DOPED GRAPHENE ELECTRODES AS INTERCONNECTS FOR FERROELECTRIC CAPACITORS

Abstract

A ferroelectric capacitor having a doped graphene bottom electrode and uses thereof are described. The doped graphene bottom electrode layer is deposited on a substrate with a ferroelectric layer deposited between the doped graphene layer and a top electrode.

Claims

1. A ferroelectric capacitor-based memory device comprising: (a) a substrate comprising a flexible polymeric substrate; (a) a bottom electrode comprising a first doped graphene layer deposited on the substrate, wherein the sheet resistance of the first doped graphene layer is less than 10 kohm/sq; (b) a ferroelectric layer deposited on the first doped graphene layer, wherein the ferroelectric layer has ferroelectric hysteresis properties; and (c) a top electrode deposited on the ferroelectric layer, wherein the top electrode comprises a second doped graphene layer; (d) wherein the bottom electrode, the ferroelectric layer, and the top electrode layer are formed on the flexible polymeric substrate as a memory cell of the ferroelectric capacitor-based memory device.

2. The ferroelectric capacitor-based memory device of claim 1, wherein the sheet resistance of the first doped graphene layer is 0.05 to 10 kohm/sq.

3. The ferroelectric capacitor-based memory device of claim 1, wherein the flexible polymeric substrate is a polyethylene terephthalate (PET), a polycarbonate (PC) family of polymers, polybutylene terephthalate (PBT), poly(1,4-cyclohexylidene cyclohexane-1,4-dicarboxylate) (PCCD), glycol modified polycyclohexyl terephthalate (PCTG), Poly(phenylene oxide) (PPO), polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), polymethylmethacrylate (PMMA), polyethyleneimine (PEI) and its derivatives, thermoplastic elastomer (TPE), terephthalic acid (TPA) elastomers, poly(cyclohexanedimethylene terephthalate) (PCT), polyethylene naphthalate (PEN), polyamide (PA), polystyrene sulfonate (PSS), or polyether ether ketone (PEEK) or combinations or blends thereof, or the flexible polymeric substrate is PET.

4. (canceled)

5. The ferroelectric capacitor-based memory device of claim 1, wherein the first doped graphene layer is doped with of p-type dopants.

6. The ferroelectric capacitor-based memory device of claim 5, wherein dopant is a p-type dopant comprising a member selected from the group consisting of tetrafluoro-tetracyanoquinodimethane (F4-TCNQ), nitrous oxide, bromine, iodide, tetracyanoethylene (TCNE), tetrasodium 1,3,6,8-pyrenetetrasulfonic acid (TPA), diazonium salts, oxygen, self-assembled monolayer of fluoroalkyltrichlorosilane molecules, bismuth, antimony and gold, nitric acid, gold, gold chloride and nitric acid, or any combination thereof, or the first doped graphene layer is doped with gold.

7. (canceled)

8. (canceled)

9. The ferroelectric capacitor-based memory device of claim 29, wherein the dopant is an n-type dopant selected from the group consisting of ethanol, ammonia, potassium, polyethyleneimine, 1, 5-naphthalenediamine, sodium amide, 9, 10-dimethylanthracene, 9, 10-dibromoanthracene and tetrasodium 1, 3, 6, 8-pyrenetetrasulfonic acid, or any combination thereof.

10. The ferroelectric capacitor-based memory device of claim 1, wherein the ferroelectric layer having ferroelectric hysteresis properties comprises a ferroelectric polymer.

11. The ferroelectric capacitor-based memory device of claim 10, wherein the ferroelectric polymer is a poly(vinylidene fluoride) (PVDF)-based polymer, polyundecanoamide (Nylon 11)-based polymer, or a blend thereof, or a PVDF-based polymer or a blend comprising the PVDF-based polymer.

12. (canceled)

13. (canceled)

14. The ferroelectric capacitor-based memory device of claim 1, wherein the ferroelectric layer having ferroelectric hysteresis properties is an inorganic layer comprising (Pb(Zr.sub.xTi.sub.1-x)O.sub.3) or BaTiO.sub.3, or a combination thereof.

15-17. (canceled)

18. The ferroelectric capacitor-based memory device of claim 1, wherein: the ferroelectric capacitor exhibits a polarization vs. electric field (P-E) hysteresis loop that is measurable as low as 1 Hz; or the ferroelectric capacitor has a total transmittance of incident light of at least 50, 60, 70 80 or 90%.

19. (canceled)

20. A method for producing a ferroelectric capacitor-based memory device of claim 1, the method comprising: (a) depositing a first doped graphene layer onto a flexible substrate or depositing a graphene layer onto a flexible substrate followed by doping the graphene layer to produce a first doped graphene layer, wherein the first doped graphene layer has a sheet resistance of less than 10 kohm/sq; (b) depositing a ferroelectric precursor layer on the first doped graphene layer and annealing the deposited ferroelectric precursor layer to obtain a ferroelectric layer having ferroelectric hysteresis properties; and (c) depositing a top electrode comprising a second doped graphene layer on the ferroelectric layer having ferroelectric hysteresis properties to obtain the ferroelectric capacitor.

