Hysteretic characteristics of pulsed laser deposited 0.5Ba(Zr0.2Ti0.8

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Surfaces, Interfaces, and Applications

Hysteretic characteristics of pulsed laser deposited 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3/ZnO bilayers José P. B. Silva, Jun Wang, Gertjan Koster, Guus Rijnders, Raluca Negrea, Corneliu Ghica, Koppole C. Sekhar, Joaquim Agostinho Moreira, and Maria J. M. Gomes ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01695 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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ACS Applied Materials & Interfaces

Hysteretic characteristics of pulsed laser deposited 0.5Ba(Zr0.2Ti0.8)O30.5(Ba0.7Ca0.3)TiO3/ZnO bilayers J. P. B. Silva1,2,*, J. Wang3, G. Koster3, G. Rijnders3, R. F. Negrea4, C. Ghica4,K. C. Sekhar5, J. Agostinho Moreira2, M. J. M. Gomes1 1

Centre of Physics, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal

2

IFIMUP and IN-Institute of

Nanoscience and Nanotechnology, Departamento de

Física e Astronomia, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal 3

Faculty of Science and Technology and MESA+ Institute for Nanotechnology,

Inorganic Materials Science, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands 4

National

Institute

of

Materials

Physics,

105

bis

Atomistilor,

077125

Magurele, Romania 5

Department of Physics, School of Basic and Applied Science, Central University of

Tamil Nadu, Thiruvarur-610 101, India *Electronic mail: [email protected] ABSTRACT: In the present work, we study the hysteretic behavior in the electric field dependent capacitance and the current characteristics of 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BCZT)/ZnO bilayers deposited on 0.7 wt% Nb-doped (001)-SrTiO3 (Nb:STO) substrates in a metal-ferroelectric-semiconductor (MFS) configuration. The x-ray diffraction measurements show that the BCZT and ZnO layers are highly oriented along the c-axis and have a single perovskite and wurzite phases, respectively, while highresolution transmission electron microscopy revealed very sharp Nb:STO/BCZT/ZnO interfaces. The capacitance-electric field (C-E) characteristics of the bilayers exhibit a memory window of 47 kV/cm and a capacitance decrease of 22%, at negative bias. The later result is explained by the formation of a depletion region in the ZnO layer. Moreover, an unusual resistive switching (RS) behavior is observed in the BCZT films, 1 ACS Paragon Plus Environment

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where the RS ratio can be 500 times enhanced in the BCZT/ZnO bilayers. The RS enhancement can be understood by the barrier potential profile modulation at the depletion region, in the BCZT/ZnO junction, via ferroelectric polarization switching of the BCZT layer. This work builds a bridge between the hysteretic behavior observed either on the C-E and current-electric field characteristics on a MFS structure.

Keywords: Metal-ferroelectric-semiconductor structures; interfacial coupling effect; resistive switching; capacitance-voltage characteristics; ferroelectric properties.

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1. INTRODUCTION Metal-ferroelectric-semiconductor (MFS) structures have been continuously investigated for various applications, such as non-volatile memories, sensors and solar cells.1-4 The outstanding properties in the semiconductor/ferroelectric bilayers stems from the polarization coupling between the non-switchable polarization in semiconductor and the switchable polarization in the ferroelectric layer. However, the poor quality of interface at ferroelectric-semiconductor junction in Si-based MFS structures degrades the charge coupling effect and thus limited the memory/photovoltaic performance.5 Several oxide semiconductors, such as (La,Ca)MnO3, BiMnO3, Pr0.7Ca0.3MnO3 and ZnO, were investigated in different MFS structures.6-9 Among them, ZnO seems to be the most promising alternative to Si on MFS structures, due to its chemical compatibility with ferroelectric oxides.10 Moreover, ZnO exhibits a wide band gap which is suitable for optoelectronic applications and for electronic devices.11 The modulation of semiconductor conductivity by the polarization reversal and the capacitance-voltage (C-V) characteristics of semiconductor-ZnO/ferroelectric bilayers, such as ZnO/Pb(Zr,Ti)O3 (PZT), ZnO/BCZT and ZnO/BaTiO3 (BTO), have been investigated separately.11-15 However, there are no reports regarding the combined study of the modulation of semiconductor conductivity by ferroelectric polarization reversal and the C-V characteristics of semiconductor/ferroelectric systems. In particular, the CV and current-voltage (I-V) properties of epitaxial ZnO/BCZT bilayers has not been previously reported. In the quest of lead free ferroelectrics to replace Pb based ones, BCZT is recognized as promising lead free material. Compared to BaTiO3, BCZT exhibit superior ferroelectric and piezoelectric performance, comparable to PZT, due to its morphotropic phase boundary (MPB).16,17 Therefore, we have chosen BCZT as ferroelectric layer in the present work.



