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MULTI-STATES NONVOLATILE OPTO-FERROELECTRIC MEMORY MATERIAL AND PROCESS FOR PREPARING THE SAME THEREOF

20170206952 ยท 2017-07-20

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    Abstract

    The invention deals with multi-states nonvolatile opto-ferroelectric memory element and method of preparing the same thereof. This invention describes multi-states nonvolatile opto-ferroelectric memory element consisting of opto-ferroelectric memory material comprised of Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3, wherein x=0.2 to 0.8 and y=0.2 to 0.6, said memory material (PBLZT) phtotovoltaic ferroelectric material is characterized by a single-phase opto-ferroelectric materials, photovoltaic and multi-states ferroelectric memory material. The invention relates to process of preparing multi-states nonvolatile opto-ferroelectric memory material by solid route, solution-gel process and pulsed laser process. It describes development of multi-states nonvolatile opto-ferroelectric memory material at room temperature. Invention describes a ferroelectric material whose polarization is switched by white light and low power LASER (10-50 mW) with wavelength (405 nm).

    Claims

    1. Multi-states nonvolatile opto-ferroelectric memory element consisting of opto-ferroelectric memory material comprised of:
    Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 wherein x=0.2 to 0.8 y=0.2 to 0.6 said memory material (PBLZT) phtotovoltaic ferroelectric material is characterized by a single-phase opto-ferroelectric materials, photovoltaic and multi-states ferroelectric memory material.

    2. The opto-ferroelectric memory element as claimed in claim 1, characterized by spontaneous polarization in the range of 4-30 C/cm.sup.2 with and without illumination of light from near UV to visible region frequencies and said probed frequencies range from 0.1 Hz to 10 Hz.

    3. The opto-ferroelectric memory element as claimed in claim 2, wherein illumination is white light tungsten filament at 20-100 mW/cm.sup.2 in or white light xenon lamp at 20-100 mW/cm.sup.2 and monochromatic light source of 405 nm at 30-50 mW.

    4. The opto-ferroelectric memory element as claimed in claim 1, characterized by 10-25% fatigue under high stress electric field (80 KV/cm) in the absence of light, and improved fatigue under high stress electric field (80 KV/cm) under illuminated light which is less than 10-100 mW/cm.sup.2.

    5. The opto-ferroelectric memory element as claimed in claim 1, characterized by ON and OFF photocurrent in the range of 4:1 to 6:1 depending on the wavelength and energy of near UV and visible light source.

    6. The opto-ferroelectric memory element as claimed in claim 1, wherein said material is characterized by photovoltaic characterization with open circuit voltage (V.sub.OC) in the range of 2-2.5 V (+/0.5 V) and short circuit current (I.sub.SC) (0.3-0.5 nA) under small stress voltage of (+/110 V and +/40 V).

    7. The opto-ferroelectric memory element as claimed in claim 1, characterized by use of white and monochromatic light to switch the polarization states, and concerned logic states for multi-states memory elements.

    8. The opto-ferroelectric memory element as claimed in claim 1, characterized by white light and 405 nm wavelength in laser source, wherein laser source is selected form complete range of near UV and visible light to realize the multi nonvolatile logic states.

    9. The opto-ferroelectric memory element as claimed in claim 1, characterized by ON and OFF states of the photocurrent under the illumination of 405 nm (10-50 mW) laser and white light.

    10. The opto-ferroelectric memory element as claimed in claim 1, characterized by dielectric properties in the range of dielectric constant (<1000) and low tangent loss (<0.05) over a wide range of temperature (<150 C.) and frequency (50 Hz-1 MHz).

    11. The opto-ferroelectric memory element as claimed in claim 1, characterized by 300-500% switching of polarization first time under 405 nm laser light depending on probe frequency and electrical stress.

    12. The opto-ferroelectric memory element as claimed in claim 1, characterized by fast switching and repeatability of polarization greater than two minutes and less than 30 minutes under 405 nm laser light, wherein the polarization is in the range of 20-50% depending on probe frequency and electrical stress.

    13. The opto-ferroelectric memory element as claimed in claim 1, characterized by multi-states memory with an electrically WRITE operation and optically READ operation and vice-versa, and/or combination of these two, and/or separately by electric field and different wavelength of monochromatic laser light in near UV and visible regions.

