Ferroelectricity-induced resistive switching in
Pb(Zr0.52Ti0.48)O3/Pr0.7Ca0.3MnO3/Nb-doped SrTiO3 epitaxial heterostructure
Sharif Md. Sadaf, El Mostafa Bourim, Xinjun Liu, Sakeb Hasan Choudhury, Dong-Wook Kim, and Hyunsang Hwang
Citation: Applied Physics Letters 100, 113505 (2012); doi: 10.1063/1.3694016 View online: http://dx.doi.org/10.1063/1.3694016
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/100/11?ver=pdfcov Published by the AIP Publishing
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Ferroelectricity-induced resistive switching in Pb(Zr
0.52Ti
0.48)O
3/
Pr
0.7Ca
0.3MnO
3/Nb-doped SrTiO
3epitaxial heterostructure
Sharif Md. Sadaf,1El Mostafa Bourim,2Xinjun Liu,1Sakeb Hasan Choudhury,3 Dong-Wook Kim,4and Hyunsang Hwang1,3,a)
1
School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong Dong, Buk-gu, Gwangju 500-712, South Korea
2
New and Renewable Energy Research Center, Ewha Womans University, Seoul 120-750, South Korea 3
Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology (GIST), 1 Oryong Dong, Buk-gu, Gwangju 500-712, South Korea
4
Department of Physics and Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, South Korea
(Received 7 December 2011; accepted 22 February 2012; published online 14 March 2012) We investigated the effect of a ferroelectric Pb(Zr0.52Ti0.48)O3(PZT) thin film on the generation of
resistive switching in a stacked Pr0.7Ca0.3MnO3 (PCMO)/Nb-doped SrTiO3 (Nb:STO)
heterostructure forming a p-n junction. To promote the ferroelectric effect, the thin PZT active layer was deposited on an epitaxially grown p-type PCMO film on a lattice-matched n-type Nb:STO single crystal. It was concluded that the observed resistive switching behavior in the all-perovskite Pt/PZT/PCMO/Nb:STO heterostructure was related to the modulation of PCMO/ Nb:STO p-n junction’s depletion width, which was caused either by the PZT ferroelectric polarization field effect, the electrochemical drift of oxygen ions under an electric field, or both simultaneously.VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3694016]
The physics and characteristics of the effect of ferroelec-tric switchable polarization in controlling the conductivity of a metal/ferroelectric Schottky contact,1 metal/ferroelectric/ semiconductor field effect (MFS-FE), and metal/ferroelectric/ insulator/semiconductor field effect (MFIS-FE) in parallel-capacitor- or coplanar-transistor-based devices continue to attract attention because of their high potential for memory device applications.2The hysteresis property of the ferroelec-tric polarization allows for a bi-stable interface polarization configuration, with the potential of using the resulting opposite electric fields to control the electronic behavior of an adjacent heterostructure in bistable mode.3–5
In this letter, we report the electrical characterization of a ferroelectrically tunable depletion zone of a p-n junction showing a resistive switching property. The investigated de-vice was composed of an all-perovskite heterostructure con-sisting of a Pr0.7Ca0.3MnO3 (PCMO)/Nb-doped SrTiO3
(Nb:STO) p-n junction on which a thin Pb(Zr0.52Ti0.48)O3
(PZT) layer was epitaxially grown. A device consisting of a simple PCMO/Nb:STO p-n junction structure, without the addition of the active PZT layer, showed only a rectifying current-voltage (I-V) characteristic without hysteresis, the existence of which is a conventional signature of resistive switching.6 Electrical characterizations of the PZT/PCMO/ Nb:STO multi-heterojunction showed low operating switch-ing voltages, good resistance state retention, high switchswitch-ing speed, and efficient pulse cycling endurance. Two kinds of mechanisms have been proposed for modulating the p-n junction resistance at the PCMO/Nb:STO interface. The first is electronic, involving the use of the ferroelectric polariza-tion field effect to modify the space charges of the p-n
junc-tion depletion zone, and the second involves the electrochemical drift of oxygen ions under the electric field.
