Switching operation of the PCRAM device employing the superlattice phase-change film. The magnetoresistance of the device to a magnetic field of 0.1 tesla changes by 2000% at room temperature. Initial operation with no magnetic field is shown in red, operation with a magnetic field present is shown in blue, and operation when the magnetic field is removed is shown in gray. Image: American Institute of Physics |
Junji
Tominaga of the Nanoelectronics Research Institute of the National
Institute of Advanced Industrial Science and Technology (AIST) and of
Collaborative Research Team Green Nanoelectronics Center, AIST, and
others have recently discovered that a superlattice phase-change film
multilayered using germanium-tellurium alloy sub-layers and
antimony-tellurium alloy sub-layers with aligned orientation axes has a
magnetoresistance effect in excess of 2000% in a temperature range from
room temperature to around 150 C.
This
giant magnetoresistance effect originates in a physical phenomenon
known as a topological insulator. It was also discovered that this
superlattice phase-change film has a magneto-optical effect in which its
optical reflectivity in relation to circularly polarized light in the
visible wavelength region (from 400 to 800 nm) changes in response to
the direction of an external magnetic field.
Thin
films of a germanium-antimony-tellurium alloy are typically used in
conventional phase-change random access memories (PCRAMs). The research
group has engaged in the development of an interfacial phase-change
memory multilayered using the germanium-tellurium alloy sub-layers and
the antimony-tellurium alloy sub-layers while aligning their orientation
axes.
The
group recently discovered that the memory has a magnetoresistance
effect of more than 2000% at room temperature and higher without
including any magnetic elements such as cobalt and platinum. The
discovery of a room-temperature giant magnetoresistance effect in a
PCRAM could lead to an entirely new ultra-high density memory, namely a
multiple-functional memory by the fusion or integration of PCRAM and
magnetic resistance memory (MRAM) technologies that are being advanced
as one of next-generation non-volatile memory technologies. In addition,
the film has the potential for high-speed switching by an electric
field, and as a result may find application in non-volatile logic
technologies.
This
research was supported by a grant from the Cabinet Office’s Funding
Program for World-Leading Innovative R&D on Science and Technology.
Details of the results will be published in a scientific journal,
“Applied Physics Letters,” Vol. 99, No. 15.
Origin of PCRAM, and competing technologies
In
the area of next-generation non-volatile memory, attention is being
focused on PCRAM, which exploits the difference in electrical resistance
between the crystalline and amorphous states of chalcogenide alloys,
MRAM, which exploits the difference in electrical resistance generated
by a magnetoresistance effect of magnetic alloys, and a resistive random
access memory (RRAM), which exploits the difference in electrical
resistance generated by the crystalline states of oxides through the
application of a strong electric field.
PCRAM
and MRAM have already been in practical use, but PCRAM has the
disadvantage of joule heating to at least the melting point of the
memory layer in order to form an amorphous state, significantly
increasing power consumption. While, in MRAM, the structural complexity
prevents the memory cells from being downsized, and the difference in
electrical resistance is significantly smaller than that obtained from
PCRAM. In addition, despite the fact that power consumption is
relatively lower for RRAM, the principle of its functioning is poorly
understood, and it has the fatal disadvantage of limited write-erase
cycles.
With
the ongoing evolution of mobile IT devices such as smart phones, the
radical reduction of the power consumption of solid-state memories and
the realization of increased component density and speed are urgent
issues, necessitating the development of next-generation non-volatile
memories. Competition in the development of solid-state memories, which
has been advanced separately, makes management decisions of companies
difficult, for example with regard to the development of multiple
products and increased manufacturing costs.
PCRAM research accelerates at AIST
The
“Development of Core Technologies for Green Nanoelectronics” project
(Core Researcher: Naoki Yokoyama) of the Cabinet Office’s Funding
Program for World-Leading Innovative R&D on Science and Technology
launched in 2010 as a collaborative R&D project among AIST, private
companies, and universities, seeking to reduce the power consumption of
next-generation electronic devices. R&D of superlattice phase-change
memories has been conducted since then as a subtheme of the research
theme “Material Research for Backend Devices” (Theme Leader: Toshimichi
Shintani).
