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RESET-first unipolar resistance switching behavior in annealed Nb2O5 films
Thin Solid Films 558 (2014) 423–429
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RESET-first unipolar resistance switching behavior in annealed Nb2O5 films Kyumin Lee a, Jonggi Kim a, In-Su Mok a, Heedo Na a, Dae-Hong Ko a, Hyunchul Sohn a,⁎, Sunghoon Lee b, Robert Sinclair c a b c
Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea Research & Development Division, SK hynix Semiconductor Inc., Icheon-si, Gyeng-gi-do 467-701, Republic of Korea Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305-2205, United States
a r t i c l e
i n f o
Article history: Received 15 April 2013 Received in revised form 3 March 2014 Accepted 3 March 2014 Available online 11 March 2014 Keywords: Resistance Switching Niobium dioxide Niobium Pentoxide Unipolar RESET-first Oxygen Annealing Crystalline phase
a b s t r a c t In this work, the effect of thermal annealing on the resistance switching behavior of Nb2O5 films was investigated in conjunction with an analysis of the chemical bonding states and crystal structure. The Nb2O5 films were deposited via reactive sputtering, and annealed via rapid thermal annealing at various temperatures up to 650 °C. The crystal structure of the as-deposited Nb2O5 films transformed from amorphous to a hexagonal Nb2O5 crystalline phase with tetragonal NbO2 following thermal annealing at 500 °C. The conductivity of the Nb2O5 films increased drastically as the annealing temperature increased. An increase in the non-lattice oxygen in the Nb2O5 films was also observed with thermal annealing. Pt/Nb2O5/Pt stacks with the as-deposited Nb2O5 showed typical unipolar resistance switching behaviors after electro-forming; however, the Nb2O5 film devices annealed at 500 °C showed RESET-first resistance switching behavior without prior electro-forming. The RESET-first resistance switching in annealed Nb2O5 is believed to be due to the nano-scale conductive path formed in the annealed Nb2O5 films. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Resistive random access memory (ReRAM) has been widely investigated for use as a future non-volatile memory device because of its low power consumption, fast switching speed, excellent scalability, and compatibility with complementary metal oxide semiconductor fabrication processes [1,2]. ReRAM exhibits bi-stable resistance switching behavior, which is reversible switching between a high-resistance state (HRS) and a low-resistance state (LRS) under an applied electrical bias. Resistance switching from LRS to HRS in this case is referred to as the RESET process; the reverse operation forms the SET process. In general, the electro-forming process is initially required to set the device to low resistance mode, where the metal oxide films go through soft breakdown and conductive paths are formed in transition metal oxides. The requirement of electro-forming with high electric bias and current is considered to be one of the serious issues for ReRAM device operation [3–6], in addition to the large SET voltage distribution and the high current required for SET operation [7–10]. It has been reported that resistance switching in oxides is related to the concentration or generation of cation defects and oxygen vacancies [11–14], as well as
⁎ Corresponding author. E-mail address: [email protected] (H. Sohn).
http://dx.doi.org/10.1016/j.tsf.2014.03.003 0040-6090/© 2014 Elsevier B.V. All rights reserved.
the formation of conductive phases in the insulating oxides such as the magnéli phase, which is based on oxygen-deficient regions [15]. Among the transition metal oxides, niobium oxide draws significant interest for use in resistive switching memory applications due to the various valence states of niobium, forming niobium oxide (NbO), niobium dioxide (NbO2), and niobium penta-oxide (Nb2O5) [16–19]. Niobium oxides can display resistance switching behaviors including unipolar, bipolar, threshold-type, and memristor-type resistance switching [19–23], depending upon composition and choice of electrode metal. In this work, the resistance switching behavior of Nb2O5 films with Pt electrodes was investigated. Particular attention was given to the effect of post-annealing temperature on the resistance switching of Nb2O5 films, in conjunction with the chemical states and changes in the microstructure of the films. We report that the annealed Nb2O5 films show RESET-first unipolar resistance switching behavior without electroforming, while the as-deposited Nb2O5 films require initial electroforming for resistance switching. 2. Experimental conditions Nb2O5-based ReRAM devices with Pt/Nb2O5/Pt structures were fabricated on Si substrates. Bottom electrodes were created by depositing 150 nm-thick Pt films onto TiN/SiO2/Si at room temperature via dc
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magnetron sputtering. 50 nm-thick Nb2O5 films were then deposited at 400 °C with an O2 partial-pressure ratio of 35% via reactive dc magnetron sputtering. Prior to the fabrication of the top electrode, the Nb2O5 films were annealed at various temperatures within the range of 450 °C to 650 °C for 120 s, via rapid thermal annealing under N2 ambient. 100 × 100 μm2 Pt top electrodes were subsequently fabricated via dc sputtering and the lift-off process. The crystallinity and microstructure of the Nb2O5 films were investigated using grazing incidence X-ray diffraction (GI XRD) and highresolution transmission electron microscopy (HRTEM). A 200 kV JEOL JEM 2100 F with a 2 nm point resolution was used for TEM analysis. The composition of the Nb2O5 films was estimated using X-ray photoelectron spectroscopy (XPS). The XPS measurements (ESCALAB 250, Thermo Electron Corporation) were carried out with a take-off angle of 90°, monochromated Al Kα source energy of 1486.6 eV, analysis area of 500 × 500 μm2, pass energy of 50.0 eV, and energy step size of 0.1 eV. For XPS curve fitting, the C 1 s peak (reference peak) is rearranged at 285 eV. And O 1 s and Nb 3d peaks were shifted accordingly. The background signals of XPS spectra were subtracted by the Shirley method. Peaks in XPS spectra were then fitted with Gaussian– Lorentzian function to find the peak positions. The GL% was set to be 98% for all the curves. And the full width half maximum (FWHM) of constituent peaks was kept constant by setting GL% to be constant. For conductive atomic force microscopy (CAFM) measurement (Nanosurf, easyScan 2 AFM), n + silicon tip with Pt/Ir-coating was used and the tip resistivity was 0.01 – 0.02 Ω · cm. The measurement was carried out at a spreading resistance mode in an atmospheric ambient with the image size of 5 × 5 μm2 and the tip force of 55 nN. And the readout voltage of 0.1 V was applied without interruption during one map which took 5 min to measure. The resistance switching behavior of the Pt/Nb2O5/Pt stacks was characterized using a two-probe measurement with an Agilent B1500A semiconductor device analyzer.
