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Ni2O3 nanoparticles in HfSiO matrix
Accepted Manuscript Resistance switching characteristics of core-shell γ-Fe2O3/Ni2O3 nanoparticles in HfSiO matrix Guangdong Zhou, Bo Wu, Xiaoqin Liu, Zhiling Li, Shuangju Zhang, Ankun Zhou, Xiude Yang PII:
S0925-8388(16)30747-2
DOI:
10.1016/j.jallcom.2016.03.163
Reference:
JALCOM 37051
To appear in:
Journal of Alloys and Compounds
Received Date: 15 December 2015 Revised Date:
5 March 2016
Accepted Date: 22 March 2016
Please cite this article as: G. Zhou, B. Wu, X. Liu, Z. Li, S. Zhang, A. Zhou, X. Yang, Resistance switching characteristics of core-shell γ-Fe2O3/Ni2O3 nanoparticles in HfSiO matrix, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.03.163. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Resistance switching characteristics of core-shell γ-Fe2 O3/Ni2O3 nanoparticles in HfSiO matrix
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Guangdong Zhoua , Bo Wub,c,∗, Xiaoqin Liua , Zhiling Lia , Shuangju Zhanga , Ankun Zhoud , Xiude Yangb a
Guizhou Institute of Technology, Guiyang, 550003, People’s Republic of China Institute of Theoretical Physics, Zunyi Normal College, Zunyi, 563002, People’s Republic of China c School of Marine Science and Technology, Northwestern Polytechnical University, Xian, 710072, People’s Republic of China d Kunming Institute of Botany, Chineses Academy Sciences, Kunming, 650201, People’s Republic of China
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b
Abstract
Core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles are synthesized by chemical co-precipitation method. Resistive switching memory behaviors, which have resistance ON/OFF ratio of ∼102 and excellent retention property, are observed in the Au/HfSiO/γ-
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Fe2 O3 /Ni2 O3 /HfSiO/Pt structure.Space charge limited current (SCLC) mech-
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anism, which is supported by the fitting current-voltage results, is employed to know the resistive switching memory effects. The transportation of Oxy-
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gen vacancy Vo2+ , oxygen ion O2− , recombination of oxygen atom and drive of external electric field are responsible for the ON or OFF states observed in device.
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Keywords: High-k HfSiO films; resistance switching characteristics; resistance random access memory(RRAM); core-shell materials.
∗
Corresponding author.Tel:+86-0851-28927153. Email address: [email protected] (Bo Wu)
Preprint submitted to Journal of Alloys and Compounds
March 22, 2016
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1. Introduction Conventional Si-based technologies applied in nonvolatile flash memory
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had made great progress and success for decades. However, further downscaling size of integrated circuit results in decreasing of charge storage den-
sity, increasing of leakage current and power consumption. According to the prediction from the international technology roadmap for semiconduc-
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tors (ITRS,2013), semiconductor manufacturing will enter an age of 5nm technology in the 2020 year, the Si-based technologies will reach its limita-
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tion. Therefore, devices with higher storage density, superior endurance, and lower power consumption are urgently developed to meet the demands from the fast development of electronic industry. Resistive random access memory (RRAM) is predicted to be the promising next generation nonvolatile flash memory due to its sample sandwich structure, fast switching speed, low power consumption and high charge storage density.[1, 2, 3, 4, 5, 6, 7, 8, 9]
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To date, resistance switching memory behaviors have been observed in
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binary metal oxides such as TiO2 , CuO, NiO, HfO2 [10, 11, 12, 13], ferromagnetic γ-Fe2O3 , Pr1−x Cax MnO3 [14, 15], ferrimagnetic Pb(Zr0.2 Ti0.8 )O3 [16],
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chalcogenide material MoS2 [17], and even organic materials[18, 19]. Recently, the resistance switching behaviors have been reported in core-shell Ni/NiO, Pt/Fe2 O3 , CeO2 , WO3 /CoWO4 , Au/CuZnSnS[20, 21, 22, 23, 24] at room
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temperature.
Several switching resistance mechanisms, including trap-controlled space
charge limited current,[25, 26] ionic conduction,[27] filament formation mechanism [28] and redox-based in multivalence [29] have been proposed to explain the RRAM behavior.What is more suitable and reasonable among those 2
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mechanisms,that is still in controversy today. In this work, the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles are embedded in
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the high-k HfSiO matrix to fabricate device with the Au/HfSiO/γ-Fe2 O3 @Ni2 O3 /HfSiO/Pt structure. The HfSiO matrix is selected to be the tunneling and control lay-
ers due to its suitable permittivity, stability against crystallization and high
energy band width, which helps to improve the dielectric property.[30] More-
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over,the resistance switching effects and mechanisms are investigated in the
2. Experimental Details
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device.
The core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles are prepared by chemical coprecipitation process. The FeCl3 ·6H2 O,NaOH,HNO3 (0.01mol/L),NiNO3 ·6H2 O, FeCl2 ·4H2 O, HCl(12mol/L) are employed to fabricate the FeOOH/Ni(OH)2 composite precursor. The FeCl3 (1mol/L), Ni(NO3 )2 (0.1mol/L), HCl(0.7mol/L),
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HNO3 (0.01mol/L), FeCl2 (0.1mol/L), NaOH(0.7mol/L) are prepared using
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above raw materials. First step, HCl (12mol/L 4L) solution is poured into Ni(NO3 )2 (0.1mol/L 10mL), which labeled it as solution I; the 30mL 1mol/L FeCl3 labeled as solution II; 400mL 0.7mol/L NaOH signed as solution III.
