The Ovshinsky Effect by Quantum Mechanics

Sept. 8, 2011 - PRLog -- Background

In 1968, Ovshinsky [1] initiated scientific research in the field of amorphous and disordered materials that continues to this day. For a history of Ovshinsky's contributions to American science,  see http://en.wikipedia.org/wiki/Stanford_R._Ovshinsky  Today, the Ovshinsky Effect where the resistance of thin film glasses called chalcogenides is significantly reduced upon the application of low voltage is of fundamental importance in phase-change random-access memory (PC-RAM) devices, the most familiar of which being the rewritable CD-RW disks shown in the thumbnail.

The PC-RAM writing and erasing processes are called SET and RESET. With the GST film in the amorphous state, it is generally [2] thought SET occurs by melting to the crystalline state by heating to temperatures above 600 C.  Memory is erased in RESET by a applying a lower-intensity heat pulse of longer duration to transform the crystalline state back to the amorphous state.

Ovshinsky’s [1] submicron thin films < 500 nm of amorphous chalcogenide GeSbTe (GST) allowed significant reversible transitions in electrical resistance in less than about 1 microsecond. Today, the GST thicknesses [3] are typically < 50 nm changing GST states in less than 100 nanoseconds, but the mechanism by which the high-resistance GST amorphous state switches to the low-resistance  crystalline state is still not understood.

Problem

Quantum mechanics (QM) requires the heat capacity of submicron GST films to vanish, and therefore the heat supplied during SET and RESET cannot be conserved by an increase in GST temperature. What this means is switching from the high to low resistance GST states is not caused by melting from the amorphous to crystalline states. In fact, Ovshinsky never thought [1] melting occurred and instead proposed the resistance changes was caused by a redistribution of charge carriers. By precluding melting, QM has re-opened the controversy between the melting and charge carrier mechanism underlying the Ovshinsky Effect.

Proposed PC-RAM Mechanism

The Ovshinsky Effect occurs by a redistribution of charge carriers as first envisioned [1] in 1968, although Ovshinsky did not identify the charge carriers to be the consequence of QM precluding conservation of heat by an increase in temperature. In fact, the charge carriers are created by QED radiation that produces excitons (holes and electrons) in amorphous GST films by the photoelectric effect.  QED stands for quantum electrodynamics. A similar process for memristors based on resistive Joule heat alone without an external heat source has been proposed. See  http://www.prlog.org/11642370-electrical-charging-of-memr...

GST Resistance by QED Induced Radiation

By the theory of QED induced radiation, electrical charge is naturally created anytime EM energy is absorbed in nanostructures. EM stands for electromagnetic. In PC-RAM devices, QM requires the heat capacity of GST films to vanish, and therefore absorbed EM energy is not conserved by an increase in GST temperature.  Conservation may only proceed by the frequency up-conversion of the absorbed EM energy to the TIR confinement frequency of the GST film. TIR stands for total internal reflection. The QED radiation creates excitons, the mobility of which reduces the resistance of the amorphous GST film. All this occurs without an increase in GST temperature. See Paper in http://www.nanoqed.org at “Ovshinsky Effect by Quantum Mechanics,”2011.    

The TIR confinement of QED photons is enhanced in GST films by the fact the absorbed EM energy is concentrated in the TIR mode because of their high surface to volume ratio. The QED photons having Planck energy beyond the ultraviolet produce excitons, the electrons and holes of which act as Ovshinsky’s charge carriers the mobility of which under the high electric field reduces the resistance of the GST film. RESET is accomplished by removing the exciton holes and electrons by electrical currents and grounding.

Discussion

Experiments Resistance measurements [3] of GST films from 3 to 30 nm sandwiched between 50 nm ZnS-SiO2 films at temperatures from ambient to 400 C did not measure the temperature of the GST film itself, but rather ASSUMED the GST film followed that of the sandwich surface. Clearly, heat was transferred to the GST film, but QM precludes any temperature increase. Simply put, the GST film remains at ambient temperature. Melting does not occur. What is actually being measured is the reduced GST resistance from increased mobility of charge carriers created from QED photons by the photoelectric effect.   GST resistance therefore has nothing to do with crystallization kinetics and attendant Avrami coefficients.      

Materials   Amorphous GST having a refractive index of 4.4 is an optimum material for PC-RAM devices because TIR confinement of QED photons is assured as C and Mo contact materials that sandwich the GST film have lower indices. This is consistent with threshold currents [4] being independent of metal-GST interfaces.

Simulations Numerical simulations of electrical, thermal, and phase change dynamics in the PC-RAM devices [5] based on classical Fourier heat conductions and finite heat capacity lack the QM constraints that negate temperature increase in nanostructures. It is far simpler and more meaningful to obviate Fourier’s law altogether and simulate only the effects of QED radiation on exciton production. .

Conclusions

1. QM precludes increases in temperature of GST films. Applying heat to the film creates QED photons that by the photoelectric effect produce the exciton charge carriers that under high electric fields reduce the GST film resistance.

2. The GST films do not melt and change phase from amorphous to crystalline state. The crystalline state is inconsequential to PC-RAM.

3. Reductions in GST film resistance need not be limited to heat, but anytime EM energy is absorbed, e.g., laser irradiation.

4. The Ovshinsky Effect is a QM size effect only occurring in submicron GST films depending on the mobility of charge carriers.  

5. QM vindicates Ovshinsky’s long-standing explanation that the reduced resistance of GST films is caused by the redistribution of charge carriers – not by melting.  

References

1. S. R. Ovshinsky, “Reversible Electrical Switching Phenomena in Disorder Structures,” Phys. Rev. Lett., 21, 1450-3 (1968).
2. M. Wuttig and N. Yamada, “Phase-change materials for rewriteable data storage,” Nature Materials, 6, 825-32 (2007).
3. X. Wei, et al., “Thickness Dependent Nano-Crystallization in Ge2Sb2Te5 Films and Its Effect on Devices,” Jap. J. Appl. Phys., 46, 2211-4 (2007),
4. S. A. Kostylev, “Threshold and Filament Current Densities in Chalcogenide-Based Switches and Phase-Change-Memory Devices,” IEEE Elect. Dev. Lett., 30, 814-6 (2009)
5. D. H. Kim, et al., “Three-dimensional simulation model of switching dynamics in phase change random access memory cells,” J. Appl. Phys., 064512 (2007).

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About QED Induced EM Radiation: Classically, absorbed EM energy is conserved by an increase in temperature. But at the nanoscale, temperature increases are forbidden by quantum mechanics. QED radiation explains how absorbed EM energy is conserved at the nanoscale by the creation of nonthermal EM radiation.