Nanoscale 3D Stackable Ag-Doped HfOx-Based Selector Devices

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Nanoscale 3D-Stackable Ag-doped HfOx-Based Selector Devices Fabricated through Low-Temperature Hydrogen Annealing Ju Hyun Park, Donghyun Kim, Dae Yun Kang, Dong Su Jeon, and Tae Geun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08166 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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ACS Applied Materials & Interfaces

Nanoscale 3D-Stackable Ag-doped HfOx-Based Selector Devices Fabricated through LowTemperature Hydrogen Annealing Ju Hyun Park, Donghyun Kim, Dae Yun Kang, Dong Su Jeon, and Tae Geun Kim* School of Electrical Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 02841, Republic of Korea Keywords: Selector device, threshold switching, crossbar array, resistive switching, nonvolatile memory.

ABSTRACT Electrochemical metallization-based threshold swtiching devices with active metal electrodes have been studied as a selector for high-density RRAM technology in crossbar array architectures. However, these devices are not suitable for integration with three-dimensional (3D) crossbar RRAM arrays due to the difficulty in vertical stacking and/or scaling into the nanometer regime as well as the asymmetric threshold switching behavior and large variation in the operating voltage. Here, we demonstrate bidirectional symmetric threshold switching behaviors from a simple Pt/Agdoped HfOx/Pt structure. While fabricating the Pt/Ag-doped HfOx/Pt film using a 250 nm hole structure, filaments composed of Ag nano clusters were constructed through a low temperature

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(~200 °C) hydrogen annealing process, where the shape of the film in a nanoscale via-hole structure was maintained for integration with 3D stackable crossbar RRAM arrays. Finite Ag filament paths in the HfOx layer led to uniform device-to-device performances. Moreover, we observed that the hydrogen annealing process reduced the delay time through the reduction of the oxygen vacancies in the HfOx layer. Consequently, the proposed Pt/Ag-doped HfOx/Pt-based nanoscale selector devices exhibited excellent performance of high selectivity (~105), ultra-low OFF current (~10 pA), steep turn-on slope (~2 mV/decade), and short delay time (3 μs).


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1. INTRODUCTION Resistive random access memory (RRAM) has attracted considerable attention as a candidate for next-generation nonvolatile memory due to its excellent scalability, rapid operation, low power consumption, and simple fabrication process.1-3 However, the current passing through the neighboring unselected cells, named sneak path current, has a significant effect on the read margin between the low- and high-resistance states, thereby limiting the reading operation and the array size.4–7 This sneak path current interrupts a high-density integration of passive crossbar arrays (CBAs). Accordingly, two-terminal rectifying devices (e.g., self-rectifying and selector devices) with high selectivity are essential for increasing the storage capacity and, in turn, overcoming the drawbacks associated with sneak path currents in passive CBAs.4–7 The energy bandgap is usually engineered to reduce the sneak path current in multi-layer selfrectifying devices.8,9 However, this method needs materials characterized by easy matching of the energy bandgap between the layers. Another method to suppress the sneak path current is to use selector devices that are connected in series with RRAM. Various selector devices, such as mixedionic-electronic-conduction devices,10 bidirectional varistor devices,11 ovonic threshold switching devices,12 and metal-insulator transition devices13, have been investigated extensively for CBA applications. However, the performance of these devices10–13 is not good enough yet to prevent such a sneak path current due to the low selectivity caused by high off-current levels or thermal instability. Recently, volatile threshold switching (TS) behavior based on electrochemical metallization (ECM) has been introduced to oxide-metal materials, such as TaOx14, TiO215, ZrO216 and SiO217. These materials are characterized by a forming-free process, low threshold voltage (107A/cm2, Selectivity~104) for 3D Bipolar Resistive Memory Arrays. Symposium on VLSI Technology Digest of Technical Papers 2012, 37–38.


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(22) Yang, Y.; Gao, P.; Li, L.; Pan, X.; Tappertzhofen, S.; Choi, S.; Waser, R.; Valov, I.; Lu, W. D. Electrochemical Dynamics of Nanoscale Metallic Inclusions in Dielectrics. Nat. Commun. 2014, 5, 4232, 1–9. (23) Wang, Z.; Joshi, S.; Savel’ev, S. E.; Jiang, H.; Midya, R.; Lin, P.; Hu, M.; Ge, N.; Strachan, J. P.; Li, Z.; Wu, Q.; Barnell, M.; Li, G.-L.; Xin, H. L.; Williams, R. S.; Xia, Q.; Yang, J. J. Memristors with Diffusive Dynamics as Synaptic Emulators for Neuromorphic Computing. Nat. Mater. 2017, 16, 101−108. (24) Wang L.-G.; Qian, X.; Cao, Y.Q.; Cao, Z.-Y.; Fang, G.-Y.; Li, A.-D.; Wu, D. Excellent Resistive Switching Properties of Atomic Layer-Deposited Al2O3/HfO2/Al2O3 Trilayer Structures for Non-Volatile Memory Applications. Nanoscale Res. Lett. 2015, 10, 1−8. (25) Al-Kuhaili, M. F.; Durrani, S. M. A.; Bakhtiari, I. A.; Dastageer, M. A.; Mekki, M. B. Influence of Hydrogen Annealing on the Properties of Hafnium Oxide Thin Films. Mater. Chem. Phys. 2011, 126, 515–523. (26) He, G.; Liu, M.; Zhu, L. Q.; Chang, M.; Fang, Q.; Zhang, L. D. Effect of Postdeposition Annealing on the Thermal Stability and Structural Characteristics of Sputtered HfO2 Films on Si (100). Surf. Sci. 2005, 576, 67–75. (27) Huang, C.-Y.; Huang, C.-Y.; Tsai, T.-L.; Lin, C.-A.; Tseng, T.-Y. Switching Mechanism of Double Forming Process Phenomenon in ZrOx/HfOy Bilayer Resistive Switching Memory Structure with Large Endurance. Appl. Phys. Lett. 2014, 104, 062901, 1–4. (28) Sun, Y.; Zhao, X.; Song, C.; Xu, K.; Xi, Y.; Yin, J.; Wang, Z.; Zhou, X.; Chen, X.; Shi, G.; Lv, H.; Liu, Q.; Zeng, F.; Zhong, X.; Wu, H.; Liu, M.; Pan, F. Performance-Enhancing Selector via Symmetrical Multilayer Design. Adv. Funct. Mater. 2019, 29, 1808376, 1–9 (29) Ji, X.; Song, L.; He, W.; Huang, K.; Yan, Z.; Zhong, S.; Zhang, Y.; Zhao, R. Super Nonlinear Electrodeposition−Diffusion-Controlled Thin-Film Selector. ACS Appl. Mater. Interfaces 2018, 10, 10165–10172 (30) Song, B.; Xu, H.; Liu, S.; Liu, H.; Li, Q. Threshold Switching Behavior of Ag-SiTe-Based Selector Device and Annealing Effect on Its Characteristics. IEEE J. Electron Devices Soc. 2018, 6, 674–679