    14. The opto-ferroelectric memory element as claimed in claim 1, wherein transformer oil or silicon oil or combination thereof is used as electrical stress medium.

    15. Opto-ferroelectric nonvolatile multistate memory cells, visible or weak UV detector, perovskite based photovoltaic and miniaturized microelectronic memory devices prepared by multi-states nonvolatile opto-ferroelectric memory element as claimed in claim 1.

    16. A process for preparation of opto-ferroelectric material,
    Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 wherein x=0.2 to 0.8 and y=0.2 to 0.6, said process comprising steps of: i) physical mixing of ingredient oxides of PbO (99.5%), ZrO.sub.2 (99.86%), TiO.sub.2 (99.96%), Bi.sub.2O.sub.3 (99.99%) and Li.sub.2CO.sub.3 (97%) in a ratio of x=0.2 to 0.8, and y=0.2 to 0.6 and further mechanical mixing in liquid alcoholic (Isopropyl alcoholIPA) media in the range of 5 to 10 ml with 99.9% purity, and mol fraction of Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 & 0.5, and y=0.2) (PBLZT) electro-ceramics; ii) calcining composites of step (i) at a temperature in the range of 800-850 C. for a period of 11-13 hours to get the desired phase followed by X-ray diffraction (XRD) study to verify the phase purity and crystallinity; iii) sintering composites of step (ii) at a temperature in the range of 1100-1200 C., to obtain desired opto-ferroelectric material; and iv) sintering composites of step (iii) were further processed for mechanical grinding and polishing to get the desired thickness in the range of 400 to 600 microns of opto-ferroelectric material.

    17. A solution-gel process of preparation of PBLZT material, said process comprising of: (a) preparing a solution of lead acetate titanium isopropoxide, zirconium isopropoxide, bismuth acetate, and lithium acetate with mol percentage as
    Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 wherein x=0.2 to 0.8 y=0.2 to 0.6 (b) preparing a solution of lead acetate in solvent comprising of 2 methoxy ethanol and acetic acid and adding a small amount of ethylene gycol in the solution for obtaining gel formation; (c) checking stability of solution to keep the solution for long time at least 30 days, and using the stable solution to prepare the thin films using spin coating technique with growth conditions, at solution molarity 0.2-0.3, spin coating speed in the range of 3000-5000 rpm, pyrolysis at 300 C. for 2 minutes to remove the organics and sintering at 600-700 C. for phase formation; and (d) obtaining final films obtained from step (c) as PBLZT material.

    18. A pulsed laser deposition process for preparing PBLZT material using one inch target of opto-ferroelectric material prepared in claim 16 to prepare the thin films under the conditions of substrate temperature 600-750 C., oxygen partial pressure 100-200 mbar, laser energy density 1-3 J/cm2, and target to substrate distance in the range of 4-5 cm, wherein high quality epitaxial thin film is prepared which directly integrate with the silicon technology for device fabrication.

    Description

    BRIEF DESCRIPTION OF THE DRAWINGS

    [0028] FIG. 1 is a schematic representation of opto-ferroelectric memory devices and characterization process. An electro-ceramic pellet in metal-insulator-metal (ITO/PBLZT/Ag) configuration is immersed in transformer oil with light gadgets on the top and transparent side of side of sandwiched ferroelectric materials to switch the polarization.

    [0029] FIGS. 2 (a) & (b) are the XRD patterns of Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) and Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.5 and y=0.2) composition, respectively represent the tetragonal crystal structure, space group P4 mm (defined crystallographic notation), & lattice parameters, a=b=3.930, c=4.124=>, & a=b=3.932, c=4.117=>, respectively.

    [0030] FIG. 3 Room temperature Raman spectra of (a) Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) compositions, (a) probed with 514 nm and 785 nm laser illumination, (b,c,&d) rationalized with maximum and minimum intensity.

    [0031] FIG. 4: SEM images of (a) Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2), (b) Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.5 and y=0.2) compositions, with compositions within the experimental limits (+/10%) of SEM/EDAX.

    [0032] FIG. 5: Temperature and frequency dependent dielectric constant and tangent loss spectra of (a &b) Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) and (c & d) Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.5 and y=0.2) electroceramics, respectively.

    [0033] FIG. 6 Opto-ferroelectric polarization vs electric field loops of unpoled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) electroceramics under illumination of various illuminating light.

