Science - USA (2021-12-10)

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By Raffaella Calarco^1 and Fabrizio Arciprete^2

R

esearchers, inspired by neurobio-

logical functions and architecture

of the human brain, have been de-

veloping neuromorphic comput-

ing by using artificial neurons and

synapses to perform processing

and storage in the same physical place.

However, no conceptual technology can

become reality without hardware. The

class of memory devices known as phase

change (PC) memories is expected to en-

able more efficient learning algorithms in

neuromorphic computers, owing to their

capability to also work as a processing unit

( 1 ). On page 1390 of this issue, Shen et al.

( 2 ) use pure elemental tellurium to build

an electrical switch with a large drive cur-

rent and rapid switching speed that can be

used to efficiently operate PC cells in the

cross-point memory architecture (see the

figure). By creating the logic gate out of

a single element, the design gets around

challenges presented by material stoichi-

ometry and selective elemental migration.

The single-element design represents a

step toward high-density, fast, and non-

volatile PC memories.

Chalcogenide alloys are the material of

choice because they have the ability to re-

versibly change their electrical resistivity

upon the application of proper electrical

stimuli and are considered to be an ideal

candidate for making memristive devices

to emulate neuronal synaptic behavior.

In these alloys—many of them tellurium-

based—the resistive switching mechanism

is thermally induced through resistive heat-

ing ( 3 ). In some materials, the change in re-

sistivity is tied to a structural phase change

from crystalline to amorphous, whereas

in others, the change in resistivity occurs

without any structural transformation. The

former kind of material is good for creating

nonvolatile PC memory, with nonvolatile re-

ferring to the ability to retain memory with-

out a power supply. The amorphous phase

has a high resistivity and represents the “0”

logic state. The crystalline phase has a low

resistivity and represents the “1” logic state.

By contrast, the latter kind of material is

good for creating ovonic threshold switches

(OTSs), which can connect and interrupt

the electrical path on demand. When a

threshold voltage is reached, both the PC

memory and the OTS would let the cur-

rent flow pass. However, once the voltage

bias is removed, the PC memory would re-

tain its highly conductive crystalline phase,

whereas the OTS would return to its initial

high-resistivity state.

Although this difference in OTS and PC

memories has been known at least since

the 1960s ( 4 , 5), there has been a renewed

research interest because of their poten-

tial application in a computer architecture

known as the crossbar array, or more spe-

cifically, the 3D XPoint ( 6 , 7 ). In this archi-

tecture, a single PC memory cell is selected

for operation by applying a proper bias to

two crossing electrode bars (see the fig-

ure). Neighboring cells, placed along the

same bars, are usually biased at half the

voltage and act as outlets for unwanted

currents to spill over. The switches—that

is, the OTSs—connected to each PC mem-

ory cell allow for current flow only to the

cell where full bias is applied. Thus, a

single XPoint element is composed of a PC

memory cell and a switch.

To successfully build hardware using

this architecture, a switch must possess

the following properties: a high on-current

density of at least 10 MA/cm^2 , enough to

drive the selected PC memory cell; a high

endurance of up to 10^8 cycles, on par with

that of PC cells; and good thermal sta-

bility to avoid heat-related failures ( 8 ).

Researchers have built OTSs using tellu-

rium-based complex alloys that can fulfill

some of the requirements, but most of

them suffer from alloy composition devia-

tion under switching cycles ( 8 ).

Instead of a classical OTS, Shen et al.

designed a pure tellurium switch that not

only satisfies the aforementioned require-

ments but also shows a switching speed

of around 10 ns, an on-off current ratio of

103 , and the ability to retain its threshold-

switching behavior up to 400°C for 30

min. The authors attribute the impressive

thermal stability of the pure tellurium

switch to its crystalline phase and not to

the amorphous one, as is typical of OTSs.

The switching mechanism is based on the

presence of a potential energy barrier at

the interface between the electrode and

the elemental-crystalline semiconduct-

ing tellurium. The rectifying characteris-

tics of the energy barrier enable the flow

of a negligible leakage current in the off

state. Once in the on state and above 2.5 V,

the tellurium film is melted and becomes

highly conductive. After removing the ap-

plied voltage, the recrystallization of liq-

uid tellurium occurs spontaneously on the

nanosecond time scale. As evidence for the

melting process, the authors observed that

the tellurium crystal was oriented along

COMPUTER ENGINEERING

Keep it simple and switch to pure tellurium

Tellurium switch operates memories through crystalline-liquid-crystalline phase changes

(^1) Institute for Microelectronics and Microsystems (IMM),
National Research Council (CNR), Via del Fosso del
Cavaliere 100, 00133 Roma, Italy.^2 Dipartimento di
Fisica, Università di Roma “Tor Vergata,” Via della Ricerca
Scientifica 1, 00133 Rome, Italy.
Email: [email protected]
Te switch
PC memory
Liquid Te
On
Crystalline Te
High
resistivity
Low
resistivity
Off
Crossbar
Selected cell
Switching between crystalline and liquid phases
Shown is a schematic of the crossbar-array memory architecture 3D XPoint. The individual cells
are connected by the crossbars (electrodes), with each cell containing a tellurium (Te) switch and
a phase change (PC) memory unit.
10 DECEMBER 2021 • VOL 374 ISSUE 6573 1321