Abstract:
The invention concerns a component of composite structure ( 10 ) consisting of a layer of ferroelectric material ( 11 ) and at least a thin film of semiconductor material or of a thin metal or supraconducting film ( 12 ) in close contact with the layer of ferroelectric material ( 11 ). It further comprises means for generating local modifications of the ferroelectric material polarization. This mechanism includes a device for applying a voltage between at least one punctiform electrode ( 13 ) arranged for selectively scanning the composite structure ( 10 ) surface facing that consisting of the semiconductor or thin metal or supraconducting film ( 12 ), and the semiconductor or thin metal or supraconducting film.

Description:
FIELD OF THE INVENTION 
     The present invention concerns an electric or electronic component, particularly an electric or electronic circuit or a non-volatile memory. 
     It also concerns the application of such a component as a non-volatile memory and as a device with acoustic surface waves. 
     BACKGROUND OF THE INVENTION 
     During the last decade, rapid progress has been made in the deposition of films with composite epitaxial, oxide-base structures, laying the groundwork for subsequent advancement in the field of copper oxide superconductors and allowing preparation of epitaxial films of materials such as magnetoresistant manganese oxides. One of the numerous other families of materials recently formed using this epitaxial approach is ferroelectric oxide. One attractive quality of these materials is their reversible, non-volatile polarization field. In addition, numerous devices with electric field effects have been developed, ranging from dynamic random access memories (DRAMs) to nanosystem models such as dual dimension electronic gases. 
     All these devices require the use of permanent metal electrodes to produce and maintain the electrical field, which poses a major problem because very delicate lithography is required to manufacture the metal electrodes. 
     U.S. Pat. No. 5,206,525 describes a device essentially equivalent to a field effect transistor with the dielectric replaced by a pi-conjugated macromolecular material mixed with a ferroelectric material. The goal of this invention is to modify conductivity using the electrical field. However, this device cannot modify local electronic properties. 
     U.S. Pat. No. 5,371,729 describes a data storage device based on the principle of locally commutating the polarization of a ferroelectric film deposited on a semi-conductor. The aim of this invention is to increase working speed by using high voltage current to generate an avalanche phenomena. 
     European Patent Application published as No. EP-O 275 881 describes a device which uses a tunnel effect microscope to read and write data on an electrically polarized film, which is primarily a polymer. The purpose of this device is not to write data using electrical impulses or to read data by detecting local modifications in electronic density. 
     The present invention proposes a novel electric or electronic component which either simplifies both the permanent electrodes and the customary lithography manufacturing process, or renders them completely superfluous. In addition, the method of making this component is non-invasive. 
     This goal is achieved by the component according to the invention, characterized in that is comprises a composite structure consisting of least one layer of ferroelectric material and at least one semi-conductor or a thin film of a metal or superconductor material, in close contact with at least one surface of said layer of ferroelectric material, as well as means for generating local polarization modifications in said ferroelectric layer either to cause local modifications, on a scale ranging from nanometers to micrometers, in the electronic density of one zone on said semi-conductor or of the thin metal film or of the superconductor, said zone being close to said surface of said ferroelectric layer; or to use a piezoelectric effect to create a local deformation in said layer of ferroelectric material, depending upon the polarization value, in order to generate an acoustic surface wave. 
     SUMMARY OF THE INVENTION 
     According to its preferred embodiments, said means for generating local polarization modifications in said layer of ferroelectric material may comprise a device for applying current between at least one selective electrode, designed to selectively scan the surface of said composite structure opposite the surface consisting of the semi-conductor or thin film of metal or superconductor material, and said semi-conductor or thin film of metal or superconductor material. 
     According to an advantageous embodiment, said composite structure may consist of a semi-conductor or a thin film of metal or superconductor material in close contact with a first surface of said layer of ferroelectric material, and a very thin film of semi-conductor, metal, or superconductor material in close contact with a second surface of the layer of ferroelectric material which is opposite said first surface of the layer of ferroelectric material. 
     According to this embodiment, the surface of the composite structure that is scanned or swept by a selective electrode is the very thin semi-conductor, metal, or superconductor film which is in close contact with the second surface layer of ferroelectric material, with current being applied between said selective electrode and said very thin film. Current may also be applied between the electrode and the semi-conductor or the thin film of metal or superconductor material in close contact with the first surface of said layer of ferroelectric material. In both these cases, it is possible to apply current simultaneously between said very thin semi-conductor, metal or superconductor film and the semi-conductor or the thin film of metal or superconductor. 
     Advantageously, said selective electrode may consist of the tip of a microscope with a local sensor. It may also consist of a source of particles. 
     According to a first embodiment, said particle source is designed to generate an electron beam. 
     According to a second embodiment, said source of particles is designed to generate an ion beam. 
     In a variation, said selective electrode may consist of a radiation source. 
     In a particularly advantageous embodiment, the polarization of said layer of ferroelectric material is represented by a characteristic that is sensitive to local modifications in the electronic density of said semi-conductor or in the thin film of metal or superconductor material. 
     Preferably, said characteristic is the tunnel conductance between a local wave scanning the surface of the composite structure opposite the surface constituting the semi-conductor or the thin film of metal or superconductor material, and said semi-conductor or thin film or metal or superconductor material. 
     One of the particularly advantageous applications of the component is in the formation of an acoustic surface wave device, specifically, a filter for frequencies typically ranging up to 20 GHz and higher. 
