**Control of the Boundary between the Gradual and Abrupt Modulation of Resistance in the Schottky Barrier Tunneling-Modulated Amorphous Indium-Gallium-Zinc-Oxide Memristors for Neuromorphic Computing**

#### **Jun Tae Jang** † **, Geumho Ahn** † **, Sung-Jin Choi, Dong Myong Kim and Dae Hwan Kim \***

School of Electrical Engineering, Kookmin University, Seoul 02707, Korea; jtjang@kookmin.ac.kr (J.T.J.);

20113145@kookmin.ac.kr (G.A.); sjchoiee@kookmin.ac.kr (S.-J.C.); dmkim@kookmin.ac.kr (D.M.K.)

**\*** Correspondence: drlife@kookmin.ac.kr; Tel.: +82-2-910-4872

† These authors are co-first author.

Received: 4 September 2019; Accepted: 23 September 2019; Published: 25 September 2019

**Abstract:** The transport and synaptic characteristics of the two-terminal Au/Ti/ amorphous Indium-Gallium-Zinc-Oxide (a-IGZO)/thin SiO2/p <sup>+</sup>-Si memristors based on the modulation of the Schottky barrier (SB) between the resistive switching (RS) oxide layer and the metal electrodes are investigated by modulating the oxygen content in the a-IGZO film with the emphasis on the mechanism that determines the boundary of the abrupt/gradual RS. It is found that a bimodal distribution of the effective SB height (ΦB) results from further reducing the top electrode voltage (*V*TE)-dependent Fermi-level (*E*F) followed by the generation of ionized oxygen vacancies (V<sup>O</sup> <sup>2</sup>+s). Based on the proposed model, the influences of the readout voltage, the oxygen content, the number of consecutive VTE sweeps on ΦB, and the memristor current are explained. In particular, the process of V<sup>O</sup> 2+ generation followed by the Φ<sup>B</sup> lowering is gradual because increasing the *V*TE-dependent *E*<sup>F</sup> lowering followed by the V<sup>O</sup> <sup>2</sup><sup>+</sup> generation is self-limited by increasing the electron concentration-dependent *E*<sup>F</sup> heightening. Furthermore, we propose three operation regimes: the readout, the potentiation in gradual RS, and the abrupt RS. Our results prove that the Au/Ti/a-IGZO/SiO2/p <sup>+</sup>-Si memristors are promising for the monolithic integration of neuromorphic computing systems because the boundary between the gradual and abrupt RS can be controlled by modulating the SiO<sup>2</sup> thickness and IGZO work function.

**Keywords:** a-IGZO memristor; Schottky barrier tunneling; non filamentary resistive switching; gradual and abrupt modulation; bimodal distribution of effective Schottky barrier height; ionized oxygen vacancy
