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The resistive switching characteristics of Ni-doped HfOx film and its application as a synapse
Journal of Alloys and Compounds 766 (2018) 918e924
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The resistive switching characteristics of Ni-doped HfOx film and its application as a synapse Tingting Tan a, *, Yihang Du a, Ai Cao a, Yaling Sun a, Gangqiang Zha a, **, Hao Lei a, Xusheng Zheng b a
State Key Laboratory of Solidification Processing, MIIT Key Laboratory of Radiation Detection Materials and Devices, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, 710072, China National Synchrotron Radiation Laboratory, University of Science and Technology of China, 230023, Hefei, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 May 2018 Received in revised form 2 July 2018 Accepted 4 July 2018 Available online 5 July 2018
The effect of Ni doping concentration on resistive switching characteristics of Au/HfOx:Ni/Pt structure was investigated, and the 3.1% Ni-doped HfOx film was used to imitate the function of biological synapse. From the X-ray absorption near edge structure analysis, the forms and the local electronic structure of Ni atoms in the HfOx films with different Ni doping concentrations were investigated. According to the Xray photoelectron spectroscopy analysis, the Ni doping introduced more oxygen vacancies in HfOx films. Gradual Reset process and concentrated distribution of resistive switching parameters were achieved for 3.1% Ni-doped HfOx films, which could be used to emulate the “learning” and “forgetting” function of the biological synapse. Multiple resistance levels can be observed in the continuously pulse number under identical pulse width of positive or negative voltage pulses. Moreover, the application of pulse-train operation scheme is an effective method to control analog synaptic devices during the Reset process, which can contribute to understand the nature of the conductive nano-filament evolution. © 2018 Elsevier B.V. All rights reserved.
Keywords: Resistive switching Ni dopants XANES Gradual reset process Pulse-train Biological synapse
1. Introduction Transition-metal-oxides (TMOs) exhibit a lot of intrinsic functionalities like high catalytic activity, magnetism, and superconductivity. In the light of its marvelous functions, TMOs have been extensively investigated in the field of energy, environment, and electronics, such as sensors [1], energy storage materials [2,3], photovoltaics [4e6], field-effect transistors (FET) and non-volatile memories (NVM) [7,8]. Transition-metal-oxide-based resistive random access memory (RRAM) devices have attracted significant attention as promising candidates for next-generation nonvolatile memory devices because of their simple structure, lower power consumption, high device density, multilevel storage capability, viability for 3D memory stacks, and good compatibility with complementary metal-oxide semiconductor (CMOS) processes [9]. Hafnium oxide (HfO2) is one of the most intensively investigated materials for the RRAM due to its large dielectric constant (~25) and
* Corresponding author. ** Corrresponding author. E-mail addresses: [email protected] (T. Tan), [email protected] (G. Zha). https://doi.org/10.1016/j.jallcom.2018.07.044 0925-8388/© 2018 Elsevier B.V. All rights reserved.
wide bandgap (~6 eV) [10]. However, further investigations are required for the commercial application of HfO2-based RRAM, such as how to improve the uniformity of the resistive switching (RS) parameters, and enhance the reliability of the device. According to previous studies, ionic doping is an effective method to improve the uniformity and reliability of RS performance, which could suppress the random distribution of oxygen filaments [11]. The doping of metal impurities into HfO2 films has been shown to improve the RS properties [12]. In particular, the Ni dopant has a significant effect on forming process in HfOx-based devices and can reduce the oxygen vacancy formation energy [13]. Although the RS properties in metal Ni doped HfO2 films have been reported previously, the forms of the impurities (Ni) and the details pertaining to the conduction band are rarely investigated. The X-ray absorption near edge structure (XANES) not only reflects the geometrical configuration of the atoms surrounding the absorbed atoms, but also reflects the structure of the electronic states near the Fermi level of condensed matter. Therefore, XANES with synchrotron radiation is a powerful and suitable tool to investigate the local arrangement of atoms in materials, providing the information of the chemical states and local electronic structure of the incorporated atoms in the host compounds even with a dilute
