EP1104494A1

EP1104494A1 - Control of crystal anisotropy for perovskite oxides on semiconductor-based substrates

Info

Publication number: EP1104494A1
Application number: EP99937553A
Authority: EP
Inventors: Rodney Allen Mckee; Frederick Joseph Walker
Original assignee: UT Battelle LLC
Current assignee: UT Battelle LLC
Priority date: 1998-07-30
Filing date: 1999-07-27
Publication date: 2001-06-06
Also published as: KR20010079590A; CA2337029A1; AU5236399A; WO2000006812A1; JP2002521309A

Abstract

A crystalline structure (20, 60 or 220) and a device (120, 140, 180 or 270) which can be suited for use in any of a number of semiconductor or electro-optic applications, such as a phase modulator or a component of an interfereometer, includes a substrate (22, 62, 142, 182, 222 or 272) of a semiconductor-based material and a thin film (24, 64, 144, 186 or 224) of a crystalline oxide material epitaxially arranged upon the surface of the substrate so that the thin film couples to the underlying substrate and so that the geometries of substantially all of the unit cells of the thin film are arranged in a predisposed orientation relative to the substrate surface. The predisposition of the geometries of the unit cells of the thin film is due to a stressed or strained condition of the lattice at the interface between the thin film material and the substrate surface and is responsible for a predisposed orientation of a directional-dependent quality, such as the dipole moment, of the unit cells. The predisposed orientation of the unit cell geometries enables a device within which the structure is embodied to take beneficial advantage of characteristics of the structure during operation.

Description

CONTROL OF CRYSTAL ANISOTROPY FOR PEROVSKITE OXIDES

ON SEMICONDUCTOR-BASED SUBSTRATES .

Background of the Invention

This invention was made with Government support under Contract No. DE-AC05-96OR22464 awarded by the U.S.

Department of Energy to Lockheed Martin Energy Research

Corporation, and the Government has certain rights in the invention.

This invention relates generally to structures and the preparation of such structures for use in semiconductor and related applications and relates, more particularly, to the growth of epitaxial thin-films upon semiconductor-based materials in the Group III-V, IV and II-VI classes such as, by way of example and not limitation, silicon, germanium or silicon-germanium alloys so that the thin films grown thereon possess desirable properties.

In U.S. Patent. No. 5,830,270, we have described how alkaline earth and perovskite oxides can be grown unstrained and commensurately upon silicon to form a structure series of (A0)_n(A^/B0₃)_B in which n and m are non-negative integer repeats of single plane commensurate oxide layers, and that this structure can be utilized in the development of a new semiconductor technology. This new technology is distinct from the device technology in common usage which is based upon amorphous Si0₂-on-silicon technology because by virtue of its amorphous state, the oxide in the commonly-used technology exhibits no directionality in its properties or its response to applied fields. Consequently, Si0₂ is treated as an isotropic material with no crystallographic anisotropy in its response to internally or externally applied electric fields. In contrast, in this new technology, there is a fundamental change to be taken advantage of since there exists "easy" directions in the crystalline oxides that promote polar phenomena and device physics that can utilize these directions in unique device designs and functions. One example of such a device is a ferroelectric field effect transistor in which tetragonal distortion of a ferroelectric oxide is utilized to modulate the channel current in the surface doped semiconductor substrate. It would therefore be desirable to take advantage of the directional-dependent qualities of crystalline oxides when grown in a thin film layup atop a semiconductor-based substrate to enhance the use of such oxides in device technology.

Anisotropic crystals are known to possess properties or qualities which differ according to the direction of movement. Particular examples are found in the anisotropy of critical phenomena like Curie ordering in magnetic and ferroelectric oxide structures. In some crystalline oxides, for example, the Curie ordering induces internal magnetic or electric fields at the onset of dipole ordering which is naturally disposed in a prescribed orientation relative to the body of the crystal. Furthermore, it is also known that the application of an externally-applied magnetic field can reorient (e.g. reverse) these induced internal magnetic or electric fields.

Heretofore and as mentioned above, oxides have been incorporated within electronic devices, such as transistors, but only in the amorphous state or in a polycrystalline microstructure, and these oxides (in the amorphous state or the polycrystalline microstructure) do not exhibit any collective anisotropic behavior. Accordingly, it would be desirable to provide a new and improved structure for use in a semiconductor device including a semiconductor-based substrate upon which is grown a thin film of crystalline oxide wherein the crystalline oxide is capable of exhibiting anisotropic properties which are beneficial for operation of the device. Furthermore, there does not exist a structure which incorporates a ferroelectric material and a silicon substrate and which has been used as an active waveguide material in a silicon-based communication system. It would be desirable, therefore, to provide a monolithic structure which incorporates a ferroelectric oxide and silicon wherein the ferroelectric material is capable for use as an active waveguide material. Accordingly, it is an object of the present invention to provide a new and improved crystalline-on-oxide structure wherein the unit cells within the thin film layers of the structure are arranged so that the directional- dependent qualities of substantially all of the unit cells are biased toward a predisposed orientation.

Another object of the present invention is to provide such a structure including a thin film of a perovskite oxide whose unit cells can be advantageously arranged so that the directional-dependent qualities of substantially all of the unit cells are arranged along a limited number of axes (i.e. either substantially in a plane parallel to the surface of the substrate or substantially normal to the surface of the substrate) . Still another object of the present invention is to provide such a structure wherein the orientation of the directional-dependent qualities of substantially all of the unit cells of the thin film material are influenced by the geometric shape of the unit cells. Yet another object of the present invention is to provide such a structure for use in a device which takes advantage of the anisotropic properties of crystalline oxides and their response to internal and external applied fields.

Yet still another object of the present invention is to provide a new and improved semiconductor device within which a crystalline oxide-on-silicon structure is incorporated and whose operation involves the application of an internally- applied or externally-applied field across the crystalline oxide. A further object of the present invention is to provide a structure involving the growth of a ferroelectric thin-film upon a semiconductor-based material, such as silicon, wherein the thin-film possesses highly desirable electro-optical characteristics. A still further object of the present invention is to provide such a structure wherein the crystalline growth of the thin-film advantageously affects the optical characteristics of the thin-film.

A yet still further object of the present invention is to provide such a structure which includes a material having the general formula AB0₃, such as, by way of example and not as limitation, a perovskite, and in particular a perovskite in the BaTi0₃ class, grown upon materials selected from the Group III-V, IV or II-VI classes of materials including, by way of example and not as limitation, a silicon or silicon-germanium substrate. One more object of the present invention is to provide such a structure which can be used as a solid state electrical component, such as a phase modulator or switch, of an electro-optic device, such as an interferometer.

Yet one more object of the present invention is to provide such a structure having a semiconductor-based substrate and a perovskite thin-film overlying the substrate wherein the substrate of the structure is utilized in the transmission of electricity and wherein the thin-film is utilized for the transmission of light. One more object of the present invention is to provide such a structure for use as a building block of a communication system through which both electricity and light are transmitted. Summary of the Invention This invention resides in a structure including a substrate of semiconductor-based material having a surface and a thin film of a crystalline oxide epitaxially overlying the substrate surface. The crystalline oxide includes unit cells which exhibit or are capable of exhibiting anisotropic behavior having a directional-dependent quality and the thin film is exposed to in-plane strain at the substrate/thin film interface so that substantially every one of the unit cells of the thin film have a geometric shape which is influenced by the in-plane strain so that the directional-dependent quality of each unit cell is arranged in a predisposed orientation relative to the substrate surface.

In one embodiment of the invention, a directional- dependent quality of each unit cell of the crystalline oxide (e.g. its dipole moment) is oriented in a plane which is parallel to the substrate surface, and in another embodiment of the invention, a directional-dependent quality of each unit cell of the crystalline oxide (e.g. its dipole moment) is oriented along lines normal to the substrate surface.

In a further embodiment of the invention, the structure of the invention is embodied in a device for a semiconductor application wherein the coupling between the crystalline oxide and the semiconductor-based substrate advantageously effects the behavioral characteristics of the semiconductor-based substrate; and in a still further embodiment of the device, the thin film is comprised of a ferroelectric, optically-clear oxide overlying the surface of the substrate wherein at least the first few atomic layers of the thin film are commensurate with the semiconductor substrate and so that substantially all of the dipole moments associated with the ferroelectric film are arranged substantially parallel to the surface of the substrate to enhance the electro-optic qualities of the structure.

One more embodiment of the invention resides in a device for a semiconductor application wherein the semiconductor capabilities of the device are required to be utilized. In this one more embodiment, the device includes a substrate of semiconductor-based material having a surface, and a thin film of anisotropic crystalline material commensurately overlying the substrate surface so as to provide, with the substrate material, a single crystal and coupling to the underlying semiconductor-based material. The thin film of the anisotropic crystalline material is comprised of unit cells commensurately arranged upon the substrate surface wherein substantially all of the unit cells of the thin film have a geometric form of tetragonal shape, and each unit cell of the thin film has a tetragonal axis which is arranged along lines normal to the substrate surface so that the polar axes of substantially all of the unit cells of the thin film are arranged along lines normal to the substrate surface .

Brief Description of the Drawings

Fig. 1 is a perspective view of a fragment of one embodiment of a crystalline structure including a silicon substrate and a layup of the perovskite BaTi0₃ grown upon the surface of the substrate.

Fig. 2 is a fragmentary cross-sectional view taken about along line 2-2 of Fig. 1.

Fig. 3 is a schematic perspective view of an undeforraed unit cell of BaTi0₃.

Fig. 4 is a schematic perspective view of the unit cell of Fig. 3 which has been misshapened due to strain.

Fig. 5 is a graph plotting lattice parameters of BaTi0₃-including structures as a function of temperature. Fig. 6 is a graph plotting thermal strain of BaTi0₃ when grown on silicon as a function of temperature.

Fig. 7 is a perspective view of a fragment of another embodiment of a crystalline structure including a silicon substrate and a layup of the perovskite SrTi0₃ grown upon the surface of the substrate.

