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. 2017 Dec;12(1):65.
doi: 10.1186/s11671-017-1847-9. Epub 2017 Jan 23.

Synaptic Plasticity and Learning Behaviors Mimicked in Single Inorganic Synapses of Pt/HfOx/ZnOx/TiN Memristive System

Lai-Guo Wang  1   2 , Wei Zhang  1 , Yan Chen  1 , Yan-Qiang Cao  1 , Ai-Dong Li  3 , Di Wu  1
Affiliations

Synaptic Plasticity and Learning Behaviors Mimicked in Single Inorganic Synapses of Pt/HfOx/ZnOx/TiN Memristive System

Lai-Guo Wang et al. Nanoscale Res Lett. 2017 Dec.

Abstract

In this work, a kind of new memristor with the simple structure of Pt/HfOx/ZnOx/TiN was fabricated completely via combination of thermal-atomic layer deposition (TALD) and plasma-enhanced ALD (PEALD). The synaptic plasticity and learning behaviors of Pt/HfOx/ZnOx/TiN memristive system have been investigated deeply. Multilevel resistance states are obtained by varying the programming voltage amplitudes during the pulse cycling. The device conductance can be continuously increased or decreased from cycle to cycle with better endurance characteristics up to about 3 × 103 cycles. Several essential synaptic functions are simultaneously achieved in such a single double-layer of HfOx/ZnOx device, including nonlinear transmission properties, such as long-term plasticity (LTP), short-term plasticity (STP), and spike-timing-dependent plasticity. The transformation from STP to LTP induced by repetitive pulse stimulation is confirmed in Pt/HfOx/ZnOx/TiN memristive device. Above all, simple structure of Pt/HfOx/ZnOx/TiN by ALD technique is a kind of promising memristor device for applications in artificial neural network.

Keywords: Atomic layer deposition; Memristor; Pt/HfOx/ZnOx/TiN; Synapse plasticity.

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Figures

Fig. 1
Fig. 1
Schematic of the memristor device of Pt/HfOx/ZnOx/TiN and its I–V characteristics. a Analogy between the biological synapse and the electronic synapse based on the Pt/HfOx/ZnOx/TiN memristor device. b I–V characteristics of the Pt/HfOx/ZnOx/TiN synapse device measured by a typical DC double sweep
Fig. 2
Fig. 2
I–V characteristics of the Pt/HfOx/ZnOx/TiN synapse device and conductance dependence on consecutive depressing or potentiating pulses. a I–V characteristics of the Pt/HfOx/ZnOx/TiN synapse device measured by a modified DC double sweep. b I–V characteristics of the memristor at positive and negative bias voltages. The voltage sweep range is from 0 to 1.4 (–0.6) V then back to 0 V, and the time for a sweep cycle is 1 s. The device conductivity continuously decreases or increases during the positive or negative voltage sweeps. c The curves of voltage and current versus time, which are plotted from the data in (b). d The curves of device conductivity versus pulse numbers. The device conductivity can be decreased or increased by consecutive depressing or potentiating pulses
Fig. 3
Fig. 3
The property of gradual current change in the context of pulse cycling of Pt/HfOx/ZnOx/TiN. a Current evolution of the device for the first 200 pulse numbers. b Endurance test up to 3 × 103 cycles. Pulse of ±2 V with 100-ms width was applied to switch the device between the LRS and the HRS, and the resistances were read out at 0.1 V at room temperature. c Current evolution of the same device after the 3 × 103 cycle endurance test in b
Fig. 4
Fig. 4
Nonlinear transmission characteristics and spike-timing-dependent plasticity (STDP) of the memristor device. a Response of a memristor device to different pulse programs; b Emulation of STDP learning rule in Pt/HfOx/ZnOx/TiN memristive device—the relative change of the memristor synaptic weight (ΔW) versus the relative spike timing (Δt). And the solid line is the fitting exponential curve to the experimental data. The insets illustrate various spike schemes. The pulse pair comprises a positive and a negative voltage pulse with amplitude of 1.0 V and width of 50 ms. The interval between the two pulses is Δt ms (t = ±10n, n = 1, 2, …, 10). The current compliance is not set in the whole emulation process. The current values are read at 0.1 V after 5 min of the spikes
Fig. 5
Fig. 5
The memory retention for synaptic weight of Pt/HfOx/ZnOx/TiN memristive device: transformation from STP to LTP induced by repetitive pulse stimulation. a–e Memory forgetting process (black solid circle in figure) was experienced by different numbers of pulse stimulation, in which the normalized data was fitted using Eq. 1. f The relaxation time (τ) changes with the number of pulses by fitting the data from Fig. 5(a–e)
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