Hsiao Yu Chang

High-performance, highly bendable MoS2 transistors with high-k dielectrics for flexible low-power systems.

While there has been increasing studies of MoS2 and other two-dimensional (2D) semiconducting dichalcogenides on hard conventional substrates, experimental or analytical studies on flexible substrates has been very limited so far, even though these 2D crystals are understood to have greater prospects for flexible smart systems. In this article, we report detailed studies of MoS2 transistors on industrial plastic sheets. Transistor characteristics afford more than 100x improvement in the ON/OFF current ratio and 4x enhancement in mobility compared to previous flexible MoS2 devices. Mechanical studies reveal robust electronic properties down to a bending radius of 1 mm which is comparable to previous reports for flexible graphene transistors. Experimental investigation identifies that crack formation in the dielectric is the responsible failure mechanism demonstrating that the mechanical properties of the dielectric layer is critical for realizing flexible electronics that can accommodate high strain. Our uniaxial tensile tests have revealed that atomic-layer-deposited HfO2 and Al2O3 films have very similar crack onset strain. However, crack propagation is slower in HfO2 dielectric compared to Al2O3 dielectric, suggesting a subcritical fracture mechanism in the thin oxide films. Rigorous mechanics modeling provides guidance for achieving flexible MoS2 transistors that are reliable at sub-mm bending radius.

On the mobility and contact resistance evaluation for transistors based on MoS2 or two-dimensional semiconducting atomic crystals

Contact resistance (Rc) can substantially obscure the extracted mobility based on standard transconductance or two-point conductance measurements of field-effect devices especially for low density of states materials such as MoS2 or similar atomic crystals. Currently, there exists a pressing need for a routine technique that can decouple mobility extraction from Rc. By combining experiments and analysis, we show that the Y-function method offers a robust route for evaluating the low-field mobility, threshold voltage and Rc even when the contact is a Schottky-barrier as is common in two-dimensional transistors. In addition, an independent modified transfer length method evaluation corroborates the Y-function analysis.

Large-Area Monolayer MoS2 for Flexible Low-Power RF Nanoelectronics in the GHz Regime.

Flexible synthesized MoS2 transistors are advanced to perform at GHz speeds. An intrinsic cutoff frequency of 5.6 GHz is achieved and analog circuits are realized. Devices are mechanically robust for 10,000 bending cycles.

Thermal Oxidation of WSe2 Nanosheets Adhered on SiO2/Si Substrates

Because of the drastically different intralayer versus interlayer bonding strengths, the mechanical, thermal, and electrical properties of two-dimensional (2D) materials are highly anisotropic between the in-plane and out-of-plane directions. The structural anisotropy may also play a role in chemical reactions, such as oxidation, reduction, and etching. Here, the composition, structure, and electrical properties of mechanically exfoliated WSe2 nanosheets on SiO2/Si substrates were studied as a function of the extent of thermal oxidation. A major component of the oxidation, as indicated from optical and Raman data, starts from the nanosheet edges and propagates laterally toward the center. Partial oxidation also occurs in certain areas at the surface of the flakes, which are shown to be highly conductive by microwave impedance microscopy. Using secondary ion mass spectroscopy, we also observed extensive oxidation at the WSe2-SiO2 interface. The combination of multiple microcopy methods can thus provide vital information on the spatial evolution of chemical reactions on 2D materials and the nanoscale electrical properties of the reaction products.

High-performance flexible nanoelectronics: 2D atomic channel materials for low-power digital and high-frequency analog devices

In this work, we report the state-of-the-art flexible devices based on graphene for radio-frequency transistors and large bandgap MoS2 for low-power digital transistors for flexible nanoelectronics. Our studies on graphene transistors feature record mobility, transit frequency, and tensile strain and the first demonstration of flexible capping layers. Our studies on MoS2 transistors yield the first comprehensive insights into the coupled electrical and mechanical properties including buckling which degrades the gate control, and the thickness dependence of electrical properties.

High-frequency prospects of 2D nanomaterials for flexible nanoelectronics from baseband to sub-THz devices

We report on the state of the art sub-μm length (L) flexible two dimensional radio frequency thin film transistors operating in the velocity saturation regime for achieving maximum carrier transport or under high-field. We realize large-area monolayer MoS₂ on flexible polyimide with 5 GHz cut-off frequency (f_T), a record value for flexible synthesized transitional metal dichalcogenides (TMDs). For higher frequency devices, flexible black phosphorus (BP) RF TFT is demonstrated for the first time with f_T ~ 17.5 GHz for L = 0.5 μm, yielding v_sat ~ 5.5 × 10⁶ cm/s. In addition, for flexible sub-THz nanosystem front-ends, we have achieved record 100 GHz graphene TFTs (v_sat ~ 8.8 × 106 cm/s) on flexible glass, 56% higher than that of graphene TFTs on polymeric substrates.

Flexible 2D electronics using nanoscale transparent polyimide gate dielectric

We report NPI as a flexible dielectric for transistors based on 2D atomic sheets such as graphene and MoS2, which features high mechanical flexibility, stable electrical performances and low roughness. NPI offers the realistic prospects for highly flexible electronics beyond the typical 2% limitation of high-κ or ceramic gate dielectrics.

INVERSION OF THE ELECTRICAL AND OPTICAL PROPERTIES OF PARTIALLY OXIDIZED HEXAGONAL BORON NITRIDE

By acoustically irradiating pristine, white, electrically insulating h-BN in aqueous environment we were able to invert its material properties. The resulting dark, electrically conductive h-BN (referred to as partially oxidized h-BN or PO-hBN) shows a significant decrease in optical transmission (>60%) and bandgap (from 5.46 eV to 3.97 eV). Besides employing a wide variety of analytical techniques (optical and electrical measurements, Raman spectroscopy, SEM imaging, EDS, X-Ray diffraction, XPS and TOF-SIMS) to study the material properties of pristine and irradiated h-BN, our investigation suggests the basic mechanism leading to the dramatic changes following the acoustic treatment. We find that the degree of inversion arises from the degree of h-BN surface or edge oxidation which heavily depends on the acoustic energy density provided to the pristine h-BN platelets during the solution-based process. This provides a facile avenue for the realization of materials with tuned physical and chemical properties that depart from the intrinsic behavior of pristine h-BN.

State-of-the-art flexible 2D nanoelectronics based on graphene and MoS 2

The authors report a flexible transistors based on 2D atomic sheets such as graphene and MoS2 that features record electrical-mechanical properties and offer the highest prospects for realizing Si-CMOS like performance on arbitrary plastic substrates. Graphene is ideal for analog RF devices while MoS2 is ideal for digital low-power FETs.

Flexible 2D nanoelectronics from baseband to sub-THz transistors and circuits

In this work, we provide an overview of the state-of-the-art flexible two dimensional radio frequency transistors based on graphene, MoS2 and black phosphorus for nanoelectronics applications from baseband to sub THz, with outstanding electrical performance obtained fulfilling the requirements of both high speed operation and low power consumptions. Flexible RF transistors based on graphene with intrinsic cutoff frequency (fT) reaching 100GHz enable sub-THz flexible nano systems. Large scale CVD grown MoS2 based flexible RF transistor was realized for the first time with record intrinsic fT ~ 5.6GHz which can afford low power RF nano systems. In addition, black phosphorus based flexible transistors and circuits have been realized with outstanding electrical performance in both DC and high frequency applications while retaining mechanical robustness.