PHASE-CHANGEMEMORY
Elemental electrical switch enabling
phase segregationÐfree operation
Jiabin Shen1,2†, Shujing Jia1,2†, Nannan Shi^3 , Qingqin Ge^3 , Tamihiro Gotoh^4 , Shilong Lv^1 , Qi Liu^5 ,
Richard Dronskowski^6 , Stephen R. Elliott7,8, Zhitang Song^1 , Min Zhu^1
Nonvolatile phase-change memory has been successfully commercialized, but further density scaling below
10 nanometers requires compositionally and structurally homogeneous materials for both the memory cell and
the associated vertically stacked two-terminal access switch. The selector switches are mostly amorphous-
chalcogenide Ovonic threshold switches (OTSs), operating with a nonlinear current response above a threshold
voltage in the amorphous state. However, they currently suffer from the chemical complexity introduced
by the quaternary or even more diverse chalcogenide compositions used. We present a single-element
tellurium (Te) volatile switch with a large (≥11 megaamperes per square centimeter) drive current density,
~10^3 ON/OFF current ratio, and faster than 20 nanosecond switching speed. The low OFF current arises
from the existence of a ~0.95–electron volt Schottky barrier at the Te–electrode interface, whereas a
transient, voltage pulse–induced crystal-liquid melting transition of the pure Te leads to a high ON current.
Our discovery of a single-element electrical switch may help realize denser memory chips.
T
he explosive growth of digital data urgent-
ly requires memory devices capable of
ultrafast programming speeds and ultra-
high densities, yet these requirements
cannotbefulfilledbymainstreamdy-
namic random-access memory (DRAM) and
flash devices ( 1 , 2 ). Nonvolatile phase-change
memory (PCM), based on reversible (crystal-
to-amorphous) structural changes of chalco-
genides, exhibits a thousand times faster
operationthanflashmemory.PCMalsohasa
storage density (in terms of the parameter 4F^2 ,
where F denotes the feature size) that is com-
parable with flash, currently one of the main
nonvolatile memory technologies ( 3 , 4 ). PCM-
based products, dubbed Optane, were suc-
cessfully launched into the market in 2017 by
Intel and Micron ( 5 ). Gigabyte-scale Optane
uses a 20-nm node technology and a three-
dimensional (3D) stacked architecture ( 5 ). Fur-
ther advances to fabricate even denser PCM
devices can be achieved by continued dimen-
sional downscaling to sub-10-nm feature sizes
( 6 , 7 ).
A full PCM device is composed of a memory
cell and an associated access-switching device
(also called a selector). Aggressive miniatur-
ization of both components is needed ( 8 ). This
requires not only state-of-the-art manufactur-
ing but also that the materials used in both
cells be (and remain) extremely composition-
ally and structurally homogeneous on the nano-
meter scale; this is necessary to avoid unwanted
performance variations and also to guarantee
long switching-operation lifetimes. Complex
Ge-Sb-Te–based chalcogenides are likely used
in the Optane PCM technology ( 9 ), but a dif-
ferent approach would be to use elemental-Sb
phase-change nonvolatile memory material that
is pure and intrinsically homogeneous, which
has been previously explored ( 10 ).
Each memory-cell operation—including the
most frequent, the read operation—needs to
access the switch; therefore, excellent switch
reliability and ultralong cycling lifetimes are
required ( 11 ). As a consequence, the switching
cell faces greater challenges for higher-density
scaling. When using chalcogenides as the
memory cell, albeit with different stoichiom-
etries, two-terminal chalcogenide Ovonic
threshold switches (OTSs) have successfully
been used in 3D PCM products instead of
charge-based Si diode and MOSFET (metal-
oxide-semiconductor field-effect transistor)
switches ( 8 , 12 , 13 ). As only amorphous chal-
cogenides exhibit the volatile OTS behavior,
elements such as As, Si, and N are often in-
troduced as dopants to improve the thermal
stability to withstand a temperature of 400°
to 450°C in the back-end-of-line (BEOL) fab-
rication process ( 14 – 16 ). OTS cells, being quat-
ernary compounds (e.g., Ge-As-Se-Si) or having
even more complex compositions, can suffer
from compositional inhomogeneities ( 17 – 19 ).