21. The method of claim 20, wherein the method comprises depositing the graphene layer onto the flexible substrate by a roll-to-roll process and/or the first graphene layer is chemical vapor deposition (CVD) graphene or liquid phase exfoliated (LPE) graphene.

22. (canceled)

23. A printed circuit board, an integrated circuit, or an electronic device comprising the ferroelectric capacitor-based memory device of claim 1.

24-28. (canceled)

29. The ferroelectric capacitor-based memory device of claim 1, wherein the first doped graphene layer is doped with n-type dopants.

30. The ferroelectric capacitor-based memory device of claim 11, the PVDF-based polymer is blended with a non-PVDF-based polymer, and wherein the non-PVDF polymer is a poly(phenylene oxide) (PPO), a polystyrene (PS), or a poly(methyl methacrylate) (PMMA), or a blend thereof; or the PVDF-based polymer is PVDF, PVDF-TrFE, or PVDF-TrFE-CtFE.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 is a 2-D cross-sectional view of ferroelectric capacitors of the present invention.

[0042] FIG. 2 is a flow chart of a process to make a ferroelectric capacitor of the present invention.

[0043] FIG. 3 is a schematic of implementation of a circuit in a semiconductor wafer or an electronic device using ferroelectric capacitors of the present invention.

[0044] FIG. 4 is a schematic of implementation of an exemplary wireless communication system in which a ferroelectric capacitor of the present invention may be advantageously employed.

[0045] FIG. 5 is a schematic of an electronic circuit that includes the ferroelectric capacitor of the present invention.

[0046] FIG. 6 is a flowchart of a method for operating an energy storage circuit that includes ferroelectric capacitor of the present invention.

[0047] FIG. 7 is a schematic of a piezoelectric sensor circuit using the ferroelectric capacitor of the present invention.

[0048] FIG. 8 is a 2-D cross-sectional representation of a ferroelectric device of the present invention with piezoelectric material.

[0049] FIG. 9 is a 2-D cross-sectional representation of a ferroelectric device of the present invention with piezoelectric material.

[0050] FIG. 10 are graphs of polarization in μC/cm.sup.2 versus electric field in megavolts per meter (MV/m) of a ferroelectric capacitor of the present invention, a comparative ferroelectric capacitor having a platinum electrodes and a comparative ferroelectric capacitor having a graphene bottom electrode.

[0051] FIG. 11 are graphs of switching time measurements dP/d(log t) vs. log (t) plot for a ferroelectric capacitor of the present invention, a comparative ferroelectric capacitor having a platinum electrodes and a comparative ferroelectric capacitor having a graphene bottom electrode.

[0052] FIG. 12 are graphs of dielectric permittivity (Constant) versus frequency (Hz) for a ferroelectric capacitor of the present invention, a comparative ferroelectric capacitor having a platinum electrodes and a comparative ferroelectric capacitor having an undoped graphene bottom electrode.

[0053] FIG. 13 are graphs of dielectric losses versus frequency (Hz) for a ferroelectric capacitor of the present invention, a comparative ferroelectric capacitor having a platinum electrodes and a comparative ferroelectric capacitor having an undoped graphene bottom electrode.

DETAILED DESCRIPTION OF THE INVENTION

[0054] The present discovery overcomes the difficulties associated with current flexible electronic devices. The discovery lies in the use of a doped graphene electrode as at least one electrode, such as the bottom electrode or interconnect, in a ferroelectric capacitor. In particular, a ferroelectric capacitor of the present invention includes a bottom electrode that can have a doped graphene layer deposited on a flexible substrate. Ferroelectric capacitors made with a doped graphene bottom electrode demonstrate lower operating voltages, faster switching times, lower dielectric loses, and low dielectric dispersion up to high frequencies. In one aspect of the present invention, ferroelectric capacitors using doped graphene electrodes with a sheet resistance of 10 kohm/sq or less, and more preferably a sheet resistance of 0.05 kohm/sq to 10 kohm/sq, have improved performance when compared with the currently available memory-based capacitors.

[0055] These and other non-limiting aspects of the present invention are discussed in further detail in the following sections.

A. Ferroelectric Capacitor

[0056] FIG. 1 is a 2-D cross-sectional view of ferroelectric capacitors 100 that include a doped graphene electrode or interconnect 102 of the present invention. The ferroelectric capacitors 100 can include a substrate 104, the doped graphene electrode 102 as the bottom electrode, a ferroelectric material 106, and top electrodes 108. Each of the top electrodes 108 may form a different ferroelectric capacitor that may be independently controlled separate from the other ferroelectric capacitors. Although shown as capacitors sharing the ferroelectric material 106 and the bottom electrode 102, the ferroelectric layer 106 and the bottom electrode 102 may be patterned to form wholly separate capacitor structures. The ferroelectric capacitors 100 can be fabricated on the substrate 104 by forming the ferroelectric material 106 between conducting electrodes 102 and 108. For the purpose of FIG. 1, the ferroelectric material 106 is in the form of a film or layer. Additional materials, layers, and coatings (not shown) known to those of ordinary skill in the art can be used with the ferroelectric capacitors 100, some of which are described below.

