Several typical bilayer-structured ultrathin memristors and their architectures.
Abstract
This chapter deals with several kinds of ultrathin bilayer-structured memristors, such as Pt/Al2O3/HfO2/TiN, Pt/HfO2/HfOx/TiN, Pt/TiO2/Ti-based maleic acid (Ti-MA)/TaN, among which the asymmetric memristive functional layers were designed and prepared by atomic layer deposition (ALD) or molecular layer deposition (MLD) technique. These bilayer memristors exhibit a typical bipolar resistive switching characteristic, in accord with the space charge limited current model. Some important biologic synaptic functions have been achieved, including nonlinear transmission characteristics, spike-timing-dependent plasticity, short−/long-term plasticity, paired-pulse facilitation, and conditioned reflex. The mechanism of bilayer memristive device has been proposed based on oxygen vacancies migration/diffusion model. Above all the ultrathin bilayer memristors fabricated by low temperature ALD/MLD are one competitive candidate for neuromorphic simulation and flexible electronic applications.
Keywords
- memristor
- atomic layer deposition
- bilayer
- synapse
- mechanism
1. Introduction
The memristor concept was first proposed as the fourth fundamental passive circuit element by Chua in 1971 based on the completeness of the circuit theory, which indicates the relationship between magnetic flux and charge [1, 2]. After thirty seven years, Strukov et al. eventually found the missing memristor in studying TiO2 cross-arrays in 2008 [2]. This draws the extensive and intensive attention from the academia and the industry. Memristor is a two-terminal electrical device whose resistance can be tuned by changing the flux or charge through it. Memristor possesses a lot of advantages, e.g., simple device architecture, high energy efficiency, better compatibility with semiconductor industry, and high integration density.
A neural synapse, as the basic unit of learning and memory in the brain, plays a critical role in biological neural networks. Electronic synapses are utilized to emulate the bio-synapses’ functions. Some researches on synapse simulation have been reported by adjusting synaptic weights so as to make an effective bio-inspired computing system [3, 4, 5, 6]. Nevertheless, most work chose transistors and capacitors to realize artificial synapse, which produced high energy consumption at high integration density and limited the programming running. The new memristor has nonlinear transfer characteristics similar to the bio-synapse and is regarded as the closest to the synaptic device [4].
Although various materials and structures exhibit memristive behavior, almost all the memristor systems are based on the structural asymmetry [7, 8]. For example, in the metal–insulator–metal (MIM) structure, the defects such as oxygen vacancy or active ions in the insulator layer can induce structural asymmetry under the action of the external field, or when one of the metal electrodes is active. Therefore, the asymmetric bilayer-structured memristors play a crucial role in constructing artificial neural networks for brain-inspired applications.
Atomic layer deposition (ALD) is a kind of commercial technology compatible with semiconductor processing. It shows unusual advantages in controllable fabrication of nano-laminate thin films due to its unique sequential self-limiting surface reaction mechanism at low growth temperature [9, 10]. In early 2001 ALD has been known as candidate technology preferred for semiconductor industry along with metalorganic chemical vapor deposition (MOCVD) and plasma-enhanced CVD by the international technology roadmap for semiconductors (ITRS) [11]. ALD has become one of the most competitive deposition techniques for microelectronics and nanotechnology owing to sub-nanometer thickness control, large-area uniformity, excellent three-dimensional conformality, and good reproducibility. Thin films with low defect density can be prepared by ALD even at room temperature (RT) with plasma assistance [12]. Evidently, low temperature or RT ALD technology can greatly widen the flexible substrate choice range, showing exciting potentials in flexible electronic device fabrication. Molecular layer deposition (MLD) can be regarded as the subtype of ALD due to the molecular nature of the deposition process, suitable for growth of organic–inorganic hybrid materials [13].
In this section, we fabricated several synaptic devices of asymmetric bilayer-structured ultrathin memristors by atomic layer deposition (ALD) and molecular layer deposition (MLD), such as Pt/AlOx/HfOx/TiN, Pt/HfO2/HfOx/TiN, Pt/TiO2/Ti-based maleic acid (Ti-MA)/TaN. Some biological synapse-like functions of long−/short-term plasticity (LTP and STP), spike-timing-dependent plasticity (STDP), and paired-pulse facilitation (PPF) have been achieved simultaneously. A memristive mechanism of an asymmetric bilayer-structured synaptic device has been proposed to explain synaptic plasticity based on the oxygen vacancy migration/diffusion model.
