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Effects of Al dopants and interfacial layer on resistive switching behaviors of HfOx film
Accepted Manuscript Effects of Al dopants and interfacial layer on resistive switching behaviors of HfOx film Tingting Guo, Tingting Tan, Zhengtang Liu, Bangjie Liu PII:
S0925-8388(17)30735-1
DOI:
10.1016/j.jallcom.2017.02.286
Reference:
JALCOM 41014
To appear in:
Journal of Alloys and Compounds
Received Date: 9 January 2017 Revised Date:
23 February 2017
Accepted Date: 27 February 2017
Please cite this article as: T. Guo, T. Tan, Z. Liu, B. Liu, Effects of Al dopants and interfacial layer on resistive switching behaviors of HfOx film, Journal of Alloys and Compounds (2017), doi: 10.1016/ j.jallcom.2017.02.286. 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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Graphic Abstract
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ACCEPTED MANUSCRIPT Effects of Al dopants and interfacial layer on resistive switching behaviors of HfOx film Tingting Guo1*, Tingting Tan*, Zhengtang Liu, Bangjie Liu
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State Key Lab of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an, 710072, China Abstract
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In this work, Al dopants were introduced into HfOx film by different methods to
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modulate the oxygen vacancies in the film or near the interface, and the resistive switching characteristics were investigated. By transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis, an interfacial layer was formed at HfOx/Al interface. The occurrence of interfacial layer resulted in the larger
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switching voltages and resistances in LRS for HfOx/Al/HfOx and HfOx/Al samples compared with HfOx:Al sample with uniform Al dopants and no interface. Besides, the different reset processes for Al-doped HfOx samples were demonstrated. Much
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uniform distribution of resistances can be observed for all Al-doped HfOx samples due
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to the control of oxygen vacancies by Al doping and the good retention properties were achieved for all samples. The models for underlying physical mechanisms were also proposed to illustrate the switching behaviors of the prepared samples. Keywords: Resistive switching; Oxygen vacancies; Al dopants; Interfacial layer. 1. Introduction Transition metal oxide-based resistive random access memory (RRAM) has been *These authors contributed equally to this work. Corresponding author. E-mail address: [email protected] (Tingting Tan) and [email protected] (Tingting Guo). 1
ACCEPTED MANUSCRIPT considered one of the most promising candidates to replace the traditional flash memory which has been rapidly approaching its fundamental scaling limit [1], owing to the simple structure and fabrication, low power consumption, and compatibility
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with complementary metal oxide semiconductor process [2-5]. Resistive switching (RS) behavior that the reversible switching between two states of low resistance state (LRS) and high resistance state (HRS) can be induced by external electric field.
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Several related switching mechanisms have been proposed to explore the origin of RS
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phenomenon, such as conductive filament [6], Poole-Frenkel [7], and Schottky barrier [8]. At present, the filamentary model has been widely accepted to illustrate the RS behaviors in metal oxides [3,9,10], and the growth of filaments plays a crucial role on the uniformity and stability of RRAM.
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Hafnium oxide (HfO2), which is a favorite dielectric material, has been attracting increasing attention in memory since the first observation of switching behavior by Lee [11]. Numerous studies show that HfO2-base memory has high density, fast
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switching speed and good retention properties [12-13]. However, the key issue that
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the random formation of filaments which resulted in the inhomogeneous distribution of switching parameters and hindered the practical application of device are still under investigation [14]. Besides, the switching voltage should be reduced. Methods including doping technology, interfacial engineering, active electrode have been proposed [15-17]. It has been proved that the introduction of low valence ions in the film can effectively improve the RS behaviors, especially uniformity [13,15]. Our previous study [18] also showed that by doing appropriate concentration of Al dopants
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ACCEPTED MANUSCRIPT in HfOx film, good switching performance, such as improved uniformity and small switching voltage, can be observed compared with undoped HfOx film, owing to the decreased formation energy of oxygen vacancies near Al dopant sites. However, the
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excess Al doping resulted in the degeneration of switching performance. In this work, three different structures for HfOx sample with Al dopants (Al-doped HfOx samples: HfOx/Al/HfOx, HfOx/Al and HfOx:Al) were designed and
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the RS characteristics were investigated. The oxygen vacancies in HfOx film or near
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interface were modulated by Al dopants in different ways and an interfacial layer can be observed at HfOx/Al interface. The effects of Al dopants and the interfacial layer on switching voltages and resistances were demonstrated. The physical mechanisms for Al-doped HfOx samples with different structures were also discussed.