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concentration [14]. With regard to the application of RRAM, the biological synapse simulation is a hot spot of the artificial intelligence (AI) [15]. The HfO2-based RRAM has been proposed to emulate the synapse due to its nonlinear transmission characteristics [16]. It is well known that the synapse is essentially a two-terminal device, including a presynaptic neuron (PRE) and a postsynaptic neuron (POST), which is remarkably similar to the metal-insulator-metal (MIM) structure of the resistive switching devices. The conductance of RRAM can be modified by the precisely regulated input pulses and by regulating charge or flux through the device, thus the conductance of the RS devices can be viewed as the biological synaptic weight [17]. According to the previous study, the application of the Ni-doped HfOx film in the synapse emulation is seldom reported. In this paper, the forms of the impurities (Ni) and the detail pertaining to the conduction band in HfOx films with different concentrations of Ni atoms were confirmed by the XANES. The Ni doping introduced more oxygen vacancies in HfOx films according to the XPS analysis. Subsequently, the RS characteristics of HfOx films doped with different concentrations of Ni atoms were investigated. Finally, the learning/forgetting application of the 3.1% Ni-doping HfOx samples in the biological synapses was implemented, and the effects of the pulse parameters on the conductance of Ni-doped HfOx samples were explored. 2. Experiments The schematics structure and the detailed deposition process are shown in Fig. 1(a) and (b). The 10 nm-thick HfOx films doped with different Ni doping concentrations were deposited on Pt substrate. The introduction of Ni was realized by loading metal Ni pieces on the Hf target, and the Ni-doping concentration was controlled by regulating the area ratio of the Ni/Hf target. During the deposition process, the flow rate of Ar and O2 was 12 and 4 sccm respectively. The working pressure was fixed at 0.3 Pa and the RF power was 70 W. The thickness of the film was measured by ellipsometer. The Ni doping concentrations in the HfOx films were analysed through X-ray photoelectron spectroscopy (XPS), which were estimated to be 1.2%, 3.1% and 6.3%. The electrical tests were performed using an Agilent B1500 semiconductor parameter analyzer in the voltage sweep mode. The bias voltage was applied to the Au top electrode (TE) while the Pt bottom electrode (BE) was always grounded during the measurement under an air atmosphere at room temperature. The XANES measurements with synchrotron radiation were performed at X-ray magnetic circular dichroism (XMCD) beamline
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at National Synchrotron Radiation Laboratory (NSRL) of China. The range of energy was 100e1000 eV, the energy resolution (E/DE) was 1000, the luminous flux was 1010 phs/s @ 244 eV, and the resolution of the spectrometer was 50 meV @ 100 eV. Finally, Keithley 4200-SCS with a pulse emitter modulator was used to adjust the resistance of the Au/HfOx: Ni/Pt device. The peak amplitude and width were 100 mVe10 V and 10 nse1 s, respectively, and the resolution was up to 1 mV. 3. Results and discussion It is well known that the XANES spectroscopy is more sensitive to the three-dimensional atomic arrangement and electronic structures around the absorbing atoms, which is able to provide more currently unavailable information on the defects. Herein the Ni L23-edge and O K-edge XANES was determined the electronic structure and the local structure of the Ni-doped HfOx thin film. The Ni L-edge corresponds to the dipole allowed transition of the 2p electron into unoccupied 3d and possibly 4s states (D[ ¼ ±1), with spin-orbit coupling causing a splitting into an L3-edge (2p3/2 / 3d) and L2-edge (2p1/2/3d) [18]. Fig. 2(a) shows the Ni L-edge XANES of the 1.2%, 3.1% and 6.3% Ni-doped HfOx films, all the Ni L-edge XANES present two remarkable characteristic peaks (A, B) owing to the spin-orbit splitting of the Ni 2p core hole [19]. The absorption peak A near 855 eV corresponds to the L3-edge, and the peak B near 872 eV corresponds to the L2-edge. The absence of the pre-edge peak in the Ni L-edge XANES spectra shows that the existential form of Ni atoms in the HfOx film is octahedral configuration. In addition, the absorption peak shifts towards higher binding energy with the Nidoping concentration increasing from 1.2% to 6.3% proving that the change of the spatial environment around the doping Ni atoms. Fig. 2(b) shows the O K-edge XANES for the Ni-doped HfOx samples with different Ni-doping concentrations. In light of the previous studies, the O K-edge XANES spectra reveal the unoccupied O 2p states, and the features are assigned to the following hybridized states [20]. The first two peaks C and D are due to the contributions from the tetrahedral environment of the oxygen atoms [21]. It is reported that the local crystal field splits the Ni d orbital into t2g and eg orbitals, which hybridized with O 2p orbital would give rise to the structure C and D in the O K-edge, respectively. The peak E at about 537 eV is assigned to a Ni band with 4sp character. Based on molecular-orbital and ligand field theory, the Ni 3d orbital mixes with the O 2p ligands form the unfilled anti-bonding 2t2g and 3eg orbitals. Therefore, the O K-edge is expected to reflect the unoccupied states that result from the hybridization between Ni 3d and
Fig. 1. (a) Schematic diagram of the Au/HfOx: Ni/Pt RRAM structure. (b) Detailed preparation process the Au/HfOx: Ni/Pt structure.
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Fig. 2. XANES spectra for Ni-doped HfOx films with doping concentrations of 1.2%, 3.1% and 6.3% respectively. (a) Ni L-edge and (b) O K-edge.