Therefore, in this paper we report on the microstructural, ferroelectric and electrical properties of Nb:STO/BCZT/Au and Nb:STO/BCZT/ZnO/Au structures. The effect of inclusion of a semiconductor ZnO thin layer between the BZCT layer and the Au electrode on the capacitance-electric field (C-E) and resistive switching (RS) characteristics in a Nb:STO/BCZT/Au structure is highlighted. Moreover, we have further discussed the C-E and RS mechanisms in the MFS structures, particularly the origin of the RS ratio enhancement and the role played by the polarization in the interfacial charge coupling effect.

2. EXPERIMENTAL DETAILS The BCZT target prepared by conventional solid state reaction as described in Silva el al.18 and the commercially available ZnO (99.99% purity from Kurt Lesker) targets were used as a source for the corresponding thin films. BCZT films and BCZT/ZnO bilayers were deposited in 0.7 wt% Nb-doped TiO2 terminated (001) SrTiO3 (Nb:STO) single-crystal substrates with RHEED-assisted pulsed laser deposition (PLD) system equipped with a KrF excimer laser source (Lambda Physik, 248 nm wavelength). The BCZT films (180 nm thick) were grown at a temperature of 750 °C in an oxygen pressure of 0.13 mbar. The laser pulse with a fluence of 1.8 Jcm-2 was incident on the BCZT target at a repetition rate of 1 Hz. Subsequently, an in-situ annealing was performed on BCZT thin films at 750 ºC in a 10 mbar oxygen atmosphere during 20 minutes. A multi-target carousel in PLD facilitates to grow the BCZT/ZnO bilayers without breaking the vacuum. The BCZT/ZnO bilayers were grown by depositing and annealing first the BCZT thin films under the same conditions. Followed by a ZnO layer with thickness of 15 nm was grown at a temperature of 400 °C in oxygen pressure of 0.05 mbar with a laser fluence of 2.0 Jcm−2 and a repetition frequency of 4 Hz. The films were cooled down to room temperature at a ramp rate of 20 °Cminute−1. 4 ACS Paragon Plus Environment

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The crystal structure of the thin films was investigated by x-ray diffraction (XRD) by using a Philips X’Pert X−ray diffractometer with the Cu Kα radiation (λ = 0.15418 nm). Atomic force microscopy (AFM) was used to investigate the surface morphology of the films with a Bruker Dimension Icon. The transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) investigations have been done on a Cs probe-corrected JEM ARM 200F analytical electron microscope equipped with a Gatan Quantum SE Image Filter for Electron Energy Loss Spectroscopy (EELS) and EELS– Spectrum Image (EELS – SI) analysis in the STEM mode. Imaging and spectral data processing have been made using specialized routines under Gatan Digital Micrograph. Cross-section TEM specimens have been prepared as described in Bucur el al.19 The electrical and polar measurements were done after depositing circular gold (Au) electrodes with a diameter of 1mm each on the upper surface of sample. The capacitance–electric field (C–E) characteristics were measured using a Precision LCR meter Agilent E4980A at an ac voltage of 50 mV. The hysteresis loop (P-E) characteristics were measured with a modified SawyerTower circuit by means of a sinusoidal signal at a frequency of 1 kHz. Current-voltage (I-V) characteristics were investigated using a Keithley 617 electrometer.

