The microstructure of the Te cell after re-
peated operations, which we investigated with
TEM (Fig. 3, A to D), explains the good ther-
mal stability. The Te layer farthest away from
the TiN plug is fully crystalline, adopting a
trigonal structure (Fig. 3B). Further analysis
of a Te device without programming (fig. S11)
directly confirmed the crystalline state of the
as-deposited Te layer, resulting from its ultra-
low crystallization temperature of−10°C ( 30 , 31 ).
The Te layer right above the TiN plug is also
crystalline but has a different orientation (Fig.
3C). Atomic EDS mapping of Te confirms the
high purity of the Te film.
We measuredI-Vcurves in the subthreshold
region at 100°, 130°, 150°, 175°, and 200°C to
answer the question why the crystalline Te-
based cell possesses a high-resistance state (with
a low leakage current) and volatile-switching
behavior (Fig. 3E). We determined the activa-
tion energy,Ea, using−@log(I)/@(1/kBT) (fig.
S12, A to C), wherekBis the Boltzmann con-
stant andTis the temperature ( 32 ). For
voltages lower than ~0.8 V, theseI-Vcurves
are characterized byIbeing proportional to
exp(V1/2) (fig. S12F) with a large activation
energyEa1of 0.85 eV at ~0 V, suggesting a
Schottky-emission mechanism ( 33 ). Above
1.25 V, however,Iis proportional to exp(V)
after the disappearance of the Schottky barrier
(Fig. 3E and fig. S12F), with an initial activa-
tion energyEa2value of ~0.19 eV. By measuring
the bandgap,Eg, the work function, and the
Fermi level,EF, of a 20-nm-thick Te film, as
well as the work function of the TiN layer (Fig.
3F), the electronic energy-band diagrams for
the Te/TiN interface are experimentally acces-
sible (Fig. 3G). We found anEgof 0.45 eV and a
work function of 4.59 eV for the 20-nm Te
layer, and we found a value of 3.82 eV for the
work function of the TiN film. These values
agree well with previously reported results:
0.33 to 0.49 eV ( 34 ), 4.65 eV ( 35 ), and 3.74 eV,
respectively ( 36 ). For a p-type Te semiconductor
(confirmed by the positive Seebeck coefficient,
8.8 mV/K) ( 37 ),the0.77eVdifferenceinthework
function between Te and TiN results in a large
(0.95 eV) Schottky barrier [= 0.77 + (EF–Ev)],
agreeing well with the value ofEa1at ~0 V (i.e.,
only ~0.1 eV higher). Because of the intrinsic
p-type doping in crystalline Te, a simple semi-
conductor-metal contact with a Schottky bar-
rier can be naturally formed between Te and
its adjacent electrode ( 38 , 39 ). A Schottky bar-
rier was also found for Te-based PCM–metal
interfaces ( 40 ). Hence, only an ultralow cur-
rent, ~0.1 nA, can pass through the Te cell with
a ~0.05 V bias, as we observed (Figs. 1F and3E). In contrast, nonvolatile memory devices
made of crystalline Sb have a low resistance
of ~1.6 kilohms, owing to an absence of the
Schottky barrier (fig. S3B). The Schottky bar-
rier of the Te switch disappears as the voltage
increases above 0.95 V, because then the semi-
conductor behavior of the crystalline Te film
becomes dominant, as indicated by the perfect
match of the 0.18 eV energy barrier (=EF–EV)
with theEa 2 value, ~0.19 eV. This observation
also implies that the OFF current (and the
ON/OFF ratio) can be further decreased (in-
creased) by using electrodes with relatively
different work functions, such as Pt (with a
5.65 eV work-function value) ( 40 ), or few-layer
Te with >1 eV bandgaps ( 41 ).
We carried out in situ TEM investigations
for monitoring the switching process of the Te
cells ( 42 ). We prepared ~100- to 200-nm-
diameter Te nanodevices (Fig. 4A) on a Cu
TEM grid (fig. S13) ( 24 ), having the same
sandwich structure (TiN/Te/TiN, seen in EDS
mapping) as the T-shaped device (Fig. 1A).
Similar to our T-shaped device, an as-deposited
Te layer is in the trigonal phase (Fig. 4C) that
we identified by the corresponding fast Fourier
transform (FFT) image (Fig. 4D). We contacted
the applied nanomanipulator, a tungsten tip,
onto the Te nanodevice (Fig. 4B, inset), through1392 10 DECEMBER 2021•VOL 374 ISSUE 6573 science.orgSCIENCE
Fig. 2. Electrical properties of 200-nm Te switching devices, before
and after thermal annealing.(A) Endurance of a Te device exceeding
2×10^8 cycles of applied voltage pulses. (B) Bidirectional threshold-switching
characteristic of Te switches. The arrows point along the switching directions.
(C) VolatileI-Vbehavior of Te devices with various Te thicknesses (5, 10,
and 20 nm) under consecutive pulses. (D)I-Vbehavior of 200-nm Te devices
annealed at different annealing temperatures (25°, 100°, 200°, 300°, and
400°C). (E) Statistical distribution of transition voltages before and after
thermal annealing. (F) The ON current slightly decreases with increasing
annealing temperature.RESEARCH | REPORTS