(a)
3.1. Chemical composition and non-lattice oxygen in Nb2O5 films The chemical composition of the as-grown NbOx films and thermallyannealed NbOx films was estimated using XPS. Fig. 1(a) and (b) show typical XPS spectra of the Nb 3d and O 1 s states, respectively, in the asgrown NbOx films. The peaks at 209.92 eV and 207.2 eV correspond to the Nb 3 d3/2 and Nb 3 d5/2 states, respectively [24]. The O 1 s peak is resolved into four sub-peaks, corresponding to the oxygen (O2−) on anion lattice sites (530.5 eV), non-lattice oxygen (531.4 eV), and O\H or O\C at 532.1 eV, respectively [25–27]. From the ratio of the O 1 s peak intensity to the Nb 3d peak intensity, the compositions of the asdeposited and thermally-annealed films are estimated to be Nb2O5 + x with x = 0.03–0.15, as summarized in Table 1. It has been reported that the non-lattice oxygen in metal oxides has a significant influence on the conductivity of oxide films [27,28]. Hence, the concentrations of non-lattice oxygen in the Nb2O5 films were estimated using XPS. For the non-lattice oxygen analysis, the O 1 s spectra were de-convoluted into four sub peaks, corresponding to the O\Nb, non-lattice oxygen ion, and the O\H or O\C, where the non-lattice oxygen ion peak site was situated between the O\Nb and O\H or O\C peaks. Fig. 1(c) and (d) show the O 1 s XPS spectra in the Nb2O5 films that were annealed at various temperatures. The estimated concentrations of the non-lattice oxygen ions are summarized in Table 1. As shown in Fig. 2, the concentration of the non-lattice oxygen increased as the annealing temperature increased. 3.2. The crystallinity of Nb2O5 films The crystal structure of the as-grown and thermally-annealed Nb2O5 films was investigated using grazing incidence X-ray diffraction (GI-XRD). Fig. 3 shows the GI-XRD pattern of the Nb2O5 films annealed
(b) Nb
5+
1.4 As-grown O 1 s
3 d 5/2
XPS intensity (counts/s)
1.4 As-grown Nb 3 d
XPS intensity (counts/s)
3. Results
1.2 5+
Nb 3 d 3/2
1.0 0.8 0.6 0.4 0.2 0.0 212
210
208
1.0 0.8 0.6 0.4 0.2
O-H or O-C
0.0
207.2eV
209.92eV
O-Nb
1.2
206
534
Binding energy (eV)
(c)
(d) o
o
1.4 RTA 600 C O 1 s O-Nb
1.2 1.0 0.8 0.6 0.4 O-H or O-C
0.0 534
Non lattice oxygen 532 530 528
Binding energy (eV)
XPS intensity (counts/s)
XPS intensity (counts /s)
Non lattice oxygen 530 528
Binding energy (eV)
1.4 RTA 500 C O 1 s
0.2
532
O-Nb
1.2 1.0 0.8 0.6 0.4 0.2
O-H or O-C
0.0 534
532
Non lattice oxygen 530 528
Binding energy (eV)
Fig. 1. XPS spectra obtained from (a) Nb 3d of Nb2O5 deposited at 400 °C, (b) O 1 s of Nb2O5 deposited at 400 °C, (c) O 1 s of Nb2O5 annealed at 500 °C in N2 ambient, and (d) O 1 s of Nb2O5 annealed at 600 °C in N2 ambient.
K. Lee et al. / Thin Solid Films 558 (2014) 423–429 Table 1 The estimated composition of Nb2O5 + x and concentrations of the non-lattice oxygen determined from the XPS spectra of the Nb2O5 + x thin films on Pt electrodes. Specimen type
x in Nb2O5 + x
Percentage of non-lattice oxygen from the O 1 s peak (%)
As-grown RTN 450 °C RTN 500 °C RTN 550 °C RTN 600 °C RTN 650 °C
0.11 0.11 0.11 0.15 0.03 0.05
4.42 8.37 10.96 11.97 12.20 12.99
425
H,T
T
650 oC 600 oC 550 oC
3.3. Resistance switching characteristics of Nb2O5 films In order to study the effect of thermal annealing on the conductivity of Nb2O5 films, the current-voltage (IV) characteristics of Nb2O5 films annealed at various temperatures under N2 ambient were measured at room temperature, up to 0.5 V, where no electro-forming occurred. The conductivities of Nb2O5 films increased drastically as the annealing temperature increased, as shown in Fig. 5.
Non-lattice oxygen (%)
15
12
9
6
3
0
As RTA RTA RTA RTA RTA grown 450 oC 500 oC 550 oC 600 oC 650 oC Fig. 2. The graph of non-lattice oxygen percentage as a function of annealing temperature.
(102) (111) (200)
(110)
(101) (440)
(100)
H-Nb 2 O5
(220)
(111)
T-NbO 2 (200)
(001)
As-grown
(400)
at various temperatures. Neither the as-grown Nb2O5 film nor the Nb2O5 film annealed at 500 °C showed any crystalline peaks, indicating an amorphous structure in the Nb2O5. The Nb2O5 films annealed at 550 °C and above showed the crystalline peaks characteristic of hexagonal Nb2O5 (H-Nb2O5) phases, with a trace peak corresponding to tetragonal NbO2 (T- NbO2). This implies that the annealed Nb2O5 films were mostly hexagonal in structure. Since the GI-XRD pattern showed a trace T- NbO2 peak, the existence of T- NbO2 phases was investigated using a High-Resolution Transmission Electron Microscope (HRTEM). Fig. 4 shows the HRTEM images of the Nb2O5 films, as well as the diffractogram obtained from the Fast Fourier transformation (FFT) of the HRTEM images. The HRTEM image of the as-grown Nb2O5 film shows an amorphous structure, as expected from the GI-XRD pattern. The HR-TEM image of the Nb2O5 film annealed at 500 °C, however, exhibits crystalline phases in the amorphous matrix, though no crystalline peaks were observed in the XRD pattern, possibly due to the weak-broad reflection of the nano-crystalline grains in an overall amorphous matrix [29]. The Nb2O5 film annealed at 600 °C was observed to be fully crystallized. In the diffractogram transformed from the HR-TEM image of the films annealed at 500 °C and 600 °C, diffraction spots from the H-Nb2O5 and T- NbO2 phases were observed. It was concluded from the GI-XRD and HR-TEM results that the hexagonal Nb2O5 (H-Nb2O5) and tetragonal NbO2 (T- NbO2) phases co-existed in the Nb2O5 films annealed at 500 °C and above. The annealed Nb2O5 films are composed primarily of the H-Nb2O5 phase, with a small amount of T- NbO2.
Intensity(arb.unit)
500 oC
C-Pt 20
30
40
50
60
70
80
2θ(o) Fig. 3. XRD pattern of the Nb2O5 thin films annealed at various temperatures in N2 ambient: hexagonal Nb2O5, tetragonal NbO2, and cubic Pt.
