Single-crystal perovskite solid solutions with indifferent points for epitaxial growth of single crystals
10844516 ยท 2020-11-24
Assignee
Inventors
Cpc classification
C30B15/36
CHEMISTRY; METALLURGY
C30B29/32
CHEMISTRY; METALLURGY
C30B11/14
CHEMISTRY; METALLURGY
C30B15/00
CHEMISTRY; METALLURGY
C30B25/00
CHEMISTRY; METALLURGY
C30B23/00
CHEMISTRY; METALLURGY
International classification
C30B11/14
CHEMISTRY; METALLURGY
C30B15/36
CHEMISTRY; METALLURGY
C30B23/00
CHEMISTRY; METALLURGY
C30B15/00
CHEMISTRY; METALLURGY
C30B11/00
CHEMISTRY; METALLURGY
C30B29/32
CHEMISTRY; METALLURGY
Abstract
Growth of single crystal epitaxial films of the perovskite crystal structure by liquid- or vapor-phase means can be accomplished by providing single-crystal perovskite substrate materials of improved lattice parameter match in the lattice parameter range of interest. Current substrates do not provide as good a lattice match, have inferior properties, or are of limited size and availability because cost of materials and difficulty of growth. This problem is solved by the single-crystal perovskite solid solutions described herein grown from mixtures with an indifferent melting point that occurs at a congruently melting composition at a temperature minimum in the melting curve in the pseudo-binary molar phase diagram. Accordingly, single-crystal perovskite solid solutions, structures, and devices including single-crystal perovskite solid solutions, and methods of making single-crystal perovskite solid solutions are described herein.
Claims
1. A single-crystal perovskite comprising a solid solution between two perovskite compounds having an indifferent melting point that occurs at a temperature minimum in a melting curve in a pseudo-binary phase diagram, wherein a perovskite tolerance factor, T, of the single-crystal perovskite is in a range of about 0.988 to about 1.006.
2. The single-crystal perovskite of claim 1, wherein the single-crystal perovskite has a cubic crystal structure at about 273 K.
3. The single-crystal perovskite of claim 1, wherein the solid solution between two perovskite compounds is selected from the group consisting of: a solid solution of barium titanate-sodium tantalate with an approximate molar chemical formula xBaTiO.sub.3-(1x)NaTaO.sub.3, wherein x is in a range of about 0.35 to about 0.75; a solid solution of barium titanate-sodium lanthanum titanate with an approximate molar chemical formula xBaTiO.sub.3-(1x)Na.sub.0.5La.sub.0.5TiO.sub.3, wherein x is in a range of about 0.15 to about 0.55; and a solid solution of sodium niobate-barium lithium niobate with an approximate molar chemical formula xNaNbO.sub.3-(1x)BaLi.sub.0.25Nb.sub.0.75O.sub.3, wherein x is in a range of about 0.21 to about 0.61.
4. The single-crystal perovskite of claim 3, wherein x is in a range of about 0.45 to about 0.65 in the solid solution of barium titanate-sodium tantalate; x is in a range of about 0.25 to about 0.45 in the solid solution of barium titanate-sodium lanthanum titanate; and x is in a range of about 0.31 to about 0.51 in the solid solution of sodium niobate-barium lithium niobate.
5. The single-crystal perovskite of claim 1, wherein the single-crystal perovskite comprises a solid solution between barium titanate and sodium niobate with an approximate molar chemical formula xBaTiO.sub.3-(1x)NaNbO.sub.3, wherein x is in a range of about 0.22 to about 0.42.
6. The single-crystal perovskite of claim 5, wherein x is in a range of about 0.27 to about 0.37.
7. The single-crystal perovskite of claim 1, wherein the single-crystal perovskite comprises a solid solution between an alkaline earth (AE) metal nickel niobate perovskite with an approximate molar chemical formula AENi.sub.1/3Nb.sub.2/3O.sub.3 with another perovskite, and wherein AE is selected from the group consisting of barium, strontium, and calcium.
8. The single-crystal perovskite of claim 7, wherein the single-crystal perovskite comprises a solid solution between barium nickel niobate and strontium nickel niobate with an approximate molar chemical formula xBaNi.sub.1/3Nb.sub.2/3O.sub.3-(1x)SrNi.sub.1/3Nb.sub.2/3O.sub.3, wherein x is in a range of about 0.2 to about 0.9.
9. The single-crystal perovskite of claim 7, wherein the single-crystal perovskite comprises a solid solution between barium nickel niobate and sodium niobate with an approximate molar chemical formula xBaNi.sub.1/3Nb.sub.2/3O.sub.3-(1x)NaNbO.sub.3, wherein x is in a range of about 0.28 to about 0.68.
10. The single-crystal perovskite of claim 1, wherein a composition of end member perovskites is within about 1 atomic percent of nominal integer values of a primitive formula unit.
11. The single-crystal perovskite of claim 1, wherein the single-crystal perovskite includes anti-site ions.
12. A structure comprising: a single-crystal perovskite comprising a solid solution between two perovskite compounds having an indifferent melting point that occurs at a temperature minimum in a melting curve in a pseudo-binary phase diagram, wherein the perovskite tolerance factor, T, of the single-crystal perovskite is in a range of about 0.98 to about 1.02; and an epitaxial single crystal epitaxially disposed on the single perovskite crystal.
13. The structure of claim 12, wherein the single-crystal perovskite excludes: a solid solution of barium titanate-calcium titanate with an approximate molar chemical formula xBaTiO.sub.3-(1x)CaTiO.sub.3, wherein x is in a range between 0 and 1; a solid solution of lanthanum aluminate-strontium aluminum tantalate with an approximate molar chemical formula xLaAlO.sub.3-(1x)SrAl.sub.0.5Ta.sub.0.5O.sub.3, wherein x is in a range between 0 and 1; and a solid solution of barium titanate-sodium niobate with an approximate molar chemical formula xBaTiO.sub.3-(1x)NaNbO.sub.3, wherein x is in a range between 0.45 and 0.65 and a perovskite tolerance factor, T, is in a range between 1.009 and 1.028.
14. The structure of claim 12, wherein the solid solution between two perovskite compounds is selected from the group consisting of: a solid solution of barium titanate-sodium tantalate with an approximate molar chemical formula xBaTiO.sub.3-(1x)NaTaO.sub.3, wherein x is in a range of about 0.35 to about 0.75; a solid solution of barium titanate-sodium lanthanum titanate with an approximate molar chemical formula xBaTiO.sub.3-(1x)Na.sub.0.5La.sub.0.5TiO.sub.3, wherein x is in a range of about 0.15 to about 0.55; a solid solution of strontium titanate-sodium tantalate with an approximate molar chemical formula xSrTiO.sub.3-(1x)NaTaO.sub.3, wherein x is in a range of about 0.2 to about 0.6; a solid solution of strontium titanate-sodium lanthanum titanate with an approximate molar chemical formula xSrTiO.sub.3-(1x)Na.sub.0.5La.sub.0.5TiO.sub.3, wherein x is in a range of about 0.05 to about 0.45; and a solid solution of sodium niobate-barium lithium niobate with an approximate molar chemical formula xNaNbO.sub.3-(1x)BaLi.sub.0.25Nb.sub.0.75O.sub.3, wherein x is in a range of about 0.21 to about 0.61.
15. The structure of claim 12, wherein the epitaxial single crystal is not congruently melting.
16. The structure of claim 12, wherein the epitaxial single crystal has a perovskite crystal structure.
17. The structure of claim 16, wherein the epitaxial single crystal having a perovskite crystal structure comprises a perovskite material selected from the group consisting of PbTiO.sub.3, PbZrO.sub.3, PbZr.sub.xTi.sub.1xO.sub.3, Pb(Mg.sub.1/3Nb.sub.2/3)O.sub.03, SrRuO.sub.3, Bi.sub.0.5Na.sub.0.5TiO.sub.3, BiFeO.sub.3, BiMnO.sub.3, BiCrO.sub.3, KNbO.sub.3, K.sub.xNa.sub.(1x)NbO.sub.3, KTa.sub.xNb.sub.(1x)O.sub.3, yttrium barium cuprate, a solid solution between (1x)Pb(Mg.sub.1/3Nb.sub.2/3)O.sub.3-xPbTi.sub.xO.sub.3, and a solid solution between xBiScO.sub.3-(1x)PbTiO.sub.3.
18. A device comprising an epitaxial single crystal according to claim 12.
19. The device of claim 18, wherein the device is selected from the group consisting of a piezoelectric transducer, a piezoelectric sensor, an electrooptic waveguide device, a magnetooptic waveguide device, an infrared detector, a ferroelectric random-access memory device, a solar cell, a photodetector, a disk read and write head, a biosensor, a microelectromechanical system, a nanoelectromechanical system, a piezoelectric field effect transistor, a piezoelectric field effect photodetector, a magneto-electronic device, a tunnel magnetoresistance sensor, a spin valve, an electrically tunable microwave filter, an electrically tunable microwave oscillator, and an electrically tunable microwave phase shifter.
