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Ceramics International

journal homepage:www.elsevier.com/locate/ceramint

Electro and photon double-driven non-volatile and non-destructive readout memory in Pt/Bi

_0.9

Eu

_0.1

FeO

₃

/Nb:SrTiO

₃

heterostructures

Maocai Wei

^a,b

, Meifeng Liu

^b

, Lun Yang

^b

, Bo Xie

^b

, Xiang Li

^b

, Xiuzhang Wang

^b,∗

,

Xiangyang Cheng

^c

, Yongdan Zhu

^c

, Zijiong Li

^a

, Yuling Su

^a

, Meiya Li

^c,^∗∗

, Zhongqiang Hu

^d

, Jun-Ming Liu

^e

aSchool of Physics and Electronic Engineering, Zhengzhou University of Light Industry, Zhengzhou, 450002, China

bInstitute for Advanced Materials, Hubei Normal University, Huangshi, 435002, China

cSchool of Physics and Technology and Key Laboratory of Artiﬁcial Micro/Nano Structures of the Ministry of Education, Wuhan University, Wuhan, 430072, China

dElectronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi'an Jiaotong University, Xi'an, 710049, China

eLaboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, China

A R T I C L E I N F O

Keywords:

Ferroelectric resistive switching Ferroelectric photovoltaic Schottky barrier

Electro and photon double-driven Non-destructive readout

A B S T R A C T

Ferroelectric resistive switching has recently attracted considerable attention as a promising candidate for next- generation non-volatile memory. In this work, we report an electro and photon double-driven bipolar resistive switching behavior in Pt /Bi_0.9Eu_0.1FeO₃(BEFO) /Nb-doped SrTiO₃(NSTO) heterostructures prepared via pulsed laser deposition. In addition to the polarization-based control of the resistive memory, a switchable photovoltaic effect is observed that can be used to detect the polarization direction non-destructively. Significantly, the electricfield-modulated interfacial barrier can be further affected by photon-generated carriers. This phenomenon is attributed to the barrier modulation in the Pt /BEFO and BEFO /NSTO interfaces by electricfield and photon excitation. These results indicate the feasibility of non-volatile and non-destructive readout from ferroelectric memory.

1. Introduction

Resistive switching (RS) devices are one class of the most promising candidates for high-density, fast, multi-level, and nonvolatile random access memory with low energy consumption [1–3]. In these devices, the resistance state can be reversibly switched between a high resistance state (HRS) and a low resistance state (LRS) by applying a voltage pulse, while the readout is non-destructive. Among the materials that display evidence of resistive switching [1,4–8], ferroelectric semiconductor oxides are promising candidates due to their sponta- neous polarization, which does not induce additional local chemical or valence changes and in which resistive switching is an intrinsically fast phenomenon [6–8]. In addition, the ferroelectric photovoltaic (PV) effect has been observed and proposed for information transfer and storage, as it can be used to non-destructively detect the polarization direction [9–11]. This simultaneous observation of ferroelectric resistive switching and PV effects in a ferroelectricfilm capacitor would imply significant potential for applications in optoelectronics,

photoelectric conversion, and photoelectric information storage [12,13].

However, traditional ferroelectric photovoltaic materials are in- sulators with large bandgaps (Eg> 3.0 eV), such as LiNbO3 [14], BaTiO3(BTO) [15], and (Pb,Zr)TiO3(PZT) [16] etc. Such properties limit the light absorption beyond the visible light range and imply weak carrier transport capabilities and small photocurrents (in the order of nA/cm²) produced.

BiFeO3 (BFO) is one of the most studied single-phase, room-temperature multiferroic materials, as it exhibits a relatively narrow bandgap (Eg~ 2.8 eV) and high residual polarization (50–150μC/cm²) [17,18]. Thus BFO has the potential to provide multiple degrees of freedom in memory applications. Choi et al. reported a switchable ferroelectric diode and visible light-induced photovoltaic effects in BFO single crystals [9]. Ji et al. found that the direction of the photocurrent in epitaxial BFO thin films can be switched using polarization, indicating that the ferroelectric depolarizationfield plays a dominant role in driving the photocurrent [17]. In addition, BFO is typically

https://doi.org/10.1016/j.ceramint.2019.10.256

Received 3 March 2019; Received in revised form 10 October 2019; Accepted 26 October 2019

∗Corresponding author.