Electro-forming was carried out first for the as-grown Nb2O5 films to investigate resistance switching. For the electro-forming, the Nb2O5 films were exposed to a dc bias above + 5 V on the top Pt electrode, with a current compliance of 10 mA, as shown in an inset of Fig. 6(a). Subsequently, the resistance switching characteristics of the Pt/Nb2O5/Pt metal–insulator–metal (MIM) stacks were investigated with voltage sweeping from 0 V to 3 V, and a current compliance of 100 mA. Measurements showed that a sudden low-resistance state (LRS) to highresistance state (HRS) transition occurred, in addition to a corresponding reduction in current near 0.75 V (RESET), as shown in Fig. 6(a). Upon further increase of the applied external bias, the resistance switched abruptly to the low resistance state (LRS) and high electric current near 2 V, with a current compliance of 10 mA (SET), as shown in Fig. 6(a). The unipolar-type resistance switching behavior exhibited by the as-grown Nb2O5 films was considered similar to studies of Nb2O5 films reported by J. Bae et al. [20] and K. Jung et al. [21]. The Nb2O5 films annealed at 500 °C and 550 °C showed significantly different resistance switching behaviors in comparison to the as-grown Nb2O5 films. The Nb2O5 films annealed at 500 °C and 550 °C showed RESET-first resistance switching, where the resistance was changed from the LRS to the HRS without the requirement of initial electroforming, as shown in Fig. 6(b). In contrast, the Nb2O5 film annealed at 600 °C displayed high electric conductivity without any resistance switching, irrespective of the applied voltage. Since electro-forming is known to produce local conductive paths in insulating oxides, it is expected that the RESET-first behavior of the Nb2O5 films annealed at 500 °C and 550 °C may be attributed to conductive paths possibly formed during thermal annealing. The endurance of resistance switching, the retention of a resistance state, and programming time are of crucial importance for the memory device application. Hence, the effect of annealing temperature on the
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Fig. 4. TEM and FFT images of cross-sections of the Nb2O5 thin films: (a) Nb2O5 deposited at 400 °C, (b) Nb2O5 annealed at 500 °C in N2 ambient, and (c) Nb2O5 annealed at 600 °C in N2 ambient (square and circle: Nb2O5 index, triangle: NbO2 index, pentagram: transmitted beam).
endurance and retention test of Nb2O5 films with a read voltage of 0.1 V were also investigated. For endurance test, DC sweeps were carried out on the MIM stacks up to 1000 times using the same bias condition described in the experiment for the resistance switching measurement. Fig. 7 shows the endurance of resistance switching for Pt/Nb2O5/Pt as a function of cycle. MIM devices with Nb2O5 annealed at 500 °C and 550 °C showed no degradation during endurance 1024 and 694, respectively but the device with as-grown Nb2O5 films failed at 101 cycles. The variance in RHRS and RLRS resistance of the annealed at 500 °C was observed to be the lowest value (3σHRS: 1.69, 3σLRS: 0.23). For the retention test, firstly the device was set to the SET state or the RESET state by applying a dc voltage. Then the resistance of a stack was measured every 10 s at a continuous readout voltage of 100 mV. Fig. 8 shows the retention of Pt/Nb2O5/Pt devices. The resistances for LRS and HRS for the Nb2O5 films did not show any significant degradation over 3.6 × 103 s at 85 °C, displaying a stable retention of resistance states. Moreover, an extrapolation showed that the resistances for LRS and HRS were stable for periods up to 106 s at 85 °C. 3.4. The distribution of conductive paths in annealed Nb2O5 films The spatial variation in the conductivity of the as-grown and annealed Nb2O5 films was investigated by probing the Nb2O5 films on the Pt bottom electrodes, using a W-probe as the top electrode. 100 RTN 650 oC RTN 600 oC RTN 550 ooC RTN 500 C
Current(A)
10-2 10-4 10-6
RTN 450 oC
10-8 As-grown
10-10 10-12 0.0
0.1
0.2
0.3
0.4
0.5
0.6
Voltage(V) Fig. 5. Initial current before the forming process of the Nb2O5 thin films annealed at various temperatures in N2 ambient.
Resistance was estimated from the current at a read voltage of 0.2 V, which was low enough to prevent resistance switching in the Nb2O5 films. Fig. 9 shows the resistance of 25 sites, measured every 25 μm along the x and y directions on 200 × 200 μm2 Nb2O5 films. The as-grown Nb2O5 films showed high resistivity at all 25 sites; however, some high-resistivity sites became highly conductive when the films were annealed. The number of conductive sites increased as the annealing temperature increased. Nb2O5 films annealed at 600 °C showed high conductivity at a majority of the tested positions, indicating that the conductivities in the annealed Nb2O5 films were non-homogeneous, but had a strong local variation in the films, as shown in Fig. 9. The distribution and scale of the conductive paths in the Nb2O5 films was investigated using CAFM. All the CAFM measurements were carried out without electroforming on samples. The as-grown Nb2O5 films showed uniform high resistivity over the entire scanned area, while the annealed Nb2O5 films showed significant local variation in current, implying a large local variation in conductivity, as shown in Fig. 10. The size of the conductive region was estimated to be on the order of a few tens of a nanometer, and the density of the conductive regions was observed to increase with annealing temperature. 4. Discussion The nature of such local conductive paths is not clearly understood yet. But the conductive region is considered closely related to the increased concentration of non-lattice oxygen and the NbO2 phase formation in the polycrystalline nature of the Nb2O5 films after thermal annealing. High electric conductivity was reported in the NbO2 phase when compared to the Nb2O5 phase with insulating property [16–18,30]. Given the data observed in the GI-XRD and HRTEM imagery, the NbO2 phase was expected to take a nano-crystalline shape and continuous NbO2 phases were not observed in annealed Nb2O5 films from TEM work. It was reported that non-lattice oxygen ions exist in transition metal oxides containing cations with multiple valence states, and are associated with O2− ions in oxygen-deficient regions [26–28]. It was reported that oxygen vacancies play important roles in the formation of the local conductive path, and non-lattice oxygen ions were closely related to oxygen vacancies [27]. Hence, the increase of oxygen vacancies will influence the RESET-first resistance switching behavior of the Nb2O5
K. Lee et al. / Thin Solid Films 558 (2014) 423–429
(a) 100
(b) 10
427
0
(2) Reset
10 (3) Set
Current(A)
Current(A)
10 -2 10 -4 10 10
10 -6
10 10
10 -8
10 10
10 -10 0
(1)
10 10
Forming Process 2 4
0
1
10
~5V
2
6
3
10
(1) Reset first
-2
(2) Set
-4
-6
-8
-10
0
1
Voltage(V)
2
3
Voltage(V)
Fig. 6. Resistance switching behaviors of the Pt/Nb2O5 (50 nm)/Pt MIM stacks with (a) Nb2O5 deposited at 400 °C with a forming process, (b) Nb2O5 annealed at 500 °C and 550 °C in N2 ambient without a forming process.
