Evolution of conduction channel and its effect on resistance switching for Au-WO₃-x-Au devices

Abstract

We performed a systematic investigation on the dynamic behavior of conduction filaments (CFs) in WO₃-x-based devices. It was found that the electric forming produced an electric structure consisted of a conductive channel (virtual cathode) started from cathode and an insulating band surrounding anode. Both the virtual cathode and the insulating region varied with repeated resistance switching. Set/reset operation affected device resistance mainly by modifying the CF, which formed in the setting process together with an insulating halo that separated it from the virtual cathode. The device resistance exhibited a sudden change exactly corresponding to the emergence/vanishing of the CF and a smooth variation corresponding to the outward/inward expansion/contraction of the insulating halo. Anode ablation occurred after repeated cycling, and it is the key factor affecting the endurance of device.

Figure 1

(a) Experiment setup for optical observation. (b) I-V relations for four electric forming processes with the current compliances of I_CC = 3 mA, 4 mA, 5 mA and 6 mA, respectively. (c) Transparent photographs corresponding to the above forming processes. All the optical microscopic images were recorded after the application of I_CC. Black shadow appears when I_CC = 3 mA, but the bright band is ambiguous until I_CC = 5 mA. Electrode separation is 50 μm.

Figure 2

(a) Transmittance image of electrically formed Au-WO_3-x–Au cell. Red circles and yellow midline indicate the locations for Raman measurements. (b) Normalized Raman spectra collected from the dark and bright regions of electrically formed WO_3-x (10 Pa). Data for pristine WO_3-δ (30 Pa) and WO_3-x (10 Pa) films are also shown for comparison. (c) Intensity variation of ~427 cm⁻¹ and ~811 cm⁻¹ peaks along the midline between two electrodes. (d) Mapping of oxygen vacancies for electrically treaded WO_3-x cell, derived from the transmittance image shown in the inset. The inset curve shows the distribution of oxygen vacancies along midline.

Figure 3

(a) I-V characteristics of a device set by current sweeping (positive biases) and reset by voltage sweeping (negative biases). Labels mark the different stages of set/reset process. (b)–(e) and (h)–(k) Two series of transmittance images corresponding to the intermediate states in the set and reset processes, respectively. (f) and (L) Relative variations of color contrast from the HRS to set1 (f) and from the LRS to reset1 (L); expansion/contraction of the fine CF and the faint halo can be clearly seen. (g) Oxygen content measured along the dashed line in (c); the dark-bright border exhibits lowest oxygen content, ~2.7 in the as-formed state, and the faint halo (marked by an arrow in (c)) owns an oxygen content of ~2.8. (m) Line-profile of the transmittance of the CF, collected along the arrow marked in the upper-right image. Shaded area marks the CF, and the number denotes the maximal transmittance drop.

Figure 4

(a) Endurance performance of the Au-WO_3-x-Au cell. Red and blue curves correspond to the devices with the Au electrodes of 40 nm and 80 nm, respectively. (b) I-V characteristics from 200 to 400 cycles. (c) Cumulative distributions of the device resistance, counted from 200 to 400 and 800 to 1000 cycles, respectively. (d)–(g) Transmittance images of the device after 10, 400, ~600 electric cycles (LRS). Random CF location appears and, sometimes, more than one CF are formed above 600 cycles. (h) and (i) Relative transmittance change after different RS cycles. Arrows in (i) mark the two strips appearing after repeated cycles. (j) Line profiles of the transmittance of the CF, measured along the direction marked by an arrow in the upper-left image.

Figure 5

Surface morphology of the LE immediately after the (a) forming process, (b) 200 switching cycles, and (c) 600 switching cycles. The ablated region in the LE expands with switching cycles. Arrow in (c) marks the location of CF.