    [0034] FIG. 7 Opto-ferroelectric polarization versus electric field loops of poled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) electroceramics under illumination of various illuminating light.

    [0035] FIG. 8 Bar diagram of Opto-ferroelectric polarization versus days to check the forth-back switching behavior of poled and unpoled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) electroceramics under illumination of various illuminating light.

    [0036] FIG. 9 shows current density versus electric field (J-E) loops of unpoled ceramics (inset shows J-E of poled ceramics).

    [0037] FIG. 10: J-E loops and photo-voltage of unpoled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) electroceramics with small voltage step and small electric field stress to check the open circuit voltage.

    [0038] FIG. 11: Switch ON and OFF photo response of unpoled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) electro-ceramics under illumination of white xenon lamp light.

    [0039] FIG. 12: Switchable polarization as function of time for unpoled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) ceramics with and without white light, (a) 80 KV/cm stress electric field and 1 Hz switching frequency, (b) 80 KV/cm stress electric field and 2 Hz switching frequency.

    [0040] FIG. 13: Switchable polarization of poled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) ceramics with and without white light (a) 80 KV/cm stress electric field and 1 Hz switching frequency.

    [0041] FIG. 14: Switchable polarization of Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) ceramic probed with PUND analysis (a) pulse wave configuration, (b) unpoled ceramics at 100 ms time, (c) unpoled ceramics at 10 ms (d) poled ceramics at 100 ms (e) poled ceramics at 10 ms time with and without white light.

    DETAILED DESCRIPTION OF THE INVENTION

    [0042] While the present invention is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the figures and will be described in detail below. It should be understood, however that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the invention as defined by the appended claims.

    [0043] Before describing in detail the various embodiments of the present invention it may be observed that the novelty and inventive step that are in accordance with the present invention resides in the construction of switch module. It is to be noted that a person skilled in the art can be motivated from the present invention and modify the various constructions of switch module. However, such modification should be construed within the scope and spirit of the invention.

    [0044] Accordingly, the drawings are showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.

    [0045] The terms comprises, comprising, including or any other variations thereof, are intended to cover a non-exclusive inclusion, such that an assembly, mechanism, setup, that comprises a list of components does not include only those components but may include other components not expressly listed or inherent to such assembly, mechanism or setup. In other words, one or more elements in switch module or assembly proceeded by comprises a does not, without more constraints, preclude the existence of other elements or additional elements in the assembly or mechanism. The following paragraphs explain present invention and the same may be deduced accordingly.

    [0046] As used hereinafter, the following terms shall have the following definitions:

    [0047] Opto-ferroelectric refers the effect of white light and/or monochromatic light on induced dielectric polarization of a material in an external stress of electric field.

    [0048] Photovoltaic refers the effect of white light and/or monochromatic light on open circuit and short circuit current.

    [0049] Remanent polarization, Pr refers to the spontaneous polarization under the application of external field of zero bias.

    [0050] Photosensitive refers the effect of white light and/or monochromatic light on ON/OFF states of the photocurrents.

    [0051] Coercive E-field, Ec refers to the electric field which is used to switch the polarization to generate the different logic state for memory applications. The E-field frequency is 1 Hz to 10 50 Hz for the purpose of this description.

    [0052] Leakage current density (J) refers to the current per unit area, measured as a function of applied electric field.

    [0053] Polarization fatigue refers to the loss in polarization after certain number of cycles of device utility.

    [0054] Room temperature refers to temperature of 296 K.

    [0055] The composite Pb1x(Bi0.5Li0.5)x(Ti1yZry)O3 (x=0.4 and y=0.2) is referred to as PBLZT (x=0.4 and y=0.2) for the purpose of this description.

    [0056] The present invention relates to a multi-states nonvolatile opto-ferroelectric memory element consisting of opto-ferroelectric memory material comprised of:


    Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 [0057] wherein x=0.2 to 0.8 [0058] y=0.2 to 0.6
    said memory material (PBLZT) phtotovoltaic ferroelectric material is characterized by a single-phase, opto-ferroelectric materials, photovoltaic and multi-states ferroelectric memory elements.

    [0059] The present invention relates to methods for producing said opto-ferroelectric memory material and uses thereof. All the terms used have the same meaning as is used in general aspect of the art, unless otherwise specified.