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Second step, the solution II and III are successively poured into the solution I to obtain a mixed solution. The mixed solution is heated to boiling
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for 10 minutes. A brown sedimentation can be gradually precipitated from the mixed solution.Then the FeOOH/Ni(OH)2 composite precursor can be obtained from this brown sedimentation washed by deionized water. Finally, the composite γ-Fe2 O3 /Ni2 O3 nanoparticles can be fabricated using
the FeOOH/Ni(OH)2 composite precursor boiled in 0.1mol/L FeCl2 solution 3
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for 30 minutes. The whole process can be described by the ionic equation:
F eOOH ↓ +Ni(OH)2 ↓
F eCl2
(1)
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F e3+ + Ni2+ + OH − → F eOOH ↓ +Ni(OH)2
→ γ − F e2 O3 /Ni2 O3
(2)
The devices with metal-insulator-metal (MIM) structure are fabricated on
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the Pt/Si(100) substrate. A 200nm HfSiO tunneling layer is first deposited
on the Pt/Si(100) substrates by radio frequency magnetron sputtering at
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300◦ C in mixed gas atmosphere(Ar:O2 =3:1) with sputtering power of 100W. The 0.1g γ-Fe2 O3 /Ni2 O3 powder is dispersed in 100 mL absolute ethanol. The dispersed liquid is covered on the HfSiO tunneling layer by spin-coating method for 60 seconds at 3000 revolutions. Then, the 200 nm HfSiO control layer are deposited on the sample, which processed the dispersed liquid. The Au top electrodes with diameter of 0.5 mm are fabricated by the means of
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sputtering.
3. Results and Discussion
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3.1. Microstructural analysis
The X-Ray Diffraction (XRD, Shimadzu-700) spectrum shows the peaks
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of 30.24◦ , 35.63◦ , 43.29◦, 53.7◦ 3, 57.27◦ and 62.92◦ corresponding to the [200], [311], [400], [422], [511] and [400] the lattice planes of the γ-Fe2 O3 crystals in the space group P4132[213] with lattice constant of a=8.351˚ A, b=8.351˚ A, c= 8.351˚ A and β=90◦ (JCPDS, NO. 39-1346), as shown in Fig.1 (a). The
lattice planes of [400], [422], [511] for the Ni2 O3 does not detected in our work due to its extremely low concentration compared with γ-Fe2 O3 . The 4
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high-resolution transmission electron microscopy (HRTEM, JEM-2010) of the γ-Fe2 O3 /Ni2 O3 nanoparticles are shown Fig.1 (a) of inset. To further
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confirm chemical component of the particles, the X-ray photoelectron spectroscopy(XPS, 250Xi) is employed. All binding energy spectrum measure-
ment in this paper are charge corrected by C 1s 284.8eV. The Fe 2p binding
energy peaks of 711.31eV and 724.93 eV are ascribed to the Fe-O bond of γ-
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Fe2 O3 ,as shown Fig.1 (b). Similarly, the Fig. 1 (c) presents the Ni 2p binding
energy peaks of 856.62eV and 875.03eV, which are originated from the Ni-O bond of Ni2 O3 . The strong binding energy peak observed at 531.2eV for O 1s
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implies the particles are oxygen-rich[31], as shown in Fig.1 (d). The distribution and diameter of core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles is investigated by the means of scanning electron microscopy (SEM, FE1450) technology, as shown in Fig.1 (e), the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles are well distributed, the area density is about 4.87×1012 cm−2 and average diameter
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is about 8.5nm.
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3.2. Resistance switching memory effects To study the resistance behaviors, a cycle sweep voltage (0V→2V→0V→-
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2V→0V) are operated on the fabricated devices at room temperature. For the device only with HfSiO matrix,the resistance switching memory phe-
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nomenon cannot be observed, as the Fig.2 (b) shown. Conversely, the obvious resistance switching memory effects can be detected in the devices with γ-Fe2 O3 /Ni2 O3 nanoparticles when performing cycle voltage at room temperature, as shown in Fig.2 (d). The schematic diagram for the devices with and without γ-Fe2O3 /Ni2 O3 nanoparticles are presented in Fig. 2 (a) and Fig.2 (b), respectively. Therefore, it can be deduced that the resistance switch5
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ing memory effects are entirely attributed to the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles rather than the HfSiO oxide matrix. It is worth noting that,
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after stressing one cycle sweep voltage, the voltage of Vset (1.96V) and Vreset(1.90V) can be detected after stressing one cycle sweep voltage. Hence, a large memory window (from -1.90 to 1.96 V) can be obtained in the device with γ-Fe2 O3 /Ni2 O3 nanoparticles.[32, 33]
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Fig.2 (d) highlights that the device is in a high resistance state (HRS or OFF) at low positive voltage (stage 1). The OFF state is well maintained as the positive voltage increasing until reaches set voltage. If the operate voltage
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over the set voltage, the current sharply increases to a stable value(stage 2), the device is in a low resistance state (LRS or ON). The ON state can be maintained when giving a reverse voltage from set voltage to reset voltage (stage 3 and stage 4). If the reverse voltage reaches the reset voltage, the current sharply decreases to a low value (stage 5), after stressing a reverse
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negative voltage, the device is stabilized in OFF stage (stage 6). To study the retention characteristics of the resistance switching behav-
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iors, several sweep cycles are operated on the device at room temperature. Fig.3 (a) exhibits that resistance switching memory effects are well main-
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tained after stressing sweep voltage for 100 cycles. The logarithmic currentvoltage are made based on the Fig. 3 (a), which demonstrates a tiny degen-
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eration of set/reset voltage after stress 100 cycles. In order to further investigate the retention properties, the datum of Fig.3 (c), which extracted from the Fig.3(b), shows a little degeneration for Vset /Vreset . The V¯ set (1.88V)and
V¯ reset (-1.75V) can be maintained after 100 switching cycles voltage. Furthermore, the resistance ON/OFF ratio is about ∼102 , as the Fig.3(d) shown.