3. RESULTS AND DISCUSSION The reflection high energy electron diffraction (RHEED) pattern of Nb:STO single crystal substrate is shown in Fig. 1a).



Figure 1. RHEED pattern of: (a) Nb:STO substrate, after growth of (b) 3.3 (c) 6.7 (d) 8.0 (e) 21.0 nm of BCZT thin film and (f) BCZT (180nm)/ZnO (15 nm) bilayers.

The central specular spot appears more intense as compared to the two side spots, which is a signature of a TiO2 terminated SrTiO3 substrate.20,21 The horizontal streaks of this pattern suggest that the exposed surface of the substrate is smooth and well-ordered. Moreover, the presence of Kikuchi lines allows one to also conclude that this order is of long-range. The growth of the BCZT thin films was monitored through in-situ RHEED. Figures 1b)-e) show the RHEED patterns of the BCZT film at the deposition time when the film thickness is 3.3, 6.7, 8.0 and 21.0 nm, respectively. The RHEED patterns of the films with thickness 3.3 and 6.7 nm display the streaky structure, suggesting the twodimensional (2D) layer-by-layer growth mode. This conclusion is supported by the RHEED intensity oscillations during the BCZT film growth, as shown in Fig. 2. From


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this Figure, the first five RHEED intensity oscillations of the growth are clearly observed. This confirms an initial 2D layer-by-layer growth mode.22

Figure 2. The RHEED intensity oscillation for the BCZT ultrathin film grown on Nb:STO substrate.

The RHEED patterns recorded for the films with thickness beyond 8 nm (Figs. 1de)) shows the spotty structure suggesting a 3D island growth mode,21 which is initiated after the structure relaxation,23 as we will discuss in the following. The in-plane lattice parameter of the BCZT thin film when the film thickness were 3.3 (8.4 unit cells (u. c.)), 6.7 (16.6 u. c.), 8.0 (20.0 u. c.), 21.0 (53.3 u. c.) and 40 nm (101.5 u. c.) was estimated by using the spacing of RHEED pattern and its dependence on the thickness is shown in Fig. 3. The in-plane lattice parameter of the BCZT film with thickness of 3.3 nm is considerably lower than that of films with the thickness ≥ 8 nm, confirming that the 8 nm thick BCZT layer begins to relax. For instance, the inplane lattice parameter increases 1.7% for the 8 nm thick BCZT layer when compared



with the 3 nm one. For the BCZT film with a thickness beyond 8 nm, the in-plane lattice parameter reaches the bulk BCZT value.18

Figure 3. Effect of thickness on in-plane lattice parameter of the BCZT film.

The growth process of ZnO on top of the 180-nm BCZT layer was also monitored by RHEED. Figure 1f) exhibit a typical RHEED pattern of the BCZT(180 nm)/ZnO(15 nm) bilayers. The RHEED spotty pattern suggests that the ZnO film grows in a 3D mode on the top of the BCZT layer. Figures 4a) and b) illustrate the XRD diffractrograms of the BCZT(180 nm) film and BCZT(180 nm)/ZnO(15 nm) bilayers, respectively. The XRD patterns reveal the epitaxial growth of the BCZT films on Nb:STO (001) substrates. The XRD diffractogram in Fig. 4a) exhibits strong (00l) reflection peaks from the BCZT and Nb:STO substrates, pointing out for the highly oriented perovskite film along the c-axis. Moreover, no other reflection peaks arising from secondary phases are observed. In Fig. 4b), besides the diffraction peak from the NB:STO substrate and BCZT thin film, only one additional Bragg peak could be observed. The later peak arises due to (002) planes


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of wurtzite ZnO, evidencing for the preferential c-orientation of the grown ZnO films.24 This result is consistent with previous reports in which ZnO also exhibit (00l) texture on perovskite substrates, such as BTO and PZT.25,26 X-ray rocking curve of (002) ZnO(15 nm) peak was recorded and is shown in Fig. 4c). The full width at half maximum (FWHM) was found to be 1.46o, which is similar to the one observed in other ferroelectric-semiconductor bilayers, such as PZT/ZnO and BaTiO3/ZnO.11,27

Figure 4. XRD patterns of (a) BCZT(180nm) film and (b) BCZT(180nm)/ZnO(15nm) bilayers. (c) X-ray rocking curve of the ZnO film.

