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Ferroelectric properties of the organic films of poly(vinylidene fluoride-trifluoroethylene blended with inorganic Pb(Zr, Ti)O3
Thin Solid Films 546 (2013) 171–175
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Ferroelectric properties of the organic films of poly(vinylidene fluoride-trifluoroethylene blended with inorganic Pb(Zr, Ti)O3 Wan-Gyu Lee a, Byung Eun Park b,⁎, Kyung Eun Park b a b
Department of Nano CMOS, National NanoFab Center, Daejeon 305-806, Republic of Korea Schoool of Electrical and Computer Engineering, University of Seoul, Seoul 130-743, Republic of Korea
a r t i c l e
i n f o
Available online 20 June 2013 Keywords: Ferroelectric P(VDF-TrFE) Pb(Zr,Ti)O3 Memory window Hysteretic characteristics
a b s t r a c t Precursor films based on poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) and P(VDF-TrFE) blended with Pb(Zr,Ti)O3 were spin-coated on Si-substrates and subsequently annealed at 170 °C. X-ray diffraction studies showed that the amorphous precursor films crystallize to the γ-phase P(VDF-TrFE) without involving the formation of other polymorphs when the P(VDF-TrFE) is blended with Pb(Zr,Ti)O3, resulting in phase mixtures composed of a crystalline γ-phase P(VDF-TrFE) and an amorphous Pb(Zr,Ti)O3. A larger memory window width and higher accumulation capacitance, as well as a lower leakage current density are induced by the blended Pb(Zr,Ti)O3 within the low operating voltage ranges from −3.0 to 3.0 V and from − 2.0 to 2.0 V for 20 wt% and 40 wt% Pb(Zr,Ti)O3 blending, respectively. These improvements not only in the hysteretic capacitance–voltage characteristics but also in the leakage current density–electric field are directly correlated with the phase mixtures, their volume fraction, dipole moments, and formation of interface layer between the blended film and Si substrate. © 2013 Elsevier B.V. All rights reserved.
1. Introduction As one of the nonvolatile memories, ferroelectric random access memory (FeRAM) has attractive features including higher speeds, much longer endurance, longer communication distance, and lower power consumption compared with its conventional counterparts such as Electrically Erasable Programming Read Only Memory and Flash [1,2]. Despite such high performance, scaling cell size has to be solved to support commercialization of high-density FeRAM products. Much of these difficulties stem mainly from the fact that the metal–insulator–metal (MIM) capacitors over bit-line integration could not be vertically etched but sloped, or the complementary metal-oxide-semiconductor circuits underlying the MIM capacitors are degraded during the subsequent crystallization process of ferroelectric materials. Researchers have searched for appropriate materials and improved processes for the realization of non-volatile FeRAM devices. On the other hand, there has been a continuous potential in ferroelectric polymers as functional devices for flexible display devices, energy transduction, information storage, and sensors [3–5]. In particular, electronic devices based on organic materials have been widely investigated because of their high flexibility, low weight, and low processing costs. In addition, these polymers could be fabricated into products at lower
⁎ Corresponding author. E-mail address: [email protected] (B.E. Park). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.06.034
temperature than ~300 °C, which may suppress the formation of interfacial layers between ferroelectric films and Si substrate. Thus, they open a path for the development of organic–inorganic combined devices with enhanced ferroelectric properties. Specifically, poly(vinylidene fluoride) P(VDF) and its copolymer with trifluoroethylene P(VDF-TrFE) are well known as ferroelectric polymers with smaller coercive strength and lower temperature crystallization compared with those of inorganic ferroelectric materials. However, the remnant polarization of organic ferroelectrics is relatively lower than that of inorganic ferroelectric materials, which could be enhanced by combining inorganic alternative with P(VDF-TrFE) [6,7]. Previous report has appeared on the remnant polarization and piezoelectricity of the copolymer (P(VDF-TrFE))-ceramic Pb(Zr,Ti)O3 (PZT) composites depending on the volume fraction for the composite of P(VDF) and PZT [8]. More recently, organic–inorganic hybrid materials have come under study as semiconducting channels in thin-film field-effect transistors because they offer both the superior carrier mobility of inorganic semiconductors and the processability of organic materials [9]. However, there have been few reports of enhanced ferroelectric properties of the organic P(VDF-TrFE) films blended with inorganic PZT. The objectives of this study were to explore the possibility of using sol–gel to deposit organic–inorganic combined thin films of (P(VDF-TrFE)1 − δ–PZTδ (0 ≤ δ ≤ 0.4) on Si-substrate (100) and to characterize the physical and electrical properties of the resulting films, as well as to investigate their ferroelectric performance in a metalferroelectric oxide-semiconductor capacitor structure.