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The resistance switching memory effects are absolutely contributed by the γ-Fe2 O3 /Ni2 O3 nanoparticles, but the excellent retention characteristics par-
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tially ascribe to the HfSiO layers[32, 33, 34]. 3.3. The mechanism discussion
The double logarithmic plot and index fitting of low resistance state(LRS)/ high resistance state(HRS) are obtained using the I-V curves in order to in-
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vestigate the resistance switching memory mechanism. The I-V1.05 fitting result demonstrates that electrons in the LRS are dominated by Ohmic con-
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duction, by contrast, the I-V2.26 , I-V0.27 , I-V2.35 fitting results illustrate that the trap-induced SCLC plays an important role in the HRS, as shown in Fig.4(a). Previous XPS analysis (Fig.1 b-d) demonstrates the core-shell γFe2 O3 /Ni2 O3 nanoparticles are in rich-oxygen. Therefore, a large proportion of oxygen ion vacancy might generate in HfSiO oxide matrix. The XPS spectrum with Ar+ depth etching in the HfSiO oxide films for 120s are presented
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in Fig.4 (b), Fig.4 (c) and Fig.4 (d). The Hf4f7/2 in Hf0 and Hf4+ stan-
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dard binding energy are 14.15 eV, 19.5 eV, respectively, however, the Hf4f7/2 binding energy of HfSiO is in between and has an obvious “red − shif t”.
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Similarly, the binding energy of Si2p located between Si0 (99.15eV) and Si4+ (103.4eV) has presented “red − shif t” phenomenon. Consequently, we de-
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duced that tremendous oxygen vacancy generated in HfSiO matrix and plays an important role in the electrons transport and the resistance switching memory process.
As the Fig.5 shown, when performing a set voltage, the numerous oxy-
gen ions O2− are gradually pumped from the bottom HfSiO oxide layer and the γ-Fe2 O3 /Ni2 O3 nanoparticles into the top HfSiO oxide layer.While, the 7
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oxygen vacancy Vo2+ in the top HfSiO oxide matrix is driven off the γFe2 O3 /Ni2 O3 nanoparticles and finally stayed in the bottom HfSiO oxide
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layer, as the process going, the “conductivef ilaments” is established between the top and bottom HfSiO layers. The set process illustrates that electrons are easily transported between the top and bottom HfSiO assisted
by the filament.Therefore, the set process is in the low resistance states. It is
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worth noting that, as oxygen vacancy Vo2+ extracted and the oxygen ion O2−
inhaled, oxygen atoms are formed in the top HfSiO oxide layer, whereas, the bottom HfSiO oxide layer is becoming poor-oxygen[35, 36]. However, when
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performing a reset voltage, the oxygen vacancy Vo2+ is driven into the top HfSiO layer, and the oxygen ion O2− returns into the bottom HfSiO layer. The conductive filament is broken with the reset process going. As a result, a high resistance state are formed between the top and bottom HfSiO layers.
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4. Conclusion
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The core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles are fabricated using the coprecipitation, and its average diameter is about 8.5nm, area density is about 4.87×1012 cm−2 . The MIM devices are prepared at room temperature and the
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devices with core-shell γ-Fe2O3 /Ni2 O3 nanoparticles present an excellently typical bipolar resistance switching memory effects: the average high-low re-
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sistance ratio of ∼102 and V¯ set (1.88V),V¯ reset (-1.75V) voltage are maintained after 100 cycle voltage sweeping. The set process is in low resistance state dominated by Ohmic conduction, and the reset process is in the high resistance state dominated by trap-induced SCLC. The generated oxygen vacancy Vo2+ , oxygen ion O2− , oxygen atom and external electric field are considered 8
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for understanding the resistance switching memory effect.
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Acknowledgments This work was partly supported by the National Natural Science Foun-
dation of China (11304410), Natural Science Foundation of Technology Department (QJHJZ-LKZS[2012]03 and QKHJZ[2014]2170) and Youth Science
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Foundation of Education Ministry (QJHKZ[2012]084) of Guizhou Province of China.
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Figure Captions Fig. 1.(a) XRD spectrum and HRTEM images (inset) for the core-shell γ-
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Fe2 O3 /Ni2 O3 nanoparticles. XPS spectrum of (b) Fe2p,(c) Ni2p and (d) O1s. (e) SEM images of the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles located
on 200 nm HfSiO oxide matrix, and its area density is 4.87×1012 cm−2 . (f) diameter distribution statistics histogram for the core-shell γ-Fe2 O3 /Ni2 O3
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nanoparticles, and its average diameter is about 8.5 nm.