The structural quality of the Nb:STO/BCZT(180nm)/ZnO(15nm) heterostructure deposited has been also investigated by TEM. As shown by the TEM image at lowmagnification in Fig. 5a), the thicknesses of the BCZT and ZnO layers were confirmed to be 180 and 15 nm, respectively. A dense column structure of BCZT is observed. Furthermore, the BCZT layer is continuous and has a very small roughness, less than 3 9 ACS Paragon Plus Environment


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nm. The interfacial chemical sharpness was investigated by EELS in the STEM mode. Figure 5(a) also depicts the EELS maps corresponding to the area marked with a green rectangle in the TEM image, disclosing the spatial distribution of Ti, Ba, O elements from the BCZT thin film, Zn and O element from the ZnO film and Sr, Ti and O from the substrate. The RGB image (down-right) was obtained by overlapping the Ba M, Sr L and Zn L maps and confirms the interfaces sharpness without atomic inter diffusion.

Figure

5.

(a)

TEM

image

at

low-magnification

showing

Nb:STO/BCZT(180nm)/ZnO(15nm) structure and EELS-SI maps showing the elemental distribution inside of green rectangle. RGB map (down-right) was obtained overlapping the Ba M, Sr L and Zn L maps; (b) SAED pattern corresponding to TEM images (a) and (c), (d) HRTEM images of Nb:STO/BCZT and BCZT/ZnO interfaces. The area marked with a green rectangle in (a) was used to depict the EELS maps. The horizontal arrows in (c) and (d) delimit the BCZT and ZnO layers.


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The selected area electron diffraction (SAED) pattern confirm the epitaxial nature of the BCZT layer on the Nb:STO substrate, as evidenced in Fig. 5b). The HRTEM image shown in Fig 5c) discloses the high crystallinity of the BCZT thin film and very sharp BCZT/Nb:STO interface. In HRTEM image from Fig. 5d) can be observed a smooth ZnO/BCZT interface and the [002] growth direction of ZnO layer on the top of BCZT thin film. The crystallographic relation between the substrate, BCZT and ZnO thin films is as follows: [001]Nb: STO||[001]BCZT||[002]ZnO. Figures 6a) and b) show the AFM topographic images of the BCZT(180 nm) film and BCZT(180 nm)/ZnO(15 nm) bilayers, respectively. The surface morphology of the BCZT film evidences a uniform microstructure, with a root-mean square (RMS) roughness value of 0.8 nm and a grain size of ≈ 40 nm. This is an important feature because a flat surface is required for the deposition of the ZnO layer. The AFM image of the ZnO surface on top of the BCZT film, shown in Fig. 6b), disclose a relatively uniform structure with grains of two different mean sizes of 80 and 50 nm. Moreover, the RMS roughness is 1.1 nm.

Figure 6. AFM images of (a) BCZT(180nm) film and (b) BCZT(180nm)/ZnO(15nm) bilayers (1x1 µm2).



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Figures 7a) and b) show the polarization-electric field (P–E) hysteresis loops of the BCZT(180nm) film and BCZT(180nm)/ZnO(15nm) bilayers, respectively, measured at room temperature.

Figure

7.

P-E

hysteresis

loops

of

the

(a)

BCZT(180nm)

and

(b)

BCZT(180nm)/ZnO(15nm) bilayers.

The typical ferroelectric behavior is clearly apparent for both systems. The positive and negative remnant polarization (Pr+, Pr-), and the positive and negative coercive field (Ec+, Ec-)are presented in Table 1.

