EP1104494A1 - Regulation de l'anisotropie du cristal destinees aux oxydes de perovskite sur des substrats a base de semi-conducteurs - Google Patents

Regulation de l'anisotropie du cristal destinees aux oxydes de perovskite sur des substrats a base de semi-conducteurs

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Publication number
EP1104494A1
EP1104494A1 EP99937553A EP99937553A EP1104494A1 EP 1104494 A1 EP1104494 A1 EP 1104494A1 EP 99937553 A EP99937553 A EP 99937553A EP 99937553 A EP99937553 A EP 99937553A EP 1104494 A1 EP1104494 A1 EP 1104494A1
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EP
European Patent Office
Prior art keywords
thin film
substrate
unit cells
bati0
plane
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP99937553A
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German (de)
English (en)
Inventor
Rodney Allen Mckee
Frederick Joseph Walker
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UT Battelle LLC
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UT Battelle LLC
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Filing date
Publication date
Priority claimed from US09/126,527 external-priority patent/US6093242A/en
Priority claimed from US09/126,526 external-priority patent/US6023082A/en
Priority claimed from US09/126,129 external-priority patent/US6103008A/en
Application filed by UT Battelle LLC filed Critical UT Battelle LLC
Publication of EP1104494A1 publication Critical patent/EP1104494A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/03—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B25/00—Single-crystal growth by chemical reaction of reactive gases, e.g. chemical vapour-deposition growth
    • C30B25/02—Epitaxial-layer growth
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02107—Forming insulating materials on a substrate
    • H01L21/02109—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates
    • H01L21/02112—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer
    • H01L21/02172—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing at least one metal element, e.g. metal oxides, metal nitrides, metal oxynitrides or metal carbides
    • H01L21/02197—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing at least one metal element, e.g. metal oxides, metal nitrides, metal oxynitrides or metal carbides the material having a perovskite structure, e.g. BaTiO3
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/40—Electrodes ; Multistep manufacturing processes therefor
    • H01L29/401—Multistep manufacturing processes
    • H01L29/4011—Multistep manufacturing processes for data storage electrodes
    • H01L29/40111—Multistep manufacturing processes for data storage electrodes the electrodes comprising a layer which is used for its ferroelectric properties
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/40—Electrodes ; Multistep manufacturing processes therefor
    • H01L29/43—Electrodes ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/49—Metal-insulator-semiconductor electrodes, e.g. gates of MOSFET
    • H01L29/51—Insulating materials associated therewith
    • H01L29/516—Insulating materials associated therewith with at least one ferroelectric layer
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B2006/12133—Functions
    • G02B2006/12142—Modulator
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B2006/12133—Functions
    • G02B2006/12159—Interferometer

