Transition metal oxide bilayers

1. A method of forming a plurality of layers comprising forming a first layer on a substrate, wherein the first layer comprises a first compound

comprising Me 1 w Me 2 x Si y O z , wherein Me 1 and Me 2 are metals, w≧0, x≧0, y≧0, and z>0, and at least one of w, x, and y is greater than zero; and

forming a second layer on a substrate, wherein the second layer comprises a second compound comprising Me 1 a Me 2 b Si c O d , wherein a≧0, b≧0, c≧0, and d>0, and at least one of a, b, and c is greater than zero;

wherein one of the first or the second layers has linear resistance and substoichiometric composition, and the other of the first or the second layer has bistable resistance and near-stoichiometric composition,

wherein the sum of the first layer thickness and the second layer thickness is between about 20 Å and about 100 Å;

wherein the layer with bistable resistance has a thickness between about 25% and about 75% of the total thickness; and

wherein at least one of the first compound or second compound further comprises nitrogen.

2. The method of claim 1 , wherein the first layer is formed using reactive sputtering in a sputtering atmosphere comprising argon and oxygen having a first oxygen concentration, and the second layer is formed using reactive sputtering in a sputtering atmosphere comprising argon and oxygen having a second oxygen concentration.

3. The method of claim 1 , wherein the first layer is formed using atomic layer deposition using a first reactant gas concentration and exposure time, and the second layer is formed using atomic layer deposition using a second reactant gas concentration and exposure time.

4. The method of claim 1 , wherein the first layer is formed using chemical vapor deposition using a first oxygen-containing gas concentration and exposure time, and the second layer is formed using chemical vapor deposition using a second oxygen-containing gas concentration and exposure time.

5. The method of claim 1 , wherein one of the metals is a transition metal.

6. The method of claim 5 , wherein the transition metal comprises hafnium, tantalum, zirconium, lanthanum, niobium, or tungsten.

7. The method of claim 1 , wherein one of the metals is selected from the group consisting of tantalum, niobium, hafnium, yttrium, zirconium, lanthanum, vanadium, titanium, tungsten, nickel, aluminum, and combinations thereof.

8. The method of claim 2 , wherein the sputtering atmosphere has a pressure of between about 1 and about 5 mTorr.

9. The method of claim 2 , wherein the oxygen concentration used to form the layer having linear resistance is less than about 5 volume percent of the sputtering atmosphere.

10. The method of claim 2 , wherein the oxygen concentration used to form the layer having bistable resistance is more than about 30 volume percent of the sputtering atmosphere.

11. The method of claim 2 , wherein the sputtering atmosphere further comprises nitrogen.

12. A method of forming a plurality of layers comprising

forming a first layer on a substrate using sputtering, wherein the first layer comprises a first compound comprising Me 1 w Me 2 x Si y , wherein Me 1 and Me 2 are metals, w≧0, x>0, y≧0, and at least one of w, x, and y is greater than zero;

forming a second layer above or below the first layer using reactive sputtering in an atmosphere comprising argon and oxygen, wherein the second layer comprises a second compound comprising Me 1 a Me 2 b Si c O d , where a≧0, b≧0, c≧0, and d>0, and at least one of a, b, and c is greater than zero; and

annealing the first and second layers such that oxygen atoms migrate from the second layer into the first layer;

wherein the annealed first layer has linear resistance and substoichiometric composition, and the annealed second layer has bistable resistance and near-stoichiometric composition,

wherein the sum of the first layer thickness and the second layer thickness is between about 20 Å and about 100 Å, and

wherein the second layer has a thickness between 25% and 75% of the total thickness.

13. The method of claim 12 , wherein the oxygen concentration of the sputtering atmosphere used to form the second layer is more than about 30 volume percent.

14. A nonvolatile memory element comprising

a first electrode layer,

a second electrode layer, and

a first oxide layer and second oxide layer disposed between the first and second electrode layers;

wherein the first oxide layer comprises a first compound comprising Me 1 w Me 2 x Si y O z , wherein Me 1 and Me 2 are metals, w≧0, x≧0, y≧0, and z>0, and at least one of w, x, and y is greater than zero;

wherein the second oxide layer comprises a second compound comprising Me 1 a Me 2 b Si c O d , wherein a≧0, b≧0, c≧0, and d>0, and at least one of a, b, and c is greater than zero,

wherein one of the oxide layers has linear resistance and substoichiometric composition, and the other oxide layer has bistable resistance and near-stoichiometric composition;

wherein the sum of the two oxide layer thicknesses is between about 20 Å and about 100 Å;

wherein the oxide layer with bistable resistance has a thickness between about 25% and about 75% of the total thickness; and

wherein one of the metals is selected from the group consisting of tantalum, niobium, hafnium, yttrium, zirconium, lanthanum, vanadium, titanium, tungsten, nickel, aluminum, and combinations thereof.

15. The nonvolatile memory element of claim 14 , wherein the first electrode layer comprises doped polycrystalline silicon and the second electrode layer comprises titanium nitride.

16. The nonvolatile memory element of claim 14 , wherein the layer with bistable resistance is adjacent to the first electrode layer.

17. The nonvolatile memory element of claim 14 , wherein the layer with bistable resistance is adjacent to the second electrode layer.

18. A ReRAM device comprising an array of the nonvolatile memory elements of claim 14 , wherein the first electrodes of each nonvolatile memory element are coplanar and arranged as a first set of parallel electrodes, and the second electrodes of each nonvolatile memory element are coplanar and arranged as a second set of parallel electrodes at an angle to the first set of parallel electrodes.