∗∗Corresponding author.

E-mail addresses:xzwang@hbnu.edu.cn(X. Wang),myli@whu.edu.cn(M. Li).

Available online 28 October 2019

T

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considered to be ap-type semiconductor due to Bi loss [6,7]. It can form ap-njunction barrier when connected to ann-type semiconductor, and this junction can be active in photon absorption and electron-hole pair generation. For instance, Zheng et al. reported that the insertion of an n⁺-type semiconductor Cu2O layer between the PZT layer and cathode Pt contact in an ITO /PZT /Pt cell hinders electron extraction and leads to an increased photocurrent increasing [19]. An a-Si ﬁlm with a narrow aids in the absorption of visible light to enhance photovoltaic performance [20]. Thus, the fascinating properties of BFO may provide great potentials for developing next-generation of optically driven ferroelectric-based electronics devices.

Since both the polarization charge and photon-generated charges can affect the interfacial barrier, non-volatile and non-destructive memory readout can be performed by using the RS and PV effects. In this paper, we report an electro and photon double-driven RS behavior and switchable PV effects in Pt /Bi0.9Eu0.1FeO3 (BEFO) /Nb-doped SrTiO3 (NSTO) heterostructures. Here, we use BEFO instead of BFO because a small amount of substitution of Bi with Eu helps with ionic defects and leakage suppression [21]. The HRS and LRS of the heterostructures can be switched by modulating the heights of the potential barriers and widths of the depletion regions in the Pt /BEFO and BEFO /NSTO interfaces. This can be further modulated using illumination with light. This provides the potential for multi-state RS behavior with non-destructive readout.

2. Experimental details

The BEFO thin films were epitaxially grown on (001)-oriented 0.7 wt% Nb-doped SrTiO3(NSTO) single crystal substrates with one- side-polished. The substrates had dimensions of 3 × 3 × 0.5 mm³and growth was achieved via pulsed laser deposition (PLD) using a KrF excimer laser (248 nm, Lambda Physik COMPex 205). During deposition, the substrate temperature and laser repetition ratio were 660 °C and 3 Hz, respectively. The deposition was performed under oxygen at 15 Pa and produced a BEFO film thickness of approximately 120 nm with a deposition rate of 2 nm/min. The oxygen was filled into the chamber at maximalflow rate right after the deposition, while thefilm was cooling at a rate of 2 °C/min. When the chamber pressure reached 1 atm, the temperature of thefilm was about 400 °C, and then cooled to room temperature at 4 °C/min. The Pt top electrodes (500μm × 500μm) were deposited via magnetron sputtering though a square shadow mask. The bottom electrode was fabricated by soldering an In pad to the back side of NSTO substrate.

The crystallographic orientation and phase purity of the BEFOfilms were characterized via X-ray diffraction (XRD, Bruker D8 Advance) in the θ-2θmode, using Cu Kα1 radiation (λ= 1.5406 Å). The positive (negative) voltage is defined as that when a positive (negative) bias is applied to the top Pt electrodes. The ferroelectric domain structures were probed using a piezoresponse force microscope (PFM, Bruker Multimode 8) with Pt-coated Si cantilevers. The current-voltage (J ~ V) curves were measured using a Keithley 4200 characterization system in the voltage sweeping mode. The maximum current was limited using a compliance current (CC) to avoid permanent hard breakdown during HRS to LRS switching. A diode laser with a wavelength of 405 nm (hν= 3.06 eV) and the power density of 200 mW/cm²was used as the light illumination source, and the measurement setup schematic is shown inFig. 1(a). Here the short circuit current density (JSC) and open circuit voltage (VOC) were obtained by the following way. First, the sample was poled using a 100 ms voltage pulse (VP) in the dark or light illumination condition. Then the sample was exposed to a low-voltage sweeping from - 0.5 V to 0.5 V, during which the J ~ V curve was obtained, noting that a voltage of ± 0.5 V is too small to change the sample's polarization state. Finally, the JSCand VOCwere extracted from the curve at V = 0 and J = 0, respectively. By setting a series of VPand performing the measurements, the JSC~ VPand VOC~ VPcurves (hysteresis loops) were obtained. Certainly, JSC can be also be probed

directly via illumination ON/OFF testing (JSC= 0 in the OFF case).