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2. Experimental details
Three types of structure for HfOx sample (HfOx/Al/HfOx, HfOx/Al and HfOx:Al) were fabricated. The Si/SiO2/Ti/Pt substrates were used as bottom electrode, and the
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HfOx film and Al film were deposited on Pt substrate by radio frequency magnetron
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sputtering using metallic target with/without reactive gas of O2. For HfOx/Al/HfOx structure, a 10-nm-thick HfOx film, a 4-nm-thick Al film and a 10-nm-thick HfOx film were sequentially deposited on Pt substrates. For HfOx/Al structure, a 20-nm-thick HfOx film was first deposited on Pt substrate and then a 4-nm-thick Al film were deposited. For HfOx:Al structure (20 nm), the introduction of Al was realized by loading metal Al pieces on the Hf target. During deposition, Ar/O2 was 12 sccm/3 sccm for HfOx film and Ar was 15 sccm for Al film, the working pressure was 0.3 Pa,
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ACCEPTED MANUSCRIPT and the sputtering power was 80 W. After deposition, a 200 °C rapid annealing treatment in N2 atmosphere were performed to manipulate the stoichiometry at HfOx/Al interface by the diffusion of Al atoms. Finally, the Cu top electrodes with
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diameter of 2 mm were deposited by evaporation using a metal shadow masks to pattern the size. The film property and structure were characterized by X-ray photoelectron spectroscope (XPS) and transmission electron microscope (TEM). The
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electrical properties were measured by an Agilent 4155C semiconductor parameter
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analyzer. During the measurement, the bias voltage was applied on Cu electrode and the bottom electrode Pt was always grounded.
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3. Results and discussion
Fig. 1 (a) TEM image of annealed HfOx/Al/HfOx structure. (b) XPS depth profile of HfOx/Al/HfOx structure before (the solid) and after (the open) annealing process. The XPS
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ACCEPTED MANUSCRIPT spectrum of Hf 4f in HfOx film, near top and bottom interface for HfOx/Al/HfOx film (c) before and (d) after annealing process.
The cross-sectional TEM image of annealed HfOx/Al/HfOx structure was
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presented in Fig. 1(a). The obvious interfacial layer can be observed at top and bottom HfOx/Al interface, which were asymmetric. Since HfOx film was deposited in O2 atmosphere, the surface of Al layer was oxidized during the deposition of top part of
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HfOx film, forming HfAlOx interfacial layer at top HfOx/Al interface. The interfacial
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layer at bottom HfOx/Al interface was formed due to the annealing process where Al atoms diffused into the bottom part of HfOx film owing to the smaller ionic radius. To further analyze the stoichiometry at HfOx/Al interface, the XPS depth etchings of HfOx/Al/HfOx samples before and after annealing process were performed, as shown
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in Fig. 1(b). Clearly seen that Al atoms diffused obviously into HfOx film after annealing process. The Hf 4f peaks in HfOx film and near interface before and after annealing process were fitted and the stoichiometry of O/Hf was calculated, as
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presented in Fig. 1(c) and (d). The Hf 4f peak in HfOx film can be fitted as a double
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peak belonging to Hf4+, and the Hf 4f peak near top and bottom interface can be deconvoluted into two or three double peaks, corresponding to Hf4+ (red line) and sub-oxide Hfx+ [19] (x<4, blue and green line). The occurrence of suboxides indicated the greater oxygen vacancy concentration at HfOx/Al interface than that in HfOx film. Besides, for annealed HfOx/Al/HfOx film (Fig. 1d), the greater intensity of suboxides near bottom interface can
be observed
obviously, indicating
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oxygen-deficient state due to the diffusion of Al atoms. The XPS analyses were
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ACCEPTED MANUSCRIPT consistent with TEM results. For HfOx/Al structure, the oxygen-deficient film can also be formed at interface due to the formation of interfacial layer by annealing
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process.
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Fig. 2 The schematic diagrams and the corresponding switching characteristics of (a)(d) HfOx/Al/HfOx, (b)(e) HfOx/Al and (c)(f) HfOx:Al samples.
The schematic diagrams of the structure for HfOx/Al/HfOx, HfOx/Al and HfOx:Al
samples were presented in Figs. 2(a)-(c), and the corresponding RS behaviors were shown in Figs. 2(d)-(f) respectively. The fresh samples were all in an insulator state and a forming process was required to activate the switching behavior with a current compliance of 10 mA to prevent the sample from breakdown. Compared with pure
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ACCEPTED MANUSCRIPT HfOx sample (FV~1.2) [18], Al-doped HfOx samples showed smaller forming voltage due to the introduction of Al atoms, especially for HfOx:Al sample, which resulted in oxygen deficiency in the HfOx film or near the interface, enhancing the ion migration
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during the forming operation. After forming process, all samples exhibited bipolar switching behaviors and the voltage was swept in a counterclockwise direction, as indicated by arrows in Fig. 2(d). Under the positive voltage, the current increased
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abruptly as the voltage increased to Vset and the sample switched from HRS to LRS.
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By applying a reverse voltage, the current decreased at Vreset where the current was the largest, switching the sample back to HRS. From Fig. 2, the ON/OFF ratio for Al-doped HfOx samples were all larger than 102. Note that the reset process for three samples showed some difference, the abrupt decrease for HfOx/Al/HfOx and HfOx/Al
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samples, and the gradual decrease for HfOx:Al sample.
Fig. 3 The distribution of (a) switching voltages and (b) resistances for Al-doped HfOx samples with different structures.
