**3. Application in organic memory devices**

In modern society, more and more electronic products are studied in both inorganic and organic electronics. Organic memory cells develop at a slow pace, but researches of organic memory devices based on PVDF and its copolymers will become extremely important to develop the future organic electronic products. In the case of copolymers including poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), the addition of one more fluorine atom provides the delicate configuration balance and always results in the polar ferroelectric β structure [32]. However, PVDF possesses higher ferroelectric transition temperature and thermal stability of remnant polarization in comparison with its copolymers [33, 34]. PVDF has a high Curie temperature of 167°C compared to the P(VDF-TrFE) copolymers with Curie temperatures from 60 to 100°C related with the TrFE content [10]. The higher Curie temperature of PVDF as a ferroelectric layer potentially allows the higher temperature operation of polymer memory than that of its copolymers. In addition, the copolymers are unlikely suitable for mass production because their synthesis is extremely complicated and thus ineffective cost [35].

where G*<sup>P</sup>* <sup>=</sup> *<sup>G</sup>*↑↑ <sup>+</sup> *<sup>G</sup>*↓↓

comparison.

*3.1.1. Au/PVDF/W tunnel junctions*

The TER is defined as:

and G*AP* <sup>=</sup> *<sup>G</sup>*↑↓ <sup>+</sup> *<sup>G</sup>*↓↑

TER <sup>=</sup> *<sup>G</sup>*<sup>←</sup> \_\_\_\_\_\_\_ <sup>−</sup> *<sup>G</sup>*<sup>→</sup>

defined by the bottom electrode isolated by a SiO<sup>2</sup>

conducted at University of Puerto Rico of USA in 2012.

two electrodes are parallel and antiparallel, respectively.

are the junction conductance, when the magnetizations of

Preparation and Device Applications of Ferroelectric β-PVDF Films

× 100% (2)

http://dx.doi.org/10.5772/intechopen.77167

139

matrix. The deposition of PVDF ultra-thin

*G*<sup>←</sup> + *G*<sup>→</sup>

where *G*← is the conductance for the PVDF polarization pointing to the left, and *G*→ the conductance for the PVDF polarization pointing to the right, and horizontal arrows indicate the polarization direction. The conductance of the Co/PVDF/O/Co(0001) MFTJ was calculated by Velev et al. using the first principles electronic structure [51]. The results are shown in **Table 1**, where also the conductance of Co/PVDF/Co MFTJ with clean interfaces is given for

Tian et al. report a robust room temperature TER of ~300% and ~1000% in organic FTJs using ultrathin PVDF films (1 and 2 layers (Ls) with the thickness of 2.2 and 4.4 nm, respectively) [58]. Three-dimensional sketch of the PVDF FTJs is shown in **Figure 5a**, and the junction sizes are

films were performed prior to the deposition of micron-size Au as top electrodes. The typical I–V characteristics of the PVDF FTJs are shown in **Figure 5b**. A Keithley 6430 sub-femtoampere source meter with a remote preamplifier was used to perform the I–V measurements [58].

**Table 1.** Conductance per unit cell area, TER, and TMR for Co/PVDF/O/Co tunneling junction. ↑↑ and ↓↓ are used to represent the conductance for parallel magnetization of the electrodes. ↑↓ + ↓↑ is the total conductance for the antiparallel magnetization. ← and → are the left and right polarization orientation of PVDF, respectively [51]. This research was

In the following report, four types of PVDF memory cells will be introduced according to the structure of memory devices. They are ferroelectric tunnel junctions, organic capacitors, field effect transistor and organic diodes [36, 37].