To fabricate the main investigated device, a 25-nm PCMO thin film was first grown epitaxially using pulsed laser deposition under an oxygen pressure of 100 mTorr at 650C on an Nb:STO (0.7 wt. % Nb) conducting substrate. A 15-nm PZT film was then deposited onto the PCMO top surface under an oxygen pressure of 250 mTorr at 600C. A good stoichiometry and substrate-to-target distance (5 cm) was maintained to ensure epitaxial growth. For the electrical device characterization, a magnetron-sputtering system was used to deposit top Pt electrodes (100 100 lm2and 100 nm thick) patterned through a stencil shadow mask and a contin-uous Pt film as a bottom electrode pad. The I-V measure-ments were carried out using a semiconductor parameter analyzer (Agilent 4155 C). A bias voltage was applied with respect to the top electrode (TE) and the bottom electrode (BE) was grounded.
The DC I-V characteristic of the Pt/PZT/PCMO/ Nb:STO/Pt heterostructure device is shown in Fig.1(a). The observed I-V hysteresis loops were measured by sweeping voltage in the sequence of 0 V! þ2 V ! 0 V ! 2 V ! 0 V with a voltage ramp rate of 0.5 V/s. The virgin fabricated device initially had a high resistance state (HRS) and evolved to a low resistance state (LRS) when the voltage was swept from zero to positive values. In the subsequent voltage sweep, in the negative voltage region, the device re-sistance switched back from the LRS to the HRS. To eluci-date the origin of this resistive switching, we fabricated, separately, different control samples with different struc-tures: Pt/PCMO(poly)/Pt, Pt/PZT(poly)/PCMO(poly)/Pt, Pt/ PCMO(epi)/Nb:STO/Pt, and Pt/PZT(epi)/Nb:STO/Pt. The I-V characteristics of these samples were determined to find the potential mechanism/s involved in the main
a)Author to whom correspondence should be addressed. Electronic mail:
all-perovskite Pt/PZT(poly)/PCMO(poly)/Nb:STO/Pt hetero-structured device being investigated. The DC I-V measure-ments performed on the Pt/PCMO(epi/poly)/Pt sample (not presented) exhibited an Ohmic like behavior, which demon-strated that both the upper and lower Pt/PCMO contacts were Ohmic.7 The Pt/PZT(poly)/PCMO(poly)/Pt sample showed symmetrical I-V hysteresis characteristics (Fig.
1(b)). This electrical conduction was probably related to ei-ther a back-to-back connection of two Schottky barriers formed at the Pt/PZT and PZT/PCMO interfaces, with the hysteresis behavior induced via the ferroelectric polarization field effect on the Schottky barrier modulation1or, as was described in our previous paper,8 to the PCMO resistance change, explained in terms of polarization-induced lattice strains in the PCMO film.