O 2p states. The intensity of the 529 eV (Peak C) is enhanced with the increasing Ni-doping concentration, which indicates that more oxygen ions ionically bond with Ni ions instead of covalently bonding with Hf ions. Simultaneously, the oxygen vacancies were introduced into the Ni-doped HfOx films for the different atomic radii and electronegativity between Ni and Hf atoms. When the oxygen vacancies were formed, the strongly distorted local structure around the oxygen vacancy probably leads to the emergence of a shallow energy level due to the energy level of the vacancy state was shallow [22]. Hence the existence of oxygen vacancies could greatly affect the physical properties originating from the change of the electron band structure. With regard to hafnium oxide, the energy position of this defect level has been estimated theoretically [23] and experimentally [24] to above the middle of the bandgap, so EF is shifted upward in the presence of oxygen vacancies, namely, the oxygen vacancy lowers the conduction band edge and increases the effective Fermi level. The results indicate that the activation energy of oxygen vacancy was reduced with the increasing of Ni doping concentrations. Furthermore, the oxygen vacancies will become one main source of intrinsic defects, which would degrade the carrier mobility. The above two factors work simultaneously on the electrical conductivity of the samples. With the increasing Nidoping concentration, on the one hand, Ni atoms capture more oxygen atoms from the surrounding HfOx film, leaving a large number of oxygen vacancies related defects, on the other hand, the Ni dopants also augment the probability of electron scattering, which decreases the carrier mobility. In order to clearly understand the variation of oxygen vacancies in Ni-doped HfOx films, the XPS analyses were performed, as shown in Fig. 3. The Au top electrodes are not deposited for this analysis. The binding energy is calibrated with the C 1s core level energy of 284.6 eV. In Fig. 3(a), the Hf 4f peaks correspond to the HfeO bonds [25]. Increasing the Ni doping concentration, the Hf 4f peaks shift toward higher binding energy, which was due to the increased number of oxygen vacancies induced by the Ni doping. It is reported that the binding energy of oxygen-deficient oxide is higher than that of the fully oxidized metal oxide [26]. In Fig. 3(b), the O 1s peak also shifts to a higher binding energy, which indicates that the number of oxygen vacancies increased with the increasing Ni doping concentration. Fig. 3(c) shows the curve of O1s spectrum of 1.2% Ni-doped HfOx film. The O1s peak can be fitted into two peaks which are attributed to the lattice oxygen at lower binding energy of 530.5 eV and non-lattice oxygen at higher binding energy of 532 eV. According to previous reports, non-lattice oxygen is related to the formation of oxygen vacancies and the proportion of nonlattice oxygen is proportional to the amount of the oxygen
vacancies [27]. Thus, the concentration of oxygen vacancies in the HfOx film can be estimated by analyzing the proportion of nonlattice oxygen in the film, as shown in the inset of (c). The proportion of non-lattice oxygen shows a positive correlation to the Ni doping concentration, indicating the increased content of oxygen vacancy by Ni doping. The electrical properties of Ni-doped HfOx samples with doping concentrations of 1.2%, 3.1% and 6.3% are shown in Fig. 4. The typical bipolar I-V curves of the Ni-doped HfOx samples are displayed in Fig. 4(a). For the RRAM devices, the large forming voltage (FV), as shown in the inset of (a), is needed to form the conductive filaments (CFs) before the electrical measurements. During the electroforming process, 1 mA compliance current was set to prevent the samples from complete breakdown and to improve the subsequent RS performance. The FV gradually decreased from 2.0 V to 1.5 V as the Ni-doping concentration increased from 1.2% to 6.3%, which is due to the increased oxygen vacancies concentration induced by the Ni doping [28]. The results are in conformity with the XANES analysis. After the electroforming process, the samples can reversibly switch between high resistance state (HRS) and low resistance state (LRS). In Fig. 4(a), the HfOx samples with different Ni doping concentrations exhibit the well-known counter-clockwise RS memory performance. It is noted that the Reset process transformed from a gradual change to an abrupt change, which may be due to the partial annihilation of the CFs with a fewer number of CFs when the Ni doping concentrations is low [29]. Conversely, when the Ni doping concentrations is high, the CFs are completely ruptured during the Reset process. Statistical distribution of the Set voltages (VSet) and the Reset voltages (VReset) of the samples are plotted in Fig. 4(b). Compared to the broad distribution of switching voltages for the 1.2% (VSet/0.14 Ve0.23 V, VReset/ 0.16 V ~ -0.24 V) and 6.3% (VSet/0.11 Ve0.17 V, VReset/-0.15 V ~ 0.23 V) Ni-doped HfOx samples, the 3.1% Ni-doped HfOx thin films show the relatively concentrated distribution of Set voltage from 0.16 V to 0.20 V and Reset voltage from 0.18V to 0.21 V. The cumulative distribution of both LRS and HRS is shown in Fig. 4(c), the 3.1% Ni-doped HfOx thin film exhibits relatively concentrated distribution of RHRS and RLRS. The retention characteristics of the HfOx samples with different Ni doping concentrations are also examined, as seen in Fig. 4(d). All samples show good retention characteristics up to 104 s without obvious degeneration in both HRS and LRS. The endurance characteristics of Ni-doped HfOx samples with different doping concentrations have been tested, and the results are shown in Fig. 4(e). After 100 program/erase cycles, the 3.1% Ni-doped HfOx samples show relatively stable RHRS and RLRS, better than that of the 1.2% and the 6.3% Ni-doped HfOx samples.
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Fig. 3. XPS spectra of HfOx: Ni films. (a) Hf 4f spectra, (b) O1s spectra and (c) The O1s core level in 1.2% Ni-doped HfOx sample. The inset in (c) shows the variations of non-lattice oxygen in the HfOx with the increasing Ni doping concentration.
