Reproducible resistive switching in the super-thin Bi

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APPLIED PHYSICS LETTERS 109, 152903 (2016)

Reproducible resistive switching in the super-thin Bi

2

FeCrO

6

epitaxial film with SrRuO

3

bottom electrode

Wenting Xu,¹ Jiao Sun,¹ Xijun Xu,¹ Guoliang Yuan,^1,2,a) Yongjun Zhang,³ Junming Liu,² and Zhiguo Liu^2,a)

1School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

2National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China ³Physics College, Jilin Normal University, Siping 136000, China

(Received 14 July 2016; accepted 27 September 2016; published online 10 October 2016)

The reproducible and reliable resistive switching is observed in the ultrathin Bi2FeCrO6 (BFCO) epitaxial film on (001) SrTiO3 substrate with SrRuO3 as the bottom electrode. The as-grown BFCO film allows its ferroelectric polarization switching under external electric field. With a 100-nm- radius tip contacting film surface, a stable bipolar resistive switching was observed through the conductive atomic force microscope. Furthermore, the resistive switching at negative bias was observed and its high/low current ratio is above 15 among a thousand of current versus voltage curves measured by the scanning tunneling microscope with a non-contacting nm-scale tip. It is argued that this transport mechanism is due to quantum tunneling, and the resistive switching in these junctions is because of ferroelectric switching. Published by AIP Publishing.

[http://dx.doi.org/10.1063/1.4964603]

Conventionally, a tunnel junction consists of two metal electrodes and an nm-thick insulating barrier layer sand- wiched inside.¹ However, ferroelectricity is found to exist even in thin films of several unit cells in thickness, which makes it possible to realize ferroelectric tunnel junctions (FTJ) by employing ultrathin ferroelectrics as barriers.^2,3 Depending on the polarizations, the electrostatic potential profile of tunneling barrier will be varied. As a result, the tunneling current can be switched between the high and low values by polarization reversal, leading to one type of resistive switching.^4,5 Resistive switching of FTJ is sensitive to both the polarization and the barrier thickness, thus the ferroelectric barrier with a higher polarization can significantly boost the high/low current ratio. In 2009, the resistive switching of FTJ was demonstrated using conducting atomic force microscopy (CAFM).^6–8 Up to recently, many previous studies focus on Pb(Zr1-xTix)O3 and BaTiO3 thin film with super-high resistance.^1,6,7,9–12

A FTJ with BiFeCrO6 (BFCO) as a ferroelectric barrier may show special properties because BFCO exhibits excel- lent multiferroic and photovoltaic properties. Although multiferroic properties and potential applications of BiFeO3 have been extensively studied,^13–16 not much works have been done on BFCO. BFCO has a double perovskite crystal structure exhibiting a clear Fe-Cr ordering along the [111]

direction, and its ferroelectricity is driven mostly by the Bi^3þ ions.^17–19 At the same time, BFCO has a saturated magnetization superior to that of BiFeO3 and the most striking char- acteristic of BFCO films is their good multiferroic properties at room temperature. For example, BFCO is promising to have a polarization of 80 lC/cm², a piezoelectric coefficient of 283 pC/N, and a magnetization of 160 emu/cm³ according

a)Authors to whom correspondence should be addressed. Electronic addresses: yuanguoliang@njust.edu.cn and liuzg@nju.edu.cn

to previous first-principle calculation.²⁰ Furthermore, the major band gap (E_g) of BFCO is 2.4–2.6 eV and the smallest Eg (i.e., 1.6 eV) can be differentiated through optimizing the interaction between Fe and Cr alternation.¹⁹ As a result, the BFCO films possess a large photovoltaic effect and a high photovoltaic conversion efficiency of 6% for red light, compared with <1% for BiFeO₃ with E_g of 2.8 eV.^21,22 Due to the high ferroelectric and photovoltaic performances of BFCO, the BFCO-based FTJ is necessary to be studied.

Here the reliable resistive switching of BFCO on (001) SrTiO3 (STO) with SrRuO3 (SRO) bottom electrodes was observed by CAFM and scanning tunneling microscope (STM), and its high/low current ratio remains over 15 among a thousand of I–V curves.