20. A method of making a single-crystal perovskite comprising: drying single-crystal perovskite reagents to remove moisture and adsorbed gases from the single-crystal perovskite reagents; combining the single-crystal perovskite reagents to provide a mixture; compacting the mixture; melting the mixture to provide a liquid solution between two perovskite compounds having an indifferent melting point; and generating a temperature gradient within the mixture configured to nucleate and grow the perovskite single crystal, wherein the single-crystal perovskite comprises a solid solution between two perovskite compounds having an indifferent melting point that occurs at a temperature minimum in a melting curve in a pseudo-binary phase diagram, wherein a perovskite tolerance factor, T, of the single-crystal perovskite is in a range of about 0.988 to about 1.006.
Description
DESCRIPTION OF THE DRAWINGS
(1) The foregoing aspects and many of the attendant advantages of claimed subject matter will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:
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DETAILED DESCRIPTION
(16) The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the disclosed subject matter and is not intended to represent the only embodiments. Each embodiment described in this disclosure is provided merely as an example or illustration and should not be construed as preferred or advantageous over other embodiments. The illustrative examples provided herein are not intended to be exhaustive or to limit the claimed subject matter to the precise forms disclosed.
(17) The present disclosure relates generally to single-crystal perovskite solid solutions, structures and devices including single-crystal perovskite solid solutions, and methods of making single-crystal perovskite solid solutions. As described further herein, epitaxial growth of single crystals frequently requires a substrate having a suitable structural and lattice parameter match to the single crystal. Prior to the present disclosure, there was a dearth of substrates for epitaxial growth of perovskite crystals having a primitive lattice parameter between about 0.387 nm and about 0.412 nm.
(18) In an embodiment, the present disclosure specifically addresses the need for substrates with the perovskite crystal structure in the range of simple ABO.sub.3 perovskite lattice parameters a.sup.o=0.387-0.407 nm or multiples thereof by an integer or, in the case of a rotated unit cell, of {square root over (2)}, {square root over (3)} or {square root over (6)}.
(19) In the following description, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments of the present disclosure. It will be apparent to one skilled in the art, however, that many embodiments of the present disclosure may be practiced without some or all of the specific details. In some instances, well-known process steps have not been described in detail in order not to unnecessarily obscure various aspects of the present disclosure. Further, it will be appreciated that embodiments of the present disclosure may employ any combination of features described herein.
(20) Single-Crystal Perovskites
(21) In one aspect, the present disclosure provides a single-crystal perovskite comprising a solid solution between two perovskite compounds having an indifferent melting point that occurs at a temperature minimum in a melting curve in a pseudo-binary phase diagram. A melting curve on a phase diagram is a compilation of the liquidus (point of last solid melting on heating and first solid formation on cooling) and solidus (point of first liquid formation on heating and last liquid solidification on cooling). If these two curves come together at a temperature minimum or maximum, this constitutes an indifferent point and represents a congruent composition. Other features may occur at lower or higher temperatures such as phase changes and solid or liquid phase separation. These are not relevant to determination of an indifferent point. A phase diagram is binary when there are only two constituents and only one compositional variable is possible. When there are more constituents, more complex constituents or more variables, such as growth atmosphere, a pseudo-binary phase diagram may be selected where all variable but one are kept constant and only the change in that variable is plotted. Complex perovskite solid solutions are generally plotted on such pseudo-ternary phase diagrams, because other variables such as pressure and oxygen stoichiometry are effectively constant. Data points on such a phase diagram may be measured by visual observation, differential scanning calorimetry or other thermodynamic metrics. However, it is not necessary to generate an entire phase diagram to demonstrate congruency. The ability to generate a solid of constant or approximately constant composition throughout the process of solidification is sufficient and may be further reinforced by a local temperature minimum or maximum.
(22) In an embodiment, the single-crystal perovskite excludes a solid solution of barium titanate-calcium titanate with an approximate molar chemical formula xBaTiO.sub.3-(1x)CaTiO.sub.3, wherein x is in a range between 0 and 1; a solid solution of lanthanum aluminate-strontium aluminum tantalate with an approximate molar chemical formula xLaA1O.sub.3-(1x)SrAl.sub.0.5Ta.sub.0.5O.sub.3, wherein x is in a range between 0 and 1; and a solid solution of barium titanate-sodium niobate with an approximate molar chemical formula xBaTiO.sub.3-(1x)NaNbO.sub.3, wherein x is in a range between 0.45 and 0.65 and a perovskite tolerance factor, T, is in a range between 1.009 and 1.028.
(23) In an embodiment, the solid solution has a cubic perovskite crystal structure. Typically, this results from a perovskite tolerance factor near 1.00. Accordingly, in an embodiment, a perovskite tolerance factor, T, of the single-crystal perovskite as calculated from Equation (1) using tabulated crystal radii is between about 0.98 and about 1.02. This can contribute to a thermodynamic energy minimum and it was found that this can shift the melting point depression curves of the end members' solid solutions and, therefore, the position of the minimum. Uniformly, the tolerance factor of the indifferent point of the single-crystal perovskite crystals described herein was, at worst, the same, and mostly closer to unity than the predicted value.
(24) In an embodiment, one or more of the end member compounds of the solid solutions described herein are not cubic, but blending compounds of higher and lower tolerance factors achieves a solid solution with T close to unity in the mixture that favors a cubic structure of the solid solution at the congruent composition. In an embodiment, a composition of end member perovskites is within about 1 atomic percent of nominal integer values of a primitive formula unit.
(25) As discussed further herein, it was found that the free energy contribution of the tolerance factor tended to make the indifferent point occur closer to a tolerance factor of unity. In an embodiment, the single-crystal perovskite is of cubic crystal structure at room temperature. In an embodiment, the single-crystal perovskite is of cubic crystal structure between at least about 273 K and about 298 K.
(26) In an embodiment, the single-crystal perovskite comprises a solid solution of barium titanate-sodium tantalate with an approximate molar chemical formula xBaTiO.sub.3-(1x)NaTaO.sub.3, wherein x is in a range between about 0.35 and about 0.75. In an embodiment, x is in a range between about 0.45 and about 0.65 in the solid solution of barium titanate-sodium tantalite.
(27) In an embodiment, the single-crystal perovskite comprises a solid solution of barium titanate-sodium lanthanum titanate with an approximate molar chemical formula xBaTiO.sub.3-(1x)Na.sub.0.5La.sub.0.5TiO.sub.3, wherein x is in a range between about 0.15 and about 0.55. In an embodiment, x is in a range between about 0.25 and about 0.45 in the solid solution of barium titanate-sodium lanthanum titanate.
(28) In an embodiment, the single-crystal perovskite comprises a solid solution of strontium titanate-sodium tantalate with an approximate molar chemical formula xSrTiO.sub.3-(1x)NaTaO.sub.3, wherein x is in a range between about 0.2 and about 0.6. In an embodiment, x is in a range between about 0.3 and about 0.5 in the solid solution of strontium titanate-sodium tantalate.
(29) In an embodiment, the single-crystal perovskite comprises a solid solution of strontium titanate-sodium lanthanum titanate with an approximate molar chemical formula xStTiO.sub.3-(1x)Na.sub.0.5La.sub.0.5TiO.sub.3, wherein x is in a range between about 0.05 and about 0.45. In an embodiment, x is in a range between about 0.15 and about 0.35 in the solid solution of strontium titanate-sodium lanthanum titanate.
(30) In an embodiment, the single-crystal perovskite comprises a solid solution of sodium niobate-barium lithium niobate with an approximate molar chemical formula xNaNbO.sub.3-(1x)BaLi.sub.0.25Nb.sub.0.75O.sub.3, wherein x is in a range between about 0.21 and about 0.61. In an embodiment, x is in a range between about 0.31 and about 0.51 in the solid solution of sodium niobate-barium lithium niobate.
(31) In an embodiment, the single-crystal perovskite comprises a solid solution between barium titanate and sodium niobate with an approximate molar chemical formula xBaTiO.sub.3-(1x)NaNbO.sub.3. In an embodiment, x is in the range between about 0.22 and about 0.42. In an embodiment, x is in the range between about 0.27 and about 0.37. In an embodiment, a perovskite tolerance factor, T, is in the range between about 0.988 and about 1.006.
(32) In an embodiment, one or both of the two perovskite compounds of the solid solution are stable end member compounds. In an embodiment, one or both of the two perovskite compounds of the solid solution are congruently melting.
(33) In an embodiment, the components of the two perovskite compounds have compatible crystal ionic sizes. For example, aluminum on the B-site is not compatible with barium on the A-site, as Al is too small for the octahedral site in a Ba-based perovskite. On the other hand, lithium is too large for the octahedral site in a Sr-based perovskite. Accordingly, in an embodiment, the two perovskite compounds have one or more common ions. In an embodiment, the two perovskite compounds have similarly-sized A-site ions. In an embodiment, the two perovskite compounds have similarly-sized sized B-site ions.
