The ON current, ~0.4 mA, was also size
independent (Fig. 1D, inset). In other words,
the ON current density,JON, quadratically in-
creased with a decrease of the device size. We
achieved aJONvalue of ~11 MA/cm^2 for a
60-nm device, which further increased to
14.9 MA/cm^2 for a 45-nm device (fig. S5).
This is comparable to Te- and Se-based OTSs
(fig. S6) and sufficient for programming PCM
cells (requiring≥10 MA/cm^2 )( 15 , 22 , 25 ). We
found largeJONvalues within ~12 ns (Fig. 1E
and fig. S7), while the time for turning off the
switch was ~13 ns. This switching time is
almost comparable to DRAM (~10 ns), sug-
gesting that Te switches may also serve in
future DRAM-like PCM devices ( 21 , 26 ). To
determine the leakage current, we performed
direct-currentI-Vmeasurements (Fig. 1F) with
a protective 1-kilohm resistor in series with the
Te device. Initially, a current of <0.1 nA passed
through the Te cell, and then the current ex-
ponentially increased upon increasing the volt-
age. We found an OFF current of ~0.45mA at
Vth/2 in the 60-nm device, resulting in a ~10^3
ON/OFF ratio.
By its very nature, the pure elemental-Te
switching material circumvents any phase-
segregation issues, which are often ob-
served in failed switching cells. We achieved a
2×10^8 – cycle endurance by using square voltage
pulses (Fig. 2A and fig. S8). Our corresponding
dynamical-response results clearly show that
theTeswitchwassuccessfullyturnedonand
off by each pulse during the 2 × 10^8 – cycle
endurance test (fig. S8). In addition, switch-
ing cells made from tellurium evidence a bi-
directional threshold-switching characteristic
(Fig. 2B), suggesting potential applications in
other emerging memory types, such as resis-
tive random-access memory (RRAM) ( 27 ). The
Te films are still switchable down to 5-nm
thickness without sacrificing the ON current
(Fig. 2C) and OFF current (fig. S9). Surpris-
ingly, the value ofVthdecreases to ~1.1 V for
both 5-nm and 10-nm Te devices, implying
that the Te device does not follow an electric
field–driven process, making it distinctly dif-
ferent from OTS cells.
Given a drive current density greater than
10 MA/cm^2 , a switching speed on the nano-
second scale, and an ON/OFF current ratio of
~1000, as well as an intrinsically homoge-
neous composition, we consider the pure
elemental-Te switch to be a very promisingselector candidate for PCM applications. More-
over, the Te material is perfectly compatible
with the materials in PCM memory cells,
which are typically Ge-Sb tellurides, there-
by enabling stacking-compatible integra-
tion ( 4 , 28 ). The final factor that needs to
be considered is the thermal stability of Te
switches; more specifically, whether they can
withstand a thermal treatment of 400°C for
30 min, as required in the BEOL process
( 14 – 16 ). To study the thermal stability, we
annealed 200-nm Te cells at 100° to 400°C
for 30 min. We measured theirI-Vcurves at
room temperature after the thermal treatment
(Fig. 2D). The Te switch still exhibits a clear
threshold-switching behavior, even after the
400°C treatment. We observed a voltage in-
crease of ~0.9 V inVfire,togetherwitha~0.25V
increase forVthandVhold. Most importantly,
the ON current decreased by only ~0.09 mA.
The difference in switching voltage and ON
current is mainly due to the increase of the
TiN resistance under high-temperature anneal-
ing (table S3) ( 29 ). That said, the ON switch-
ing speed remains at ~15 ns (fig. S10). These
results therefore directly show that our pure
Te switch is BEOL compatible.SCIENCEscience.org 10 DECEMBER 2021•VOL 374 ISSUE 6573 1391
Fig. 1. Structure and performance of Te switching devices.(A) Cross-
sectional TEM image of a Te cell with a ~60-nm TiN plug and corresponding
EDS elemental mappings. The Te layer is ~20 nm thick. (B) VolatileI-V
behavior of Te devices with various device sizes (60, 120, 150, and 200 nm)
under consecutive voltage pulses.DBEis the diameter of the bottom electrode
(BE). (C) Statistical distribution of transition voltages (Vfire,Vth, andVhold) in
more than 100 Te devices. (D) The ON current density increases quadratically
as the device size is reduced. A 60-nm cell already exhibits an ON current
density of ~11 MA/cm^2. The inset shows the statistical distribution of ON
currents measured atVth, appearing to be size independent. (E) ON and OFF
switching speeds, ~12 and ~13 ns, respectively, obtained from the measured
currents (orange) by applying ~4-V triangular voltage pulses with rising and
falling edges of 1ms (blue). (F) DCI-Vcurves of Te devices in series with
a 1-kilohm resistor. ON and OFF currents (measured atVthandVth/2) are
~0.5 mA and ~0.5mA, respectively. The ratio of ON and OFF currents is thus
~10^3 and remains at this value while the electrode size changes.RESEARCH | REPORTS