Oxygen migration during resistance switching and failure of hafnium oxide memristors

Abstract

Metal-oxide memristors, or resistive random access memory (RRAM) switches, in particular utilizing HfO x as the resistive switching material, have seen significant interest recently for nonvolatile memory and computation applications. [1][2][3][4][5] There has been particular interest in understanding the role of migration of oxygen atoms in determining the operation of memristors. 6-11 Similar recent advances in understanding the localized nanoscale physico-chemical changes underlying resistance switching 4,12-15 have opened up fresh interests into studying the effect of atomic movements on extended device operation and the nanoscale material behavior during eventual failure and possible techniques to mitigate such failure. [16][17][18] To enable scanning transmission x-ray microscopy (STXM) measurements, each device was built on a 200 nm low-stress Si 3 N 4 film suspended over 50 µm x 50 µm holes etched through a silicon substrate. 13 We fabricated crosspoint HfO x devices with an active area of 2 µm x 2 µm ( Figure 1a) by depositing a bottom electrode (15 nm Pt), a blanket layer of 6 nm HfO 2 , followed by the top electrode (10 nm TiN and 15 nm Pt). Typical currentvoltage plots of these devices (Figure 1b) exhibited the well-recognized resistance switching behavior, or pinched hysteresis loop, that characterizes a memristor. 19 During operation, high and low non-volatile resistance states (also called OFF and ON, respectively) were repeatedly accessed using bipolar voltage pulses. STXM experiments were performed using resonantly tuned x-ray beams mostly in the O K-edge region, with spectral resolution of ~70 meV and a beam diameter <30 nm. 20 The device was electrically connected inside the chamber of the system to enable in-situ operation and ON/OFF cycling to emulate ageing of the memristor. The x-ray absorption spectrum of the material stack within a device crosspoint before its operation (Figure 2a) revealed oxygen bonds to both Hf and Ti, suggesting oxidation of Ti upon sputter deposition of TiN onto HfO 2 and a resulting mixture of Ti and Hf oxides. We used the absorption of the pre O K-edge at 522 eV to monitor total thickness and other structural modulations (especially electrode distortions), the intensity of the 531 eV peak (the lowest conduction band of the stack) as an indicator of the relative conductivity within the crosspoint, 1,21 and the post O K-edge at 570 eV to determine the local oxygen concentration in the film.

Analysis of Performance Instabilities of Hafnia‐Based Ferroelectrics Using Modulus Spectroscopy and Thermally Stimulated Depolarization Currents

“…[45,46] However, diffusion of charged oxygen vacancies as sole cause is unlikely since the applied field during cycling is rather low and alternating. During this electric cycling, a strong dependence of wake-up and fatigue on electric field strength was observed, and the influence of internal charges was suspected as cause.…”

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

“…Region (i). This region is as important for the resistive switching as the suboxide regions (ii) and (iii), because the processes in which oxygen diffuses into the metallic electrodes and the subsequent formation of an interface layer between the oxide and electrodes are both important for the durability and reproducibility of cycle-to-cycle parameters, as well as for the aging with subsequent device failure [9]. Recently, it was shown [5,7,50] that introducing an oxygen scavenger layer results in a better device performance by facilitating the formation of conducting filaments, and reduces the dependence of device's behavior on the microstructure in its "on" state [51].…”

“…So far, several important points are commonly accepted [1]. It is now well-established that the resistance switching in metal oxide memristors is mediated by oxygen migration [8,9], resulting in formation of the conductive filaments, while the details of the filament formation as well as the type of switching (bipolar vs unipolar) depend on the nature of electrodes [10] and on the interfacial metal-oxide interactions [5].…”

A New Simplified Model and Parameter Estimations for a HfO2-Based Memristor †

“…The main interest in the present case is associated with new and not very well analyzed types of memristors, such as transition-metal oxide memristors, especially hafnium oxide memristors [10][11][12]. Several basic models of HfO 2 -based memristors exist in the technical literature [10][11][12][13]. To the best of the author's knowledge, some of them are either very complex [10] or not sufficiently precise [11].…”