Sung Kwan Lim

Highly flexible and transparent multilayer MoS2 transistors with graphene electrodes.

A highly flexible and transparent transistor is developed based on an exfoliated MoS2 channel and CVD-grown graphene source/drain electrodes. Introducing the 2D nanomaterials provides a high mechanical flexibility, optical transmittance (∼74%), and current on/off ratio (>10(4)) with an average field effect mobility of ∼4.7 cm(2) V(-1) s(-1), all of which cannot be achieved by other transistors consisting of a MoS2 active channel/metal electrodes or graphene channel/graphene electrodes. In particular, a low Schottky barrier (∼22 meV) forms at the MoS2 /graphene interface, which is comparable to the MoS2 /metal interface. The high stability in electronic performance of the devices upon bending up to ±2.2 mm in compressive and tensile modes, and the ability to recover electrical properties after degradation upon annealing, reveal the efficacy of using 2D materials for creating highly flexible and transparent devices.

Chemical Sensing of 2D Graphene/MoS2 Heterostructure device

We report the production of a two-dimensional (2D) heterostructured gas sensor. The gas-sensing characteristics of exfoliated molybdenum disulfide (MoS2) connected to interdigitated metal electrodes were investigated. The MoS2 flake-based sensor detected a NO2 concentration as low as 1.2 ppm and exhibited excellent gas-sensing stability. Instead of metal electrodes, patterned graphene was used for charge collection in the MoS2-based sensing devices. An equation based on variable resistance terms was used to describe the sensing mechanism of the graphene/MoS2 device. Furthermore, the gas response characteristics of the heterostructured device on a flexible substrate were retained without serious performance degradation, even under mechanical deformation. This novel sensing structure based on a 2D heterostructure promises to provide a simple route to an essential sensing platform for wearable electronics.

Au nanoparticle-decorated graphene electrodes for GaN-based optoelectronic devices

We studied GaN-based optoelectronic devices such as light-emitting diodes (LEDs) and solar cells (SCs) with graphene electrodes. A decoration of Au nanoparticles (NPs) on multi-layer graphene films improved the electrical conductivity and modified the work function of the graphene films. The Au NP-decorated graphene film enhanced the current injection and electroluminescence of GaN-based LEDs through low contact resistance and improved the power conversion efficiency of GaN-based SCs through additional light absorption and energy band alignment. Our study will enhance the understanding of the role of Au NP-decorated graphene electrodes for GaN-based optoelectronic device applications.

Effects of multi-layer graphene capping on Cu interconnects

The benefits of multi-layer graphene (MLG) capping on Cu interconnects have been experimentally demonstrated. The resistance of MLG capped Cu wires improved by 2-7% compared to Cu wires. The breakdown current density increased by 18%, suggesting that the MLG can act as an excellent capping material for Cu interconnects, improving the reliability characteristics. With a proper process optimization, MLG capped Cu interconnects could become a promising technology for high density back end-of-line interconnects.

Enhanced Current Drivability of CVD Graphene Interconnect in Oxygen-Deficient Environment

Graphene has been considered as a candidate for interconnect metal due to its high carrier mobility and current drivability. In this letter, the breakdown mechanism of single-layer chemical-vapor-deposited (CVD) graphene and triple-layer CVD graphene has been investigated at three different conditions (air exposed, vacuum, and dielectric capped) to identify a failure mechanism. In vacuum, both single- and triple-layer graphenes demonstrated a breakdown current density as high as ~108 A/cm2, which is similar to that of exfoliated graphene. On the other hand, the breakdown current of graphene exposed to air was degraded by one order of magnitude from that of graphene tested in vacuum. Thus, oxidation initiated at the defect sites of CVD graphene was suggested as a major failure mechanism in air, while Joule heating was more dominant with dielectric capping and in vacuum.

Characteristics of a pressure sensitive touch sensor using a piezoelectric PVDF-TrFE/MoS2 stack

A new touch sensor device has been demonstrated with molybdenum disulfide (MoS2) field effect transistors stacked with a piezoelectric polymer, polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE). The performance of two device stack structures, metal/PVDF-TrFE/MoS2 (MPM) and metal/PVDF-TrFE/Al2O3/MoS2 (MPAM), were compared as a function of the thickness of PVDF-TrFE and Al2O3. The sensitivity of the touch sensor has been improved by two orders of magnitude by reducing the charge scattering and enhancing the passivation effects using a thin Al2O3 interfacial layer. Reliable switching behavior has been demonstrated up to 120 touch press cycles.

Metal Decoration Effects on the Gas-Sensing Properties of 2D Hybrid-Structures on Flexible Substrates.

We have investigated the effects of metal decoration on the gas-sensing properties of a device with two-dimensional (2D) molybdenum disulfide (MoS2) flake channels and graphene electrodes. The 2D hybrid-structure device sensitively detected NO2 gas molecules (>1.2 ppm) as well as NH3 (>10 ppm). Metal nanoparticles (NPs) could tune the electronic properties of the 2D graphene/MoS2 device, increasing sensitivity to a specific gas molecule. For instance, palladium NPs accumulate hole carriers of graphene/MoS2, electronically sensitizing NH3 gas molecules. Contrarily, aluminum NPs deplete hole carriers, enhancing NO2 sensitivity. The synergistic combination of metal NPs and 2D hybrid layers could be also applied to a flexible gas sensor. There was no serious degradation in the sensing performance of metal-decorated MoS2 flexible devices before/after 5000 bending cycles. Thus, highly sensitive and endurable gas sensor could be achieved through the metal-decorated 2D hybrid-structure, offering a useful route to wearable electronic sensing platforms.

Quantitatively estimating defects in graphene devices using discharge current analysis method.

Defects of graphene are the most important concern for the successful applications of graphene since they affect device performance significantly. However, once the graphene is integrated in the device structures, the quality of graphene and surrounding environment could only be assessed using indirect information such as hysteresis, mobility and drive current. Here we develop a discharge current analysis method to measure the quality of graphene integrated in a field effect transistor structure by analyzing the discharge current and examine its validity using various device structures. The density of charging sites affecting the performance of graphene field effect transistor obtained using the discharge current analysis method was on the order of 1014/cm2, which closely correlates with the intensity ratio of the D to G bands in Raman spectroscopy. The graphene FETs fabricated on poly(ethylene naphthalate) (PEN) are found to have a lower density of charging sites than those on SiO2/Si substrate, mainly due to reduced interfacial interaction between the graphene and the PEN. This method can be an indispensable means to improve the stability of devices using a graphene as it provides an accurate and quantitative way to define the quality of graphene after the device fabrication.

Electrical characteristics of wrinkle-free graphene formed by laser graphitization of 4H-SiC

Wrinkle free few layer graphene was demonstrated by a graphitization of 4H-SiC substrates using a high power pulsed KrF laser. Wrinkles often observed after thermal graphitization were eliminated with a short heat cycle using a pulse laser anneal. Few layer graphene formed by the laser graphitization appears to have a non-Bernal stack, which leads to on-off ratio of ∼2 even at a few layer graphene. Drive current of 143 μA/μm was obtained at Vd = 100 mV and field effect mobility was 374 cm2/Vs.

Contact resistance reduction using Fermi level de-pinning layer for MoS 2 FETs

Achieving a low contact resistance for 2D materials is a critical challenge for device applications. In this work, the contact resistance of MoS2 FETs has been drastically reduced by five times from the reference data using an optimized TiO2 Fermi level de-pinning layer which reduced the effective Schottky barrier height to 0.1 eV. As a result, a very low contact resistance ~5.4 kΩ·μm was achieved without any doping technique.