(Above: Top,
2H-MoTe2 Bottom, 1T’-MoTe2)
- A New Technique for
Making 2D Transistor from dual-phase TMD Crystal -
(August 7, 2015)
Molybdenum ditelluride (MoTe2) is
a crystalline compound that if pure enough can be used as a transistor. Its molecular structure is an atomic sandwich
made up of one molybdenum atom for every two tellurium atoms. It was first made in the 1960’s via several
different fabrication methods, but until last year it had never been made in a
pure enough form to be suitable for electronics.
Last year a multi-discipline research team led by South
Korea’s Institute for Basic Science (IBS) Center for Integrated Nanostructure
Physics at Sungkyunkwan University (SKKU) director Young Hee Lee devised a
fabrication method for the creation of pure MoTe2. Not only did they succeed in making MoTe2 in
pure form, they were able to make two types of it — a semiconducting variety
called 2H-MoTe2 (2H because of its hexagonal shape) and a metallic variety
called 1T'-MoTe2 (1T’ because it has an octahedral shape) — which are both stable at room temperature.
(Above: A
simulation of the process of converting the
2H-MoTe2
into 1T'-MoTe2 with laser-irradiation)
Making MoTe2 in a pure form was very difficult and it was
seen by some as a black sheep of the transition metal dichalcogenides (TMD)
family and purposefully ignored. TMDs
are molecules that can be made exceedingly thin, only several atomic layers,
and have an electrical property called a band gap, which makes them ideal for
making electrical components, especially transistors.
A TMD crystal follows an MX2 format: there is one transition
metal, represented by M (M can be Tungsten, Molybdenum, etc.) and two
chalcogenides, the X2 (Sulfur, Selenium, or Tellurium). These atoms form a thin, molecular sandwich
with the one metal and two chalcogenides, and depending on their fabrication
method can exist in several slightly different shaped atomic arrangements.
(Above: the
2H-MoTe2 and 1T'-MoTe2 transition line and metal electrodes
attached to the 1T'-MoTe2)
The overwhelming majority of microchips that exist in
electronics now are made from silicon, and they work extremely well. However, as devices get smaller there is an
increasing demand to shrink the size of the logic chips that make those devices
work. As the chips approach single or
several atom thickness, (commonly referred to as 2-dimensional), silicon no
longer works as well as it does in a larger, 3-dimensional (3D) scale. As the scale approaches 2 dimensions (2D),
the band gap of silicon changes (higher band gap than that of its 3D form) and
the contact points with metal connections on silicon are no longer smooth
enough to be used efficiently in electrical circuits.