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Fig. 2.(a) Schematic diagram of MIM structures with the HfSiO oxide matrix and (b) its leakage current-voltage(J-V) curves. (c)Schematic diagram of MIM devices with the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles and (d) its leakage current-voltage(J-V) curves, which clearly presented the switching resistance memory effects.
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Fig. 3.(a) Leakage current-voltage(J-V) characteristics after performing 100 cycles sweep voltage at roomtemperature.(b) the logarithmic current-voltage
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curves after performing 100 cycles sweep voltage from -2V to 2V at room temperature, an excellent Vset /Vreset voltage and resistance switching mem-
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ory are presented.(c) The average set/reset voltage, which extracted from the Fig.3.(b), presented a stable properties after 100 cycles. (d) The retention
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characteristics of high resistance states (HRS) and low resistance states(LRS) after 100 cycles, and the RH/RL is above ∼102 at -1V reading voltage. Fig. 4.(a) Double logarithmic plot of LRS and HRS, the slope of LRS indicates Ohmic conduction, while, the slopes of HRS demonstrate space-chargelimited current (SCLC), respectively. XPS spectrum of (b)Hf4f, (c)Si2p and 13
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(d)O1s for the HfSiO oxied matrix, the “red − shif t” and “blue − shif t” illustrate that the HfSiO are in poor-oxygen.
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Fig.5. Schematic graph of the resistance switching process for the MIM
devices with the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles, the set/LRS and
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reset/HRS are dominated by the oxygen ion and oxygen vacancy.
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Fe 2p
3/2
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(b)
Fe 2p
710
720
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700
1/2
730
740
540
545
Binding energy(eV)
O 1s
(d)
Ni 2p
850
3/2
Ni 2p
860
1/2
870
880
890
525
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Binding energy(eV)
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(c)
535
Binding energy(eV)
18 (f) 16
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(e)
530
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Average diameter 8.5nm
Nd/N(%)
12 10 8
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6 4 2 0
1 2
3 4
5 6
7
8 9 10 11 12 13 14 15
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Diameter(nm)
Figure 1: (a) XRD spectrum and HRTEM images (inset) for the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles. XPS spectrum of (b) Fe2p,(c) Ni2p and (d) O1s. (e) SEM images of the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles located on 200 nm HfSiO oxide matrix, and its
area density is 4.87×1012cm−2 . (f) diameter distribution statistics histogram for the coreshell γ-Fe2 O3 /Ni2 O3 nanoparticles, and its average diameter is about 8.5 nm.
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(b)
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0.01 1E-3 1E-4
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Current (A)
0.1
-2
(d)
-1
0 Gata Voltage(V)
1
2
1
3
4
0.1
Current (A)
5 reset
set 2
0.01
1E-3
6
1
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1E-4
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1E-5 -2
-1
0 Voltage(V)
1
2
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Figure 2: (a) Schematic diagram of MIM structures with the HfSiO oxide matrix and (b) its leakage current-voltage(J-V) curves. (c)Schematic diagram of MIM devices with the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles and (d) its leakage current-voltage(J-V) curves,
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which clearly presented the switching resistance memory effects.
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(b)
(a) 0.6
1
0.1
0.4 0.2
Current (A)
Current (A)
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0.0
0.01 1E-3
-0.2
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1E-4 -0.4 1E-5 -2
-1
0 Voltage(V)
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-2
2
-1
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-0.6
(d)
(c) 2.0
600
0.5
Resistance(O)
Voltage(V)
1.0 Vset= 1.88V
0.0
Vrest= -1.75V
-0.5 -1.0 -1.5
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RH/RL~10
2
500 400 300 200
Reading voltage @-1V
100
-2.0
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HRS: ~687O LRS: ~6O
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40 60 Switching cycles
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Figure 3: (a) Leakage current-voltage(J-V) characteristics after performing 100 cycles sweep voltage at roomtemperature.(b) the logarithmic current-voltage curves after per-
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forming 100 cycles sweep voltage from -2V to 2V at room temperature, an excellent Vset /Vreset voltage and resistance switching memory are presented.(c) The average set/reset voltage, which extracted from the Fig.3.(b), presented a stable properties af-
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ter 100 cycles. (d) The retention characteristics of high resistance states (HRS) and low resistance states(LRS) after 100 cycles, and the RH/RL is above ∼102 at -1V reading voltage.
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(a) I-V
1.05
LRS
0.01 I-V I-V
1E-3
I-V
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Current (A)
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0.27
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Voltage(V)
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HRS
1E-4 0.1
Figure 4: (a) Double logarithmic plot of LRS and HRS, the slope of LRS indicates Ohmic
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conduction, while, the slopes of HRS demonstrate space-charge-limited current (SCLC), respectively. XPS spectrum of (b)Hf4f, (c)Si2p and (d)O1s for the HfSiO oxied matrix, the “red − shif t” and “blue − shif t” illustrate that the HfSiO are in poor-oxygen.
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Figure 5: Schematic graph of the resistance switching process for the MIM devices with
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the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles, the set/LRS and reset/HRS are dominated
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by the oxygen ion and oxygen vacancy.