(34) In an embodiment, the components of the two perovskite compounds have low to moderate vapor pressure. In this regard, a liquid solution including the two perovskite compounds is stabilized at elevated temperatures, such as during making the single perovskites described herein. In an embodiment, the components of the two perovskite compounds lose no more than 10% of their total initial weight during melting, equilibration, and crystal growth at the congruently melting minimum temperature. Melting and equilibration generally occur no more than 100 C. above the crystal growth temperature. In practice, the vapor pressures of species, such as sodium and potassium, can be compensated for by adding an extra amount of these components above a minimum amount required for a congruently melting composition, adjusting the oxygen concentration of the atmosphere, or using a closed vessel. In an embodiment, the vapor pressure tin and indium are too high to be used. In an embodiment, nickel oxide may be used without excessive evaporation for compounds that melt below 1600 C. and can be grown from a platinum crucible in air.
(35) In an embodiment, the two perovskite compounds are not excessively refractory. Zirconates, for example, are well known perovskites, but have such high melting points that they can be difficult to grow in bulk by practical means. Scandium- and magnesium-containing compounds can be, likewise, difficult to grow using bulk methods. Reasons for this are multi-fold. 1) An indifferent point cannot be achieved if the two components have too high a difference in melting temperatures. 2) Constituents such as Ti, Nb, and Ta can be reduced at high temperatures, so a melting temperature is desired <1800 C. and preferentially <1600 C. where a platinum crucible and air or other oxidizing atmosphere can be used. 3) Higher temperatures increase the vapor pressure of moderately volatile constituents.
(36) In an embodiment, the lattice parameter of the perovskite solid solution is between about 0.385 and about 0.412 nm. In an embodiment, the lattice parameter of the perovskite solid solution is between about 0.390 nm and about 0.41 nm. In this regard, the single-crystal perovskites described herein meet the need for commercially-available substrates for epitaxial growth.
(37) In an embodiment, the two perovskite compounds have limited concentrations of ions observed to result in distortion or phase separation. In an embodiment, the two perovskite compounds have limited concentrations of Al in Ba-containing compounds, due to phase separation. In an embodiment, the two perovskite compounds have limited concentrations of Ga, due to phase separation. In an embodiment, the two perovskite compounds have limited concentrations of Li in Sr-containing compounds, due to non-cubic distortion.
(38) In an embodiment, the ions of the two perovskite compounds do not include the same valence on a single site. While BT-CT is a successful solid solution with an indifferent point, having ions of the same valence on a single site was generally not a good strategy for more complex solutions, particularly if it involved same-valence cations on both the A and B sites. Only one such same valence solid solution between Ba and Sr was found to have a congruent composition.
(39) In an embodiment, the solid solution has a melting temperature less than about 1600 C. This permits growth from, for example, a crucible comprising platinum, a platinum alloy, a platinum composite, or a platinum alloy composite and, therefore, growth in air, oxygen, or other oxidizing atmosphere.
(40) In an embodiment, the single-crystal perovskite has a diameter of between about 10 mm and about 100 mm. In an embodiment, the single-crystal perovskite has a diameter of between about 10 mm and about 50 mm. In an embodiment, the single-crystal perovskite has a diameter of between about 10 mm and about 25 mm.
(41) In an embodiment, the single-crystal perovskite has a length of between about 10 mm and about 100 mm. In an embodiment, the single-crystal perovskite has a length of between about 10 mm and about 50 mm. In an embodiment, the single-crystal perovskite has a length of between about 10 mm and about 20 mm. single-crystal perovskite
(42) In the foregoing, the term approximately is as defined previously. It will be understood by those knowledgeable in the art that, although integer and integer fraction values are given for the various components, in real materials there is some entropic disorder including vacancies (mainly oxygen) and anti-site ions that can vary these stoichiometry values by up to 10 atomic percent of the five atoms in the primitive formula unit in some perovskites, but typically only up to 1 atomic percent in the perovskites described herein. Therefore, in another embodiment, the composition of the liquid solution and the solid solution of these single-crystal perovskites can vary up to 1 atomic percent for each constituent from the claimed composition of all ions in the primitive formula unit.
(43) TABLE I contains a list of certain compounds tested or that are otherwise discussed herein. In an embodiment, the two perovskite compounds of the solid solutions described herein are chosen from the compounds listed in TABLE I.
(44) TABLE-US-00001 TABLE I a b c a.sub.pc a.sub.hs t.sub.m Abb Formula CS (nm) (nm) (nm) (nm) (nm) T ( C.) Cong NN NaNbO.sub.3 o 0.5513 0.5571 0.7766 0.3907 0.4013 0.967 1425 co NT NaTaO.sub.3 o 0.5481 0.5524 0.7795 0.3893 0.4013 0.967 1810 co KN KNbO.sub.3 o 0.5695 0.5721 0.3974 0.4015 0.4190 1.054 1071 p KT KTaO.sub.3 cu 0.3988 0.3988 0.4190 1.054 1368 p CT CaTiO.sub.3 o 0.5404 0.5422 0.7651 0.3827 0.3942 0.966 1975 co ST SrTiO.sub.3 cu 0.3903 0.3903 0.4013 1.002 2060 co BT BaTiO.sub.3 tet 0.4000 0.4024 0.4008 0.4133 1.062 1625 co LA LaAlO.sub.3 r 0.5359 0.3789 0.3887 1.009 2110 co LG LaGaO.sub.3 o 0.5491 0.5523 0.7773 0.3892 0.3972 0.966 1698 co SAN SrAl.sub.0.5Nb.sub.0.5O.sub.3 cu 0.7795 0.3898 0.3996 1.010 1790 co SAT SrAl.sub.0.5Ta.sub.0.5O.sub.3 cu 0.7795 0.3898 0.3996 1.010 1980 co SGT SrGa.sub.0.5Ta.sub.0.5O.sub.3 cu 0.7898 0.3949 0.4038 0.989 1820 co BGN BaGa.sub.0.5Nb.sub.0.5O.sub.3 pc 0.4038 0.4038 0.4158 1.048 1500 sl BGT BaGa.sub.0.5Ta.sub.0.5O.sub.3 pc 0.4038 0.4038 0.4158 1.048 1690 sl NLT Na.sub.0.5La.sub.0.5TiO.sub.3 o 0.5479 0.5487 0.7747 0.3876 0.3967 0.979 1800* u KLT K.sub.0.5La.sub.0.5TiO.sub.3 pc 0.3914 0.3914 0.4056 1.023 1360 n SLT SrLi.sub.0.25Ta.sub.0.75O.sub.3 r 0.9811 0.9811 1.1206 0.4005 0.4078 0.970 1905 co BLT BaLi.sub..25Ta.sub..75O.sub.3 h 0.5802 0.5802 1.9085 0.4112 0.4198 1.028 1620* u BLN BaLi.sub..25Nb.sub..75O.sub.3 h 0.5803 0.5803 1.9076 0.4112 0.4198 1.028 1350* u certain targeted end member compounds and their properties, including: abbreviation used herein (Abb); chemical formula; crystal structure (CS: cu = cubic, tet = tetragonal, o = orthorhombic, r = rhombohedral, h = hexagonal and pc = pseudo-cubic where the exact crystal structure is undetermined); lattice parameters, a, b and c; pseudo-cubic lattice parameter, apc; calculated hard sphere lattice parameter, ahs (not adjusted for tolerance factor T or ionic size); tolerance factor, T; melting temperature, tm; and congruency (Cong: co = congruent, p = peritectic, sl = slightly incongruent, n = not congruent, and u = unknown). Starred melting temperatures are estimated.
(45) In an embodiment, the two perovskite compounds have similar melting temperatures, with a difference in melting temperatures of no greater than about 300 C. For components with the same melting temperature, a first approximation is that either an indifferent point or a eutectic will be near the equimolar point in the center of the phase diagram. As the melting temperature difference increases, there is a tendency for the temperature minimum to move closer to the lower melting constituent. At a large enough difference, the minimum reaches the melting point of the end member constituent or is so close as not to have a significantly different set of properties from the end member. Empirically this occurs for differences in end member melting temperatures >300 C.
(46) A discussion of how the combination of the heat of mixing and the difference in the end member melting temperatures influences the melting behavior will help to explain the last point. Phase diagram construction from free energy curves for a simple two component system C-D is shown in
G.sub.L=tS.sub.m(7)
(47) The free energy of the solid G.sub.s is given by the linear combination of the temperature dependent free energies of the end members (1x)G.sub.C+xG.sub.D plus the free energy of mixing G.sub.m=H.sub.mtS.sub.m.