#### **3.1. Ferroelectric tunnel junctions**

The idea of ferroelectric tunnel junctions (FTJs) was derived from the observation of spindependent tunneling phenomenon. FTJ is a device with two ferromagnetic metal layers separated by insulating barrier [38]. Perovskite-type ABO<sup>3</sup> metal oxides (e.g., BaTiO<sup>3</sup> or KNbO3 ) are widely known as ferroelectric materials for their excellent piezoelectric response and strong polarization [39, 40]. However, their applications are limited by brittleness, heavy weight, high cost, and large thermal budget [41]. The electric polarization of ferroelectric PVDF is comparable to that of perovskite oxides. The use of organic ferroelectrics PVDF could solve the problem of thermal budget because PVDF can be processed at 200°C or lower temperature [19]. Furthermore, PVDF films are very appealing due to their flexible, light weight, low cost, and nontoxic characteristics [41–43]. PVDF thin films are very promising and used as barriers in FTJs for these properties [44]. Ferroelectric tunnel barriers allow switching of the tunneling conductance between two stable states because their spontaneous polarization can be reversed under a bias voltage. This phenomenon is known as tunneling electro resistance (TER). In the past years, extensive studies in theory have confirmed the possibility to fabricate PVDF FTJs [45–55]. By contrast, the progress in experimental investigations is not so good owing to the challenges in the fabrication of PVDF ultra-thin films with high purity of ferroelectric active phase.

On the basis of first principles calculations, simultaneous TER and TMR effects and multiple resistance states were demonstrated [51, 56, 57]. The TMR is defined as:

$$\text{TMR} = \frac{G\_p - G\_{\Lambda\rho}}{G\_p + G\_{\Lambda\rho}} \times 100\% \tag{1}$$

where G*<sup>P</sup>* <sup>=</sup> *<sup>G</sup>*↑↑ <sup>+</sup> *<sup>G</sup>*↓↓ and G*AP* <sup>=</sup> *<sup>G</sup>*↑↓ <sup>+</sup> *<sup>G</sup>*↓↑ are the junction conductance, when the magnetizations of two electrodes are parallel and antiparallel, respectively.

The TER is defined as:

**3. Application in organic memory devices**

138 Ferroelectrics and Their Applications

effect transistor and organic diodes [36, 37].

rated by insulating barrier [38]. Perovskite-type ABO<sup>3</sup>

**3.1. Ferroelectric tunnel junctions**

roelectric active phase.

In modern society, more and more electronic products are studied in both inorganic and organic electronics. Organic memory cells develop at a slow pace, but researches of organic memory devices based on PVDF and its copolymers will become extremely important to develop the future organic electronic products. In the case of copolymers including poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), the addition of one more fluorine atom provides the delicate configuration balance and always results in the polar ferroelectric β structure [32]. However, PVDF possesses higher ferroelectric transition temperature and thermal stability of remnant polarization in comparison with its copolymers [33, 34]. PVDF has a high Curie temperature of 167°C compared to the P(VDF-TrFE) copolymers with Curie temperatures from 60 to 100°C related with the TrFE content [10]. The higher Curie temperature of PVDF as a ferroelectric layer potentially allows the higher temperature operation of polymer memory than that of its copolymers. In addition, the copolymers are unlikely suitable for mass produc-

tion because their synthesis is extremely complicated and thus ineffective cost [35].

In the following report, four types of PVDF memory cells will be introduced according to the structure of memory devices. They are ferroelectric tunnel junctions, organic capacitors, field

The idea of ferroelectric tunnel junctions (FTJs) was derived from the observation of spindependent tunneling phenomenon. FTJ is a device with two ferromagnetic metal layers sepa-

widely known as ferroelectric materials for their excellent piezoelectric response and strong polarization [39, 40]. However, their applications are limited by brittleness, heavy weight, high cost, and large thermal budget [41]. The electric polarization of ferroelectric PVDF is comparable to that of perovskite oxides. The use of organic ferroelectrics PVDF could solve the problem of thermal budget because PVDF can be processed at 200°C or lower temperature [19]. Furthermore, PVDF films are very appealing due to their flexible, light weight, low cost, and nontoxic characteristics [41–43]. PVDF thin films are very promising and used as barriers in FTJs for these properties [44]. Ferroelectric tunnel barriers allow switching of the tunneling conductance between two stable states because their spontaneous polarization can be reversed under a bias voltage. This phenomenon is known as tunneling electro resistance (TER). In the past years, extensive studies in theory have confirmed the possibility to fabricate PVDF FTJs [45–55]. By contrast, the progress in experimental investigations is not so good owing to the challenges in the fabrication of PVDF ultra-thin films with high purity of fer-