[0057] The ferroelectric capacitors in FIG. 1 are said to have “memory” because, at zero applied volts, it has two remnant polarization states that do not decay back to zero. These polarization states can be used to represent a stored value, such as binary 0 or 1, and are read by applying a sense voltage between the electrodes 102 and 108 and measuring a current that flows between the electrodes 102 and 108. Also, the amount of charge needed to flip the polarization state to the opposite state can be measured and the previous polarization state is revealed. This means that the read operation changes the polarization state, and can be followed by a corresponding write operation, in order to write back the stored value by again altering the polarization state.

[0058] 1. Substrate

[0059] The substrate 104 may be used as a support. The substrate 104 can be made from material that is not easily altered or degraded by heat or organic solvents. Non-limiting examples of such materials include inorganic materials such as silicon, plastic, paper, banknotes substrates, which include polyethylene terephthalate, polycarbonates, and polyetherimide substrates. In a preferred aspect, the substrate is flexible. Other examples of substrates include those that have low glass transition temperatures (T.sub.g) (e.g., polyethylene terephthalate (PET), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), or polypropylene (PP)).

[0060] 2. Bottom Electrode or Interconnect

[0061] The bottom electrode or interconnect 102 of the present invention is made of a doped graphene layer. Graphene is a two-dimensional monolayer of sp.sup.2-bonded hexagonal carbon atoms. The graphene layer can be manufactured using known methods, for example, chemical vapor deposition or liquid phase exfoliation. The graphene layer can be deposed on the substrate 104 using known methods for transferring graphene to substrate materials such as vacuum filtration, polymethylmethacrylate (PMMA), thermal release tapes, or the like. The graphene coated substrate can be doped with a p-type dopant such as nitromethane, nitric acid, gold (Au), gold tetrachloride (AuCl.sub.3), sulfuric acid, or thionyl chloride. FIG. 2 is a schematic representing the method 200 for preparing a doped graphene layer electrode of the invention. An electrochemically exfoliated graphene is collected on a material 202 (for example, a copper foil or a nylon material). The substrate 104 (for example, a polyethylene terephthalate film) is placed in contact with the graphene loaded material 202 and subjected to conditions sufficient to transfer or deposit the graphene from the material to the substrate 104 to obtain a stack 204 containing graphene layer 206 deposed on the substrate 104. The graphene layer 206 can be transferred on to the substrate 104 using lamination conditions, roll-to-roll conditions or other known methods for transferring graphene to polymeric substances. The graphene layer 206 can be doped with either a p-type or n-type dopant by contact of the graphene layer 206 with a dopant or dopant solution to form the stack 208 having doped the graphene electrode layer 102 deposed on the substrate 104 (non-limiting examples of p-type and n-type dopants are provided in the Summary of the Invention section of this application. Additional n-type and p-type dopants known to those of skill in the art are also contemplated as being useful in the context of the capacitors of the present invention). For example, the stack 204 can be immersed in a solution of dopant (for example, an alcoholic solution of gold chloride). The sheet resistance of the doped graphene electrode 102 of the stack 208 can be less than 10 kohm/sq, less than 5 kohm/sq, less than 1 kohm/sq, or less than 0.5 kohm/sq. In other embodiments, the stack 208 can be deposited on the surface of other substrates. The sheet resistance can be modified or tuned as desired by increasing the dopant content (to decrease the sheet resistance) or decreasing the dopant content (to increase sheet resistance). By way of example only, immersing the stack 204 in the dopant solution for a longer period of time can serve to increase the dopant level in the graphene layer 206. Conversely, immersing stack 204 for a shorter period of time in the dopant solution can serve to decrease the dopant level in the graphene layer 206.