G.sub.S=(1x)G.sub.C+xG.sub.D+H.sub.mtS.sub.m(8)
(48) This solid free energy curve shifts up and down as temperature is varied. The form of the enthalpy of mixing H.sub.m is assumed to be proportional to x(1x) as in Equation (4). The form of the entropy of mixing S.sub.m is assumed to be proportional to xln(x)+(1x)ln(1x) as in Equation (5). The difference among the forms of these functions and their sum and difference account for a variety of phase diagram phenomena.
(49) In
(50) In
(51) In
(52) In
(53) Therefore, an initial estimate of the melting point t.sub.m(est) and indifferent composition x.sub.est was made based on the melting temperatures of the two components with initial assumptions that 1) the melting point depressions of both compounds were the same and given a value from the average of literature values and 2) the temperature difference wherein and melting minimum would reach the lower melting temperature end member is 400 C. Measured approximate values of the indifferent composition x.sub.m and melting temperature t.sub.m(meas) (where determined) differ from these somewhat with x.sub.m in particular being closer to the value where T=1. In the one case where there is literature data for the indifferent composition x.sub.l and melting temperature t.sub.m(lit), those data are recorded as well, but they differ significantly from the current determination. It was considered that any melting temperature difference greater than 300 C. would not give a congruent composition far enough away from lowest melting end member to be worth pursuing.
(54) Those knowledgeable in the art will understand that there is a much higher level of complexity in a xABO.sub.3-(1x)ABO.sub.3 solid solution than in the simple C-D example. The entropy of mixing of the liquid will be inherently higher than the solid because of the site constraints of all three types of ions in the solid and the possibilities of oxide dissociation and multiple types of cation species with various oxygen coordinations. This changes its functional form and curve shape. The enthalpy of mixing of the solid will depend on nearest and next-nearest neighbors among the A- and B-sites. The tolerance factor T will contribute a free energy that is at a minimum for T=1.00. This will reinforce the entropy of mixing in the solid solution and can counteract an excess of positive enthalpy of mixing so as to eliminate phase separation. However, it will have a different functional form than any of the other free energies and is not completely understood. As will be discussed, the free energy contribution of the perovskite tolerance factor is seen to influence the melting temperature minimum to be closer to or equal to the composition where T=1.00. Edge and volume constraints can result in internal stress within the unit cell that provides a contribution to free energy that is cannot be fully modeled without intensive computing.
(55) The various functional forms, therefore, allow an even higher number of free energy phase diagrams than are shown in
(56) A surprisingly large number of pairings were found to form solid solutions readily and melting point depression was measured in four of the six cases. When slowly cooled to room temperature, these compounds retained the desired cubic phase. Based, in part, on the criteria used to choose the pairs, these were mostly solid solutions with barium titanate (BT) or strontium titanate (ST). All contained Na as one of the A-site ions. All the starting chemicals are low to moderate cost.
(57) TABLE-US-00002 TABLE II Examples of solid solution pairs that form indifferent minimum points. a t.sub.m (meas) x.sub.m (nm) t.sub.m (est) ( C.) t.sub.m (lit) x.sub.est (0.1) x.sub.l T (est) T (1%) ( C.) (100 C.) ( C.) xBT (1 x)NN 0.25 0.32 0.5-0.6 0.990 0.997 0.3958 1362 1285 1290-1325 xBT (1 x)NT 0.73 0.55 1.036 1.019 0.3970 1553 1440 xBT (1 x)NLT 0.72 0.35 1.038 1.008 0.3920 1547 1625 xST (1 x)NT 0.19 0.40 0.974 0.981 0.3908 1772 xST (1 x)NLT 0.18 0.25 0.983 0.984 0.3872 1766 xNN (1 x)BLN 0.41 0.41 1.004 1.004 0.4071 1198 1300
(58) The tolerance factor at the actual indifferent point was at worst the same and mostly closer to unity than at the predicted indifferent point suggesting that the tolerance factor has a thermodynamic influence on the energy minimum. This is explicitly a new discovery and a specific teaching of this patent application.
(59) It will be understood by those knowledgeable in the art that solid solution pairs that form indifferent points include, but are not limited to, these compositions. These constitute a broad and important class of compounds not previously described.
(60) The compounds that failed substantially did so through formation of second phases or completely different phases altogether from the perovskites because of different combinations of the ions used. Eutectic behavior with separation into two end member perovskites was also observed in the failed pairs. However, cryoscopic examination of melted charges of the successful compositions 0.32BaTiO.sub.3-0.68NaNbO.sub.3 and 0.41NaNbO.sub.3-0.59BaLi.sub.0.25Nb.sub.0.75O.sub.3 returned to room temperature did not reveal the lamellar structure typical of eutectic crystallization.
(61) Nickel-Containing Solid Solution Perovskites
(62) Perovskites containing VIB-VIIIB transition metals from the fourth row of the periodic table (i.e. Cr to Ni) are known, but these perovskites tend to be unstable because these ions can take on multiple valences and be subject to evaporation at high temperatures and reducing/neutral atmospheres. These elements are magnetic in varying forms and take on a variety of valences in oxides and other compounds. The reduced temperatures allowed by indifferent points with congruently melting minima may allow solid solutions of these compounds to be stabilized. This is especially true if the compounds can be grown in air or an oxidizing atmosphere, which requires use of a platinum crucible, which in turn also requires a lower melting temperature. Nickel compounds with nickel in the 2+ valence state are among the most stable both against reduction and evaporation. It was previously found that calcium nickel niobate CaNi.sub.1/3Nb.sub.2/3O.sub.3 (CNiN) is congruently melting at approximately 1650 C. and valence-stable against reduction with a bright green color even in a 3% oxygen atmosphere (V. J. Fratello, G. W. Berkstresser, C. D. Brandle and A. J. Ven Graitis, Nickel Containing Perovskites, Journal of Crystal Growth Volume 166, pp. 878-882 (1996)). Solid solutions incorporating alkaline earth nickel niobates AENi.sub.1/3Nb.sub.2/3O.sub.3 (where AE can be Ba, Sr, Ca or some solid solution among them) potentially have attractive substrate lattice parameters, especially those containing barium. However, because of unfavorable tolerance factors, none of these materials has a cubic crystal structure at room temperature. The discovery in this work that congruent melting occurs most readily near T=1.00 suggested testing solid solutions with x optimized for this condition. Table IV lists two such solid solutions based on barium nickel niobate BaNi.sub.1/3Nb.sub.2/3O.sub.3 (BNiN). Only a single composition was prepared for each composition with the x calculated to give T=1.00. This composition was somewhat close to the estimate from melting temperatures for the solid solution with NaNbO.sub.3 (NN), but was quite different for SrNi.sub.1/3Nb.sub.2/3O.sub.3 (SNiN). Samples were prepared from component oxides that were dried, weighed to proper proportions, mixed, fired at 1000 C., ground, fired at 1200 C., reground, compressed, re-fired at 1200 C., reground and measured by x-ray diffraction. The resultant samples were both single phase cubic as is shown for BNiN-NN in Table V. While this does not establish congruent melting, it is suggestive that these compounds fill the requirements of this application. The solid solution Ba.sub.xSr.sub.1xNi.sub.1/3Nb.sub.2/3O.sub.3 (BNiN-SNiN) does not contain a high vapor pressure constituent and is therefore more suitable for open crucible bulk single crystal growth methods such as the Czochralski method. This was the only solid solution with two A species of the same valence that appeared to have an indifferent point. Since niobium and tantalum are similar in valence, size and chemical behavior, past experience suggests nickel containing tantalates may behave similarly to the niobate compounds recited here. Substrates containing magnetic ions such as nickel are not suitable for some applications.
(63) TABLE-US-00003 TABLE IV Nickel-containing solid solutions that may have indifferent minimum points. x.sub.est T (est) t.sub.m (est) x.sub.est (from melting (T = a (nm) x (1 x) temperatures) 1.00) (calc.) (meas.) BaNi.sub.1/3Nb.sub.2/3O.sub.3 SrNi.sub.1/3Nb.sub.2/3O.sub.3 0.89 1.028 1545 C. 0.40 0.4012 0.4013 BaNi.sub.1/3Nb.sub.2/3O.sub.3 NaNbO.sub.3 0.33 0.990 1320 C. 0.48 0.4012 0.4014
(64) TABLE-US-00004 TABLE V X-ray powder diffraction pattern taken with a Siemens D-8 diffractometer for the BNiN.sub.0.48NN.sub.0.52 cubic solid solution including inter-planar d-spacings, intensity percentage with respect to the maximum (1, 1, 0) line and planar (h, k, l) indices. The d spacings have an error bar of 2% and the intensities should not be considered as precise. d(nm) I % (h, k, l) 0.4005 5% (1, 0, 0) 0.2838 100% (1, 1, 0) 0.2320 3% (1, 1, 1) 0.2007 35% (2, 0, 0) 0.1798 2% (2, 1, 0) 0.1640 31% (2, 1, 1) 0.1420 16% (2, 2, 0) 0.1270 11% (3, 1, 0) 0.1159 4% (2, 2, 2)
(65) Accordingly, in an embodiment, the single-crystal perovskites described herein comprise a solid solution comprising a solid solution of an alkaline earth (AE) metal nickel niobate perovskite with an approximate molar chemical formula AENi.sub.1/3Nb.sub.2/3O.sub.3 with another perovskite, wherein the single-crystal perovskite has a perovskite tolerance factor, T, between about 0.98 and about 1.02. In an embodiment, the AE metal is selected from the group consisting of Ba, Sr and Ca. In an embodiment, the single-crystal perovskite comprises a solid solution between barium nickel niobate and strontium nickel niobate with an approximate molar chemical formula xBaNi.sub.1/3Nb.sub.2/3 O.sub.3-(1x)SrNi.sub.1/3Nb.sub.2/3O.sub.3, wherein x is in the range between about 0.2 and about 0.9. In an embodiment, the single-crystal perovskite comprises a solid solution between barium nickel niobate and sodium niobate with an approximate molar chemical formula xBaNi.sub.1/3Nb.sub.2/3O.sub.3-(1x)NaNbO.sub.3, wherein x is in the range between about 0.28 and about 0.68.
