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Suspended Molybdenum Disulphide Benefits From Gas Annealing

With an intrinsic band gap and two-dimensional (2D) nature, single- and few-layer molybdenum disulphide (MoS2) are promising candidates for digital applications. Despite this, a significant drawback in MoS2 electronics is its limited mobility. This is in part due to the trapping of charges by the substrate. Reporting in Nanotechnology, researchers study the effect of removing the substrate on MoS2 field effect transistor (FET) devices. A new gas annealing technique is developed which improves device conductance by three to four orders of magnitude.

MoS2 not only has an intrinsic band gap and 2D nature. It also provides a platform for manipulating spin and valley degrees of freedom. This is due to its large spin-orbit coupling and the two valleys having oppositely spin-split bands. However, most devices to date are fabricated on SiO2 substrates and suffer from low carrier mobility. Until this report from researchers at the University of California, Riverside (UCR) the intrinsic properties free of substrate influence were yet to be explored.

Free-standing devices

Here, MoS2 devices become fully free-standing after release from the underlying SiO2 substrate by wet etching. A unique double layer metal deposition method ensures good electrical contacts while protecting the device from the acidic solution during wet etching. The researchers treat the as-fabricated devices with flowing argon and hydrogen mixture gases at 200°C for 40 minutes. They then observe a dramatic improvement in the device conductance and mobility.

Identifying the bottleneck

By performing electronic transport on these suspended devices, the researchers conclude that the substrate is not the mobility bottleneck for MoS2 devices. Instead, the culprits are likely the defects and/or Schottky barriers at the metal-MoS2 interfaces. These suspended MoS2 devices with reasonably high mobility constitute a powerful platform for investigating properties and applications of MoS2 that are best performed in the absence of substrates. For example, thermal expansion, thermal conductivity, strain engineering, nanomechanical resonators and chemical and biological sensors.


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