Optimal stoichiometry for nucleation and growth of conductive filaments in HfO_x

Abstract

We present a first principles based investigation into the nucleation and growth of metal-rich precipitates in substoichiometric hafnium dioxide with relevance to applications in resistive switching memories. We identify an optimal HfO x stoichiometry, with x in the range 1.50-1.75, for efficient nucleation and growth of stable Hf-rich clusters which may serve as precursors for the growth of conductive filaments during forming. We also show that filaments with sub-nm diameter posses remarkably metallic character. These results provide invaluable atomistic insight into forming in resistive switching films and demonstrate that precise regulation of stoichiometry is essential in order to ensure uniformity in the nucleation and subsequent growth of a conductive filament during the forming process.

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

“…This peak, however, is shifted to larger lattice spacing due to octahedral interstitial oxygen. Recent calculations suggested that a phase separation into a mixture of m ‐HfO 2 and hcp‐HfO 0.2 is thermodynamically more favorable for substoichiometric HfO x , while a tetragonal phase was claimed to be stable only in a narrow window near x = 1 . However, density functional methods have also shown that the presence of oxygen vacancies that usually arise to neutralize dopants in HfO x greatly stabilizes the higher symmetric tetragonal and cubic phases .…”

Statistical analysis of random telegraph noise in HfO2-based RRAM devices in LRS

“…current compliance), the conductive filament exhibits quasi-ohmic I-V characteristics indicating that it is formed by an Hf-rich (oxygen deficient) region in the dielectric [24]. The CF formation is a two-step process involving the Hf-O bond breakage and the subsequent diffusion of the released oxygen ions out of the forming CF region [5,24,25]. O ions distribution is, in turn, the critical factor controlling the subsequent reset process.…”

“…This is due to the relatively high conductivity of the undisrupted CF portion, which makes the most of the voltage applied during set to drop across the oxidized CF tip (dielectric barrier). Thus, voltages as low as $0.5-1 V allow the electric field in the dielectric barrier to approach its intrinsic dielectric strength, thus resulting in a fast Hf-O bond breakage and release of the oxygen ions that restores the characteristics of the CF [24,25]. Since the major properties (e.g.…”

Routes for increasing endurance and retention in HfO2 -based resistive switching memories

“…The conductivity can be destroyed by recombination with oxygen, which restores the insulating state. Recent first-principles simulations were performed to identify an optimal HfO x stoichiometry, where 1.5 < x < 1.75, and the critical radius for the nucleation and growth of a stable conductive Hf-rich cluster [27]. It was further predicted theoretically that the stability of oxygen vacancies in monoclinic HfO 2 is enhanced at grain boundaries as compared to the bulk crystal, which helps the formation of the conductive filaments [28].…”