2. Bilayer-structured ultrathin memristors
2.1 Fabrication processing
Asymmetric bilayer-structured ultrathin memristor based on Pt/A/B/TiN or TaN was fabricated on SiO2/Si substrates by thermal-ALD (TALD), MLD and plasma-enhanced ALD (PEALD), as illustrated in Figure 1a. Herein A and B act as asymmetric memristive functional layer, PEALD TiN or sputtered TaN as bottom electrode, sputtered Pt as top electrode with a spot size in diameter of 150 μm. Table 1 gives several typical bilayer ultrathin memristors and their architectures. The related deposition conditions have been listed in Table 2, including used metal precursors and reactants, source temperature and deposition temperature, and growth per cycle (GPC).
Device structure | A (thickness) | B (thickness) |
---|---|---|
Pt/HfOx/ZnOx/TiN | HfOx (5 nm) | ZnOx (5 nm) |
Pt/AlOx/HfOx/TiN | AlOx (5 nm) | HfOx (5 nm) |
Pt/TiO2/Ti-MA/TaN | Ti-MA (4 nm) | TiO2 (4 nm) |
Material | Metal precursor | Precursor temperature | Reactant | Deposition temperature | GPC (Å/cycle) | |
---|---|---|---|---|---|---|
TALD | HfO2 | TEMAH | 155°C | H2O | 250°C | 1 |
AlOx | TMA | RT | H2O | 250°C | 1 | |
ZnOx | DEZ | RT | H2O | 250°C | 1.3 | |
TiO2 | TiCl4 | RT | H2O | 250°C | 0.3 | |
PEALD | HfOx | TEMAH | 155°C | H2 plasma | 250°C | 1 |
TiN | TiCl4 | RT | NH3 plasma | 400°C | 0.5 | |
MLD | Ti-MA | TiCl4 | RT | MA(135°C) | 160°C | 1.4 |
2.2 Electrical performances and synaptic functions
The electrical properties were measured under DC sweep and pulse modes using semiconductor parameter analyzer on probe station. The bottom electrode of memristors was set on ground and all the voltage signals were applied to the top electrode. The asymmetric bilayer ultrathin memristors were exploited to mimic some important synaptic functions such as long-term potentiation/depression, the transition from STP to LTP, PPF and STDP.
2.2.1 TiN/ZnOx/HfOx/Pt inorganic memristor
The
A series of pulse signals were designed and applied to the memristor to test the important STDP rule in the Hebbian learning theory, as seen in the insets of I and III of Figure 2, including the V−/V+ = −1.0 V/1.0 V pulse pair signal as a presynaptic and postsynaptic spike with the 3 s interval time. Such design can prevent from the disturbance of excitatory postsynaptic current [16]. The time interval between the final presynaptic spike and the initial postsynaptic spike is defined as the relative time of Δt. The relative change of the synaptic weights (ΔW) is defined as:
The initial postsynaptic or presynaptic current I1 was used as the control value. After the spike pair was applied and over for 5 min, the measured presynaptic or postsynaptic current was I2.
The dependence of Δ
In addition, Pt/HfOx/ZnOx/TiN device also exhibits the nonlinear transmission efficiency, and the transition from STP to LTP (not shown here) [15].
2.2.2 TiN/HfOx/AlOx/Pt inorganic memristor
The memristor device based on TiN/HfOx/AlOx/Pt can also emulate the biological synapse. Usually, the synapse operates under pulse signals rather than DC bias sweep voltage. The LTPo and LTD phenomena can be observed in Pt/AlO
Further experiments have demonstrated that a pulse signal from amplitude of 1.0–1.5 V and pulse width of 50–100 ms leads to various current responses in Figure 3b. That is to say, the larger pulse amplitude, the longer pulse width, and the more pulse number will produce more significant response current change, which is analogous to long-term potentiation/depression of the human brain.
Synaptic plasticity can be divided into STP and LTP according to the timelines of enhanced synaptic connections. The repeated stimulation induced STP to LTP transition is illustrated in Figure 4. With increasing the rehearsal pulse number (
An exponential decay equation was employed to depict the relaxation process:
where
The STDP rule has been mimicked in TiN/HfOx/AlOx/Pt memristor, as indicated in Figure 5. The schematic of another training pulse signal with various amplitudes is shown in Figure 5a, different from the pulse design in Figure 2. A set of pulses (1 V, −0.5 V, −0.45 V, −0.4 V, −0.35 V, −0.3 V, −0.25 V)/(0.5 V, −1 V, −0.9 V, −0.8 V, −0.7 V, −0.6 V, −0.5 V) were used as pre-synaptic/post-synaptic stimulation signals, respectively. Some different pulse signals designed at various spike timings (Δ
The memristive mechanism of asymmetric TiN/HfOx/AlOx/Pt memristor has been deeply investigated with the aid of x-ray photoelectron spectroscopy (XPS) depth analyses, which will be discussed in the following Section 2.3.
