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Scientists
at the Naval Research Laboratory have demonstrated, for the first time,
the use of graphene as a tunnel barrier—an electrically insulating
barrier between two conducting materials through which electrons tunnel
quantum mechanically. They report fabrication of magnetic tunnel
junctions using graphene, a single atom thick sheet of carbon atoms
arranged in a honeycomb lattice, between two ferromagnetic metal layers
in a fully scalable photolithographic process. Their results demonstrate
that single-layer graphene can function as an effective tunnel barrier
for both charge and spin-based devices, and enable realization of more
complex graphene-based devices for highly functional nanoscale circuits,
such as tunnel transistors, non-volatile magnetic memory and
reprogrammable spin logic. These research results are published in the
online issue of Nano Letters.
The
research initiates a “paradigm shift in tunnel barrier technology for
magnetic tunnel junctions (MTJs) used for advanced sensors, memory and
logic,” explains NRL’s Dr. Berend Jonker. Graphene has been the focus of
intense research activity because of its remarkable electronic and
mechanical properties. In the past, researchers focused on developing
graphene as a conductor, or perhaps a semiconductor, where the current
flows in-plane parallel to the carbon honeycomb sheet. In contrast, the
NRL researchers show that graphene serves as an excellent tunnel barrier
when current is directed perpendicular to the plane, and in fact, also
preserves the spin polarization of the tunneling current.
Tunnel
barriers are the basis for many electronic (charge-based) and
spintronic (spin-based) device structures. Fabrication of ultra-thin and
defect-free barriers is an ongoing challenge in materials science.
Typical tunnel barriers are based on metal oxides (e.g. aluminum oxide
or magnesium oxide), and issues such as non-uniform thicknesses,
pinholes, defects and trapped charge compromise their performance and
reliability. Such oxide tunnel barriers have several limitations which
hinder future performance. For example, they have high resistance-area
(RA) products which results in higher power consumption and local
heating; they allow interdiffusion at the interfaces, which reduces
their performance and can lead to catastrophic failure; and their
thickness is generally non-uniform, resulting in “hot spots” in the
current transport. In contrast, Dr. Jonker explains, the inherent
material properties of graphene make it an ideal tunnel barrier.
Graphene is chemically inert and impervious to diffusion even at high
temperatures. The atomic thickness of graphene represents the ultimate
in tunnel barrier scaling for the lowest possible RA product, lowest
power consumption and fastest switching speed.
This
discovery by NRL researchers is significant because MTJs are widely
utilized as read heads in the hard disk drive found in every computer,
and as memory elements in non-volatile magnetic random access memory
(MRAM) which is rapidly emerging as a universal memory replacement for
the many varieties of conventional semiconductor-based memory. They are
also considered to be lead contenders as reprogrammable, non-volatile
elements for a universal logic block.
Although
there has been significant progress, the emerging generation of
MTJ-based MRAM relies upon spin-transfer torque switching, and is
severely limited by the unacceptably high current densities required to
switch the logic state of the cell. The accompanying issues of power
consumption and thermal dissipation prevent scaling to higher densities
and operation at typical CMOS voltages.
The 2011 International Technology Roadmap for Semiconductors (ITRS) states that “all
of the existing forms of nonvolatile memory face limitations based on
material properties. Success will hinge on finding and developing
alternative materials and/or developing alternative emerging
technologies … development of electrically accessible non-volatile
memory with high speed and high density would initiate a revolution in
computer architecture … and provide a significant increase in
information throughput beyond the traditional benefits of scaling when
fully realized for nanoscale CMOS devices” (ITRS 2011 Executive Summary, p28; and Emerging Research Devices, p. 4).
NRL
researchers believe that the graphene-based magnetic tunnel junctions
they have demonstrated will eclipse the performance and ease of
fabrication of existing oxide technology. These graphene-based MTJs
would be a breakthrough for nascent spin-based technologies like MRAM
and spin logic, and enable the electrically accessible non-volatile
memory required to initiate a revolution in computer architecture. These
results also pave the way for utilization of other two-dimensional
materials such as hexagonal boron nitride for similar applications.
The
NRL research team includes Dr. Enrique Cobas, Dr. Adam Friedman, Dr.
Olaf van ‘t Erve, and Dr. Berend Jonker from the Materials Science and
Technology Division, and Dr. Jeremy Robinson from the Electronics
Science and Technology Division.
Graphene As a Tunnel Barrier: Graphene-Based Magnetic Tunnel Junctions
Source: Navy Research Laboratory