[0062] 3. Top Electrode or Contact

[0063] The top electrode or contact 108 can be disposed on the ferroelectric material 106 by thermal evaporation through a shadow mask. The material used for the top electrode 108 can be conductive. Non-limiting examples of such materials include metals, metal oxides, and conductive polymers (e.g., polyaniline, polythiophene, etc.) and polymers made conductive by inclusion of conductive micro- or nano-structures. In addition, non-limiting examples of conductive polymer materials include conducting polymers (such as PEDOT: PSS, Polyaniline, graphene etc.), and polymers made conductive by inclusion of conductive micro- or nano-structures (such as gold nanowires). The top electrode 108 can be a single layer or laminated layers formed of materials each having a different work function. Further, it may be an alloy of one or more of the materials having a low work function and at least one selected from the group consisting of gold, silver, platinum, copper, manganese, titanium, cobalt, nickel, tungsten, and tin. Examples of the alloy include a lithium-aluminum alloy, a lithium-magnesium alloy, a lithium-indium alloy, a magnesium-silver alloy, a magnesium-indium alloy, a magnesium-aluminum alloy, an indium-silver alloy, and a calcium-aluminum alloy. In some embodiments, the top electrode or contact is metal oxide, for example, indium-tin-oxide (ITO). The film thickness of the top electrode or contact 108 is typically between 20 nm to 500 nm, or 50 nm to 100 nm. In some embodiments, the tope electrode is deposited on ferroelectric material 106 by spray coating, ultra sonic spray coating, roll-to-roll coating, ink jet printing, screen printing, drop casting, spin coating, dip coating, Mayer rod coating, gravure coating, slot die coating, doctor blade coating or extrusion coating

[0064] 4. Ferroelectric Material

[0065] Referring back to FIG. 1, the ferroelectric material 106 can be interposed between the lower electrode 102 and the top electrode 108. In one instance, the ferroelectric material 106 can be obtained from a blend of a ferroelectric polymer and a polymer having a low dielectric constant, wherein the polymers have been solubilized in the same solvent or solvent system. In one instance, the ferroelectric material 106 can be obtained from a ferroelectric precursor material, which can include a ferroelectric polymer, copolymer, terpolymer, or a polymer blend that includes a ferroelectric polymer, copolymer, or terpolymer or combinations thereof. Non-limiting examples of ferroelectric polymers include PVDF-based polymers, polyundecanoamide (Nylon 11)-based polymers, or blends of PVDF-based polymers or polyundecanoamide (Nylon 11)-based polymers. The PVDF-based polymer can be a homopolymer, a copolymer, or a terpolymer, or a blend thereof. A non-limiting example of a PVDF-based homopolymer polymer is PVDF. Non-limiting examples of PVDF-based copolymers are poly(vinylidene fluoride-tetrafluoroethylene) (PVDF-TrFE), poly(vinylidene-fluoride-co-hexafluoropropene) (PVDF-HFP), poly(vinylidene-fluoride-chlorotrifluoroethylene) (PVDF-CTFE) or poly(vinylidene-fluoride-chlorofluoroethylene) (PVDF-CFE). Non-limiting examples of PVDF-based terpolymers include poly(vinylidene-fluoride-trifluoroethylene-chlorotrifluoroethylene) (PVDF-TrFE-CTFE) or poly(vinylidene-fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TrFE-CFE). The ferroelectric polymer can be blended with a non-ferroelectric polymer. Examples of non-ferroelectric polymers include a poly(phenylene oxide) (PPO), a polystyrene (PS), or a poly(methyl methacrylate) (PMMA), or blends thereof. In preferred aspects, the polymers in the precursor material are solubilized in a solvent or melt such that they do not exhibit ferroelectric hysteresis properties but can be deposited on the doped graphene electrode 102, and then transformed via annealing by, for example heat, to exhibit ferroelectric hysteresis properties. Referring back to FIG. 2, the precursor ferroelectric material 210 can be deposited on the doped graphene electrode 102 via spin-coating, spray coating, ultra sonic spray coating, roll-to-roll coating, ink jet printing, screen printing, drop casting, dip coating, Mayer rod coating, gravure coating, slot die coating, doctor blade coating, extrusion coating, flexography, gravure, offset, rotary screen, flat screen, ink-jet, or laser ablation. A non-limiting example includes solubilizing a ferroelectric precursor material in a polar solvent to form a thin film. The stack 208 may be placed on a spin coater and the thin film can be applied to the center of the doped graphene electrode layer 102 while spinning at high speed (for example, 2000 rpm) for about 1 minute to spread the ferroelectric precursor material 210 over the doped graphene layer to form stack 212. The stack 212 coated with ferroelectric material precursor 210 can be heated to about 80° C. or more to anneal the precursor and form the stack 214 that contains the substrate 104, the bottom doped graphene electrode layer 102, and the ferroelectric material layer 106. The applied heat converts the precursor material 210 into the ferroelectric material 106, which has ferroelectric hysteresis properties. This annealing step allows for the formulation of a crystalline phase via chemical restricting of the ferroelectric precursor material or removal of the solvent from the ferroelectric precursor material, or both. Subsequently, the stack 214 can be further processed by deposing the top electrode 108 onto at least a surface of the ferroelectric material 106 via a deposition device. The top electrode 108, if needed, can be further cured.