Fig. 8 is a fragmentary cross-sectional view taken about along line 8-8 of Fig. 7.

Fig. 9 is a schematic perspective view of a unit cell of SrTi0₃ which has been misshapened due to compression. Fig. 10 is a Z-contrast image of SrTi0₃ on (100) silicon.

Fig. 11 is a plot of capacitance versus voltage for a SrTi0₃ capacitor.

Fig. 12 is a schematic cross-sectional view of a fragment of a ferroelectric field effect transistor (FFET) utilizing a perovskite thin film as a layer of a gate dielectric.

Fig. 13 is a schematic cross-sectional view of a fragment of a capacitor utilizing a perovskite thin film as a dielectric layer.

Fig. 14 is a plan view of an electro-optic device within which features of the present invention are embodied. Fig. 15 is a cross-sectional view taken about along line 15-15 of Fig. 14.

Fig. 16 is a schematic perspective view of a fragment of a silicon wafer upon which a film of BaTi0₃ is constructed for use in a waveguide structure.

Fig. 17 is transverse cross-sectional view of the Fig. 16 wafer taken about line 17-17 of Fig. 16.

Fig. 18 is a graph of lattice parameter versus temperature for the materials BaTi0₃ in bulk form and BaTi0₃ thin films clamped to a silicon substrate.

Fig. 19 is a plan view of an electro-optic device within which features of the present invention are embodied.

Fig. 20 is a cross-sectional view taken about along line 20-20 of Fig. 19. Detailed Description of the Illustrated Embodiments

Turning now to the drawings in greater detail, there is shown in Figs. 1 and 2 a monolithic crystalline structure, generally indicated 20, comprised of a substrate 22 of silicon and a layup 24 of the perovskite BaTi0₃ epitaxially covering the surface of the substrate 22. BaTi0₃ is a ferroelectric oxide material which when combined with the silicon substrate 22 in the form of an overlying and epitaxial thin film enables the crystalline structure 20 to take advantage of the semiconductor, as well as the ferroelectric properties, of the structure 20. The structure 20 is a crystalline oxide-on- silicon (COS) structure, but it will be understood that in accordance with the principles of the present invention, a structure can involve a film of crystalline oxide grown upon an alternative semiconductor-based substrate constructed, for example, of silicon-germanium, germanium or other materials in the Group III-V, IV and II-IV classes. Along these lines, the substrate can be comprised of silicon-germanium wherein the variable "y" could range from 0.0 to 1.0.

The crystalline form of BaTi0₃ is anisotropic in that each of its unit cells has a directional-dependent quality, and as will be apparent herein, this directional dependent quality of each unit cell of BaTiθ₃ in the layup 24 is arranged in a predisposed orientation relative to the substrate surface. For present purposes, the directional- dependent quality of each BaTi0₃ unit cell of the exemplary layup 24 is its polar axis or, in other words, its permanent spontaneous electric polarization (i.e. the electric dipole moment). Accordingly, in the exemplary structure 20, it is the dipole moment of substantially every BaTi0₃ unit cell which is arranged in a predisposed orientation relative to the substrate surface. Although the structure 20 is described herein as including an epitaxial film of BaTi0₃, a ferroelectric film- including structure in accordance with the broader aspects of this invention does not require commensurate periodicity in order that the directional-dependent qualities (e.g. the dipole moments) of the unit cells of the materials are arranged in a predisposed orientation. Instead, it is the geometric influence (described herein) to which the unit cells are exposed, rather than any commensurate periodicity, that arranges the directional-dependent qualities of the unit cells in a predisposed orientation. Accordingly, the principles of the present invention can be variously applied.

In addition, the composition of the structure 20 is a member of our general series of commensurate structures designated as (A0)_n(A B0₃)_B (and described in our U.S. Patent No. 5,835,270) in which n and m are the integer repeats (i.e. non-negative integer repeats) of single plane commensurate oxide layers. If n=l, then the perovskite is grown directly as AB0₃ from the suicide truncation of silicon beginning at the AO plane. If n>l, the face-centered NaCl-type structure is grown at the interface then truncated with the B0₂ plane to transition with the perovskite structure. Accordingly, the principles of the present invention can be applied to any member of our structures designated as (A0)_n(A'B0₃)_B.

An exemplary process whose steps can basically be followed in order to construct the crystalline structure 20 of Figs. 1 and 2 is set forth our referenced U.S. Patent No. 5,835,270 and the disclosure of which is incorporated herein by reference, so that a detailed description of the construction process is not believed to be necessary. Briefly however and with reference to Fig. 2, in order to produce a structure having a BaTi0₃ film whose lattice structure is in a positively-strained (i.e. an in-plane, tensile-strained) condition atop the silicon substrate and so that a directional-dependent quality, i.e. the dipole moment, of each BaTi0₃ unit cell is oriented parallel to, or in the plane of, the thin film, initial steps are taken to cover the surface, indicated 26 of the silicon substrate 22 (or wafer) with a thin alkaline earth oxide film 28 of Ba_o.725Sr₀.₂₇₅0. An initial step in the deposition of this oxide atop the silicon substrate 22 involves the deposition of a fraction of a monolayer (e.g. one-fourth of a monolayer) of an alkaline earth metal atop the silicon as is described in our U.S. Patent 5,225,031, the disclosure of which is incorporated herein by reference. The Ba₀.₇₂₅Sr₀.₂₇₅0 film 28 is thereafter covered with a thin perovskite (template) film 30 of Ca_0β4Sr₀.₃₆Ti0₃, and then to cover the film 30 with the desired (multi-stratum) film 32 of BaTi0₃ to provide the layup 24 and thus the resultant structure. Each of the alkaline earth oxide film and the template film and at least the first few layers of the BaTi0₃ film is constructed in somewhat of a single plane-layer-by-single-plane layup fashion to ensure commensurate periodicity throughout the build-up of the exemplary COS structure 20 and wherein the films are selected for used in the build-up of the structure for the lattice parameters of the unit cells of the films. In other words, to promote thermal strain at the thin film/silicon interface, the thin film materials are selected so the lattice parameters of adjacent films are not equal, but instead are slightly different from one another to promote lattice strain between the adjacent layers.

The differences between the coefficients of thermal expansion (i.e. linear thermal expansion) of the constituents of the structure strongly effect the coupling of the ferroelectric material to the semiconductor substrate in the resultant structure. In other words, the coupling of the unit cells of BaTi0₃ to the underlying substrate (in this case, silicon) can advantageously effect the performance of the substrate during use in an electronic application. In this regard, the coefficient of thermal expansion of silicon is smaller than that of BaTi0₃ so that a uniform heating (or cooling) of the resultant structure results in a tendency of the BaTi0₃ film to misshapen, as will be described herein, and the tendency for an appreciable in-plane strain to develop within the BaTi0₃ film.

Applicants have discovered, however, that the relative thermal expansion (or contraction) between silicon and BaTi0₃ is of less consequence during the build-up of the film than it is when the film is subsequently cooled during a cool-down period following of the deposition of the BaTi0₃ film atop the silicon. In this connection, the steps involving the deposition of BaTi0₃ are carried out at a relatively high (growth) temperature of about 600°C, and at this temperature, the deposited film of BaTi0₃ is substantially free of in-plane strain. Following this deposition process, however, the resulting structure is subsequently cooled to a lower temperature, such as about 40°C (closer to room temperature) , and it is during this cooling process that the differences in thermal expansion (or contraction) characteristics between the silicon and the BaTi0₃ come into play. Phase transformations may or may not occur within the BaTi0₃ film during cooling depending upon the extent to which the lattice is constrained during cooling. In other words, by using the techniques described herein to induce lattice strain within the oxide overlayer, phase transformations during cooling of the oxide overlayer can be minimized.

More specifically, as the resultant structure is cooled as aforedescribed, the differences in thermal expansion (or contraction) of the BaTi0₃ film and the silicon effects a greater shrinkage of the BaTiθ₃ film than the silicon. In other words, as the resultant structure is cooled from the deposition temperature of about 600°C, the number of BaTi0₃ unit cells per unit area at the Si/BaTi0₃ interface remain proportional to the number of Si unit cells per unit area at the Si/BaTi0₃ interface while the atoms of the BaTi0₃ film tend to move closer together than do the atoms of the silicon substrate. As the BaTi0₃ film attempts to contract more than the silicon, it is constrained by the constraint of the film of BaTi0₃ at the Si/BaTi0₃ interface. Thus, upon reaching the lower temperature, e.g. room temperature, the BaTi0₃ film is constrained to a larger in-plane area than it would otherwise if it were not so constrained. Consequently, the contraction of the BaTi0₃ film effects a shortening of the out-of-plane lattice parameter of the BaTi0₃ film as a path is traced therethrough from the surface of the silicon. As the lattice parameter tends to reduce in size as a path is traced through the aforementioned transition layers, the lattice of the BaTi0₃ film is exposed to an appreciable amount of biaxial tensile strain induced within the plane of the layers of the film, i.e. in a plane generally parallel to the surface of the silicon, as well as along the sides of the lattice, i.e. along a path normal to the surface of the silicon. With reference to Figs. 3 and 4 wherein a unit cell of BaTi0₃ is denoted 42, this induced strain tends to misshapen the form of the BaTi0₃ unit cell from a cubic form, as depicted in Fig. 3, to the somewhat distorted cubic form (i.e. to a tetragonal form) as depicted in Fig. 4 wherein the lattice form of the Fig. 4 unit cell has a top which possesses an area which is slightly smaller than the area of its base. Along the same lines, the width of the Fig. 4 unit cell 42 as measured across the base thereof is slightly larger than the height thereof as the volume of the unit cell 42 tends to remain relatively constant as its shape is altered.