On the basis of first principles calculations, simultaneous TER and TMR effects and multiple

*GP* + *GAP*

resistance states were demonstrated [51, 56, 57]. The TMR is defined as:

TMR <sup>=</sup> *GP* \_\_\_\_\_\_ <sup>−</sup> *GAP*

metal oxides (e.g., BaTiO<sup>3</sup>

× 100% (1)

or KNbO3

) are

$$\text{TER} = \frac{G\_{\text{\\_}} - G\_{\text{\\_}}}{G\_{\text{\text}} + G\_{\text{\text}}} \times 100\% \tag{2}$$

where *G*← is the conductance for the PVDF polarization pointing to the left, and *G*→ the conductance for the PVDF polarization pointing to the right, and horizontal arrows indicate the polarization direction. The conductance of the Co/PVDF/O/Co(0001) MFTJ was calculated by Velev et al. using the first principles electronic structure [51]. The results are shown in **Table 1**, where also the conductance of Co/PVDF/Co MFTJ with clean interfaces is given for comparison.

#### *3.1.1. Au/PVDF/W tunnel junctions*

Tian et al. report a robust room temperature TER of ~300% and ~1000% in organic FTJs using ultrathin PVDF films (1 and 2 layers (Ls) with the thickness of 2.2 and 4.4 nm, respectively) [58]. Three-dimensional sketch of the PVDF FTJs is shown in **Figure 5a**, and the junction sizes are defined by the bottom electrode isolated by a SiO<sup>2</sup> matrix. The deposition of PVDF ultra-thin films were performed prior to the deposition of micron-size Au as top electrodes. The typical I–V characteristics of the PVDF FTJs are shown in **Figure 5b**. A Keithley 6430 sub-femtoampere source meter with a remote preamplifier was used to perform the I–V measurements [58].


**Table 1.** Conductance per unit cell area, TER, and TMR for Co/PVDF/O/Co tunneling junction. ↑↑ and ↓↓ are used to represent the conductance for parallel magnetization of the electrodes. ↑↓ + ↓↑ is the total conductance for the antiparallel magnetization. ← and → are the left and right polarization orientation of PVDF, respectively [51]. This research was conducted at University of Puerto Rico of USA in 2012.

**Figure 5.** TER in submicron Au/PVDF/W tunnel junctions. (a) Three-dimensional sketch of PVDF FTJs. (b) I–V curves in the 1 L and 2Ls PVDF FTJs. The arrows show the direction of the voltage sweeps [58]. This research was conducted at Chinese Academy of Sciences in 2016.

The transport mechanisms and electro resistive effects in the FTJs with a 5 nm thick barrier can be divided into Fowler-Nordheim tunneling, direct tunneling, and thermionic injection currents [59]. The larger TER response for thicker barriers agrees with electrostatics models based on a direct tunneling process [60, 61]. The devices show a room temperature TER effect as high as 1000%, which will open a new route for low cost, silicon-compatible, or potentially rollable organic devices.

ferroelectric polarizations under 10 mV at 10 K. The positive TMR will vanish with increasing

**Figure 6.** Morphology and characterization of LSMO/PVDF/Co. (a) Schematics of the LSMO/PVDF/Co device. (b) AFM topography of the PVDF barrier surface. (c) Amplitude hysteresis loops and representative local PFM phase measured on the PVDF surface. (d) PFM phase image recorded on the PVDF surface. (e) Tunneling magneto-resistance of the device measured after +1.2 V and − 1.5 V polarized. The insert graphs denote four different resistance states associated with the orientations of magnetization and polarization [57]. This research was conducted at CNRS-Université de

Preparation and Device Applications of Ferroelectric β-PVDF Films

http://dx.doi.org/10.5772/intechopen.77167

141

Ferroelectric PVDF was also introduced to control the magneto transport and the spin transport through the spinterface of ferromagnet/organic is studied. A LSMO/PVDF/ MgO (0.5 nm)/Co device shows a much higher junction resistance in comparison with the devices without MgO layer indicating the insertion of MgO likely suppresses the diffusion of Co top layer. There is no coupling for PVDF/Co spinterface with MgO insertion layer, and the MgO/Co interface totally modifies the spin polarization [57]. The TMR sign with the MgO insertion layer is consistent with the reports of Co/MgO/Co magnetic tunnel

is one potential electrode because of its high spin polarization efficiency (100%) and