As is known, PCMO and PZT commonly present p-type semiconductor properties,9while Nb:STO presents an n-type semiconductor characteristic.10The measured DC I-V char-acteristics for these layers, which were stacked in a manner to form a p-n junction, showed, for the Pt/PCMO/Nb:STO system, a p-n diode-like behavior without any resistance hys-teresis (Fig.1(c)), while in the Pt/PZT/Nb:STO p-n system, a diode-like behavior was also observed but was accompanied by a tight hysteresis loop and a high rectification ratio (not shown). In the former system, the electrical transport is cer-tainly dominated by the formed p-n junction because the Pt/ PCMO contact is Ohmic. However, in the latter system, according to the results of Blomet al.,1where the Schottky contact resistance modulation through a ferroelectric field effect demonstrated a small rectification ratio, we deduce that the current transport is not controlled by the Pt/PZT con-tact, and the resulting current transport is due to the PZT/ Nb:STO p-n junction in the Pt/PZT/Nb:STO system. Further-more, the absence of the Pt/PZT junction effect can be seen in a leakage current comparison for the device structures with and without a PZT layer. The I-V leakage current back-ground measured with a Pt/PCMO/Nb:STO junction under-went no change after inserting the active PZT layer. This presumes that, in the Pt/PZT/PCMO/Nb-STO device, only the PCMO/Nb:STO p-n junction resistance dominates the current transport, and the resistance effects of both the Pt/ PZT and PZT/PCMO contacts are negligible. Generally, epi-taxially grown PZT/PCMO films would generate clean and well-defined interfaces that are highly free of interface state defects. All of these characteristics result in lowering the Pt/ PZT and PZT/PCMO junction resistances and promote the conclusion that the PCMO/Nb:STO interface is the main
ori-gin of the observed I-V asymmetry. C-V characterizations (Fig.2) showed a typical p-n junction diode-like behavior in the C-V curve tendency; the capacitance increases when the voltage is swept from reverse bias (negative voltages) to for-ward bias (positive voltages). Such a behavior could be mainly governed by the PCMO/Nb:STO interface capaci-tance evolution under a DC bias sweep. The abnormally drastic decrease in the capacitance evolving to negative val-ues, observed in the positive voltage region (Fig.2) when the forward bias reaches a certain threshold, implies that the p-n junction depletion width becomes too narrow to trigger direct electron tunneling. Thus, the Pt/PZT/PCMO/Nb:STO heterostructure, at a high applied positive bias voltage, acts more like a resistor than a capacitor.11This also explains the high leakage current in the positive voltage direction and the absence of the symmetrical butterfly-shape C-V curve com-monly recorded for a PZT ferroelectric film sandwiched between two symmetrical electrodes.12Moreover, it is worth noting that the hysteresis loop demonstrated by the positive capacitance values (inset of Fig. 2) is directly linked to the reversal of the ferroelectric domains in the active PZT layer.
Based on the electronic mechanism by which the PCMO/Nb:STO interface resistance is modulated by the PZT ferroelectric polarization field effect, the resistive mem-ory in the Pt/PZT/PCMO/Nb:STO heterostructured device takes place as follows. When the ferroelectric polarization in PZT is downward directed under a positive bias voltage upon the Pt TE, the charge screening of the positive
FIG. 1. (Color online) Typical I-V curve characteristics of (a) epitaxial Pt/PZT/ PCMO/Nb:STO heterostructure, (b) poly-crystalline Pt/PZT/PCMO/Pt heterostruc-ture, and (c) epitaxial Pt/PCMO/Nb:STO heterostructure p-n junction.
FIG. 2. (Color online) Typical C-V characteristics for Pt/PZT/PCMO/ Nb:STO device. The inset shows the hysteresis loop formed by positive ca-pacitance values, which is a signature of the ferroelectric domain switching of the PZT in the stacked heterostructure.
polarization bound charges at the PZT/PCMO interface, through the ferroelectric polarization field effect acting dis-tantly in drifting the negative minority carriers from the PCMO p-type film and the majority electron carriers from the Nb:STO n-type, results in a decrease in the depleted p-n region width of the PCMO/Nb:STO junction. Thus, the sys-tem evolves to a stable LRS (Fig.3(a)). In contrast, when the polarization of PZT is upward directed under a negative bias upon the Pt TE, the screening of negative polarization bound charges, with the induced field, by attracting majority hole carriers from the PCMO film and repealing the majority elec-trons from the Nb:STO interface to the crystal bulk, results in a rise in the depleted p-n region width of the PCMO/ Nb:STO junction. Thus, the system switches to a stable HRS (Fig.3(b)). The resistive switching speed, which was meas-ured in the order of 100 ns in the investigated device, sup-ports the above electronic mechanism, in which the switching speed is related to the fast charging of the polar-ization screening at the PZT/PCMO interface during the PZT ferroelectric polarization reversal. Note that the PZT layer in the investigated structures is made to be sufficiently thin insomuch as to be transparent to charge transport through it.
