microscopy. An original method is implemented to localize and investigate the conductive region by combining focused ion beam, scanning spreading resistance microscopy and scanning transmission electron microscopy. Results clearly show the presence of a conductive filament extending over 20 nm. Its size and shape is mainly tuned by the corresponding HfO(2) crystalline grain. Oxygen vacancies together with localized states in the HfO(2) band gap are highlighted by electron energy loss spectroscopy. Oxygen depletion is seen mainly in the central part of the conductive filament along grain boundaries. This is associated with partial amorphization, in particular at both electrode/oxide interfaces. Our results are a direct confirmation of the filamentary conduction mechanism, showing that oxygen content modulation at the nanometre scale plays a major role in resistive switching.

The Ti/HfO2 interface plays a major role for resistance switching performances. However, clear interface engineering strategies to achieve reliable and reproducible switching have been poorly investigated. For this purpose, we present a comprehensive study of the Ti/HfO2 interface by a combined experimental-theoretical approach. Based on the use of oxygen-isotope marked Hf*O2, the oxygen scavenging capability of the Ti layer is clearly proven. More importantly, in line with ab initio theory, the combined HAXPES-Tof-SIMS study of the thin films deposited by MBE clearly establishes a strong impact of the HfO2 thin film morphology on the Ti/HfO2 interface reactivity. Low-temperature deposition is thus seen as a RRAM processing compatible way to establish the critical amount of oxygen vacancies to achieve reproducible and reliable resistance switching performances.

Articles you may be interested inIn-operando hard X-ray photoelectron spectroscopy study on the impact of current compliance and switching cycles on oxygen and carbon defects in resistive switching Ti/HfO2/TiN cells Resistive switching mechanisms relating to oxygen vacancies migration in both interfaces in Ti/HfOx/Pt memory devices Hard x-ray photoelectron spectroscopy study of the electroforming in Ti/HfO2-based resistive switching structures Appl.

Articles you may be interested inResistive switching mechanisms relating to oxygen vacancies migration in both interfaces in Ti/HfOx/Pt memory devices J. Appl. Phys. 113, 064510 (2013); 10.1063/1.4791695In-operando and non-destructive analysis of the resistive switching in the Ti/HfO2/TiN-based system by hard xray photoelectron spectroscopy Appl. Phys. Lett. 101, 143501 (2012) In-operando hard X-ray photoelectron spectroscopy study on the impact of current compliance and switching cycles on oxygen and carbon defects in resistive switching Ti/HfO 2 /TiN cells In this study, direct experimental materials science evidence of the important theoretical prediction for resistive random access memory (RRAM) technologies that a critical amount of oxygen vacancies is needed to establish stable resistive switching in metal-oxide-metal samples is presented. In detail, a novel in-operando hard X-ray photoelectron spectroscopy technique is applied to non-destructively investigates the influence of the current compliance and direct current voltage sweep cycles on the Ti/HfO 2 interface chemistry and physics of resistive switching Ti/HfO 2 /TiN cells. These studies indeed confirm that current compliance is a critical parameter to control the amount of oxygen vacancies in the conducting filaments in the oxide layer during the RRAM cell operation to achieve stable switching. Furthermore, clear carbon segregation towards the Ti/HfO 2 interface under electrical stress is visible. Since carbon impurities impact the oxygen vacancy defect population under resistive switching, this dynamic carbon segregation to the Ti/HfO 2 interface is suspected to negatively influence RRAM device endurance. Therefore, these results indicate that the RRAM materials engineering needs to include all impurities in the dielectric layer in order to achieve reliable device performance. V C 2014 AIP Publishing LLC.