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2. Experimental details 2.1. PZT blended P(VDF-TrFE) film preparation Amorphous precursor films were deposited from mixed solutions using a spin-coating process. One of the precursor solutions, P(VDF-TrFE) was prepared by dissolving 0.15 g of the vinylidene fluoridetrifluoroethylene P(VDF-TrFE) copolymer powder (99.9%, SOLVAY) in 2.5 mL of dimethyl formamide (99.9%, Aldrich) and stirring for 24 h at 60 °C. The resulting solution had an atomic VDF/TrFE ratio of 52:48, which was a composition approximately midway between VDF and TrFE. The others were prepared by blending the P(VDF-TrFE) solution with 20 wt.% and 40 wt.% of a 0.4 mol PZT solution (99.9%, Inostek Inc.). The resulting precursor solutions had atomic Pb/Zr/Ti ratios of 110/35/65, which is the ratio typically applied for FeRAM devices. (100)-oriented Si single-crystal substrates (1 × 1 cm) doped with B were coated with the P(VDF-TrFE) precursor solution at 500 rpm for 5 s, and with the PZT blended P(VDF-TrFE) precursor solutions at 1500 rpm for 40 s under ambient conditions, to make the annealed films almost with the same thickness of 100 nm. For the measurement of polarization versus electric field (P–E), the substrates were coated with 150 nm Pt film on Si single-crystal substrates prior to spincoating. The precursor films were subsequently pyrolyzed and annealed in a box furnace at 170 °C for 0.5 h. Au top electrodes were deposited on the formed ferroelectric films using electron-beam evaporation through a metal shadow mask with a diameter of 30 μm. 2.2. PZT blended P(VDF-TrFE) film characterization The thicknesses were measured using a field emission-scanning electron microscopy after annealing the precursor films. X-ray diffraction (XRD) was used to assess the phases and crystallographic textures of the product films. Routine analysis was performed using a 2θ diffractometer (Rigaku, Tokyo, Japan) with monochromatized Cu-K radiation that was supplied by a rotating anode generator operating at 40 kV and 300 mA with a scanning speed of 2°/min and a step size of 0.01°. The capacitance versus voltage (C–V) characteristics was measured by sweeping the gate voltage from inversion to accumulation at room temperature. The leakage current density versus electric field (J–E) characteristics was measured by sweeping the gate voltage from 5.0 V to −5.0 V at room temperature. An electric field up to 3 MV/cm was applied to the films at a frequency of 10 Hz during the polarization versus electric field (P–E) measurement. The synthesized capacitors were also investigated by transmission electron microscopy (JEM ARM 200 F, JEOL) operating at 200 kV, energy dispersive spectroscopy (EDS) for further evaluation and examination of the Au/0, 40 wt.% PZT-blended P(VDF-TrFE) films/Si substrate capacitors. The samples were prepared by precision ion polishing system (Gatan, Model 691) having double (top and bottom) ion guns operating at 3.6 keV ion beam energy with 6° milling angle under a vacuum level of 5 × 10−5 Torr for the double guns.
Fig. 1. XRD patterns of the P(VDF-TrFE) and 20, and 40 wt.% PZT blended P(VDF-TrFE) films annealed at 170 °C: (a) P(VDF-TrFE); (b) 20 wt.% PZT blended P(VDF-TrFE); (c) 40 wt.% PZT blended P(VDF-TrFE).