Fig. 3(a) shows the statistical distribution of switching voltages (VSet and VReset) for HfOx/Al/HfOx, HfOx/Al and HfOx:Al samples. Seen from Fig. 3(a), HfOx:Al sample exhibited smaller switching voltages with narrow distribution compared to
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ACCEPTED MANUSCRIPT HfOx/Al/HfOx and HfOx/Al samples. This may be due to that the oxygen deficiency occurred in the film for HfOx:Al sample, while only at the interface for HfOx/Al/HfOx and HfOx/Al samples, as indicated in Fig. 1. In addition, according to the refs 20-21,
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the distribution of applied field across AlOx/HfOx layer was closely related to their dielectric constants. Owing to the larger dielectric constant of HfO2 (25) than that of Al2O3 (9), the electric field distributed in HfOx film was much smaller than that in
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AlOx film. As a result, under the same voltage, for HfOx/Al/HfOx and HfOx/Al
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samples, most of the electric fields were distributed in AlOx layer and the electric field distributed in HfOx layer was not large enough to switch the sample. Therefore the lager switching voltages were required to drive the migration of oxygen ions in HfOx layer for HfOx/Al/HfOx and HfOx/Al samples due to the formation of interfacial layer
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compared to HfOx:Al sample with uniform Al dopants and no interface. In addition, the distribution of VReset for HfOx/Al/HfOx and HfOx/Al samples was relatively scattered, which may be related to the abrupt reset process. The variations of
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resistance in LRS and HRS were presented in Fig. 3(b). Much concentrated
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distribution of resistances, especially in LRS, can be observed for all Al-doped HfOx samples, indicating the good uniformity of resistance for the prepared samples. Note that the resistance in LRS for HfOx/Al/HfOx and HfOx/Al samples were larger compared to HfOx:Al sample. This was related to the formation of interfacial layer which can act as a series resistance to LRS value [22], leading to the lower LRS of HfOx:Al sample. For HfOx/Al/HfOx sample, lower resistance in HRS can be observed. It may be due to the much thinner thickness of HfOx film (10 nm) on both sides of Al
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ACCEPTED MANUSCRIPT layer. Thus, the diffusion of Al atoms can occur in HfOx film rather than only at interface like HfOx/Al sample, which enlarged the chemical mismatch of HfOx film and increased the conductivity of the film [23], resulting in the lower HRS of the film
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finally. During the reset process, the larger distributed electric field at interfacial layer or enhanced local electric field by Al atoms may result in the easier rupture of filaments near interface, which was responsible for the abrupt reset process for
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HfOx/Al/HfOx and HfOx/Al samples. Overall, results indicated that Al dopants were
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beneficial to improve the RS performance of HfOx sample, especially uniformity, compared with HfOx samples which exhibited large switching voltages with scattered
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distribution.
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Fig. 4 Current conduction properties for Al-doped HfOx samples.
To explore the switching behaviors of three Al-doped HfOx samples, the I-V
curves were replotted in double-log scales, as presented in Fig. 4. The fitting results for three samples with different structures were similar. For LRS, the current and voltage can be fitted as a straight line, indicating the ohmic characteristics, as shown in the inset of Fig. 4. While for HRS, the current and voltage was found to follow ohmic mechanism at low voltage region, and Child’ law at higher voltage, which can
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ACCEPTED MANUSCRIPT be well explained by the trap-controlled space charge limited current effect [1,24]. According to XPS and electrical analysis, oxygen vacancy acted as a dominant defect
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in HfOx film and was responsible for the switching behavior.
Fig. 5 The schematic diagram of the switching mechanisms for Al-doped HfOx samples.
Based on above analysis, the switching behaviors of Al-doped HfOx samples with
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different structures can be dominantly attributed to the formation and rupture of
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oxygen vacancy filaments in the film. Fig. 5 shows the schematic diagrams of switching mechanism of Al-doped HfOx samples. For HfOx/Al/HfOx sample, as presented in Fig. 5(a), the annealing process already introduced oxygen vacancies in the film. By applying a positive voltage, oxygen ions in the film were easier to migrated towards anode and more oxygen vacancies were generated. The formation of oxygen vacancy chains between electrodes switched the sample from HRS to LRS. It is surmised that Al diffusion may contribute to the switch of resistance due to the
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ACCEPTED MANUSCRIPT thinner film thickness (10 nm), which need further study however. Under the negative voltage, the rupture of these oxygen vacancy filaments by the diffusion of oxygen ions resulted in the HRS of the sample. It was speculated that the filaments at the
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interface may rupture firstly due to the larger distributed electric field at interfacial layer or enhanced local electric field by Al atoms [23]. which was responsible for the observed abruptly reset process. For HfOx/Al sample (Fig. 5b), under the positive
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voltage, the oxygen ions moved towards anode and the Al layer acted as an oxygen
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reservoir, thus leaving oxygen vacancies in HfOx film. With the increase of voltage, the oxygen vacancy chains were formed and assisted the hopping conduction of electrons, switching the sample from LRS to HRS. By applying the negative voltage, the oxygen ions were released from the interfical layer and recovered with oxygen
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vacancies, leading to the rupture of filaments. Also, the easier rupture of filaments due to the enhanced electric field led to the abrupt reset process HfOx/Al samples. The schematic diagram of switching mechanism for HfOx:Al sample was presented in Fig.
