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SCIENCE CHINA Information Sciences, Volume 61, Issue 8: 081302(2018) https://doi.org/10.1007/s11432-018-9404-2

From octahedral structure motif to sub-nanosecond phase transitions in phase change materials for data storage

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  • ReceivedFeb 9, 2018
  • AcceptedMar 30, 2018
  • PublishedJul 5, 2018

Abstract

Phase change random access memory (PCRAM) has been successfully applied in the computer storage architecture, as storage class memory, to bridge the performance gap between DRAM and Flash-based solid-state drive due to its good scalability, 3D-integration ability, fast operation speed and compatible with CMOS technology. Focusing on phase change materials and PCRAM for decades, we have successfully developed 128 Mb embedded PCRAM chips, which can meet the requirements of most embedded systems. 3D Xpoint (3D PCRAM), invented by Intel and Micron, has been regarded as a new breakthrough in the last 25 years since the application of NAND in 1989, which represents state-of-the-art memory technology. This technology has some remarkable features, such as the confined device structure with 20 nm size, the metal crossbar electrodes to reduce the resistance variations in PCRAM arrays, and the ovonic threshold switching selector that can provide a high drive current and a low leakage current. A good understanding of phase change mechanism is of great help to design new phase change materials with fast operation speed, low power consumption and long-lifetime. In this paper, we firstly review the development of PCRAM and different understandings on phase change mechanisms in recent years, and then propose a new view on the mechanism, which is based on the octahedral structure motifs and vacancies. Octahedral structure motifs are generally found in both amorphous and crystalline phase change materials. They are considered to be the basic units during phase transition, which are severely defective in the amorphous phase. These configurations turn into more ordered ones after minor local rearrangements, the growth of which results in the crystallization of rocksalt (RS) phase with a large amount of vacancies in the cation sites. Further driven by thermodynamic driving force, these vacancies move and layer along certain directions; consequently, the metastable RS structure transforms into the stable hexagonal (HEX) structure. Based on our results, we find that reversible phase transition between amorphous phase and RS phase, without further changing into HEX phase, would greatly decrease the required power consumption. Robust octahedra and plenty of vacancies in both amorphous and RS phase, respectively avoiding large atomic rearrangement and providing necessary space, are crucial to achieve the nanosecond or even sub-nanosecond operation of PCRAM.


Acknowledgment

This work was supported by National Key Research and Development Program of China (Grant No. 2017YFB0206101).


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  • Figure 1

    (Color online) Diagram of the operating principle of PCRAM [15]@Copyright 2007 Springer Nature.

  • Figure 2

    (Color online) Diagram of the operating principle of PCRAM [15]@Copyright 2005 IEEE.

  • Figure 3

    (Color online) The threshold switching phenomenon of chalcogenide device.

  • Figure 4

    Umbrella flip model proposed by Kolobov et al. [33,34]. Local structure arrangement of Ge atom inprotect łinebreak (a) crystalline GST and (c) amorphous GST; (b) shows the structure evolution of Ge atom from octahedral site (crystalline phase) to tetrahedral site (amorphous phase) @Copyright 2006 Elsevier.

  • Figure 5

    (Color online) Structure and performance of interfacial phase change memory (iPCM) proposed by Tominaga et al [35,36]. (a) TEM image of iPCM cell in the Reset state (highly resistive); (b)–(e) TEM images of active phase change material in the upper left, lower left, upper right, and lower right regions; (f) performances of iPCM cell operated for the first time and after $10^6$ cycles @Copyright 2011 Springer Nature.

  • Figure 6

    (Color online) Schematic diagram of Sb atoms. (a) and (c) without resonant bonding, and (b) with resonant bonding [37]. Resonant bonding is defined as the unsaturated bonding formed by sharing three p-orbit electrons of a given atom with six neighboring atoms @Copyright 2008 Springer Nature.

  • Figure 7

    (Color online) Ti$_{0.43}$Sb$_2$Te$_3$-based PCRAM and device performances [45]. (a) Ti, Sb and Te element distributions of active phase change area after cycling operation; (b) operation speed of GST and TST-based PCRAM @Copyright 2014 Springer Nature.

  • Figure 8

    (Color online) Structural and chemical identifications of crystalline TST along $\langle~100~\rangle$ direction [49]. (a)–(c) Theprotect łinebreak HAADF image, the respective EDS mapping for Ti/Te, and Sb/Te; (d) STEM-HAADF image zooming into the field marked in (a). Scale bar represents 1 nm. @Copyright 2015 Springer Nature.