    [0060] The present invention provides a class of single phase, opto-ferroelectric materials belonging to the perovskite crystal structure and prepared with the solid state reaction route. An aspect of the present invention provides a composition Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.2 to 0.8, and y=0.2 to 0.6) (PBLZT) that exhibits switching of remanent polarization under illumination white light and monochromatic laser source with 405 nm wavelength.

    [0061] An aspect of the present invention provides a room temperature Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.2 to 0.8, and y=0.2 to 0.6) (PBLZT) opto-ferroelectric used as opto-electric logic states, which implies nonvolatile memory elements with an electric voltage for WRITE/READ operation and various optical signals for READ/WRITE operation and vice-versa and dual operation together.

    [0062] An aspect of the present invention provides opto-ferroelectric samples of Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.2 to 0.8, and y=0.2 to 0.6) (PBLZT) fabricated by a solid state reaction method (method 1) comprising steps of: appropriate analytical-purity oxides, mechanical mixing; calcinations of physical mixture at 800-850 C. for 12 h to obtain the perovskite phase; sintering the pellets in a temperature range of 1100-1200 C. for 2 to 4 h; and further pressure polishing to reduced the thickness near 500 m.

    [0063] An aspect of the present invention provides the transparent ITO coating on the top surface of opto-ferroelectric materials, the above said electro-ceramic pellets were top coated by transparent conducting Indium tin oxide (ITO) electrode using rf magnetron sputtering method and bottom by the conducting silver paints to further opto-electrical characterization; In several cases semi transparent silver paint was also used for opto-electrical measurement.

    [0064] Another aspect of the present invention provides a composition Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.2 to 0.8, and y=0.2 to 0.6) (PBLZT) that exhibits ON/OFF states of photocurrent under illumination of white light and monochromatic laser source with 405 nm wavelength. ON/OFF states of photocurrent were tested under small stress electric voltage (+/110 V and +/40 V @ 500 m thick pellets).

    [0065] Another aspect of the present invention provides an opto-ferroelectric material with in-built spontaneous polarization of 4-30 C/cm.sup.2 with and without illumination of light from near UV to visible region frequencies i.e different probe frequencies (0.1 Hz-10 Hz) under the electric field stress of +/80 KV/cm and with various strength of the illuminating light. FIG. 1 shows the P-E hysteresis of unpoled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) with and without illumination of light. A large change, in the range of 300-500% in switching of polarization under 40 mW 405 nm laser light and in the range of 100-250% in switching of polarization under white light was obtained depending of probe frequency, electrical stress, and energy of the light.

    [0066] The compositions PBLZT (x=0.5 and y=0.2) also shows 10-20% less polarization switching compared to Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2), other compositions are also contemplated.

    [0067] Accordingly, the present invention also provides a process for the preparation of opto-ferroelectric material, comprising process steps of: [0068] I. physical mixing of ingredient oxides of PbO (99.5%), ZrO.sub.2 (99.86%), TiO.sub.2 (99.96%), Bi.sub.2O.sub.3 (99.99%) and Li.sub.2CO.sub.3 (97%) (Alfa Aesar) in a ratio in the range of x=0.2 to 0.8, and y=0.2 to 0.6 with further mechanical mixing in liquid alcoholic (IPA) media in range of 5 to 10 ml with 99.9% purity, and mol fraction of electro-ceramics; [0069] II. calcining composites of step (i) at a temperature in the range of 800-850 C. for a period of 11-13 hours to get the desired phase followed by X-ray diffraction (XRD) study to verify the phase purity and crystallinity; [0070] III. sintering composites of step (ii) at a temperature in the range of 1100-1200 C., to obtain desired opto-ferroelectric material; and [0071] IV. sintered composites of step (iii) were further processed for mechanical grinding and polishing to get the desired thickness in the range of 400 to 600 microns of opto-ferroelectric material.