PVDF/Co/Al stacking structure shown in **Figure 7a** was successfully fabricated very recently [31]. The magnetoresistance (MR) ratio at room temperature can achieve approximately 2.6% for the FTJs with 3-layer PVDF barrier (~7 nm thick). In **Figure 7b**, it is noted that the MR ratios increase with reducing measurement temperature. This is attributed to the reduction in spin scattering. The standard four-probe method is used to measure MR curves. The I–V curves were measured by A nanovoltmeter (model 2182A, Keithley Inst. Inc.) and an AC and DC current source (model 6221, Keithley Inst. Inc.). In addition, the MR response decreases with increasing PVDF layer numbers likely owing to the change of spin transport mechanism

O4 /AlO/

high Curie temperature (∼850 K) in theory [64, 65]. The PVDF spin devices with Fe<sup>3</sup>

temperature up to 120 K. The negative TMR effect can survive over 250 K.

*/AlO/PVDF/Co/Al stacking structure*

junctions [63].

*O4*

Lorraine of France in 2016.

*3.1.3. Fe3*

Fe<sup>3</sup> O4

#### *3.1.2. La0.6Sr0.4MnO3 /PVDF/Co structures*

The observation of a large 300% TMR in La0.7Sr0.3MnO<sup>3</sup> /Alq3 /Co tunnel junctions is a particularly interesting result, where the interfaces of ferromagnetic metal/organic hybrid play a key role for spin injection in spintronics [62]. Liang et al. fabricated La0.6Sr0.4MnO<sup>3</sup> (LSMO)/PVDF/ Co FTJ organic structures with PVDF as a tunneling barrier to study the ferroelectric control of the spin-polarization of the spacer interfaces [57]. **Figure 6a** shows the device structure. The surface morphology of PVDF barrier investigated by AFM (Asylum Research, MFP-3D, USA) in **Figure 6b** reveals a smooth surface with 3.12 nm RMS roughness in the range of 1 × 1 μm<sup>2</sup> . Amplitude hysteresis loops and representative local piezoresponse force microscopy (PFM) phase are shown in **Figure 6c**. **Figure 6d** shows the PFM phase image recorded on the PVDF surface.

It has been demonstrated that tuning the ferroelectric polarization of PVDF can control the spin-polarization of the PVDF/Co spinterface actively at low temperatures. Two polarization states are possibly related to either F-C-H/Co or H-C-F/Co interface (see the inset of **Figure 6e**). **Figure 6e** shows the magneto-response of the LSMO/PVDF/Co device for two different

**Figure 6.** Morphology and characterization of LSMO/PVDF/Co. (a) Schematics of the LSMO/PVDF/Co device. (b) AFM topography of the PVDF barrier surface. (c) Amplitude hysteresis loops and representative local PFM phase measured on the PVDF surface. (d) PFM phase image recorded on the PVDF surface. (e) Tunneling magneto-resistance of the device measured after +1.2 V and − 1.5 V polarized. The insert graphs denote four different resistance states associated with the orientations of magnetization and polarization [57]. This research was conducted at CNRS-Université de Lorraine of France in 2016.

ferroelectric polarizations under 10 mV at 10 K. The positive TMR will vanish with increasing temperature up to 120 K. The negative TMR effect can survive over 250 K.

Ferroelectric PVDF was also introduced to control the magneto transport and the spin transport through the spinterface of ferromagnet/organic is studied. A LSMO/PVDF/ MgO (0.5 nm)/Co device shows a much higher junction resistance in comparison with the devices without MgO layer indicating the insertion of MgO likely suppresses the diffusion of Co top layer. There is no coupling for PVDF/Co spinterface with MgO insertion layer, and the MgO/Co interface totally modifies the spin polarization [57]. The TMR sign with the MgO insertion layer is consistent with the reports of Co/MgO/Co magnetic tunnel junctions [63].