films blended with 20, 40 wt.% PZT resembled that of an amorphous film, in which there was an increase in the intensity of the broadening with the blended PZT content, as shown in Fig. 1(b) and (c). With blending PZT, a two-phase mixture was formed in the annealed films blended with 20 wt.% and 40 wt.% PZT; one was a γ-phase P(VDF-TrFE), the other was an amorphous PZT. The relative amount of each phase was contrary to the relative change of other phase, i.e., the volume fraction of γ phase (1-XPZT) decreases with the increase of PZT volume fraction (XPZT), as confirmed by the relative intensity change of X-ray
3. Results and discussion 3.1. Phase development in the blended P(VDF-TrFE)–PZT films X-ray diffraction patterns of the films annealed for 30 min at 170 °C are presented in Fig. 1. In Fig. 1(a), the crystalline phase was detected in the P(VDF-TrFE) film that had 0 wt.% PZT. The pattern can be identified as belonging to the orthorhombic γ-phase (C2cm, a = 0.497, b = 0.966, c = 0.918 nm) (020) and/or (110) of P(VDF-TrFE) at 2θ = 19.1° [10,11]. No shift in peak position was evident following the blended films with 20 wt.% and 40 wt.% PZT. An increase in PZT composition led to a decrease in the peak intensity from I = 1263 to I = 850 and I = 803 at 2θ = 19.1°, and to the appearance of an interesting broad maximum of the precursor films. The XRD patterns obtained from the
Fig. 2. C–V characteristics of Au/P(VDF-TrFE) blended with 0, 20, and 40 wt.% PZT/Si capacitors annealed at 170 °C.
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Fig. 3. J–E characteristics of Au/P(VDF-TrFE) blended with 0, 20, and 40 wt.% PZT/Si capacitors annealed at 170 °C.
diffraction patterns, inasmuch as an ordinary two-phase mixture shows the same behavior in its temperature-composition phase diagram as this study does. 3.2. Ferroelectric properties of the blended P(VDF-TrFE)–PZT films Fig. 2 shows the typical C–V curves of Au/0, 20, 40 wt.% PZTblended P(VDF-TrFE) films/Si-substrate capacitors annealed at 170 °C. Clockwise hysteretic behavior was observed in all the films. The capacitance versus voltage characteristics were enhanced by blending Pb(Zr,Ti)O3, as shown in Fig. 2. These improvements included: (1) a
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stronger polarization effect than that in the single P(VDF-TrFE) film only; (2) an increase in memory window width with the blended PZT; and (3) a lower operating voltage. As pointed out earlier [12], ferroelectric behavior stems primarily from the crystallization of γ-phase P(VDF-TrFE) rather than from the amorphous phase. Furthermore, it can also be seen in Fig. 2 that the ferroelectric property corresponding to the accumulation capacitance is mainly determined by the linear combination of γ-phase P(VDF-TrFE) and amorphous PZT with their volume fraction. For example, the accumulated capacitance is about 40, 50, and 55 nF/cm2 at −3 V, corresponding to the crystallization of γ-phase P(VDF-TrFE) (black curve in Fig. 2), the combined γ-phase P(VDF-TrFE) and amorphous PZT with different volume fraction of each phase (red and blue curves in Fig. 2), respectively. However, the memory window width does not show the exactly same trend as the accumulated capacitance, which suggests that the memory window width is not a function of the degree of P(VDF-TrFE) crystallization, the presence of PZT, and their volume fraction, but of the combined physical state of dipole moments in the phase mixture of crystalline γ-phase P(VDF-TrFE) and amorphous PZT. The leakage current density versus electric field (J–E) characteristics for the Au/0, 20, and 40 wt.% PZT-blended P(VDF-TrFE) films/Si substrate capacitors is shown in Fig. 3. The leakage current of the films at an electric field of − 1.0 MV/cm was approximately 3.0 × 10− 6, 4.0 × 10− 8, and 3.5 × 10− 8 A/cm2 for the PZT composition of 0, 20, and 40 wt.%, respectively. As can be seen in Fig. 3, it is clear that the leakage current density of the films containing 20 wt.%, 40 wt.% PZT is relatively low in comparison with that of γ-phase P(VDF-TrFE) film, and higher-percentage of PZT blending provides better leakage current property than lower-percentage of PZT blending in the phase mixture of γ-phase and an amorphous PZT phase. These lower levels of leakage current density at a −1.0 MV/cm are
Fig. 4. Cross-sectional HRTEM images of the Au/0, 40 wt.% PZT blended P(VDF-TrFE)/Si capacitors: (a) P(VDF-TrFE); (b) 40 wt.% PZT blended P(VDF-TrFE). EDX spectrum of the interface layer between PZT blended P(VDF-TrFE) films and Si substrates, in which the measuring points P1, P2, and P3 are marked by colored circles: (c), (d), and (e), respectively.
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comparable to the lowest results, approximately 1 × 10−7 A/cm2 of inorganic strontium bismuth tantalite (SBT) films on an HfO2 [13]. Furthermore, they show the electric field independence from −5.0 V to +5.0 V with a transition from an asymmetric feature to symmetric feature of leakage current density versus electric field.