     Another application of the component is to use it as an electronic circuit, particularly as a non-volatile memory. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more readily understood from the following description of a preferred embodiment of the component according to the invention and its variations, with reference to the drawings, in which: 
     FIGS. 1 and 2 are schematic perspectives showing two modes of the electric or electronic component of the invention; and 
     FIG. 3 shows the application of the component as an acoustic surface wave device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The component of the invention as shown in FIG. 1 consists primarily of a composite structure  10  comprising a layer of ferroelectric material  11  in the form of a film, and a semi-conductor or a film of metal or superconductor material  12 , in close contact with one of the surfaces of the layer of ferroelectric material  11 . Moreover, the component comprises a means of generating local polarization modifications in the layer of ferroelectric material  11 . The objective is to modify electronic density locally in one zone of said semi-conductor or said thin film of metal or superconductor material  12  near the surface of the layer of ferroelectric material  11 , or to create a local deformation in the layer of ferroelectric material in order to generate an acoustic surface wave. 
     Said means consists of a device for applying current between said semi-conductor or the thin film of metal or superconductor material  12  and at least one selective electrode  13  designed to selectively scan the surface of said layer of ferroelectric material  11  opposite the surface supporting thin film  12 . Said selective electrode may consist, for example, of the tip of a microscope with a local sensor. It may also consist of a particle source. Said particle source may generate either an electron beam or an ion beam. It may also consist of a radiation source. 
     In a first particularly advantageous application of the invention, the component is used to write an electrical circuit. According to a second application, the component is used to make data storage devices. Writing takes place on the nanometer scale with the tip of a microscope having a local sensor, which serves as a selective electrode or a particle source causing local modifications in the polarity of the ferroelectric material. The polarity (±) of the material in the ferroelectric layer modifies the electronic density of the metal or of the semi-conductor near the surface, and this modification defines the electronic circuit or storage device. 
     To achieve this, in an ideal context, an atomic force microscope (AFM) with a conductive tip scans the surface of the composite structure, known as the ferroelectric epitaxial heterostructure, which in this case consists of the following materials—Pb(ZrO.52 TiO.48)O3 (PZT)/SrRuO3—while current is applied between the tip and the semiconductor or the film of metal or superconductor material. When the electric field between the tip and the conductor film is larger than the coercive field in the layer of ferroelectric material  11 , the polarization in this layer fluctuates locally below the tip and causes a flux of charges in the screen in the layer of metal or semi-conductor material, thus modifying the total density of the carriers and consequently the resistance of the conductor, which can be measured using standard techniques for measuring electronic transport. In an ideal context, the semi-conductor or the thin metal or superconductor film has a low carrier density and an electric or electronic characteristic that is sensitive to changes in the density of an electronic state. 
     According to a second advantageous embodiment, shown in FIG. 2, structure  10  is completed by a semi-conductor, metal, or superconductor material  14 , deposited on the second surface of the layer of ferroelectric material  11  opposite thin film  12 . Said material is preferably in the form of a very thin film in close contact with ferroelectric material  11 . It may be in the form of a continuous film or it may consist of islands. Said film constitutes both a means of applying current to the layer of ferroelectric material  11  in combination with thin film  12  and a means of passivating the surface of polarized ferroelectric material. 
     The means for generating local polarization modifications consists of a device for applying current between at least one selective electrode  13 , designed to selectively scan the layer of ferroelectric material  11  or the film  14 , and said very thin film  14  or said thin film  12 . Current may also be simultaneously applied between said very thin film  14  and said thin film  12 . The ferroelectric material can be polarized by applying current impulses to the selective electrode, possibly in the presence of current passing through the layer of ferroelectric material of lower strength then the coercive current. 
     This component permits the non-invasive establishment of a local field effect, while also creating electronic nanostructures without electrical contacts made using conventional lithograph methods. This approach is of interest because it offers a completely novel perspective on the manufacture of field-effect devices and small electronic structures. The ability to change domains vertically using a nearby microscope to produce an electronic contrast that is not susceptible to passivation is especially advantageous. 
     The principal advantage to this approach is that it permits writing directly to submicronic electronic structures with no chemical substitution and no lithography and/or permanent electrodes. The field effect is local, reversible, non-volatile, and non-invasive. This technique has another advantage: it allows the use of a mechanism for detecting reading/writing of ferroelectric domains that is not sensitive to contrast deterioration due to electrostatic passivation. 
     In practice, it is possible to use any local sensor which will alter the polarization of the ferroelectric material. For example, a tunnel effect microscope may be used to polarize the ferroelectric material if the latter is covered with a sufficiently thin layer of conductive material. Said tunnel effect microscope can also be used to measure and read changes in density of the carriers produced by the electric field, for example, by measuring modifications in tunnel conductive (density of states). 
     To read small ferroelectric domains, it is possible to use other contrast mechanisms based on the piezoelectric and pyroelectric properties of the ferroelectrics. Much of the same as with the preceding approaches, these techniques would also be subject to passivation. 
     FIG. 3 is a schematic illustration of an application of the component described above as an acoustic surface wave device. Composite structure  10  comprises, in this case, perhaps one layer of ferroelectric material  11  which has been locally polarized, for example, periodically, and a semi-conductor or a thin film of metal or superconductor material  12 . An alternating signal uses a piezo-electric effect to provoke a deformation, the sign of which depends upon the polarization, and excites an acoustic surface wave if the frequency of this signal is equal to the speed of the surface wave divided by the wave length. 
     In devices known in the art, lithographic techniques impose frequency limitations of about 1 Ghz. 
     A single contact  22  on the structure of the composite surface  10  respectively generates and detects frequencies as high as 20 Ghz and above, using an excitation device  20  and a detector  21 . 
     The device has very high spatial resolution, allowing it to attain elevated techniques that have not been possible using conventional devices with contacts deposited by lithography. 
     On direct use is as a high frequency filter.