Toward Multiple-Bit-Per-Cell Memory Operation With Stable Resistance Levels in Phase Change Nanodevices

Abstract

Resistance drift of the amorphous states of multilevel phase change memory (PCM) cells is currently a great challenge for the commercial implementation of a reliable multiple-bit-per-cell memory technology. This paper reports observation of a stable intermediate state for a multilevel PCM cell that is achieved through nonuniform heating with a square current injection top electrode. Drift coefficient of the intermediate state is an order of magnitude lower than reset and has weaker temperature dependence. Using finite-element simulations and an analytical model for the subthreshold current-voltage characteristics, based on thermally activated hopping of charge carriers across Coulombic donor-like traps, we conclude that the defect density is two orders of magnitude larger in the intermediate state. We attribute the low drift coefficient of the intermediate state to a large number of stable interfacial defects which dominate the electron transport. Current findings give way to a more stable ultrahigh-density PCM device. Index Terms-Multiple-bit-per-cell memory, phase change memory (PCM), resistance drift.

Three-Dimensional Magnetic Page Memory

“…The device was measured 2 to be 0.026W/m.K, yielding more than two orders of magnitude reduction in the thermal conductivity. Such high resistance to heat flow at the interface between a metal and an insulator is an indication of a poor overlap between the phonon states providing few phonon modes available for heat transfer 3 . At 220ºC it was found that the interfacial thermal conductivity displays a modest increase by 10% to ~0.03 W/m.K.…”

“…This approach is already used in optical recording such as CDs (compact discs) and DVDs (digital versatile discs) where the heat source is a laser pulse that causes a change in the crystallinity of the chalcogenide recording medium leading to marked modifications of the optical reflectivity 1 . If the laser heat source is replaced by resistive heating from electrical pulses and the physical property being monitored is the electrical resistivity of the chalcogenide material then the resulting device (phase-change memory) is now being utilized in storage-class memory applications 2,3 . In magnetic recording, in order to overcome the limitations in scaling imposed by the thermal stability of small grains 4 , thermallyassisted recording with a laser beam coupled to a near field transducer as the heat source instantaneously reduces the reversal fields required to write high-anisotropy media, allowing thermally stable grain sizes down to a few nm 5 , 6 .…”

“…Such a memory design with three dimensional storage capability is very desirable from a technology point of view 10 , 11 , 12 since it is expected to provide a low-cost per bit, high-speed, and ultra-dense memory solution. In addition, such a three dimensional magnetic memory does not suffer from loss of information in the long run due to resistance drift in PCM 3 and charge leakage in flash memories. This high potential inspired the industry to investigate alternative approaches such as three dimensional magnetic domain-wall racetrack memory 13 , 14 , 15 based on the interaction of a spin-polarized current pulse with a magnetic domain-wall 16 .…”

“…and an insulator is an indication of a poor overlap between the phonon states providing few phonon modes available for heat transfer3 . At 220ºC it was found that the interfacial thermal conductivity displays a modest increase by 10% to ~0.03 W/m.K.…”