A more detailed investigation of the Au/0, 40 wt.% PZT-blended P(VDF-TrFE) films/Si-substrate capacitors was performed by analyzing high-resolution transmission electron microscopy (HRTEM) images as in Fig. 4(a) and (b). High-resolution image acquired from a 40 wt.% PZT-blended P(VDF-TrFE)/Si-substrate shows what appear to be an interfacial layer between the PZT blended P(VDF-TrFE) and Si-substrate even if low temperature annealing was applied. Lowering of leakage current level and of strong voltage dependence in both the 20 wt.% and 40 wt.% PZT blended films is attributed to the formation of the interfacial layer between the PZT blended P(VDF-TrFE) film and Si-substrate that is also an amorphous phase, corresponding to the XRD result of Fig. 1. Energy dispersive X-ray spectroscopy (EDS) was used to further evaluate the composition of the interface film formed during the low temperature annealing at 170 °C, as shown in Fig. 4(c), (d), and (e). The probing points are marked by the color circles in Fig. 4(b). All these EDS data clearly indicate that Pb oxide layer was formed between the PZT blended P(VDF-TrFE) films and Si-substrate with the thickness of less than 4 nm, which is the main cause of improved leakage current density versus electric field (J–E) characteristics for the Au/20, 40 wt.% PZT-blended P(VDF-TrFE) films/Si-substrate capacitors. The polarization versus electric field (P–E) characteristics for the 0, 20, and 40 wt.% PZT blended P(VDF-TrFE) films is presented in Fig. 5. Compared with P(VDF-TrFE) film, PZT blended films show tilted P–E hysteresis loops. Although all these capacitors have similar values of coercive field (Ec) 0.35 MV/cm, the remnant polarization (Pr) decreases from 1.3 μC/cm2 for the 0 wt.% PZT blended film to 0.7 μC/cm2 for both 20 and 40 wt.% PZT blended films, which are in contrast to the results observed from C–V, and J–E as shown in Figs. 2 and 3. It should be noted that the formation of interfacial layer, Pb oxide between the PZT blended films and Si substrates would induce the depolarization field and hence tends to reduce the Pr value of the PZT blended P(VDF-TrFE) capacitors [14]. 4. Conclusions Thin films of crystalline γ-phase P(VDF-TrFE) blended with 0, 20 and 40 wt.% amorphous PZT on Si (100) were formed at an annealing temperature of 170 °C, and their phase development and ferroelectric properties were investigated to apply them to one-transistor FeRAM. The X-ray diffraction data showed that all the precursor films crystallize to the γ-phase P(VDF-TrFE) without involving the formation of any other polymorphs, and transform to a phase mixture composed of the γ-phase P(VDF-TrFE) and an amorphous PZT when they are blended with PZT. Remnant polarization decreases with the blending of PZT due to the presence of PZT and the formation of interfacial layer between the PZT blended P(VDF-TrFE) films and Si-substrate. Nevertheless, the improvement in the hysteretic C–V characteristics includes a larger memory window width, and higher accumulation capacitance within the low operating voltage ranges from −3.0 to 3.0 V and from −2.0 to 2.0 V for the 20 wt.%, 40 wt.% PZT blending, respectively. Compared to the P(VDF-TrFE) films, the bias voltage independence of leakage current and reduction in the much lower level of leakage current, 2.5 × 10−7 and 4.7 × 10−8 were also enhanced at 1.0 MV/cm in the 20, 40 wt.% PZT blended capacitors, respectively. These enhancements in the low-voltage operation, low leakage current, and high performance capacitance with the γ-phase and an amorphous PZT lead to the ways to extend the commercial market of nonvolatile highdensity FeRAM devices and their potential applications as functional devices. Acknowledgments
Fig. 5. P–E characteristics of the Au/P(VDF-TrFE) blended with 0, 20, and 40 wt.% PZT/Si capacitors annealed at 170 °C: (a) P(VDF-TrFE); (b) 20 wt.% PZT blended P(VDF-TrFE); (c) 40 wt.% PZT blended P(VDF-TrFE).
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0021507).