2.2.3 TiN/HfOx/HfOx/Pt inorganic memristor
In the previous work on Pt/HfOx/ZnOx/TiN and TiN/HfOx/AlOx/Pt memristors, the asymmetric memristive functional layers of A and B are different materials. Next, we will focus on Pt/HfO2/HfOx/TiN bilayer-structured memristor, as illustrated in Figure 6a. 4 nm-thick non-stoichiometric HfOx films were prepared by PEALD using the H2 plasma and 2 nm-thick stoichiometric HfO2 films by TALD using the H2O precursor, in basically consistent with the measured result by the cross-sectional high angle annular dark field (HAADF)-scanning transmission microscopy (STEM) in Figure 6b. The energy dispersive x-ray spectroscopy (EDS) elemental mapping images of Pt/HfO2/HfOx/TiN are shown in Figure 6c, revealing the stacking structure. In addition, XPS composition analyses show that the atomic ratio of Hf:O in the HfO2 and HfOx layers is 1:2.04 and 1:1.84, respectively, indicating that stoichiometric HfO2 and nonstoichiometric HfOx bilayer-structured memristors have been obtained [20]. Hence A and B herein represent HfO2 and HfOx with various oxygen contents, respectively. This device unit based on TiN/HfOx/HfO2/Pt memristor can also simulate the biological synapse learning rule of STDP, as indicated in Figure 6d. When the shortest spike timing of 10 ms is applied to the memristor device, the pulse train responses give rise to the largest Δ
The paired-pulse facilitation (PPF) is a phenomenon wherein the post-synaptic response induced by the spike increases when the time interval of the two spikes is very close [20]. PPF index can be defined as follows:
Evidently Pt/HfO2/HfOx/TiN memristor displays the marked dependence of synaptic weight on pulse interval Δ
Pt/HfO2/HfOx/TiN also mimics a classical conditioning under different pulse stimuli, as illustrated in Figure 8. In the famous experiment [21], a dog salivates (unconditioned response, UR) when watching the food (unconditioned stimulus, US) (Figure 8a), when it does not salivate (conditioned response, CR) on hearing the ring (conditioned stimulus, CS) alone (Figure 8b). Nonetheless, after some rehearsals,
Before rehearsing, the memristor has a low resistance state of 5 kΩ. The +4 V stimulus (US) causes a high resistance state of 3 MΩ (UR) (Figure 8f), when the −1.3 V stimulus (CS) only results in a low resistance state of 5 kΩ before training (Figure 8g). In Pavlov’s experiments, the food and the ring exist simultaneously to reinforce the correlation between US and CS. In our experiments, the +2.7 V stimulus was exerted to the memristor, the same as the simultaneous stimuli of −1.3 V and + 4 V pulse signals. When two rehearsing sequences with +2.7 V pulse, the device becomes the high resistance output of 2 MΩ (CR) (Figure 8h). When removing the +4 V signal, the memristor continues to keep in a high resistance state under a series of −1.3 V stimuli alone and then returns to a low resistance state (Figure 8i), implying the setup and vanish of the classical conditional reflex.
The energy consumption is one important indicator for a practical electronic synaptic device in neuromorphic network. Pt/HfO2/HfOx/TiN memristor can be set in less than 100 ns and reset in less than 10 ns, indicating the rapid switching speed, as recorded in Figure 9a.
The current response curves versus the time after the applied programming signal during the set or reset operation are plotted in Figure 9b and c, respectively. The current rises after a waiting time of about 260 ns when a − 2 V/1 ms stimulus is applied, indicating the beginning of the set process (Figure 9b). The memristor resistance decreases from the initial high resistance state (∼1 MΩ) to low resistance state (∼800 Ω). Similarly, the current reduces after a waiting time of about 70 ns when a +3 V/1 ms signal is exerted, showing the occurrence of the reset process. The energy consumption per operation can be calculated to be 520 pJ for the set process and 1.05 nJ for the reset process by considering the pulse waveforms (time, response current, and pulse voltage), corresponding to the maximum energy consumption in one set or reset operation, as the memristor has been set in the lowest resistance state with the highest response current. Nevertheless, the actual operation of the electronic synapse is generally in the mediate resistance states (∼80 kΩ). The response current of the memristor is inversely proportional to the resistance value of the synaptic device with a first-order approximation. So, the evaluated actual energy consumption per operation will decline in the range of around ten picojoules.
