A neuromorphic device includes a synapse. The synapse includes a first electrode, a second electrode spaced apart from the first electrode, an oxygen-containing layer disposed between the first electrode and the second electrode, the oxygen-containing layer including oxygen ions, and a stack structure disposed between the oxygen-containing layer and the second electrode, the stack structure including a plurality of reactive metal layers alternately arranged with a plurality of oxygen diffusion-retarding layers. The plurality of reactive metal layers are capable of reacting with oxygen ions of the oxygen-containing layer. The plurality of oxygen diffusion-retarding layers interfere with a movement of the oxygen ions from the oxygen-containing layer to the plurality of reactive metal layers.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A synapse comprising: a first electrode; a second electrode spaced apart from the first electrode; an oxygen-containing layer disposed between the first electrode and the second electrode, the oxygen-containing layer including oxygen ions; and a stack structure disposed between the oxygen-containing layer and the second electrode, the stack structure including a plurality of reactive metal layers alternately arranged with a plurality of oxygen diffusion-retarding layers, wherein the plurality of reactive metal layers are capable of reacting with oxygen ions of the oxygen-containing layer, and wherein the plurality of oxygen diffusion-retarding layers interfere with a movement of the oxygen ions from the oxygen-containing layer to the plurality of reactive metal layers. 2. The synapse of claim 1, wherein a dielectric oxide layer is formed or disappears according to a voltage or current applied to the first electrode and the second electrode, the dielectric oxide layer being formed in at least one of the plurality of reactive metal layers at an interface with a corresponding one of the plurality of oxygen diffusion-retarding layers when the at least one of the plurality of reactive metal layers reacts with the oxygen ions. 3. The synapse of claim 2, wherein as a thickness of the dielectric oxide layer increases, an electrical conductivity of the synapse decreases, and as the thickness of the dielectric oxide layer decreases, the electrical conductivity of the synapse increases. 4. The synapse of claim 2, wherein a thickness of the dielectric oxide layer increases when a number of first electrical pulses applied to the synapse increases, the first electrical pulses having a first polarity, the first electrical pulses being applied to the synapse through the first and second electrodes, and wherein the thickness of the dielectric oxide layer decreases when a number of second electrical pulses applied to the synapse increases, the second electrical pulses having a second polarity opposite to the first polarity, the second electrical pulses being applied to the synapse through the first and second electrodes. 5. The synapse of claim 4, wherein the first electrical pulses have a constant width and a constant magnitude, and the second electrical pulses have a constant width and a constant magnitude. 6. The synapse of claim 1, wherein each of the oxygen diffusion-retarding layers has a thickness incompletely blocking the movement of the oxygen ions. 7. The synapse of claim 1, wherein each of the oxygen diffusion-retarding layers comprises a dielectric material, a semiconductor material, or a combination thereof. 8. The synapse of claim 1, further comprising: a resistance layer located between the first electrode and the oxygen-containing layer, or between the second electrode and the stack structure, or both, the resistance layer increasing a resistance value of the synapse. 9. The synapse of claim 8, wherein the resistance layer comprises a dielectric material, a semiconductor material, or a combination thereof. 10. The synapse of claim 1, wherein the synapse undergoes a depression operation when an electrical conductivity of the synapse decreases as a number of first electrical pulses applied to the synapse increases, the first electrical pulses having a first polarity and being applied to the synapse through the first and second electrodes, and wherein the synapse undergoes a potentiation operation when the electrical conductivity of the synapse increases as a number of second electrical pulses increases, the second electrical pulses having a second polarity opposite to the first polarity and being applied to the synapse through the first and second electrodes. 11. The synapse of claim 10, wherein a change in the electrical conductivity of the synapse in the potentiation operation is substantially symmetric with a change in the electrical conductivity of the synapse in the depression operation. 12. The synapse of claim 10, wherein a rate of change in the electrical conductivity of the synapse in each of the potentiation operation and the depression operation is substantially constant. 13. The synapse of claim 10, wherein the first electrical pulses have a constant width and a constant magnitude, and the second electrical pulses have a constant width and a constant magnitude. 14. The synapse of claim 10, wherein the electrical conductivity of the synapse is constant when at least one of a width and a magnitude of each of third electrical pulses applied to the synapse is less than a certain critical value, the third electrical pulses having the first polarity or the second polarity. 15. A neuromorphic device comprising: a first neuron; a second neuron; a first line connected to the first neuron, the first line extending in a first direction; a second line connected to the second neuron, the second line extending in a second direction crossing the first direction; and a synapse disposed between the first line and the second line in a third direction, the synapse being located at an intersection region between the first line and the second line, the third direction being perpendicular to the first and second directions, wherein the synapse comprises: an oxygen-containing layer including oxygen ions; and a stack structure disposed between the oxygen-containing layer and the second line, the stack structure including a plurality of reactive metal layers alternately arranged with a plurality of oxygen diffusion-retarding layers, wherein the plurality of reactive metal layers are capable of reacting with oxygen ions of the oxygen-containing layer, and wherein the plurality of oxygen diffusion-retarding layers interfere with a movement of the oxygen ions from the oxygen-containing layer to the plurality of reactive metal layers. 16. The neuromorphic device of claim 15, wherein a dielectric oxide layer is formed or disappears according to a voltage or current applied to the synapse through the first line and the second line, the dielectric oxide layer being generated in at least one of the plurality of reactive metal layers at an interface with a corresponding one of the plurality of oxygen diffusion-retarding layers when the at least one of the plurality of reactive metal layers reacts with the oxygen ions. 17. The neuromorphic device of claim 16, wherein an electrical conductivity of the synapse decreases as a thickness of the dielectric oxide layer increases, and the electrical conductivity of the synapse increases as the thickness of the dielectric oxide layer decreases. 18. The neuromorphic device of claim 16, wherein the first neuron and the second neuron are configured to drive, through the first line and the second line, the synapse with first electrical pulses having a first polarity or second electrical pulses having a second polarity opposite to the first polarity, wherein a thickness of the dielectric oxide layer increases with when a number of the first electrical pulses applied to the synapse increases, and wherein the thickness of the dielectric oxide layer decreases when a number of the second electrical pulses applied to the synapse increases. 19. The neuromorphic device of claim 18, wherein the first electrical pulses have a constant width and a constant magnitude, and the second electrical pulses have a constant width and a constant magnitude. 20. The neuromorphic device of claim 15, wherein the synapse further comprises: a resistance layer located between the first line and the oxygen-containing layer, or between the second line and the stack structure, or both, the resistance layer increasing a resistance value of the synapse.