  • Figure 9

    (Color online) Comparison of Ti-centred atomic motifs in amorphous and crystalline TST [45]. (a) The amorphous structure of TST; (b) the coordination number distribution of Ge/Te atoms in the amorphous GST/TST; (c) Th bond angle distributions of Ge/Ti atoms in amorphous GST/TST; (d) structural evolutions of Ti-centred atomic motifs upon crystallization @Copyright 2014 Springer Nature.

  • Figure 10

    (Color online) Phase transition of Sb$_2$Te$_3$ from RS to HEX [53]. (a) and (b) are the atomic structure of RS and HEX phases, respectively; (c) and (d) are the STEM-HAADF images of RS and HEX phases, respectively; (e) andprotect łinebreak (f) are the corresponding HAADF intensities of regions of interest in (c) and (d) @Copyright 2016 Springer Nature.

  • Figure 11

    (Color online) Rocksalt SST. Temperature dependence of the sheet resistance of (a) GST and (b) SST films. Selected area electron diffraction pattern and TEM images of rocksalt (c)–(d) Sb$_2$Te$_3$ and (e)–(f) SST films [55]@Copyright 2017 American Association for the Advancement of Science.

  • Figure 12

    (Color online) Set speed and endurance of SST-based PCRAM device [55]. (a) Comparison of SST and GST-based device. SST-based one can achieve the set operation during 700 ps; (b) endurance of SST-based device. The SST device can be repeatedly operated exceeding $\sim$10$^5$ times @Copyright 2017 American Association for the Advancement of Science.

  • Figure 13

    (Color online) The amorphous-RS-HEX crystallization process of Ge$_2$Sb$_2$Te$_5$. (a)–(c) are the atomic structures of amorphous, RS and HEX GST. The light blue, dark green, and brown spheres represent Ge, Sb and Te atoms, respectively.

  • Table 1   The technical performance comparison of various semiconductor memories
    Memory DRAM NOR NAND FeRAM MRAM RRAM PCRAM
    Feature size (nm) 50 65 19 180 130 180 20
    Cell size 6F$^2$ 6F$^2$ 5F$^2$ 22F$^2$ 45F$^2$ 4F$^2$ 4F$^2$
    Memory density 8 Gb 2 Gb 128 Gb 128 Mb 64 Mb 64 Mb 8 Gb
    Read time (ns) $<$10 10 50 45 50 25 60
    Erasing time $<$10 ns 1 us/10 ms 1/0.1 ms 10 ns 50 ns 8.2 ns 100/150 ns
    Operation cycles $>$1E16 $>$1E5 $>$1E5 $>$1E16 $>$1E14 1E6 1E12
    Data retention 64 ms $>$10 yr $>$10 yr $>$10 yr $>$10 yr 10 yr $>$10 yr
    Multi-level storage Yes Yes Yes No Yes Yes Yes
  • Table 2   The technical performance comparison of various semiconductor memories
    Time Events
    September 1966 Stanford Ovshinsky applies first patent on phase-change technology
    September 1970 Gordon Moore publishes an research paper on the technology
    February 2000 Intel invests in Ovonyx, to develop PCRAM
    August 2004 Samsung announces successful fabrication of 64 Mbit PCRAM
    September 2005 Samsung announces successful 256 Mbit PRAM array
    September 2006 Samsung announces a prototype 512 PCRAM device
    October 2006 Intel and STMicroelectronics demonstrate a 128 Mbit PCRAM chip
    December 2006 IBM Research Labs shows a prototype 3 by 20 nanometer
    February 2008 Intel and STMicroelectronics revel the first multilevel PCRAM chip
    September 2009 Samsung announces mass production start of 512 Mbit PCRAM device
    April 2010 Numonyx releases Omneo PCRAM Series (P8P and P5Q), both in 90 nm
    April 2010 Samsung releases 512 Mbit PCRAM with 65 nm process, in multi-chip-package
    February 2011 Samsung presents 58 nm 1.8 V 1 Gb PCRAM
    July 2011 Intel announces multi-phase PCRAM with 90 nm process
    September 2011 SIMIT together with SMIC announce successful 8 Mbit PCRAM chip
    February 2012 Samsung presents 20 nm 1.8 V 8 Gb PCRAM
    July 2012 Micron announces volume production of PCRAM for mobile devices
    May 2016 SIMIT announces 130 nm PCRAM chip as a control part in the printer
    July 2016 Intel presents 3D Xpoint chip with the memory capacity of 128 Gbit
    November 2017 Intel and Micron expands manufacturing for 3D Xpoint (PCRAM)
    December 2017 SIMIT presents 128 Mbit embedded chip

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