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5(c). The uniform doping of Al atoms in HfOx film decreased the formation energy of
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oxygen vacancy near Al dopants and induced more oxygen vacancies in HfOx film. These oxygen vacancies near Al dopants were easier to form more fixed conductive filaments, resulting in the small switching voltages and uniform distribution of switching parameters. By reversing the voltage, the oxygen vacancy filaments ruptured by recovering with oxygen ions. Seen from Fig. 3(a), the voltage fluctuation was only partially suppressed for HfOx:Al sample. The much scattered distribution of reset voltage for HfOx/Al/HfOx and HfOx/Al samples may be related to the abrupt
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reset process.
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Fig. 6 The retention properties of Al-doped HfOx samples with different structures at 85 °C. Read
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at 0.1 V.
Retention property is an important index to evaluate the memory performance. Fig. 6 shows the retention characteristics of HfOx/Al/HfOx, HfOx/Al and HfOx:Al samples measured at 85 °C. For all samples, the currents in HRS and LRS can
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maintain for over 2×103 s without serious degeneration. Although somewhat fluctuations of the currents can be observed with the increasing time, which may be caused by the variation of defects in the film under higher temperature and need
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further investigation, the least ON/OFF ratio larger than 10 was large enough to avoid
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the misreading. The results showed that Al-doped HfOx samples was promising for application in non-volatile memory. 4. Conclusions
In summary, the RS characteristics of HfOx/Al/HfOx, HfOx/Al and HfOx:Al
samples were investigated and the switching behaviors were attributed to the formation and rupture of oxygen vacancy filaments. An interfacial layer was formed at HfOx/Al interface for HfOx/Al/HfOx and HfOx/Al samples by TEM and XPS
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ACCEPTED MANUSCRIPT analysis, and the oxygen vacancies in HfOx film or near interface can be modulated by Al dopants in different ways, leading to the easier formation of conductive filaments. The interfacial layer resulted in the larger switching voltages and
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resistances in LRS compared with HfOx:Al sample with uniform Al dopants and no interface. Besides, the larger distributed electrical field at interfacial layer caused the abrupt reset process of HfOx/Al/HfOx and HfOx/Al samples, which may be
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responsible for the scattered distribution of reset voltage. Although Al dopants were
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introduced into HfOx films by different ways, good RS performance including large ON/OFF ratio, good uniformity and retention properties can be observed for all prepared samples, especially for HfOx:Al sample, indicating that Al dopants were beneficial to improve the RS performance of HfOx sample.
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Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (No. 51202196), the Fundamental Research Funds for the Central
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Universities (No. 3102014JCQ01032), the 111 Project (No. B08040), the Innovation
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Foundation for Doctor Dissertation of Northwestern Polytechnical University (No. CX201612), the Excellent Doctorate Foundation of Northwestern Polytechnical University and the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (No. 155-QP-2016). References [1] Y. C. Yang, F. Pan, Q. Liu, M. Liu, F. Zeng, Nano Lett. 9 (2009) 1636-1643. [2] Y. S. Fan, P. T. Liu, L. F. Teng, C. H. Hsu, Appl. Phys. Lett. 101 (2012) 052901.
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ACCEPTED MANUSCRIPT [3] X. Zhu, W. Su, Y. Liu, B. Hu, L. Pan, W. Lu, J. Zhang, R. W. Li, Adv. Mater. 24 (2012) 3941-3946. [4] S. Gao, F. Zeng, M. Wang, G. Wang, C. Song, F. Pan, Phys. Chem. Chem. Phys. 17
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(2015) 12849-12856. [5] R. Fang, W. Chen, L. Gao, W. Yu, S. Yu, IEEE Electron Device Lett. 36 (2015) 567-569.
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[6] B. Sun, L. Wei, H. Li, X. Jia, J. Wu, P. Chen, J. Mater. Chem. C (2015) 3
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12149-12155.
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[9] U. Celano, Y. Y. Chen, D. J. Wouters, G. Groeseneken, M. Jurczak, W. Vandervorst, Appl. Phys. Lett. 102 (2013) 121602.
[10] M. J. Rozenberg, M. J. Sánchez, R. Weht, C. Acha, F. Gomez-Marlasca, P. Levy,
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Phys. Rev. B 81 (2010) 115101.
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[11] H. Y. Lee, P. S. Chen, C. C. Wang, S. Maikap, P. J. Tzeng, C. H. Lin, L. S. Lee, M. J. Tsai, Jpn. J. Appl. Phys. 46 (2007) 2175. [12] Y. Wang, Q. Liu, S. Long, W. Wang, Q. Wang, M. Zhang, S. Zhang, Y. Li, Q. Zuo, J. Yang, M. Liu, Nanotechnology 21 (2010) 045202. [13] H. Zhang, L. Liu, B. Gao, Y. Qiu, X. Liu, J. Lu, R. Han, J. Kang, B. Yu, Appl. Phys. Lett. 98 (2011) 042105. [14] Q. Zhou, J. Zhai, AIP Adv. 3 (2013) 032102.