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References [1] J.F. Scott, C.A. Paz de Araujo, Science 246 (1989) 1400. [2] C.A. Pas De Araujo, J.D. Cuchiaro, L.D. McMillan, M.C. Scott, J.F. Scott, Nature 374 (1995) 627. [3] S. Fujisaki, H. Ishiwara, Y. Fujisaki, Appl. Phys. Express 1 (2008) 081801. [4] R.C.G. Naber, K. Asadi, P.W.M. Blom, D.M. de Leeuw, B. de Boer, Adv. Mater. 22 (2010) 933. [5] Q.M. Zhang, H. Li, M. Poh, F. Xia, Z.-Y. Cheng, H. Xu, C. Huang, Nature 419 (2002) 284.
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Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Ferroelectric properties of the organic films of poly(vinylidene fluoride-trifluoroethylene blended with inorganic Pb(Zr, Ti)O3 Wan-Gyu Lee a, Byung Eun Park b,⁎, Kyung Eun Park b a b
Department of Nano CMOS, National NanoFab Center, Daejeon 305-806, Republic of Korea Schoool of Electrical and Computer Engineering, University of Seoul, Seoul 130-743, Republic of Korea
a r t i c l e
i n f o
Available online 20 June 2013 Keywords: Ferroelectric P(VDF-TrFE) Pb(Zr,Ti)O3 Memory window Hysteretic characteristics
a b s t r a c t Precursor films based on poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) and P(VDF-TrFE) blended with Pb(Zr,Ti)O3 were spin-coated on Si-substrates and subsequently annealed at 170 °C. X-ray diffraction studies showed that the amorphous precursor films crystallize to the γ-phase P(VDF-TrFE) without involving the formation of other polymorphs when the P(VDF-TrFE) is blended with Pb(Zr,Ti)O3, resulting in phase mixtures composed of a crystalline γ-phase P(VDF-TrFE) and an amorphous Pb(Zr,Ti)O3. A larger memory window width and higher accumulation capacitance, as well as a lower leakage current density are induced by the blended Pb(Zr,Ti)O3 within the low operating voltage ranges from −3.0 to 3.0 V and from − 2.0 to 2.0 V for 20 wt% and 40 wt% Pb(Zr,Ti)O3 blending, respectively. These improvements not only in the hysteretic capacitance–voltage characteristics but also in the leakage current density–electric field are directly correlated with the phase mixtures, their volume fraction, dipole moments, and formation of interface layer between the blended film and Si substrate. © 2013 Elsevier B.V. All rights reserved.
1. Introduction As one of the nonvolatile memories, ferroelectric random access memory (FeRAM) has attractive features including higher speeds, much longer endurance, longer communication distance, and lower power consumption compared with its conventional counterparts such as Electrically Erasable Programming Read Only Memory and Flash [1,2]. Despite such high performance, scaling cell size has to be solved to support commercialization of high-density FeRAM products. Much of these difficulties stem mainly from the fact that the metal–insulator–metal (MIM) capacitors over bit-line integration could not be vertically etched but sloped, or the complementary metal-oxide-semiconductor circuits underlying the MIM capacitors are degraded during the subsequent crystallization process of ferroelectric materials. Researchers have searched for appropriate materials and improved processes for the realization of non-volatile FeRAM devices. On the other hand, there has been a continuous potential in ferroelectric polymers as functional devices for flexible display devices, energy transduction, information storage, and sensors [3–5]. In particular, electronic devices based on organic materials have been widely investigated because of their high flexibility, low weight, and low processing costs. In addition, these polymers could be fabricated into products at lower
⁎ Corresponding author. E-mail address: [email protected] (B.E. Park). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.06.034
temperature than ~300 °C, which may suppress the formation of interfacial layers between ferroelectric films and Si substrate. Thus, they open a path for the development of organic–inorganic combined devices with enhanced ferroelectric properties. Specifically, poly(vinylidene fluoride) P(VDF) and its copolymer with trifluoroethylene P(VDF-TrFE) are well known as ferroelectric polymers with smaller coercive strength and lower temperature crystallization compared with those of inorganic ferroelectric materials. However, the remnant polarization of organic ferroelectrics is relatively lower than that of inorganic ferroelectric materials, which could be enhanced by combining inorganic alternative with P(VDF-TrFE) [6,7]. Previous report has appeared on the remnant polarization and piezoelectricity of the copolymer (P(VDF-TrFE))-ceramic Pb(Zr,Ti)O3 (PZT) composites depending on the volume fraction for the composite of P(VDF) and PZT [8]. More recently, organic–inorganic hybrid materials have come under study as semiconducting channels in thin-film field-effect transistors because they offer both the superior carrier mobility of inorganic semiconductors and the processability of organic materials [9]. However, there have been few reports of enhanced ferroelectric properties of the organic P(VDF-TrFE) films blended with inorganic PZT. The objectives of this study were to explore the possibility of using sol–gel to deposit organic–inorganic combined thin films of (P(VDF-TrFE)1 − δ–PZTδ (0 ≤ δ ≤ 0.4) on Si-substrate (100) and to characterize the physical and electrical properties of the resulting films, as well as to investigate their ferroelectric performance in a metalferroelectric oxide-semiconductor capacitor structure.