films post-annealed at 500 °C and 550 °C. It was expected that the increased non-lattice oxygen in the annealed Nb2O5 films might have led to increased conductivity in the Nb2O5 films. The formation of the local conductive paths in the Nb2O5 films could not be explained by the non-lattice oxygen ions alone since the distribution of the non-lattice oxygen is expected to be homogeneous in the annealed Nb2O5 films or the NbO2 phase formed in the thermally
(a)
10 12 10
101(As-grown)
LRS HRS
10
annealed Nb2O5 films since NbO2 phases were isolated from each other. From TEM and XRD, the local conductive paths were possibly attributed to the conducting NbO2 phases connected by leaky Nb2O5 phases with high concentration of non-lattice oxygen. Also, the grain boundaries, defective in nature, are known to highly conductive and play important roles in resistance switching [31–36]. High resolution TEM work showed that the grain size was 4 nm – 8 nm in Nb2O5 films annealed at 500 °C and was increased to 4 – 18 nm by
(a) 100M
As-grown Nb 2 O5 RHRS
10 8
RLRS
1M
10 6 10k
10 4 10 2 10
100
0
0
50
100
1
Cycles (#)
1
10
100
1k
10k
100k
1M
Time (seconds)
(b)
10
1024 ( N 2 500 oC)
LRS HRS
10 12
(b) 1M 10k
10 2
100
0
0
200
400
600
800
1000
1
Cycles (#)
(c) 10 12 10
RHRS
6
10 4
10
N 2 500 C Nb2 O5
RLRS
10 8 10
o
100M
10
10
100
1k
10k
100k
1M
Time (seconds)
694 ( N 2 550 oC)
LRS HRS
1
(c)
o
100M
10
10 8
RHRS
N 2 550 C Nb2 O5
RLRS
1M
10 6 10k
10 4 10 2 10
100
0
0
100
200
300
400
500
600
700
Cycles (#)
1
1
10
100
1k
10k
100k
1M
Time (seconds) Fig. 7. The endurance of resistance switching for Pt/Nb2O5/Pt as a function of cycle (a) Nb2O5 deposited at 400 °C, (b) Nb2O5 annealed at 500 °C in N2 ambient, and (c) Nb2O5 annealed at 550 °C in N2 ambient.
Fig. 8. The retention of Pt/Nb2O5/Pt devices (a) Nb2O5 deposited at 400 °C, (b) Nb2O5 annealed at 500 °C in N2 ambient, and (c) Nb2O5 annealed at 550 °C in N2 ambient.
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(a)
(b) Initial resistance
4 3 2
5
Y-Position
Y-Position
5
1
Initial resistance
4 3 2 1
1
2
3
4
5
1
X-Position
3
4
5
X-Position
(d) 5
Initial resistance
4 3 2 1
5
Y-Position
(c) Y-Position
2
Initial resistance
4 3 2 1
1
2
3
4
5
X-Position
1
2
3
4
5
X-Position
Fig. 9. Distribution of the initial resistance states before forming process for each sample: (a) Nb2O5 deposited at 400 °C, (b) Nb2O5 annealed at 500 °C in N2 ambient, (c) Nb2O5 annealed at 550 °C in N2 ambient, and (d) Nb2O5 annealed at 600 °C in N2 ambient.
the annealing at 550 °C (data not shown). The increased non-lattice oxygen after thermal annealing above 500 °C was expected to cause the conductivity along grain boundary to increase, possibly resulting in conductive paths along grain boundaries and the RESET-first resistance switching behavior. According to the work of Kim et al. [37] and Chang et al. [38] on the local heating during forming process, the local temperature around filaments during forming could be higher than ~ 530 °C (~ 800 K) by the Joule heating. Therefore, it is expected that there would be strong correlation between the observation of RESET-first behavior of annealed Nb2O5 films and the filament formation during forming process. As observed in thermally annealed Nb2O5 films, the formation of conducting filaments could be attributed to the significant increase of defects such as non-lattice oxygen ions and secondary phases such as NbO2 phases during forming of Nb2O5 films. Also, filament formation could be enhanced during forming by the high electrical process, causing the directional move of defects related with non-lattice oxygen ions.
5. Conclusion In this work, we investigated the effects of post-annealing on the chemical bonding states, crystallinity, and resistance switching behavior of Nb2O5 films. While the as-grown Nb2O5 films required an initial electro-forming process for resistance switching, Nb2O5 films annealed at 500 °C and 550 °C under N2 ambient showed RESET resistance switching without the initial electro-forming step. Post-annealing over 500 °C caused a change in microstructure from an amorphous to a crystalline structure, an increase in electrical conductivity, and an increase in the concentration of non-lattice oxygen ions in the annealed Nb2O5 films. RESET-first resistance switching was believed to be induced by the formation of local conductive paths during thermal annealing over 500 °C. The conductive paths were considered to be closely related to the increased concentration of non-lattice oxygen and the NbO2 phase formation in Nb2O5 films with the polycrystalline nature of the Nb2O5 films after thermal annealing.
Fig. 10. Spreading resistance at 0.1 V of the Nb2O5 thin films (a) as-deposited and (b) annealed at 500 °C in N2 ambient.