#### *3.1.3. Fe3 O4 /AlO/PVDF/Co/Al stacking structure*

The transport mechanisms and electro resistive effects in the FTJs with a 5 nm thick barrier can be divided into Fowler-Nordheim tunneling, direct tunneling, and thermionic injection currents [59]. The larger TER response for thicker barriers agrees with electrostatics models based on a direct tunneling process [60, 61]. The devices show a room temperature TER effect as high as 1000%, which will open a new route for low cost, silicon-compatible, or potentially

**Figure 5.** TER in submicron Au/PVDF/W tunnel junctions. (a) Three-dimensional sketch of PVDF FTJs. (b) I–V curves in the 1 L and 2Ls PVDF FTJs. The arrows show the direction of the voltage sweeps [58]. This research was conducted at

larly interesting result, where the interfaces of ferromagnetic metal/organic hybrid play a key

Co FTJ organic structures with PVDF as a tunneling barrier to study the ferroelectric control of the spin-polarization of the spacer interfaces [57]. **Figure 6a** shows the device structure. The surface morphology of PVDF barrier investigated by AFM (Asylum Research, MFP-3D, USA) in **Figure 6b** reveals a smooth surface with 3.12 nm RMS roughness in the range of

copy (PFM) phase are shown in **Figure 6c**. **Figure 6d** shows the PFM phase image recorded

It has been demonstrated that tuning the ferroelectric polarization of PVDF can control the spin-polarization of the PVDF/Co spinterface actively at low temperatures. Two polarization states are possibly related to either F-C-H/Co or H-C-F/Co interface (see the inset of **Figure 6e**). **Figure 6e** shows the magneto-response of the LSMO/PVDF/Co device for two different

. Amplitude hysteresis loops and representative local piezoresponse force micros-

role for spin injection in spintronics [62]. Liang et al. fabricated La0.6Sr0.4MnO<sup>3</sup>

/Alq3

/Co tunnel junctions is a particu-

(LSMO)/PVDF/

rollable organic devices.

Chinese Academy of Sciences in 2016.

140 Ferroelectrics and Their Applications

*/PVDF/Co structures*

The observation of a large 300% TMR in La0.7Sr0.3MnO<sup>3</sup>

*3.1.2. La0.6Sr0.4MnO3*

1 × 1 μm<sup>2</sup>

on the PVDF surface.

Fe<sup>3</sup> O4 is one potential electrode because of its high spin polarization efficiency (100%) and high Curie temperature (∼850 K) in theory [64, 65]. The PVDF spin devices with Fe<sup>3</sup> O4 /AlO/ PVDF/Co/Al stacking structure shown in **Figure 7a** was successfully fabricated very recently [31]. The magnetoresistance (MR) ratio at room temperature can achieve approximately 2.6% for the FTJs with 3-layer PVDF barrier (~7 nm thick). In **Figure 7b**, it is noted that the MR ratios increase with reducing measurement temperature. This is attributed to the reduction in spin scattering. The standard four-probe method is used to measure MR curves. The I–V curves were measured by A nanovoltmeter (model 2182A, Keithley Inst. Inc.) and an AC and DC current source (model 6221, Keithley Inst. Inc.). In addition, the MR response decreases with increasing PVDF layer numbers likely owing to the change of spin transport mechanism

**Figure 7.** (a) Diagrammatic sketch of the Fe<sup>3</sup> O4 /AlO/PVDF/Co/Al FTJs structure. The magnetic field along the long direction of the Fe<sup>3</sup> O4 strip was applied to measurement the magnetoresistance. (b) Magnetoresistance ratio dependence on different measurement temperature and various layer PVDF for spin valves [31]. This research was conducted at Northeastern University of China in 2017.

from tunneling to hopping transport. This study is valuable to realize the design of flexible spin devices using PVDF barriers operated at room temperature.