    [0072] In an embodiment of the present invention, opto-ferroelectric materials, wherein the optimized Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) and Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.5 and y=0.2) compositions have shown very high change in polarization under illumination of white light and 405 nm monochromatic light for opto-ferroelectric memory applications. [0073] i. as obtained electro-ceramic pellets in step (iv) were further characterized by XRD to check the crystal structure, and surface morphology and chemical compositions by scanning electron microscope (SEM); [0074] ii. as obtained electro-ceramic pellets in step (iv) were polished by mechanical pressure technique to make it thin slab of 500 m; [0075] iii. the above said electro-ceramic pellets were top coated by transparent conducting Indium tin oxide (ITO) electrode using RF magnetron sputtering and bottom by the conducting silver paints to further opto-electrical characterization; [0076] iv. the above said electro-ceramic pellets were kept under transformer oil to avoid electrical spark under high voltage opto-electrical testing; [0077] v. the above said electro-ceramic pellets were tested for optical characterization under tungsten filament bulb white source, 405 nm monochromatic laser light, and near 70% efficient Xenon lamp; [0078] vi. as designed and constructed electro-ceramic pellets in step (vii & Viii) were tested for switchable polarization, displacement current and photocurrent under ON and OFF states of near UV and visible light source; [0079] vii. as designed and constructed electro-ceramic pellets in step (vii & Viii) were tested for small stress voltage for (+/110 V and +/40 v with small step size to get the open circuit voltage (V.sub.OC) and short circuit current (I.sub.SC) to realize the photovoltaic characterization of above said embodiments.

    [0080] In yet another embodiment of the present invention, use of white and monochromatic light to switch the polarization states, and hence concerned logic states are provided.

    [0081] According to various embodiments of the present invention, opto-polarization memory, photovoltaic, optoelectronics are provided.

    [0082] In yet another embodiment of the present invention, white light and 405 nm wavelength laser source are selected form complete range of near UV and visible light to realize the multi nonvolatile logic states.

    [0083] In yet another embodiment of the present invention, the ON and OFF states of the photocurrent are selected from various light sources for near UV and visible light detector.

    [0084] In yet another embodiment of the present invention, electrical stress medium was selected from transformer oil or silicon oil or combination thereof.

    [0085] According to various embodiments of the present invention, high dielectric constant (<1000) and low tangent loss (<0.05) over a wide range of temperature (<150 C.) and frequency (50 Hz-1 MHz) are provided.

    [0086] In yet another embodiment of the present invention, photo-luminance of the Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 & x=0.5 and y=0.2) electro ceramic is high with good blue emission spectra.

    [0087] In yet another embodiment of the present invention, switching of polarization of Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 & and y=0.2) first time under 405 nm laser light is in the range 300-500% depending of probe frequency and electrical stress. The above said experiment was tested for several days and found reproducible after 24 hours;

    [0088] In yet another embodiment of the present invention, fast switching and repeatability of polarization greater than two minutes and less than 30 minutes under 405 nm laser light is in the range 20-50% depending of probe frequency and electrical stress.

    [0089] Another aspect of the present invention provides the tetragonal crystal structure with space group P4mn using XRD (see FIG. 2) and local crystal structure using Raman spectroscopy (FIG. 3) for the composition Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2).

    [0090] Another aspect of the present invention provides surface morphology of grains and grain boundaries distribution of (a) Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2), (b) Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.5 and y=0.2) compositions on the top surface/illuminating surface of the pellets, and composition within the experimental limits of SEM/EDAX (FIG. 4).

    [0091] FIG. 5 depicts the dielectric aspect of the present invention that provides temperature and frequency dependent dielectric constant and tangent loss behavior of (a & b) Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) (c & d) Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.5 and y=0.2) electro-ceramics in dark condition, respectively. It shows high dielectric constant (250 to 500) and low tangent loss (<0.05) near room temperature; however significant dielectric dispersion were observed in the large frequency span and at elevated temperature till 450 C. A kink in dielectric anomaly was observed from 300 C. to 390 C. due to ferroelectric phase transitions depending on A and B-site compositions.

    [0092] FIG. 6 (a-d) depicts the variation of polarization versus applied electric field (P-E) and displacement current without light (dark) and with monochromatic 405 nm light and white light for various probe frequencies (a) 1 Hz, (b) 2 Hz, (c) 5 Hz, (d) 10 Hz of unpoled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) electro-ceramics. Proper P-E loops can be obtained for bulk polycrystalline ceramics and single crystal for low probe frequencies. At low probe frequency, displacement current and switchable polarization (technique used: positive-up-negative-down (PUND)) was more prominent and noteworthy under the illumination of light. The displacement current used to READ and WRITE the logic states are more prominent under the illumination of light and vary frequently and fast to create more logic states.

