Control of Switching Modes and Conductance Quantization in Oxygen Engineered HfO_x based Memristive Devices

Abstract

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.

Controllable and Stable Quantized Conductance States in a Pt/HfOx/ITO Memristor

“…The as-deposited HfO x layer is monoclinic in structure, and has a thickness of ≈10 nm with root-mean-square roughness of <1 nm and O/Hf ratio of ≈2 ( Figure S2, Supporting Information). [14] As such, accurate control of the APC size can be made possible via atom-by-atom oxygen ion manipulation, giving rise to noiseless, continuous, and stable quantized conductance pheno menon (stage V). Upon reversing the applied voltage polarity, oxygen anions will be injected from ITO electrode (rather than from the atmosphere) back into the HfO x matrix and recombine with the filament (stage IV).…”

“…[6,7] Eliminating the frequent data fetching and movement between the separated memory and processing units, memristors with the simplicity of a resistor structure can lead to extremely compact crossbar array for highly efficient hardware systems. [10][11][12][13][14] The stepwise development of device conductance in the unit of conductance quantum G 0 = 2e 2 /h = 77.5 µS (where e is the elemental charge of electrons and h is the Planck's constant), [10] associated with the continuous yet precise atomic Quantum-level manipulation of atomic configuration offers a excellent platform for the construction of exotic nanostructures that exhibit unusual solid-state physics and electronic properties. [10][11][12][13][14] The stepwise development of device conductance in the unit of conductance quantum G 0 = 2e 2 /h = 77.5 µS (where e is the elemental charge of electrons and h is the Planck's constant), [10] associated with the continuous yet precise atomic Quantum-level manipulation of atomic configuration offers a excellent platform for the construction of exotic nanostructures that exhibit unusual solid-state physics and electronic properties.…”

Direct Epitaxial Growth of Polar (1 – x)HfO2–(x)ZrO2 Ultrathin Films on Silicon

“…Such distinguishing characteristics 2 lead to an upsurge in application-oriented research as well as in curiosity-driven fundamental research to solve questions such as why these materials are capable of sustaining the unconventional ferroelectricity 13-32 , how these materials negate the effects of depolarization fields 33,34 , and whether such a new type of ferroelectricity can be replicated in other simple oxide systems.A prominent feature of hafnia-based materials is polymorphism 35 . While the ground state in the bulk HfO2 is a non-polar monoclinic (m-, P21/c) phase, a plethora of low-volume both polar and non-polar metastable states can be stabilized at ambient conditions via a combination of strategies such as cationic and anionic doping 1,[25][26][27]29,32 , thermal and inhomogeneous stresses 36,37 , nanostructuring 38 , epitaxial strain 16,20,22,23,26,29,[39][40][41][42] , and oxygen vacancy engineering 43,44 , all of which can be suitably engineered into thin-film geometries.Based on first-principles calculations 15,39,[45][46][47] at least five polar polymorphs (with space groups Pca21, Cc, Pmn21, R3 and R3m) can be identified as those that can be experimentally obtained.Owing to its relatively low energy, the orthorhombic (o-) Pca21 phase is widely observed in hafnia-based films grown via atomic layer deposition (ALD) 1,17,24,25 , chemical solution deposition (CSD) 28 , RF sputtering on Si 18,21 and pulsed-laser deposition (PLD) on selected substrates 19,23,26,31,[40][41][42] . A slightly higher energy rhombohedral (r-) phase (R3m or R3) has been recently observed on epitaxial Hf1/2Zr1/2O2 films grown on SrTiO3 (STO) 39 .…”

Dual‐Functional Nonvolatile and Volatile Memory in Resistively Switching Indium Tin Oxide/HfOx Devices

“…As shown in Figure b, a bipolar TS behavior was observed under the negative/positive polarity (−0.5 V/0.5 V). TS has been reported to occur in some cases depending on filament morphology, operating temperature, and oxygen stoichiometry . The I – V curves of the TS devices have a lower threshold voltage (approximately 0.5 V) and higher “on/off” ratio (approximately 10 5 ), and are more complex than those discussed in previous works.…”

“…In the TS mode, Joule heating, which is dominant, leads to a temperature‐gradient‐driven thermophoresis, as represented by the horizontal (mazarine) arrows in T1 and T2. During the TS mode (T1 ⇔ T2), the thermally driven forces dominates during the set process compared with electric field . The threshold operation can be explained in that a reduced heat dissipation could lead to drive more oxygen ions into CFs .…”