Table 1. Positive and negative remnant polarization and coercive field values. Films

Pr+ (µC/cm2) Pr- (µC/cm2) Ec+ (kV/cm)

Ec- (kV/cm)

BCZT

22.1±0.1

-19.3±0.1

52.7±0.4

-51.4±0.4

BCZT/ZnO

18.1±0.1

-7.9±0.1

25.7±0.4

-49.5±0.4


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The average remnant polarization (Pr) and coercive field (Ec) for BCZT films are 20.7 µC/cm2 and 52.1 kV/cm, respectively. The average Pr value is 31% higher than the values reported for BCZT thin films grown on SrTiO3 substrates, either by PLD or rfmagnetron sputtering.28,29 Moreover, the observed coercive field of BCZT film is similar to the reported values.29 The slight asymmetry of Ec, observed in Nb:STO/BCZT(180nm)/Au, is attributed to the Nb:STO and Au electrodes work functions difference.30 The P-E loop for the BCZT(180nm)/ZnO(15nm) bilayers is significantly asymmetric when compared to the BCZT(180nm) thin film. The Pr+ and Ec- are close to the ones observed in the BCZT thin film, but Pr- and Ec+ are significantly lower. The evident asymmetry of the P-E loop is due to the insertion of the ZnO layer, which indicated that the polarization play a role in the interfacial coupling effect. When a positive voltage is applied to the bottom electrode, ZnO layer is highly conductive owing to the charge accumulation at the interface. As a result, the potential falls at the BCZT layer. Therefore, the device behaves like a typical MFM capacitor. When a negative voltage is applied, a depletion region is formed in ZnO film. Thus, most of the potential falls in the ZnO layer. Thus, the potential in the BCZT layer is not strong enough to reverse the ferroelectric polarization.13 This effect is consistent with that observed in other ferroelectric-semiconductor bilayers, such as PZT/ZnO and BTO/ZnO.13,30 Figure 8a) depicts the frequency dependence of the capacitance-electric field (CE) characteristics for the BCZT(180nm) film.



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Figure 8. Frequency dependence of the C-E characteristics of the (a) BCZT(180nm) and (b) BCZT(180nm)/ZnO(15nm) bilayers.

The butterfly features of field dependent capacitance characteristics confirm the ferroelectric nature of the BCZT film. The fields corresponding to two peaks can be considered as coercive field (Ec). The value of Ec ≈ 54.0±0.4 kV/cm obtained from the C-E measurements agrees well with the one obtained from the P-E loops. Moreover, the C-E curves are well stable in the 1 kHz - 100 kHz frequency range, which confirms the negligible contribution of defects to the polarization reversal process.31 The

frequency

dependence

of

the

C-E

curves

measured

for

the

BCZT(180nm)/ZnO(15nm) bilayers is shown in Fig. 8b). In metal-ferroelectricsemiconductor configuration, the memory window width is defined as the flat-band voltage (VFB) shift of two direction of C-E curves.32 The BCZT/ZnO bilayers exhibits a memory window width of 47 kV/cm (0.6 V), at room temperature, which is comparable to the ones observed in other MFS structures grown on Si.5,33 Furthermore, the BCZT/ZnO bilayers exhibits a typical C-E hysteresis curve of a MFS structure. The C-E window in MFS devices is due to the polarization flipping.11 However, the C-E curve is asymmetric on either side of the two branches, when compared to the C-E curve of the


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BCZT thin film. The absence of butterfly features of C-E curves in a MFS structure is due the formation of depletion region in the BCZT/ZnO heterojunction.8,14 Under a forward bias in a MFS structure, the heterojunction is in charge accumulation, and therefore the device works like a MFM capacitor. Thus, the electric field corresponding to the C-E peak coincides with the Ec obtained from the P-E loop. Whereas under the reverse bias, a potential drop occurs at the depletion layer and thus shifts the peak position accordingly. Choi et al.34 have shown that the contribution of depletion region to the capacitance decreases with the enhancement of charge carriers in a MFS structure making the C-E curves similar to the one of a MFM structure. Therefore, the 22% capacitance decrease under negative bias is due to the development of a depletion region in ZnO.35 Since ZnO is a n-type semiconductor, the majority carriers are electrons;36 when a negative voltage is applied to the bottom electrode, the polarization vector in the ferroelectric layer points towards the bottom electrode. This causes the depletion of electrons in the semiconductor near the BCZT/ZnO interface.13 Therefore, this depletion region contributes with an additional capacitance (CD) in series with that of the ferroelectric layer (CBCZT) reducing the total capacitance.35 However, when a positive voltage is applied, the polarization vector points up to the ZnO layer, accumulating electrons in the semiconductor. In this case, no capacitance other than that contributed by the ferroelectric layer exists and a higher total capacitance is expected.35 Nevertheless,