Finally, the impact of oxygen vacancy concentration in non-stoichiometric HfOx layers on resistive switching properties of Pt/HfO2/HfOx/TiN bilayer ultrathin memristor has been investigated. The memristor with 12.1% oxygen vacancy concentration in the HfOx layer exhibits comprehensively better performances such as the optimal pulse energy consumption, reset switching speed, and DC endurance and retention characteristics [20].
2.2.4 TaN/Ti-MA/TiO2/Pt organic: inorganic hybrid memristor
As mentioned above, we mainly elucidated the bio-synaptic functions of three asymmetric inorganic bilayer-structured ultrathin memristors. In this part, organic–inorganic hybrid bilayer memristors of TaN/Ti-MA/TiO2/Pt were prepared by low temperature MLD/ALD at 160°C. The synaptic plasticity has been explored deeply. Some superb synaptic functions, such as nonlinear transmission characteristics, STP/LTP, PPF, and STDP have been achieved in the hybrid memristors [22].
First the narrow-scan XPS and Fourier transform infrared (FTIR) spectroscopy were used to detect the chemical composition and organic group of Ti-based maleic acid (Ti-MA) hybrid film, as shown in Figure 10a–d. The C 1 s XPS peaks at 284.6 eV and 288.4 eV (Figure 10a) result from the C-C (backbone chain carbon) bond and the O-C=O bond from carboxyl, respectively, suggesting the occurrence of organic component in Ti-MA films. The doublet at 458.7 eV and 464.5 eV with the spin orbit splitting energy of 5.8 eV can be assigned to the Ti 2p1
The resistive switching characteristics of the hybrid bilayer memristor of TaN/Ti-MA/TiO2/Pt have been examined for 100 times, as seen in in Figure 11a. The typical bipolar resistive switching behavior has been confirmed with narrow distribution of set voltage of −1.6 ± 0.2 V (red line) or reset voltage of 1 ± 0.1 V (black line). The double-logarithmic
The PPF and STDP functions have also been characterized in hybrid memristor, as shown in Figures 12 and 13, respectively. A pair of pulses (−1 V, 400 ns) with different Δ
The STDP rule was emulated in hybrid memristor by using a pair of 0.8 V and −0.8 V pulses with 120
The measured data can be fitted well.
In addition, the conditioned reflex has been mimicked in hybrid film memristor, similar to the results of Pt/HfO2/HfOx/TiN memristor in Figure 8.
By comparison with inorganic bilayer memristors, it can be found that the organic–inorganic hybrid bilayer memristor has similar bio-synaptic functions with comparable switching speed and energy consumption. Moreover organic–inorganic hybrid materials may possess both the advantages of organic and inorganic components with excellent flexibility and tunability. Inorganic compounds have better electrical characteristics and thermal stability. Organic compounds own various functional groups, larger stretchability and low processing temperature. By means of the synergetic and complementary effects between organic and inorganic components, the comprehensive properties of hybrid memristive materials could be expected for significant improvement. The hybrid bilayer ultrathin memristor derived by low temperature MLD/ALD is one competitive candidate for flexible neuroscience applications.
2.3 Memristive mechanism
In Section 2.2, we focused on the electrical Performance and synaptic functions of several bilayer ultrathin memristors. In this section, the asymmetric memristive mechanism of the bilayer-structured memristors on TiN or TaN will be studied carefully. Taking Pt/AlO
Figure 14a–d records the high-resolution Al 2p, Hf 4f and O 1 s peaks in AlO
The XPS depth data of Pt/AlO
Oxygen vacancy concentration | A | B | C | D | E |
---|---|---|---|---|---|
Position | Pt/AlOx | AlOx | AlOx/HfOx | HfOx | HfOx/TiN |
Initial | 9.5% | 0.7% | 17.7% | 8.7% | 21.1% |
LRS | 8.8% | 4.1% | 10.4% | 7.1% | 17.3% |
HRS | 7.2% | 3.0% | 11.2% | 8.0% | 16.7% |
Medium | 6.6% | 4.0% | 11.3% | 7.5% | 17.0% |
In general, the resistive switching mechanism of metal oxide memristors is related to the connection and rupture of conductive filaments of oxygen vacancies. But the simple increase of oxygen vacancy concentration is not always effective. The non-uniform distribution of oxygen vacancies in memristors is the critical factor affecting the resistive switching behavior of memristive devices [30].