The 3.1% Ni-doped HfOx thin film shows favorable RS behaviors and stable electrical properties. Moreover, the 3.1% Ni-doped sample also exhibits the gradual reset process, which is caused by the gradual rupture of the CFs. In the Reset process, the oxygen ions would gradually recombine with oxygen vacancies due to the concentration gradient dominated diffusion of oxygen ion, which leads to the gradual rupture of the filament [30]. According to previous researches, one prime requirement of RRAM to be applicable for the synaptic applications is the feasibility of gradual Reset characteristics [31]. Considering the above factors, the 3.1% Nidoped HfOx sample can be used to emulate the function of the biological synapse. The current-voltage characteristics of the sample under positive and negative bias voltages were shown in Fig. 5. The structure diagram of two-terminal Ni-doped HfOx-based RRAM and the schematic illustration of the concept of using the RRAM as synapses between neurons are shown in the inset of (a). The potentiation with four cycles is presented in Fig. 5(a), and the applied positive DC sweeping voltage is from 0 to 0.10 V, which is lower than the Set voltage to impede the abrupt Set process. For the synaptic response, the gradually enhancing of current level after each sweep suggests the gradual growth of the filament, which can imitate the “learning” process in biological synapse. Note that the current level can grow further after 4th cycle, which means that at least five states are available for the potentiation based on the DC sweep. The depression with four cycles is presented in Fig. 5(b), and the applied negative DC voltage sweep ranges from 0 to 0.15 V. The damping current level after each sweep indicates that the oxygen ions can gradually recombine with the vacancies during the Reset process, which can imitate the “forgetting” process. Compared with the DC sweep, pulse programming is a more effective method for practical applications owing to the high operational speed and energy efficiency. Fig. 6 presents the multiple resistance levels of the device during the sequential application
of identical pulses. In Fig. 6(a), the resistance of the device continuously decrease with the increased number of positive voltage pulse, which corresponds to the potentiation process. Once the negative voltages pulses were applied on the device, the resistance increases with the increased pulse number, which corresponds to the depression process, as shown in Fig. 6(b). The phenomenon is analogous to the biological synapse, and the conductance of the memristor can be incrementally modified by modulating the charge flow through the device [32]. If the electric conductance of the device is considered as the synaptic weight, the phenomena show a close similarity to the transmission characteristics of biological. The positive and negative pulses are used to excite and inhibit the synapse, respectively. The regulation of the conductance can be viewed as the result of the motion of the electric field induced oxygen ions. Based on the aforementioned results, the growth or the annihilation of CFs can be regulated by the applied voltage pulses. More significantly, substantial amount of intermediate states between HRS and LRS can be obtained during the Reset process, which is related to the oxygen ions can gradually be pulled back to recombine with oxygen vacancies [31]. In the light of previous studies, pulse-train has been suggested to minimize the Reset failures of RRAM [33], hence it emerges as a nature method to control analog synaptic devices for neuromorphic application [34]. Multiple resistance levels can be achieved by tuning either the duration (widths) or amplitudes of the applied Reset pulses. The variation of the resistances during the sequential application of 50 identical pulses with different widths (from 100 ns to 2 ms) is presented in Fig. 7(a). Interestingly, three distinct regimes are observed in the three resistance curves under different pulse width. Initially, the resistance changes slowly, then begins to increase rapidly at a threshold pulse width, and finally becomes saturated around a certain value. Similar to the output characteristics of MOSFET, the
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Fig. 4. The electrical properties of Ni-doped HfOx samples with doping concentrations of 1.2%, 3.1% and 6.3%, respectively. (a) The typical bipolar resistive switching behaviors under DC voltage sweep mode. (b) The cumulative probabilities of VSet/VReset. (c) The cumulative probabilities of RHRS/RLRS. (d) The retention properties of HRS and LRS at room temperature under 0.1V. (e) The endurance characteristics of HRS and LRS at room temperature under 0.1 V.
phenomenon of resistance depending on pluses width in the switching process can be explained by the ionic drift/diffusion model in the oxygen vacancies CFs [33]. The change of the resistances during the sequential application of 50 identical pulses with various amplitudes (from 0.10 V to 0.14 V) is plotted in Fig. 7(b), suggesting that the characteristics of the conductivity has strong connection with the evolution of the CFs in the HfOx-based system during switching process [35]. The above-mentioned two modes adjusting the resistance suggest that the Reset states are not only determined by the value of applied voltage but also determined by the duration time of the Reset process. For the biological synapse, the result indicates that the nerve synapses depends more on the duration of input signals.
4. Conclusions XANES with synchrotron radiation was applied to investigate the local electronic structure of Ni atoms in the virgin state of HfOx samples with different Ni doping concentrations, which provides direct evidence that the activation energy of oxygen vacancy was reduced with the increasing Ni doping concentrations. The Ni dopants introduced more oxygen vacancies in HfOx films from the XPS analysis, and the decrease of FV and VSet were related to the reduction of the activation energy of oxygen vacancies induced by Ni doping. The RS characteristics of HfOx samples with different Ni doping concentrations were studied, and the 3.1% Ni-doped sample was selected to emulate “learning” and “forgetting” functions of the
T. Tan et al. / Journal of Alloys and Compounds 766 (2018) 918e924
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Fig. 5. The transmission characteristics of the Ni-doped HfOx RRAM. (a) Potentiation behaviors of the device under positive voltage. (b) Depression behaviors of the device under negative voltage.
Fig. 6. The resistance of Ni-doped HfOx device vs. biasing pulse number: (a) Set and (b) Reset process.
Fig. 7. The resistance changes upon the application of 50 identical pulses with different (a) pulse width and (b) pulse amplitude in the Reset process.