K. Lee et al. / Thin Solid Films 558 (2014) 423–429
Acknowledgment This work was supported by the industry–university cooperation project of Samsung Electronics and by the second stage of the Brain Korea 21 project (BK21). References [1] H. Xie, Q. Liu, Y. Li, H. Lv, M. Wang, K. Zhang, S. Long, S. Liu, M. Liu, Effect of low constant current stress treatment on the performance of the Cu/ZrO2/Pt resistive switching device, Semicond. Sci. Technol. 27 (2012) 105007. [2] A. Younis, D. Chu, S. Li, Oxygen level: the dominant of resistive switching characteristics in cerium oxide thin films, J. Phys. D. Appl. Phys. 45 (2012) 355101. [3] J.J. Yang, F. Miao, M.D. Pickett, D.A.A. Ohlberg, D.R. Stewart, C.N. Lau, R.S. Williams, The mechanism of electroforming of metal oxide memristive switches, Nanotechnology 20 (2009) 215201. [4] Z. Fang, H.Y. Yu, X. Li, N. Singh, G.Q. Lo, D.L. Kwong, HfOx/TiOx/HfOx/TiOx multilayerbased forming-free RRAM devices with excellent uniformity, IEEE Electron Device Lett. 32 (2011) 566. [5] W. Banerjee, Sk.Z. Rahaman, S. Maikap, Excellent uniformity and multilevel operation in formation-free low power resistive switching memory using IrOx/AlOx/W cross-point, Jpn. J. Appl. Phys. 51 (2012) 04DD10. [6] D. Jeong, H. Schroeder, U. Breuer, R. Waser, Characteristic electroforming behavior in Pt/TiO2/Pt resistive switching cells depending on atmosphere, J. Appl. Phys. 104 (2008) 123716. [7] K.C. Ryoo, S. Kim, J.H. Oh, S. Jung, H. Jeong, B.G. Park, Novel protruded-shape unipolar resistive random access memory structure for improving switching uniformity through excellent conductive filament controllability, Jpn. J. Appl. Phys. 51 (2012) 6FE06. [8] W. Lee, J. Park, S. Kim, J. Woo, J. Shin, D. Lee, E. Cha, H. Hwang, Improved switching uniformity in resistive random access memory containing metal-doped electrolyte due to thermally agglomerated metallic filaments, Appl. Phys. Lett. 100 (2012) 142106. [9] K.C. Ryoo, J.H. Oh, S. Jung, B.G. Park, Novel U-shape resistive random access memory structure for improving resistive switching characteristics, Jpn. J. Appl. Phys. 50 (2012) 04DD15. [10] J.J. Huang, T.H. Hou, C.W. Hsu, Y.M. Tseng, W.H. Chang, W.Y. Jang, C.H. Lin, Flexible one diode–one resistor crossbar resistive-switching memory, Jpn. J. Appl. Phys. 51 (2012) 04DD09. [11] S. Zhang, S. Long, W. Guan, Q. Liu, Q. Wang, M. Liu, Resistive switching characteristics of MnOx-based ReRAM, J. Phys. D. Appl. Phys. 42 (2009) 055112. [12] X. Cao, X.M. Li, X.D. Gao, Y.W. Zhang, X.J. Liu, Q. Wang, L.D. Chen, Effects of the compliance current on the resistive switching behavior of TiO2 thin films, Appl. Phys. A 97 (2009) 883. [13] X. Cartoixà, R. Rurali, J. Suňé, Transport properties of oxygen vacancy filaments in metal/crystalline or amorphous HfO2/metal structures, Phys. Rev. B 86 (2012) 165445. [14] T. Bertaud, M. Sowinska, D. Walczyk, S. Thiess, A. Gloskovskii, C. Walczyk, T. Schroeder, In-operando and non-destructive analysis of the resistive switching in the Ti/HfO2/TiN-based system by hard X-ray photoelectron spectroscopy, Appl. Phys. Lett. 101 (2012) 143501. [15] D.H. Kwon, K.M. Kim, J.H. Jang, J.M. Jeon, M.H. Lee, G.H. Kim, X.S. Li, G.S. Park, B. Lee, S. Han, M. Kim, C.S. Hwang, Atomic structure of conducting nanofilaments in TiO2 resistive switching memory, Nat. Nanotechnol. 5 (2010) 148. [16] W. Gao, J.F. Conley Jr., Y. Ono, NbO as gate electrode for n-channel metal-oxidesemiconductor field-effect-transistors, Appl. Phys. Lett. 84 (2004) 4666. [17] Y. Zhao, Z. Zhang, Y. Lin, Optical and dielectric properties of a nanostructured NbO2 thin film prepared by thermal oxidation, J. Phys. D. Appl. Phys. 37 (2004) 3392.
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RESET-first unipolar resistance switching behavior in annealed Nb2O5 films Kyumin Lee a, Jonggi Kim a, In-Su Mok a, Heedo Na a, Dae-Hong Ko a, Hyunchul Sohn a,⁎, Sunghoon Lee b, Robert Sinclair c a b c
Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea Research & Development Division, SK hynix Semiconductor Inc., Icheon-si, Gyeng-gi-do 467-701, Republic of Korea Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305-2205, United States
a r t i c l e
i n f o
Article history: Received 15 April 2013 Received in revised form 3 March 2014 Accepted 3 March 2014 Available online 11 March 2014 Keywords: Resistance Switching Niobium dioxide Niobium Pentoxide Unipolar RESET-first Oxygen Annealing Crystalline phase
a b s t r a c t In this work, the effect of thermal annealing on the resistance switching behavior of Nb2O5 films was investigated in conjunction with an analysis of the chemical bonding states and crystal structure. The Nb2O5 films were deposited via reactive sputtering, and annealed via rapid thermal annealing at various temperatures up to 650 °C. The crystal structure of the as-deposited Nb2O5 films transformed from amorphous to a hexagonal Nb2O5 crystalline phase with tetragonal NbO2 following thermal annealing at 500 °C. The conductivity of the Nb2O5 films increased drastically as the annealing temperature increased. An increase in the non-lattice oxygen in the Nb2O5 films was also observed with thermal annealing. Pt/Nb2O5/Pt stacks with the as-deposited Nb2O5 showed typical unipolar resistance switching behaviors after electro-forming; however, the Nb2O5 film devices annealed at 500 °C showed RESET-first resistance switching behavior without prior electro-forming. The RESET-first resistance switching in annealed Nb2O5 is believed to be due to the nano-scale conductive path formed in the annealed Nb2O5 films. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Resistive random access memory (ReRAM) has been widely investigated for use as a future non-volatile memory device because of its low power consumption, fast switching speed, excellent scalability, and compatibility with complementary metal oxide semiconductor fabrication processes [1,2]. ReRAM exhibits bi-stable resistance switching behavior, which is reversible switching between a high-resistance state (HRS) and a low-resistance state (LRS) under an applied electrical bias. Resistance switching from LRS to HRS in this case is referred to as the RESET process; the reverse operation forms the SET process. In general, the electro-forming process is initially required to set the device to low resistance mode, where the metal oxide films go through soft breakdown and conductive paths are formed in transition metal oxides. The requirement of electro-forming with high electric bias and current is considered to be one of the serious issues for ReRAM device operation [3–6], in addition to the large SET voltage distribution and the high current required for SET operation [7–10]. It has been reported that resistance switching in oxides is related to the concentration or generation of cation defects and oxygen vacancies [11–14], as well as
⁎ Corresponding author. E-mail address: [email protected] (H. Sohn).
http://dx.doi.org/10.1016/j.tsf.2014.03.003 0040-6090/© 2014 Elsevier B.V. All rights reserved.