3. Results and discussion

Fig. 1(a) is the schematic diagram of the Pt /BEFO /NSTO heterostructure.Fig. 1(b) shows the XRDθ-2θscan of the BEFOfilm grown on an NSTO (001) substrate. Strong BEFO (00l) (l= 1, 2) diffraction peaks appear and no diffraction peaks of other orientations or impurities are found, suggesting a single phase and highly (00l) oriented BEFOfilm.

The fourfold symmetrical diffraction peaks of the BEFO (011) reflection appear at the same angles as those of the NSTO (011), indicating good epitaxy of the BEFOfilm on the NSTO substrate. The ferroelectric domain structure observed in the as-grown (fresh) state is shown in Fig. 1(c), indicating a typical random domain pattern. Further, typical phase hysteresis loop is shown inFig. 1(d), with the coercive voltages of about−2.6 V and 1.8 V, revealing good ferroelectric property of BEFO film.

The J ~ V curves of the Pt /BEFO /NSTO heterostructure in the dark and under illumination are shown inFig. 1(e). In these tests, the BEFO film is in the nonpoled (fresh) state. In the dark, a distinct diode-like current rectification can be observed, suggesting ap-njunction barrier at the BEFO /NSTO interface. We note that NSTO containsn-type carriers [6]. Significantly, the J ~ V curve does not pass through the origin in the dark, as shown in the inset ofFig. 1(e). This indicates that there is a built-in electricfield in the heterostructure. Under illumination, the fresh BEFOfilm exhibits a significant photovoltaic response, which also serves as an indirect evidence of the built-in electricfield [22]. The JSC

and VOCmeasured in this case are - 7.0μA/cm² and 0.38 V, respectively. This suggests that photo-generated charges generated under illumination can affect the built-in electricfield [23,24]. Such an effect indicates good retention properties. This is supported by the JSCdata generated using a set of illumination on/offtests with an interval of 20 s, as shown inFig. 1(f).

To investigate the polarization (P)-modulated PV effect and RS behavior, the heterostructure was pre-poled using a pulse of VP= 2 V and - 4 V (pulse width 100 ms) respectively so that the BEFO film had downward-P and upward-P states. The J ~ V curves measured under the illumination in the fresh, upward-P, and downward-P states, are plotted inFig. 2(a). While the polarized state of the BEFOfilm reduces the VOC from 0.38 V (in the fresh state) to 0.1 V (P < 0) and 0.01 V (P > 0), the remarkably enhancement of JSCfrom−7.0μA/cm² to 9.8μA/cm²(P < 0) and 30.1μA/cm²(P > 0) was observed. Sub- sequently, we performed a series of sequential measurements in which different VPwere set and the JSC~ VPand VOC~ VPhysteresis loops are plotted inFig. 2(b). The hysteresis loops correspond well with the phase hysteresis loop of the BEFOfilm (Fig. 1(d)), indicating that the ferroelectric polarization plays a key role in the switchable PV effect. The significant asymmetry of the JSC~ VPand VOC~ VPcurves also suggests that the photovoltaic response is dominated by the polarization- modulated interfacial barriers rather than the bulk photovoltaic effect (BPE) [25].