It will be understood that amorphous oxides (heretofore used in oxide-on-silicon structures) do not behave in the aforedescribed manner. In other words, since amorphous oxides do not possess unit cells whose geometries can be altered in the aforedescribed manner, the anisotropy of the structure of amorphous oxides cannot be effected by geometric influences as is the case with the structure of. the instant invention. Furthermore, unlike the thin film layers of the structure of the instant invention which couple to the underlying semiconductor substrate for purposes of advantageously effecting the behavior of the substrate during an electronic application, amorphous oxides do not couple to the underlying substrate. Accordingly, the instant invention is advantageous in this respect. With reference to Fig. 5, there is shown a plot of the lattice parameter of BaTi0₃/Si and bulk BaTi0₃ structures as a function of temperature. It can be seen from the Fig. 5 plot that as the lattice parameter of each structure decreases as the structure temperature decreases , but that the lattice parameter of BaTi0₃/Si does not increase as much as does the bulk BaTi0₃ through this temperature range. Such a phenomena can be explained in that during a cooling of each of the respective structures, the lattice of Si (having a mass which is much greater than the mass of the overlying thin film) does not permit the lattice parameter of BaTi0₃ to reduce to the extent permitted during the cooling of bulk BaTi0₃.

Along the same lines, there is shown in Fig. 6 a plot of the in-plane thermal strain induced upon a thin film of BaTi0₃ grown upon a substrate of silicon versus the temperature of the structure. Generally, the in-plane strain induced within the BaTi0₃ film of the BaTi0₃/Si structure increases as the temperature falls from a temperature of about 1030°K. As is the case with a cubic unit cell of an unpoled ferroelectric material, such as the cubic BaTi0₃ unit cell 42 schematically illustrated in Fig. 3, there exists an electric dipole moment which may be oriented along any of three (X, Y or Z) coordinate axes. Therefore, when a layup of BaTi0₃ material is in an unstrained (or uncompressed) condition within an epitaxial crystalline layup, the dipole moments of the unit cells may be oriented in any of six directions (corresponding with the number of faces in the unit cell). However, the in-plane strain induced within the BaTi0₃ unit cells of the layup 24 of the structure 20 and the consequential geometric misshapening of the unit cells of BaTi0₃ into the aforedescribed tetragonal form orients the tetragonal axis (i.e. the longer axis) of the cells parallel to the plane of the substrate and thereby prevents the dipole moment in each unit cell from being naturally established in a direction normal to the plane of the substrate surface. Consequently, the in-plane strain induced within the BaTi0₃ layup limits the permissible orientations of the dipole moments in the BaTi0₃ unit cells of the layup to those orientations which correspond to the X and Y (Fig. 4) coordinate axes so that substantially all of the dipole moments of the BaTi0₃ unit cells in the layup 24 are arranged in a plane oriented parallel to the substrate surface 26.

With reference to Figs. 7 and 8, there is shown an alternative embodiment of a structure 60 comprising a silicon substrate 62 and a commensurate layup 64 of the perovskite SrTi0₃ covering the substrate surface. As is the structure 20 of Figs. 1 and 2, the structure 60 of Figs. 7 and 8 is a member of our general series of commensurate structures designated as (A0)_n(A^/B0₃)_ιn described earlier. However, unlike the structure 20 of Figs. 1 and 2, the structure 60 of Figs. 7 and 8 is grown with in-plane compression induced therein, arising entirely from the commensurate, epitaxial strain through the substrate.

A process used to construct the crystalline structure 60 of Figs. 7 and 8 is comparable to the construction process described above in conjunction with the construction of the Fig. 1 structure 20 except that different elements are used during the construction process. In particular and with reference to Fig. 7, an appreciable portion of the SrTi0₃ film 64 is constructed in somewhat of a single plane-layer-by-single plane-layer fashion to ensure commensurate periodicity throughout the build up of the structure 60. In other words, steps are taken to cover the surface, indicated 66, of the silicon substrate 62 with a thin alkaline earth oxide film 68 of one monolayer thickness (following the deposition of a fraction of a monolayer of an alkaline earth oxide metal), then to cover the alkaline earth oxide film 68 with a thin perovskite (template) film 70 of any desired thickness (i.e. one monolayer or greater), and then to cover the perovskite film 70 with the desired (multi- stratum) film 72 of SrTi0₃ to provide the layup and thus the resultant structure 60. To maintain the desired periodicity between the SrTi0₃ film 72 and the silicon substrate 62 during the growth process, the SrTi0₃ unit cells are rotated 45^° with respect to the unit cells of the underlying silicon substrate (as the lattice parameter [3.91 angstroms] of SrTi0₃ seeks to match the lattice parameter [5.43 angstroms] divided by the square root of 2.0 [or 3.84 angstroms]). If it is desired to magnify the desired negative strain effect upon the lattice of the overlying SrTi0_3/ various film layers (such as the template film 70) may be omitted or the film layer composition can be altered during the build-up process. Each of the alkaline earth oxide film 68 and the template film 70 and an appreciable portion of the SrTi0₃ film 72 is constructed in somewhat of a single plane-layer-by- single plane-layer fashion to ensure commensurate periodicity throughout the build up of the structure 60 and wherein the layer-construction processes take into account the crystalline form of the material out of which the film is desired to be constructed. Again, for a more detailed description of the construction of the epitaxial film 64 directly atop the surface of the silicon substrate 62, reference can be had to the earlier-referenced U.S. Patent No. 5,835,270. It will be understood, however, that while the referenced U.S. Patent No. 5,835,270 deals with the build up of commensurate structures in which lattice parameters of adjacent layers match one another so that no strain is induced within the unit cells of the last-constructed oxide layer, materials are selected for the construction of structures in accordance with the present invention whose lattice parameters are different and thereby induce a desired strain (positive or negative) within the unit cells of the last-constructed oxide layer of the structure. With reference still to the structure 60 of Figs. 7 and 8, it follows that by virtue of the commensurate periodicity of the thin film 64 of SrTi0₃ atop the silicon substrate 62, the atoms of the film 64 are effectively clamped to those of the underlying silicon and the lattice of the film 64 must conform (e.g. contract or expand) in accordance with that of the underlying silicon. As mentioned above, at room temperature, the 3.84 angstrom figure is what the lattice parameter of each unit cell of SrTiθ₃ seeks to match in the structure 60. Therefore, at room temperature, the SrTi0₃ film 64 is strained by about -2% (or, more specifically, compressed by about 2%) from its unstrained (or uncompressed) condition at the silicon/thin film interface, and this induced strain on the SrTi0₃ lattice influences the geometry of the SrTi0₃ unit cells.

In particular, the strain which exists at the silicon/film interface exerts an in-plane compression within the SrTi0₃ so that the SrTi0₃ unit cells are deformed out-of- plane. In other words, as the base of each SrTi0₃ unit cell is compressed in-plane, the geometry of the SrTi0₃ distorts to a condition (as best illustrated by the SrTi0₃ unit cell 80 shown in Fig. 9) wherein the dimensions of the base of the unit cell 80 are smaller than those measured across the top of the cell. Moreover, as the volume of the unit cell 80 is maintained relatively constant as it is deformed, the height of the unit cell increases as the base is compressed. This distortion of the SrTi0₃ unit cells into a tetragonal form with the tetragonal axis (i.e. the longer axis) thereof arranged out of the plane of the thin film. Consequently, this strain-induced influence on the geometry of the SrTi0₃ unit cells predisposes the tetragonal axis of, and thus the directional-dependent quality of, each unit cell out of the plane of the thin film.

Due to the analogy between SrTi0₃ and ferroelectrics in the perovskite series, it follows that ferroelectric oxides can be grown upon a semiconductor-based substrate, such as silicon, so that the directional-dependent qualities, such as the dipole moments, of substantially all of the unit cells of the ferroelectric oxides are oriented out-of-plane relative to the substrate surface. In either event, the thin film materials are selected for the construction process wherein an oxide having a larger lattice parameter is built upon an underlying material having a smaller lattice parameter so that a negative (i.e. compressive) strain is induced within the unit cells of the overlying oxide which geometrically distorts the shape of the unit cells and predisposes the directional- dependent qualities thereof out of the plane of the thin film (i.e. along lines normal to the substrate surface).

It follows from the foregoing that the characteristics of a thin film of crystalline oxide material on Si can be affected when the directional-dependent qualities of the unit cells of the oxide are arranged in a predisposed orientation relative to the substrate surface, and that the predisposed direction of the directional-dependant qualities of the oxide can be controlled through geometric constraint upon the unit cells thereof. In the aforedescribed examples, a strain (i.e. a positive strain in one instance and a negative strain in the other instance) is induced at the silicon/film interface as a consequence of the differences in lattice parameters between the silicon and the crystalline material of the film layup. Thus, by appropriately selecting an anisotropic oxide for growth upon a silicon substrate whose unit cells are placed in either a strained or compressed condition (e.g. within a strained condition of between about ±2%) when cooled to an appropriate temperature below the growth temperature, the directional-dependent qualities of the unit cells will be predisposed along directions oriented in a plane which is parallel to the surface of the silicon substrate or along lines normal to the substrate surface. Moreover, this invention is particularly advantageous in that the overlying anisotropic material couples to the underlying semiconductor substrate for affecting the electronic capabilities of the substrate. In other words, with the geometry of the unit cells of the anisotropic material constrained so that the directional- dependent qualities of the unit cells are oriented either in a plane which is parallel to the substrate surface or along lines normal to the substrate surface, the coupling of the anisotropic material to the semiconductor substrate enables the substrate to behave in an advantageous and reproducible manner during an electronic application. Therefore, the structure of this invention can be embodied in semiconductor devices which use the coupling of the anisotropic thin film material with the underlying semiconductor material during an electronic application.

