-based capacitors with Ti and Pt top electrodes

APPLIED PHYSICS LETTERS 111, 252901 (2017)

Resistive switching and photovoltaic effects in ferroelectric BaTiO

₃

-based capacitors with Ti and Pt top electrodes

Hua Fan,^1,2 Chao Chen,¹ Zhen Fan,^1,a) Luyong Zhang,¹ Zhengwei Tan,¹ Peilian Li,¹ Zhifeng Huang,¹ Junxiang Yao,¹ Guo Tian,¹ Qiuyuan Luo,¹ Zhongwen Li,¹ Xiao Song,¹ Deyang Chen,¹ Min Zeng,¹ Jinwei Gao,¹ Xubing Lu,¹ Yue Zhao,² Xingsen Gao,^1,a) and Jun-Ming Liu^1,3

1Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Optical Information Materials and Technology, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China

2Institute for Quantum Science and Engineering and Department of Physics, South University of Science and Technology of China, Shenzhen 518055, China

3Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

(Received 12 August 2017; accepted 5 December 2017; published online 18 December 2017) We have comparatively studied the dielectric, ferroelectric, conduction, and photovoltaic properties of Ti/BaTiO3 (BTO)/SrRuO3 (SRO) and Pt/BTO/SRO capacitors. The resistive switching (RS) is observed in the Pt/BTO/SRO capacitor while it is absent in the Ti/BTO/SRO capacitor, which may be attributed to the interfacial layer existing between Pt and BTO and the Ti/BTO Ohmic interface, respectively. Further analyses on the conduction mechanisms suggest that the RS may be caused by the opening/closing of conduction paths in the Pt/BTO interfacial layer, whereas the polarization is ruled out as the origin of RS because of the inconsistency between the RS switching voltages and coercive voltages. On the other hand, it is observed that the photovoltaic effects (PVEs) in both Ti/BTO/SRO and Pt/BTO/SRO capacitors are electrically unswitchable and the open-circuit voltages of the two capacitors are similar in magnitude, implying that the PVE is driven by an internal bias field rather than the polarization-induced field. The exis-tence of such an internal bias field is indicated by the self-polarization and imprint phenomena. Our study demonstrates that the interfacial layer and the internal bias field can be the major causes for the RS and PVE in certain ferroelectric capacitors, respectively, whereas the polarization may not necessarily play a role.

Published by AIP Publishing. https://doi.org/10.1063/1.4999982

Utilizing the ferroelectric polarization as a degree of freedom to tune the charge transport properties, such as resis- tive switching (RS) and photovoltaic effect (PVE), has become a focus of attention recently.^1,2 The RS in ferroelec- trics holds great promise for non-volatile memory applica- tions, owing to its ultrafast switching speed,³ low power consumption,⁴ and non-destructive readout.^5,6 On the other hand, the PVE makes the ferroelectrics an attractive candi- date for optoelectronic applications, such as solar cells,⁷ pho- todetectors,⁸ and electrically written, optically read memories.⁹ Despite great advances in device performances, the mechanisms of both RS and PVE in ferroelectrics are still unclear.

More specifically, it was initially thought that the RS in ferroelectrics originated from the polarization modulation of the interface barrier,^10–12 but a few recent studies showed that the RS was also related to the defects (e.g., oxygen vacancies) at the electrode/ferroelectric interface.^13–15 For example, Kim et al.¹⁴ reported that the oxygen vacancy accumulation/dissipation in a CoOx layer formed at the Co/BaTiO3 (BTO) interface could lead to the memristive RS behavior. Qin et al.¹⁵ observed almost identical RS behaviors in the tunnel junctions based on different oxide barriers [e.g.,

a)Authors to whom correspondence should be addressed: fanzhen@m.scnu.

edu.cn and xingsengao@scnu.edu.cn

ferroelectric BTO, PbZr0.2Ti0.8O3 (PZT), and non- ferroelectric SrTiO3 (STO)], and they further proposed a uni- versal mechanism, i.e., voltage-driven oxygen vacancy migration across the oxide/La2/3Sr1/3MnO3 interface. As seen above, the RS depends strongly on the electrical and chemical properties of the electrode/ferroelectric interface. It is therefore of great importance to investigate the electrode dependence of RS, which is critical for understanding the true mechanism of RS.