Epitaxial BFCO films were grown on (001) STO sub- strates by pulse laser deposition (PLD) and then its crystal structure, transmittance, ferroelectric, and resistive switching were characterized. A KrF excimer laser was used for film growth and its wavelength, frequency, and energy per pulse were 248 nm, 2 Hz, and 55 mJ. Pure BFCO film ( 50 nm) was deposited on the double-sided polished STO substrate at 680 C and 0.01 Pa oxygen pressure. Besides, BFCO films ( 6 nm) with either La0.7Sr0.3MnO3 (LSMO) or SRO buffer layer as bottom electrodes were prepared at 680 C and

0.01 Pa oxygen pressure. All as-grown films were po st- anneal

ed

at 1000 Pa oxygen pressure

during cooling

wi th 5

C/min.

Crystal structure of

BF CO

was measured

wi th X-ray diffraction (XRD, Brucker D8) and the transmittance of BFCO film on double-sided polished STO was characterized by a UV-Visible Spectrometer (Shimadzu Co.). The morphology and piezoelectric force microcopy (PFM) studies were conducted on an Atomic Force Microscope (Multimode 8, Bruker Co.). An a.c. voltage of 2 V at 41 kHz was applied to the conductive tip (Brucker MESP-RC, Co/Cr coating, 35 nm tip radius) to obtain the domain image. The

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152903-2 ^{Xu et al.}

morphology and current-voltage (I–V) curves of BFCO films were characterized by CAFM with the other conductive tip (Nano world CDT, diamond coating, 100 nm radius) and by STM with a non-contacting tip (Brucker TP10, nm-scale radius). The maximum current was limited by compliance current (CC) to avoid permanent hard breakdown when OFF switches ON. The high/low current ratio was derived from I–V curves at a certain voltage.

A high-quality epitaxial BFCO film was grown on the (001) STO substrate and its main E_g was 2.6 eV according

to transmittance spectra. The crystal constant of BFCO (i.e.,

is slightly larger than those of STO

a b c 0.397 nm)23,24 The XRD patterns of the 50-

(a ¼ b ¼ c 0.391 nm nm).

BFCO film on STO shows only (00 l) diffraction peaks of STO and BCFO film, indicating the epitaxial growth of BFCO (Figure 1(a)). BFCO crystal crystallizes in a rhombo- hedrally distorted perovskite structure with space group R3c.²³ The light transmittance of STO/BFCO film (T) can estimate the Eg according to the formula of [-ln(T)hv/ a]² ¼ [ahv]² ¼ C(hv-Eg), where a is film thickness, h is Plank constant, v is the light’s frequency, a ¼ -ln(T)/a and C is constant.²⁵ As shown in Figure 1(b), the main Eg of BFCO is 2.6 eV, being consistent with the previous studies.¹⁹

The ferroelectric polarization of the 50-nm BFCO film with SRO buffered layer can be switched by an external elec-tric field. Figure 1(c) shows an out-of-plane (OP) PFM image in a 5 5 lm² BFCO region, in which a 4 V bias was applied by the PFM tip to the central 3 3 lm², followed by a 4 V bias applied to the central 1 1 lm². The 4 V dark region has upward polarization and the 4 V yellow region has down-ward polarization. Most area of the as-grown BFCO film is yellow which suggests a national downward polarization in most regions. PFM phase and amplitude hysteresis loops are shown in Figure 1(d). These loops suggest the local coercive voltages (V_C) of about 1.7 V and þ1.9 V, respectively.

A bipolar resistive switching was differentiated in the I–V curves of STO/LSMO/BFCO measured by CAFM.

Appl. Phys. Lett. 109, 152903 (2016)

Figure 2(a) shows the 1 0.5 lm² morphology image (top) and CAFM current image (bottom), where three I–V curves were measured at the three points marked by white dots. The surface is nm-scale smooth and the current fluctuates from 22.9 pA to 21.1 pA at 0.5 V bias (Figure 2(b)). There is a clear character of bipolar resistive switching. The weak resistive switching mainly comes from two factors. At first, the LSMO thin film shows a relatively high electrical resistivity of 3 10 ³ X cm at 300 K.²⁶ Second, there is a high-

resistance depletion layer at the interface between n-type BFCO film and p-type LSMO buffer layer. These high resis- tances in series seriously screen the resistive switching of BFCO film.

The reproducible bipolar resistive switching with the high/low current ratio between 15 and 122 was observed in the STO/SRO/BFCO structure using CAFM. The resistivity of SRO is 6.3 10 ⁴ X cm at 300 K, and there is no high-resistance depletion layer between n-type BFCO film and n-type SRO buffer layer.²⁷ After that, we characterized the STO/SRO/BFCO film with CAFM. Figure 2(c) shows the 1 0.5 lm² morphology image (top) and the current image (bottom), where three I–V curves were measured at the three points marked with white dot.