Definitions

  • 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.
  • Si0 2 is treated as an isotropic material with no crystallographic anisotropy in its response to internally or externally applied electric fields.
  • 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.
  • 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.
  • 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 AB0 3 , such as, by way of example and not as limitation, a perovskite, and in particular a perovskite in the BaTi0 3 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.
  • 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.
  • 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.
  • 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 .
  • 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 BaTi0 3 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 BaTi0 3 .
  • 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 BaTi0 3 -including structures as a function of temperature.
  • Fig. 6 is a graph plotting thermal strain of BaTi0 3 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 SrTi0 3 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 SrTi0 3 which has been misshapened due to compression.
  • Fig. 10 is a Z-contrast image of SrTi0 3 on (100) silicon.
  • Fig. 11 is a plot of capacitance versus voltage for a SrTi0 3 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.
  • FFET ferroelectric field effect transistor
  • 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 BaTi0 3 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 BaTi0 3 in bulk form and BaTi0 3 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
  • a monolithic crystalline structure generally indicated 20, comprised of a substrate 22 of silicon and a layup 24 of the perovskite BaTi0 3 epitaxially covering the surface of the substrate 22.
  • BaTi0 3 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 BaTi0 3 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 BaTi ⁇ 3 in the layup 24 is arranged in a predisposed orientation relative to the substrate surface.
  • the directional- dependent quality of each BaTi0 3 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 BaTi0 3 unit cell which is arranged in a predisposed orientation relative to the substrate surface.
  • 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.
  • 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.725 Sr 0 . 275 0.
  • 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 Ba 0 . 725 Sr 0 The Ba 0 . 725 Sr 0 .
  • a thin perovskite (template) film 30 of Ca 0 ⁇ 4 Sr 0 . 36 Ti0 3 and then to cover the film 30 with the desired (multi-stratum) film 32 of BaTi0 3 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 BaTi0 3 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.
  • the thin film materials are selected so
  • 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.
  • the coupling of the unit cells of BaTi0 3 to the underlying substrate in this case, silicon
  • the coefficient of thermal expansion of silicon is smaller than that of BaTi0 3 so that a uniform heating (or cooling) of the resultant structure results in a tendency of the BaTi0 3 film to misshapen, as will be described herein, and the tendency for an appreciable in-plane strain to develop within the BaTi0 3 film.
  • the relative thermal expansion (or contraction) between silicon and BaTi0 3 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 BaTi0 3 film atop the silicon.
  • the steps involving the deposition of BaTi0 3 are carried out at a relatively high (growth) temperature of about 600°C, and at this temperature, the deposited film of BaTi0 3 is substantially free of in-plane strain.
  • 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 BaTi0 3 come into play.
  • Phase transformations may or may not occur within the BaTi0 3 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.
  • the differences in thermal expansion (or contraction) of the BaTi0 3 film and the silicon effects a greater shrinkage of the BaTi ⁇ 3 film than the silicon.
  • the resultant structure is cooled from the deposition temperature of about 600°C, the number of BaTi0 3 unit cells per unit area at the Si/BaTi0 3 interface remain proportional to the number of Si unit cells per unit area at the Si/BaTi0 3 interface while the atoms of the BaTi0 3 film tend to move closer together than do the atoms of the silicon substrate.
  • the BaTi0 3 film attempts to contract more than the silicon, it is constrained by the constraint of the film of BaTi0 3 at the Si/BaTi0 3 interface.
  • the BaTi0 3 film upon reaching the lower temperature, e.g. room temperature, the BaTi0 3 film is constrained to a larger in-plane area than it would otherwise if it were not so constrained. Consequently, the contraction of the BaTi0 3 film effects a shortening of the out-of-plane lattice parameter of the BaTi0 3 film as a path is traced therethrough from the surface of the silicon.
  • the lattice of the BaTi0 3 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.
  • this induced strain tends to misshapen the form of the BaTi0 3 unit cell from a cubic form, as depicted in Fig.
  • Fig. 4 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.
  • 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.
  • amorphous oxides do not behave in the aforedescribed manner.
  • 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.
  • amorphous oxides do not couple to the underlying substrate. Accordingly, the instant invention is advantageous in this respect.
  • Fig. 6 a plot of the in-plane thermal strain induced upon a thin film of BaTi0 3 grown upon a substrate of silicon versus the temperature of the structure.