B. Applications for Ferroelectric Capacitors

[0066] Any one of the ferroelectric capacitors of the present invention can be used in a wide array of technologies and devices including but not limited to: smartcards, RFID cards/tags, piezoelectric sensors, piezoelectric transducers, piezoelectric actuators, pyroelectric sensors, memory devices, non-volatile memory, standalone memory, firmware, microcontrollers, gyroscopes, acoustics sensors, actuators, micro-generators, power supply circuits, circuit coupling and decoupling, radio frequency filtering, delay circuits, radio frequency tuners, passive infra-red sensors (“people detectors”), infrared imaging arrays and fingerprint sensors. If implemented in memory, including firmware, functions may be stored in the ferroelectric capacitors as one or more instructions or code, such that the ferroelectric capacitor-based memory operates as a computer-readable medium. Examples include computer-readable media encoded with a data structure and computer-readable media encoded with a computer program. Computer-readable media includes physical computer storage media. Combinations of the above should also be included within the scope of computer-readable media.

[0067] In many of these applications thin films of ferroelectric materials are typically used, as this allows the field required to switch the polarization to be achieved with a moderate voltage, such as less than 5 Volts, preferably less than 3 Volts, and more preferably less than 1.8 Volts. Although some specific circuitry has been set forth, it will be appreciated by those skilled in the art that not all of the disclosed circuitry is required to practice the disclosure. Moreover, certain well known circuits have not been described, to maintain focus on the disclosure.

[0068] FIG. 3 is block diagram illustrating implementation of an integrated circuit in a semiconductor wafer or an electronic device according to one embodiment. In one case, a ferroelectric capacitor 100 may be manufactured on a wafer 302. The wafer 302 may be singulated into one or more dies that may contain the ferroelectric capacitor 100. Additionally, the wafer 302 may experience further semiconductor manufacturing before singulation. For example, the wafer 302 may be bonded to a carrier wafer, a packaging bulk region, a second wafer, or transferred to another fabrication facility. Alternatively, an electronic device 304 such as, for example, a personal computer, may include a memory device 306 that includes the ferroelectric capacitor 100. Additionally, other parts of the electronic device 304 may include the ferroelectric capacitor 100 such as a central processing unit (CPU), a digital-to-analog converter (DAC), an analog-to-digital converter (ADC), a graphics processing unit (GPU), a microcontroller, or a communications controller.

[0069] FIG. 4 is a block diagram showing an exemplary wireless communication system 400 in which an embodiment of the disclosure may be advantageously employed. For purposes of illustration, FIG. 4 shows three remote units 402, 404, and 406 and two base stations 408. It will be recognized that wireless communication systems may have many more remote units and base stations. Remote units 402, 404, and 406 include circuit devices 402A, 402C and 402B, which may include integrated circuits or printable circuit boards that include the disclosed ferroelectric capacitor made by the processes of the present invention. It will be recognized that any device containing an integrated circuit or printable circuit board may also include the ferroelectric capacitor disclosed herein, including the base stations, switching devices, and network equipment. FIG. 4 shows forward link signals 410 from the base station 408 to the remote units 402, 404, and 406 and reverse link signals 412 from the remote units 402, 404, and 406 to base stations 408.

[0070] In FIG. 4, remote unit 402 is shown as a mobile telephone, remote unit 406 is shown as a portable computer, and remote unit 404 is shown as a fixed location remote unit in a wireless local loop system. For example, the remote units may be mobile phones, hand-held personal communication systems (PCS) units, portable data units such as personal data assistants, GPS enabled devices, navigation devices, set upper boxes, music players, video players, entertainment units, fixed location data units such as meter reading equipment, tablets, or any other device that stores or retrieves data or computer instructions, or any combination thereof. Although FIG. 4 illustrates remote units according to the teachings of the disclosure, the disclosure is not limited to these exemplary illustrated units. Embodiments of the disclosure may be suitably employed in any device which includes the ferroelectric capacitor 100 made by the processes disclosed by the present invention.

C. Use of Ferroelectric Components as Memory Cells

[0071] Ferroelectric components, such as the ferroelectric capacitors described throughout this application, may be operated as memory cells to store data, such as information, code, or instructions. For example, a single ferroelectric capacitor may store a single bit of information, e.g., ‘1’ or ‘0.’ This ‘1’ or ‘0’ value may be stored as a binary polarization direction of the ferroelectric layer in the ferroelectric component. For example, when the ferroelectric layer is polarized from top to bottom, the ferroelectric component stores a ‘1’, and when the ferroelectric layer is polarized from bottom to top, the ferroelectric component stores a ‘0.’ This mapping of polarization states is only one example. Different polarization levels may be used to represent the ‘1’ and ‘0’ data bits in different embodiments of the present invention.