In addition, the mechanism of PVE in ferroelectrics is also elusive, and both bulk effects (including lattice non- centrosymmetry^16–18 and depolarization field^19–21) and inter- face effects (interface Schottky barrier^22–24) have been proposed. To distinguish the bulk and interface effects, one needs to create different interface contacts (Schottky-type and Ohmic-type) for the ferroelectric capacitors and then compare the capacitors’ PVEs. This points to the necessity of studying the effects of different electrode materials on the PVE.

Hence, to elucidate the mechanisms of both RS and PVE in ferroelectrics, a comparative study on the ferroelec-tric capacitors with different electrodes is needed. Herein, we choose the heterostructures comprising BTO films ( 350 nm), SrRuO3 (SRO) bottom electrodes ( 40 nm), and Ti and Pt top electrodes as the model systems. The 350-nm BTO films typically exhibit polarization-voltage (P-V) hys-teresis loops with small coercive voltages (V_c),²⁵ allowing us

0003-6951/2017/111(25)/252901/5/$30.00 111, 252901-1 Published by AIP Publishing.

to affirmatively switch the polarization using only a small voltage. Ti and Pt have different work functions [4.3 eV (Ref. 26) versus 5.6 eV (Ref. 27)], which could form Ohmic and Schottky contacts with the BTO,¹² respectively. By com- paring the RS and PVE in the Ti/BTO/SRO and Pt/BTO/

SRO capacitors, we find that the RS results from the open- ing/closing of conduction paths in the interfacial layer while the PVE is driven by an unswitchable internal bias field.

Quite unexpectedly, the ferroelectric polarization seems to play a minor role in both the RS and PVE.

The epitaxial BTO thin films were grown on the (001)c

STO substrates with 40-nm SRO buffer layers, using the pulsed laser deposition (PLD) with the flowing O2 pressure of 1 Pa and the temperature of 700 C. Then, the circular Pt and Ti electrodes of 50 lm in diameter and 15 nm in thick- ness were deposited on the BTO films by PLD at room tem- perature and vacuum ( 10 ⁴ Pa). The crystal structures were investigated by X-ray diffraction (XRD; PANalytical X’Pert PRO diffractometer). The polarization-voltage (P-V) hyster- esis loops were measured with a ferroelectric tester (Radiant Precision Multiferroic). The conduction and dielectric char- acteristics were studied using a Keithley 6430 SourceMeter and an Agilent E4980A LCR meter, respectively. An ultravi- olet light-emitting diode (LED) with a light wavelength of 365 nm and an intensity of 300 mW/cm² was used as the light source. Piezoresponse force microscopy (PFM) studies were performed using a scanning probe microscope (Cypher, Asylum Research, USA) and Pt/Ir-coated silicon cantilever tips (EFM, Nanoworld).

Figure 1(a) shows the XRD h–2h diffraction pattern of the BTO/SRO bilayers grown on the STO substrate. Only (00l) peaks from the BTO, SRO, and STO are seen, and no impurity phases are detected. The (002) peak of BTO splits into two, suggesting the coexistence of two tetragonal phases (T1 and T2 phases). The c-axis lengths of the BTO T1 and T2 phases are calculated to be 0.41 nm and 0.43 nm, respec- tively. The coexistence of T1 and T2 phases may be due to the strain relaxation in the BTO thick films.²⁸

Figure 1(b) presents the topography image of the BTO film, revealing a relatively smooth surface (roughness:

0.55 nm). The PFM phase image [Fig. 1(c)] was obtained after electrically writing two adjacent areas with 64 V. The 4 V-written region shows 180 phase contrast compared with the þ4 V-written region, indicating that the domains are switchable. In addition, the domains in the as-grown region are largely pointing downward. This self-polarization phe- nomenon is typically caused by an internal bias field (E_in) across the ferroelectric film, whose formation may be due to stress gradients,²⁹ asymmetric electrostatic boundary condi- tions,^30,31 and non-uniformly distributed space charges.³² As inferred from the direction of self-polarization, E_in is point- ing downward.

