August 7, 2015

New 2D Transistor Material Made Using Precision Lasers

(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.

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