To distinguish the RS behavior better, the J ~ V curves measured in the dark and under illumination after poling by various voltage pulses are plot in semi-log scale inFig. 2(c). For VP= 2 V, the J ~ V curves are symmetric and overlap in the dark and under illumination. The curves become highly asymmetric when VP= - 4 V and the two curves measured in the dark and under illumination are markedly different from each other. The measured J at VP= 2 V is larger than that at VP= - 4 V, a strong signal of the resistance switching from the LRS to the HRS when the polarization is reversed from the downward direction to the upward one. In other words, one sees that a sequence of alternating electric biases of - 4 V and 2 V can drive the LRS/HRS switching. In addition, illumination has little effect on LRS, but significant tuning for HRS is noted. This effect can be further illustrated by the measured resistance as a function of VPin the dark and under illumination, as shown inFig. 2(d). The result is similar to the phase hysteresis loop of

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Fig. 1.(a) Schematic diagram of the Pt /BEFO /NSTO heterostructure. (b) XRDθ-2θscan of the BEFO /NSTO heterostructure, and the inset isΦ-scan of the BEFO (011) and NSTO (011) planes. (c) PFM phase image (2μm × 2μm) of BEFOﬁlm in the as-grown state. (d) PFM phase hysteresis loops. (e) The J ~ V curves of the Pt/

BEFO/NSTO heterostructure in unpoled state in the dark and under illumination. The inset is the current-voltage characteristics of BEFO in the dark. (f) J_SCas a function of time for the Pt /BEFO /NSTO device.

Fig. 2.(a) J ~ V curves for the Pt /BEFO /NSTO heterostructure in light illumination, where the BFOﬁlm is in the as-grown, positively poled (+2 V) and negatively poled (−4 V) states, respectively. The inset shows the expanded region of the J ~ V curves. (b) J_SCand Voc as a function of the voltage pulse (V_P) under light illumination. (c) J ~ V curves for the Pt /BEFO /NSTO heterostructure in dark and under illumination, where the BFOﬁlm is in positively poled and negatively poled states. (d) Bipolar resistance switching hysteresis loops for the Pt /BEFO /NSTO heterostructure in the dark and under light illumination.

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the BEFOﬁlm (Fig. 1(d)), which suggests that polarization reversal is closely related to RS in the heterostructure. Illumination reduces the high /low resistance ratio from 425 in the dark condition to 62, indicating that the resistive states can be modulated via illumination.

Such complementary modulations of the resistance states by electric ﬁeld and illumination produce additional resistance states and enhance the memory density.

In this section, we discuss the stability and retention properties of the heterostructure with respect to the PV effect and RS effect, as shown inFig. 3(a) and (b), respectively. The PV effect is persistent and can be switched via the polarizationflipping, which suggests that the polarization states can be readout non-destructively by detecting JSC. By combining the PV and RS effects, one can achieve the electro and photon double-driven RS behavior, as shown inFig. 3(b). When alter- natively poling the sample at VP= - 4 V and 2 V in sequence, where the sample's resistance R is read using a bias of - 0.3 V, one can use an illumination ON/OFF sequence to demonstrate the excellent stability and retention characteristics of the resistance. More interestingly, the RS switching remains quite stable during the cycling under various illumination. Here, the current in the LRS is large and thus the resistance can't be modulated. The double-driven RS behavior exhibits a significant tri-state feature. Further, the electro and photon double-driven RS behavior can be detected using the JSC(or VOC), which provides a non-destructive and reliable detection route. We note that the illumination does not change the devices polarization states.

Nevertheless, the remarkable JSC~ VP, VOC~ VP and R ~ VPhys- teresis characteristics seem to indicate more ingredient of physics in addition to the polarization state and interfacial barrier for determining the J ~ V behaviors of the heterostructure. To explain the transport behaviors and PV effects within such a heterostructure, we discuss several possible mechanisms proposed for ferroelectricfilms [26]. We plot the J ~ V curves of the LRS and HRS asln(J) ~ln(V) in the dark and under illumination inFig. 3(c) and (d), respectively. The data canfit the predictions of the space-charge-limited bulk conduction (SCLC) mechanism, expressed by J ~ Vⁿwhere n is a scaling exponent that is close to 1.0 in the low-field range (Ohmic conduction behavior) and larger

than 2.0 in the high-ﬁeld range (contributions from electricﬁeld-induced polarization reversal and interfacial charge redistribution).

Moreover, the exponent n is similar in the dark and under illumination in the LRS. In the HRS, the exponent is smaller under illumination than in the dark. The latter phenomenon occurs due to the reduced barrier width in the Pt /BEFO and BEFO /NSTO interfaces, because polarization-modulated interfacial bound charge distribution can be further modulated by photon-generated charges.