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(31) Hua, Q.; Wu, H.; Gao, B.; Zhao, M.; Li, Y.; Li, X.; Hou, X.; Chang, M.-F.; Zhou, P.; Qian, H. A Threshold Switching Selector Based on Highly Ordered Ag Nanodots for X-Point Memory Applications. Adv. Sci. 2019, 6, 1900024, 1–9


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Figure Captions Figure 1. (a) Schematic illustration (left) and fabrication flows (right) of the Ag-doped HfOx selector devices. SEM top view images of the Pt/Ag-doped HfOx/Pt selector fabricated in the 250 nm via-hole structure with (b) 0 °C, (c) 200 °C, and (d) 300 °C annealing processes. Crosssectional TEM images of the corresponding Pt/Ag-doped HfOx/Pt selectors annealed at (e) 200 °C (inset figure; EDX mapping image for three Pt, Ag, and Ti elements. Right side figure; film states of amorphous and crystal structure in the two regions.) and (f) 300 °C (inset figure; EDX mapping image for three Pt, Ag, and Ti elements.), respectively.

Figure 2. 3D AFM images captured at the surface of the Ag-doped HfOx films on SiO2 and Pt substrates, respectively, (a, d) without annealing and with annealing at (b, e) 200 °C and (c, f) 300 °C, respectively.

Figure 3. (a) Schematic showing the position of the five selected devices in a sample. Currentvoltage (I−V) characteristics of (b) the non-annealed and (c) annealed devices in 200 °C hydrogen gas environments in the positive and negative voltage regions, at a compliance current (Icc) of 1 μA, in the five devices (D1, D2, D3, D4, and D5).

Figure 4. (a) I−V curves of the annealed selector device at different compliance currents. (b) Stable threshold switching characteristics of the annealed selector device with operation up to 100 cycles. (c) Statistical characteristics of the threshold and hold voltage in the negative and positive regions.


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Figure 5. Schematic illustration on the mechanism of filament formation when a voltage is applied in (a) non-annealed and (b) annealed devices. Ag atoms or clusters are distributed sporadically at the initial state, and produce conducting bridges in the form of filament for the non-annealed device, as shown in the left side of Figure 5 (a). However, during the annealing process, Ag clusters are more formed in the Ag-doped HfOx film, leading to a reduction in the path of the filament.

Figure 6. Atomic concentration of the four elements (i.e., Hf, O, Ag, Pt) in the Pt/Ag-doped HfOx/Pt thin film layers at a temperature of 200 °C. The concentrations were measured from AES depth profiles collected from three randomly selected regions (a, b, and c) of the device.

Figure 7. XPS spectra of the O1s peaks at the surface of the (a) non-annealed and (b) annealed selector devices. The inset figures show the respective Hf 4f peaks. Voltage vs. time plots of the (c) non-annealed and (d) annealed devices subjected to AC measurement conditions (1.25 V/20 μs pulse).

Table 1. Performance comparison with other ECM-based selector devices using Ag as active metal.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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Selectors

Ag/TaOx/TaOy/TaOx/Ag [28]

Sacked layers Icc ON/OFF ratio Vth (DC) Endurance Switching slop Delay time

3 layers 1 mA 1010 0.1-0.9 V (Tunable) 50 cycles < 1 mV/decade < 100 ns

TiW/Ag alloyed Ge2Sb2Te5/Pt [29] 1 layer

TiN/SiTe/Ag [30]

Pt/Ag-nanodots /HfO2/Pt [31]

Pt/HfOx:Ag/Pt (This work)

1 layer

Nanodots/1 layer

1 layer

1 μA ~105 ~1 V 105 cycles < 5 mV/decade -

100 μA 104 ~0.6 V 105 cycles < 2 mV/decade

1 μA ~105 ~0.6 V 105 cycles < 2 mV/decade

~5.13 μs

100 μA 108 0.23-0.28 V 108 cycles < 1 mV/decade < 100 ns

Cell size

Circular

Square

Square

Square

(diameter 50 μm)

(width 4 μm)

(width 5 μm)

(width 5 μm)

~3 μs Circular (diameter 250 nm)

Process T.

RT

RT

Need annealing process, but not known.

200 ℃

200 ℃

Table 1


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Graphic for manuscript


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Nanoscale 3D Stackable Ag-Doped HfOx-Based Selector Devices

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