    [0093] Another aspect of the present invention depicts the variation of polarization versus applied electric field (P-E) and displacement current without light (dark) and with monochromatic 405 nm light and white light for various probe frequencies (a) 1 Hz, (b) 2 Hz, (c) 5 Hz, (d) 10 Hz of poled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) electro-ceramics (FIG. 7 (a-d)). Proper P-E loops can be obtained for bulk polycrystalline ceramics and single crystal for low probe frequencies. The displacement current used to READ and WRITE the logic states are more prominent under the illumination of light and vary frequently and fast to create more logic states using different illuminating monochromatic light energy/wavelength.

    [0094] A bar diagram has shown in FIG. 8 which represents the effective switchable polarization of Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) over a period of time. Since the invention was carried out on bulk polycrystalline electro-ceramic samples, the big question was whether we would be able to switch the polarization back to normal. Similar magnitude of switchable dark polarization and under the illumination of white and laser light polarization was obtained after 24 hours within the experimental/human errors of the light focusing and energy distribution. In yet another embodiment of the present invention, switching of polarization first time under 405 nm laser light is in the range 300-500% depending of probe frequency and electrical stress. In yet another embodiment of the present invention, switching of polarization first time under white light is in the range 150-250% depending of probe frequency and electrical stressing (FIG. 7).

    [0095] In yet another embodiment of the present invention, these experiments were repeated to check the back and forth polarization switching and related displacement current under the illumination of light and within the speed of experimental process (2-3 minutes), it was found that around 20-50% polarization can switch back and forth depending on the probe frequencies and nature of light source (FIG. 8).

    [0096] Another aspect of the present invention provides polarization switch back and forth within the different intensities and energies of the illuminated light. The peaks of displacement current for different light energy and frequency for near UV and visible light are different and more prominent compared to the dark switchable polarization, and hence different logic states can be created with suitable use of light frequency and energy. Monochromatic light with various wavelengths from 400 nm to 450 nm will more suitable for logic states.

    [0097] Another aspect of the present invention provides current density versus electric field (J-E) loops (FIG. 9) of unpoled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) electro-ceramics. The induced photocurrents were one-two orders of magnitude higher than the dark current, however the same samples after 12 hours of electrical poling (80 KV/cm) shows effectively less photocurrents (inset shows J-E of poled ceramics). The magnitude of photocurrents was good enough to design and disseminate the photo-diode and photo-detector. For electrical measurements, both the electrodes were dissimilar, such as top transparent ITO and bottom silver paint, under the large stress electric field (+/80 KV/cm), almost similar dark and light current behavior were obtained. Initially, diode like behavior was observed for very low electric field stress (+/110 V) condition under the illumination of white light. The trends regain back to normal while switching back for photocurrent (FIG. 10).

    [0098] Another aspect of the present invention provides photocurrent and photo-voltage of unpoled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) electroceramics with small voltage step and small electric field stress. It shows short circuit current (I.sub.SC) in the range of 0.3 nA and an open circuit voltage of (V.sub.OC) in the range of 2 V, below than the bandgap of systems (inset of FIG. 10). The main advantage of invented V.sub.OC over the normal solar cell V.sub.OC is the presence of both second and forth quadrant photocurrent. It depicts that ferroelectric photovoltaic solar cell photo currents depend on the direction of switchable polarization as indicated in the inset of FIG. 10.

    [0099] Another aspect of the present invention provides the large photo response with and without illumination of white xenon lamp light (70mw). Nearly four times enhancement in the photocurrents were observed for unpoled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) electro-ceramics when light was switch ON with a bias voltage of +/110 V. It took nearly 3 to 4 seconds to reach the saturation of the photocurrent, however decay was fast compared to the growth of photocurrent as can be seen in the FIG. 11.

    [0100] Another aspect of the present invention provides a unpoled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) electro-ceramics with switchable spontaneous polarization fatigue almost 15-25 percent (a) 1 Hz, & (b) 2 Hz, probe frequency, under the combined electric field stress of +/80 KV/cm in dark condition (FIG. 12). Both the polarity shows similar trends of fatigue under dark current. Another aspect of the present invention provides the said material with switchable spontaneous polarization improved under the combined electric field stress of +/80 KV/cm and illumination of white light.