D. Operation of a Controller for a Ferroelectric Memory Device for Storing Multiple Bits of Information in Memory Cells of the Ferroelectric Memory Device

[0072] A ferroelectric memory device may be constructed with an array of ferroelectric memory capacitors described above, in which each capacitor includes a ferroelectric memory cell. Read and write operations to the ferroelectric memory device may be controlled by a memory controller coupled to the array of multi-level ferroelectric memory cells. One example of a write operation performed by the controller to store information in a single ferroelectric memory cell is described below. A method may include receiving a bit and an address for writing to the addressed ferroelectric memory cell. The bit may be, for example ‘0’ or ‘1.’ Then, a write pulse of a predetermined voltage may be applied across the top and bottom electrodes of the memory cell. The write pulse may create a certain level of remnant polarization in the ferroelectric layer of the ferroelectric memory cell. That remnant polarization affects characteristics of the ferroelectric memory cell, which may be measured at a later time to retrieve the bit that was stored in the ferroelectric memory cell. The cell programming may also include other variations in the write pulse. For example, the controller may generate multiple write pulses to apply to the memory cell to obtain the desired remnant polarization in the ferroelectric layer. In some embodiments, the controller may be configured to follow a write operation with a verify operation. The verify operation may be performed with select write operations, all write operations, or no write operations. The controller may also execute a read operation to obtain the bit stored in the ferroelectric memory cell.

[0073] In an array of ferroelectric memory cells, the array may be interconnected by word lines extending across rows of memory cells and bit lines extending across columns of memory cells. The memory controller may operate the word lines and bit lines to select particular memory cells from the array for performing read and/or write operations according to address received from a processor or other component requesting data from the memory array. Appropriate signals may then be applied to the word lines and bit lines to perform the desired read and/or write operation.

E. Operation as a Decoupling Capacitor and as an Energy Storage Device

[0074] The ferroelectric capacitor of the present invention can be used to decouple one part of an electrical network (circuit) from another. FIG. 5 is a schematic of circuit 500 that includes the ferroelectric capacitor 100. Ferroelectric capacitor 100 is coupled to power voltage line 502 and a ground voltage line 504. Power noise generated by the power voltage and the ground voltage is shunted through the capacitor, and thus reducing the overall power noise in the circuit 506. The ferroelectric capacitor 100 can provide local energy storage for the device by providing releasing charge to the circuit when the voltage in the line drops. FIG. 6 is a flowchart of a method for operating an energy storage circuit that includes ferroelectric capacitor 100. The ferroelectric capacitor 100 can provide electrical power to a consuming device when electrical power from a primary source is unavailable. Method 600 of FIG. 6 begins at block 602 with defining a target energy level for the ferroelectric capacitor. The target energy level may be, for example, 1 μF to 20 μF. After the target energy level is defined, at block 604 the ferroelectric capacitor 100 is charged to the defined energy level. At block 606, a first amount of energy that is stored in the ferroelectric capacitor 100 is measured. When the first amount of energy stored in the ferroelectric capacitor 100 reaches the target energy level, the charging is terminated at block 608. At block 610, when electrical power becomes unavailable from the primary source (for example, a voltage source), the ferroelectric capacitor 100 will discharge energy into the consuming device (for example, a smart phone, computer, or tablet).

[0075] FIG. 7 is a schematic of a piezoelectric sensor circuit using the ferroelectric capacitor 100. When a piezoelectric sensor is at rest, the dipoles formed by the positive and negative ions cancel each other due to the symmetry of the crystal or polymer structure used in the device, and an electric field is not observed. When stressed, the crystal or polymer deforms, symmetry is lost, and a net dipole moment is created. The dipole moment creates an electric field across the crystal. The materials generate an electrical charge that is proportional to the pressure applied. As shown in FIG. 7, the piezoelectric sensor 700 includes the ferroelectric capacitor 100 as a piezoelectric component in the sensor. It is also envisioned that the ferroelectric capacitor 100 can be used as the decoupling capacitor in the same circuit. FIG. 8 is a 2-D cross-sectional representation of a ferroelectric device of the invention that includes a piezoelectric material. As shown in FIG. 8, piezoelectric material 802 can be disposed between doped graphene electrode 102 and top electrode 108 in a piezoelectric transducer, and, when stressed create a net dipole moment. The piezoelectric material 802 is, in some aspects, the same as the ferroelectric material described throughout this specification. A method of using a ferroelectric capacitor of the present invention as a piezoelectric capacitor includes sending a vibrational pulse to the piezoelectric capacitor; comparing the capacitor voltage to a reference voltage and adjusting the vibration pulses in response to the comparison. FIG. 9 is a 2-D cross-sectional representation of the ferroelectric device 100 that includes pyroelectric material. As shown in FIG. 9, pyroelectric material 902 can be disposed between doped graphene electrode 102 and top electrode 108 in a pyroelectric device, and then will generate a charge when exposed to infrared light. The pyroelectric material 902 is, in some aspects, the same as the ferroelectric material described throughout this specification. A method of using a ferroelectric capacitor of the present invention as a pyroelectric capacitor includes sending heat pulse to the pyroelectric capacitor; comparing the capacitor voltage to a reference voltage; and adjusting the heat pulses in response to the comparison.

EXAMPLES

[0076] The present invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes only, and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of noncritical parameters which can be changed or modified to yield essentially the same results.