In accordance with the foregoing, we have experimentally established that perovskite oxides can be grown in perfect registry with the (100) face of silicon while totally avoiding the amorphous silica phase that forms when silicon is exposed to an oxygen-containing environment. In particular, a metal-oxide-semiconductor (MOS) capacitor has been constructed using the perovskite SrTi0₃ (as an alternative to amorphous Si0₂) and wherein its SrTi0₃ layer is 150 angstroms in thickness and the underlying silicon is p- type silicon. A Z-contrast image (taken at atomic scale) of a cross section of the constructed capacitor is shown in Fig. 10 illustrating the arrangement of atoms at the oxide/silicon interface. The epitaxy that is apparent from the Fig. 10 image is (100) SrTi0₃// (001) Si and SrTiO₃[110]//Si[100] . On the left side of the image is an insert model of the perovskite/silicon projection. Inasmuch as the perfection of the physical structure of the constructed MOS capacitor couples directly to the electrical structure, we obtain for the constructed capacitor (having the oxide thickness of 150 angstroms) an equivalent oxide thickness of less than 10 angstroms (i.e. about 8.8 angstroms) wherein the equivalent oxide thickness, t_βq, can be defined for a metal-oxide-semiconductor (MOS) capacitor as:

wherein e_aloa and e₀ are the dielectric constants of silica and the permitivity of free space, respectively, and (C/A)_ox is the specific capacitance of the MOS capacitor. Fig. 11 shows a plot of our data for specific capacitance against voltage for our constructed capacitor. This capacitor exhibits a C/A value of 40 fF/um² at negative voltages where the field is across the oxide. The interface trap density, obtained from the frequency dependence of the capacitance data, is sharply peaked at 0.11 ev above the valence band with values that range from 10^lo/cm². An analysis of this data suggests that the interface registry is so perfect that the original silicon surface step interactions can be identified as the interface trap states. The relatively small equivalent oxide thickness for this capacitor is an unparalleled result for MOS capacitors and suggests that crystalline oxides-on-silicon (COS) can potentially replace silica in transistor gate technology.

It will be understood that numerous modifications and substitutions can be had to the aforedescribed embodiments without departing from the spirit of the invention. For example, while much of the foregoing discussion has focused upon the ferroelectric qualities of a perovskite constructed on a semiconductor-based material, it will be understood by one skilled in the art that many comparable structures can be constructed in accordance with the principles of the present invention which possess other desirable characteristics. For example, comparable devices can be constructed which are piezoelectric in nature, pyroelectric in nature or electro- optic in nature. In the paragraphs which follow, exemplary structures are described in which a crystalline oxide is utilized in a thin film layup which has been grown upon a semiconductor- based substrate so that the unit cells of the crystalline oxide thin film are epitaxially, and in some cases commensurately, arranged upon the substrate and wherein the unit cells of the crystalline thin film are exposed to an in- plane strain (which may be a positive strain or a negative strain) at the substrate/thin film interface. Furthermore, the in-plane strain to which the unit cells are exposed arranges a directional-dependent quality of the unit cells of the thin film along predisposed axes which render the resulting structure advantageous for a number of semiconductor device applications in which a magnetic, electric or optic field is applied to the device. Even in an instance in which a crystalline oxide whose unit cells are naturally isotropic is used as the thin film overlayer, anisotropic behavior can be induced in the oxide by way of the in-plane strain, and this behavior be used beneficially within a semiconductor structure. As used herein, a crystalline oxide is said to be capable of exhibiting anisotropic behavior if it possesses a directional-dependent quality, such as a dipole moment or dielectric constant. Turning again to the drawings, there is shown in

Fig. 12 a schematic cross section of a fragment of a ferroelectric field effect transistor (FFET), indicated 120, embodying features of the present invention. To this end, the transistor 120 includes a base, or substrate 122 of silicon, a source electrode 124, a drain electrode 126, and a gate dielectric 128. Incorporated within the gate dielectric 128 is a thin film layer 130 of the perovskite BaTi0₃ which directly overlies the substrate 122 and is disposed beneath the remainder of the gate dielectric 128 so as to be positioned adjacent the epilayer 132 of the transistor 120. The transistor 120 includes a crystalline oxide-on-silicon (COS) structure, but it will be understood that in accordance with the principles of the present invention, a structure can involve a film of crystalline oxide grown upon an alternative semiconductor-based substrate constructed, for example, of silicon-germanium, pure germanium or other materials in the Group III-V, IV and II-IV classes. Since ferroelectric materials possess a permanent spontaneous electric polarization (electric dipole moment per unit centimeter) that can be reversed by an electric field, the disposition of the BaTi0₃ thin film layer 130 in the transistor 120 in the manner described permits the ferroelectric dipoles to be switched, or flipped, in a manner which aids in the modulation of the charge density and channel current of the transistor 120. For example, the transistor 120 can be turned ON or OFF by the action of the ferroelectric polarization, and if used as a memory device, the transistor 120 can be used to read the stored information (+ or -, or "1" or "0") without ever switching or resetting (hence no fatigue) .

It is known that unit cells of the perovskite oxide BaTi0₃, when in a crystalline form, are anisotropic in that its unit cells possess directional-dependent qualities, such as a dipole moment. However, previous semiconductor devices have not relied upon anisotropic response of the gate oxide. Within the transistor 120, however, the thin film 130 of BaTi0₃ consists of BaTi0₃ in a crystalline form and the unit cells of BaTi0₃ are arranged upon the silicon substrate 122 so that the dipole moments of the unit cells are arranged in a limited number of directions. In particular, the dipole moments of substantially all of the unit cells of BaTiθ₃ in the thin film 130 are disposed along lines oriented normal to the surface of the substrate 122, and as such, the charge density and channel currents are controlled by the internal fields set up by the predisposition of the dipole moments oriented normal to the substrate material. As will be apparent herein, the thin film of BaTi0₃ is grown upon the underlying silicon substrate 122 in a manner so that a negative strain (i.e. compression) is induced within the plane of the thin film (i.e. in a plane parallel to the substrate surface) so that the consequential misshapening of the unit cells by this negative strain condition predisposes the directional dependent quality, i.e. the dipole moment, of each unit cell along lines normal to the surface of the silicon substrate 122.

For purposes of constructing the transistor 120 (and, in particular, the build-up of BaTi0₃ atop silicon so that the unit cells of BaTi0₃ are epitaxially arranged upon the silicon substrate), reference can be had to the description of our crystalline-on-silicon (COS) deposition process set forth in our co-pending U.S. Patent Application Serial No. 08/868,076, filed June 3, 1997. Briefly, by way of example, steps can be taken to cover the surface of the silicon substrate 122 with a thin alkaline earth oxide film of Ba₀.₇₂₅Sr₀.₂₇₅0, then to cover the earth oxide film with a thin perovskite (template) film of SrTi0₃, and then to cover the template film with the desired (multi-stratum) film of BaTi0₃. Each of the alkaline earth oxide film and the template film and at least the first few initial layers of the BaTi0₃ film is constructed in somewhat of a single plane-layer-by-single plane layup fashion to ensure commensurate periodicity throughout the build up of the COS structure and wherein the films are selected for use in the build-up of the structure for the lattice parameters of the unit cells of the films. For example, the template film of SrTi0₃ will induce a -2% strain in the BaTi0₃ unit cells at the interface with the BaTi0₃ film, while the omission of the template film of SrTi0₃ (between the Ba_o.₇₂₅Sr₀.₂₇₅0 and the BaTi0₃) will induce a strain of -4% in BaTi0₃ unit cells. Accordingly, in an alternative embodiment of a COS structure in which greater strain is desired to be induced within the BaTi0₃ unit cells, the SrTi0₃ template can be omitted.

The negative strain which is induced within the BaTi0₃ thin film is due in part to the differences in the lattice parameters between the unit cells of silicon and those of the thin film materials used in the construction process and the commensurate periodicity of the unit cells in the thin film layup. More specifically, the lattice parameter of each of silicon and Ba_o.₇₂₅Sr_o.₂₇₅0 is 0.543 nm while the lattice parameter of SrTi0₃ is about 0.392 nm (which is slightly greater than the 0.543 figure divided by the square root of 2.0). Consequently, there will exist no measurable strain at the interface of the unit cells of silicon and Ba_o.₇₂₅Sr_o.₂₇₅0, while the unit cells of the SrTi0₃ film have an orientation which is rotated 45° with respect to the underlying unit cells of Ba_o.₇₂₅Sr_o._27S0 and are in a state of negative strain (i.e. compression) due to the difference between the lattice parameter (0.392 nm) of SrTi0₃ when in an unstrained condition and the lattice parameter (0.543 nm) of silicon divided by the square root of 2.0 (or 0.384). Furthermore, the lattice parameter of BaTi0₃ is about 0.4 nm so that unit cells of BaTi0₃ are also in a state of negative strain (i.e. compression) as they overlie the SrTi0₃ film.