biological synapse. In the simulation process of biological synapse, multiple resistance levels of the device were achieved in the continuous pulse number. Intermediate states between HRS and LRS can be obtained by regulating the duration widths or amplitudes of the applied reset pulse-train during the Reset process. The results suggest that 3.1% Ni-doped HfOx sample was available to imitate the partial function of the biological synapse. Acknowledgements This work was financially supported by the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China
(Grant No. 155-QP-2016), the Fundamental Research Funds for the Central Universities (No. 3102018gxc017) and the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2018JM5059). References [1] B. Miccoli, V. Cauda, A. Bonanno, A. Sanginario, K. Bejtka, F. Bella, F. Marco, D. Demarchi, Sci. Rep. 6 (2016), 29763. [2] B.G.S. Raj, J.J. Wu, A.M. Asiri, S. Anandan, RSC Adv. 6 (2016) 33361e33368. ~ oz, M. Giuseppina, L. Andreaamberti, D. Matteo estro, [3] F. Bella, B. Ana, G. Mun P. Michele, G. Claudio, Nano Res. 10 (2017) 2891e2903. [4] F. Bella, J. Popovic, A. Lamberti, E. Tresso, C. Gerbaldi, J. Maier, ACS Appl. Mater. Interfaces 9 (2017) 37797e37803. [5] M. Gerosa, A. Sacco, A. Scalia, F. Bella, A. Chiodoni, M. Quaglio, E. Tresso,
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The resistive switching characteristics of Ni-doped HfOx film and its application as a synapse Tingting Tan a, *, Yihang Du a, Ai Cao a, Yaling Sun a, Gangqiang Zha a, **, Hao Lei a, Xusheng Zheng b a
State Key Laboratory of Solidification Processing, MIIT Key Laboratory of Radiation Detection Materials and Devices, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, 710072, China National Synchrotron Radiation Laboratory, University of Science and Technology of China, 230023, Hefei, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 May 2018 Received in revised form 2 July 2018 Accepted 4 July 2018 Available online 5 July 2018
The effect of Ni doping concentration on resistive switching characteristics of Au/HfOx:Ni/Pt structure was investigated, and the 3.1% Ni-doped HfOx film was used to imitate the function of biological synapse. From the X-ray absorption near edge structure analysis, the forms and the local electronic structure of Ni atoms in the HfOx films with different Ni doping concentrations were investigated. According to the Xray photoelectron spectroscopy analysis, the Ni doping introduced more oxygen vacancies in HfOx films. Gradual Reset process and concentrated distribution of resistive switching parameters were achieved for 3.1% Ni-doped HfOx films, which could be used to emulate the “learning” and “forgetting” function of the biological synapse. Multiple resistance levels can be observed in the continuously pulse number under identical pulse width of positive or negative voltage pulses. Moreover, the application of pulse-train operation scheme is an effective method to control analog synaptic devices during the Reset process, which can contribute to understand the nature of the conductive nano-filament evolution. © 2018 Elsevier B.V. All rights reserved.
Keywords: Resistive switching Ni dopants XANES Gradual reset process Pulse-train Biological synapse
1. Introduction Transition-metal-oxides (TMOs) exhibit a lot of intrinsic functionalities like high catalytic activity, magnetism, and superconductivity. In the light of its marvelous functions, TMOs have been extensively investigated in the field of energy, environment, and electronics, such as sensors [1], energy storage materials [2,3], photovoltaics [4e6], field-effect transistors (FET) and non-volatile memories (NVM) [7,8]. Transition-metal-oxide-based resistive random access memory (RRAM) devices have attracted significant attention as promising candidates for next-generation nonvolatile memory devices because of their simple structure, lower power consumption, high device density, multilevel storage capability, viability for 3D memory stacks, and good compatibility with complementary metal-oxide semiconductor (CMOS) processes [9]. Hafnium oxide (HfO2) is one of the most intensively investigated materials for the RRAM due to its large dielectric constant (~25) and
* Corresponding author. ** Corrresponding author. E-mail addresses: [email protected] (T. Tan), [email protected] (G. Zha). https://doi.org/10.1016/j.jallcom.2018.07.044 0925-8388/© 2018 Elsevier B.V. All rights reserved.
wide bandgap (~6 eV) [10]. However, further investigations are required for the commercial application of HfO2-based RRAM, such as how to improve the uniformity of the resistive switching (RS) parameters, and enhance the reliability of the device. According to previous studies, ionic doping is an effective method to improve the uniformity and reliability of RS performance, which could suppress the random distribution of oxygen filaments [11]. The doping of metal impurities into HfO2 films has been shown to improve the RS properties [12]. In particular, the Ni dopant has a significant effect on forming process in HfOx-based devices and can reduce the oxygen vacancy formation energy [13]. Although the RS properties in metal Ni doped HfO2 films have been reported previously, the forms of the impurities (Ni) and the details pertaining to the conduction band are rarely investigated. The X-ray absorption near edge structure (XANES) not only reflects the geometrical configuration of the atoms surrounding the absorbed atoms, but also reflects the structure of the electronic states near the Fermi level of condensed matter. Therefore, XANES with synchrotron radiation is a powerful and suitable tool to investigate the local arrangement of atoms in materials, providing the information of the chemical states and local electronic structure of the incorporated atoms in the host compounds even with a dilute
