Three Resistance States Achieved by Nanocrystalline Decomposition in Ge‐Ga‐Sb Compound for Multilevel Phase Change Memory

Abstract

technologies encodes digital information via fast and reversible transition between amorphous and crystalline phases of chalcogenide phase-change materials. [1][2][3][4][5][6][7] The phase transitions are precisely controlled by electrical pulses to realize the SET (crystallization) and the RESET (melt-quenched amorphization) operations. PCM has the characteristics of fast switching speed (<1 ns), [8] long data retention (>10 years), [9,10] and good scalability, [11,12] bridging to some extent the performance gap between the slow solid-state drive (SSD) and the fast dynamic random access memory (DRAM).The large resistance contrast (over 2-3 orders of magnitude) between amorphous and crystalline phases allows intermediate resistance states to be populated in, and thus multiple bits of data can be stored in a single memory cell, also known as multilevel cell (MLC) technology. The MLC increases the storage capacity of PCMs exponentially without changing the physical volume, and thus the cost per bit of data is remarkably reduced. Over the past years, many methods have been developed to enable MLC PCMs, including: 1) stacking multiple layers: multiple resistance levels are obtained owing to the designed heat profile inducing phase transition of chalcogenide layers sequentially; [13] 2) write programming strategy: modulating the programming pulse protocols to systematically control the volume ratio of amorphous to crystalline phases; [14,15] and 3) materials engineering: to design novel materials with multiple crystallization stages to achieve MLC. [16,17] Prototypical phase change material Ge 2 Sb 2 Te 5 (GST) has been explored for multi-bit date storage. However, the relatively low thermal stability (T c = 150 °C) and narrow voltage programming margin (i.e., the resistance is very sensitive to the change of voltage) are key issues that limit the adoption of GST in MLC applications. [18] It has been recently reported that phase change material based on pseudobinary Ga-Sb-Ge system shows fast switching speed (80 ns), good endurance (10 9 cycles), and high reliability (10 years at 220 °C). [10] Moreover, Ge-Sb, Ga-Sb, and Ge-Ga-Sb alloys were found to have a certain degree of phase separation at different temperatures, [19][20][21][22][23] which may result in variations of resistance. Based *on the above features of Phase-change memory (PCM), using the fast and reversible transition between crystal and glass to store binary data, is a promising candidate for next-generation information storage and computing technologies. Recording more than one bit of information on each memory cell, known as multilevel cell (MLC) technology, can greatly increase the data density of PCMs. In this paper, the MLC capability in a phase change material Ge-Ga-Sb (GGS) is explored. Using the "SET" operation with increasing voltage amplitudes on this PCM cell in a 250 nm pillar structure device, three resistance levels are achieved and can be stabilized within large operating voltage windows, allowing large tolerance of SET voltage variation ...

Unraveling the Atomic Structure of Bulk Binary Ga–Te Glasses with Surprising Nanotectonic Features for Phase-Change Memory Applications

“…Germanium–tellurium binary and ternary phase-change materials (PCMs) belong to flagship PCMs and are widely used for optical storage and nonvolatile memory applications and recently for the development of brain-inspired computing technologies. − The maximum working temperatures of the PCM elements of this type are limited by low- T glass transition and crystallization phenomena. Consequently, automotive and similar high- T applications need novel PCM systems capable of performing reliable temperature-resistant operation. − …”

Design of all-phase-change-memory spiking neural network enabled by Ge-Ga-Sb compound

“…We expect to improve this performance upon further scaling-down of the device size [43], and the variability of the device has been estimated in the emulation of the neural network. Note that the electric response to short pulses (18 ns) of GGS is totally different from the previous study [31], which used much longer stimulant pulses (150 ns), so that the materials are directly annealed to the LRS without multiple intermediate phases. The short-pulse operation method allows more resistance states in GGS, enabling its applications in neuromorphic devices beyond multi-level data storage.…”

“…3a, a partial crystallization is observed. This is because the short-width pulses could not generate enough heat to crystallize the entire GGS area in the device, unlike early experiments [31]. Fig.…”

“…To this end, we took advantage of an unconventional phase change material Ge 15 Ga 25 Sb 60 (GGS) which can realize both neuronal and synaptic functions. We focused on this material because previous studies showed that the GGS compound can achieve multiple resistance levels with low resistance drift, high thermal stability, and large dynamic range [31]. In this work, we applied this material to the neuromorphic applications, instead of mere data storage.…”

Sputter‐grown GeTe/Sb 2 Te 3 superlattice interfacial phase change memory for low power and multi‐level‐cell operation

“…It should, however, be noted that this multi‐level feature is difficult to be implemented in the case of a GST PCM as it is difficult to control the uniformity of the thermal distribution applied to the phase change material. In addition, since the major obstacle of data retention in multi‐level GST PCM is the resistance drift caused by structural relaxation of amorphous GST, the resistance drift coefficient was calculated by using Equation (), where R 0 is the initial resistance at t 0 , and v is the fitted resistance drift coefficient to confirm the unique operation mechanism of iPCM [10]: …”