From the capacitance-frequency (C-f) characteristics [Fig. 2(a)], the respective capacitances of Ti/BTO/SRO and Pt/BTO/SRO capacitors are 140 pF and 70 pF at 10⁶ Hz, corresponding to the effective dielectric constants of 2180 and 1060, respectively. Note that the bulk BTO has a dielectric constant of 2800 at 10⁶ Hz,³³ which is similar to that of the Ti/BTO/SRO capacitor but twice larger than that of the Pt/BTO/SRO capacitor. These observations suggest that the Ti/BTO interface is Ohmic while an interfacial layer exists between Pt and BTO.¹² The formation of the Pt/BTO interfacial layer may be attributed to the depolarization effect,³⁴ metal deposition-induced disorder/damage,³⁵ accu- mulation of oxygen vacancies,³⁶ and inter-diffusion between metal and oxide.³⁷ Assuming a dielectric constant of 26 for the interfacial layer,³⁸ its thickness may be estimated as 4.5 nm.

The capacitance-voltage (C-V) curves of both Ti/BTO/

SRO and Pt/BTO/SRO capacitors show double peaks, which are the typical features of ferroelectrics [Figs. 2(b) and 2(c)].

However, the C-V curve of the Ti/BTO/SRO capacitor shifts toward the negative voltage axis whereas that of the Pt/BTO/

SRO capacitor shifts toward the opposite direction. The neg- ative voltage shift in the Ti/BTO/SRO capacitor is an indica- tor of an Ein pointing downward [see the inset in Fig. 2(b)], which is also suggested by the downward self-polarization mentioned above. In the Pt/BTO/SRO capacitor, however, the high work function of Pt may create a built-in field (E_bi)

FIG. 1. (a) XRD h-2h scan of the 350-nm BTO film grown on the SRO buffered STO substrate. (b) Topography and (c) PFM phase images of the BTO film after the electrical writing.

FIG. 2. (a) C-f characteristics of Ti/BTO/SRO and Pt/BTO/SRO capacitors.

C-V characteristics of (b) Ti/BTO/SRO and (c) Pt/BTO/SRO capacitors at 1 MHz. Insets in (b) and (c) schematically show the field distributions in the two capacitors, respectively. (d) P-V loops of the Ti/BTO/SRO and Pt/BTO/

SRO capacitors (frequency ¼ 10 kHz).

252901-3 Fan et al. Appl. Phys. Lett. 111, 252901 (2017)

in the Pt/BTO interfacial layer whose direction is opposite to that of Ein [inset in Fig. 2(c)].³⁹ The overall voltage shift in the Pt/BTO/SRO capacitor therefore depends on the sum of the effects of E_bi and E_in, which may have a positive direc- tion. Similar imprint behaviors are observed in the P-V hysteresis loops [Fig. 2(d)], where the Ti/BTO/SRO and Pt/BTO/SRO capacitors exhibit the negative and positive voltage shifts, respectively. The pinched positive branch of the P-V loop observed in the Ti/BTO/SRO capacitor may be correlated with the downward self-polarization. The reduced remanent polarization of the Pt/BTO/SRO capacitor may be attributed to the interfacial layer-induced polarization degradation.³⁸

As shown above, there are significant differences between the Ti/BTO and Pt/BTO interfaces, enabling us to study the interface effects on the RS behaviors. Figure 3(a) shows the quasi-static I-V curves of the Ti/BTO/SRO and Pt/BTO/SRO capacitors measured with the rate of 0.032 V/s and the sequence of þ8 V ! 0 ! 8 V ! 0 ! þ8 V. Here, the applied voltage is termed as positive if a positive bias is applied on the top electrode, and the current flowing from top to bottom has a positive direction. As clearly seen, negli- gible hysteresis is observed in the Ti/BTO/SRO capacitor, although the applied voltage is far beyond the V_c of BTO.

(Note that the inconsistency between the voltages corre- sponding to the two current minima is probably due to the dielectric relaxation rather than the RS;⁴⁰ see Fig. S1 in the supplementary material.). If the conduction is mainly con- trolled by the polarization-modulated interface barrier, a sig- nificant RS with an ON/OFF ratio on the order of 10³ is expected in spite of the small polarization of BTO (see Sec.