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1. Bipolar resistance switching effects are detected in core-shell of γ-Fe2O3@Ni2O3。 2. The Ohimc conduction and space-charge-limited current play an important role in Low/High field. 3. Rapture of filament assisted by Vo2+, O2- and O2 recombination is responsible for switching. 4. Resistance switching memory highlights excellent retention properties after stress 100 cycles.
S0925-8388(16)30747-2
DOI:
10.1016/j.jallcom.2016.03.163
Reference:
JALCOM 37051
To appear in:
Journal of Alloys and Compounds
Received Date: 15 December 2015 Revised Date:
5 March 2016
Accepted Date: 22 March 2016
Please cite this article as: G. Zhou, B. Wu, X. Liu, Z. Li, S. Zhang, A. Zhou, X. Yang, Resistance switching characteristics of core-shell γ-Fe2O3/Ni2O3 nanoparticles in HfSiO matrix, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.03.163. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Resistance switching characteristics of core-shell γ-Fe2 O3/Ni2O3 nanoparticles in HfSiO matrix
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Guangdong Zhoua , Bo Wub,c,∗, Xiaoqin Liua , Zhiling Lia , Shuangju Zhanga , Ankun Zhoud , Xiude Yangb a
Guizhou Institute of Technology, Guiyang, 550003, People’s Republic of China Institute of Theoretical Physics, Zunyi Normal College, Zunyi, 563002, People’s Republic of China c School of Marine Science and Technology, Northwestern Polytechnical University, Xian, 710072, People’s Republic of China d Kunming Institute of Botany, Chineses Academy Sciences, Kunming, 650201, People’s Republic of China
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b
Abstract
Core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles are synthesized by chemical co-precipitation method. Resistive switching memory behaviors, which have resistance ON/OFF ratio of ∼102 and excellent retention property, are observed in the Au/HfSiO/γ-
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Fe2 O3 /Ni2 O3 /HfSiO/Pt structure.Space charge limited current (SCLC) mech-
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anism, which is supported by the fitting current-voltage results, is employed to know the resistive switching memory effects. The transportation of Oxy-
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gen vacancy Vo2+ , oxygen ion O2− , recombination of oxygen atom and drive of external electric field are responsible for the ON or OFF states observed in device.
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Keywords: High-k HfSiO films; resistance switching characteristics; resistance random access memory(RRAM); core-shell materials.
∗
Corresponding author.Tel:+86-0851-28927153. Email address: [email protected] (Bo Wu)
Preprint submitted to Journal of Alloys and Compounds
March 22, 2016
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1. Introduction Conventional Si-based technologies applied in nonvolatile flash memory
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had made great progress and success for decades. However, further downscaling size of integrated circuit results in decreasing of charge storage den-
sity, increasing of leakage current and power consumption. According to the prediction from the international technology roadmap for semiconduc-
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tors (ITRS,2013), semiconductor manufacturing will enter an age of 5nm technology in the 2020 year, the Si-based technologies will reach its limita-
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tion. Therefore, devices with higher storage density, superior endurance, and lower power consumption are urgently developed to meet the demands from the fast development of electronic industry. Resistive random access memory (RRAM) is predicted to be the promising next generation nonvolatile flash memory due to its sample sandwich structure, fast switching speed, low power consumption and high charge storage density.[1, 2, 3, 4, 5, 6, 7, 8, 9]
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To date, resistance switching memory behaviors have been observed in
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binary metal oxides such as TiO2 , CuO, NiO, HfO2 [10, 11, 12, 13], ferromagnetic γ-Fe2O3 , Pr1−x Cax MnO3 [14, 15], ferrimagnetic Pb(Zr0.2 Ti0.8 )O3 [16],
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chalcogenide material MoS2 [17], and even organic materials[18, 19]. Recently, the resistance switching behaviors have been reported in core-shell Ni/NiO, Pt/Fe2 O3 , CeO2 , WO3 /CoWO4 , Au/CuZnSnS[20, 21, 22, 23, 24] at room
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temperature.
Several switching resistance mechanisms, including trap-controlled space
charge limited current,[25, 26] ionic conduction,[27] filament formation mechanism [28] and redox-based in multivalence [29] have been proposed to explain the RRAM behavior.What is more suitable and reasonable among those 2
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mechanisms,that is still in controversy today. In this work, the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles are embedded in
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the high-k HfSiO matrix to fabricate device with the Au/HfSiO/γ-Fe2 O3 @Ni2 O3 /HfSiO/Pt structure. The HfSiO matrix is selected to be the tunneling and control lay-
ers due to its suitable permittivity, stability against crystallization and high
energy band width, which helps to improve the dielectric property.[30] More-
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over,the resistance switching effects and mechanisms are investigated in the
2. Experimental Details
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device.
The core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles are prepared by chemical coprecipitation process. The FeCl3 ·6H2 O,NaOH,HNO3 (0.01mol/L),NiNO3 ·6H2 O, FeCl2 ·4H2 O, HCl(12mol/L) are employed to fabricate the FeOOH/Ni(OH)2 composite precursor. The FeCl3 (1mol/L), Ni(NO3 )2 (0.1mol/L), HCl(0.7mol/L),
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HNO3 (0.01mol/L), FeCl2 (0.1mol/L), NaOH(0.7mol/L) are prepared using
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above raw materials. First step, HCl (12mol/L 4L) solution is poured into Ni(NO3 )2 (0.1mol/L 10mL), which labeled it as solution I; the 30mL 1mol/L FeCl3 labeled as solution II; 400mL 0.7mol/L NaOH signed as solution III.