  • the in-plane strain induced within the BaTi0 3 film of the BaTi0 3 /Si structure increases as the temperature falls from a temperature of about 1030°K.
  • a cubic unit cell of an unpoled ferroelectric material such as the cubic BaTi0 3 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.
  • 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).
  • the in-plane strain induced within the BaTi0 3 unit cells of the layup 24 of the structure 20 and the consequential geometric misshapening of the unit cells of BaTi0 3 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.
  • the in-plane strain induced within the BaTi0 3 layup limits the permissible orientations of the dipole moments in the BaTi0 3 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 BaTi0 3 unit cells in the layup 24 are arranged in a plane oriented parallel to the substrate surface 26.
  • a structure 60 comprising a silicon substrate 62 and a commensurate layup 64 of the perovskite SrTi0 3 covering the substrate surface.
  • the structure 60 of Figs. 7 and 8 is a member of our general series of commensurate structures designated as (A0) n (A / B0 3 ) ⁇ n described earlier.
  • 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.
  • an appreciable portion of the SrTi0 3 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.
  • 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 SrTi0 3 to provide the layup and thus the resultant structure 60.
  • a thin alkaline earth oxide film 68 of one monolayer thickness followeding the deposition of a fraction of a monolayer of an alkaline earth oxide metal
  • a thin perovskite (template) film 70 of any desired thickness (i.e. one monolayer or greater)
  • the perovskite film 70 with the desired (multi- stratum) film 72 of SrTi0 3 to provide the layup and thus the resultant structure
  • the SrTi0 3 unit cells are rotated 45 ° with respect to the unit cells of the underlying silicon substrate (as the lattice parameter [3.91 angstroms] of SrTi0 3 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 SrTi0 3 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.
  • 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.
  • 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.
  • the 3.84 angstrom figure is what the lattice parameter of each unit cell of SrTi ⁇ 3 seeks to match in the structure 60. Therefore, at room temperature, the SrTi0 3 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 SrTi0 3 lattice influences the geometry of the SrTi0 3 unit cells.
  • the strain which exists at the silicon/film interface exerts an in-plane compression within the SrTi0 3 so that the SrTi0 3 unit cells are deformed out-of- plane.
  • the geometry of the SrTi0 3 distorts to a condition (as best illustrated by the SrTi0 3 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.
  • 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.
  • 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.
  • 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).
  • a strain i.e. a positive strain in one instance and a negative strain in the other instance
  • a strain 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.
  • 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%
  • 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.
  • 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.
  • 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.
  • 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.
  • MOS metal-oxide-semiconductor
  • a metal-oxide-semiconductor (MOS) capacitor has been constructed using the perovskite SrTi0 3 (as an alternative to amorphous Si0 2 ) and wherein its SrTi0 3 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) SrTi0 3 // (001) Si and SrTiO 3 [110]//Si[100] .
  • On the left side of the image is an insert model of the perovskite/silicon projection.
  • 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
  • the equivalent oxide thickness, t ⁇ q can be defined for a metal-oxide-semiconductor (MOS) capacitor as:
  • e aloa and e 0 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/um 2 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 /cm 2 .
  • 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.
  • COS crystalline oxides-on-silicon
  • 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.
  • an 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.
  • 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.
  • 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.
  • Fig. 12 a schematic cross section of a fragment of a ferroelectric field effect transistor (FFET), indicated 120, embodying features of the present invention.
  • the transistor 120 includes a base, or substrate 122 of silicon, a source electrode 124, a drain electrode 126, and a gate dielectric 128.
  • a thin film layer 130 of the perovskite BaTi0 3 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 BaTi0 3 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.
  • COS crystalline oxide-on-silicon
  • 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) .
  • unit cells of the perovskite oxide BaTi0 3 when in a crystalline form, are anisotropic in that its unit cells possess directional-dependent qualities, such as a dipole moment.
  • previous semiconductor devices have not relied upon anisotropic response of the gate oxide.
  • the thin film 130 of BaTi0 3 consists of BaTi0 3 in a crystalline form and the unit cells of BaTi0 3 are arranged upon the silicon substrate 122 so that the dipole moments of the unit cells are arranged in a limited number of directions.
