Reduction of the Operating Current Range of Analog Resistive Switching in Pt/TaO_x/Ta₂O₅/Pt Cells by Controlling the Supply of Oxygen Vacancies

Memristors are one of the strong candidates for synaptic devices. Transition metal oxides such as titanium oxide (TiO₂) and tantalum oxide (Ta₂O₅) are broadly used as materials of memristors. However, these transition-metal-oxide-based memristors often show an abrupt decrease in the cell resistance (set process), resulting in the difficulty of the analog control of the cell resistance [1]. Although some studies have reported gradual set processes, the mechanism of the gradual set processes has not been clarified [2]. Therefore, any guidelines to improve the analog resistive switching (RS) characteristics, including reduction of power consumption, have not been established. In our previous study, analog RS characteristics were demonstrated at both set and reset processes after a semi-forming phenomenon, which is defined as an initial transition process to a higher resistance state (~ 1 kΩ) than the conventional low resistance state (&lt; 100 Ω) [3], in Ta₂O₅-based RS cells with tantalum sub-oxide (TaO<i>_x</i>) as an oxygen vacancy (V_O) reservoir. However, there is an inevitable trade-off relationship between the current range of analog RS operation and the occurrence frequency of the semi-forming phenomenon, as long as the oxygen composition of the TaO<i>_x</i> layer is widely varied. In this study, the trade-off relationship was optimized by adjusting the oxygen composition and the TaO<i>_x</i> thickness.<br/>Platinum (Pt)/TaO<i>_x</i>/Ta₂O₅/Pt stack cells were fabricated on a SiO₂/Si substrate. A 60-nm-thick Pt bottom electrode (BE) was deposited by DC sputtering. Then, a 10-nm-thick Ta₂O₅ layer and a TaO<i>_x</i> layer with different TaO<i>_x</i> thicknesses (10 nm, 30 nm, and 60 nm) were deposited by reactive RF sputtering. The oxygen compositions of the TaO<i>_x</i> and Ta₂O₅ layers were controlled by appropriately adjusting the oxygen gas flow rate. (TaO<i>_x</i>: 0.8–1.2 sccm, Ta₂O₅: 2.0 sccm) Finally, 25-nm-thick Pt top electrodes (TE) with a diameter of 100 μm were formed by electron beam evaporation. Forming and RS operations in the Pt/TaO<i>_x</i>/Ta₂O₅/Pt cells at room temperature were characterized.<br/>We investigated the dependences of initial resistance (<i>R</i>_ini) and relative occurrence frequency of the semi-forming phenomenon (<i>F</i>_semi) on the oxygen composition <i>x</i> and the TaO<i>_x</i> thickness. When the TaO<i>_x</i> thickness is 30 nm, as <i>x</i> linearly increases from 1.3 to 2.0, <i>R</i>_ini exponentially increases from 1 kΩ to 1 GΩ and <i>F</i>_semi tends to decrease from 0.8 to 0. The increase in <i>x</i> means the decrease in the density of V_Os supplied from the TaO<i>_x</i> layer to the Ta₂O₅ layer. In other words, while the semi-forming phenomenon is more likely to occur as more V_Os are supplied to the Ta₂O₅ layer, the more supplied V_Os cause the decrease in <i>R</i>_ini, which means a trade-off relationship. Moreover, compared with the analog RS characteristics when the oxygen composition <i>x</i> is 1.3 and 1.7, it was revealed that the decrease in <i>R</i>_ini leads to lowering the resistance range of RS operation from 6–40 kΩ to 0.1–1 kΩ. Furthermore, the dependence of <i>R</i>_ini and <i>F</i>_semi on the TaO<i>_x</i> thickness was investigated. Under the oxygen gas flow rate of 1.0 sccm, corresponding to the oxygen composition <i>x</i> of 1.7, when the TaO<i>_x</i> thickness is 10 nm, 30 nm, and 60 nm, <i>R</i>_ini is 30 MΩ, 2 MΩ, and 1 MΩ, and <i>F</i>_semi is 0.1, 0.6, 0.9, respectively. These results indicate that <i>F</i>_semi increases while keeping high <i>R</i>_ini and resistance range of analog RS operations by increasing the TaO<i>_x</i> thickness, that is, the trade-off relationship can be improved. Analog RS characteristics could be demonstrated in the resistance range of 3–10 kΩ also when <i>x</i> is 1.7 and the TaO<i>_x</i> thickness is 60 nm.<br/>In summary, by adjusting the TaO<i>_x</i> thickness and the oxygen composition <i>x</i> appropriately, the current range of analog RS after the semi-forming can be lowered for reduction of power consumption.<br/>[1] D. Ielmini, <i>Microelectronic Engineering</i>, <b>190</b>, 44 (2018).<br/>[2] R. Waser <i>et al.</i>,<i> Faraday Discuss.</i> <b>213</b>, 11 (2019).<br/>[3] T. Miyatani <i>et al.</i>, <i>2020 virtual MRS spring/fall meeting</i>, F. NM07. 06. 06.