This challenge becomes more serious when
decreasing the scale, as reduced dimensions re-
sult in a drastic degradation of thermal sta-
bility caused by interfacial hetero-crystallization( 20 , 21 ).Thismaymaketheconventionaldo-
ping strategy ineffective, unpredictable, and
time consuming. However, the continuing ex-
ploration of Ge-, Si-, Al-, B-, and C-Te OTSs (es-
pecially Te-rich Ge) has provided some valuable
clues for improving stability ( 15 , 22 , 23 ). Be-
cause pure elemental Ge, Si, Al, B, and C mate-
rials are widely known as semiconductors or
metals, without any intrinsic switching ten-
dency, this suggests that elemental tellurium
plays a dominant role in the volatile-switching
process. The crucial question then rests on the
abilityofpureTetoswitch.
In this study, we prepared T-shaped switch-
ing devices with between 60- and 200-nm-
sized electrodes made of pure Te films (with
≥99.99% purity). One cross-sectional trans-
mission electron microscopy (TEM) image
shows a Te device with a 60-nm TiN plug,
above which we stacked a ~20-nm Te layer
(Fig. 1A). Before deposition of the Te, we etched
the device with Ar ions for 20 min to clean the
surface oxide layer. Cleaning was confirmed by
the fact that Te-free cells had a low resistance
of between 150 ohms and 2 kilohms [see ( 24 )
for fabrication details and tables S1 and S2].
The Te layer is intrinsically homogeneous,
without any oxide layer, which we confirmed
by the overlapped corresponding energy-
dispersive spectroscopy (EDS) mappings of
W, Ti, Te, and Si (fig. S1). We measured the
current-voltage (I-V) curves of Te devices
under consecutive pulses (Fig. 1B). Initially,
the Te device was in a high-resistance state
(OFF state) through which a negligible
current passed. As we increased the voltage
to ~2.5 V (the firing or forming voltage,Vfire),
the current suddenly increased to ~1 mA,
and a negative differential resistance behav-
ior appeared. Upon reducing the voltage to
below ~0.9 V, the so-called holding volt-
age (Vhold), the cell returned to the high-
resistance state. In the second sweep, we
only needed ~1.2 V (threshold voltage,Vth)
to trigger the transition from the OFF state
to the ON state, and we obtained an ON
current of ~0.4 mA. The switching behav-
ior was completely reproducible, with aVth
variation of ~0.4 V (fig. S2). The typical,
volatileI-Vbehavior directly proves that pure
Te devices behave as switches ( 15 , 22 ). Intrin-
sically and completely different from the Te
cells, elemental-Sb devices, with the same
T-shaped device structure and 20 nm film
thickness, exhibit a typically nonvolatile mem-
ory feature characterized by a nonvolatileI-V
behavior (fig. S3), as previously reported ( 10 ).
Interestingly, the values of programming volt-
ages for the Te cells, includingVfire,Vth, and
Vhold, hardly vary with the electrode size (Fig.
1C), with a maximum variation of 0.7 V (for
Vfire) for more than 100 cells (fig. S4), there-
by indicating predictable parameters for cir-
cuit design upon scaling.1390 10 DECEMBER 2021•VOL 374 ISSUE 6573 science.orgSCIENCE
(^1) State Key Laboratory of Functional Materials for Informatics,
Shanghai Institute of Microsystem and Information
Technology, Chinese Academy of Sciences, 200050
Shanghai, China.^2 University of the Chinese Academy of
Sciences, Beijing 100029, China.^3 Thermo Fisher Scientific
China, Shanghai 200050, China.^4 Department of Physics,
Graduate School of Science and Technology, Gunma
University, Maebashi 3718510, Japan.^5 Frontier Institute of
Chip and System, Fudan University, Shanghai 200433,
China.^6 Institute of Inorganic Chemistry, Chair of Solid-
State and Quantum Chemistry, RWTH Aachen University,
Aachen 52056, Germany.^7 Trinity College, University of
Cambridge, Cambridge CB2 1TQ, UK.^8 Physical and
Theoretical Chemistry Laboratory, University of Oxford, Oxford
OX1 3QZ, UK.
*Corresponding author. Email: [email protected] (M.Z.);
[email protected] (Z.S.)
These authors contributed equally to this work.
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