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Posted: Feb 22, 2017

A new contact to the two-dimensional world

(Nanowerk Spotlight) Your laptop or cellphone becomes very hot after intensive use like playing video games. This is because much of the electric energy is wasted in the form of heat, particularly at the contact between metal and semiconductor.
Metal and semiconductors are important components in most electronic devices, so is their contact. To minimize the energy dissipation and improve the device performance, it is critical to reduce the contact resistance.
However, this is challenging for next-generation electronics based on two-dimensional (2D) semiconductors, which have a significantly higher resistance than conventional silicon-based electronics.
Two-dimensional semiconductors, such as MoS2 and WSe2, have a thickness of only one or few atomic layers. This extreme thickness makes the materials largely exposed to the environment, so one can have a better control on the materials by using external electric field. This property makes them promising for next-generation electronics.
However, their development is significantly limited by the high contact resistance between the metal electrode and the 2D semiconductor. Writing in Journal of the American Chemical Society ("Schottky-Barrier-Free Contacts with Two-Dimensional Semiconductors by Surface-Engineered MXenes"), Yuanyue Liu, Hai Xiao and William A. Goddard III from the California Institute of Technology, propose that this problem could be solved by using a new class of metal MXene.
the potential of MXenes as Schottky-barrier-free metal contacts to 2D semiconductors
The team demonstrates the potential of MXenes as Schottky-barrier-free metal contacts to 2D semiconductors, providing a solution to the contact-resistance problem in 2D electronics. (Reprinted with permission by American Chemical Society)
MXene is a class of 2D metal carbides or nitrides, which has shown great promise in many applications especially energy storage (also see our previous Nanowerk Spotlights: "Graphene was only the beginning; now MAX phases get two-dimensional as well" and "MXene - A new family of 2-D metal carbides and nitrides").
In this new work, the authors show that they are also promising metals that can achieve low contact resistance with 2D semiconductors. This benefits from two factors:
  • 1) their surface can be tuned from O termination, to OH termination, depending on the synthesis condition. These surface groups create a dipole that strongly modulates the work function of MXene; and
  • 2) the interaction between MXene and the 2D semiconductor is a weak van der Waals interaction, different from the strong bonding if using conventional metals.
  • This weak interaction suppress the formation of interface electronic states, and allows for effective tuning of the Schottky barrier – a major contributor to the contact resistance (see also: "Van der Waals metal-semiconductor junction: Weak Fermi level pinning enables effective tuning of Schottky barrier").
    This study enhances the understanding of the correlation between surface chemistry and electronic/transport properties of 2D materials, and also gives practical predictions for improving 2D electronics.
    Source: Materials and Process Simulation Center at California Institute of Technology

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