the

higher

capacitance

under

a

positive

voltage

in

Nb:STO/BCZT/ZnO/Au heterostructure as compared to Nb:STO/BCZT/Au structure is due to low barrier height and width at BCZT/ZnO interface, due to charge accumulation, as compared to BCZT/Au interface under positive bias condition. Therefore, the depletion layer at the BCZT/Au may cause the smaller capacitance.37 The maximum depletion layer width (Wmax) in a semiconductor is given by34



= (

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/ )

(1)

where ε0 is the free space permittivity, εZnO is the static dielectric permittivity of ZnO,

is the surface potential, q is unit charge, and Na is space-charge density of the channel.

To

estimate

εZnO,

the

static

capacitance

of

BCZT(180nm)

and

BCZT(180nm)/ZnO(15nm) bilayers were measured, at a frequency of 100 kHz and with an amplitude of 50 mV, and were found to be 2.53 nF and 1.87 nF, respectively. The total capacitance of the MFS structure, Ctotal, is given by34

= ( + !" )

(2)

where the CZnO and CBCZT are the capacitances of the ZnO and BCZT, respectively. This allows us to calculate the CZnO and estimate εZnO, which was found to be ≈15. This value agrees well with the value found on literature for ZnO films (εZnO ≈12).38,39 Furthermore, from the equation 2, and with the assumption of ≈ 1% and & ≈

10( )*+,35,40 the thickness of the depletion region in ZnO is ≈12 nm. Moreover, we should mention that for a capacitance decrease of ≈17% in a MFS structure, a depletion layer of ≈18 nm is predicted to form, which means that our results agree well with the theoretical models for MFS structures.34 The decrease in capacitance of BCZT(180nm)/ZnO(15nm) bilayers can be calculated by using Eq. (1). As mentioned before, the total capacitance at a positive voltage (C+) is given by35

, ≈ (

!-. /

) =

!" (3)


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while the total capacitance at a negative voltage (C-) can be calculated by35

= (!

-. / , !1

)

(4)

By combining the Equations 3 and 4, the decrease in capacitance at a negative voltage can be expressed as:

1−

1 ≈1− , 1 + !" ⁄3

≈ 1 −

,-. /51 ⁄1 5-. /

(5)

where εBCZT is the static dielectric permittivity of BCZT(180nm) film estimated from CBCZT, dBCZT is the thickness of the BCZT film and εD and dD are the static dielectric permittivity (15) and thickness (12 nm) of the depletion layer. From this model, a capacitance decrease of 20% is predicted, which is similar to the measured value of 22%. Figure 9 depicts the current-electric field (I-E) characteristics of the BCZT(180nm) and BCZT(180nm)/ZnO(15nm) bilayers, measured at room temperature. The electric field was first swept from –Emax to +Emax, in forward direction and then vice-versa in reverse direction.



Figure 9. I-E curves of (a) BCZT(180nm) film and (b) BCZT(180nm)/ZnO(15nm) bilayers.

From Fig. 9 it is possible to conclude that the I-E curves are significantly different for BCZT (180nm) film and BCZT (180nm)/ZnO(15nm) bilayers, besides their similar growth conditions and electrode configurations. The I-E curves show rectifying characteristics, with a rectification ratio (IEmax/IEmin) of 0.8 and 26, in the I-E curves of the BCZT and BCZT/ZnO bilayers, respectively. The drastic change in the selfrectifying effect, when a thin ZnO layer is inserted between the BCZT and the Au electrode, is assigned to the band bending at the BCZT(180nm)/ZnO(15nm) interface,41 and dominates the charge transport in the bilayers. As illustrated in Fig. 9, both I-E curves display the electroforming-free resistive switching (RS) behavior. The resistance of the ON state and OFF state was read-out at 50 kV/cm and the RS ratio [ROFF/RON] was found to be ≈66 and ≈3.3x104 for the BCZT thin films and BCZT/ZnO bilayers, respectively. The observed RS ratio in the bilayers is superior than the one observed in other bilayers and in epitaxial ferroelectric thin films.12,42-48