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Second step, the solution II and III are successively poured into the solution I to obtain a mixed solution. The mixed solution is heated to boiling
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for 10 minutes. A brown sedimentation can be gradually precipitated from the mixed solution.Then the FeOOH/Ni(OH)2 composite precursor can be obtained from this brown sedimentation washed by deionized water. Finally, the composite γ-Fe2 O3 /Ni2 O3 nanoparticles can be fabricated using
the FeOOH/Ni(OH)2 composite precursor boiled in 0.1mol/L FeCl2 solution 3
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for 30 minutes. The whole process can be described by the ionic equation:
F eOOH ↓ +Ni(OH)2 ↓
F eCl2
(1)
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F e3+ + Ni2+ + OH − → F eOOH ↓ +Ni(OH)2
→ γ − F e2 O3 /Ni2 O3
(2)
The devices with metal-insulator-metal (MIM) structure are fabricated on
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the Pt/Si(100) substrate. A 200nm HfSiO tunneling layer is first deposited
on the Pt/Si(100) substrates by radio frequency magnetron sputtering at
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300◦ C in mixed gas atmosphere(Ar:O2 =3:1) with sputtering power of 100W. The 0.1g γ-Fe2 O3 /Ni2 O3 powder is dispersed in 100 mL absolute ethanol. The dispersed liquid is covered on the HfSiO tunneling layer by spin-coating method for 60 seconds at 3000 revolutions. Then, the 200 nm HfSiO control layer are deposited on the sample, which processed the dispersed liquid. The Au top electrodes with diameter of 0.5 mm are fabricated by the means of
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sputtering.
3. Results and Discussion
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3.1. Microstructural analysis
The X-Ray Diffraction (XRD, Shimadzu-700) spectrum shows the peaks
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of 30.24◦ , 35.63◦ , 43.29◦, 53.7◦ 3, 57.27◦ and 62.92◦ corresponding to the [200], [311], [400], [422], [511] and [400] the lattice planes of the γ-Fe2 O3 crystals in the space group P4132[213] with lattice constant of a=8.351˚ A, b=8.351˚ A, c= 8.351˚ A and β=90◦ (JCPDS, NO. 39-1346), as shown in Fig.1 (a). The
lattice planes of [400], [422], [511] for the Ni2 O3 does not detected in our work due to its extremely low concentration compared with γ-Fe2 O3 . The 4
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high-resolution transmission electron microscopy (HRTEM, JEM-2010) of the γ-Fe2 O3 /Ni2 O3 nanoparticles are shown Fig.1 (a) of inset. To further
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confirm chemical component of the particles, the X-ray photoelectron spectroscopy(XPS, 250Xi) is employed. All binding energy spectrum measure-
ment in this paper are charge corrected by C 1s 284.8eV. The Fe 2p binding
energy peaks of 711.31eV and 724.93 eV are ascribed to the Fe-O bond of γ-
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Fe2 O3 ,as shown Fig.1 (b). Similarly, the Fig. 1 (c) presents the Ni 2p binding
energy peaks of 856.62eV and 875.03eV, which are originated from the Ni-O bond of Ni2 O3 . The strong binding energy peak observed at 531.2eV for O 1s
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implies the particles are oxygen-rich[31], as shown in Fig.1 (d). The distribution and diameter of core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles is investigated by the means of scanning electron microscopy (SEM, FE1450) technology, as shown in Fig.1 (e), the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles are well distributed, the area density is about 4.87×1012 cm−2 and average diameter
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is about 8.5nm.
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3.2. Resistance switching memory effects To study the resistance behaviors, a cycle sweep voltage (0V→2V→0V→-
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2V→0V) are operated on the fabricated devices at room temperature. For the device only with HfSiO matrix,the resistance switching memory phe-
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nomenon cannot be observed, as the Fig.2 (b) shown. Conversely, the obvious resistance switching memory effects can be detected in the devices with γ-Fe2 O3 /Ni2 O3 nanoparticles when performing cycle voltage at room temperature, as shown in Fig.2 (d). The schematic diagram for the devices with and without γ-Fe2O3 /Ni2 O3 nanoparticles are presented in Fig. 2 (a) and Fig.2 (b), respectively. Therefore, it can be deduced that the resistance switch5
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ing memory effects are entirely attributed to the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles rather than the HfSiO oxide matrix. It is worth noting that,
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after stressing one cycle sweep voltage, the voltage of Vset (1.96V) and Vreset(1.90V) can be detected after stressing one cycle sweep voltage. Hence, a large memory window (from -1.90 to 1.96 V) can be obtained in the device with γ-Fe2 O3 /Ni2 O3 nanoparticles.[32, 33]
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Fig.2 (d) highlights that the device is in a high resistance state (HRS or OFF) at low positive voltage (stage 1). The OFF state is well maintained as the positive voltage increasing until reaches set voltage. If the operate voltage
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over the set voltage, the current sharply increases to a stable value(stage 2), the device is in a low resistance state (LRS or ON). The ON state can be maintained when giving a reverse voltage from set voltage to reset voltage (stage 3 and stage 4). If the reverse voltage reaches the reset voltage, the current sharply decreases to a low value (stage 5), after stressing a reverse
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negative voltage, the device is stabilized in OFF stage (stage 6). To study the retention characteristics of the resistance switching behav-
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iors, several sweep cycles are operated on the device at room temperature. Fig.3 (a) exhibits that resistance switching memory effects are well main-
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tained after stressing sweep voltage for 100 cycles. The logarithmic currentvoltage are made based on the Fig. 3 (a), which demonstrates a tiny degen-
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eration of set/reset voltage after stress 100 cycles. In order to further investigate the retention properties, the datum of Fig.3 (c), which extracted from the Fig.3(b), shows a little degeneration for Vset /Vreset . The V¯ set (1.88V)and
V¯ reset (-1.75V) can be maintained after 100 switching cycles voltage. Furthermore, the resistance ON/OFF ratio is about ∼102 , as the Fig.3(d) shown.
