Memory devices with reduced operational energy in phase change material and methods of operation

1. A method of operating a memory device, comprising:

performing one of a read operation, a write operation, and a reset operation on a memory cell while a magnitude of an activation energy barrier between a stable phase and a metastable phase of a phase change material of the memory cell is at a first level;

applying a lateral stress to the phase change material to selectively change the magnitude of the activation energy barrier from the first level to a second level differing from the first level; and

performing another of the read operation, the write operation, and the reset operation on the memory cell while the magnitude of the activation energy barrier is at the second level.

2. The method of claim 1 , wherein the first level of the magnitude of the activation energy barrier is higher than the second level of the magnitude of the activation energy barrier.

3. The method of claim 2 , wherein performing one of the read operation, the write operation, and the reset operation on the memory cell while the magnitude of the activation energy barrier is at the first level comprises performing the reset operation.

4. The method of claim 3 , wherein performing the reset operation comprises switching a phase of the phase change material from the stable phase to the metastable phase.

5. The method of claim 4 , wherein switching the phase of the phase change material from the stable phase to the metastable phase comprises switching the phase of the phase change material from a stable crystalline phase to a metastable amorphous phase.

6. The method of claim 3 , wherein performing another of the read operation, the write operation, and the reset operation on the memory cell while the magnitude of the activation energy barrier is at the second level comprises performing the read operation.

7. The method of claim 1 , wherein applying a lateral stress to the phase change material comprises:

inducing strain in a piezoelectric material using an electrical field; and

using the strain in the piezoelectric material to apply physical lateral stress to the phase change material.

8. The method of claim 1 , wherein applying a lateral stress to the phase change material comprises:

inducing strain in a thermal expansion material by at least one of heating and cooling the thermal expansion material; and

using the strain in the thermal expansion material to apply physical lateral stress to the phase change material.

9. The method of claim 1 , wherein applying a lateral stress to the phase change material to selectively change the magnitude of the activation energy barrier from the first level to the second level comprises selectively changing the magnitude of the activation energy barrier by about 0.05 eV or more.

10. A memory device, comprising:

a phase change memory cell comprising:

a first electrode;

a second electrode; and

a phase change material disposed between the first electrode and the second electrode having a first coefficient of thermal expansion; and

a thermal expansion material at least partially surrounding the phase change material of the phase change memory cell, the thermal expansion material having a second coefficient of thermal expansion differing from the first coefficient of thermal expansion of the phase change material, the thermal expansion material selected to selectively stress the phase change material upon heating or cooling of the thermal expansion material and the phase change material during operation of the memory device.

11. The memory device of claim 10 , wherein the thermal expansion material is selected to generate stress within the phase change material sufficient to reduce a magnitude of an activation energy barrier between a stable phase and a metastable phase of the phase change material by about 0.05 eV or more.

12. The memory device of claim 10 , wherein the second coefficient of thermal expansion of the thermal expansion material is at least about one and one-half (1.5) times the first coefficient of thermal expansion of the phase change material.

13. A method of operating a phase change memory device, comprising:

selectively stressing a phase change material in at least one phase change memory cell to maintain a resistance of the phase change material above a threshold level; and

passing current through the phase change material while maintaining the resistance of the phase change material above the threshold level to directly heat the phase change material at least primarily by joule heating within the phase change material.

14. The method of claim 13 , wherein selectively stressing a phase change material in at least one phase change memory cell comprises placing the phase change material under tensile stress.

15. The method of claim 14 , further comprising, after passing the current through the phase change material, placing the phase change material under compressive stress.

16. The memory device of claim 10 , wherein the thermal expansion material further at least partially surrounds the first electrode.

17. The memory device of claim 10 , wherein:

the phase change material comprises GeSbTe; and

the thermal expansion material comprises benzocyclobutene.

18. The memory device of claim 10 , wherein the thermal expansion material is not in physical contact with the first electrode and the second electrode.

19. The memory device of claim 10 , wherein the phase change material extends laterally beyond the first electrode and the second electrode.