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concentration [14]. With regard to the application of RRAM, the biological synapse simulation is a hot spot of the artificial intelligence (AI) [15]. The HfO2-based RRAM has been proposed to emulate the synapse due to its nonlinear transmission characteristics [16]. It is well known that the synapse is essentially a two-terminal device, including a presynaptic neuron (PRE) and a postsynaptic neuron (POST), which is remarkably similar to the metal-insulator-metal (MIM) structure of the resistive switching devices. The conductance of RRAM can be modified by the precisely regulated input pulses and by regulating charge or flux through the device, thus the conductance of the RS devices can be viewed as the biological synaptic weight [17]. According to the previous study, the application of the Ni-doped HfOx film in the synapse emulation is seldom reported. In this paper, the forms of the impurities (Ni) and the detail pertaining to the conduction band in HfOx films with different concentrations of Ni atoms were confirmed by the XANES. The Ni doping introduced more oxygen vacancies in HfOx films according to the XPS analysis. Subsequently, the RS characteristics of HfOx films doped with different concentrations of Ni atoms were investigated. Finally, the learning/forgetting application of the 3.1% Ni-doping HfOx samples in the biological synapses was implemented, and the effects of the pulse parameters on the conductance of Ni-doped HfOx samples were explored. 2. Experiments The schematics structure and the detailed deposition process are shown in Fig. 1(a) and (b). The 10 nm-thick HfOx films doped with different Ni doping concentrations were deposited on Pt substrate. The introduction of Ni was realized by loading metal Ni pieces on the Hf target, and the Ni-doping concentration was controlled by regulating the area ratio of the Ni/Hf target. During the deposition process, the flow rate of Ar and O2 was 12 and 4 sccm respectively. The working pressure was fixed at 0.3 Pa and the RF power was 70 W. The thickness of the film was measured by ellipsometer. The Ni doping concentrations in the HfOx films were analysed through X-ray photoelectron spectroscopy (XPS), which were estimated to be 1.2%, 3.1% and 6.3%. The electrical tests were performed using an Agilent B1500 semiconductor parameter analyzer in the voltage sweep mode. The bias voltage was applied to the Au top electrode (TE) while the Pt bottom electrode (BE) was always grounded during the measurement under an air atmosphere at room temperature. The XANES measurements with synchrotron radiation were performed at X-ray magnetic circular dichroism (XMCD) beamline
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at National Synchrotron Radiation Laboratory (NSRL) of China. The range of energy was 100e1000 eV, the energy resolution (E/DE) was 1000, the luminous flux was 1010 phs/s @ 244 eV, and the resolution of the spectrometer was 50 meV @ 100 eV. Finally, Keithley 4200-SCS with a pulse emitter modulator was used to adjust the resistance of the Au/HfOx: Ni/Pt device. The peak amplitude and width were 100 mVe10 V and 10 nse1 s, respectively, and the resolution was up to 1 mV. 3. Results and discussion It is well known that the XANES spectroscopy is more sensitive to the three-dimensional atomic arrangement and electronic structures around the absorbing atoms, which is able to provide more currently unavailable information on the defects. Herein the Ni L23-edge and O K-edge XANES was determined the electronic structure and the local structure of the Ni-doped HfOx thin film. The Ni L-edge corresponds to the dipole allowed transition of the 2p electron into unoccupied 3d and possibly 4s states (D[ ¼ ±1), with spin-orbit coupling causing a splitting into an L3-edge (2p3/2 / 3d) and L2-edge (2p1/2/3d) [18]. Fig. 2(a) shows the Ni L-edge XANES of the 1.2%, 3.1% and 6.3% Ni-doped HfOx films, all the Ni L-edge XANES present two remarkable characteristic peaks (A, B) owing to the spin-orbit splitting of the Ni 2p core hole [19]. The absorption peak A near 855 eV corresponds to the L3-edge, and the peak B near 872 eV corresponds to the L2-edge. The absence of the pre-edge peak in the Ni L-edge XANES spectra shows that the existential form of Ni atoms in the HfOx film is octahedral configuration. In addition, the absorption peak shifts towards higher binding energy with the Nidoping concentration increasing from 1.2% to 6.3% proving that the change of the spatial environment around the doping Ni atoms. Fig. 2(b) shows the O K-edge XANES for the Ni-doped HfOx samples with different Ni-doping concentrations. In light of the previous studies, the O K-edge XANES spectra reveal the unoccupied O 2p states, and the features are assigned to the following hybridized states [20]. The first two peaks C and D are due to the contributions from the tetrahedral environment of the oxygen atoms [21]. It is reported that the local crystal field splits the Ni d orbital into t2g and eg orbitals, which hybridized with O 2p orbital would give rise to the structure C and D in the O K-edge, respectively. The peak E at about 537 eV is assigned to a Ni band with 4sp character. Based on molecular-orbital and ligand field theory, the Ni 3d orbital mixes with the O 2p ligands form the unfilled anti-bonding 2t2g and 3eg orbitals. Therefore, the O K-edge is expected to reflect the unoccupied states that result from the hybridization between Ni 3d and
Fig. 1. (a) Schematic diagram of the Au/HfOx: Ni/Pt RRAM structure. (b) Detailed preparation process the Au/HfOx: Ni/Pt structure.
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Fig. 2. XANES spectra for Ni-doped HfOx films with doping concentrations of 1.2%, 3.1% and 6.3% respectively. (a) Ni L-edge and (b) O K-edge.