There is a strong bipolar resis-tive switching as shown in Figure 2(d), where the VSET/ VRESET is read from the intersection point of compliance cur-rent (CC) and the current in stage 1/4 (labeled in the dashed circle). The voltage when high/low resistance status (OFF/ ON) transforms to low/high resistance status (ON/OFF) is defined as VSET/VRESET.²⁸ It is argued that the switch of polarization causes the resistive switching and the OFF

transforms to ON at VSET þVC and the ON transforms to OFF at V_RESET V_C during the 0 ! 3 ! 0 ! 3 ! 0 V scan in this FTJ.^4,6,12 The schematic polarization domain

structure and the interfacial band alignment for OFF and ON resistance status of the BFCO film is shown in Figure 2(f), where Vi is built-in voltage in the depletion layer, Vd is depolarization voltage of BFCO ferroelectric layer, EF is the

FIG. 1. (a) XRD pattern and (b) [ahv]² versus hv curve of STO/50-nm-BFCO derived from the transmittance spectra (Inset). (c) OP PFM image where upward/downward polarization was induced by the 4/ 4 V bias, and (d) phase and the amplitude loops of 50- nm-BFCO film on STO with SRO buffered layer.

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152903-3 Xu et al. Appl. Phys. Lett. 109, 152903 (2016)

FIG. 2. (a) Morphology and current images of the 6-nm BFCO film with LSMO electrode and its (b) I–V curves measured by CAFM. (c) Morphology and current images of the BFCO film with SRO electrode, (d) I–V curves with voltage between 3 V and 3 V, (e) I–V curves with voltage between 1 V to 1.6 V, 1 V to 0 V and 0 to 1.2 V (Inset), and (f) the schematic polarization domain structure and interfacial band alignment for OFF and ON resistance status of the BFCO film.

Fermi level, EC is the bottom of the conduction band, and EV

is the top of the valence band. As can be seen in the graph, there exists a depletion layer between the p-type diamond tip and the n-type BFCO, so the switching progress of the downward polarization to the upward changes the direction of depolarization field and further affects the built-in electric field in the depletion layer, which introduces a lower potential barrier for electrons quantum tunnel across the ferroelectric layer.^6,12,29 The polarization switching from downward to upward induces a lower potential barrier and high current status at 1.8 V during voltage scanning from 0 to 3 V.

Similarly, the polarization switching from upward to downward introduces a higher potential barrier and low current status at about 1.8 V during voltage scanning from 0 to 3 V.

What is more, the I–V curve does not show an obvious resistive switching when the voltage bias was limited between 1 V and 1.6 V (Figure 2(e)), between 1 and 0 V or between 0 and 1.5 V (Inset of Figure 2(e)) in which, the polarization does not switch.^6,12 It is worth mentioning that although the thickness of the BFCO film is 6 nm, it does not mean that the tunneling effect occurs throughout the whole film, instead, the electrons only need to tunnel through the depletion layer between the tip and film, and the deple-tion width is smaller than the thickness of the film, which

can also be easily seen from Figure 2(f). What is more, the electron tunneling effect has also been observed in a 50-nm Pb(Zr0.2Ti0.8)O3 film in the previous study.⁶

The resistive switching in STO/SRO/BFCO film shows a good repeatability. I–V curves of one particular point on the film were measured for 100 times by CAFM. The first three and the last three I–V curves are shown in Figures 3(a) and 3(b), respectively. It is noted that the last three I–V curves are similar to the first three ones, which reveal the good repeatability of the BFCO film. What is more, we counted and analyzed V_SET and V_RESET of all I–V curves, and both V_SET and V_RESET are normally distributed in Figure 3(c).⁶ Furthermore, high and low current values at a certain voltage are derived from I–V curves (Figure 3(a)). The high/ low current ratio of all I–V curves are shown in Figure 3(d). The ratio keeps a value up to 15 and over 50% values are above 100, which is comparable to the other results reported before.⁷ The data reveal a good performance and a favorable repeatability of the resistive switching. The reproducibility of I–V curves across the surface distinguishes our results from those of the filamentary conduction because the spatial distribution of conducting filaments was very non- uniform.³⁰ Also, the nonlinear I–V characteristics are distinct from the previous study of a macroscopic ferroelectric capacitor,

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152903-4 Xu et al. Appl. Phys. Lett. 109, 152903 (2016)

FIG. 3. (a) The first three and (b) the last three I–V curves of the 6-nm BCFO with SRO electrodes which were repeatedly measured by CAFM for 100 times at a fixed dot. (c) The distribution of V_SET and V_RESET and (d) the high/low current ratio derived from these I–V curves.