The commensurate periodicity of the layup of thin film layers constrains (and clamps) the lattice structures together at the interface of each successive silicon/thin film or thin film/thin film interface so that during the cooldown of the resultant COS structure from a relatively high deposition temperature, the lattice structures of the unit cells of the thin films must conform in size to the lattice structure of the dominant material , which in this case is silicon. Consequently, at the end of a cooldown period to a temperature (e.g. about room temperature) which is appreciably lower than the deposition temperature at which the various thin-film layers were deposited upon the silicon, each unit cell of BaTi0₃ at the SrTi0₃/BaTi0₃ interface (which is, in essence, the Si/BaTi0₃ interface) assumes an in-plane compressed condition as its lattice parameter (as measured in- plane of the film) seeks to match the 0.384 nm figure discussed above. In other words, upon achieving a lower temperature, e.g. room temperature, following cooldown of a COS structure from its growth temperature, the unit cells of the BaTi0₃ film adjacent the Si/BaTi0₃ interface are constrained to a smaller in-plane area than would be the case if it were not so constrained. Consequently, the in-plane contraction of the unit cells of BaTi0₃ effects a lengthening of the out-of-plane lattice parameter of the unit cells of the BaTi0₃ film as a path is traced therethrough from the surface of the silicon (as the unit cell seeks to maintain a constant volume) so that each unit cell assumes a tetragonal form (i.e. a somewhat distorted cubic shape) with its tetragonal axis (i.e. its longer axis) disposed normal to the surface of the silicon. This misshapening of the geometric shape of the unit cells of BaTi0₃ by the constraint at the Si/BaTi0₃ interface prevents the dipole moment of each unit cell from being naturally established along directions which are parallel to the surface of the substrate (i.e. normal to the longitudinal axis of the misshapened unit cell of BaTi0₃). Consequently, the geometric influence to which the BaTi0₃ unit cells are exposed predisposes the dipole moments of the unit cells of BaTi0₃ along axes, or lines, oriented generally normal to the surface of the substrate 122. For a more detailed description of the COS deposition process involving a layup of BaTi0₃ upon silicon, reference can be had to the referenced application Serial No. 08/868,076, as well as U.S. Patent No. 5,835,270, the disclosures of which are incorporated herein by reference. As mentioned earlier, the ferromagnetic nature of the BaTi0₃ thin film 130 in the Fig. 12 FFET 120 enables the polarization of the unit cells of the film 130 to be switched by the application of an electric field between the gate dielectric 124 and the substrate 122 and thereby control the modulation of the charge density and channel current of the transistor 120. By utilizing the BaTi0₃ thin film 130 in the transistor 120 so that the dipole moments of the unit cells of the film 130 are arranged in planes oriented normal to the substrate surface, in-plane mechanical restraint through the BaTi0₃ thin film 130 is so robust that a relatively large threshold voltage must be applied across the gate dielectric 124 and the substrate 122 to effect the switching of the dipole moments of the thin film 130 to an in-plane orientation. Such a feature is advantageous in that the dielectric response needs to be insensitive to an applied field over some (i.e. a minimum) range in order to allow design or optimization of its use in a capacitor for charge storage such as might be found in a DRAM application.

Although the aforedescribed FFET 120 of Fig. 12 has been described as including a thin-film comprised of unit cells of an anisotropic perovskite whose directional-dependent qualities, e.g. its dipole moments, are oriented along lines arranged normal to the silicon substrate, a field-effect transistor (FET) can be constructed with a thin-film of anisotropic perovskite whose directional-dependent qualities, e.g. it dipole moments, are arranged in-plane of the thin film and are switched out-of-plane upon the application of an electric field between the gate dielectric and the substrate. Briefly, the perovskite BaTi0₃ is grown commensurately upon a silicon substrate in such a manner so that the unit cells of the BaTi0₃ are exposed to a positive (i.e. tensile-strained) in-plane strain which misshapens the BaTi0₃ unit cells to a tetragonal shape so that the dipole moments thereof are prevented from naturally orienting themselves out-of-plane (i.e. normal to the plane of the silicon substrate). In order to permit the dipole moments to be poled out-of-plane with the application of an externally-applied field, the edges of selected regions of the BaTi0₃ thin film are either cut away or otherwise treated to relieve some of the in-plane strain to which the unit cells of BaTi0₃ is exposed. For a more complete description of a transistor within which a thin film of BaTi0₃ is employed and wherein the dipoles of the unit cells of BaTi0₃ are predisposed by the constraints (those induced commensurately and thermally) of the underlying silicon lattice to orientations within the plane of the film, reference can be had to the referenced co-pending U.S. Patent Application Ser. No. 08/868,076, the disclosure of which is incorporated herein by reference.

Furthermore, although the aforedescribed FFET 120 of Fig. 12 has been described as including a thin-film of BaTi0₃ material whose unit cells naturally exhibit anisotropic behavior, a structure in accordance with the present invention may include a thin film crystalline oxide layer whose unit cells, while not naturally anisotropic, can be made to exhibit anisotropic behavior by the in-strain (e.g. either positive or negative strain) induced within the unit cells of the thin film layer grown upon a semiconductor-based substrate. For example, the crystalline form of SrTi0₃ has unit cells which are naturally isotropic, and a useful property of the unit cells (i.e. its dielectric constant) may naturally exhibit a directional dependence. However, by inducing a desired strain within the unit cells of the SrTi0₃ thin film material so that the unit cells of SrTi0₃ are distorted to a tetragonal shape whose smallest side area is closest to the semiconductor substrate, the dielectric constant will exhibit uniaxial behavior associated with the tetragonal distortion.

For example, by constructing a COS structure wherein the perovskite SrTi0₃ is intended to be the active layer which couples to the underlying silicon, SrTi0₃ can be deposited upon silicon so that a cooling of the resultant structure from the high deposition temperature to a lower temperature (e.g. about room temperature), the negative in-plane (compressive) strain induced at the Si/SrTi0₃ interface distorts the geometric (normally cubic) shape of the unit cells of the SrTi0₃ to a tetragonal shape so that the dielectric constant of each unit cell is biased by the strain and thereby alters the dielectric constant of the COS structure. It follows that by controlling the amount of in-plane strain at the Si/SrTi0₃ interface, the electric (or dielectric) response of the resulting structure can be controlled.

With reference to Fig. 13, there is schematically shown a capacitor 140 for dynamic random access memory (DRAM) circuit including a silicon layer 142 and an oxide (dielectric) layer 144 comprised of a thin film of BaTi0₃ which are in superposed relationship and which are sandwiched between a gate 146 and a ground terminal 148. In use, an information-providing signal is collected from the capacitor 140 by measuring the current of the capacitor 140 during a discharge cycle. Therefore, the greater the dielectric constant exhibited by the oxide layer 144, the greater the charge-storage capacity of the capacitor 140. It is a feature of the capacitor 140 that the dipole moments of substantially all of the unit cells of the BaTi0₃ are oriented along a plane which is generally parallel to the surface of the silicon layer 142 upon which it overlies. While it was positive in-plane strain which was responsible for orienting the dipole moments of the BaTi0₃ normal to the substrate surface in the FFET 120 of Fig. 12, it is negative (e.g. tensile-strained) in-plane strain induced at the Si/Bi0₃ interface which is responsible for orienting the dipole moments of the BaTi0₃ unit cells parallel to the substrate surface in the capacitor 140 of Fig. 13. In other words, due to the in-plane strain induced in the BaTi0₃ crystals at the Si/BaTi0₃ interface, the dipole moments of the BaTi0₃ crystals of the dielectric layer 144 are oriented along a plane which is generally parallel to the surface of the silicon layer 142 upon which it overlies.

Such a feature is advantageous in that the dielectric constant for the non-polar axis of the tetrahedral unit cell is ten to one-hundred times larger than the dielectric constant along the polar axis. Therefore, since the stored charge in a capacitor is directly proportional to the dielectric constant, more charge can be stored in the capacitor if its polar axis is normal to the applied electric field. Again, for a description of the construction of a structure, such as the capacitor 140, comprised of a thin film of BaTi0₃ overlying a silicon substrate wherein the dipoles of the unit cells of BaTi0₃ are predisposed by the constraints of the underlying silicon lattice to orientations within the plane of the film, reference can be had to the referenced co- pending U.S. Patent Application Ser. No. 08/868,076.

With reference to Figs. 14 and 15, there is illustrated an electro-optic structure 180 having a substrate 182 of silicon, an intermediate thin film 184 of MgO and a thin film layer 186 of the perovskite BaTi0₃ overlying the MgO layer 184. The unit cells of the BaTi0₃ layer 186 are geometrically influenced by the lattice structure of the underlying silicon substrate 182 so that the dipoles of the unit cells of BaTi0₃ are oriented within the plane of the BaTi0₃ film 186 (i.e. parallel to the surface of the silicon substrate 182). In the structure 180, the orientation (and exemplary directions) of the polar axes of the unit cells in the plane of the BaTi0₃ film 186 are indicated with the arrows 178, and the light which is directed through the film 186 is directed therethrough along the indicated "z" direction and is polarized in-plane. As will be apparent herein, the electric field which is applied between electrodes 190 and 192 is arranged at about an ninety degree angle within respect to the direction of light directed through the BaTi0₃ film and forty-five degrees relative to the orientation of the polar axes of the BaTi0₃ unit cells. To construct the electro-optic device 180, reference can be had to the combined teachings of U.S. Patent Nos. 5,225,031 and 5,846,667 (each of which identifies as inventors the named inventors of the instant application) , the disclosures of which are incorporated herein by reference, so that a detailed description is not believed to be necessary. Briefly, however, and in accordance with the teachings of the referenced Patent No. 5,225,031, the MgO film 184 is grown in a layer-by-layer fashion upon the underlying silicon substrate 182 following an initial growth procedure involving the growth of a fraction (e.g. one-fourth) of a monolayer of an alkaline earth metal (e.g. Ba, Sr, Ca or Mg) upon the silicon at appropriate growth conditions to form ASi₂ wherein "A" in this compound is the alkaline earth metal. Upon achieving a MgO film 184 whose thickness is about 0.2-0.3 μm (1000 to 3000 monolayers), steps can be taken in accordance with the teachings of the referenced U.S. Patent No. 5,846,667 to construct the BaTi0₃ film 186 upon the MgO film 184. For example, the build-up of BaTi0₃ is initiated by growing a first single plane layer comprised of Ti0₂ over the surface of the MgO film 184 and then growing a second single plane layer comprised of BaO over the previously-grown Ti0₂ layer. The growth of the BaO plane is followed by the sequential steps of growing a single-plane layer of Ti0₂ directly upon the previously-grown plane of BaO and then growing a single plane layer of BaO upon the previously-grown layer of Tiθ₂ until a desired thickness (e.g. 0.2-0.6 μm) of the BaTi0₃ film 186 is obtained.

Within the electro-optic structure 180, the BaTi0₃ film 186 is intended to serve as a waveguide for light transmitted through the structure 180, while the MgO film 184 serves to optically isolate the BaTi0₃ film 186 from the silicon substrate, as well as provide a stable structure upon which the BaTi0₃ is grown.