  • the dipole moments of substantially all of the unit cells of BaTi ⁇ 3 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.
  • the thin film of BaTi0 3 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.
  • the transistor 120 For purposes of constructing the transistor 120 (and, in particular, the build-up of BaTi0 3 atop silicon so that the unit cells of BaTi0 3 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 Ba 0 . 725 Sr 0 .
  • COS crystalline-on-silicon
  • each of the alkaline earth oxide film and the template film and at least the first few initial layers of the BaTi0 3 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.
  • the template film of SrTi0 3 will induce a -2% strain in the BaTi0 3 unit cells at the interface with the BaTi0 3 film, while the omission of the template film of SrTi0 3 (between the Ba o . 725 Sr 0 . 275 0 and the BaTi0 3 ) will induce a strain of -4% in BaTi0 3 unit cells. Accordingly, in an alternative embodiment of a COS structure in which greater strain is desired to be induced within the BaTi0 3 unit cells, the SrTi0 3 template can be omitted.
  • the negative strain which is induced within the BaTi0 3 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 . 725 Sr o . 275 0 is 0.543 nm while the lattice parameter of SrTi0 3 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 . 725 Sr o .
  • the unit cells of the SrTi0 3 film have an orientation which is rotated 45° with respect to the underlying unit cells of Ba o . 725 Sr o . 27S 0 and are in a state of negative strain (i.e. compression) due to the difference between the lattice parameter (0.392 nm) of SrTi0 3 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).
  • the lattice parameter of BaTi0 3 is about 0.4 nm so that unit cells of BaTi0 3 are also in a state of negative strain (i.e. compression) as they overlie the SrTi0 3 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.
  • each unit cell of BaTi0 3 at the SrTi0 3 /BaTi0 3 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.
  • a lower temperature e.g.
  • the unit cells of the BaTi0 3 film adjacent the Si/BaTi0 3 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 BaTi0 3 effects a lengthening of the out-of-plane lattice parameter of the unit cells of the BaTi0 3 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.
  • in-plane mechanical restraint through the BaTi0 3 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.
  • 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.
  • 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
  • FET field-effect transistor
  • the perovskite BaTi0 3 is grown commensurately upon a silicon substrate in such a manner so that the unit cells of the BaTi0 3 are exposed to a positive (i.e. tensile-strained) in-plane strain which misshapens the BaTi0 3 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).
  • a positive (i.e. tensile-strained) in-plane strain which misshapens the BaTi0 3 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).
  • the edges of selected regions of the BaTi0 3 thin film are either cut away or otherwise treated to relieve some of the in-plane strain to which the unit cells of BaTi0 3 is exposed.
  • 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.
  • the in-strain e.g. either positive or negative strain
  • the crystalline form of SrTi0 3 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.
  • the dielectric constant will exhibit uniaxial behavior associated with the tetragonal distortion.
  • SrTi0 3 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/SrTi0 3 interface distorts the geometric (normally cubic) shape of the unit cells of the SrTi0 3 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/SrTi0 3 interface, the electric (or dielectric) response of the resulting structure can be controlled.
  • 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 BaTi0 3 which are in superposed relationship and which are sandwiched between a gate 146 and a ground terminal 148.
  • DRAM dynamic random access memory
  • 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.
  • the dipole moments of substantially all of the unit cells of the BaTi0 3 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 BaTi0 3 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/Bi0 3 interface which is responsible for orienting the dipole moments of the BaTi0 3 unit cells parallel to the substrate surface in the capacitor 140 of Fig. 13.
  • the dipole moments of the BaTi0 3 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.
  • 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 BaTi0 3 overlying the MgO layer 184.
  • the unit cells of the BaTi0 3 layer 186 are geometrically influenced by the lattice structure of the underlying silicon substrate 182 so that the dipoles of the unit cells of BaTi0 3 are oriented within the plane of the BaTi0 3 film 186 (i.e. parallel to the surface of the silicon substrate 182).
  • the orientation (and exemplary directions) of the polar axes of the unit cells in the plane of the BaTi0 3 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.
  • 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 BaTi0 3 film and forty-five degrees relative to the orientation of the polar axes of the BaTi0 3 unit cells.
  • 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 ASi 2 wherein "A" in this compound is the alkaline earth metal.
  • a fraction e.g. one-fourth
  • an alkaline earth metal e.g. Ba, Sr, Ca or Mg
  • steps can be taken in accordance with the teachings of the referenced U.S. Patent No. 5,846,667 to construct the BaTi0 3 film 186 upon the MgO film 184.