where the I–V curves became ohmic after polarization switching.³¹

The reproducible resistive switching at negative bias was observed in STO/SRO/BFCO by STM with a non- contacting nm-scale tip. The morphology image of a 420 420 nm² region at 0.1 V bias is shown in Figure 4(a), and three different tiny regions are chosen for the measure-ment of I–V curves. These I–V curves just show resistive

FIG. 4. (a) Morphology and (b) I–V curves of the 6-nm BFCO film with SRO electrode measured by STM. (c) Morphology and (d) I–V curves of pure SRO film measured by STM. (e) VRESET distribution, where the inset shows the interfacial band alignment, and (f) high/low current ratio of thou- sands of I–V curves of BFCO.

switching in the range of 1 to 1 V, where high current sta-tus transforms to low current status at 0.65 V and then the low current status transforms to high current status at positive bias. The resistive switching at negative bias is mainly due to three factors. First, the size of the tips used in CAFM and STM is completely different. The radius of the CAFM tip is 100 nm. However, in consideration of the fact that STM tip can observe graphite crystal lattice, the radius of the STM tip is believed to be 1 nm, which is much smaller than that of the CAFM tip. As a result, the upward polarization of the tiny region introduced by STM tip is embedded among the matrix of downward polarization or disordered status, and prefers to bounce back in a very short time. Second, electrons quantum tunnel through air layer and then quantum tunnel through BFCO ferroelectric film, and the resistive switching mechanism of two tunnels are much more complex than the single quantum tunnel of FTJ mea-sured by CAFM. Third, the quantum tunnels under positive bias is quite different from that under negative bias, partially because STM tip emits electrons to air easier than the BFCO film emits electrons to air. What is more, it can be seen easily from the I–V curves of CAFM and STM measurement that VRESET of STM is smaller than that of the CAFM measure-ment. This is probably because of the different band struc-ture of tip/film/SRO-electrode in the two measurements. In the CAFM measurement, there exists a depletion layer between the p-type diamond and the n-type BFCO, and the width of the depletion layer can be up to a few nanometers. However, in the STM measurement, despite the resistance of the air layer is relatively higher, the width of the high-resistive air layer is much smaller than that of the depletion layer between the CAFM tip and the BFCO film.

So that, it is easier for the electrons tunneling the super-thin high-resis-tive air layer. For comparison, the morphology image (Figure 4(c)) and the I–V curves (Figure 4(d)) of pure SRO film on STO substrate were measured by STM, too.

The cur-rent arrives in 120 nA at less than 0.05 V, and there is no obvious resistive switching according to the I–V curves mea-sured at different positions. This suggests that the resistive switching of STO/SRO/BFCO comes from the BFCO ferro-electric layer rather than the SRO electrode.

The resistive switching at negative bias is reproducible and reliable though a nm-scale region was characterized by the STM tip. I–V curves were measured for 1000 times at a fixed position though a natural nm-scale movement cannot be excluded within the limitations of STM. The resistive switch-ing was observed in all I–V curves. The V_RESET of these I–V curves and the interfacial band alignment (inset) are counted and shown in Figure 4(e). It is noted that the VRESET shows a normal distribution near 0.57 V.

Furthermore, high/low cur-rent ratio of these I–V curves are counted and shown in Figure 4(f), where the high/low current status is derived at B and A point in Figure 4(a). The ratio between 15 and 122 is similar to the high/low current ratio of I–V curves measured by CAFM. In a word, the reliable resistive switching was achieved in STO/SRO/BFCO measured by the nm-scale STM tip.

In conclusion, reproducible and reliable resistive switching was observed in the super-thin BFCO epitaxial film on STO substrate with SRO electrode. A stable bipolar resistive

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152903-5 ^{Xu et al.}

switching with the high/low current ratio of 15–122 was observed by CAFM when a 100-nm-radius tip contacting film surface, and the resistive switching did not show an obvious decay even after I–V curves were measured for 100 times. The VSET and VRESET are close to þVC and VC, and the ferroelectric polarization switching explains the bipolar resistive switching. What is more, the resistive switching at negative bias was also observed in the I–V curves measured by STM with a nm-scale non-contacting tip, and its high/low current ratio is over 15 among a thousand of I–V curves.

The research has received funding from the National Natural Science Foundation of China (11134004, 51431006, and 51472118) and the Fundamental Research Funds for the Central Universities (30916011104).

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