We then consult the interfacial band structure and discuss the effect of polarization in the heterostructure. This effect is believed to be crucial to the separation of photon-generated carriers. The net build-in electricfieldEbican be described as

= − + + −

Ebi Ebi s Edp Ebi d

where the built-infield (Ebi-s) is based on the Pt /BEFO Schottky barrier, Edprepresents the depolarization field caused by ferroelectric polarization in the BEFOfilm, and the build-infield (Ebi-d) of the depletion region is based on the BEFO /NSTO interfacial barrier.

To further understand the electro and photon double-driven non- volatile resistance states and PV effect, we sketch a schematic of the interfacial band diagrams inFig. 4. The bandgap and electron affinity of BFO are 2.8 and 3.3 eV [27], respectively. For NSTO, they are 3.2 eV and 4.0 eV [28], respectively. The work function of Pt is 5.65 eV [29], thus, a Schottky barrier can be formed at the Pt /BFO interface. If there is no ferroelectric polarization, a depletion region with a given width forms across the BEFO /NSTO interface after dynamic equilibrium is reached. The band structure of the heterostructure is described in Fig. 4(a). An induced built-in electricfield can be formed, which is the summation ofEbi-SandEbi-d. When the polarization of BEFO is downward (upward), the negative majority carriers (electrons) in then-type NSTO are attracted (repelled) by the positive (negative) bound charges, resulting in a decrease (increases) in the depletion width. In addition, the charge screening effect causes the energy band to bend at the interface leading to the resistive switching behavior shown inFig. 4(c) and (e). More importantly, light can impact the barrier between the Pt /BEFO and BEFO /NSTO interfaces via photon-generated carriers, as Fig. 3.(a) Shows the J_SCas a function of time for the Pt /BEFO /NSTO heterostructure when the light was turned on and off, respectively, with BEFO in upward polarization and downward polarization. (b) Retention and switching cycling characteristics of the heterostructure with - 4 V and + 2 V pulse voltages in the dark and under various illumination. (c) and (d) present LRS and HRS conduction mechanisms in the dark and under illumination, respectively.

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shown inFig. 4(b), (d), and (f). In addition, although the magnitude of the built-in electricfield can be modulated significantly by ferroelectric polarization and the PV effect, the direction remains unchanged. When the BEFO layer is in the LRS, the direction ofEdpis parallel toEin, which enhances the transport of photon-generated holes to the bottom of the BEFO layer. In the HRS, polarization is anti-parallel toEin, suppressing the mobility of photon-generated holes.

4. Conclusion

In summary, we achieved an electro and photo double-driven tri- state RS behavior and switchable ferroelectric photovoltaic eﬀect in the Pt /BEFO /NSTO heterostructure. The high /low resistance ratio de- creased from 425 in the dark to 62 under illumination. Polarizing the BEFOﬁlm reduced the VOCto 0.1 V (P < 0) and 0.01 V (P > 0), and enhanced the JSC to 9.8μA/cm² (P < 0) and 30.1μA/cm² (P > 0) from JSC~ - 7.0μA/cm² and VOC~ 0.38 V in the nonpoled state.

Further, the electro and photon double-driven RS behavior can be detected using JSC(or VOC), which provides a non-destructive and reliable route to determining the polarization direction. Key to this phenomenon is that either the polarization charge or a photon-generated charge can aﬀect the barrier in the Pt /BEFO and BEFO /NSTO interfaces.

These results will open a door for producing and non-destructively sensing multiple non-volatile electronic states.

Declaration of competing interest

The authors declare that they have no known competingﬁnancial interests or personal relationships that could have appeared to inﬂu- ence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant Nos. 51702093, 61571403, 51802248, 11704109, 51801059, 51871091, and 11804088), the Scientiﬁc and Technological Research Program of Education Department of Hubei Province (Grant No. B2017139), and the School Doctor Foundation of Zhengzhou University of Light Industry (Grant No. 2018BSJJ057).

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