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2. Experimental details 2.1. PZT blended P(VDF-TrFE) film preparation Amorphous precursor films were deposited from mixed solutions using a spin-coating process. One of the precursor solutions, P(VDF-TrFE) was prepared by dissolving 0.15 g of the vinylidene fluoridetrifluoroethylene P(VDF-TrFE) copolymer powder (99.9%, SOLVAY) in 2.5 mL of dimethyl formamide (99.9%, Aldrich) and stirring for 24 h at 60 °C. The resulting solution had an atomic VDF/TrFE ratio of 52:48, which was a composition approximately midway between VDF and TrFE. The others were prepared by blending the P(VDF-TrFE) solution with 20 wt.% and 40 wt.% of a 0.4 mol PZT solution (99.9%, Inostek Inc.). The resulting precursor solutions had atomic Pb/Zr/Ti ratios of 110/35/65, which is the ratio typically applied for FeRAM devices. (100)-oriented Si single-crystal substrates (1 × 1 cm) doped with B were coated with the P(VDF-TrFE) precursor solution at 500 rpm for 5 s, and with the PZT blended P(VDF-TrFE) precursor solutions at 1500 rpm for 40 s under ambient conditions, to make the annealed films almost with the same thickness of 100 nm. For the measurement of polarization versus electric field (P–E), the substrates were coated with 150 nm Pt film on Si single-crystal substrates prior to spincoating. The precursor films were subsequently pyrolyzed and annealed in a box furnace at 170 °C for 0.5 h. Au top electrodes were deposited on the formed ferroelectric films using electron-beam evaporation through a metal shadow mask with a diameter of 30 μm. 2.2. PZT blended P(VDF-TrFE) film characterization The thicknesses were measured using a field emission-scanning electron microscopy after annealing the precursor films. X-ray diffraction (XRD) was used to assess the phases and crystallographic textures of the product films. Routine analysis was performed using a 2θ diffractometer (Rigaku, Tokyo, Japan) with monochromatized Cu-K radiation that was supplied by a rotating anode generator operating at 40 kV and 300 mA with a scanning speed of 2°/min and a step size of 0.01°. The capacitance versus voltage (C–V) characteristics was measured by sweeping the gate voltage from inversion to accumulation at room temperature. The leakage current density versus electric field (J–E) characteristics was measured by sweeping the gate voltage from 5.0 V to −5.0 V at room temperature. An electric field up to 3 MV/cm was applied to the films at a frequency of 10 Hz during the polarization versus electric field (P–E) measurement. The synthesized capacitors were also investigated by transmission electron microscopy (JEM ARM 200 F, JEOL) operating at 200 kV, energy dispersive spectroscopy (EDS) for further evaluation and examination of the Au/0, 40 wt.% PZT-blended P(VDF-TrFE) films/Si substrate capacitors. The samples were prepared by precision ion polishing system (Gatan, Model 691) having double (top and bottom) ion guns operating at 3.6 keV ion beam energy with 6° milling angle under a vacuum level of 5 × 10−5 Torr for the double guns.
Fig. 1. XRD patterns of the P(VDF-TrFE) and 20, and 40 wt.% PZT blended P(VDF-TrFE) films annealed at 170 °C: (a) P(VDF-TrFE); (b) 20 wt.% PZT blended P(VDF-TrFE); (c) 40 wt.% PZT blended P(VDF-TrFE).
