Electronic threshold switching of As-embedded SiO2 selectors: charged oxygen vacancy model

Abstract

Sneak current issues in crossbar arrays of non-volatile memories can be effectively alleviated using threshold switching (TS)-based selectors. However, 1-selector-1-resistor integration requires coherence between the constituent materials and operational parameters of the two components. Here, we propose a highly coherent selector via in-depth investigation of the operation process of a fab-friendly As-SiO₂ selector unit. The structural and electrical characteristics of an As-embedded SiO₂ selector are analyzed, and the TS-on and -off operational mechanism is presented. Further, the critical control elements governing the selector operation are identified, including the electron charging into the oxygen vacancies in the SiO₂ matrix and energy band alignment between the As cluster and charged oxygen vacancies in SiO₂. Consequently, practical control strategies for the TS behavior are proposed with a pulse scheme applicable to actual device operation. The proposed TS operational mechanism and analytical methodology can contribute to interpreting and integrating various memory/selector components, thereby advancing their operational and integrative research.

Keywords: Charged oxygen vacancy; Crossbar array; Mechanism; Pulse scheme; Selector device; Threshold switching.

Fig. 1

As-embedded SiO₂ selector device structure and DC/AC characteristics. (a) STEM image of the As-SiO₂ selector and magnified image with the (b) corresponding EDS mapping results of As and O. (c) EELS O K-edge spectra at (1) As cluster (represented by the black line), (2) As cluster-SiO₂ interface (red line), and (3) SiO₂ (blue line) in the As cluster-embedded in the SiO₂ layer. The EELS O K-edge spectrum of pure SiO₂ is included for comparison (d) As 3d and (e) Si 2p core level XPS spectra of As cluster-embedded SiO₂ layer compared to that of pure SiO₂ layer. (f) Schematic structure of the As-SiO₂ selector and oxidation–reduction reaction near the As cluster in the film. (g) Typical DC I–V curves of the As-SiO₂ selectors. (h) Cumulative probability of DC-based operating voltages; forming voltage (V_F), TS-on voltage (V_th), and TS-off voltage (V_h), including (i) those as a function of the operating temperature ranging from 113–300 K. (j) Transient TS characteristics of the As-SiO₂ selector under single pulse and (k) typical I–V curve based on the pulse amplitude sweep. On-current of the selectors as a function of the (l) device area (pulse amplitude = 3.5 V) and (m) operating temperatures (pulse amplitude = 4 V)

Fig. 2

TS-on operation mechanism based on charged oxygen vacancy model. (a) TS-on characteristics of the As-SiO₂ selector with ① a typical voltage linear sweep and ② voltage sweep with a constant voltage at 0.5 V. (b) Schematic illustrating the energy levels of the constituent materials and defect states in the As-SiO₂ layer. Electronic band diagrams explaining the operational component TS-on process; (c) Charging of neutral dimer (V_O⁰) to form the negative charged state (V_O^-) and (d) Band alignment between the As cluster and electron tunneling path in the SiO₂. (e) TS-on operation with various constant voltage magnitudes and (f) total charging time of TS-on as a function of the constant voltage magnitude. (g) Double logarithmic I–V curve of the As-SiO₂ selector for the evolution of current conduction mechanisms in the sub-threshold region with the (h) corresponding band diagram for the V_O⁰ charging process: SCLC and P–F emission to trap-assisted tunneling. (i) Verification of the current conduction mechanism evolution process using a CV at 0.5 V for 180 and 365 s

Fig. 3

Controlled TS-on characteristics of the As-SiO₂ selector using the two-step pulse scheme. (a) Typical two-step single pulse comprising the pre-charging step with a low amplitude at 0.5 V for 1 µs, which is utilized for V_O⁰ charging, and a primary pulse step with a high amplitude at 3.5 V for 3 µs for TS-on. (b) Dependence of V_th as a function of the pre-charging times from 0–50 µs of the two-step single pulses. (c) Typical two-step pulse train comprising pre-charging steps at 0.5 V for 1 µs and primary pulse steps with a high amplitude from 0–4 V for 3 µs. (d) Dependence of the off current as a function of the pre-charging times from 0–3.5 µs of the two-step pulse trains

Fig. 4

Exploration and verification of the TS-off mechanism. (a) Typical I–V curves of the As-SiO₂ selector with the linear voltage range from 0 V to before and after V_th. (b) Band diagram illustrating the abnormal TS-on state caused by the electronic dipole formation in the theoretical TS-off voltage region (V_h< V< V_align). The applied DC voltage and corresponding current over time: (c) TS-on state is abnormally maintained for a period, even with the constant voltage below V_align, as highlighted in the red box region, which is caused by electronic dipole formation in (b). (d) TS-on occurs with the constant voltage between V_align and V_th for some time, as highlighted in the green box, owing to electron charging, while TS-off occurs with a constant voltage slightly below V_align, as highlighted in the red box, which is similarly to (c). (e) Typical I–V curves with a linear voltage sweep and those corresponding to (d); considerably decrease in the difference between V_th and V_h