Example 1

Fabrication of Ferroelectric Capacitors with Doped Graphene Electrodes

[0077] A ferroelectric capacitor of the present invention was fabricated using a doped graphene as the bottom electrode using the following method.

[0078] Doped Graphene Electrode.

[0079] High performance PET substrates were cleaned with acetone, ethanol and deionized water prior to device fabrication. Liquid phase exfoliated (LPE) graphene bottom electrodes were made by vacuum filtration of the electrochemically exfoliated graphene solution on a nylon filter of 0.2 μm of pore diameter to form a graphene film. The graphene film was transferred onto polyethylene terephthalate (PET, SABIC) by a roll-to roll method. The sheet resistance was calculated by a four probe measurement unit. Gold tetrachloride (AuCl.sub.3, 10 mM) was dispersed in ethanol and the graphene coated PET was immersed in the AuCl.sub.3-ethanol solution for thirty (30) seconds. The procedure was repeated for samples 2 and 3. The sheet resistance values of three doped graphene electrodes of the present invention are listed in Table 1.

TABLE-US-00001 TABLE 1 Sheet Resistance after doping Transmittance Sample # (kohm/sq) (%) 1 0.9 57 2 8 60

[0080] Ferroelectric Material and Top Electrode.

[0081] A 3 wt. % solution of ferroelectric PVDF-TrFE was prepared in methyl-ethyl-ketone (MEK) solvent. The ferroelectric thin film was spun on the doped graphene electrode at 2000 rpm for 60 seconds to form a ferroelectric layer of about 400 nm thickness. The film was annealed on a hotplate for 30 minutes at 80° C. prior to annealing in a vacuum furnace at 130° C. for 2 hours to improve the crystallinity. A 100 nm thick top Au electrode was deposited by thermal evaporation through a shadow mask.

Comparative Example 2

Fabrication of a Comparative Ferroelectric Capacitor with an Undoped Graphene Bottom Electrode

[0082] Comparative ferroelectric capacitors were fabricated on flexible substrates with undoped graphene as the bottom electrode.

[0083] Graphene Electrode.

[0084] High performance PET substrates were cleaned with acetone, ethanol and deionized water prior to device fabrication. The liquid phase exfoliated (LPE) graphene bottom electrodes were made by vacuum filtration of the electrochemically exfoliated graphene solution on a nylon filter of 0.2 μm of pore diameter to form a graphene film. The graphene film was transferred onto polyethylene terephthalate (PET, SABIC) by a roll-to roll method. The sheet resistance values of the comparative graphene electrodes are listed in Table 2.

TABLE-US-00002 TABLE 2 Sheet Resistance Transmittance Sample # kohm/sq) (%) C1 12 57 C2 20 60

[0085] Ferroelectric Material and Top Electrode.

[0086] A 3 wt. % solution of ferroelectric PVDF-TrFE was prepared in methyl-ethyl-ketone (MEK) solvent. The ferroelectric thin film was spun on the graphene electrode at 2000 rpm for 60 seconds to form a ferroelectric layer of about 400 nm thickness. The film was annealed on a hotplate for 30 minutes at 80° C. prior to annealing in a vacuum furnace at 130° C. for 2 hours to improve the crystallinity. A 100 nm thick top Au electrode was deposited by thermal evaporation through a shadow mask.

Example 3

Operation of Devices of the Present Invention and Comparative Devices

[0087] Hysteresis Measurements.

[0088] Hysteresis loops for the ferroelectric capacitors of the present invention made in Example 1 (Sample number 1), the comparative graphene ferroelectric capacitors made in Example 2 (sample number, Cl) and a comparative ferroelectric capacitor with a platinum bottom electrode (Pt device) and having a 400 nm thick ferroelectric layer were measured at a frequency of 10 Hz and are depicted in FIG. 10. FIG. 10 is a graphical depiction of the polarization (μC/cm.sup.2) versus electric field (MV/m) at 10 Hz for a ferroelectric capacitor of the present invention, a comparative ferroelectric capacitor having a platinum electrodes and a comparative ferroelectric capacitor having a graphene bottom electrode. Data 1002 was the comparative platinum capacitor, data 1004 was the doped graphene capacitor of the present invention and data 1006 was the comparative undoped graphene capacitor. As shown in FIG. 10 well-saturated loops with saturation polarization (P.sub.s) of 8.6 μC/cm.sup.2 and remnant polarization (P.sub.r) of ˜7.2 μC/cm.sup.2 were produced, which indicated good crystallinity of the PVDF-TrFE film irrespective of the electrodes used. To achieve the same polarization as the Pt device, higher saturation fields were applied to the capacitors with graphene electrodes. A consistent increase in the positive and negative switching fields was observed for the graphene capacitors as compared to the Pt capacitor. Without wising to be bound by theory, it is believed that the increase in coercive fields (+E.sub.c/−E.sub.c) from [+581-53] MV/m for the Pt capacitor and to [+82/−70 MV/m] for the comparative graphene capacitor can be attributed to the poor conductivity of the undoped (comparative) graphene, which led to a large voltage drop across the un-patterned bottom electrode. The poorly conducting graphene electrode acted as a large resistor connected in series with the ferroelectric capacitor and the intrinsic coercive field of the ferroelectric film itself did not change. Instead, a higher voltage was needed to reach the required coercive field to switch the dipoles in the ferroelectric copolymer film. In contrast to the comparative graphene capacitor, the doped graphene capacitor of the present invention displayed coercive fields closer to the Pt capacitor (+72 MV/m/−61 MV/m). This difference was a substantial improvement in coercive fields compared to the comparative graphene capacitor.

