Heterolayered ferroelectric thin films and methods of forming same

Abstract

Heterolayered thin films having ferroelectric/piezoelectric layers of alternating crystal structures and methods of their preparation are provided. In the ferroelectric/piezoelectric thin film, a first layer has a rhombohedral crystal structure and a second layer adjacent the first layer has a tetragonal crystal structure. The layers have a (100) preferred orientation with a-axis normal to the surface of the film. The first layer can be a Zr-rich lead ziroconate titanate layer (e.g. PbZr 0.8 Ti 0.2 O 3 ) and the second layer can be a Ti-rich PZT layer (e.g., PbZr 0.2 Ti 0.8 O 3 ). Heterolayered ferroelectric/piezoelectric thin film comprising a plurality of such first and second layers in alternating sequence exhibits particularly improved electrical properties.

1 . A heterolayered thin film having a generally flat surface, comprising: 
 a first ferroelectric/piezoelectric layer having a rhombohedral crystal structure;    a second ferroelectric/piezoelectric layer adjacent said first layer and having a tetragonal crystal structure;    said first and second layers having a (100) preferred orientation with a-axis normal to said surface.    
   
   
       2 . The thin film of  claim 1  wherein each of said first and second layers contains Pb(Zr,Ti)O 3 . Pb(Mg,Nb)O 3 , Pb(Zn,Nb)O 3 —PbTiO 3 ,or Pb(Mg,Nb)O 3 —PbTiO 3 .  
   
   
       3 . The thin film of  claim 1  wherein each of said first and second layers contains lead ziroconate titanate (PZT).  
   
   
       4 . The thin film of  claim 3  comprising a plurality of said first and second layers in alternating sequence.  
   
   
       5 . The thin film of  claim 4  wherein the PZT in said first layer has a chemical formula PbZr x Ti (1-x) O 3 , where 0.52<x<0.9, the PZT in said second layer has a chemical formula PbZr y Ti (1-y) O 3 , where 0<y<0.52.  
   
   
       6 . The thin film of  claim 5  wherein x=0.8 and y=0.2.  
   
   
       7 . The thin film of  claim 6  formed on a substrate.  
   
   
       8 . The thin film of  claim 7  wherein said substrate is a Pt-passivated silicon wafer.  
   
   
       9 . The thin film of  claim 8  wherein the layer adjacent the substrate is said first layer.  
   
   
       10 . The thin film of  claim 9  comprising six alternating layers of PbZr 0.8 Ti 0.2 O 3  and PbZr 0.2 Ti 0.8 O 3 .  
   
   
       11 . The thin film of  claim 10  having a remanent polarization of 71 μC/cm 2  and a coercivity field of 223 kV/cm.  
   
   
       12 . The thin film of  claim 9  comprising four alternating layers of PbZr 0.8 Ti 0.2 O 3  and PbZr 0.2 Ti 0.8 O 3 .  
   
   
       13 . The thin film of  claim 12  having a remanent polarization of 53 μC/cm 2  and a coercivity field of 265 kV/cm.  
   
   
       14 . The heterolayered ferroelectric/piezoelectric thin film of  claim 1 , formed by a method comprising: 
 (a) forming a first precursor layer capable of being crystallized into a rhombohedral structure of a first ferroelectric/piezoelectric material;    (b) baking said first precursor layer without crystallizing said first precursor layer;    (c) forming a second precursor layer capable of being crystallized into a tetragonal structure of a second ferroelectric/piezoelectric material on said first precursor layer;    (d) baking said second precursor layer at a temperature for a period of time sufficient to crystallize said second precursor layer without crystallizing said first precursor layer; and    (e) after said second precursor layer is crystallized, annealing said first and second precursor layers, thus crystallizing said first precursor layer and forming said thin film.    
   
   
       15 . The heterolayered PZT thin film of  claim 1 , formed by a method comprising: 
 (a) forming a first precursor layer containing PZT having a chemical formula of PbZr x Ti (1-x) O 3 , wherein 0.52<x<0.9;    (b) baking said first precursor layer without crystallizing said first precursor layer;    (c) forming on said first precursor layer, a second precursor layer containing PZT having a chemical formula of PbZr y Ti (1-y) O 3 , wherein 0<y<0.52;    (d) baking said second precursor layer at a temperature for a period of time sufficient to crystallize said second precursor layer without crystallizing said first precursor layer; and    (e) after said second precursor layer is crystallized, annealing said first and second precursor layers, thus crystallizing said first precursor layer and forming said thin film.    
   
   
       16 . The heterolayered PZT thin film of  claim 15 , wherein the steps (a) to (d) are repeated twice.  
   
   
       17 . The heterolayered PZT thin film of  claim 15 , wherein the steps (a) to (d) are repeated three times.  
   
   
       18 . The heterolayered PZT thin film of  claim 15 , wherein the steps (a) to (d) are repeated more than three times.