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ACCEPTED MANUSCRIPT [15] S. Yu, B. Gao, H. Dai, B. Sun, L. Liu, X. Liu, R. Han, J. Kang, B. Yu, Electrochem. Solid State Lett. 13 (2010) H36-H38. [16] Q. Liu, S. Long, W. Wang, Q. Zuo, S. Zhang, J. Chen, M. Liu, IEEE Electron
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Device Lett. 30 (2009) 1335-1337. [17] C. Dou, K. Kakushima, P. Ahmet, K. Tsutsui, A. Nishiyama, N. Sugii, K. Natori, T. Hattori, H. Iwai, Microelectron. Reliab. 52 (2012) 688-691.
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[19] S. U. Sharath, T. Bertaud, J. Kurian, E. Hildebrandt, C. Walczyk, P. Calka, P. Zaumseil, M. Sowinska, D. Walczyk, A. Gloskovskii, T. Schroeder, L. Alff, Appl. Phys. Lett. 104 (2014) 063502.
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[22] S. Mondal, H. Y. Chen, J. L. Her, F. H. Ko, T. M. Pan, Appl. Phys. Lett. 101
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ACCEPTED MANUSCRIPT Highlights: Three structures of HfOx film with Al dpoants were prepared. Oxygen vacancies in the film or at interface were modulated by Al dpoant.
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An interfacial layer can be observed at HfOx/Al interface by XPS and TEM analysis.
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A physical mechanism model based on oxygen vacancy filaments was proposed.
1
S0925-8388(17)30735-1
DOI:
10.1016/j.jallcom.2017.02.286
Reference:
JALCOM 41014
To appear in:
Journal of Alloys and Compounds
Received Date: 9 January 2017 Revised Date:
23 February 2017
Accepted Date: 27 February 2017
Please cite this article as: T. Guo, T. Tan, Z. Liu, B. Liu, Effects of Al dopants and interfacial layer on resistive switching behaviors of HfOx film, Journal of Alloys and Compounds (2017), doi: 10.1016/ j.jallcom.2017.02.286. 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.
ACCEPTED MANUSCRIPT
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Graphic Abstract
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ACCEPTED MANUSCRIPT Effects of Al dopants and interfacial layer on resistive switching behaviors of HfOx film Tingting Guo1*, Tingting Tan*, Zhengtang Liu, Bangjie Liu
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State Key Lab of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an, 710072, China Abstract
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In this work, Al dopants were introduced into HfOx film by different methods to
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modulate the oxygen vacancies in the film or near the interface, and the resistive switching characteristics were investigated. By transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis, an interfacial layer was formed at HfOx/Al interface. The occurrence of interfacial layer resulted in the larger
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switching voltages and resistances in LRS for HfOx/Al/HfOx and HfOx/Al samples compared with HfOx:Al sample with uniform Al dopants and no interface. Besides, the different reset processes for Al-doped HfOx samples were demonstrated. Much
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uniform distribution of resistances can be observed for all Al-doped HfOx samples due
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to the control of oxygen vacancies by Al doping and the good retention properties were achieved for all samples. The models for underlying physical mechanisms were also proposed to illustrate the switching behaviors of the prepared samples. Keywords: Resistive switching; Oxygen vacancies; Al dopants; Interfacial layer. 1. Introduction Transition metal oxide-based resistive random access memory (RRAM) has been *These authors contributed equally to this work. Corresponding author. E-mail address: [email protected] (Tingting Tan) and [email protected] (Tingting Guo). 1
ACCEPTED MANUSCRIPT considered one of the most promising candidates to replace the traditional flash memory which has been rapidly approaching its fundamental scaling limit [1], owing to the simple structure and fabrication, low power consumption, and compatibility
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with complementary metal oxide semiconductor process [2-5]. Resistive switching (RS) behavior that the reversible switching between two states of low resistance state (LRS) and high resistance state (HRS) can be induced by external electric field.
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Several related switching mechanisms have been proposed to explore the origin of RS
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phenomenon, such as conductive filament [6], Poole-Frenkel [7], and Schottky barrier [8]. At present, the filamentary model has been widely accepted to illustrate the RS behaviors in metal oxides [3,9,10], and the growth of filaments plays a crucial role on the uniformity and stability of RRAM.