  • the build-up of BaTi0 3 is initiated by growing a first single plane layer comprised of Ti0 2 over the surface of the MgO film 184 and then growing a second single plane layer comprised of BaO over the previously-grown Ti0 2 layer.
  • the growth of the BaO plane is followed by the sequential steps of growing a single-plane layer of Ti0 2 directly upon the previously-grown plane of BaO and then growing a single plane layer of BaO upon the previously-grown layer of Ti ⁇ 2 until a desired thickness (e.g. 0.2-0.6 ⁇ m) of the BaTi0 3 film 186 is obtained.
  • a desired thickness e.g. 0.2-0.6 ⁇ m
  • the BaTi0 3 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 BaTi0 3 film 186 from the silicon substrate, as well as provide a stable structure upon which the BaTi0 3 is grown.
  • the dipoles of the unit cells of the BaTi0 3 thin film 186 are oriented within the plane of the thin film 186, and the dipoles of the BaTi0 3 unit cells are arranged in this manner because of the in-plane thermoelastic strain induced within the unit cells of the BaTi0 3 film by the silicon.
  • 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.
  • the coefficient of thermal expansion of silicon is smaller than that of BaTi0 3 so that a uniform cooling of a structure comprised of a BaTi0 3 film-on-silicon results in the development of an appreciable in-plane strain within the unit cells of the BaTi0 3 film and the consequential transformation of the BaTi0 3 unit cells of the film to a tetragonal form.
  • the form of a unit cell of BaTi0 3 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.
  • each BaTi0 3 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 BaTi0 3 unit cells in a plane which is substantially parallel to the surface of the substrate.
  • 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).
  • the last- mentioned factor i.e. the orientation of the crystal
  • the electro-optic (EO) response (corresponding to the aforediscussed electro-optic coefficient) for a crystalline material, such as BaTi0 3 , 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 BaTi ⁇ 3 film and at about a forty-five degree angle with respect to the orientations of the polar axes of the BaTi0 3 unit cells to allow the mode of propagation in the device.
  • the structure 180 can be used as a phase modulator wherein the BaTi0 3 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 BaTi0 3 film.
  • the overlying oxide material couples to the underlying semiconductor material for affecting the electronic capabilities of the substrate.
  • 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.
  • advantageous behavior can involve the modulation of charge density and channel current in a transistor.
  • 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)
  • 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.
  • 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.)
  • 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.
  • the aforedescribed embodiments are intended for the purpose of illustration and not as limitation.
  • a monolithic crystalline structure generally indicated 220, comprised of a substrate 222 of silicon, an intermediate thin film layer
  • MgO magnesium oxide
  • BaTi0 3 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.
  • the growth (or formation) of the structure 220 predisposes the electric dipole moments of the BaTi0 3 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.
  • the silicon substrate 222 can be used for transmission of electrical signals in a communication system which employs both optical and electrical signals.
  • 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.
  • the BaTi0 3 perovskite film is one of a number of materials having the general formula AB0 3
  • the silicon substrate is one of a number of a Group III, IV or II-VI materials upon which the AB0 3 material can be epitaxially grown for use of the AB0 3 material as a waveguide.
  • the element A in BaTi0 3 (having the general formula AB0 3 ) is Ba
  • another material such as the Group IVA elements Zr or Hf
  • 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.
  • 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.
  • MBE molecular beam epitaxy
  • steps are taken in accordance with the teachings of the referenced U.S. Patent No. 5,846,667 to construct the BaTi0 3 film 224 upon the underlying MgO film 223.
  • the build-up of BaTi0 3 is initiated by growing a first epitaxial (preferably commensurate) single- plane layer comprised of Ti0 2 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 Ti0 2 layer.
  • the growth of the BaO plane is followed by the sequential steps of growing a single-plane layer of Ti0 2 directly upon the previously-grown plane of BaO and then growing a single-plane layer of BaO upon the previously-grown layer of Ti0 2 until a desired thickness (e.g. 0.2-0.6 ⁇ m) of the BaTi0 3 film 224 is obtained.
  • a desired thickness e.g. 0.2-0.6 ⁇ m
  • the MgO film 223 serves to optically isolate the BaTi0 3 film 224 from the silicon substrate 222, as well as provide a stable structure upon which the BaTi0 3 is grown.
  • the index of refraction of BaTi0 3 is about 2.3 whereas the index of refraction of silicon is on the order of about 4.0.
  • 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 BaTi0 3 film 224, the amount of light which is transmitted along the BaTi0 3 film 224 and which could otherwise be lost into the silicon is reduced.
  • the thickness of the MgO film 223 is selected to provide satisfactory optical isolation for the BaTi0 3 film.
  • 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.