#### **3.2. Field effect transistor**

Recently, ferroelectric field effect transistor (FeFET) device architectures have been formed by using ferroelectric thin films as gate insulators because of the various advantages such as easily integrated structure, small cell sizes, low operating voltages, and nondestructive readout capability [14, 36]. The high ON/OFF ratio at zero gate voltage must be ensured to distinguish the different data recognition for nonvolatile ferroelectric memory. In generally, both ON and OFF source–drain current are strongly dependent on the charge density of semi-conducting layer, and the operating voltage of FeFET depends on the thickness and dielectric constant of the gate dielectric [66].

The composition of organic transistor has three main components: three electrodes including source, drain and gate; an active semiconductor layer; and a dielectric layer [37]. A FeFET was successfully fabricated utilizing 100 nm thick PVDF/PMMA (80:20) films as a gate insulator [14]. The capacitor with a 160 nm thick β-phase PVDF film exhibited the fairly large remanent polarization (Pr) of 7 μC cm−2 with the coercive voltage (V<sup>c</sup> ) of 8 V corresponding to coercive electric field (Ec) of ∼50MV/m (as shown in **Figure 8a**). A typical ferroelectric hysteresis with the drain current bistability at zero gate bias was shown in **Figure 8b** for a FeFET with the PVDF as gate dielectric and Agilient technologies E5270B parameter analyzer was used to characterize this transistor in a dark environment [10].

The high performance TFT device with a PVDF/GOnP nanocomposite as insulator has been

cross-section of FeFET device [67]. This research was conducted at Research Center Jülich of Germany in 2010.

**Figure 8.** (a) Polarization vs. applied voltage (P-V) hysteresis loops of a PVDF film with thickness of 160 nm. (b) ID–VG transfer curve of organic thin film transistors (OTFTs) with PVDF–PVP as the bilayer gate dielectric. The hysteresis direction is denoted by arrows. The inset is the diagrammatic sketch of FeFET device structure [10]. This research was

The metal/ferroelectric polymer/metal (MFM) capacitor is a basic ferroelectric polarization storage component in memory storage devices, in which a ferroelectric polymer film is sandwiched between metal electrodes. Organic ferroelectric material PVDF has attracted much interest recently for its applications in low operating voltage, nonvolatile memory storage

, and a field effect mobility of 1.1 cm<sup>2</sup> V−1 s, which

Preparation and Device Applications of Ferroelectric β-PVDF Films

http://dx.doi.org/10.5772/intechopen.77167

143

/p-Si FeFET. Inset: The

found to have an ON/OFF current ratio of 10<sup>5</sup>

conducted at Yonsei University of Korea in 2008.

**3.3. Organic capacitors**

can make the OTFTs operated successfully at voltages below 2 V [68].

**Figure 9.** Source–drain current ISD curves depend on the gate voltage values for an Au/PVDF/SiO<sup>2</sup>

devices. Many researches are focused on MFM capacitors [69].

The Au/PVDF/SiO<sup>2</sup> /p-Si stack is successfully prepared by Gerber et al., and the various dimension Au gate electrodes were deposited by vacuum evaporation. The inset diagram denotes the cross-section of the device in **Figure 9**. The FeFET exhibited excellent current-voltage characteristics for the various gate voltage V<sup>g</sup> from 0 to +3.5 V [67]. The FeFET measurements were accomplished using a semiconductor parameter analyzer in a shielded metal box at room temperature.

**Figure 8.** (a) Polarization vs. applied voltage (P-V) hysteresis loops of a PVDF film with thickness of 160 nm. (b) ID–VG transfer curve of organic thin film transistors (OTFTs) with PVDF–PVP as the bilayer gate dielectric. The hysteresis direction is denoted by arrows. The inset is the diagrammatic sketch of FeFET device structure [10]. This research was conducted at Yonsei University of Korea in 2008.

**Figure 9.** Source–drain current ISD curves depend on the gate voltage values for an Au/PVDF/SiO<sup>2</sup> /p-Si FeFET. Inset: The cross-section of FeFET device [67]. This research was conducted at Research Center Jülich of Germany in 2010.

The high performance TFT device with a PVDF/GOnP nanocomposite as insulator has been found to have an ON/OFF current ratio of 10<sup>5</sup> , and a field effect mobility of 1.1 cm<sup>2</sup> V−1 s, which can make the OTFTs operated successfully at voltages below 2 V [68].

