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1 A fast, high endurance and scalable non-volatile memory device made from asymmetric Ta 2 O 5-x /TaO 2-x bilayer structures Myoung-Jae Lee 1, Chang Bum Lee 1, Dongsoo Lee 1, Seung Ryul Lee 1, Man Chang 1, Ji Hyun Hur 1, Young-Bae Kim 1, Chang-Jung Kim 1, David H. Seo 1, Sunae Seo 2, U-In Chung 1, In-Kyeong Yoo 1 and Kinam Kim 3 1 Semiconductor Device Laboratory, Samsung Advanced Institute of Technology, Gyeonggi , Korea 2 Department of Physics, Sejong University, Seoul , Korea 3 Samsung Advanced Institute of Technology, Samsung Electronics, Yongin, Gyeonggido , Korea NATURE MATERIALS 1

2 Supplementary Figures Figure S1 SEM images of before and after the lift-off process for definition of 30 nm lines used in the crosspoint structure. a, Shows images from after developing the 2-layer ZEP/LOR e-beam resist and depositing Pt/Ti. b, After lift-off using an ER stripper showing the 21 nm Pt line and larger contact pad. c, After completing the top 30 nm Pt line above the Ta 2 O 5-x /TaO 2-x layer for the crossbar device. 2 NATURE MATERIALS

3 SUPPLEMENTARY INFORMATION Figure S2 Electroforming process. The initialization of resistance switching, the socalled electroforming process in metal-insulator-base-metal (MIMB) structure is usually performed by applying a dc voltage sweep from 0 V to about - 3 V on the top electrode with a compliance current of 100 µa. At a certain voltage during the sweep, so-called forming voltage, an abrupt increase of current is observed and the voltage sweep process is terminated at the preset current compliance set value. At this time, conducting filaments are formed in the highly insulating Ta 2 O 5-x layer. NATURE MATERIALS 3

4 Figure S3 Raw data (I-V sweeps) for devices by different cell sizes. 4 NATURE MATERIALS

5 SUPPLEMENTARY INFORMATION Figure S4 Anti-serial HRTEM image of Pt/TaO 2-x /Ta 2 O 5-x /Pt/Ta 2 O 5-x /TaO 2-x /Pt. In the current structure the middle Pt (TE1) is a continues line which extends out of the plane. For the final anti-serial structure the Pt line must either be removed or etched into an embedded floating electrode. NATURE MATERIALS 5

6 Figure S5 The stray leakage path problem. a, Reading interference in an array consisting of 2 2 cells without switch elements. b, Rectified reading operation in an array consisting of 2 2 cells with switch elements. A well known issue for crossbar type devices is the need for a rectifying element. In order for any memory cell to be randomly accessed, each memory element must be connected with a switch element to prevent reading interference between neighboring cells. Figure S5a shows a typical erroneous operation of the simplest 2 2 crossbar cell array without switch elements. Although we want to read the information of the cell in the HRS surrounded by three cells in the LRS, the reading current can easily flow through the surrounding cells in the LRS and thus transmit erroneous LRS information. By adding a switch element with rectifying behaviors, alternate paths are rectified by applying an appropriate voltage to all other unselected cells and the reading current only flows through the selected cell we want to access as shown in Fig. S5b. The rectifying element was conventionally a diode or transistor. However we have used neither in our device by taking advantage of the asymmetric structure and I-V behavior. By combining two asymmetric devices into a combined anti-serial device we can overcome the stray leakage path problem. 6 NATURE MATERIALS

7 SUPPLEMENTARY INFORMATION 250 o 275 o C 85 o C 300C o 320 o C C fail Figure S6 Retention measurements. The measured activation energy was 1.47 ev, leading to a retention value of over 10 years at 85. NATURE MATERIALS 7

8 Figure S7 Photomask and optical images from fabricated devices. a, CAD image of the photomask. b-g, Optical images of 8 8 cell array in different cell sizes (b), µm 2 (c), 1 1 µm 2 (d), 2 2 µm 2 (e), 5 5 µm 2 (f), µm 2 (g), µm 2 8 NATURE MATERIALS

9 SUPPLEMENTARY INFORMATION Figure S8 Operation of a 64 cell array ( µm 2 ) and 10 cell array (30 30 nm 2 ). 64 of 64 cells and 10 of 10 cells were operational for each respective array. NATURE MATERIALS 9