films blended with 20, 40 wt.% PZT resembled that of an amorphous film, in which there was an increase in the intensity of the broadening with the blended PZT content, as shown in Fig. 1(b) and (c). With blending PZT, a two-phase mixture was formed in the annealed films blended with 20 wt.% and 40 wt.% PZT; one was a γ-phase P(VDF-TrFE), the other was an amorphous PZT. The relative amount of each phase was contrary to the relative change of other phase, i.e., the volume fraction of γ phase (1-XPZT) decreases with the increase of PZT volume fraction (XPZT), as confirmed by the relative intensity change of X-ray
3. Results and discussion 3.1. Phase development in the blended P(VDF-TrFE)–PZT films X-ray diffraction patterns of the films annealed for 30 min at 170 °C are presented in Fig. 1. In Fig. 1(a), the crystalline phase was detected in the P(VDF-TrFE) film that had 0 wt.% PZT. The pattern can be identified as belonging to the orthorhombic γ-phase (C2cm, a = 0.497, b = 0.966, c = 0.918 nm) (020) and/or (110) of P(VDF-TrFE) at 2θ = 19.1° [10,11]. No shift in peak position was evident following the blended films with 20 wt.% and 40 wt.% PZT. An increase in PZT composition led to a decrease in the peak intensity from I = 1263 to I = 850 and I = 803 at 2θ = 19.1°, and to the appearance of an interesting broad maximum of the precursor films. The XRD patterns obtained from the
Fig. 2. C–V characteristics of Au/P(VDF-TrFE) blended with 0, 20, and 40 wt.% PZT/Si capacitors annealed at 170 °C.
W.-G. Lee et al. / Thin Solid Films 546 (2013) 171–175
Fig. 3. J–E characteristics of Au/P(VDF-TrFE) blended with 0, 20, and 40 wt.% PZT/Si capacitors annealed at 170 °C.
diffraction patterns, inasmuch as an ordinary two-phase mixture shows the same behavior in its temperature-composition phase diagram as this study does. 3.2. Ferroelectric properties of the blended P(VDF-TrFE)–PZT films Fig. 2 shows the typical C–V curves of Au/0, 20, 40 wt.% PZTblended P(VDF-TrFE) films/Si-substrate capacitors annealed at 170 °C. Clockwise hysteretic behavior was observed in all the films. The capacitance versus voltage characteristics were enhanced by blending Pb(Zr,Ti)O3, as shown in Fig. 2. These improvements included: (1) a
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stronger polarization effect than that in the single P(VDF-TrFE) film only; (2) an increase in memory window width with the blended PZT; and (3) a lower operating voltage. As pointed out earlier [12], ferroelectric behavior stems primarily from the crystallization of γ-phase P(VDF-TrFE) rather than from the amorphous phase. Furthermore, it can also be seen in Fig. 2 that the ferroelectric property corresponding to the accumulation capacitance is mainly determined by the linear combination of γ-phase P(VDF-TrFE) and amorphous PZT with their volume fraction. For example, the accumulated capacitance is about 40, 50, and 55 nF/cm2 at −3 V, corresponding to the crystallization of γ-phase P(VDF-TrFE) (black curve in Fig. 2), the combined γ-phase P(VDF-TrFE) and amorphous PZT with different volume fraction of each phase (red and blue curves in Fig. 2), respectively. However, the memory window width does not show the exactly same trend as the accumulated capacitance, which suggests that the memory window width is not a function of the degree of P(VDF-TrFE) crystallization, the presence of PZT, and their volume fraction, but of the combined physical state of dipole moments in the phase mixture of crystalline γ-phase P(VDF-TrFE) and amorphous PZT. The leakage current density versus electric field (J–E) characteristics for the Au/0, 20, and 40 wt.% PZT-blended P(VDF-TrFE) films/Si substrate capacitors is shown in Fig. 3. The leakage current of the films at an electric field of − 1.0 MV/cm was approximately 3.0 × 10− 6, 4.0 × 10− 8, and 3.5 × 10− 8 A/cm2 for the PZT composition of 0, 20, and 40 wt.%, respectively. As can be seen in Fig. 3, it is clear that the leakage current density of the films containing 20 wt.%, 40 wt.% PZT is relatively low in comparison with that of γ-phase P(VDF-TrFE) film, and higher-percentage of PZT blending provides better leakage current property than lower-percentage of PZT blending in the phase mixture of γ-phase and an amorphous PZT phase. These lower levels of leakage current density at a −1.0 MV/cm are
Fig. 4. Cross-sectional HRTEM images of the Au/0, 40 wt.% PZT blended P(VDF-TrFE)/Si capacitors: (a) P(VDF-TrFE); (b) 40 wt.% PZT blended P(VDF-TrFE). EDX spectrum of the interface layer between PZT blended P(VDF-TrFE) films and Si substrates, in which the measuring points P1, P2, and P3 are marked by colored circles: (c), (d), and (e), respectively.