[0089] Switching Characteristics.

[0090] Switching characteristics of the capacitors are shown in FIG. 11. Switching characteristics were obtained by a time domain measurement of the charge density or polarization (P) response. Switching times (t.sub.s) were estimated from the time of the maximum of dP/d(log t) vs. log (t) plot. FIG. 11 are graphical depictions of time measurements dP/d(log t) vs. log (t) plot for the ferroelectric capacitor of the present invention (Sample 1 in Example 1), the comparative ferroelectric capacitor having a platinum electrodes and the comparative ferroelectric capacitor having a graphene bottom electrode (Sample C1). Data 1100 is the Pt capacitor, data 1102 is the doped graphene capacitor of the present invention, and data 1104 is the comparative graphene capacitor. At applied fields of 140 MV/m, the comparative Pt capacitor exhibited a switching time of 12.88 ms while the comparative graphene capacitor and the capacitor of the present invention exhibited switching times of 15.45 ms and 13.8 ms respectively. Without wishing to be bound by theory, it is believed that the switching time, t.sub.s, is dependent on the applied electric field.

[0091] Switching time can be represented by Equation (1):

t.sub.s=t.sub.s∞e.sup.(Ea/E)  (1)

where t.sub.s∞ is the limited switching time, E.sub.a is the activation field, and E is applied electric field.
Applying Equation I, the switching time increases for lower applied fields in the ferroelectric capacitors. For the comparative graphene capacitor, the voltage drop across the bottom electrode had lower applied fields across the ferroelectric film, which led to a higher switching time relative to the Pt capacitor. The ferroelectric capacitor of the present invention had a switching time of 13.8 ms, which approached the switching time of the Pt device. Thus, the ferroelectric capacitor of the present invention demonstrates enhanced switching time as compared to the comparative graphene capacitors.

[0092] Dielectric Permittivity/Dielectric Constant Measurements.

[0093] FIG. 12 are graphical representations of relative dielectric permittivity (constant) versus frequency (Hz) of the ferroelectric capacitor of the present invention having a doped graphene bottom electrode (Sample 1 of Example 1), the comparative ferroelectric capacitor having a platinum electrode and the comparative ferroelectric capacitor having a graphene bottom electrode (Sample C1). Data 1200 is for the Pt capacitor, data 1202 is for the ferroelectric capacitor of the present invention graphene device, and data 1204 is for comparative capacitor with an undoped graphene electrode. FIG. 13 are graphical representations of dielectric losses versus frequency (Hz) for the ferroelectric capacitor of the present invention, the comparative ferroelectric capacitor having a platinum electrodes, and the comparative ferroelectric capacitor having a graphene bottom electrode. Data 1300 is the Pt capacitor, data 1302 is the doped graphene capacitor of the present invention, and data 1304 is the comparative graphene capacitor. As shown, the permittivity of the comparative graphene capacitor was less than the permittivity of the ferroelectric capacitor of the present invention. The dielectric response of ferroelectric capacitors were represented with an equivalent circuit consisting of a resistor in series with a parallel RC circuit. Ferroelectric capacitors with poorly conducting electrodes demonstrated lower permittivity and higher dielectric losses as compared to a Pt capacitor, which is depicted in FIGS. 12 and 13, respectively. The dielectric loss, for the comparative undoped graphene capacitor (0.19 at 100 KHz) was higher than the dielectric loss for the doped graphene capacitor of the present invention (0.14 at 100 KHz) and the Pt capacitor for the comparative Pt capacitor (˜0.11 at 100 KHz). Thus, the ferroelectric capacitor of the present invention demonstrated improved dielectric permittivity and dielectric constant measurements as compared to the comparative graphene capacitor.

[0094] As shown in the data, ferroelectric capacitors of the present invention have similar electrical properties as ferroelectric capacitors with Pt bottom electrodes and better electrical properties as compared to the comparative ferroelectric capacitors having an undoped graphene bottom electrode. Thus, ferroelectric capacitors of the present invention have improved durability, operating voltage, switching time, dielectric loss, low dielectric dispersion, which beneficial for use in flexible electronic devices.

[0095] Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the technology of the disclosure as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.