Electrochemically induced oxygen ion transfer under the electric field could also be an alternative mechanism con-trolling the p-n junction width modulation at the PCMO/ Nb:STO interface. PCMO is a hole-doped p-type semicon-ductor, which can have its conductivity raised by an excess
of oxygen ions. On the other hand, Nb:STO is an electron-doped semiconductor, which can have its conductivity enhanced by inducing an excess of oxygen ion vacancies. Upon the application of a positive bias voltage on the Pt TE, oxygen ions will migrate from the Nb:STO interface to the PCMO interface (Fig.3(c)). Such ion transfer simultaneously increases the conduction in both semiconductors by reducing and increasing the oxygen vacancy density in PCMO and Nb:STO, respectively. Oxygen ion displacement conse-quently lowers the p-n depletion width of the PCMO/ Nb:STO junction. Hence, the device evolves to the LRS. Furthermore, the PZT ferroelectric polarization switching helps to force the oxygen transfer and ensures, through the field effect, its stability at the interface when the applied field ceases. In contrast, a negative bias voltage on the Pt TE has the opposite effect, changing resistance state from the LRS to HRS (Fig. 3(d)). Note that in a fabricated device with a PCMO thickness of >100 nm, the I-V hysteresis aspect was not observed anymore (not shown). This behavior favors the electronic model, in which the ferroelectric field effect becomes less effective in modulating the relatively far-located fixed space charge of the depleted p-n zone in the PCMO/Nb:STO junction.
The most intriguing properties of this investigated PZT/ PCMO/Nb:STO heterostructured device are its excellent re-sistance state retention at room temperature as well as at 85C (Fig.4(a)and its inset), switching endurance up to 108
FIG. 3. (Color online) Schematic diagrams: elec-tronic models of (a) LRS and (b) HRS and electro-chemical models of (c) LRS and (d) HRS.
FIG. 4. (Color online) (a) Retention properties of Pt/PZT/PCMO/Nb:STO device switched in LRS and HRS at room temperature (retention at 85C is shown in upper inset). The time depend-ent fitting data at RT of the initial LRS drift using the double exponential decay law is traced in solid line (blue color). (b) Pulse endurance test of the Pt/PZT/ PCMO/Nb:STO device continuously pulsed for over 108cycles.
cycles, and very fast switching speed (100 ns) (Fig. 4(b)). Such good performance characteristics are probably related to the epitaxially tailored p-n junction, which makes its resist-ance modulation effective via the electric field generated from the PZT film ferroelectric polarization on the p-n junc-tion charge carrier environment. However, the initial reten-tion degradareten-tion in the LRS, observed drifting at room temperature measurement, is believed to be due to the oxygen vacancy relaxation in PCMO,13along with mechanical strain relaxation in PZT.14We fitted the time dependence of the ini-tial LRS current part (blue line) with a double exponenini-tial decay function, I(t)¼ Io þ A1exp(t/s1) þ A2exp(t/s2),
where Io, A1, and A2are constants, and s1and s2 are time
constants, as shown in Fig.4(a). The determined time con-stant values, s1¼ 47 s, s2¼ 513 s, also support our above
assumptions.
In summary, we experimentally demonstrated the effect of ferroelectric PZT on an epitaxial PCMO/Nb:STO p-n junction. A device formed of a PCMO/Nb:STO p-n junction structure without using the active PZT layer showed only a rectifying I-V characteristic, without any hysteresis. The integration of ferroelectric PZT with the epitaxially grown PCMO/Nb:STO heterostructure demonstrated excellent resistive switching performance, overcoming the inherent slow switching, unstable resistance state retention, and high power device operation that have been reported in metal/ Nb:STO Schottky contact structure15 or in metal/ferroelec-tric/Nb:STO structure16 or in reactive metal/PCMO based memory devices.17
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea
government (MEST) (No. 2011-001864) and by R&D Pro-gram of the Ministry of Knowledge Economy. The authors thank Alex Ignatiev (University of Houston) for his useful suggestions and comments about the work.
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