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Hafnium oxide (HfO2), which is a favorite dielectric material, has been attracting increasing attention in memory since the first observation of switching behavior by Lee [11]. Numerous studies show that HfO2-base memory has high density, fast
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switching speed and good retention properties [12-13]. However, the key issue that
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the random formation of filaments which resulted in the inhomogeneous distribution of switching parameters and hindered the practical application of device are still under investigation [14]. Besides, the switching voltage should be reduced. Methods including doping technology, interfacial engineering, active electrode have been proposed [15-17]. It has been proved that the introduction of low valence ions in the film can effectively improve the RS behaviors, especially uniformity [13,15]. Our previous study [18] also showed that by doing appropriate concentration of Al dopants
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ACCEPTED MANUSCRIPT in HfOx film, good switching performance, such as improved uniformity and small switching voltage, can be observed compared with undoped HfOx film, owing to the decreased formation energy of oxygen vacancies near Al dopant sites. However, the
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excess Al doping resulted in the degeneration of switching performance. In this work, three different structures for HfOx sample with Al dopants (Al-doped HfOx samples: HfOx/Al/HfOx, HfOx/Al and HfOx:Al) were designed and
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the RS characteristics were investigated. The oxygen vacancies in HfOx film or near
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interface were modulated by Al dopants in different ways and an interfacial layer can be observed at HfOx/Al interface. The effects of Al dopants and the interfacial layer on switching voltages and resistances were demonstrated. The physical mechanisms for Al-doped HfOx samples with different structures were also discussed.
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2. Experimental details
Three types of structure for HfOx sample (HfOx/Al/HfOx, HfOx/Al and HfOx:Al) were fabricated. The Si/SiO2/Ti/Pt substrates were used as bottom electrode, and the
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HfOx film and Al film were deposited on Pt substrate by radio frequency magnetron
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sputtering using metallic target with/without reactive gas of O2. For HfOx/Al/HfOx structure, a 10-nm-thick HfOx film, a 4-nm-thick Al film and a 10-nm-thick HfOx film were sequentially deposited on Pt substrates. For HfOx/Al structure, a 20-nm-thick HfOx film was first deposited on Pt substrate and then a 4-nm-thick Al film were deposited. For HfOx:Al structure (20 nm), the introduction of Al was realized by loading metal Al pieces on the Hf target. During deposition, Ar/O2 was 12 sccm/3 sccm for HfOx film and Ar was 15 sccm for Al film, the working pressure was 0.3 Pa,
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diameter of 2 mm were deposited by evaporation using a metal shadow masks to pattern the size. The film property and structure were characterized by X-ray photoelectron spectroscope (XPS) and transmission electron microscope (TEM). The
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electrical properties were measured by an Agilent 4155C semiconductor parameter
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analyzer. During the measurement, the bias voltage was applied on Cu electrode and the bottom electrode Pt was always grounded.
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3. Results and discussion
Fig. 1 (a) TEM image of annealed HfOx/Al/HfOx structure. (b) XPS depth profile of HfOx/Al/HfOx structure before (the solid) and after (the open) annealing process. The XPS
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ACCEPTED MANUSCRIPT spectrum of Hf 4f in HfOx film, near top and bottom interface for HfOx/Al/HfOx film (c) before and (d) after annealing process.
The cross-sectional TEM image of annealed HfOx/Al/HfOx structure was
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presented in Fig. 1(a). The obvious interfacial layer can be observed at top and bottom HfOx/Al interface, which were asymmetric. Since HfOx film was deposited in O2 atmosphere, the surface of Al layer was oxidized during the deposition of top part of
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HfOx film, forming HfAlOx interfacial layer at top HfOx/Al interface. The interfacial
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layer at bottom HfOx/Al interface was formed due to the annealing process where Al atoms diffused into the bottom part of HfOx film owing to the smaller ionic radius. To further analyze the stoichiometry at HfOx/Al interface, the XPS depth etchings of HfOx/Al/HfOx samples before and after annealing process were performed, as shown
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in Fig. 1(b). Clearly seen that Al atoms diffused obviously into HfOx film after annealing process. The Hf 4f peaks in HfOx film and near interface before and after annealing process were fitted and the stoichiometry of O/Hf was calculated, as
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presented in Fig. 1(c) and (d). The Hf 4f peak in HfOx film can be fitted as a double
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peak belonging to Hf4+, and the Hf 4f peak near top and bottom interface can be deconvoluted into two or three double peaks, corresponding to Hf4+ (red line) and sub-oxide Hfx+ [19] (x<4, blue and green line). The occurrence of suboxides indicated the greater oxygen vacancy concentration at HfOx/Al interface than that in HfOx film. Besides, for annealed HfOx/Al/HfOx film (Fig. 1d), the greater intensity of suboxides near bottom interface can
be observed
obviously, indicating
the larger
oxygen-deficient state due to the diffusion of Al atoms. The XPS analyses were
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ACCEPTED MANUSCRIPT consistent with TEM results. For HfOx/Al structure, the oxygen-deficient film can also be formed at interface due to the formation of interfacial layer by annealing
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process.
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Fig. 2 The schematic diagrams and the corresponding switching characteristics of (a)(d) HfOx/Al/HfOx, (b)(e) HfOx/Al and (c)(f) HfOx:Al samples.