#### **3.3. Organic capacitors**

from tunneling to hopping transport. This study is valuable to realize the design of flexible

on different measurement temperature and various layer PVDF for spin valves [31]. This research was conducted at

strip was applied to measurement the magnetoresistance. (b) Magnetoresistance ratio dependence

Recently, ferroelectric field effect transistor (FeFET) device architectures have been formed by using ferroelectric thin films as gate insulators because of the various advantages such as easily integrated structure, small cell sizes, low operating voltages, and nondestructive readout capability [14, 36]. The high ON/OFF ratio at zero gate voltage must be ensured to distinguish the different data recognition for nonvolatile ferroelectric memory. In generally, both ON and OFF source–drain current are strongly dependent on the charge density of semi-conducting layer, and the operating voltage of FeFET depends on the thickness and dielectric constant of

The composition of organic transistor has three main components: three electrodes including source, drain and gate; an active semiconductor layer; and a dielectric layer [37]. A FeFET was successfully fabricated utilizing 100 nm thick PVDF/PMMA (80:20) films as a gate insulator [14]. The capacitor with a 160 nm thick β-phase PVDF film exhibited the fairly large remanent

electric field (Ec) of ∼50MV/m (as shown in **Figure 8a**). A typical ferroelectric hysteresis with the drain current bistability at zero gate bias was shown in **Figure 8b** for a FeFET with the PVDF as gate dielectric and Agilient technologies E5270B parameter analyzer was used to

sion Au gate electrodes were deposited by vacuum evaporation. The inset diagram denotes the cross-section of the device in **Figure 9**. The FeFET exhibited excellent current-voltage char-

were accomplished using a semiconductor parameter analyzer in a shielded metal box at

/p-Si stack is successfully prepared by Gerber et al., and the various dimen-

) of 8 V corresponding to coercive

from 0 to +3.5 V [67]. The FeFET measurements

/AlO/PVDF/Co/Al FTJs structure. The magnetic field along the long

spin devices using PVDF barriers operated at room temperature.

O4

polarization (Pr) of 7 μC cm−2 with the coercive voltage (V<sup>c</sup>

characterize this transistor in a dark environment [10].

acteristics for the various gate voltage V<sup>g</sup>

**3.2. Field effect transistor**

direction of the Fe<sup>3</sup>

**Figure 7.** (a) Diagrammatic sketch of the Fe<sup>3</sup>

O4

142 Ferroelectrics and Their Applications

Northeastern University of China in 2017.

the gate dielectric [66].

The Au/PVDF/SiO<sup>2</sup>

room temperature.

The metal/ferroelectric polymer/metal (MFM) capacitor is a basic ferroelectric polarization storage component in memory storage devices, in which a ferroelectric polymer film is sandwiched between metal electrodes. Organic ferroelectric material PVDF has attracted much interest recently for its applications in low operating voltage, nonvolatile memory storage devices. Many researches are focused on MFM capacitors [69].

The structure of ferroelectric device has been shown in **Figure 10a**, in which a PEDOT:PSS layer as a hole-only (electron blocking) transport layer is inserted between the Al top electrode and the PVDF LB nanofilm. The chemical structure of PEDOT:PSS is shown in **Figure 10b**. Ferroelectricity was characterized at different voltage amplitudes (**Figure 5c**), which shows asymmetric displacement-electric field hysteresis loops. The hysteresis loops were characterized by a traditional Sawyer–Tower method. The Pr values depend on the electric field (**Figure 10d**) [70]. The Pr values increase with the increasing electric field, which is accordance with reference report [13]. The dipole moments of PVDF in the films are slightly oriented at a low electric field, and the dipole moments are highly oriented at a high electric field, thereby leading to the increase of Pr.

composites, [75] or by dispersing nanomaterials [76]. The simple metal-insulator-metal (MIM) structure is shown in **Figure 11a**, in which the insulator part composed of graphene nano flakes (GR) embedded in PVDF layer was sandwiched between Platinum (Pt) top electrode and ITO bottom electrode. The nonvolatile resistive memory switching was investigated using this structure. The I–V characteristic curves under various compliance current of 1, 10 and 100 μA are shown in **Figure 11b**. The I–V curves were measured by a Keithley