As mentioned above, the dipoles of the unit cells of the BaTi0₃ thin film 186 are oriented within the plane of the thin film 186, and the dipoles of the BaTi0₃ unit cells are arranged in this manner because of the in-plane thermoelastic strain induced within the unit cells of the BaTi0₃ film by the silicon. Briefly, the phase transformations and the thermal contractions (i.e. the linear thermal expansion or contraction) of the materials comprising the layers of the structure 180 during cooldown of the structure from the relatively high temperature at which film deposition occurs induces a positive (e.g. tensile) strain across the lattice structure of the BaTi0₃ unit cells within the plane of the film in a manner which affects the ferroelectric characteristics of the BaTi0₃ in a desired manner. To this end, the coefficient of thermal expansion of silicon is smaller than that of BaTi0₃ so that a uniform cooling of a structure comprised of a BaTi0₃ film-on-silicon results in the development of an appreciable in-plane strain within the unit cells of the BaTi0₃ film and the consequential transformation of the BaTi0₃ unit cells of the film to a tetragonal form.

More specifically, when the BaTi0₃ film transforms to the ferroelectric phase at the temperature of about 120 °C, the form of a unit cell of BaTi0₃ is transformed to a tetragonal, or elongated, form wherein the lattice parameter as measured along one edge of the unit cell is larger than the lattice parameter as measured along a second edge of the unit cell wherein the second edge is arranged at a right angle with respect to the first edge. As the BaTi0₃-on-silicon structure is cooled from the deposition temperature, each BaTi0₃ unit cell naturally oriented so that the longest edge the cell is arranged parallel to the plane of the silicon substrate (in a natural attempt to minimize the in-plane strain to which the unit cell is exposed) . This orientation of the unit cells also predisposes the polar axes (and hence the dipole moments) of the BaTi0₃ unit cells in a plane which is substantially parallel to the surface of the substrate.

It is known that the electro-optic response of a single crystal waveguide is effected by such factors as the orientation of the electric field, the orientation of polarization of light directed through the material, and the orientation of the crystal (with respect to the beam directed therethrough). Of the aforementioned factors, the last- mentioned factor (i.e. the orientation of the crystal) relates to the orientation of the dipoles of the crystal. Along the same lines, the electro-optic (EO) response (corresponding to the aforediscussed electro-optic coefficient) for a crystalline material, such as BaTi0₃, is described by a third rank tensor. The largest EO response in the thin film 186 of the structure 180 is achieved when the polarization is along the indicated "x-z" direction and a component of the electric field is in the "x" direction. Accordingly, an electric field is applied between the electrodes 190 and 192 positioned across the film 186, and the applied electric field is arranged at about a ninety degree angle with respect to the direction of light directed through the BaTiθ₃ film and at about a forty-five degree angle with respect to the orientations of the polar axes of the BaTi0₃ unit cells to allow the mode of propagation in the device. Accordingly, the structure 180 can be used as a phase modulator wherein the BaTi0₃ thin film 186 serves as the waveguide through which a beam of light is transmitted and across which an electric field is applied (and appropriately controlled) for the purpose of altering the phase of the light beam transmitted through the BaTi0₃ film.

It follows from the foregoing that embodiments of structure have been described which include a thin film of crystalline oxide which has been grown upon an underlying semiconductor-based substrate and wherein an in-plane strain is induced within the thin film which influences, and alters, the geometric shape of the unit cells of the film. This altering of the geometric shape arranges a directional- dependent quality of the unit cells in a predisposed orientation with respect to the underlying substrate. When these structures are embodied in semiconductor devices which are exposed to an internally-applied or externally-applied field during operation, the predisposed orientation of the directional-dependent quality of the unit cells can beneficially contribute to the operation of the device.

For example, the overlying oxide material couples to the underlying semiconductor material for affecting the electronic capabilities of the substrate. In other words, with the geometry of the unit cells of the anisotropic material constrained so that the directional-dependent qualities of the unit cells are oriented either in a plane which is parallel to the substrate surface or along lines normal to the substrate surface, the coupling of the anisotropic material to the semiconductor substrate enables the substrate to behave in an advantageous and reproducible manner during an electronic application. As discussed above, such advantageous behavior can involve the modulation of charge density and channel current in a transistor. Thus, the structure of this invention can be advantageously embodied in semiconductor devices which use the coupling of the anisotropic thin film material with the underlying semiconductor application during an electronic application.

Although in-plane strain has been induced within the aforedescribed structures 120, 140 or 180 principally by either commensurate strain (e.g. wherein the lattice structure of the unit cells at the substrate/thin film interface are clamped to the underlying lattice structure of the substrate so that the unit cells of the thin film are in a strained or compressed condition as they are forced to conform in size to the lattice parameter of an underlying material) or thermally (wherein the differences in expansion coefficients between the thin film layer and the underlying substrate and the effects of phase transformations during cooling are principally responsible for the induced strain) , it will be understood that in-plane strain can be induced by other means, such as mechanical means (e.g. by a physical bending of the structure) so that a desired in-plane strain is induced at the substrate/thin film interface.

It will be understood that numerous modifications and substitutions can be had to the aforedescribed embodiments of Figs. 12-15 without departing from the spirit of the invention. For example, although aforediscussed embodiments have described as utilizing the perovskites BaTi0₃ or SrTi0₃ as the active thin film layer in a COS device, any crystalline oxide which is capable of exhibiting anisotropic behavior can be employed. For example, other perovskite oxides or spinels or oxides of similarly-related cubic structures can be used as the crystalline oxide thin film wherein the oxide is adapted to exhibit ferroelectric, piezoelectric, pyroelectric, electro-optic, ferromagnetic, anti-ferromagnetic, magneto- optic or large dielectric properties within the structure, (perovskites and spinels are similarly-related in the sense that the unit cells of each oxide possesses a cubic structure.) Further still, although the aforedescribed semiconductor devices 120, 140, 180 have been described as a transistor, a capacitor or an electro-optic device, the principles of the present invention can be embodied in other semiconductor devices wherein the thin film overlayer synergistically couples to the underlying substrate, such as silicon. Thus, the aforedescribed embodiments are intended for the purpose of illustration and not as limitation.

Considering now the electro-optic advantages of the invention, there is shown in Figs. 16 and 17 a monolithic crystalline structure, generally indicated 220, comprised of a substrate 222 of silicon, an intermediate thin film layer

223 of magnesium oxide (MgO) and a thin film layer 224 of the perovskite BaTi0₃ overlying the MgO layer 223. Both the MgO and BaTi0₃ layers 223 and 224 are epitaxially arranged upon the underlying silicon substrate 222. BaTi0₃ is a ferroelectric oxide material which when combined with the silicon substrate 222 in the form of an overlying thin film enables the crystalline structure to take advantage of the semiconductor (as mentioned earlier), as well as the optical properties, of the structure 220. As will be apparent herein, the growth (or formation) of the structure 220 predisposes the electric dipole moments of the BaTi0₃ film 224 in the plane of the film 224 so that the optical properties of the thin film 224 are advantageously affected and so that the structure 220 can be used in waveguide applications. Hence, the BaTi0₃ film

224 enables optical signals to be transmitted wherein the silicon substrate 222 can be used for transmission of electrical signals in a communication system which employs both optical and electrical signals.

As will be apparent herein, it is the influence of the in-plane thermoelastic strain that attends the ferroelectric thin film 224 which predisposes the dipole moments of the film 224 within the plane of the film 224 and advantageously effects the optical properties of the thin film 224.

Furthermore, it will be understood that the BaTi0₃ perovskite film is one of a number of materials having the general formula AB0₃, and that the silicon substrate is one of a number of a Group III, IV or II-VI materials upon which the AB0₃ material can be epitaxially grown for use of the AB0₃ material as a waveguide. For example, whereas the element A in BaTi0₃ (having the general formula AB0₃) is Ba, another material, such as the Group IVA elements Zr or Hf , can provide the element A in alternative forms of a AB0₃ material capable of being epitaxially grown upon the substrate in accordance with the broad principles of the present invention. A detailed discussion of the crystalline characteristics of the material having the general formula AB0₃ and in particular, the general series of structures designated as (AO)_n(A^/BO)_n in which n and m are the integer repeats of single plane oxide layers, and the relationship between the successively-grown layers of the AB0₃ and the underlying substrate can be found in our above-referenced U.S. Patent No. 5,835,270, the disclosure of which is incorporated herein by reference.

A process used to construct the structure 220 is set forth in the combined teachings of U.S. Patent Nos. 5,225,031 and 5,846,667 (each of which identifies as inventors the named inventors of the instant application) , the disclosures of which are incorporated herein by reference, so that a detailed description of the construction process is not believed to be necessary. Briefly, however, and in accordance with the teachings of the referenced Patent No. 5,225,031, the MgO film 223 is grown in a layer-by-layer fashion upon the underlying silicon substrate 222 with molecular beam epitaxy (MBE) methods and including an initial step of depositing a fraction of a monolayer (e.g. one-fourth of a monolayer) of the metal Mg atop of the silicon substrate.