  • the coefficient of thermal expansion of silicon (Si) is smaller than that of BaTi0 3 so that a uniform cooling of a structure comprised of a BaTi0 3 film-on-silicon results in the development of an appreciable in-plane strain within the unit cells of the BaTi0 3 film and the consequential transformation of the BaTi0 3 unit cells of the film to a tetragonal form as will be described herein.
  • Fig. 18 a graph which plots the lattice parameter of the BaTi0 3 and silicon unit cell in a BaTi0 3 -on-silicon structure versus temperature. It can be seen that when the BaTi0 3 film is grown upon the silicon at a temperature of about 600 °C, the lattice parameter of the BaTi0 3 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 BaTi0 3 contracts at the same rate as that of the silicon, and an in-plane strain (i.e.
  • a change in length, l begins to develop in the BaTi0 3 film.
  • the developed in-plane strain is uniform throughout the BaTi0 3 film.
  • the form of the BaTi0 3 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.
  • the lattice parameter as measured along the c-axis increases while the a-axis lattice parameter decreases.
  • the lattice parameter of BaTi0 3 as measured along the c-axis is larger than it is as measured along the a-axis. Consequently, if the BaTi0 3 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 BaTi0 3 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 BaTi0 3 crystal when measured along the a-axis is larger than it is when measured along the c-axis.
  • 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 BaTi0 3 unit cells in the film of a BaTi0 3 -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 BaTi0 3 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 BaTi0 3 unit cells in a plane which is substantially parallel to the surface of the substrate.
  • the characteristics of BaTi0 3 unit cells are the same as aforedescribed when a thin film of MgO is interposed between the silicon substrate and the BaTi0 3 film.
  • the thickness of the silicon substrate is so large in comparison to that of either the MgO or BaTi0 3 film that it is the strain of the silicon which dominates during the contraction of the overlying films, rather than vise-versa.
  • the interposed layer of MgO is simply too thin to alter the characteristics of contraction of the BaTi0 3 when overlying silicon from those previously described. Therefore, in a structure like that of the structure 220 of Figs.
  • the BaTi0 3 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 BaTi0 3 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.
  • 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 BaTi0 3 film in response to a change in the electric field applied across the film.
  • 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).
  • the last- mentioned factor i.e. the orientation of the crystal
  • the orientation of the crystal relates to the orientation of the dipoles of the crystal.
  • the electro-optic (EO) response (corresponding to the aforediscussed electro-optic coefficient) for a crystalline material, such as BaTi0 3 , is described by a third rank tensor.
  • the elements of the tensor relate the change in refractive index, ⁇ n, for the crystalline material to the applied electric field, E, in accordance with the following equation:
  • 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.
  • a tetragonal unit cell such as BaTi0 3 of the aforedescribed (cooled) BaTi0 3 -on-silicon structure
  • n orthogonal directions
  • SUBSTTTUTESHEET(RULE26) is in the "x" direction
  • Electric fields which are applied in other directions are just superpositions of these three x, y and z orthogonal directions.
  • a crystal type a tetragonal unit cell
  • 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, r 51 .
  • 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 BaTi0 3 unit cells of the aforedescribed BaTi0 3 -on-silicon structure are determined by the aforediscussed differences in thermal expansion (or contraction) between BaTi0 3 and silicon.
  • the polar axis of the particular domain structure that forms in the BaTi0 3 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 BaTi0 3 on silicon.
  • a BaTi0 3 -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 BaTi0 3 overlying the MgO layer 223.
  • MgO magnesium oxide
  • the orientation (and exemplary directions) of the polar axes of the plane of the BaTi0 3 film are indicated with the arrows 276, the light directed through the BaTi0 3 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 BaTi0 3 film and at about a 45° angle with respect to the orientations of the polar axes of the BaTi0 3 unit cells.
  • the mode of propagation in the device 270 is allowed and is called a transverse electric mode.
  • BaTi0 3 -on-Si structure which has beneficial optical qualities. While BaTi0 3 , 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.
  • the resultant BaTi0 3 -on-Si structure can be used as a phase modulator wherein the BaTi0 3 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 BaTi0 3 film.
  • the phase modulator can function as an intensity modulator.
  • the BaTi0 3 -on-Si structure has been described above as including an intermediate layer of MgO having a thickness which is sufficient to optically isolate the BaTi0 3 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.
  • exemplary structures have been described as including BaTi0 3 in a pure form, the principles of this invention are intended to cover applications wherein the ferroelectric perovskite, such as BaTi0 3 , 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.
  • 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.
  • 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.
  • 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.