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The resistance switching memory effects are absolutely contributed by the γ-Fe2 O3 /Ni2 O3 nanoparticles, but the excellent retention characteristics par-
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tially ascribe to the HfSiO layers[32, 33, 34]. 3.3. The mechanism discussion
The double logarithmic plot and index fitting of low resistance state(LRS)/ high resistance state(HRS) are obtained using the I-V curves in order to in-
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vestigate the resistance switching memory mechanism. The I-V1.05 fitting result demonstrates that electrons in the LRS are dominated by Ohmic con-
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duction, by contrast, the I-V2.26 , I-V0.27 , I-V2.35 fitting results illustrate that the trap-induced SCLC plays an important role in the HRS, as shown in Fig.4(a). Previous XPS analysis (Fig.1 b-d) demonstrates the core-shell γFe2 O3 /Ni2 O3 nanoparticles are in rich-oxygen. Therefore, a large proportion of oxygen ion vacancy might generate in HfSiO oxide matrix. The XPS spectrum with Ar+ depth etching in the HfSiO oxide films for 120s are presented
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in Fig.4 (b), Fig.4 (c) and Fig.4 (d). The Hf4f7/2 in Hf0 and Hf4+ stan-
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dard binding energy are 14.15 eV, 19.5 eV, respectively, however, the Hf4f7/2 binding energy of HfSiO is in between and has an obvious “red − shif t”.
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Similarly, the binding energy of Si2p located between Si0 (99.15eV) and Si4+ (103.4eV) has presented “red − shif t” phenomenon. Consequently, we de-
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duced that tremendous oxygen vacancy generated in HfSiO matrix and plays an important role in the electrons transport and the resistance switching memory process.
As the Fig.5 shown, when performing a set voltage, the numerous oxy-
gen ions O2− are gradually pumped from the bottom HfSiO oxide layer and the γ-Fe2 O3 /Ni2 O3 nanoparticles into the top HfSiO oxide layer.While, the 7
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oxygen vacancy Vo2+ in the top HfSiO oxide matrix is driven off the γFe2 O3 /Ni2 O3 nanoparticles and finally stayed in the bottom HfSiO oxide
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layer, as the process going, the “conductivef ilaments” is established between the top and bottom HfSiO layers. The set process illustrates that electrons are easily transported between the top and bottom HfSiO assisted
by the filament.Therefore, the set process is in the low resistance states. It is
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worth noting that, as oxygen vacancy Vo2+ extracted and the oxygen ion O2−
inhaled, oxygen atoms are formed in the top HfSiO oxide layer, whereas, the bottom HfSiO oxide layer is becoming poor-oxygen[35, 36]. However, when
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performing a reset voltage, the oxygen vacancy Vo2+ is driven into the top HfSiO layer, and the oxygen ion O2− returns into the bottom HfSiO layer. The conductive filament is broken with the reset process going. As a result, a high resistance state are formed between the top and bottom HfSiO layers.
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4. Conclusion
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The core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles are fabricated using the coprecipitation, and its average diameter is about 8.5nm, area density is about 4.87×1012 cm−2 . The MIM devices are prepared at room temperature and the
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devices with core-shell γ-Fe2O3 /Ni2 O3 nanoparticles present an excellently typical bipolar resistance switching memory effects: the average high-low re-
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sistance ratio of ∼102 and V¯ set (1.88V),V¯ reset (-1.75V) voltage are maintained after 100 cycle voltage sweeping. The set process is in low resistance state dominated by Ohmic conduction, and the reset process is in the high resistance state dominated by trap-induced SCLC. The generated oxygen vacancy Vo2+ , oxygen ion O2− , oxygen atom and external electric field are considered 8
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for understanding the resistance switching memory effect.
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Acknowledgments This work was partly supported by the National Natural Science Foun-
dation of China (11304410), Natural Science Foundation of Technology Department (QJHJZ-LKZS[2012]03 and QKHJZ[2014]2170) and Youth Science
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Foundation of Education Ministry (QJHKZ[2012]084) of Guizhou Province of China.
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Figure Captions Fig. 1.(a) XRD spectrum and HRTEM images (inset) for the core-shell γ-
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Fe2 O3 /Ni2 O3 nanoparticles. XPS spectrum of (b) Fe2p,(c) Ni2p and (d) O1s. (e) SEM images of the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles located
on 200 nm HfSiO oxide matrix, and its area density is 4.87×1012 cm−2 . (f) diameter distribution statistics histogram for the core-shell γ-Fe2 O3 /Ni2 O3
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nanoparticles, and its average diameter is about 8.5 nm.