    [0101] In still another embodiment of the invention, the opto-ferroelectric memory element is characterized by 10-25% fatigue under high stress electric field (80 KV/cm) in the absence of light, and improved fatigue under high stress electric field (80 KV/cm) under illuminated of light less than 10-100 mW/cm.sup.2.

    [0102] Another aspect of the present invention provides a poled Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(T.sub.1-yZr.sub.y)O.sub.3 (x=0.4 and y=0.2) electro-ceramics with switchable spontaneous polarization fatigue almost 15-25 percent under the combined electric field stress of +/80 KV/cm in dark condition (FIG. 13). Both the polarity shows similar trends of fatigue under dark current. Another aspect of the present invention provides the said material with switchable spontaneous polarization improved under the combined electric field stress of +/80 KV/cm and illumination of white light. Both unpoled and poled electro-ceramic show similar fatigue behavior within the said polarization magnitude.

    [0103] Another aspect of the present invention provides switchable polarization probed with PUND analysis (a) pulse wave configuration, (b) unpoled ceramics at 100 ms time, (c) unpoled ceramics at 10 ms (d) poled ceramics at 100 ms (e) poled ceramics at 10 ms time with and without white light. The magnitude of PUND data was higher for slow pulse frequency (FIG. 14).

    EXAMPLES

    [0104] The following examples are given by way of illustration of the working of invention in actual practice and should not be construed to limit the scope of the present invention in any way.

    [0105] The developed materials may be fabricated as a simple one transistor one capacitor (1T1C) nonvolatile random access memory (NVRAM) ferroelectric cell which may write data bit using external electrical voltage and read the data bit optically using laser light. External electric field will be employed to write the data bit and later optically read out even in the absence of power supply. Different magnitude of laser power and associated polarization may be utilized for different logic states of memory elements, photo-detector, Radio-frequency identification (RF ID) readers, and ferroelectric photovoltaic cells. The developed materials may be useful as ferroelectric photovoltaic devices which can generate power under illumination of sunlight.

    Example-1

    [0106] Preparation of Opto-Ferroelectric Material by Solid Route

    [0107] Opto-ferroelectric Pb.sub.1-x(Bi.sub.0.5Li.sub.0.5).sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (x=0.2 to 0.8, and y=0.2 to 0.6) (PBLZT) electro-ceramic samples were prepared by a solid state route. Analytical-purity oxides, lead oxide (PbO) (99.5%), zirconium oxide (ZrO.sub.2) (99.86%), Titanium oxide (TiO.sub.2) (99.96%), Bismuth oxide (Bi.sub.2O.sub.3) (99.99%) and lithium carbonate (Li.sub.2CO.sub.3) (97%) (Alfa Aesar) were used as raw materials. The powder of the above mentioned metal oxides were mechanically mixed at least two hours in the liquid alcoholic medium such as isopropyl alcohol (IPA) and ethanol, and then it was calcined at 800-850 C. for 12 h in a closed alumina crucible. 10% excess of PbO and 2% Bi.sub.2O.sub.3 and 2% Li.sub.2CO.sub.3 was added to each composition to compensate Pb, Bi, and Li deficiency during the high temperature processing. Poly (vinyl alcohol) solutions (1%) were added to the calcined powders as a binder. The dried powders were first filtered using a 150 m-mesh sieve and then pressed using a hydrostatic press 6-8 Ton into pellets of 10 mm diameter. The pressed pellets were sintering at 1100-1200 C. for 2 h. All heat treatments were performed in air to prevent the PbO loss during high temperature sintering and to maintain desired stoichiometric. An equilibrium PbO vapor pressure was established by fully covered alumina crucible with PbZrO.sub.3 as medium on the top of alumina power where these pellets were kept for sintering.

    Example-2

    [0108] Preparation of PBLZT Electro-Ceramic Samples by a Solution-Gel Technique

    [0109] PBLZT electro-ceramic samples can be prepared by a solution-gel technique. In this process, the solution of lead acetate, titanium isopropoxide, zirconium isopropoxide, bismuth acetate, and lithium acetate with mol percentage mentioned in the formula is prepared in the solvent 2 methoxy ethanol, and acetic acid, later for gel formation, a small amount of ethylene glycol may be added in the solution. The stability of solution can be checked to keep the solution for long time at least 30 days. The stable solution is used to prepare the thin films using spin coating technique with growth conditions such as: solution molarity 0.2-0.3, spin coating speed in the range of 3000-5000 rpm, pyrolysis at 300 C. for 2 minutes to remove the organics and sintering at 600-700 C. for phase formation. Final films may be used directly for device fabrication process.