The schematic diagrams of the structure for HfOx/Al/HfOx, HfOx/Al and HfOx:Al
samples were presented in Figs. 2(a)-(c), and the corresponding RS behaviors were shown in Figs. 2(d)-(f) respectively. The fresh samples were all in an insulator state and a forming process was required to activate the switching behavior with a current compliance of 10 mA to prevent the sample from breakdown. Compared with pure
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ACCEPTED MANUSCRIPT HfOx sample (FV~1.2) [18], Al-doped HfOx samples showed smaller forming voltage due to the introduction of Al atoms, especially for HfOx:Al sample, which resulted in oxygen deficiency in the HfOx film or near the interface, enhancing the ion migration
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during the forming operation. After forming process, all samples exhibited bipolar switching behaviors and the voltage was swept in a counterclockwise direction, as indicated by arrows in Fig. 2(d). Under the positive voltage, the current increased
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abruptly as the voltage increased to Vset and the sample switched from HRS to LRS.
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By applying a reverse voltage, the current decreased at Vreset where the current was the largest, switching the sample back to HRS. From Fig. 2, the ON/OFF ratio for Al-doped HfOx samples were all larger than 102. Note that the reset process for three samples showed some difference, the abrupt decrease for HfOx/Al/HfOx and HfOx/Al
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samples, and the gradual decrease for HfOx:Al sample.
Fig. 3 The distribution of (a) switching voltages and (b) resistances for Al-doped HfOx samples with different structures.
Fig. 3(a) shows the statistical distribution of switching voltages (VSet and VReset) for HfOx/Al/HfOx, HfOx/Al and HfOx:Al samples. Seen from Fig. 3(a), HfOx:Al sample exhibited smaller switching voltages with narrow distribution compared to
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ACCEPTED MANUSCRIPT HfOx/Al/HfOx and HfOx/Al samples. This may be due to that the oxygen deficiency occurred in the film for HfOx:Al sample, while only at the interface for HfOx/Al/HfOx and HfOx/Al samples, as indicated in Fig. 1. In addition, according to the refs 20-21,
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the distribution of applied field across AlOx/HfOx layer was closely related to their dielectric constants. Owing to the larger dielectric constant of HfO2 (25) than that of Al2O3 (9), the electric field distributed in HfOx film was much smaller than that in
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AlOx film. As a result, under the same voltage, for HfOx/Al/HfOx and HfOx/Al
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samples, most of the electric fields were distributed in AlOx layer and the electric field distributed in HfOx layer was not large enough to switch the sample. Therefore the lager switching voltages were required to drive the migration of oxygen ions in HfOx layer for HfOx/Al/HfOx and HfOx/Al samples due to the formation of interfacial layer
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compared to HfOx:Al sample with uniform Al dopants and no interface. In addition, the distribution of VReset for HfOx/Al/HfOx and HfOx/Al samples was relatively scattered, which may be related to the abrupt reset process. The variations of
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resistance in LRS and HRS were presented in Fig. 3(b). Much concentrated
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distribution of resistances, especially in LRS, can be observed for all Al-doped HfOx samples, indicating the good uniformity of resistance for the prepared samples. Note that the resistance in LRS for HfOx/Al/HfOx and HfOx/Al samples were larger compared to HfOx:Al sample. This was related to the formation of interfacial layer which can act as a series resistance to LRS value [22], leading to the lower LRS of HfOx:Al sample. For HfOx/Al/HfOx sample, lower resistance in HRS can be observed. It may be due to the much thinner thickness of HfOx film (10 nm) on both sides of Al
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ACCEPTED MANUSCRIPT layer. Thus, the diffusion of Al atoms can occur in HfOx film rather than only at interface like HfOx/Al sample, which enlarged the chemical mismatch of HfOx film and increased the conductivity of the film [23], resulting in the lower HRS of the film
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finally. During the reset process, the larger distributed electric field at interfacial layer or enhanced local electric field by Al atoms may result in the easier rupture of filaments near interface, which was responsible for the abrupt reset process for
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HfOx/Al/HfOx and HfOx/Al samples. Overall, results indicated that Al dopants were
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beneficial to improve the RS performance of HfOx sample, especially uniformity, compared with HfOx samples which exhibited large switching voltages with scattered
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distribution.
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Fig. 4 Current conduction properties for Al-doped HfOx samples.
To explore the switching behaviors of three Al-doped HfOx samples, the I-V
curves were replotted in double-log scales, as presented in Fig. 4. The fitting results for three samples with different structures were similar. For LRS, the current and voltage can be fitted as a straight line, indicating the ohmic characteristics, as shown in the inset of Fig. 4. While for HRS, the current and voltage was found to follow ohmic mechanism at low voltage region, and Child’ law at higher voltage, which can
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ACCEPTED MANUSCRIPT be well explained by the trap-controlled space charge limited current effect [1,24]. According to XPS and electrical analysis, oxygen vacancy acted as a dominant defect
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in HfOx film and was responsible for the switching behavior.
Fig. 5 The schematic diagram of the switching mechanisms for Al-doped HfOx samples.