the formation of conductive phases in the insulating oxides such as the magnéli phase, which is based on oxygen-deficient regions [15]. Among the transition metal oxides, niobium oxide draws significant interest for use in resistive switching memory applications due to the various valence states of niobium, forming niobium oxide (NbO), niobium dioxide (NbO2), and niobium penta-oxide (Nb2O5) [16–19]. Niobium oxides can display resistance switching behaviors including unipolar, bipolar, threshold-type, and memristor-type resistance switching [19–23], depending upon composition and choice of electrode metal. In this work, the resistance switching behavior of Nb2O5 films with Pt electrodes was investigated. Particular attention was given to the effect of post-annealing temperature on the resistance switching of Nb2O5 films, in conjunction with the chemical states and changes in the microstructure of the films. We report that the annealed Nb2O5 films show RESET-first unipolar resistance switching behavior without electroforming, while the as-deposited Nb2O5 films require initial electroforming for resistance switching. 2. Experimental conditions Nb2O5-based ReRAM devices with Pt/Nb2O5/Pt structures were fabricated on Si substrates. Bottom electrodes were created by depositing 150 nm-thick Pt films onto TiN/SiO2/Si at room temperature via dc
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magnetron sputtering. 50 nm-thick Nb2O5 films were then deposited at 400 °C with an O2 partial-pressure ratio of 35% via reactive dc magnetron sputtering. Prior to the fabrication of the top electrode, the Nb2O5 films were annealed at various temperatures within the range of 450 °C to 650 °C for 120 s, via rapid thermal annealing under N2 ambient. 100 × 100 μm2 Pt top electrodes were subsequently fabricated via dc sputtering and the lift-off process. The crystallinity and microstructure of the Nb2O5 films were investigated using grazing incidence X-ray diffraction (GI XRD) and highresolution transmission electron microscopy (HRTEM). A 200 kV JEOL JEM 2100 F with a 2 nm point resolution was used for TEM analysis. The composition of the Nb2O5 films was estimated using X-ray photoelectron spectroscopy (XPS). The XPS measurements (ESCALAB 250, Thermo Electron Corporation) were carried out with a take-off angle of 90°, monochromated Al Kα source energy of 1486.6 eV, analysis area of 500 × 500 μm2, pass energy of 50.0 eV, and energy step size of 0.1 eV. For XPS curve fitting, the C 1 s peak (reference peak) is rearranged at 285 eV. And O 1 s and Nb 3d peaks were shifted accordingly. The background signals of XPS spectra were subtracted by the Shirley method. Peaks in XPS spectra were then fitted with Gaussian– Lorentzian function to find the peak positions. The GL% was set to be 98% for all the curves. And the full width half maximum (FWHM) of constituent peaks was kept constant by setting GL% to be constant. For conductive atomic force microscopy (CAFM) measurement (Nanosurf, easyScan 2 AFM), n + silicon tip with Pt/Ir-coating was used and the tip resistivity was 0.01 – 0.02 Ω · cm. The measurement was carried out at a spreading resistance mode in an atmospheric ambient with the image size of 5 × 5 μm2 and the tip force of 55 nN. And the readout voltage of 0.1 V was applied without interruption during one map which took 5 min to measure. The resistance switching behavior of the Pt/Nb2O5/Pt stacks was characterized using a two-probe measurement with an Agilent B1500A semiconductor device analyzer.
(a)
3.1. Chemical composition and non-lattice oxygen in Nb2O5 films The chemical composition of the as-grown NbOx films and thermallyannealed NbOx films was estimated using XPS. Fig. 1(a) and (b) show typical XPS spectra of the Nb 3d and O 1 s states, respectively, in the asgrown NbOx films. The peaks at 209.92 eV and 207.2 eV correspond to the Nb 3 d3/2 and Nb 3 d5/2 states, respectively [24]. The O 1 s peak is resolved into four sub-peaks, corresponding to the oxygen (O2−) on anion lattice sites (530.5 eV), non-lattice oxygen (531.4 eV), and O\H or O\C at 532.1 eV, respectively [25–27]. From the ratio of the O 1 s peak intensity to the Nb 3d peak intensity, the compositions of the asdeposited and thermally-annealed films are estimated to be Nb2O5 + x with x = 0.03–0.15, as summarized in Table 1. It has been reported that the non-lattice oxygen in metal oxides has a significant influence on the conductivity of oxide films [27,28]. Hence, the concentrations of non-lattice oxygen in the Nb2O5 films were estimated using XPS. For the non-lattice oxygen analysis, the O 1 s spectra were de-convoluted into four sub peaks, corresponding to the O\Nb, non-lattice oxygen ion, and the O\H or O\C, where the non-lattice oxygen ion peak site was situated between the O\Nb and O\H or O\C peaks. Fig. 1(c) and (d) show the O 1 s XPS spectra in the Nb2O5 films that were annealed at various temperatures. The estimated concentrations of the non-lattice oxygen ions are summarized in Table 1. As shown in Fig. 2, the concentration of the non-lattice oxygen increased as the annealing temperature increased. 3.2. The crystallinity of Nb2O5 films The crystal structure of the as-grown and thermally-annealed Nb2O5 films was investigated using grazing incidence X-ray diffraction (GI-XRD). Fig. 3 shows the GI-XRD pattern of the Nb2O5 films annealed
(b) Nb
5+
1.4 As-grown O 1 s
3 d 5/2
XPS intensity (counts/s)
1.4 As-grown Nb 3 d
XPS intensity (counts/s)
3. Results
1.2 5+
Nb 3 d 3/2
1.0 0.8 0.6 0.4 0.2 0.0 212
210
208
1.0 0.8 0.6 0.4 0.2
O-H or O-C
0.0
207.2eV
209.92eV
O-Nb
1.2
206
534
Binding energy (eV)
(c)
(d) o
o
1.4 RTA 600 C O 1 s O-Nb
1.2 1.0 0.8 0.6 0.4 O-H or O-C
0.0 534
Non lattice oxygen 532 530 528
Binding energy (eV)
XPS intensity (counts/s)
XPS intensity (counts /s)
Non lattice oxygen 530 528
Binding energy (eV)
1.4 RTA 500 C O 1 s
0.2
532
O-Nb
1.2 1.0 0.8 0.6 0.4 0.2
O-H or O-C
0.0 534
532
Non lattice oxygen 530 528
Binding energy (eV)
Fig. 1. XPS spectra obtained from (a) Nb 3d of Nb2O5 deposited at 400 °C, (b) O 1 s of Nb2O5 deposited at 400 °C, (c) O 1 s of Nb2O5 annealed at 500 °C in N2 ambient, and (d) O 1 s of Nb2O5 annealed at 600 °C in N2 ambient.