2 in the supplementary material). The absence of RS sug- gests two possibilities: (i) the polarization may be completely screened and (ii) other conduction mechanism(s) may work in the Ti/BTO/SRO capacitor. To unravel the

FIG. 3. (a) I-V curves of the Ti/BTO/SRO and Pt/BTO/SRO capacitors. Plots of ln(I/V) versus V^1/2 for (b) Ti/BTO/SRO and (c) Pt/BTO/SRO capaci-

tors. (d) Plot of ln(I/V²) versus (1/V) for the Pt/BTO/SRO capacitor at the negative biases. Insets in (b) schematically show the conduction mechanism in the Ti/BTO/SRO capacitor, where the PFE controls the conductions in both positive and negative voltage regimes. Insets in (c) and (d) schemati- cally show the conduction mechanisms in the Pt/BTO/SRO capacitor, where the PFE and FNT control the conductions in the positive and negative volt- age regimes, respectively. The pink line in the inset of (c) represents the opened conduction paths.

conduction mechanism, we have fitted the I-V curves using a series of models (Fig. S2 in the supplementary material).

Among them, a bulk-limited conduction model, i.e., Poole- Frenkel emission (PFE), can best fit the experimental results.

The PFE model describes the J-V relationship as⁴¹

J AE

exp

pffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffi

3

;

(1)

2 _U t e eE=pKe0

¼ 4 5

kBT

where A is a constant, E is the electric field, U_t is the trap ionization energy, e is the electron charge, e0 is the vacuum permittivity, K is the optical dielectric constant, kB is the Boltzmann constant, and T is the absolute temperature. As can be seen from Fig. 3(b), the ln(I/V) V^0.5 curves in both positive and negative voltage regimes exhibit linearity.

Moreover, the K values extracted from the curve slopes are 6.4–6.7, consistent with the experimental K values of BTO ( 6.3).⁴² Therefore, the PFE is considered as the dominant conduction mechanism in the Ti/BTO/SRO capacitor.⁴³ The PFE is a bulk effect and is thus insensitive to the polarization modulation, which may be one reason for the absence of RS.

In addition, the complete polarization screening may also lead to the absence of RS, even in the ferroelectric capacitors whose dominant conduction mechanism is interface-limited (Figs. S3–S5 in the supplementary material).

In contrast, the Pt/BTO/SRO capacitor shows hysteretic I-V characteristics [Fig. 3(a)]. The voltage of the switching from the high resistance state (HRS) to the low resistance state (LRS) is þ5 V, while that of the LRS ! HRS switching is 4 V. These voltages far exceed the Vc of the Pt/BTO/SRO capacitor, excluding the polarization switching as the origin of RS. Because the BTO bulk region in the Pt/BTO/SRO capacitor is basically the same as that in the Ti/BTO/SRO capacitor, the emergence of RS in the former capacitor may thus be related to the Pt/BTO interfa-cial layer. We therefore conjecture that the RS may origi-nate from the opening/closing of conduction paths in the interfacial layer.^44–47 When a large positive voltage (e.g., þ5 V) is applied on the Pt electrode, the electrons could be detrapped from the trapping states in the interfacial layer. With the assistance of unoccupied trapping states, the electrons are able to resonantly tunnel through the interfa-cial layer, which can be viewed as the opening of conduc-tion paths. In this case, the PFE shall limit the overall conduction, similar to that in the Ti/BTO/SRO capacitor. This conjecture could be supported by the observation of similar I-V curves in the two capacitors when the voltage is swept from þ8 V to þ3 V [Figs. 3(a)–3(c)]. When a nega-tive voltage is applied, the trapping states start to capture electrons and eventually become filled at a large negative voltage (e.g., 4 V). Hence, the conduction paths are closed, and the high potential barrier in the interfacial layer may mainly control the current flow, which can be sup-ported by the fitting of the I-V curves ( 4 V ! 8 V) to the Fowler-Nordheim tunneling (FNT) model [Fig. 3(d)]. The current due to the FNT is given by

p ^{ffiffiffiffiffiffiffiffi}ffiffiffi

3

=

2 !

;

8 I ¼

BE² ex

p p

3h eE

U

i (2)

252901-4 Fan et al. Appl. Phys. Lett. 111, 252901 (2017)

where B is a constant, meff is the effective electron mass, and Ui

is the potential barrier height in the interfacial layer.^41,48 Assuming that meff is 1–5m0 (m0 is the free electron mass),¹⁰ the Ui is estimated to be 0.45–0.78 eV based on the fitting, consistent with those reported previously.⁴⁹ Note that the PFE model has also been used for fitting, but it yields an unreasonably large K value of 10 [Fig. 3(c)].