(66) Epitaxial Single Crystal-Single-Crystal Perovskite Solid Solution Composites
(67) As described further herein, prior to the work described in the present disclosure, there was a dearth of appropriate substrates for epitaxial growth of crystals having primitive lattice parameters between about 0.387 nm and about 0.412 nm. In an embodiment, the present disclosure provides single-crystal perovskites having primitive lattice parameters within this range useful, for example, for epitaxial growth of single crystals.
(68) Accordingly, in an aspect, the present disclosure provides a structure comprising a single-crystal perovskite as described further herein; and an epitaxial single crystal epitaxially disposed on the single perovskite crystal.
(69) In an embodiment, the epitaxial single crystal is not congruently melting.
(70) In an embodiment, the epitaxial single crystal has a perovskite crystal structure. In an embodiment, the epitaxial single crystal having a perovskite crystal structure comprises a perovskite material selected from the group consisting of PbTiO.sub.3, PbZrO.sub.3, PbZr.sub.xTi.sub.1xO.sub.3, Pb(Mg.sub.1/3Nb.sub.2/3)O.sub.3SrRuO.sub.3, Bi.sub.0.5Na.sub.0.5TiO.sub.3, BiFeO.sub.3, BiMnO.sub.3, BiCrO.sub.3, KNbO.sub.3, K.sub.xNa.sub.(1x)NbO.sub.3, KTa.sub.xNb.sub.(1x)O.sub.3, yttrium barium cuprate, a solid solution between (1x)Pb(Mg.sub.1/3Nb.sub.2/3)O.sub.3-xPbTi.sub.xO.sub.3, and a solid solution between xBiScO.sub.3-(1x)PbTiO.sub.3.
(71) In an embodiment, the epitaxial single crystal includes a perovskite material that is one or more of ferroelectric, electrooptic, ferromagnetic, ferrimagnetic, antiferromagnetic, multiferroic, piezoelectric, pyroelectric, magnetoresistive, colossal magnetoresistive (CMR), magnetooptic, photovoltaic, photoluminescent, insulating, conducting, semiconducting, superconducting, ferroelastic, catalytic and combinations thereof.
(72) In an embodiment, the epitaxial single crystal has an as-grown area between about 10 mm and about 100 mm square or diameter. In an embodiment, the epitaxial single crystal has an as-grown area between about 20 mm and about 50 mm square or diameter. In an embodiment, the epitaxial single crystal has an as-grown area between about 25 mm and about 50 mm square or diameter.
(73) Devices
(74) In another aspect, the present disclosure provides a device comprising an epitaxial single crystal, as described further herein.
(75) In an embodiment, the epitaxial single crystal is free-standing or otherwise separate from the single-crystal perovskite. The epitaxial crystal can be made free-standing according to the methods described herein.
(76) In an embodiment, the device is a device selected from the group consisting of a sensor, an active component, a passive component, a system, and combinations thereof. Because perovskite compounds include ferroelectric, electrooptic, ferromagnetic, ferrimagnetic, antiferromagnetic, multiferroic, piezoelectric, pyroelectric, magnetoresistive, colossal magnetoresistive (CMR), magnetooptic, photovoltaic, photoluminescent, insulating, conducting, semiconducting, superconducting, ferroelastic, catalytic and other materials, many devices, applications and systems that are dependent on the availability of the perovskite material in single crystal form, either as a thin film on a substrate or a free-standing crystal removed from the substrate, are possible because of the single-crystal perovskites described herein.
(77) Such innovative devices, in turn enable higher order systems, applications and services. Many of these applications are currently limited by size, cost and availability of perovskite substrate materials. Accordingly, the single-crystal perovskites described herein provide a substrate for making larger scale perovskite and other single crystals useful in such devices, as described further herein.
(78) In an embodiment, the device is selected from the group consisting of a piezoelectric transducer, a piezoelectric sensor, an electrooptic waveguide device, a magnetooptic waveguide device, an infrared detector, a ferroelectric random-access memory device, a solar cell, a photodetector, a disk read and write head, a biosensor, a microelectromechanical system, a nanoelectromechanical system, a piezoelectric field effect transistor, a piezoelectric field effect photodetector, a magneto-electronic device, a tunnel magnetoresistance sensor, a spin valve, an electrically tunable microwave filter, an electrically tunable microwave oscillator, and an electrically tunable microwave phase shifter.
(79) Methods of Making Single-Crystal Perovskites
(80) In another aspect, the present disclosure provides a method of making a single-crystal perovskite. In an embodiment, the method includes: drying single-crystal perovskite reagents to remove moisture and adsorbed gases from the single-crystal perovskite reagents; combining the single-crystal perovskite reagents to provide a mixture; compacting the mixture; melting the mixture to provide a liquid solution between two perovskite compounds having an indifferent melting point; and generating a temperature gradient within the mixture configured to nucleate and grow the perovskite single crystal.
(81) In an embodiment, melting the mixture to provide a liquid solution between two perovskite compounds includes heating the mixture in a reactor. In an embodiment, the reactor is a reactor as discussed further herein with respect to
(82) In an embodiment, melting the mixture to provide a liquid solution between two perovskite compounds includes melting the mixture in a crucible comprising a material selected from the group consisting of platinum, a platinum alloy, a platinum composite or a platinum alloy composite.
(83) In an embodiment, generating a temperature gradient within the mixture includes configuring the radiofrequency (RF) coil or furnace winding, zones, insulation, cooling water coils and pedestal such that the desired temperature gradient occurs within a specific portion of the furnace where the crucible is located, applying after-heater(s) and/or baffle(s) to control the gradient further and observing the fluid flow in the melt as is known to those well versed in the art.
(84) In an embodiment, the single-crystal perovskite comprises a solid solution between two perovskite compounds having an indifferent melting point that occurs at a temperature minimum in a melting curve in a pseudo-binary phase diagram.
(85) In an embodiment, the single-crystal perovskite is grown by a bulk growth method from a congruently melting or near congruently melting liquid solution. In an embodiment, a composition of the liquid solution and the single-crystal perovskite are within 1 atomic percent of all ions in a primitive formula unit for each constituent.
(86) In another embodiment, the liquid solution between the two perovskite compounds has a nearly-congruent composition. In an embodiment, the liquid solution between two perovskite compounds P1 and P2 has an approximate general molar formula P1.sub.xP2.sub.1x, wherein x is within 0.2 mole of a molar chemical formula of the single-crystal perovskite. In an embodiment, x is within 0.1 mole of a molar chemical formula of the single-crystal perovskite. In an embodiment, x is within 0.05 mole of a molar chemical formula of the single-crystal perovskite. A wider growth stoichiometric range beyond a mixture having a composition exactly equal to a congruently melting composition is possible because the congruently melting composition is at a temperature minimum in the free energy curve. Therefore, the melt and crystal composition move toward the congruently melting composition as the crystal grows, contrary to the case of conventional solid solutions and congruent compositions that are at a temperature maximum in the free energy curve where the composition moves away from congruency as the crystal grows.
(87) Because of the relatively-low growth temperatures, many of the single-crystal perovskites described herein may be grown from, for example, a crucible comprising a material selected from the group comprising platinum, a platinum alloy, a platinum composite and a platinum alloy composite in air at atmospheric pressure or a more oxidizing atmosphere with a higher partial pressure of oxygen by a number of bulk growth methods selected from the group consisting of Bridgman/Stockbarger, Czochralski, Stepanov, Heat Exchanger Method (HEM), Vertical-Horizontal Gradient Freezing (VHGF), Edge-Defined Film-Fed Growth (EFG), Kyropoulos and Bagdasarov growth methods among others. The Czochralski and Stepanov methods have advantages in that the crystal does not contact the crucible, but the Bridgman-Stockbarger method is better for suppressing evaporation of more volatile oxides. All these methods have the primary steps of drying all the constituent chemicals, weighing them in the proper stoichiometric weight ratios to yield the congruently melting or near congruently melting molar composition (or a specifically targeted composition to take into account constituent volatility), mixing and compacting them, melting them to provide a liquid solution, and generating a temperature gradient that allows the crystal to nucleate and grow from, for example, a specifically determined seed or interface.