K. Lee et al. / Thin Solid Films 558 (2014) 423–429 Table 1 The estimated composition of Nb2O5 + x and concentrations of the non-lattice oxygen determined from the XPS spectra of the Nb2O5 + x thin films on Pt electrodes. Specimen type
x in Nb2O5 + x
Percentage of non-lattice oxygen from the O 1 s peak (%)
As-grown RTN 450 °C RTN 500 °C RTN 550 °C RTN 600 °C RTN 650 °C
0.11 0.11 0.11 0.15 0.03 0.05
4.42 8.37 10.96 11.97 12.20 12.99
425
H,T
T
650 oC 600 oC 550 oC
3.3. Resistance switching characteristics of Nb2O5 films In order to study the effect of thermal annealing on the conductivity of Nb2O5 films, the current-voltage (IV) characteristics of Nb2O5 films annealed at various temperatures under N2 ambient were measured at room temperature, up to 0.5 V, where no electro-forming occurred. The conductivities of Nb2O5 films increased drastically as the annealing temperature increased, as shown in Fig. 5.
Non-lattice oxygen (%)
15
12
9
6
3
0
As RTA RTA RTA RTA RTA grown 450 oC 500 oC 550 oC 600 oC 650 oC Fig. 2. The graph of non-lattice oxygen percentage as a function of annealing temperature.
(102) (111) (200)
(110)
(101) (440)
(100)
H-Nb 2 O5
(220)
(111)
T-NbO 2 (200)
(001)
As-grown
(400)
at various temperatures. Neither the as-grown Nb2O5 film nor the Nb2O5 film annealed at 500 °C showed any crystalline peaks, indicating an amorphous structure in the Nb2O5. The Nb2O5 films annealed at 550 °C and above showed the crystalline peaks characteristic of hexagonal Nb2O5 (H-Nb2O5) phases, with a trace peak corresponding to tetragonal NbO2 (T- NbO2). This implies that the annealed Nb2O5 films were mostly hexagonal in structure. Since the GI-XRD pattern showed a trace T- NbO2 peak, the existence of T- NbO2 phases was investigated using a High-Resolution Transmission Electron Microscope (HRTEM). Fig. 4 shows the HRTEM images of the Nb2O5 films, as well as the diffractogram obtained from the Fast Fourier transformation (FFT) of the HRTEM images. The HRTEM image of the as-grown Nb2O5 film shows an amorphous structure, as expected from the GI-XRD pattern. The HR-TEM image of the Nb2O5 film annealed at 500 °C, however, exhibits crystalline phases in the amorphous matrix, though no crystalline peaks were observed in the XRD pattern, possibly due to the weak-broad reflection of the nano-crystalline grains in an overall amorphous matrix [29]. The Nb2O5 film annealed at 600 °C was observed to be fully crystallized. In the diffractogram transformed from the HR-TEM image of the films annealed at 500 °C and 600 °C, diffraction spots from the H-Nb2O5 and T- NbO2 phases were observed. It was concluded from the GI-XRD and HR-TEM results that the hexagonal Nb2O5 (H-Nb2O5) and tetragonal NbO2 (T- NbO2) phases co-existed in the Nb2O5 films annealed at 500 °C and above. The annealed Nb2O5 films are composed primarily of the H-Nb2O5 phase, with a small amount of T- NbO2.
Intensity(arb.unit)
500 oC
C-Pt 20
30
40
50
60
70
80
2θ(o) Fig. 3. XRD pattern of the Nb2O5 thin films annealed at various temperatures in N2 ambient: hexagonal Nb2O5, tetragonal NbO2, and cubic Pt.
Electro-forming was carried out first for the as-grown Nb2O5 films to investigate resistance switching. For the electro-forming, the Nb2O5 films were exposed to a dc bias above + 5 V on the top Pt electrode, with a current compliance of 10 mA, as shown in an inset of Fig. 6(a). Subsequently, the resistance switching characteristics of the Pt/Nb2O5/Pt metal–insulator–metal (MIM) stacks were investigated with voltage sweeping from 0 V to 3 V, and a current compliance of 100 mA. Measurements showed that a sudden low-resistance state (LRS) to highresistance state (HRS) transition occurred, in addition to a corresponding reduction in current near 0.75 V (RESET), as shown in Fig. 6(a). Upon further increase of the applied external bias, the resistance switched abruptly to the low resistance state (LRS) and high electric current near 2 V, with a current compliance of 10 mA (SET), as shown in Fig. 6(a). The unipolar-type resistance switching behavior exhibited by the as-grown Nb2O5 films was considered similar to studies of Nb2O5 films reported by J. Bae et al. [20] and K. Jung et al. [21]. The Nb2O5 films annealed at 500 °C and 550 °C showed significantly different resistance switching behaviors in comparison to the as-grown Nb2O5 films. The Nb2O5 films annealed at 500 °C and 550 °C showed RESET-first resistance switching, where the resistance was changed from the LRS to the HRS without the requirement of initial electroforming, as shown in Fig. 6(b). In contrast, the Nb2O5 film annealed at 600 °C displayed high electric conductivity without any resistance switching, irrespective of the applied voltage. Since electro-forming is known to produce local conductive paths in insulating oxides, it is expected that the RESET-first behavior of the Nb2O5 films annealed at 500 °C and 550 °C may be attributed to conductive paths possibly formed during thermal annealing. The endurance of resistance switching, the retention of a resistance state, and programming time are of crucial importance for the memory device application. Hence, the effect of annealing temperature on the
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Fig. 4. TEM and FFT images of cross-sections of the Nb2O5 thin films: (a) Nb2O5 deposited at 400 °C, (b) Nb2O5 annealed at 500 °C in N2 ambient, and (c) Nb2O5 annealed at 600 °C in N2 ambient (square and circle: Nb2O5 index, triangle: NbO2 index, pentagram: transmitted beam).
endurance and retention test of Nb2O5 films with a read voltage of 0.1 V were also investigated. For endurance test, DC sweeps were carried out on the MIM stacks up to 1000 times using the same bias condition described in the experiment for the resistance switching measurement. Fig. 7 shows the endurance of resistance switching for Pt/Nb2O5/Pt as a function of cycle. MIM devices with Nb2O5 annealed at 500 °C and 550 °C showed no degradation during endurance 1024 and 694, respectively but the device with as-grown Nb2O5 films failed at 101 cycles. The variance in RHRS and RLRS resistance of the annealed at 500 °C was observed to be the lowest value (3σHRS: 1.69, 3σLRS: 0.23). For the retention test, firstly the device was set to the SET state or the RESET state by applying a dc voltage. Then the resistance of a stack was measured every 10 s at a continuous readout voltage of 100 mV. Fig. 8 shows the retention of Pt/Nb2O5/Pt devices. The resistances for LRS and HRS for the Nb2O5 films did not show any significant degradation over 3.6 × 103 s at 85 °C, displaying a stable retention of resistance states. Moreover, an extrapolation showed that the resistances for LRS and HRS were stable for periods up to 106 s at 85 °C. 3.4. The distribution of conductive paths in annealed Nb2O5 films The spatial variation in the conductivity of the as-grown and annealed Nb2O5 films was investigated by probing the Nb2O5 films on the Pt bottom electrodes, using a W-probe as the top electrode. 100 RTN 650 oC RTN 600 oC RTN 550 ooC RTN 500 C
Current(A)
10-2 10-4 10-6
RTN 450 oC
10-8 As-grown
10-10 10-12 0.0
0.1
0.2
0.3
0.4
0.5
0.6
Voltage(V) Fig. 5. Initial current before the forming process of the Nb2O5 thin films annealed at various temperatures in N2 ambient.
