A neuromorphic device includes a substrate; a first electrode and a second electrode that are disposed over the substrate, extend in a first direction, and are spaced apart in a second direction; a stack structure between the first electrode and the second electrode, which includes reactive metal layers alternately stacked with one or more insulating layers; an oxygen-containing layer between the first electrode and the stack structure, which includes oxygen ions; and an oxygen diffusion-retarding layer between the stack structure and the oxygen-containing layer. The first direction is perpendicular to a top surface of the substrate, and the second direction is parallel to the top surface of the substrate. Each reactive metal layer may react with the oxygen ions to form a dielectric oxide layer. The oxygen diffusion-retarding layer interferes with a movement of the oxygen ions. A thickness of the oxygen diffusion-retarding layer varies along the first direction.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A neuromorphic device comprising: a substrate; a first electrode and a second electrode that are disposed over the substrate, the first and second electrodes extending in a first direction and being spaced apart from each other in a second direction, the first direction being perpendicular to a top surface of the substrate, the second direction being parallel to the top surface of the substrate; a stack structure disposed between the first electrode and the second electrode, the stack structure including a plurality of reactive metal layers which are alternately stacked with one or more insulating layers, wherein each of the reactive metal layers is capable of reacting with oxygen ions to form a dielectric oxide layer; an oxygen-containing layer disposed between the first electrode and the stack structure, the oxygen-containing layer including the oxygen ions; and an oxygen diffusion-retarding layer disposed between the stack structure and the oxygen-containing layer, the oxygen diffusion-retarding layer interfering with a movement of the oxygen ions from the oxygen-containing layer to the reactive metal layers, and wherein a thickness of the oxygen diffusion-retarding layer varies along the first direction. 2. The neuromorphic device of claim 1, wherein the thickness of the oxygen diffusion-retarding layer increases from a top of the oxygen diffusion-retarding layer to a bottom of the oxygen diffusion-retarding layer. 3. The neuromorphic device of claim 1, wherein the thickness of the oxygen diffusion-retarding layer decreases from a top of the oxygen diffusion-retarding layer to a bottom of the oxygen diffusion-retarding layer. 4. The neuromorphic device of claim 1, wherein the second electrode includes a plurality of second electrodes, which are spaced apart from each other, and wherein the stack structure includes a plurality of stack structures, which are spaced apart from each other, each of the plurality of stack structures corresponding to a respective one of the plurality of the second electrodes. 5. The neuromorphic device of claim 4, further comprising: an interlayer insulating layer disposed between two adjacent second electrodes and between two adjacent stack structures corresponding to the two adjacent second electrodes. 6. The neuromorphic device of claim 5, further comprising: a slit penetrating through the interlayer insulating layer and extending in a direction crossing a region between the two adjacent second electrodes and between the two adjacent stack structures, the slit extending from the first electrode. 7. The neuromorphic device of claim 6, further comprising: a filling layer disposed in the slit, the filling layer including the same material as at least one of the oxygen-containing layer and the oxygen diffusion-retarding layer. 8. The neuromorphic device of claim 1, wherein thicknesses of the plurality of reactive metal layers are different from each other. 9. The neuromorphic device of claim 2, wherein thicknesses of the plurality of reactive metal layers increase along a direction from a top of the stack structure to a bottom of the stack structure. 10. The neuromorphic device of claim 3, wherein thicknesses of the reactive metal layers decrease along a direction from a top of the stack structure to a bottom of the stack structure. 11. The neuromorphic device of claim 1, wherein the oxygen-containing layer is disposed against a side surface of the first electrode, the oxygen-containing layer encircling the first electrode. 12. The neuromorphic device of claim 11, wherein the oxygen diffusion-retarding layer is disposed against a side surface of the oxygen-containing layer, the oxygen diffusion-retarding layer encircling the oxygen-containing layer. 13. The neuromorphic device of claim 1, wherein a reset operation is performed when a reset voltage having a first polarity is applied through the first electrode and the second electrode, the reset operation including forming or thickening the dielectric oxide layer in at least one of the plurality of reactive metal layers at an interface with the oxygen diffusion-retarding layer, and wherein a set operation is performed when a set voltage having a second polarity opposite to the first polarity is applied through the first electrode and the second electrode, the set operation including disappearing or thinning the dielectric oxide layer. 14. The neuromorphic device of claim 13, wherein the dielectric oxide layer is formed the fastest in a first reactive metal layer of the plurality of reactive metal layers, the first reactive metal layer being separated from the oxygen-containing layer by a thinnest region of the oxygen diffusion-retarding layer. 15. The neuromorphic device of claim 13, wherein thicknesses of the plurality of reactive metal layers are different from each other, and the dielectric oxide layer is formed the fastest in a first reactive metal layer of the plurality of reactive metal layers, the first reactive metal layer having the smallest thickness of the plurality of reactive metal layers. 16. The neuromorphic device of claim 13, wherein the dielectric oxide layer includes one or more dielectric oxide layers, and wherein an electrical conductivity decreases as a number of the one or more dielectric oxide layers increases or a thickness of each the one or more dielectric oxide layers increases, and the electrical conductivity increases as the number of the one or more dielectric oxide layers decreases or the thickness of each of the one or more dielectric oxide layers decreases. 17. The neuromorphic device of claim 13, wherein the dielectric oxide layer includes one or more dielectric oxide layers, and wherein, during the reset operation, a number of the one or more dielectric oxide layers increases or a thickness of each of the one or more dielectric oxide layers increases when a number of pulses of the reset voltage applied through the first electrode and the second electrode increases, and wherein, during the set operation, the number of the one or more dielectric oxide layers decreases or a thickness of each of the one or more dielectric oxide layers decreases when a number of pulses of the set voltage applied through the first electrode and the second electrode increases. 18. The neuromorphic device of claim 17, wherein, during the reset operation, the pulses of the reset voltage have a constant width and magnitude, and wherein, during the set operation, the pulses of the set voltage have a constant width and magnitude. 19. The neuromorphic device of claim 1, wherein the oxygen diffusion-retarding layer has a thickness that incompletely blocks the movement of the oxygen ions. 20. The neuromorphic device of claim 1, wherein the oxygen diffusion-retarding layer comprises a dielectric material, a semiconductor material, or a combination of the dielectric material and the semiconductor material.