(88) In an embodiment, combining the single-crystal perovskite reagents to provide a mixture includes combining the single-crystal perovskite reagents in a ratio to provide a congruent molar composition. In an embodiment, combining the single-crystal perovskite reagents to provide a mixture includes adding one or more higher-vapor pressure constituents in excess of what is strictly required for a congruently melting composition.
(89) In an embodiment, the method includes slicing the single-crystal perovskite into one or more wafers. In an embodiment, the method further includes lapping and polishing one or more sides of the one or more wafers. In this regard, the method provides a plurality of substrates, for example, for further epitaxial deposition.
(90) Epitaxial Perovskite Crystal Growth on Perovskite Substrates
(91) As above described above, there is a dearth of commercially-available perovskite substrates having a primitive lattice parameter between about 0.387 nm and about 0.412 nm. In an embodiment, the single-crystal perovskites described herein have primitive lattice parameters within this range. Accordingly, the substrates described herein are capable of serving as substrates for epitaxial growth of commercially-relevant single crystal and, in an embodiment, the methods described herein further include epitaxially growing an epitaxial single crystal on one or more surfaces of the single-crystal perovskites described herein.
(92) In an embodiment, epitaxially growing the epitaxial single crystal includes vapor phase methods of epitaxial growth. In an embodiment, vapor phase methods of epitaxial growth include sputtering, thermal evaporation, physical vapor deposition (PVD), chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD), vapor phase epitaxy (VPE), organo-metallic vapor phase epitaxy (OMVPE), molecular beam epitaxy (MBE), chemical beam epitaxy (CBE), atomic layer epitaxy (ALE), pulsed laser deposition (PLD), and combinations thereof. In an embodiment, such processes are conducted in a vacuum chamber. As a group, they typically allow the substrate to be at a significantly-reduced temperature during epitaxial crystal growth compared to growth techniques from the liquid, especially bulk growth.
(93)
(94) In an embodiment, epitaxially growing the epitaxial single crystal includes an epitaxial growth method including liquid phase methods of epitaxial growth. In an embodiment, liquid phase methods of epitaxial growth include liquid phase epitaxy (LPE) from a high temperature solution (HTS). In an embodiment, liquid phase methods of epitaxial growth include methods selected from the group consisting of flux growth, hydrothermal growth, and sol-gel processing from a metal-organic sol.
(95)
(96) Successful liquid phase epitaxy can include selection of a substrate having one or more of a good structural, chemical, lattice parameter, and thermal expansion match to the desired perovskite composition. Accordingly, in an embodiment, the epitaxial single crystal is a single-crystal perovskite with a primitive perovskite unit cell lattice parameter within 2% or 0.008 nm of the single-crystal perovskite lattice parameter to prevent the formation of misfit dislocations.
(97) Those knowledgeable in the art will understand that the film may have a cubic crystal structure at the growth temperature and a non-destructive phase transition between the growth temperature and room temperature. The phase transition to a lower symmetry structure resulting in non-degeneracy of the crystal directions and non-destructive domain twinning will proceed on cooling according to the cooling program, the anisotropic stress field of a disc or square, and the stress between the film and the substrate. The latter will evolve to minimize the stress energy and give the best possible match. Following growth, poling in an electric field may be used for ferroelectric materials to improve or impose a specific a-b-c orientation match. Choice of the lattice match of the substrate and film may template the film to grow preferentially in the a-b plane depending on the two-dimensional lattice match between the substrate and film.
(98) The facet directions of the perovskites are the (001), (010) and (100) directions of the primitive perovskite unit cell. These are the slowest growing directions and growth typically occurs by atomic/molecular addition at steps, kinks, and dislocations, which can create an as-grown surface with bunched islands, micro- and macro-steps, spirals, growth hillocks and possible impurity segregation at the steps and dislocations. This is undesirable for LPE where a fast-growing direction perpendicular to the substrate and a smooth uniform growth surface is desired. This best occurs for an atomically rough non-facet growth direction where almost all surface sites are growth sites.
(99) Non-facet surfaces can be achieved by growing on substrates cut at an intentional misorientation from a crystallographically oriented facet, typically 0.5-2.0 off the facet direction, for example the (001) direction in perovskite. This can be preferable if the crystallographic properties of the facet direction are desired in the final crystal. Alternatively, growth can be performed on a crystallographic orientation that is not a facet, for example the atomically rough (111) direction as is used in LPE of garnets. This makes characterization of the resultant films by x-ray diffraction more practical than misoriented substrates.
(100) However, some applications require that the exact facet surface be the growth surface to achieve the desired properties of the resulting crystal film. In this case the growth conditions may be optimized to achieve kinetic roughening at the atomic or micro level, but have a relatively smooth film at the macro level.
(101) In certain embodiments, it may be useful to have the epitaxial crystal separated from or physically independent of the single-crystal perovskite from which it is grown. Accordingly, in an embodiment, the methods described herein include removing the epitaxial single crystal from the one or more surface of the single-crystal perovskite to provide a free-standing single crystal.
(102) The epitaxial crystals described herein are grown epitaxially on the single-crystal perovskite described herein. Accordingly, in an embodiment, the epitaxial single crystal has a perovskite crystal structure that can grow disposed upon and in registry with the single-crystal perovskite with lower lattice strain. In an embodiment, the single crystal having a perovskite crystal structure is selected from the group consisting of PbTiO.sub.3, PbZrO.sub.3, PbZr.sub.xTi.sub.1xO.sub.3, Pb(Mg.sub.1/3Nb.sub.2/3)O.sub.3, SrRuO.sub.3, Bi.sub.0.5Na.sub.0.5TiO.sub.3, BiFeO.sub.3, BiMnO.sub.3, BiCrO.sub.3, KNbO.sub.3, K.sub.xNa.sub.(1x)NbO.sub.3, KTa.sub.xNb.sub.(1x)O.sub.3, yttrium barium cuprate, a solid solution between (1x)Pb(Mg.sub.1/3Nb.sub.2/3)O.sub.3-xPbTi.sub.xO.sub.3, and a solid solution between xBiScO.sub.3-(1x)PbTiO.sub.3.
(103) In an embodiment, the epitaxial single crystal includes a perovskite material that is one or more of ferroelectric, electrooptic, ferromagnetic, ferrimagnetic, antiferromagnetic, multiferroic, piezoelectric, pyroelectric, magnetoresistive, colossal magnetoresistive (CMR), magnetooptic, photovoltaic, photoluminescent, insulating, conducting, semiconducting, superconducting, ferroelastic, catalytic, and combinations thereof.
(104) Other materials that have perovskite-related structures or even unrelated structures may also have a lattice match to the single-crystal perovskites described herein so that they can be grown epitaxially. Accordingly, in an embodiment, the epitaxial single crystal does not have a perovskite crystal structure.
(105) In an embodiment, the epitaxial single crystal is not congruently melting. While the epitaxial crystal structure can be grown from a congruently melting solution, it can also be grown in other ways, as described herein.
EXAMPLES
(106) All chemicals used in the examples are presumed to have been dried previously in air at standard pressure at appropriate temperature to remove moisture and adsorbed gases without decomposing or otherwise changing the constituent. Such drying temperatures commonly range from 150 C. for sodium carbonate to 1000 C. for tantalum oxide.
Example 1
(107) Barium Titanate-Sodium Niobate Substrate Crystal Perovskite Solid Solution Growth by the Czochralski Method
(108) Pre-dry in air suitable quantities of barium carbonate BaCO.sub.3 at 850 C., titanium oxide TiO.sub.2 at 850 C., sodium carbonate Na.sub.2CO.sub.3 at 150 C., and niobium oxide Nb.sub.2O.sub.5 at 850 C. Mix a charge consisting of 146.77 grams of barium carbonate BaCO.sub.3, 59.41 grams of titanium oxide TiO.sub.2, 87.95 grams of sodium carbonate Na.sub.2CO.sub.3 and 210.06 grams of niobium oxide Nb.sub.2O.sub.5 in a ball mill, press it into a solid body in an isostatic press, place it in a platinum crucible of dimensions 50 mm diameter by 50 mm high and put it into a resistance-heated furnace in a pure oxygen atmosphere. This composition intentionally contains a 5% excess of sodium carbonate Na.sub.2CO.sub.3 to allow for volatilization of sodium species during growth. A crystal of approximate molar chemical formula Ba.sub.0.32Na.sub.0.68Ti.sub.0.32Nb.sub.0.68O.sub.3 can be grown from this melt by the Czochralski method with a pulling rate of 1 mm/hour using a (100) oriented seed of the same material produced in a previous growth run on a platinum wire seed. This crystal has a favorable lattice parameter of approximately 0.3958 nm and remains cubic all the way to room temperature. Orient the resultant boule to the (100) direction using x-ray diffraction methods, grind to a cylinder and cut crosswise into slices that are edge finished and polished to an epitaxial finish.
