Variable resistive element, method for producing the same, and nonvolatile semiconductor memory device including the variable resistive element
A variable resistive element configured to reduce a forming voltage while reducing a variation in forming voltage among elements, a method for producing it, and a highly integrated nonvolatile semiconductor memory device provided with the variable resistive element are provided. The variable resistive element includes a resistance change layer (first metal oxide film) and a control layer (second metal oxide film) having contact with a first electrode sandwiched between the first electrode and a second electrode. The control layer includes a metal oxide film having a low work function (4.5 eV or less) and capable of extracting oxygen from the resistance change layer. The first electrode includes a metal having a low work function similar to the above metal, and a material having oxide formation free energy higher than that of an element included in the control layer, to prevent oxygen from being thermally diffused from the control layer.
1. A variable resistive element comprising
a first metal oxide film which is an oxide film of a first metal and sandwiched between a first electrode and a second electrode, wherein
through a forming process, a resistance state between the first and second electrodes of the variable resistive element is changed from an initial high resistance state before the forming process to a variable resistance state,
the resistance state in the variable resistance state is changed between two or more different resistance states by applying an electric stress between the first electrode and the second electrode of the variable resistive element in the variable resistance state, and one resistance state after the change is used for storing information,
a control layer containing oxygen is inserted between the first electrode and the first metal oxide film, and formed of a second metal capable of extracting oxygen from the first metal oxide film to prevent oxygen from being thermally diffused from the first metal oxide film to the first electrode,
the second metal included in the control layer is different from the first metal, or same as the first metal,
when the second metal is same as the first metal, a concentration distribution is provided in such a manner that an oxygen concentration of the control layer and the first metal oxide film becomes lower from the first metal oxide film toward the control layer across a boundary between the first metal oxide film and the control layer,
oxide formation free energy of at least one element included in the control layer except for oxygen is lower than oxide formation free energy of an element included in the first electrode, and
both work functions of the second metal and the first electrode are 4.5 eV or less.
2. The variable resistive element according to claim 1 , wherein
a concentration distribution is provided in such a manner that an oxygen concentration of the control layer becomes lower from a side of the first metal oxide film toward a side of the first electrode.
3. The variable resistive element according to claim 1 , wherein
a concentration distribution is provided in such a manner that an oxygen concentration of the first metal oxide film becomes lower from a side of the second electrode toward a side of the control layer.
4. The variable resistive element according to claim 1 , wherein
the first metal oxide film comprises an n-type metal oxide.
5. The variable resistive element according to claim 4 , wherein
the first metal oxide film comprises an oxide of any one of elements of Hf, Zr, Ti, Ta, V, Nb, and W, or a strontium titanate.
6. The variable resistive element according to claim 1 , wherein
the second metal comprises any one of elements of Ti, V, Al, Hf, and Zr.
7. The variable resistive element according to claim 1 , wherein
a work function of the second electrode is 4.5 eV or more.
8. The variable resistive element according to claim 7 , wherein
the second electrode comprises a Ti nitride.
9. The variable resistive element according to claim 1 , wherein
the control layer is thinner than the first electrode.
10. The variable resistive element according to claim 9 , wherein
a film thickness of the control layer is 20 nm or less.
11. A method for producing the variable resistive element according to claim 1 , comprising:
depositing a second electrode material on a substrate, and forming the second electrode;
depositing a first metal oxide film material, a second metal material, and a first electrode material in this order;
forming the first metal oxide film and the first electrode by patterning the first metal oxide film material, the second metal material, and the first electrode material with a common resist mask;
performing a heat treatment; and
applying a forming voltage between the first electrode and the second electrode to perform a forming process, whereby oxygen in the first metal oxide film being partially moved toward the second metal material, the second metal material being changed into the control layer, a resistance state of the variable resistive element being changed from the initial high resistance state to the variable resistance state.
12. A nonvolatile semiconductor memory device comprising a memory cell array having the variable resistive elements according to claim 1 , the variable resistive elements being arranged in at least a column direction of a row direction and the column direction.
13. The nonvolatile semiconductor memory device according to claim 12 , wherein
the memory cell array is provided in such a manner that
the first electrode extends in the column direction to connect the variable resistive elements adjacent in the column direction, and
the control layer extends in the column direction.