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Fig. 2.(a) Schematic diagram of MIM structures with the HfSiO oxide matrix and (b) its leakage current-voltage(J-V) curves. (c)Schematic diagram of MIM devices with the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles and (d) its leakage current-voltage(J-V) curves, which clearly presented the switching resistance memory effects.
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Fig. 3.(a) Leakage current-voltage(J-V) characteristics after performing 100 cycles sweep voltage at roomtemperature.(b) the logarithmic current-voltage
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curves after performing 100 cycles sweep voltage from -2V to 2V at room temperature, an excellent Vset /Vreset voltage and resistance switching mem-
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ory are presented.(c) The average set/reset voltage, which extracted from the Fig.3.(b), presented a stable properties after 100 cycles. (d) The retention
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characteristics of high resistance states (HRS) and low resistance states(LRS) after 100 cycles, and the RH/RL is above ∼102 at -1V reading voltage. Fig. 4.(a) Double logarithmic plot of LRS and HRS, the slope of LRS indicates Ohmic conduction, while, the slopes of HRS demonstrate space-chargelimited current (SCLC), respectively. XPS spectrum of (b)Hf4f, (c)Si2p and 13
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(d)O1s for the HfSiO oxied matrix, the “red − shif t” and “blue − shif t” illustrate that the HfSiO are in poor-oxygen.
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Fig.5. Schematic graph of the resistance switching process for the MIM
devices with the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles, the set/LRS and
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reset/HRS are dominated by the oxygen ion and oxygen vacancy.
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Fe 2p
3/2
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(b)
Fe 2p
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720
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700
1/2
730
740
540
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Binding energy(eV)
O 1s
(d)
Ni 2p
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Ni 2p
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890
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Binding energy(eV)
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(c)
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Binding energy(eV)
18 (f) 16
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Average diameter 8.5nm
Nd/N(%)
12 10 8
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5 6
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8 9 10 11 12 13 14 15
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Figure 1: (a) XRD spectrum and HRTEM images (inset) for the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles. XPS spectrum of (b) Fe2p,(c) Ni2p and (d) O1s. (e) SEM images of the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles located on 200 nm HfSiO oxide matrix, and its
area density is 4.87×1012cm−2 . (f) diameter distribution statistics histogram for the coreshell γ-Fe2 O3 /Ni2 O3 nanoparticles, and its average diameter is about 8.5 nm.
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(b)
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Current (A)
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2
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Figure 2: (a) Schematic diagram of MIM structures with the HfSiO oxide matrix and (b) its leakage current-voltage(J-V) curves. (c)Schematic diagram of MIM devices with the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles and (d) its leakage current-voltage(J-V) curves,
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which clearly presented the switching resistance memory effects.
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(b)
(a) 0.6
1
0.1
0.4 0.2
Current (A)
Current (A)
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0.0
0.01 1E-3
-0.2
SC
1E-4 -0.4 1E-5 -2
-1
0 Voltage(V)
1
-2
2
-1
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-0.6
(d)
(c) 2.0
600
0.5
Resistance(O)
Voltage(V)
1.0 Vset= 1.88V
0.0
Vrest= -1.75V
-0.5 -1.0 -1.5
2
RH/RL~10
2
500 400 300 200
Reading voltage @-1V
100
-2.0
1
HRS: ~687O LRS: ~6O
700
1.5
0 Voltage(V)
0
20
40 60 Switching cycles
80
0
100
20
40 60 Switching cycles
80
100
TE
D
0
Figure 3: (a) Leakage current-voltage(J-V) characteristics after performing 100 cycles sweep voltage at roomtemperature.(b) the logarithmic current-voltage curves after per-
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forming 100 cycles sweep voltage from -2V to 2V at room temperature, an excellent Vset /Vreset voltage and resistance switching memory are presented.(c) The average set/reset voltage, which extracted from the Fig.3.(b), presented a stable properties af-
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ter 100 cycles. (d) The retention characteristics of high resistance states (HRS) and low resistance states(LRS) after 100 cycles, and the RH/RL is above ∼102 at -1V reading voltage.
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(a) I-V
1.05
LRS
0.01 I-V I-V
1E-3
I-V
2.26
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Current (A)
0.1
2.35
0.27
1
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Voltage(V)
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HRS
1E-4 0.1
Figure 4: (a) Double logarithmic plot of LRS and HRS, the slope of LRS indicates Ohmic
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conduction, while, the slopes of HRS demonstrate space-charge-limited current (SCLC), respectively. XPS spectrum of (b)Hf4f, (c)Si2p and (d)O1s for the HfSiO oxied matrix, the “red − shif t” and “blue − shif t” illustrate that the HfSiO are in poor-oxygen.
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Figure 5: Schematic graph of the resistance switching process for the MIM devices with
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the core-shell γ-Fe2 O3 /Ni2 O3 nanoparticles, the set/LRS and reset/HRS are dominated
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by the oxygen ion and oxygen vacancy.
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1. Bipolar resistance switching effects are detected in core-shell of γ-Fe2O3@Ni2O3。 2. The Ohimc conduction and space-charge-limited current play an important role in Low/High field. 3. Rapture of filament assisted by Vo2+, O2- and O2 recombination is responsible for switching. 4. Resistance switching memory highlights excellent retention properties after stress 100 cycles.