Example 2
(109) Barium Titanate-Sodium Niobate Single-Crystal Perovskite Solid Solution Growth by the Bridgman Method
(110) Pre-dry in air suitable quantities of barium carbonate BaCO.sub.3 at 850 C., titanium oxide TiO.sub.2 at 850 C., sodium carbonate Na.sub.2CO.sub.3 at 150 C., and niobium oxide Nb.sub.2O.sub.5 at 850 C. Mix a charge consisting of 146.77 grams of barium carbonate BaCO.sub.3, 59.41 grams of titanium oxide TiO.sub.2, 83.76 grams of sodium carbonate Na.sub.2CO.sub.3 and 210.06 grams of niobium oxide Nb.sub.2O.sub.5 in a ball mill, load into a platinum Bridgman crucible and seal under a pure oxygen atmosphere to suppress the evaporation of sodium atoms in various forms. Put the loaded crucible into a resistance-heated furnace. A crystal of approximate molar chemical formula Ba.sub.0.32Na.sub.0.68Ti.sub.0.32Nb.sub.0.68O.sub.3 can be grown from this melt by the Bridgman method (also known as the Bridgman-Stockbarger method). The form of the Bridgman crucible results in the growth of a single crystal by promoting growth of a single nucleated crystal. This crystal has a favorable lattice parameter of approximately 0.3958 nm and remains cubic all the way to room temperature. Orient the resultant boule to the (100) direction using x-ray diffraction methods, grind to a cylinder and cut crosswise into slices that are edge finished and polished to an epitaxial finish.
Example 3
(111) Barium Titanate-Sodium Niobate Single-Crystal Perovskite Solid Solution Characterization
(112) A number of polycrystalline solid solutions of barium titanate and sodium niobate with an approximate molar chemical formula of xBaTiO.sub.3-(1x)NaNbO.sub.3, where x=0.3-0.65 were prepared by furnace cooling from a melt prepared as described in Examples 1 and 2, having approximate chemical composition xBaTiO.sub.3-(1x)NaNbO.sub.3, where x=0.3-0.65. Additional powder compositions were prepared by solid state reaction of the constituents at 1200 C. In an embodiment, the growth range was between x=0.22-0.42, where there was the least amount of second phase and the perovskite tolerance factor T is between 0.988 and 1.006. In an embodiment, the growth range, where highest uniformity may be achieved, was immediately adjacent to the determined congruent composition, wherein x=0.320.05. This compositional range is in direct contradiction of earlier publications and the perovskite tolerance factor is T=0.997 at the indifferent point, which is closer to the preferred value of unity than the compositions previously published.
(113) The structure remained cubic at room temperature for all samples measured from x=0.3-0.65 and the range may prove broader with additional measurements. Taking these factors into account, a preferred crystal growth range is x=0.22 to 0.42 and the most preferred growth range is x=0.27-0.37. The x=0.32 compound BT.sub.0.32-NN.sub.0.68 has a simple perovskite lattice parameter of approximately 0.3958 nm and indexes to a simple perovskite body-centered-cubic (BCC) cell (see TABLE III). Cryoscopic examination of a melted charge of 0.32BaTiO.sub.3-0.68NaNbO.sub.3 returned to room temperature did not reveal the lamellar structure typical of eutectic crystallization.
(114) TABLE-US-00005 TABLE III X-ray powder diffraction pattern taken with a Siemens D-8 diffractometer for the BT.sub.0.32NN.sub.0.68 cubic solid solution including inter-planar d-spacings, intensity percentage with respect to the maximum (1, 1, 0) line and planar (h, k, l) indices. The (1, 1, 1) and (3, 1, 1) lines have zero intensity, but are included because they have non-zero intensities for higher values of x. The d spacings have an error bar of 2% and the intensities should not be considered as precise. d(nm) I % (h, k, l) 0.3938 21% (1, 0, 0) 0.2789 100% (1, 1, 0) 0% (1, 1, 1) 0.1975 41% (2, 0, 0) 0.1763 9% (2, 1, 0) 0.1616 25% (2, 1, 1) 0.1400 13% (2, 2, 0) 0.1252 10% (3, 1, 0) 0% (3, 1, 1) 0.1142 6% (2, 2, 2)
(115) The (1,0,0), (1,1,1) and (3,1,1) lines would not appear if the solid solution was fully disordered with a primitive unit cell. The (1,0,0) line becomes more intense at lower x near the indifferent point, while the (1,1,1) and (3,1,1) lines decrease and disappear. Ordering can appear during cooling, but since these samples were furnace quenched, the fictive (equilibrium) temperature is near 1200 C. This could indicate that an ordering transition, probably second order, is responsible for the difference in the two phases on either side of the minimum, therefore satisfying the phase rule. This could result from partial ordering or clustering resulting from the positive enthalpy of mixing. It could also be a consequence of the differing x-ray cross sections of the substituting ions.
Example 4
(116) Barium Titanate-Sodium Tantalate Single-Crystal Perovskite Solid Solutions
(117) A perovskite single crystal comprising a solid solution of barium titanate and sodium tantalate with approximate molar chemical formula of xBaTiO.sub.3-(1x)NaTaO.sub.3, where x=0.35-0.75, can be grown from a melt of approximate chemical composition xBaTiO.sub.3-(1x)NaTaO.sub.3, where x=0.35-0.75. In an embodiment, the growth range, where highest uniformity was achieved, is immediately adjacent to the congruent composition, wherein x=0.550.05. Ta is chemically similar to Nb and has the same ionic size. The tantalates are somewhat more refractory with 300-400 C. higher melting temperatures for the end members, which would tend to push the indifferent point toward the BT side of the phase diagram. However, the tolerance factor contribution to free energy pushes the minimum back toward the center as seen in
Example 5
(118) Barium Nickel Niobate-Sodium Niobate Substrate Crystal Growth by the Czochralski Method
(119) Pre-dry in air suitable quantities of barium carbonate BaCO.sub.3 at 850 C., sodium carbonate Na.sub.2CO.sub.3 at 150 C., nickel (II) dioxide NiO at 850 C., and niobium oxide Nb.sub.2O.sub.5 at 850 C. Mix a charge consisting of 187.78 grams of barium carbonate BaCO.sub.3, 57.92 grams of sodium carbonate Na.sub.2CO.sub.3, 37.55 grams of nickel (II) dioxide NiO and 221.06 grams of niobium oxide Nb.sub.2O.sub.5 in a ball mill, press it into a solid body in an isostatic press, place it in a platinum crucible of dimensions 50 mm diameter by 50 mm high and put it into a resistance-heated furnace in an oxygen atmosphere. This composition intentionally contains a 5% excess of sodium carbonate Na.sub.2CO.sub.3 to allow for volatilization of sodium species during melting and growth. A crystal of approximate molar chemical formula Ba.sub.0.48Na.sub.0.52Ni.sub.0.16Nb.sub.0.84O.sub.3 can be grown from this melt by the Czochralski method with a pulling rate of 1 mm/hour using a (100) oriented seed of the same material produced in a previous growth run on a platinum wire seed. This crystal has a favorable lattice parameter of approximately 0.4014 nm and remains cubic all the way to room temperature. Orient the resultant boule to the (100) direction using x-ray diffraction methods, grind to a cylinder and cut crosswise into slices that are edge finished and polished to an epitaxial finish.
Prior Art Example 1
(120) SLT Substrate Crystal Growth
(121) A charge consisting of 205.43 grams of strontium carbonate SrCO.sub.3, 12.85 grams of lithium carbonate Li.sub.2CO.sub.3 and 230.61 grams of tantalum oxide Ta.sub.2O.sub.5, where all constituents were previously dried, was mixed in a ball mill and pressed into a solid body in an isostatic press. The charge was placed in an iridium crucible of dimensions 50 mm diameter by 50 mm high and put into an RF heated furnace. The melting point of this composition was 1915 C. A crystal of approximate molar chemical formula SrLi.sub.0.25Ta.sub.0.75O.sub.3 (SLT) was grown from this melt by the Czochralski method with a pulling rate of 1 mm/hour using a (100) oriented seed of the same material that had been produced in a previous growth run on an iridium wire seed. The crystal had a favorable measured lattice parameter of 0.4005 nm. The resultant boule was oriented to the (100) direction, ground to a cylinder and cut crosswise into slices that were then polished to an epitaxial finish. It was determined that this material has a destructive phase transition between room temperature and 900 C. that prevented it from being used successfully as a substrate material. Films and substrates cracked up completely on cooling. (V. J. Fratello, G. W. Berkstresser, A. J. Ven Graitis and I. Mnushkina, A New Perovskite Substrate Material, Strontium Lithium Tantalate, invited talk presented at the 14.sup.th International Conference on Crystal Growth, Grenoble, France, Aug. 9-13, 2004).