    Example 3

    [0110] Preparation of PBLZT Film by Pulsed Laser Deposition

    [0111] The pulsed laser deposition (PLD) technique can also be used to prepare the PBLZT thin films. In this process, the one inch target of the developed material (PBLZT) is prepared by the solid-state reaction technique using the similar conditions as mentioned in the process method. The prepared target is used in PLD to prepare the thin films. The growth conditions may as follows: substrate temperature 600-750 C., oxygen partial pressure 100-200 mbar, Laser energy density 1-3 J/cm2, and target to substrate distance in the range of 4-5 cm. In this process high quality epitaxial thin film may be prepared which may directly integrate with the silicon technology for device fabrication.

    [0112] The thin films of the aforementioned materials can be prepared by various other thin film growth techniques such as rf-magnetron sputtering, chemical vapor deposition process, metalorganic chemical vapour deposition (MOCVD), atomic layer deposition etc., respectively. Optimized parameters and growth conditions depend on the ambient conditions and fabrication processes.

    Example-4

    [0113] Preparation of Memory Material Using PBLZT Opto-Ferroelectric Materials

    [0114] The above said PBLZT opto-ferroelectric materials in their thin film form with 1T1C memory cell configuration may be used for practical ferroelectric random access memory (FeRAM) device applications with extra degree of freedom such as optically tunable logic states. It is expected to have 128 Mbit memory cell using the developed material with high logic states density. One can fabricate high quality PBLZT thin films which can be directly integrated to the silicon technology for logic and memory devices. The thick PBLZT films with suitable top (semi transparent Au or ITO) and bottom electrodes may be useful for bulk ferroelectric photovoltaic devices for power generation.

    [0115] The aforementioned material shows high ON and OFF light assisted switchable current which indicates its suitability as photo-detector near UV light (300 to 400 nm). FeRAM based Radio-frequency identification (RFID) is a useful for data carrier applications, factory-automation, maintenance, asset-management, and logistic-tracking applications. This system is also resistant to gamma-ray sterilization which makes it useful in the medical, pharmaceutical, biomedical, foods, and cosmetic industries applications. The high endurance greater than 10.sup.10 write and read process make it suitable for memory elements. It is also useful for high frequency (HF 13.56 MHz) and ultra high frequency (UHF 860 MHz-960 MHz) applications.

    [0116] The thin films of the aforementioned materials can be prepared by various other thin film growth techniques such as rf-magnetron sputtering, chemical vapor deposition process, metalorganic chemical vapour deposition (MOCVD), atomic layer deposition etc., respectively. Optimized parameters and growth conditions depend on the ambient conditions and fabrication processes.

    ADVANTAGES OF THE PRESENT INVENTION

    [0117] The main advantages of the present invention are: [0118] 1) The present invention provides optical switch and opto-electric memory devices which are in high demand for the next generation NVRAM (nonvolatile random access memory) applications. [0119] 2) The present invention provides electrically write and optically read FeRAM with exceptionally high density memory bits and less power consumption, low leakage current and less heat dissipation. [0120] 3) The present invention provides photo-ferroelectric materials showing exceptionally high opto-ferroelectric effect. [0121] 4) In the present invention, multi-states memory logic is created with suitable use of monochromatic laser wavelength for writing and reading the logic states. [0122] 5) It shows characteristic features with V.sub.OC2 V and I.sub.SC0.3 nA. [0123] 6) In the present invention, use of weak white light or monochromatic light or near UV light induce large amount of photocurrent and the said effect can be utilized for design and fabrication of photo-diode and photodetector. [0124] 7) In the present invention, the ratio of ON/OFF states of dark and light current possesses significant magnitude to design and develop the multi-states photocurrent based microelectronic devices. [0125] 8) In the present invention, photo-ferroelectric materials show significantly low fatigue in dark light and improved polarization under illumination satisfying the basic requirement of memory devices.