Based on above analysis, the switching behaviors of Al-doped HfOx samples with
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different structures can be dominantly attributed to the formation and rupture of
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oxygen vacancy filaments in the film. Fig. 5 shows the schematic diagrams of switching mechanism of Al-doped HfOx samples. For HfOx/Al/HfOx sample, as presented in Fig. 5(a), the annealing process already introduced oxygen vacancies in the film. By applying a positive voltage, oxygen ions in the film were easier to migrated towards anode and more oxygen vacancies were generated. The formation of oxygen vacancy chains between electrodes switched the sample from HRS to LRS. It is surmised that Al diffusion may contribute to the switch of resistance due to the
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ACCEPTED MANUSCRIPT thinner film thickness (10 nm), which need further study however. Under the negative voltage, the rupture of these oxygen vacancy filaments by the diffusion of oxygen ions resulted in the HRS of the sample. It was speculated that the filaments at the
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interface may rupture firstly due to the larger distributed electric field at interfacial layer or enhanced local electric field by Al atoms [23]. which was responsible for the observed abruptly reset process. For HfOx/Al sample (Fig. 5b), under the positive
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voltage, the oxygen ions moved towards anode and the Al layer acted as an oxygen
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reservoir, thus leaving oxygen vacancies in HfOx film. With the increase of voltage, the oxygen vacancy chains were formed and assisted the hopping conduction of electrons, switching the sample from LRS to HRS. By applying the negative voltage, the oxygen ions were released from the interfical layer and recovered with oxygen
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vacancies, leading to the rupture of filaments. Also, the easier rupture of filaments due to the enhanced electric field led to the abrupt reset process HfOx/Al samples. The schematic diagram of switching mechanism for HfOx:Al sample was presented in Fig.
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5(c). The uniform doping of Al atoms in HfOx film decreased the formation energy of
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oxygen vacancy near Al dopants and induced more oxygen vacancies in HfOx film. These oxygen vacancies near Al dopants were easier to form more fixed conductive filaments, resulting in the small switching voltages and uniform distribution of switching parameters. By reversing the voltage, the oxygen vacancy filaments ruptured by recovering with oxygen ions. Seen from Fig. 3(a), the voltage fluctuation was only partially suppressed for HfOx:Al sample. The much scattered distribution of reset voltage for HfOx/Al/HfOx and HfOx/Al samples may be related to the abrupt
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reset process.
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Fig. 6 The retention properties of Al-doped HfOx samples with different structures at 85 °C. Read
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at 0.1 V.
Retention property is an important index to evaluate the memory performance. Fig. 6 shows the retention characteristics of HfOx/Al/HfOx, HfOx/Al and HfOx:Al samples measured at 85 °C. For all samples, the currents in HRS and LRS can
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maintain for over 2×103 s without serious degeneration. Although somewhat fluctuations of the currents can be observed with the increasing time, which may be caused by the variation of defects in the film under higher temperature and need
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further investigation, the least ON/OFF ratio larger than 10 was large enough to avoid
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the misreading. The results showed that Al-doped HfOx samples was promising for application in non-volatile memory. 4. Conclusions
In summary, the RS characteristics of HfOx/Al/HfOx, HfOx/Al and HfOx:Al
samples were investigated and the switching behaviors were attributed to the formation and rupture of oxygen vacancy filaments. An interfacial layer was formed at HfOx/Al interface for HfOx/Al/HfOx and HfOx/Al samples by TEM and XPS
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ACCEPTED MANUSCRIPT analysis, and the oxygen vacancies in HfOx film or near interface can be modulated by Al dopants in different ways, leading to the easier formation of conductive filaments. The interfacial layer resulted in the larger switching voltages and
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resistances in LRS compared with HfOx:Al sample with uniform Al dopants and no interface. Besides, the larger distributed electrical field at interfacial layer caused the abrupt reset process of HfOx/Al/HfOx and HfOx/Al samples, which may be
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responsible for the scattered distribution of reset voltage. Although Al dopants were
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introduced into HfOx films by different ways, good RS performance including large ON/OFF ratio, good uniformity and retention properties can be observed for all prepared samples, especially for HfOx:Al sample, indicating that Al dopants were beneficial to improve the RS performance of HfOx sample.
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Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (No. 51202196), the Fundamental Research Funds for the Central
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Universities (No. 3102014JCQ01032), the 111 Project (No. B08040), the Innovation
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Foundation for Doctor Dissertation of Northwestern Polytechnical University (No. CX201612), the Excellent Doctorate Foundation of Northwestern Polytechnical University and the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (No. 155-QP-2016). References [1] Y. C. Yang, F. Pan, Q. Liu, M. Liu, F. Zeng, Nano Lett. 9 (2009) 1636-1643. [2] Y. S. Fan, P. T. Liu, L. F. Teng, C. H. Hsu, Appl. Phys. Lett. 101 (2012) 052901.
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ACCEPTED MANUSCRIPT Highlights: Three structures of HfOx film with Al dpoants were prepared. Oxygen vacancies in the film or at interface were modulated by Al dpoant.
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An interfacial layer can be observed at HfOx/Al interface by XPS and TEM analysis.
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A physical mechanism model based on oxygen vacancy filaments was proposed.
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