Resistance was estimated from the current at a read voltage of 0.2 V, which was low enough to prevent resistance switching in the Nb2O5 films. Fig. 9 shows the resistance of 25 sites, measured every 25 μm along the x and y directions on 200 × 200 μm2 Nb2O5 films. The as-grown Nb2O5 films showed high resistivity at all 25 sites; however, some high-resistivity sites became highly conductive when the films were annealed. The number of conductive sites increased as the annealing temperature increased. Nb2O5 films annealed at 600 °C showed high conductivity at a majority of the tested positions, indicating that the conductivities in the annealed Nb2O5 films were non-homogeneous, but had a strong local variation in the films, as shown in Fig. 9. The distribution and scale of the conductive paths in the Nb2O5 films was investigated using CAFM. All the CAFM measurements were carried out without electroforming on samples. The as-grown Nb2O5 films showed uniform high resistivity over the entire scanned area, while the annealed Nb2O5 films showed significant local variation in current, implying a large local variation in conductivity, as shown in Fig. 10. The size of the conductive region was estimated to be on the order of a few tens of a nanometer, and the density of the conductive regions was observed to increase with annealing temperature. 4. Discussion The nature of such local conductive paths is not clearly understood yet. But the conductive region is considered closely related to the increased concentration of non-lattice oxygen and the NbO2 phase formation in the polycrystalline nature of the Nb2O5 films after thermal annealing. High electric conductivity was reported in the NbO2 phase when compared to the Nb2O5 phase with insulating property [16–18,30]. Given the data observed in the GI-XRD and HRTEM imagery, the NbO2 phase was expected to take a nano-crystalline shape and continuous NbO2 phases were not observed in annealed Nb2O5 films from TEM work. It was reported that non-lattice oxygen ions exist in transition metal oxides containing cations with multiple valence states, and are associated with O2− ions in oxygen-deficient regions [26–28]. It was reported that oxygen vacancies play important roles in the formation of the local conductive path, and non-lattice oxygen ions were closely related to oxygen vacancies [27]. Hence, the increase of oxygen vacancies will influence the RESET-first resistance switching behavior of the Nb2O5
K. Lee et al. / Thin Solid Films 558 (2014) 423–429
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Fig. 6. Resistance switching behaviors of the Pt/Nb2O5 (50 nm)/Pt MIM stacks with (a) Nb2O5 deposited at 400 °C with a forming process, (b) Nb2O5 annealed at 500 °C and 550 °C in N2 ambient without a forming process.
films post-annealed at 500 °C and 550 °C. It was expected that the increased non-lattice oxygen in the annealed Nb2O5 films might have led to increased conductivity in the Nb2O5 films. The formation of the local conductive paths in the Nb2O5 films could not be explained by the non-lattice oxygen ions alone since the distribution of the non-lattice oxygen is expected to be homogeneous in the annealed Nb2O5 films or the NbO2 phase formed in the thermally
(a)
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LRS HRS
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annealed Nb2O5 films since NbO2 phases were isolated from each other. From TEM and XRD, the local conductive paths were possibly attributed to the conducting NbO2 phases connected by leaky Nb2O5 phases with high concentration of non-lattice oxygen. Also, the grain boundaries, defective in nature, are known to highly conductive and play important roles in resistance switching [31–36]. High resolution TEM work showed that the grain size was 4 nm – 8 nm in Nb2O5 films annealed at 500 °C and was increased to 4 – 18 nm by
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Time (seconds) Fig. 7. The endurance of resistance switching for Pt/Nb2O5/Pt as a function of cycle (a) Nb2O5 deposited at 400 °C, (b) Nb2O5 annealed at 500 °C in N2 ambient, and (c) Nb2O5 annealed at 550 °C in N2 ambient.
Fig. 8. The retention of Pt/Nb2O5/Pt devices (a) Nb2O5 deposited at 400 °C, (b) Nb2O5 annealed at 500 °C in N2 ambient, and (c) Nb2O5 annealed at 550 °C in N2 ambient.
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(a)
(b) Initial resistance
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Fig. 9. Distribution of the initial resistance states before forming process for each sample: (a) Nb2O5 deposited at 400 °C, (b) Nb2O5 annealed at 500 °C in N2 ambient, (c) Nb2O5 annealed at 550 °C in N2 ambient, and (d) Nb2O5 annealed at 600 °C in N2 ambient.
the annealing at 550 °C (data not shown). The increased non-lattice oxygen after thermal annealing above 500 °C was expected to cause the conductivity along grain boundary to increase, possibly resulting in conductive paths along grain boundaries and the RESET-first resistance switching behavior. According to the work of Kim et al. [37] and Chang et al. [38] on the local heating during forming process, the local temperature around filaments during forming could be higher than ~ 530 °C (~ 800 K) by the Joule heating. Therefore, it is expected that there would be strong correlation between the observation of RESET-first behavior of annealed Nb2O5 films and the filament formation during forming process. As observed in thermally annealed Nb2O5 films, the formation of conducting filaments could be attributed to the significant increase of defects such as non-lattice oxygen ions and secondary phases such as NbO2 phases during forming of Nb2O5 films. Also, filament formation could be enhanced during forming by the high electrical process, causing the directional move of defects related with non-lattice oxygen ions.
5. Conclusion In this work, we investigated the effects of post-annealing on the chemical bonding states, crystallinity, and resistance switching behavior of Nb2O5 films. While the as-grown Nb2O5 films required an initial electro-forming process for resistance switching, Nb2O5 films annealed at 500 °C and 550 °C under N2 ambient showed RESET resistance switching without the initial electro-forming step. Post-annealing over 500 °C caused a change in microstructure from an amorphous to a crystalline structure, an increase in electrical conductivity, and an increase in the concentration of non-lattice oxygen ions in the annealed Nb2O5 films. RESET-first resistance switching was believed to be induced by the formation of local conductive paths during thermal annealing over 500 °C. The conductive paths were considered to be closely related to the increased concentration of non-lattice oxygen and the NbO2 phase formation in Nb2O5 films with the polycrystalline nature of the Nb2O5 films after thermal annealing.
Fig. 10. Spreading resistance at 0.1 V of the Nb2O5 thin films (a) as-deposited and (b) annealed at 500 °C in N2 ambient.
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