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comparable to the lowest results, approximately 1 × 10−7 A/cm2 of inorganic strontium bismuth tantalite (SBT) films on an HfO2 [13]. Furthermore, they show the electric field independence from −5.0 V to +5.0 V with a transition from an asymmetric feature to symmetric feature of leakage current density versus electric field.
A more detailed investigation of the Au/0, 40 wt.% PZT-blended P(VDF-TrFE) films/Si-substrate capacitors was performed by analyzing high-resolution transmission electron microscopy (HRTEM) images as in Fig. 4(a) and (b). High-resolution image acquired from a 40 wt.% PZT-blended P(VDF-TrFE)/Si-substrate shows what appear to be an interfacial layer between the PZT blended P(VDF-TrFE) and Si-substrate even if low temperature annealing was applied. Lowering of leakage current level and of strong voltage dependence in both the 20 wt.% and 40 wt.% PZT blended films is attributed to the formation of the interfacial layer between the PZT blended P(VDF-TrFE) film and Si-substrate that is also an amorphous phase, corresponding to the XRD result of Fig. 1. Energy dispersive X-ray spectroscopy (EDS) was used to further evaluate the composition of the interface film formed during the low temperature annealing at 170 °C, as shown in Fig. 4(c), (d), and (e). The probing points are marked by the color circles in Fig. 4(b). All these EDS data clearly indicate that Pb oxide layer was formed between the PZT blended P(VDF-TrFE) films and Si-substrate with the thickness of less than 4 nm, which is the main cause of improved leakage current density versus electric field (J–E) characteristics for the Au/20, 40 wt.% PZT-blended P(VDF-TrFE) films/Si-substrate capacitors. The polarization versus electric field (P–E) characteristics for the 0, 20, and 40 wt.% PZT blended P(VDF-TrFE) films is presented in Fig. 5. Compared with P(VDF-TrFE) film, PZT blended films show tilted P–E hysteresis loops. Although all these capacitors have similar values of coercive field (Ec) 0.35 MV/cm, the remnant polarization (Pr) decreases from 1.3 μC/cm2 for the 0 wt.% PZT blended film to 0.7 μC/cm2 for both 20 and 40 wt.% PZT blended films, which are in contrast to the results observed from C–V, and J–E as shown in Figs. 2 and 3. It should be noted that the formation of interfacial layer, Pb oxide between the PZT blended films and Si substrates would induce the depolarization field and hence tends to reduce the Pr value of the PZT blended P(VDF-TrFE) capacitors [14]. 4. Conclusions Thin films of crystalline γ-phase P(VDF-TrFE) blended with 0, 20 and 40 wt.% amorphous PZT on Si (100) were formed at an annealing temperature of 170 °C, and their phase development and ferroelectric properties were investigated to apply them to one-transistor FeRAM. The X-ray diffraction data showed that all the precursor films crystallize to the γ-phase P(VDF-TrFE) without involving the formation of any other polymorphs, and transform to a phase mixture composed of the γ-phase P(VDF-TrFE) and an amorphous PZT when they are blended with PZT. Remnant polarization decreases with the blending of PZT due to the presence of PZT and the formation of interfacial layer between the PZT blended P(VDF-TrFE) films and Si-substrate. Nevertheless, the improvement in the hysteretic C–V characteristics includes a larger memory window width, and higher accumulation capacitance within the low operating voltage ranges from −3.0 to 3.0 V and from −2.0 to 2.0 V for the 20 wt.%, 40 wt.% PZT blending, respectively. Compared to the P(VDF-TrFE) films, the bias voltage independence of leakage current and reduction in the much lower level of leakage current, 2.5 × 10−7 and 4.7 × 10−8 were also enhanced at 1.0 MV/cm in the 20, 40 wt.% PZT blended capacitors, respectively. These enhancements in the low-voltage operation, low leakage current, and high performance capacitance with the γ-phase and an amorphous PZT lead to the ways to extend the commercial market of nonvolatile highdensity FeRAM devices and their potential applications as functional devices. Acknowledgments
Fig. 5. P–E characteristics of the Au/P(VDF-TrFE) blended with 0, 20, and 40 wt.% PZT/Si capacitors annealed at 170 °C: (a) P(VDF-TrFE); (b) 20 wt.% PZT blended P(VDF-TrFE); (c) 40 wt.% PZT blended P(VDF-TrFE).
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0021507).
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