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Posted: Aug 01, 2014

Interface engineering for high-performance top-gated MoS2 field-effect transistors

(Nanowerk News) Following the success of graphene, a group of two-dimensional (2-D) materials known as the transition metal dichalcogenides (TMDs) are attracting considerable attention due to their unique electronic, optical and mechanical properties. Among them, molybdenum disulphide (MoS2) is probably one of the most explored TMDs. In contrast to graphene, the presence of a bandgap (a thickness-dependent bandgap of 1.2-1.8 eV) in MoS2 allows the fabrication of transistors that can be turned off and used as switches for thin film transistor applications.
However, the most of these efforts to date employ a silicon substrate as a global back gate and silicon oxide as the gate dielectric, which is limited by the difficulty in deposition uniform and compact dielectrics onto MoS2.
Researchers have shown that surface functionalization of MoS2 channels with oxygen plasma or ultraviolet ozone promotes the reactivity of MoS2 with ALD precursors, but the energetic oxygen species may inevitably damage the 2-D channels inducing defects to deteriorate their corresponding electrical properties.
In new work coming out of the Lei Liao group at the Wuhan University, researchers have now taken another method towards the further optimization of interface quality between MoS2 and HfO2 dielectric. The results have been published in the July 28, 2014 online edition of Advanced Materials ("Interface Engineering for High-Performance Top-Gated MoS2 Field-Effect Transistors").
The researchers employ an ultrathin metal oxide buffer layer inserted between the ALD-HfO2 and MoS2 channel in order to achieve conformal HfO2/MoS2 interfaces with the minimal interface defect density.
They also tried oxygen plasma and ultraviolet ozone, but the degradation in device performance seems unavoidable, in contrast, the proper interface engineering with metal oxide buffer layer improve the electrical properties greatly. For testing, researchers try different ultrathin metal oxide (MgO, Al2O3 and Y2O3) film. The advantages in higher melting point and better wetting with MoS2 make Y2O3 more suitable for buffer layer.
Utilizing the MoS2/Y2O3/HfO2 stack, the fabricated devices exhibit high electron mobility of 63.7 cm2/V•s and near-ideal sub-threshold slope of 65 mV/decade. In particular, as the channel length is reduced to 400 nm, the device shows the highest saturation current (526 µA/µm) of any MoS2 transistor reported to date, which is comparable to the same scaled state-of-the-art Si MOSFETs.
As HfO2 dielectric thickness is reduced down to 9 nm, the versatility of this interface engineering technique is further illustrated with the construction of high-performance MoS2 integrated circuits such as inverters with a large voltage gain of 16. Using the proper buffer layer, MoS2 devices could meet the requirement of aggressive device scaling.
Source: Wuhan University
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