Development
of PCRAM that operate with low power consumption actually commenced in
2007. In 2004, analysis of the switching mechanism of phase change in
germanium-antimony-tellurium alloys used in conventional PCRAM at the
atomic level revealed that the valence numbers of germanium atoms are
the cause of changes in electrical resistance (AIST press release on
September 29, 2004). Since then, the research has been conducted in
order to exploit this principle by synthesizing compounds in which
change in the valence numbers of germanium atoms is controlled by the
atomic displacement with one-dimensionally aligned orientations rather
than relying on random three-dimensional transfers, and electrical
resistance could be changed without melting. This research has been
positioned as one of the core themes of the Funding Program for
World-Leading Innovative R&D on Science and Technology since 2010,
and the discovery of the giant magnetoresistance effect was made during
the course of this research.
Detailed examination of the latest research
The
memory layer of the low power consumption PCRAM was a crystalline
laminated film formed by multilayering germanium-tellurium and
antimony-tellurium crystal sub-layers. The thickness of each crystal
sub-layer was determined by quantum mechanics-based simulations known as
the first principle simulation. Germanium-tellurium crystals are
distorted cubic crystals, while antimony-tellurium crystals have a
hexagonal crystal structure. When these two crystal phases are
alternately layered, they naturally layer so as to align the c axes (the
longitudinal axes) of the hexagonal crystals and the axes connecting
the diagonals of the apices of the cubic crystals with an identical
direction, which is normal to a substrate surface.
Precise
control of the thickness of the sub-layers of each alloy based on the
simulations enabled a superlattice phase-change film to be formed from
different alloys. PCRAM using the superlattice phase-change film was
employed in devices, and it was demonstrated that it could be operated
using one-tenth or less power than conventional PCRAM.
PCRAM
using the superlattice phase-change film has an electrical resistance
one order of the magnitude higher than that of PCRAM using the alloy
with the same composition, in spite of its high crystallinity. An
analysis of this effects showed that the cause was the fact that the
antimony-tellurium crystal layers (in a composition ratio of 2:3)
possess a special property: topological insulating. Topological
insulators have a special pair of cross bands (Draic cones) as part of
their electron band structure, enabling electrons to migrate from the
valence band to the conduction band without scattering, generating spin
current on the surface or the interface.
The
interesting characteristics of topological insulators are induced by
spin-orbit coupling of internal electrons of which velocity is close to
light speed, and this effect occurs in materials including heavy
elements. In other words, superlattice phase-change films are able to
internally generate spin-orbit coupling and an intrinsic magnetic field
without including any ferromagnetic element such as cobalt and iron.
Up
to now, physical properties as a topological insulator has only been
demonstrated at ultra-low temperatures, and there have been no reported
examples emerged at room temperature. However, through first principle
simulations, the group found that some of the electron bands in the
germanium atoms in germanium-tellurium layers (not a topological
insulator) would sprint into the bands with an energy difference of a
maximum of 200 meV (the Rashba effect).
Measurement
of the change in electrical resistance in a PCRAM employing the
superlattice phase-change film in response to a magnetic field at room
temperature revealed a giant magnetoresistance effect, with a change in
resistance of more than 2000% in response to a 0.1 tesla magnetic field.
In addition, it was confirmed that the voltage required for phase
change can be controlled to between 1 and 2 V. The current-voltage
characteristic does not obey Ohm’s law and stepped changes were
confirmed.
A
superlattice phase-change film was fabricated on a silicon substrate
and differences in the spin-polarized current at the surface of the film
due to the Rashba effect were measured. The reflectance of light in the
visible wavelength region, from 400 nm to 800 nm, which are far shorter
than the band gap energy was measured at room temperature. When a
magnetic field was applied, the reflectance of circularly polarized
light changed, irrespective of wavelength. In addition, the reflectance
changed even when the magnetic field of different polarization was
applied.
This
research fabricated the world’s first topological insulator functioning
at room temperature and higher, in addition to providing the world’s
first demonstration of the feasibility of spintronics exploiting the
Rashba effect, using an artificial superlattice phase-change film. A
magnetoresistance effect exceeding 2000 % has never been obtained at
room temperature, even when using ferromagnetic materials. The developed
superlattice phase-change films are expected to be employed in new
devices going beyond memory devices, for example in readout-heads for
next-generation hard disk drives.