O 2p states. The intensity of the 529 eV (Peak C) is enhanced with the increasing Ni-doping concentration, which indicates that more oxygen ions ionically bond with Ni ions instead of covalently bonding with Hf ions. Simultaneously, the oxygen vacancies were introduced into the Ni-doped HfOx films for the different atomic radii and electronegativity between Ni and Hf atoms. When the oxygen vacancies were formed, the strongly distorted local structure around the oxygen vacancy probably leads to the emergence of a shallow energy level due to the energy level of the vacancy state was shallow [22]. Hence the existence of oxygen vacancies could greatly affect the physical properties originating from the change of the electron band structure. With regard to hafnium oxide, the energy position of this defect level has been estimated theoretically [23] and experimentally [24] to above the middle of the bandgap, so EF is shifted upward in the presence of oxygen vacancies, namely, the oxygen vacancy lowers the conduction band edge and increases the effective Fermi level. The results indicate that the activation energy of oxygen vacancy was reduced with the increasing of Ni doping concentrations. Furthermore, the oxygen vacancies will become one main source of intrinsic defects, which would degrade the carrier mobility. The above two factors work simultaneously on the electrical conductivity of the samples. With the increasing Nidoping concentration, on the one hand, Ni atoms capture more oxygen atoms from the surrounding HfOx film, leaving a large number of oxygen vacancies related defects, on the other hand, the Ni dopants also augment the probability of electron scattering, which decreases the carrier mobility. In order to clearly understand the variation of oxygen vacancies in Ni-doped HfOx films, the XPS analyses were performed, as shown in Fig. 3. The Au top electrodes are not deposited for this analysis. The binding energy is calibrated with the C 1s core level energy of 284.6 eV. In Fig. 3(a), the Hf 4f peaks correspond to the HfeO bonds [25]. Increasing the Ni doping concentration, the Hf 4f peaks shift toward higher binding energy, which was due to the increased number of oxygen vacancies induced by the Ni doping. It is reported that the binding energy of oxygen-deficient oxide is higher than that of the fully oxidized metal oxide [26]. In Fig. 3(b), the O 1s peak also shifts to a higher binding energy, which indicates that the number of oxygen vacancies increased with the increasing Ni doping concentration. Fig. 3(c) shows the curve of O1s spectrum of 1.2% Ni-doped HfOx film. The O1s peak can be fitted into two peaks which are attributed to the lattice oxygen at lower binding energy of 530.5 eV and non-lattice oxygen at higher binding energy of 532 eV. According to previous reports, non-lattice oxygen is related to the formation of oxygen vacancies and the proportion of nonlattice oxygen is proportional to the amount of the oxygen
vacancies [27]. Thus, the concentration of oxygen vacancies in the HfOx film can be estimated by analyzing the proportion of nonlattice oxygen in the film, as shown in the inset of (c). The proportion of non-lattice oxygen shows a positive correlation to the Ni doping concentration, indicating the increased content of oxygen vacancy by Ni doping. The electrical properties of Ni-doped HfOx samples with doping concentrations of 1.2%, 3.1% and 6.3% are shown in Fig. 4. The typical bipolar I-V curves of the Ni-doped HfOx samples are displayed in Fig. 4(a). For the RRAM devices, the large forming voltage (FV), as shown in the inset of (a), is needed to form the conductive filaments (CFs) before the electrical measurements. During the electroforming process, 1 mA compliance current was set to prevent the samples from complete breakdown and to improve the subsequent RS performance. The FV gradually decreased from 2.0 V to 1.5 V as the Ni-doping concentration increased from 1.2% to 6.3%, which is due to the increased oxygen vacancies concentration induced by the Ni doping [28]. The results are in conformity with the XANES analysis. After the electroforming process, the samples can reversibly switch between high resistance state (HRS) and low resistance state (LRS). In Fig. 4(a), the HfOx samples with different Ni doping concentrations exhibit the well-known counter-clockwise RS memory performance. It is noted that the Reset process transformed from a gradual change to an abrupt change, which may be due to the partial annihilation of the CFs with a fewer number of CFs when the Ni doping concentrations is low [29]. Conversely, when the Ni doping concentrations is high, the CFs are completely ruptured during the Reset process. Statistical distribution of the Set voltages (VSet) and the Reset voltages (VReset) of the samples are plotted in Fig. 4(b). Compared to the broad distribution of switching voltages for the 1.2% (VSet/0.14 Ve0.23 V, VReset/ 0.16 V ~ -0.24 V) and 6.3% (VSet/0.11 Ve0.17 V, VReset/-0.15 V ~ 0.23 V) Ni-doped HfOx samples, the 3.1% Ni-doped HfOx thin films show the relatively concentrated distribution of Set voltage from 0.16 V to 0.20 V and Reset voltage from 0.18V to 0.21 V. The cumulative distribution of both LRS and HRS is shown in Fig. 4(c), the 3.1% Ni-doped HfOx thin film exhibits relatively concentrated distribution of RHRS and RLRS. The retention characteristics of the HfOx samples with different Ni doping concentrations are also examined, as seen in Fig. 4(d). All samples show good retention characteristics up to 104 s without obvious degeneration in both HRS and LRS. The endurance characteristics of Ni-doped HfOx samples with different doping concentrations have been tested, and the results are shown in Fig. 4(e). After 100 program/erase cycles, the 3.1% Ni-doped HfOx samples show relatively stable RHRS and RLRS, better than that of the 1.2% and the 6.3% Ni-doped HfOx samples.
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Fig. 3. XPS spectra of HfOx: Ni films. (a) Hf 4f spectra, (b) O1s spectra and (c) The O1s core level in 1.2% Ni-doped HfOx sample. The inset in (c) shows the variations of non-lattice oxygen in the HfOx with the increasing Ni doping concentration.