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Before the discussion of the physical mechanism of the RS behavior, it is important to unravel the governing conduction mechanisms. Several leakage current models were considered to interpret the I-E curves in the BCZT thin film. As shown in Fig. 10a) for HRS, Ln (J/T2) shows a linear increase with E0.5, suggesting the dominant role of Schottky emission (SE) in the transport in the negative bias region. Usually SE occurs due to the thermionic injection of electron from electrode to ferroelectric thin layer.

Figure 10. (a) Ln(J/T2) versus E0.5 plot and linear fitting to the HRS state. (b) Linear fitting on a logarithmic scale to the LRS state for the BCZT film.

According to the Schottky emission model, the current density is given by49

6 = 7∗ 9 :;< =

( - >? ⁄@A 5 B"

C

(6)

where J is the leakage current density, A* is the effective Richardson constant, T is the temperature, is the Schottky barrier height, V is the applied voltage, D is the optical dielectric constant, and E is the Boltzmann constant. For the BCZT film, the data 19 ACS Paragon Plus Environment


handling in the case of SE gives the appropriate values of refractive index (n) = D F.H = 2.05, which is similar to the reported values for BaTiO3-based thin films (n = 2.23).50 For the LRS state, the current follows simply the Ohmic behavior as shown in Fig. 10b). The fitting of LRS by Ohm law and HRS by SE is a clear signature of an interface based mechanism51 and a similar behavior was also observed in the positive bias region. This clearly evidences that the polarization flipping might play a role in the RS effect. As previously reported in diodes based on ferroelectric thin films, the RS behavior is related with the modulation of the Schottky barriers at the top and bottom metal/ferroelectric interfaces.45 In the case of our BCZT film, the current when the polarization point to the bottom electrode is always higher than when the polarization point to the top electrode. Although the present RS behavior is different from the one observed in ferroelectric diodes, where the RS behavior is dependent of the polarity of the readout electric field, similar polarization-dependent RS behavior have also been reported for other ferroelectric thin films and ferroelectric-semiconductor bilayers.40,48,52 Moreover, in Fig. S1 (see Supporting Information) is shown an overlap of the P-E loops and the I-E. It is possible to observe that the coercive field and switching field are similar. This confirms the coupling between the RS and the polarization flipping in both structures.53 Therefore, the physical mechanism for the RS behavior can be understood as follows:41,43,51,52 When the direction of polarization is towards the Nb:STO surface (downward polarization state), the BCZT/Nb:STO interface is in charge accumulation, due to the depolarization field, and causes the barrier height and width reduction. At the same time, at the other BCZT side, the polarization charges spread the space depletion region and raise the barrier height/width of the Au/BCZT junction, as schematically shown in Fig. 11a).


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Figure 11. Schematic charge distribution and energy-band diagrams of BCZT film at ON state (a) and OFF state (b), as well as for the BCZT/ZnO bilayers at ON state (c) and OFF state (d).

As a result, the BCZT film is in the ON state. When the polarization direction is towards the Au (upward polarization state), the polarization accumulate the electrons at the BCZT side and consequently reduce the barrier of Au/BCZT junction, as schematically shown in Fig. 11b). Thus, the BCZT film is switched to the OFF state. In the case of the BCZT/ZnO bilayers, a 500-fold RS ratio enhancement was observed compared to the BCZT film. This enhancement is due to the insertion of a thin layer of ZnO between the BCZT film and the Au electrode, since ZnO shows a lower screening efficiency as compared to the Au and Nb:STO electrodes. As schematically shown in Figs. 11c) and d), the polarization flipping in BCZT leads to the formation of the ON and OFF states, due to the accumulation or depletion of electrons in the ZnO layer at the interface. For the upward polarization state, electrons are tending to accumulate at the interface and thus the heterojunction is in the ON state.13 For the 21 ACS Paragon Plus Environment