Upon achieving a MgO film 223 whose thickness is about 0.2-0.3 μm (1000 to 3000 monolayers), steps are taken in accordance with the teachings of the referenced U.S. Patent No. 5,846,667 to construct the BaTi0₃ film 224 upon the underlying MgO film 223. The build-up of BaTi0₃ is initiated by growing a first epitaxial (preferably commensurate) single- plane layer comprised of Ti0₂ over the surface of the MgO film 223 and then growing a second epitaxial (preferably commensurate) single-plane layer comprised of BaO over the previously-grown Ti0₂ layer. The growth of the BaO plane is followed by the sequential steps of growing a single-plane layer of Ti0₂ directly upon the previously-grown plane of BaO and then growing a single-plane layer of BaO upon the previously-grown layer of Ti0₂ until a desired thickness (e.g. 0.2-0.6 μm) of the BaTi0₃ film 224 is obtained. Again, for a more detailed description of the growth of the MgO film 223 atop the silicon substrate 222 and the growth of the BaTi0₃ film 224 atop the MgO film 223, reference can be had to the referenced Patent Nos. 5,225,031 and 5,846,667, the disclosures of which are incorporated herein by reference. As will be apparent herein, since the BaTi0₃ film

224 is intended to serve as a waveguide for light transmitted through the structure 220, the MgO film 223 serves to optically isolate the BaTi0₃ film 224 from the silicon substrate 222, as well as provide a stable structure upon which the BaTi0₃ is grown. The index of refraction of BaTi0₃ is about 2.3 whereas the index of refraction of silicon is on the order of about 4.0. On the other hand, the index of refraction of MgO is about 1.74 so that by positioning the MgO film 223 between the silicon substrate 222 and the BaTi0₃ film 224, the amount of light which is transmitted along the BaTi0₃ film 224 and which could otherwise be lost into the silicon is reduced. Accordingly, the thickness of the MgO film 223 is selected to provide satisfactory optical isolation for the BaTi0₃ film. During the process of constructing a structure whose composition is comparable to that of the structure 220 of Fig. 16, the ferroelectric characteristics of the structure are strongly affected by the phase transformations and the thermal contraction (i.e. the linear thermal expansion or contraction) of the materials comprising the layers of the structure. In this regard, the coefficient of thermal expansion of silicon (Si) is smaller than that of BaTi0₃ so that a uniform cooling of a structure comprised of a BaTi0₃ film-on-silicon results in the development of an appreciable in-plane strain within the unit cells of the BaTi0₃ film and the consequential transformation of the BaTi0₃ unit cells of the film to a tetragonal form as will be described herein.

Along the lines of the foregoing, there is shown in Fig. 18 a graph which plots the lattice parameter of the BaTi0₃ and silicon unit cell in a BaTi0₃-on-silicon structure versus temperature. It can be seen that when the BaTi0₃ film is grown upon the silicon at a temperature of about 600 °C, the lattice parameter of the BaTi0₃ takes on the value that it has in bulk. However, as the structure begins to cool from the growth temperature, the in-plane lattice parameter of the thin-film BaTi0₃ contracts at the same rate as that of the silicon, and an in-plane strain (i.e. a change in length, l , divided by the original length, 1₀) begins to develop in the BaTi0₃ film. It should be noted that the developed in-plane strain is uniform throughout the BaTi0₃ film. When the BaTi0₃ film transforms to the ferroelectric phase at the temperature of about 120°C, the form of the BaTi0₃ unit cell is changed from a cubic shape to a tetragonal, or elongated, form wherein the lattice parameter as measured along the indicated c-axis is larger than that as measured along the indicated a-axis. Furthermore, as the temperature of the structure continues to cool from 120°C, the lattice parameter as measured along the c-axis increases while the a-axis lattice parameter decreases. Meanwhile, the volume of each BaTi0₃ unit cell as a function of temperature is equal to the square of the cell length along the a-axis times the cell length along the c-axis (i.e. V(T) = l_c(l_a)²).

It is noteworthy from the Fig. 18 graph that at about room temperature, the lattice parameter of BaTi0₃ as measured along the c-axis is larger than it is as measured along the a-axis. Consequently, if the BaTi0₃ unit cell is arranged upon the substrate so that its a-axis is oriented parallel to the surface of the substrate, the in-plane strain induced within the BaTi0₃ unit cell is larger than would be the case if the c-axis were oriented parallel to the substrate surface because the difference between the lattice parameter of the silicon and that of the BaTi0₃ crystal when measured along the a-axis is larger than it is when measured along the c-axis. However, as the BaTi0₃-on-structure is cooled and the BaTi0₃ unit cells experience a transformation in shape, the unit cells tend to naturally orient themselves so that the c- axis is parallel to the surface of the substrate and thereby maintain the in-plane strain as small as possible. It follows that the BaTi0₃ unit cells in the film of a BaTi0₃-on-silicon structure which has been cooled from the growth temperature of about 600°C to a lower temperature of about room temperature orient themselves upon the silicon so that the longest dimension of the unit cell is arranged parallel to the plane of the silicon substrate. The orientation of the BaTi0₃ unit cells, which are naturally oriented so that the (longer) c-axis of the ferroelectric unit cell is parallel to the silicon substrate, also predispose the polar axes (and hence the dipole moments) of the BaTi0₃ unit cells in a plane which is substantially parallel to the surface of the substrate.

Although the foregoing discussion has focused upon the characteristics of BaTi0₃ unit cells in a BaTi0₃-on- silicon structure, the characteristics of BaTi0₃ unit cells are the same as aforedescribed when a thin film of MgO is interposed between the silicon substrate and the BaTi0₃ film. The thickness of the silicon substrate is so large in comparison to that of either the MgO or BaTi0₃ film that it is the strain of the silicon which dominates during the contraction of the overlying films, rather than vise-versa. In other words, the interposed layer of MgO is simply too thin to alter the characteristics of contraction of the BaTi0₃ when overlying silicon from those previously described. Therefore, in a structure like that of the structure 220 of Figs. 16 and 17 wherein a thin film 223 of MgO is interposed between a silicon substrate 222 and a film 224 of BaTi0₃ and the resulting structure is cooled from the growth temperature of 600°C, the BaTi0₃ unit cells are arranged over the silicon substrate so that the longest dimensions thereof are oriented parallel to the plane of the silicon substrate. Consequently, the polar axes (and the dipole moments) of the BaTi0₃ unit cells in a structure like that of the structure 220 of Figs. 16 and 17 are also arranged in a plane which is substantially parallel to the surface of the silicon substrate.

For use of the BaTi0₃ film of the structure described herein in a waveguide application, such as in a phase modulator or interferometer, it is desirable that the electro-optic coefficient (designated "r" in the known Pockels Effect equation) be optimized (i.e. be as large as practically possible) to effect a greater phase shift in a beam of light directed through the BaTi0₃ film in response to a change in the electric field applied across the film. In this connection, it is known that the electro-optic coefficient of a single crystal waveguide is effected by such factors as the orientation of the electric field, the orientation of polarization of light directed through the material, and the orientation of the crystal (with respect to the beam directed therethrough). Of the aforementioned factors, the last- mentioned factor (i.e. the orientation of the crystal) relates to the orientation of the dipoles of the crystal.

Along the same lines, the electro-optic (EO) response (corresponding to the aforediscussed electro-optic coefficient) for a crystalline material, such as BaTi0₃, is described by a third rank tensor. The elements of the tensor relate the change in refractive index, Δn, for the crystalline material to the applied electric field, E, in accordance with the following equation:

0 0 13 0 0 J5 0 0 '33

Δn^-j-n 0 ^■fi. (1)

^LSI 0

0

0 0 0

The EO response is described by a third rank tensor because there exists three different sets of directions that can describe the EO response figure of merit, r_M. These three sets of directions correspond to the aforementioned 1) direction of the applied electric field, 2) direction of the polarization of light directed through the material, and 3) orientation, or polar axis, of the crystal with respect to the beam directed therethrough. For a tetragonal unit cell, such as BaTi0₃ of the aforedescribed (cooled) BaTi0₃-on-silicon structure, there are three orthogonal directions, n, in which the electric field can be oriented. More specifically, n=l

SUBSTTTUTESHEET(RULE26) is in the "x" direction, n=2 is in the "y" direction, and n=3 is in the "z" direction. Electric fields which are applied in other directions are just superpositions of these three x, y and z orthogonal directions. For this same crystal type (a tetragonal unit cell) , there are four distinct directions along which light can be polarized. They are, as for the electric field directions, the x, y and z directions having a subscript m of 1 , 2 and 3. The other two come from the fact that the unit cell can be polarized up or down the c-axis of the unit cell. The two optical polarizations have equal components along the y and z directions and are assigned the subscripts m=5 and 4. As can be seen in the third rank tensor for BaTi0₃ (the matrix appearing between the brackets in Equation ( 1 ) above) , the largest EO response is achieved when the polarization is along the x-z direction and a component of the electric field is in the x direction, r₅₁.

To take advantage of the large r₅₁ in the thin film waveguide structures, the directions of the fields have to be oriented in a prescribed manner relative to the ferroelectric domains that form in the thin film structure. The domains that form in the BaTi0₃ unit cells of the aforedescribed BaTi0₃-on-silicon structure are determined by the aforediscussed differences in thermal expansion (or contraction) between BaTi0₃ and silicon. The polar axis of the particular domain structure that forms in the BaTi0₃ can be oriented in any of four directions (corresponding to directions along either of the x or y coordinates), but these polar axes are all arranged parallel to the plane of the silicon substrate due to the thermal strain influences on the cubic-tetragonal phase transformation of the BaTi0₃ on silicon.

A BaTi0₃-on-silicon device which achieves practical realization of the optimum EO and the electro-optic coefficient is shown in Figs. 19 and 20 and indicated 270. The device 270 includes a substrate 272 of silicon, and intermediate thin film layer 273 of magnesium oxide (MgO) and a thin film layer 274 of the perovskite BaTi0₃ overlying the MgO layer 223. In the device 270, the orientation (and exemplary directions) of the polar axes of the plane of the BaTi0₃ film are indicated with the arrows 276, the light directed through the BaTi0₃ film 274 is directed therethrough along the indicated "x-z" direction and is polarized in-plane. An electric field is applied between electrodes 278 and 280 positioned across the film 274, and the applied electric field is arranged at about a 90° angle with respect to the direction of light directed through the BaTi0₃ film and at about a 45° angle with respect to the orientations of the polar axes of the BaTi0₃ unit cells. The mode of propagation in the device 270 is allowed and is called a transverse electric mode.

It is noteworthy to point out that arbitrary polarizations of the BaTi0₃ unit cells of the film 274 cannot be supported by a thin-film waveguide structure. The polarization must either be in the plane of the film or (mostly) out of the plane. Because of this limitation, the in-plane poling induced by the thermal expansion misfit is the optimum poling that provides access to the large r₅₁ coefficient of BaTi0₃.