Preparation and Device Applications of Ferroelectric β-PVDF Films

http://dx.doi.org/10.5772/intechopen.77167

145

The I–V characteristics of the Hg/PVDF/Au device were obtained by DC voltage sweep program (**Figure 12a**). The quick transition from high resistance state to low resistance state was realized at the set voltage. The resistance versus voltage (R-V) curves are shown in **Figure 12b**. A maximum resistance ratio of 25 may be practical for application in nonvolatile memory devices [34]. Several phenomena in capacitor structures exist in the literature used to explain the bistability using the filamentary conduction along with Schottky emission, trap charging and discharging, space charge limited current and Poole-Frenkel emission [78]. The I–V cycles

The device structure of FeFETs would complicate the construction of a larger integrated memory technology. In addition, ferroelectric capacitors with relatively simple device structure have a limited scaling capability. Ferroelectric diodes can combine the advantages of FeFETs and capacitors. Therefore, there is an ongoing research activity in a diode structure to realize

The semilogarithmic forward and reverse I–V characteristics of the Au/n-InP and Au/PVDF/n-InP Schottky diodes are shown in **Figure 13**. The I–V was measured using a Keithley source measuring unit 2400. The reverse leakage current of the Au/n-InP diode (6.809 × 10−5 A at −1 V)

**Figure 11.** (a) Schematic diagram of the Pt/PVDF/GR/PVDF/ITO memory device fabricated layer by layer. (b) Typical I–V characteristic curves of the device under different compliance currents varying from 1 to 100 μA [77]. This research

2401 in top–bottom configuration.

**3.4. Organic diodes**

the resistive switching [4].

were performed using a Keithley 238 SMU unit.

was conducted at University of Puerto Rico of USA in 2014.

The asymmetric hysteresis loops for the capacitor with the PEDOT:PSS layer are different from that of the Al/PVDF LB nanofilms/Al device, which shows symmetric hysteresis loops [11]. Memory devices utilize the hysteresis by associating the positive remanent polarization (+Pr) and negative remanent polarization (−Pr) with a Boolean 1 and 0 [4]. Asadi et al. reported a diode with a Ag/PVDF:P3HT/LiF/Au capacitor, which possesses asymmetric hole accumulation properties under positive or negative polarization [22].

The low-density storage capability and slow switching speed are the major problems for most organic memory devices [71–73]. The performance of these devices can be greatly enhanced by several methods such as forming hybrid organic structures, [74] organic/inorganic

**Figure 10.** (a) Layer structure of the PEDOT:PSS ferroelectric device, (b) The chemical structure of PEDOT:PSS, (c) The hysteresis loops of the device at different applied voltage amplitudes, and (d) Pr values dependence of the electric field [70]. This research was conducted at Tohoku University of Japan in 2015.

composites, [75] or by dispersing nanomaterials [76]. The simple metal-insulator-metal (MIM) structure is shown in **Figure 11a**, in which the insulator part composed of graphene nano flakes (GR) embedded in PVDF layer was sandwiched between Platinum (Pt) top electrode and ITO bottom electrode. The nonvolatile resistive memory switching was investigated using this structure. The I–V characteristic curves under various compliance current of 1, 10 and 100 μA are shown in **Figure 11b**. The I–V curves were measured by a Keithley 2401 in top–bottom configuration.

The I–V characteristics of the Hg/PVDF/Au device were obtained by DC voltage sweep program (**Figure 12a**). The quick transition from high resistance state to low resistance state was realized at the set voltage. The resistance versus voltage (R-V) curves are shown in **Figure 12b**. A maximum resistance ratio of 25 may be practical for application in nonvolatile memory devices [34]. Several phenomena in capacitor structures exist in the literature used to explain the bistability using the filamentary conduction along with Schottky emission, trap charging and discharging, space charge limited current and Poole-Frenkel emission [78]. The I–V cycles were performed using a Keithley 238 SMU unit.