The 3.1% Ni-doped HfOx thin film shows favorable RS behaviors and stable electrical properties. Moreover, the 3.1% Ni-doped sample also exhibits the gradual reset process, which is caused by the gradual rupture of the CFs. In the Reset process, the oxygen ions would gradually recombine with oxygen vacancies due to the concentration gradient dominated diffusion of oxygen ion, which leads to the gradual rupture of the filament [30]. According to previous researches, one prime requirement of RRAM to be applicable for the synaptic applications is the feasibility of gradual Reset characteristics [31]. Considering the above factors, the 3.1% Nidoped HfOx sample can be used to emulate the function of the biological synapse. The current-voltage characteristics of the sample under positive and negative bias voltages were shown in Fig. 5. The structure diagram of two-terminal Ni-doped HfOx-based RRAM and the schematic illustration of the concept of using the RRAM as synapses between neurons are shown in the inset of (a). The potentiation with four cycles is presented in Fig. 5(a), and the applied positive DC sweeping voltage is from 0 to 0.10 V, which is lower than the Set voltage to impede the abrupt Set process. For the synaptic response, the gradually enhancing of current level after each sweep suggests the gradual growth of the filament, which can imitate the “learning” process in biological synapse. Note that the current level can grow further after 4th cycle, which means that at least five states are available for the potentiation based on the DC sweep. The depression with four cycles is presented in Fig. 5(b), and the applied negative DC voltage sweep ranges from 0 to 0.15 V. The damping current level after each sweep indicates that the oxygen ions can gradually recombine with the vacancies during the Reset process, which can imitate the “forgetting” process. Compared with the DC sweep, pulse programming is a more effective method for practical applications owing to the high operational speed and energy efficiency. Fig. 6 presents the multiple resistance levels of the device during the sequential application
of identical pulses. In Fig. 6(a), the resistance of the device continuously decrease with the increased number of positive voltage pulse, which corresponds to the potentiation process. Once the negative voltages pulses were applied on the device, the resistance increases with the increased pulse number, which corresponds to the depression process, as shown in Fig. 6(b). The phenomenon is analogous to the biological synapse, and the conductance of the memristor can be incrementally modified by modulating the charge flow through the device [32]. If the electric conductance of the device is considered as the synaptic weight, the phenomena show a close similarity to the transmission characteristics of biological. The positive and negative pulses are used to excite and inhibit the synapse, respectively. The regulation of the conductance can be viewed as the result of the motion of the electric field induced oxygen ions. Based on the aforementioned results, the growth or the annihilation of CFs can be regulated by the applied voltage pulses. More significantly, substantial amount of intermediate states between HRS and LRS can be obtained during the Reset process, which is related to the oxygen ions can gradually be pulled back to recombine with oxygen vacancies [31]. In the light of previous studies, pulse-train has been suggested to minimize the Reset failures of RRAM [33], hence it emerges as a nature method to control analog synaptic devices for neuromorphic application [34]. Multiple resistance levels can be achieved by tuning either the duration (widths) or amplitudes of the applied Reset pulses. The variation of the resistances during the sequential application of 50 identical pulses with different widths (from 100 ns to 2 ms) is presented in Fig. 7(a). Interestingly, three distinct regimes are observed in the three resistance curves under different pulse width. Initially, the resistance changes slowly, then begins to increase rapidly at a threshold pulse width, and finally becomes saturated around a certain value. Similar to the output characteristics of MOSFET, the
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Fig. 4. The electrical properties of Ni-doped HfOx samples with doping concentrations of 1.2%, 3.1% and 6.3%, respectively. (a) The typical bipolar resistive switching behaviors under DC voltage sweep mode. (b) The cumulative probabilities of VSet/VReset. (c) The cumulative probabilities of RHRS/RLRS. (d) The retention properties of HRS and LRS at room temperature under 0.1V. (e) The endurance characteristics of HRS and LRS at room temperature under 0.1 V.
phenomenon of resistance depending on pluses width in the switching process can be explained by the ionic drift/diffusion model in the oxygen vacancies CFs [33]. The change of the resistances during the sequential application of 50 identical pulses with various amplitudes (from 0.10 V to 0.14 V) is plotted in Fig. 7(b), suggesting that the characteristics of the conductivity has strong connection with the evolution of the CFs in the HfOx-based system during switching process [35]. The above-mentioned two modes adjusting the resistance suggest that the Reset states are not only determined by the value of applied voltage but also determined by the duration time of the Reset process. For the biological synapse, the result indicates that the nerve synapses depends more on the duration of input signals.
4. Conclusions XANES with synchrotron radiation was applied to investigate the local electronic structure of Ni atoms in the virgin state of HfOx samples with different Ni doping concentrations, which provides direct evidence that the activation energy of oxygen vacancy was reduced with the increasing Ni doping concentrations. The Ni dopants introduced more oxygen vacancies in HfOx films from the XPS analysis, and the decrease of FV and VSet were related to the reduction of the activation energy of oxygen vacancies induced by Ni doping. The RS characteristics of HfOx samples with different Ni doping concentrations were studied, and the 3.1% Ni-doped sample was selected to emulate “learning” and “forgetting” functions of the
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Fig. 5. The transmission characteristics of the Ni-doped HfOx RRAM. (a) Potentiation behaviors of the device under positive voltage. (b) Depression behaviors of the device under negative voltage.
Fig. 6. The resistance of Ni-doped HfOx device vs. biasing pulse number: (a) Set and (b) Reset process.
Fig. 7. The resistance changes upon the application of 50 identical pulses with different (a) pulse width and (b) pulse amplitude in the Reset process.
biological synapse. In the simulation process of biological synapse, multiple resistance levels of the device were achieved in the continuous pulse number. Intermediate states between HRS and LRS can be obtained by regulating the duration widths or amplitudes of the applied reset pulse-train during the Reset process. The results suggest that 3.1% Ni-doped HfOx sample was available to imitate the partial function of the biological synapse. Acknowledgements This work was financially supported by the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China
(Grant No. 155-QP-2016), the Fundamental Research Funds for the Central Universities (No. 3102018gxc017) and the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2018JM5059). References [1] B. Miccoli, V. Cauda, A. Bonanno, A. Sanginario, K. Bejtka, F. Bella, F. Marco, D. Demarchi, Sci. Rep. 6 (2016), 29763. [2] B.G.S. Raj, J.J. Wu, A.M. Asiri, S. Anandan, RSC Adv. 6 (2016) 33361e33368. ~ oz, M. Giuseppina, L. Andreaamberti, D. Matteo estro, [3] F. Bella, B. Ana, G. Mun P. Michele, G. Claudio, Nano Res. 10 (2017) 2891e2903. [4] F. Bella, J. Popovic, A. Lamberti, E. Tresso, C. Gerbaldi, J. Maier, ACS Appl. Mater. Interfaces 9 (2017) 37797e37803. [5] M. Gerosa, A. Sacco, A. Scalia, F. Bella, A. Chiodoni, M. Quaglio, E. Tresso,
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