which we recorded theI-Vcurves and corre-
sponding times (Fig. 4B). The microstructural
evolution of the Te layer during the switch-
ing process was synchronously recorded by
a high-speed camera in the TEM (movies S1
and S2). The recording time occurs on the
seconds time scale, much longer than the
nanosecond-scale switching process. Still,
the switching processes could be captured
by the applied direct-current voltage (~100 ms
every step).
At the beginning of the switching process
(Fig. 4, G to J, and movie S1), the Te device
was in the OFF state with negligible passing
current. As time proceeded, we measured the
corresponding currents (Fig. 4B, red arrows).
The lattice of the Te film was slightly blurred
owing to a slight trembling of the nanodevice
during switching. Nonetheless, the Te film ap-
peared to be melted as the voltage went be-
yond2.5V(Fig.4,HtoJ),whichcanbe
directly observed in movies S1 and S2. The
flow of the liquid caused a small void to ap-
pear in this unconfined nanodevice (Fig. 4E,
red arrow). This unwanted void does not ap-
pear in the T-shape device after operation
(Fig. 3A). Tellurium easily melts by electri-
cal pulse–induced Joule heating owing to its
low melting point of ~445°C ( 37 ). As liquid
Te exhibits a metal-like low resistivity of ~3 ×
10 −^4 ohms·cm ( 42 ), a submilliampere ON cur-
rent can then pass through the device, as in
the T-shaped Te device (Fig. 1). The 0.28 mA
reduction in the ON current was caused by
the larger protective resistor (10 kilohms) in
series with the Te nanodevice. After remov-
ing the applied voltage, the liquid Te layer
spontaneously recrystallized back into the
trigonal structure (Fig. 4E) owing to its ul-
tralow crystallization temperature (–10°C),
with a nanosecond-scale crystallization speed
( 29 , 43 ), which in turn resulted in the re-
currence of the OFF state. The recrystallized
Te had a noticeably different crystalline ori-
entation before (Fig. 4D) and after (Fig. 4F)
switching, further confirming the crystalline-
liquid-crystalline phase-transition mecha-
nism in the pure elemental-Te switch. This
process is also responsible for the ~31.5%
thickness increase in the active Te layer
above the TiN plug in the T-shaped device
we observed and explained ( 24 ) after 3 ×
108 cycles of operation when the device failed
(fig. S14).TheunderlyingmechanismofthepureTe
volatile switch intrinsically differs from that of
chalcogenide-based nonvolatile PCMs, which
depend on the amorphous-to-crystalline phase
transition (fig. S3). The large Schottky barrier
formed at the elemental-Te–electrode inter-
face enables a low leakage current required for
the switch cell, which is absent in the elemental-
Sb nonvolatile memory device. Our Te devices
are also unlike the OTS, in which the chalco-
genidematerialsremainfrozenintheamor-
phous state throughout the switching process.
Not only may the elemental-Te switch we pre-
sented be considered a new switching type, we
also expect more elements and/or compounds
with low crystallization temperature and fast
crystallization ability to possess such volatile
switchability, but only if there is a sufficiently
large difference in properties of the ON and
OFF states. In Te, this difference is due to the
metallic-like–liquid and semiconducting-crystal
behavior. Integration of pure Te (as the switch-
ing selector cell) with pure Sb (as the non-
volatile memory cell) ( 10 ) would be an interesting
combination, offering the potential for fabri-
cating entirely single-element–based memory
arrays. We have demonstrated that the deviceSCIENCEscience.org 10 DECEMBER 2021•VOL 374 ISSUE 6573 1393
Fig. 3. Origin of the low OFF current in the Te switch.(A) Cross-sectional
TEM image of the Te device after pulsed switching. (B) TEM image of the Te layer
farther away from the TiN electrode. As-deposited Te is in the crystalline state
owing to its very low crystallization temperature, and the inset shows Te to be
trigonal (a=b=4.512Åandc= 5.960 Å). (C)TEMimageoftheTelayerabovethe
TiN electrode. After switching, the Te layer remains trigonal but with a different
crystal-growth orientation. Corresponding atomic mapping is shown in (D). (E)I-V
characteristics of a Te switch in the subthreshold region, measured at 100°, 130°,
150°, 175°, and 200°C, from which the activation energy,Ea, can be obtained.
TwoI-Vrelationships,Iºexp(V1/2) andIºexp(V), are found (fig. S12).
(F) Bandgap, work functions, and Fermi level of a 20-nm-thick Te film. A Tauc
plot of the optical-absorption coefficient,a, that is (ahn)1/2, versus photon
energy,hn, of a crystalline Te film indicates a bandgap of 0.45 eV. The work
function is 4.59 eV for a Te film, whereas it is 3.82 eV for the TiN electrode, as
obtained from ultraviolet (UV) photoelectron spectra. UV-excited valence-band
spectra show the Fermi-level energy (EF) to be 0.18 eV for Te. a.u., arbitrary
units. (G) Experimentally determined energy-band diagrams of the p-type Te/TiN
interface, obtained from (F).Ea1arises from the Schottky barrier, whereasEa2
corresponds to the energy required to move hole carriers fromEFto the valence-
band edge (Ev).Evac, vacuum level;EC, conduction-band edge.RESEARCH | REPORTS