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Abstract

L'invention concerne une structure cristalline (20, 60 ou 220) et un dispositif (120, 140, 180 ou 270), ladite structure, qui peut être adaptée à plusieurs applications électro-optiques ou de semi-conducteurs telles qu'un modulateur de phase ou un composant d'un interféromètre, comprenant un substrat (22, 62, 142, 182, 222 ou 272) d'un matériau à base de semi-conducteur et un film mince (24, 64, 144, 186 ou 224) d'un matériau d'oxyde cristallin disposé de manière épitaxiale à la surface du substrat de manière à ce qui le film mince soit relié au substrat sous-jacent et que la disposition de pratiquement toutes les cellules unités du film mince se caractérise par une orientation prédéterminée par rapport à la surface de substrat. La prédisposition géométrique des cellules unités du film mince est due à l'état tendu ou précontraint du réseau à la jonction entre le matériau en film mince et la surface du substrat. Cette prédisposition géométrique est responsable d'une orientation prédisposée d'une qualité dépendant de la direction telle que le moment dipolaire des cellules unités. L'orientation prédéterminée de la géométrie des cellules unités permet de créer un dispositif comportant une structure qui permet de bénéficier des caractéristiques avantageuses de ladite structure pendant le traitement.
EP99937553A 1998-07-30 1999-07-27 Regulation de l'anisotropie du cristal destinees aux oxydes de perovskite sur des substrats a base de semi-conducteurs Withdrawn EP1104494A1 (fr)

Applications Claiming Priority (7)

Application Number Priority Date Filing Date Title
US126526 1987-11-30
US09/126,527 US6093242A (en) 1996-08-05 1998-07-30 Anisotropy-based crystalline oxide-on-semiconductor material
US09/126,526 US6023082A (en) 1996-08-05 1998-07-30 Strain-based control of crystal anisotropy for perovskite oxides on semiconductor-based material
US126129 1998-07-30
US126527 1998-07-30
US09/126,129 US6103008A (en) 1998-07-30 1998-07-30 Silicon-integrated thin-film structure for electro-optic applications
PCT/US1999/017050 WO2000006812A1 (fr) 1998-07-30 1999-07-27 Regulation de l'anisotropie du cristal destinees aux oxydes de perovskite sur des substrats a base de semi-conducteurs

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WO2002082551A1 (fr) * 2001-04-02 2002-10-17 Motorola, Inc. Structure de semi-conducteur a courant de fuite attenue
US10437082B2 (en) 2017-12-28 2019-10-08 Tetravue, Inc. Wide field of view electro-optic modulator and methods and systems of manufacturing and using same
KR102496956B1 (ko) * 2020-10-28 2023-02-06 포항공과대학교 산학협력단 박막 트랜지스터 및 이의 제조 방법

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GB8913450D0 (en) * 1989-06-12 1989-08-02 Philips Electronic Associated Electrical device manufacture,particularly infrared detector arrays
US5295218A (en) * 1992-09-29 1994-03-15 Eastman Kodak Company Frequency conversion in inorganic thin film waveguides by quasi-phase-matching
US5666305A (en) * 1993-03-29 1997-09-09 Olympus Optical Co., Ltd. Method of driving ferroelectric gate transistor memory cell
US5576879A (en) * 1994-01-14 1996-11-19 Fuji Xerox Co., Ltd. Composite optical modulator
US5654229A (en) * 1995-04-26 1997-08-05 Xerox Corporation Method for replicating periodic nonlinear coefficient patterning during and after growth of epitaxial ferroelectric oxide films
US5830270A (en) * 1996-08-05 1998-11-03 Lockheed Martin Energy Systems, Inc. CaTiO3 Interfacial template structure on semiconductor-based material and the growth of electroceramic thin-films in the perovskite class

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See references of WO0006812A1 *

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KR20010079590A (ko) 2001-08-22
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WO2000006812A1 (fr) 2000-02-10
CA2337029A1 (fr) 2000-02-10

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