It follows from the foregoing that a BaTi0₃-on-Si structure has been described which has beneficial optical qualities. While BaTi0₃, itself, is known to provide a waveguide structure, its utilization as a waveguide structure when built upon a silicon wafer (or any other substrate) has not heretofore been realized. Once constructed, the resultant BaTi0₃-on-Si structure can be used as a phase modulator wherein the BaTi0₃ film serves as the waveguide through which a beam of light is transmitted and across which an electric field is applied (and appropriately controlled) for the purpose of altering the phase of the light beam transmitted through the BaTi0₃ film. By utilizing the same phase modulator in one branch of an interferometer, the phase modulator can function as an intensity modulator.

It will be understood that numerous modifications and substitutions can be had to the aforedescribed embodiment without departing from the spirit of the invention. For example, arrangements within devices other than that of Figs. 19 and 20 can be fabricated to optimize coupling of the electric field to the ferroelectric film as well as match the microwave coupling of the electric field to the ferroelectric film as well as match the microwave phase velocity traveling along the electrodes with the speed of the guided light.

Still further, although the BaTi0₃-on-Si structure has been described above as including an intermediate layer of MgO having a thickness which is sufficient to optically isolate the BaTi0₃ from the underlying silicon substrate, it may be desirable for some applications that the intermediate MgO layer be thin enough to permit some amount of the transmitted light to be lost into the silicon substrate. In other words, the thickness of the intermediate MgO should be considered as a design parameter chosen with regard to the application in which the structure is to be used.

Further still, although foregoing exemplary structures have been described as including BaTi0₃ in a pure form, the principles of this invention are intended to cover applications wherein the ferroelectric perovskite, such as BaTi0₃, has been doped with a material, such as Erbium, thereby enabling the perovskite to amplify light transmitted therethrough due to the optic interactions of transmitted light with the doping material. Of course, a light amplifier of such construction may find application as a gain component of an electro-optic system (which gain component can compensate for loss of light transmitted around a bend) or even a laser. Such a laser device includes 1) a channel waveguide of low loss, 2) mirrors on both ends, or gratings, to form a laser cavity, 3) a pump light source or, alternatively, an electrical pumping system consisting of other dopants that act as reservoirs of electrons and couple to the lasing levels, and 4) electrodes, as in a phase modulator, to vary the index of refraction within the laser cavity and perform a tuning function. While these (four) laser features have been proposed in the art, our contribution here permits the integration of such a laser on silicon.

Further still, the thickness of the light-guiding layer in a structure of this invention can be selected for transmitting different modes of light which may, for example, enable the transmission of several types of signals by the same light-guiding structure. Alternatively, the materials of the light-guiding structure render possible the construction of a structure wherein the optimizing of a structure to provide a single mode guide or a multiple mode (i.e. multiplex) guide.

Accordingly, the aforedescribed embodiments are intended for the purpose of illustration and not as limitation.

Claims

1. A structure comprising: a substrate of semiconductor-based material having a surface; and a thin film of a crystalline oxide epitaxially overlying the substrate surface wherein the crystalline oxide includes unit cells which exhibit or are capable of exhibiting anisotropic behavior having a directional-dependent quality and the thin film is exposed to in-plane strain at the substrate/thin film interface so that substantially every one of the unit cells of the thin film have a geometric shape which is influenced by the in-plane strain so that the directional-dependent quality of each unit cell is arranged in a predisposed orientation relative to the substrate surface.

2. The structure as defined in Claim 1 wherein the directional-dependent quality of each unit cell of the thin film is oriented in either a plane which is parallel to the substrate surface or along lines normal to the substrate surface.

3. The structure as defined in Claim 2 wherein the thin film material is in a state of biaxial tension so that the geometric shape of each unit cell of the thin film is characterized by a width as measured in the plane of the thin film which is greater than its height as measured normal to the plane of the thin film and so that the directional- dependent quality of each unit cell is arranged in a plane which is parallel to the substrate surface.

4. The structure as defined in Claim 2 wherein the thin film material is in a state of biaxial compression so that the geometric shape of each unit cell of the thin film is characterized by a width as measured in the plane of the thin film which is smaller than its height as measured normal to the plane of the thin film and so that the directional- dependent quality of each unit cell is arranged along a line which is generally normal to the substrate surface.

5. The structure as defined in Claim 2 wherein the thin film material is a ferroelectric oxide having unit cells whose dipole moment provides the directional-dependent quality of the unit cells, and the predisposed orientation of the geometries of the unit cells arranges the dipole moments of substantially all of the unit cells in either a plane which is parallel to the substrate surface or along lines normal to the substrate surface.

6. The structure as defined in Claim 1 wherein the in-plane strain induced within the unit cells is at least one of a thermal strain, a commensurate strain or a mechanical strain.

7. The structure as defined in Claim 1 wherein the crystalline oxide synergistically couples to the underlying semiconductor-based substrate so that the application of an externally-applied or an internally-applied field to the structure affects the semiconductor characteristics of the substrate.

8. The structure as defined in Claim 8 wherein the crystalline oxide is a perovskite, a spinel or an oxide of similarly-related cubic structure which is adapted to exhibit ferroelectric, piezoelectric, pyroelectric, electro-optic, ferromagnetic, antiferromagnetic, magneto-optic or large dielectric properties within the structure.

9. The structure as defined in Claim l wherein the crystalline oxide provides a ferroelectric, optically-clear thin film overlying the surface of the substrate and the dipole moments of substantially every one of the unit cells of the thin film are arranged substantially parallel to the surface of the substrate.

10. A device for a semiconductor application wherein the device includes a structure comprising: a substrate of semiconductor-based material having a surface; and a thin film of crystalline oxide epitaxially overlying the substrate surface wherein each unit cell of the crystalline oxide exhibits or is capable of exhibiting anisotropic behavior having a directional-dependent quality and wherein substantially all of the unit cells of the thin film are exposed to an in-plane strain which influences the geometric shape of the unit cells, and the influence upon the geometric shape of the unit cells imparts a predisposed orientation to the directional-dependent quality of the unit cells, and wherein the oxide film couples to the underlying semiconductor-based material for use of the semiconductor capabilities of the substrate during a semiconductor application.

11. A structure comprising: a substrate of semiconductor-based material having a surface; a thin film of anisotropic crystalline oxide epitaxially overlying the substrate surface wherein the thin film consists of unit cells of an AB0₃ material having at least one AO constituent plane and at least one B0₂ constituent plane and wherein the film is arranged upon the surface of the substrate so that a first single plane consisting of a single atomic layer of said AO constituent of the AB0₃ material overlies the surface of the substrate and a second single plane consisting of a single atomic layer of said B0₂ constituent of the AB0₃ material overlies the first single plane of AO and the AO and B0₂ constituent planes form unit cells of the AB0₃ material wherein each unit cell possesses or is capable of possessing a directional-dependent quality, and wherein the unit cells of the crystalline oxide thin film is exposed to an in-plane strain which influences the geometry of the unit cells so that substantially every unit cell of the thin film has a geometry which is conformed to a tetragonal shape and so that the tetragonal axis of each conformed unit cell is arranged in a predisposed orientation relative to the substrate surface and so that the predisposed orientation of the geometries of the thin film orients any directional-dependent quality of substantially every one of the thin film unit cells in either a plane which is parallel to the substrate surface or along lines normal to the substrate surface.

12. The structure as defined in Claim 11 wherein the thin film material is a ferroelectric oxide having unit cells whose dipole moment provides the directional-dependent quality of the unit cells, and the predisposed orientation of the geometry of the thin film arranges the dipole moments of substantially all of the unit cells in either a plane which is parallel to the substrate surface or along lines normal to the substrate surface.

13. The structure as defined in Claim 11 wherein the semiconductor substrate includes silicon and the thin film includes a perovskite of the BaTi0₃ class of perovskites.

14. A device for a semiconductor application wherein the semiconductor capabilities of the device are required to be utilized, the device including a structure comprising: a substrate of semiconductor-based material having a surface; a thin film of anisotropic crystalline material commensurately overlying the substrate surface so as to provide, with the substrate material, a single crystal and coupling to the underlying semiconductor-based material wherein the thin film is comprised of unit cells commensurately arranged upon the substrate surface and substantially all of the unit cells of the thin film have a geometric form of tetragonal shape and each unit cell of the thin film has a tetragonal axis which is arranged along lines normal to the substrate surface so that the polar axes of substantially all of the unit cells of the thin film are arranged along lines normal to the substrate surface.

15. The device as defined in Claim 14 wherein the device is a ferroelectric field effect transistor.

16. The device as defined in Claim 14 wherein the semiconductor-based material is silicon, germanium or a silicon-germanium alloy.

17. The device as defined in Claim 16 wherein the device is a field effect transistor.

18. A crystalline structure comprising: a semiconductor substrate having a surface; a ferroelectric, optically-clear thin film overlying the surface of the substrate wherein at least the first few atomic layers of the thin film are commensurate with the semiconductor substrate and so that substantially all of the dipole moments associated with the ferroelectric film are arranged substantially parallel to the surface of the substrate to enhance the electro-optic qualities of the structure.

19. The structure as defined in Claim 18 wherein there is interposed between the semiconductor substrate and the ferroelectric thin film an intermediate thin film which provides an optical isolation layer between the substrate and the ferroelectric thin film.

20. The structure as defined in Claim 19 wherein the semiconductor substrate includes silicon and the ferroelectric thin film includes a perovskite of the BaTi0₃ class of perovskites.

21. The structure as defined in Claim 20 further including a thin film layer of MgO interposed between the silicon substrate and the thin film of perovskite.

22. The structure as defined in Claim 18 wherein the ferroelectric thin film is doped with an element which alters the optical properties of the thin film.

23. The structure as defined in Claim 22 wherein the doping element is Erbium for imparting optical amplification qualities to the thin film.