Prior Art Example 2
(122) Thin Film LPE of KNbO.sub.3 on a SrTiO.sub.3 Substrate
(123) This epitaxial film was grown by a self-flux method from an excess of K.sub.2O. Dried powders of K.sub.2CO.sub.3 (99.99% pure) and Nb.sub.2O.sub.5 (99.9% pure) were mixed in a molar ratio of 60/40. The mixture was placed in a platinum crucible, then heated to 1100 C., which is above the liquidus, in a three-zone spiral resistor vertical furnace designed for minimization of temperature fluctuations. The melt was cooled to 970 C. A mirror polished (0,0,1) SrTiO.sub.3 substrate was mounted vertically on a platinum holder and cleaned in tetrahydrofuran, acetone and ethanol followed by rinsing in deionized water and blowing off with filtered high-pressure air. The platinum holder was attached to a thin alumina tube. This was lowered slowly to just above the melt surface and allowed to come to thermal equilibrium. The alumina tube was rotated slowly and the assembly was dipped in the melt until the entire substrate was submerged. After growth for the desired length of time, the tube and holder were raised to move the substrate and epitaxial film to the upper area of the furnace. It was cooled there to room temperature at a rate of 1.5 C./min. A very thin 100 nm film of KNbO.sub.3 grew epitaxially on the substrate with a monoclinic structure that differs from the bulk orthorhombic structure. This structural distortion occurred because of the large 0.0122 nm lattice parameter mismatch between the film lattice parameter and the substrate. Also because of the mismatch, only 100 nm of material could be grown without nucleation of non-epitaxial bulk crystals. At more preferred KNbO.sub.3 growth conditions where bulk crystals would not have nucleated, the poor lattice match resulted in epitaxial growth being delayed and some etch pits forming in the SrTiO.sub.3 surface. (K. Kakimoto, T. Hibino, I. Masuda and H. Ohsato, Development of Transparent Single-Crystalline KNbO.sub.3 Thin Film by LPE Technique, Science and Technology of Advanced Materials 6 (2005) pp. 61-65)
Example 6
(124) Liquid-Phase Epitaxy of Potassium Tantalum Niobate on Barium Titanate-Sodium Niobate Single-Crystal Perovskite Solid Solution Substrates
(125) BT-NN substrates with growth and preparation as defined above in Example 1 or 2 can be used for liquid phase epitaxial growth of epitaxial crystal films of the perovskite crystal structure with lattice parameter match within 0.008 nm or in the primitive perovskite unit cell range 0.388-0.404 nm, which comprises epitaxial films of the perovskites BiFeO.sub.3, KNN, KTN, PZT and many other important materials.
(126) Mix a charge consisting of 74.38 grams potassium fluoride KF, 7.69 grams potassium carbonate K.sub.2CO.sub.3, 7.87 grams tantalum oxide Ta.sub.2O.sub.5 and 10.06 grams niobium oxide Nb.sub.2O.sub.5. Place the charge in a 100 ml volume platinum crucible, in turn placed inside a vertical resistance-heated tube furnace with an air atmosphere per
(127) This is a vertical tube furnace into which the substrate may be lowered into the melt with the substrate held horizontally (pictured) or vertically. Baffles can be inserted to control convection, temperature gradients and flux evaporation. An exhaust system is used to remove toxic vapors. The furnace may be water cooled or uncooled. The furnace temperature is controlled with a thermocouple(s) on the windings and a thermocouple can be inserted into the melt or the furnace outside the melt to monitor and possibly control the process temperature.
(128) Heat the melt to 980 C. and stir 10-16 hours with a platinum paddle to dissolve the KTN. Cool the melt to 930 C. and dip a horizontally oriented 25 mm diameter substrate of (1,0,0)-oriented BT-NN into the melt. Rotate the substrate at 40-100 rpm, reversing every 1-5 rotations. This is an undercooled melt and so crystals may be grown at a constant temperature at a growth rate of approximately 0.5 micron/minute. After 10 hours growth, remove the substrate and KTN crystal grown on it from the melt and spin above the hot melt at 1000 rpm to remove residual melt. Cool the crystal slowly to room temperature. Any remaining solvent may be dissolved in hot water. Remove the substrate from the crystal by back-lapping and polishing the crystal to the desired thickness.
Prior Art Example 3
(129) Molecular Beam Epitaxy of Lead Titanate on a Lanthanum Aluminate Substrate
(130) A commercial Varian EPI 930 oxide reactive molecular beam epitaxy (MBE) system with all molybdenum parts replaced by stainless steel was used. This system can operate up to eight component sources simultaneously including an effusion cell for lead and a heated titanium source that sublimes this more refractory component. A (1,0,0) oriented LaAlO.sub.3 substrate was introduced into the system through an airlock and placed on a substrate block that can be heated radiatively with quartz lamps. The substrate was heated to 600 C. A very low pressure of purified ozone was used to supply oxygen in the growth chamber without impacting the high vacuum required for MBE. In-situ atomic absorption spectroscopy that is part of the MBE system was utilized to monitor the coverage of the surface by each of the metallic components. In this way, atomic layer epitaxy (ALE) was achieved with monolayer control by controlling the opening and closing of the Ti shutter. Using a lead overpressure assures films with proper PbTiO.sub.3 1:1 stoichiometry. This method on LaAlO.sub.3 substrates resulted in epitaxial PbTiO.sub.3 films up to 1000 thick with a mixture of a- and c-axis orientations. (C. D. Theis and D. G. Schlom, Epitaxial lead titanate grown by MBE, Journal of Crystal Growth 174, (1997) pp. 473-479). Lanthanum aluminate goes through a cubic-rhombohedral phase change as it cools that results in the substrate becoming multi-domain. The problematic mixed orientations of the PT domains most likely result from this.
Example 7
(131) Molecular Beam Epitaxy of Lead Titanate on a Strontium Titanate-Sodium Titanate Single-Crystal Perovskite Solid Solution Substrate
(132) Utilize the method of Prior Art Example 3 in all respects except for using a ST-NT substrate of composition as defined in TABLE II. Proper lattice matching with a cubic substrate and no phase change can give a single domain orientation in the film. Films of this type have potential utility in ferroelectric memories.
Prior Art Example 4
(133) Sputtering of Yttrium Barium Cuprate on Magnesium Oxide
(134) A MgO substrate was placed in an ion beam sputtering system equipped with a thermocouple-controlled substrate heater. The substrate heater was set to 750 C. and the substrate was cleaned in situ by treating with an oxygen plasma source discharged at a current of 20 mA and discharge voltage of 1.2 kV so the oxygen partial pressure comes to 4 mTorr. The substrate was cleaned in this way for three hours. After cleaning, the substrate heater was set to 600 C. The oxygen plasma was set to 1 kV and 9.6 mA at 60 Hz to achieve an oxygen pressure of 2 mTorr. A YBa.sub.1.5Cu.sub.2.3O.sub.x (YBCO) target was sputtered with 4 keV Ar+ ions so the sputtered particles were deposited on the heated substrate. Sputtering was carried out for a period of six hours. Because of the poor structural match of the film to the substrate, the film was amorphous. (T. Endo, K. I. Itoh, A. Hashizume, H. Kohmoto, E. Takahashi, D. Morimoto, V. V Srinivasu, T. Masui, K. Niwano, H. Nakanishi, Mechanism of a-c oriented crystal growth of YBCO thin films by ion beam sputtering, Journal of Crystal Growth 229 (2001) pp. 321-324).
Example 8
(135) Sputtering of High Temperature Superconductor on ST-NLT
(136) Utilize the method of Prior Art Example 4 using a ST-NLT substrate of composition described in TABLE II. ST-NLT is a good structural and lattice parameter match to YBCO allowing true crystal growth and epitaxy and elimination of the amorphous phase.
(137) While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.
(138) It should be noted that for purposes of this disclosure, terminology such as upper, lower, vertical, horizontal, inwardly, outwardly, inner, outer, front, rear, etc., should be construed as descriptive and not limiting the scope of the claimed subject matter. Further, the use of including, comprising, or having and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms connected, coupled, and mounted and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. The term about means plus or minus 5% of the stated value.
(139) The principles, representative embodiments, and modes of operation of the present disclosure have been described in the foregoing description. However, aspects of the present disclosure that are intended to be protected are not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. It will be appreciated that variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present disclosure. Accordingly, it is expressly intended that all such variations, changes, and equivalents fall within the spirit and scope of the present disclosure, as claimed.