Based on the oxygen vacancy concentration and distribution in the Pt/AlO
There are much more random oxygen vacancies in the HfO
After applying the +2.5 V reset voltage to the LRS device, the memristor transfers from LRS (600 Ω) to HRS (1 MΩ) (Figure 15c). During the reset process, the oxygen vacancies migrate from the AlO
Because the synapse device usually operates under pulse mode to get more intermediate resistance states, a continuous pulse experiment was exploited to alter the device from a LRS (600 Ω) to a medium resistance state (50 kΩ) by imposing 40 pulses (+1.5 V, 10 ms) (Figure 15d). The XPS result (Figure 14f) indicates that the oxygen vacancy concentration curve of medium resistance state devices lies approximately between HRS and LRS in the AlO
The oxygen vacancy migration/diffusion model can be used to explain the transition from STP to LTP in bilayer memristive device (Figure 4a). When imposing the +1.6 V pulse, the oxygen vacancies move from the AlOx layer to the HfOx layer with the reduced response current. When the voltage is removed, some oxygen vacancies may stay in a new steady position, however some oxygen vacancies may diffuse back to the old position owing to the gradient of oxygen concentration. This leads to the device conductance change with a reduced synaptic weight during the relaxation time. After applying repetitive pulse stimuli, the subsequent voltage forces the reversely diffused oxygen vacancies to move forward again so as to improve the migration efficiency until most oxygen vacancies attain new equilibrium positions. The remaining synaptic weight gradually increases with the increasing pulse number. This process is called repeated training and learning, corresponding to the transformation from STP to LTP [17].
The memristive mechanism from Pt/AlO
3. Conclusion
Our asymmetric bilayer-structured memristors fabricated by ALD/MLD and their main memristive features are summarized in Table 4, including set/reset voltage, ON/OFF ratio, and some important synaptic functions. Some similar work with asymmetric bilayer structure has also been listed in Table 4 for comparison. It can be seen that all memristors with asymmetric bilayer structure exhibit better resistive switching performance. Our memristors have relatively thinner functional layers, relatively smaller ON/OFF ratio and emulate more artificial synaptic functions such as LTPo, LTD, the transition from STP to LTP, PPF, STDP, and conditional reflex (CR). The memristive mechanism of our bilayer-structured ultrathin device has been proposed to explain the synaptic plasticity based on oxygen vacancies migration/diffusion model. The non-uniform distribution of oxygen vacancies in asymmetric bilayer memristors plays the crucial role in affecting the linkage/rupture of conductive filaments.
Device structure | Thickness (nm) | Set/reset voltage (V) | ON/OFF ratio | Synaptic functions | References |
---|---|---|---|---|---|
Pt/HfO2/ZnO/TiN | ∼10 | −1.7/+1.4 | ∼30 | LTPo, LTD, STP/LTP, STDP | Our works |
Pt/Al2O3/HfO2/TiN | ∼10 | −1.4/ +1.3 | ∼610 | LTPo, LTD, STP/LTP, PPF, STDP | |
Pt/HfO2/HfOx/TiN | ∼6 | −1.6/+1.1 | ∼954 | LTPo, LTD, STP/LTP, PPF, STDP, CR | |
Pt/TiO2/Ti-MA/TaN | ∼8 | −1.5/+1 | ∼230 | LTPo, LTD, STP/LTP, PPF, STDP, CR | |
Ni/SiNx/AlOy/TiN | ∼11.5 | +4/−3.5 | ∼500 | LTPo, LTD, STDP | [35] |
TiN/HfO2/Al2O3/Pt | ∼10 | +1.4/−1.3 | ∼105 | STDP | [36] |
W/AlOx/Al2O3/TiN | ∼10 | +1.05/−1.25 | ∼103 | — | [37] |
Ag/ZrO2/WS2/Pt | ∼100 | +0.16/−0.06 | >105 | PPF, STDP | [38] |
In light of these promising results and the fabrication compatibility with semiconductor industry, the ALD/MLD-derived bi-layer ultrathin memristor devices have tremendous potential as billions of electronic synapses in next-generation artificial neural network and flexible electronics.
Acknowledgments
This work is supported by the Natural Science Foundation of China (52073142, 51721001) and Jiangsu Province (BK20201252). Mr. Chang Liu also acknowledges the support from the Nanjing University Innovation and Creative Program for PhD candidate (CXCY18-27).
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