Abstract:
A layer of chemically compatible conductive material is applied to ferroelectric semiconductor material having a permanent polarization below its Curie temperature. The materials are heated to a temperature above the Curie temperature of the ferroelectric semiconductor material and allowed to cool. The result is a low resistance contact. The ferroelectric semiconductive material may be a layer on a non ferroelectric semiconductor material with a matching work function or matching dopant levels.

Description:
BACKGROUND OF THE INVENTION 
     This invention pertains to methods of making ohmic contacts to semiconductors and more particular is concerned with a method of making ohmic contact to ferroelectric semiconductors. 
     One known method to make ohmic contact electrodes uses a metal or metal alloy which has a work function which matches that of the semiconductor which need to be electroded. Because the work functions of both materials match, no electrical barrier height is formed at the electrode junction and an ohmic contact electrode is thus obtained. It is, however, difficult to find an electrode material which has matching work function to that of a particular semiconducting material and still has satisfactory engineering properties. 
     Another method calls for having the surface of the semiconducting material heavily doped (either n type or p type), such that the electrical barrier at the electrode junction created due to the mismatch of work function of both materials has a narrow profile and charge carriers such as electrons and holes have no difficulty quantum-tunnelling through the potential barrier. The difficulty is that not all semiconducting materials can be heavily doped without having the dopant charge compensation mechanism changed. If the dopant charge compensation mechanism is changed and the dopants can be charge compensated by defects other than free electrons or holes, a narrow profile of electrical barrier will not be built and no ohmic contact can be formed. 
     An object of the invention is to provide a method of forming ohmic contact electrodes to semiconductors which does not require work-function matching nor heavy doping. 
     SUMMARY OF THE INVENTION 
     According to the method of the invention, a layer of chemically compatible conductive material is applied to ferroelectric semiconductor material having a permanent polarization below its Curie temperature. The materials are heated to a temperature above the Curie temperature of the ferroelectric semiconductor material and then allowed to cool. The result is a low resistance contact. 
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the method of the invention, a semiconducting ferroelectric material which has permanent polarization below the Curie temperature of the material (ferro-para transition) is used as an ohmic contact substrate material. A known example of such a material is barium titanate, BaTiO 3 , which is semiconducting when doped with one or more alio-valent dopant. See for example, W. Heywang &#34;Semiconducting Barium Titanates&#34;, J. Materials Science 6, (1971) pp. 1214-1226. An alio-valent dopant has a valence different than that of the host. Suitable alio-valent dopants are Y, Sb, Sc, V, W, Cr, Ni, Rh, Ru, and any element of the lanthanide group (doping percentage 0.-1 m/0). The Curie temperature can be changed by adding one or more moderating dopants to the host ferroelectric material. Dopants which result in Curie temperature modulation are Ca, Zr, Sn, Pb, Hf, and Sn (doping percentage 0.-60 m/0). On top of this semiconducting ferroelectric material, an electrode material will be applied. The electrode material can be any type of material as long as it has reasonable electrical conductivity and is chemically compatible, i.e., will not chemically react with the ferroelectric material to form insulating products. 
     Such electrode material can be any metal or metal alloy. Other examples of electrode materials are electrically-conducting ceramics (such as TiN, ZnO, SnO 2-x , and doped In 2  O 3 ), and electronic-conducting organic polymers. It is known that SnO 2  may be reduced during the doping process. The method of electrode deposition can be by any physical deposition method such as physical vaporization, thermal vacuum deposition, arc spray, plasma, arc plasma, DC and AC sputter, brush, or screen-printing. Chemical vaporization method, such as metallic organic chemical vapor deposition can also be used for electrode deposition. The only preclusion for possible electrode material applied methods is that no insulating material should be formed during the applied procedure due to chemical interaction between the electrode material and the ferroelectric semiconducting materials. During or after the electrode application, the materials are exposed to a temperature higher than the ferroelectric material&#39;s Curie point and lower than what would cause chemical reactions or physical degradation. After this temperature treatment, the procedure for ohmic contact is completed and the materials are allowed to cool. The heating-up treatment could use a hot furnace, hot plate, hot air gun, or thermal irradiation. The temperature treatment is very important as the junction resistance is modulated by the temperature to which it is exposed. 
    
    
     EXAMPLE 
     
         ______________________________________Ferroelectrical material            BaTiO.sub.3 Semiconducting            properties are given by            0.3 m/o Y dopant. The Curie            temperature is moderated by            16 m/o Ca dopant and 3.5 m/o            Sn dopant.Curie temperature            120° C.Electrode material            AuMethod of Au deposition            DC sputtering method            Cathode: Au            Gas: Ar            Pressure: 3 × 10.sup.-1 mbar            Voltage: 1 kV            Current: 40 mA______________________________________ 
    
     During deposition, the sample was water-cooled. The impedance of the Au/BaTiO 3  junction was studied by using the AC complex impedance method. Right after the Au deposition, the junction resistance was as high as 690 Ohm-cm 2 . The sample was heated to various exposed temperatures and allowed to cool to room temperature. Repeated exposure indicated that when the maximum exposed temperatures were gradually increased, the room temperature junction resistance shrank and finally became close to zero after the exposed temperature exceeded the sample&#39;s Curie temperature (120° C.). 
     The fact that ohmic contacts are formed by this method can be explained by the trapped charges at electrode junction being compensated by the permanent dipole movement of the ferroelectric material, which leaves no potential barrier at the junction, so that the electrode makes ohmic contact with the ferroelectric material. 
     This method can be applied to form an ohm contact to nonferroelectric, semiconducting material. The procedure involves depositing (by CVD or PVD method) a ferroelectric semiconducting material on top of a nonferroelectric-semiconducting material; the dopant level on both sides of the materials needs to be selected so that no major potential barrier will be created. Then electrode materials can be applied to the top of the ferroelectric semiconducting material according to the previously described procedure to form an ohmic contact junction. Alternatively, the non-ferroelectric material and ferroelectric material are selected to have matching work functions. 
     The preferred embodiment and best mode of practicing the invention has been described. With these teachings variations of the method will be apparent to those skilled in the art, particularly is substitutions of materials. Accordingly, the scope of the invention is to be determined by the following claims.