Next, the PVEs of the Ti/BTO/SRO and Pt/BTO/SRO capacitors are comparatively studied. Prior to the photovol- taic measurements, the polarization down (up) state was set by applying þ8 V ( 8 V) pulses (pulse width ¼ 0.1 ms) on the top electrode. Figures 4(a) and 4(b) present the dark and illuminated I-V curves for the Ti/BTO/SRO and Pt/BTO/

SRO capacitors, respectively. The open circuit voltage (V_OC) of the Ti/BTO/SRO capacitor is 0.88 V, close to that of the Pt/BTO/SRO capacitor ( 0.79 V). This suggests that the PVE mainly occurs in the BTO bulk region whereas that occurring in the interfacial layer is negligible. In addition, the V_OCs of both capacitors in the different polarization states are almost unchanged. Therefore, the driving force for separating photo-generated electron-hole pairs in the BTO bulk region appears to be an unswitchable electric field, rem- iniscent of the internal bias field Ein. As discussed above, the E_in, which is also responsible for the self-polarization and imprint phenomena, has a downward orientation. As a result of the downward E_in, the energy band of the BTO is tilted down at the top side with respect to the bottom side [insets in Figs. 4(a) and 4(b)], consistent with the negative sign of the V_OC. In contrast to the E_in, the polarization weakly influences the energy band, probably due to the complete screening.⁵⁰ If the polarization mainly controls the energy band, the downward (upward) polarization will tilt the bottom side down (up) with respect to the top side, resulting in a positive (negative) VOC (Fig. S6 in the supplementary material). This however contradicts our observation, i.e., a negative V_OC in the polarization down state. Also note that the high work function of Pt may induce an upward band bending and a large built-in field E_bi in the Pt/BTO interfacial layer. However, as mentioned earlier, the PVE in the interfacial layer is negligible due to its very small thickness ( 4.5 nm, as estimated by the C-f results).

Figures 4(a) and 4(b) also show that the short-circuit currents (I_SC) of the Ti/BTO/SRO and Pt/BTO/SRO capaci- tors are 1.2 nA and 0.3 nA, respectively. The suppressed I_SC in the Pt/BTO/SRO capacitor may be due to the less effi- cient charge extraction at the Pt/BTO interface.

FIG. 4. I-V curves of (a) Ti/BTO/SRO and (b) Pt/BTO/SRO capacitors under the dark and illumination conditions. Insets in (a) and (b) show the schematic energy band diagrams of the two capacitors under the short circuit condition, respectively.

In summary, the Ti/BTO/SRO and Pt/BTO/SRO capaci- tors were fabricated by PLD, and their dielectric, ferroelec- tric, RS, and PVE properties were studied comparatively.

The C-f results suggest that the Ti/BTO interface is Ohmic while an interfacial layer forms between the Pt and BTO.

This interfacial layer, in which the opening/closing of con- duction paths may occur, seems to be the origin of RS observed in the Pt/BTO/SRO capacitor. However, in the Ti/BTO/SRO capacitor without the interfacial layer effect, negligible RS is observed and the dominant conduction mechanism is identified as PFE. On the other hand, the com- bined results of C-V, P-V, and PFM show the self- polarization and imprint phenomena, revealing the existence of E_in in the BTO films. This E_in may be responsible for the charge separation in the PVEs in both the Ti/BTO/SRO and Pt/BTO/SRO capacitors. As a result, the V_OCs of the two capacitors are similar in magnitude and they are electrically unswitchable. Our study therefore suggests that in certain ferroelectric capacitors, the interfacial layer and the internal bias field can cause the RS and PVE, respectively, whereas the polarization may play a minor role.

See supplementary material for rate-dependent I-V curves of the Pt/BTO/SRO capacitor, estimation of the RS ON/OFF ratio using the Schottky emission model, fittings of the I-V characteristics, and supplementary experiments on the PZT-based capacitors.

The authors thank the National Key Research Program of China (Nos. 2016YFA0201002 and 2016YFA0300101), the State Key Program for Basic Researches of China (No.

2015CB921202), the National Natural Science Foundation of China (Nos. 51602110, 11674108, and 51272078), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014), the Science and Technology Planning Project of Guangdong Province (No.

2015B090927006), the Natural Science Foundation of Guangdong Province (No. 2016A030308019), the Guangdong Innovative and Entrepreneurial Research Team Program (No.

2016ZT06D348), and the Shenzhen Science and Technology Innovation Commission (No. JCYJ20160613160524999).

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