downward polarization state, electrons are depleted from the interface and thus the bilayers switches to the OFF state.13 Therefore, we should mention that in the case of the BCZT/ZnO bilayers the current in the downward polarization state is always lower, which is the reverse behavior when compared to the BCZT film. In fact, the insertion of the ZnO layer between the BCZT film and the Au electrode induced a polarization controlled depletion region at the BCZT/ZnO interface, which was already demonstrated by the C-E measurements. Thus, this depletion region was responsible for the change in the rectification direction and the overall RS behavior of the bilayers.54 The performance of the BCZT/ZnO bilayers has been studied by investigating the endurance characteristics and is shown in Fig. 12. Endurance characteristics revealed that both HRS and LRS states are stable up to 100 cycles. After the endurance test, a RS ratio of ≈104 is observed, which is higher than the one observed in other bilayers and epitaxial ferroelectric thin films, before the fatigue test. For instance, a RS ratio of 103 in Au/PZT/NSTO,44 30 in Au/BiFeO3/SrRuO3,45 400 in Au/BiFeO3/La0.6Sr0.4MnO3,43 102 in BaTiO3/La0.8Ca0.2MnO3,42 and 103 in La0.67Sr0.33MnO3/BaTiO341 was observed. Therefore, the bilayers display a reliable resistive switching behavior. Moreover, the switching field (≈53 kV/cm) in present bilayers is smaller as compared to other thin films, such as HfO2-based oxides, which are being extensively investigated for RS memory devices.55-57 Therefore, the present BCZT/ZnO bilayers can be considered as potential candidates for next generation memory applications.


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Figure 12. Endurance characteristics for the BCZT(180nm)/ZnO(15nm) bilayers.

4. CONCLUSIONS In summary, epitaxial based BCZT/ZnO MFS structures, with high-quality interfaces, were deposited by pulsed laser deposition on single crystalline Nb:STO (001) substrates. With an insertion of a 15 nm ZnO layer between the BCZT film and the Au top electrode, the shape of the C-E curves changes dramatically, exhibiting a significant asymmetry due to the existence of a 12 nm depletion region in the ZnO layer. Moreover, a memory window of 47 kV/cm was observed. An unusual RS behavior was also observed in BCZT films, where the RS ratio can be significantly enhanced by introducing the ZnO layer. The RS behavior observed in the MFS structure is attributed to the modulation of the barrier potential profile of the depletion layer of the BCZT/ZnO junction by the polarization flipping of the BCZT layer. Thus, these results, by combining both C-E and I-E hysteretic effects, offer a new perspective in using MFS structures for next generation memory devices.



SUPPORTING INFORMATION An overlap of the P-E loops and the I-E curves of the (a) BCZT(180nm) thin films and (b) BCZT(180nm)/ZnO(15nm) bilayers. “This material is available free of charge via the Internet at http://pubs.acs.org”

ACKNOWLEDGEMENTS This research was supported by: (i) Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2013 and (ii) Project Norte-070124-FEDER-000070 Nanomateriais Multifuncionais. This research was partially supported by the COST Action MP1308 "Towards Oxide-Based Electronics (TO-BE)". The authors acknowledge the CERIC-ERIC Consortium for access to experimental facilities and financial support under proposal 20157018. The author J.P.B.S. is grateful for financial support through the FCT Grant SFRH/BPD/92896/2013. The author K.C.S. acknowledge UGC and DST-SERB, Govt. of India for the funds through the grant No.F.4-5(59-FRP/2014(BSR)) and ECR/2017/000068, respectively. The authors R.F.N. and C.G. acknowledge the financial support from Romanian Ministry of Research and Innovation in the frame of the Core Program PN18-11. The authors would also like to acknowledge P. B. Tavares, from Centro de Química da Universidade de Trás-os-Montes e Alto Douro, for the supply of the BCZT PLD target.

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