Sang Kyung Lee

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.

Characteristics of CVD graphene nanoribbon formed by a ZnO nanowire hardmask

A graphene nanoribbon (GNR) is an important basic structure to open a bandgap in graphene. The GNR processes reported in the literature are complex, time-consuming, and expensive; moreover, the device yield is relatively low. In this paper, a simple new process to fabricate a long and straight graphene nanoribbon with a high yield has been proposed. This process utilizes CVD graphene substrate and a ZnO nanowire as the hardmask for patterning. 8 µm long and 50-100 nm wide GNRs were successfully demonstrated in high density without any trimming, and ∼ 10% device yield was realized with a top-down patterning process. After passivating the surfaces of the GNRs using a low temperature atomic layer deposition (ALD) of Al(2)O(3), high performance GNR MOSFETs with symmetric drain-current-gate-voltage (I(d)-V(g)) curves were demonstrated and a field effect mobility up to ∼ 1200 cm(2) V(-1) s(-1) was achieved at V(d) = 10 mV.

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.

Highly sensitive wide bandwidth photodetectors using chemical vapor deposited graphene

A photodetector generating a nearly constant photocurrent in a very wide spectral range from ultraviolet (UV) to infrared has been demonstrated using chemical vapor deposited (CVD) graphene. Instability due to a photochemical reaction in the UV region has been minimized using an Al2O3 passivation layer, and a responsivity comparable to that of Highly Ordered Pyrolytic Graphite graphene photodetectors of ∼8 mA/W has been achieved at a 0.1 V bias, despite high defect density in the CVD graphene. A highly sensitive multi-band photodetector using graphene has many potential applications including optical interconnects, multi-band imaging sensors, highly sensitive motion detectors, etc.

Intrinsic photocurrent characteristics of graphene photodetectors passivated with Al2O3

The intrinsic photo-response of chemical vapor deposited (CVD) graphene photodetectors were investigated after eliminating the influence of photodesorption using an atomic layer deposited (ALD) Al₂O₃ passivation layer. A general model describing the intrinsic photocurrent generation in a graphene is developed using the relationship between the device dimensions and the level of intrinsic photocurrent under UV illumination.

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.

Variability and feasibility of CVD graphene interconnect

Graphene and its derivatives (graphite, CNT) have very high conductivity and critical current density higher than 108 A/cm2, which can be utilized in interconnect applications. Theoretically, a doped graphene is predicted to have better performance than Cu as an interconnect conductor. However, the feasibility of graphene interconnect has not been experimentally examined systematically. In this paper, the critical current density of single layer and multilayer graphene are studied to provide insights about the feasibility of graphene interconnect technology.

Contact resistance improvement by the modulation of peripheral length to area ratio of graphene contact pattern

High contact resistance between graphene and metal is a major huddle for high performance electronic device applications of graphene. In this work, a method to improve the contact resistance of graphene is investigated by varying the ratio of peripheral length and area of graphene pattern under a metal contact. The contact resistance decreased to 0.8 kΩ·μm from 2.1 kΩ·μm as the peripheral length increased from 312 to 792 μm. This improvement is attributed to the low resistivity of edge-contacted graphene, which is 8.1 × 105 times lower than that of top-contacted graphene.

Demonstration of Complementary Ternary Graphene Field-Effect Transistors

Strong demand for power reduction in state-of-the-art semiconductor devices calls for novel devices and architectures. Since ternary logic architecture can perform the same function as binary logic architecture with a much lower device density and higher information density, a switch device suitable for the ternary logic has been pursued for several decades. However, a single device that satisfies all the requirements for ternary logic architecture has not been demonstrated. We demonstrated a ternary graphene field-effect transistor (TGFET), showing three discrete current states in one device. The ternary function was achieved by introducing a metal strip to the middle of graphene channel, which created an N-P-N or P-N-P doping pattern depending on the work function of the metal. In addition, a standard ternary inverter working at room temperature has been achieved by modulating the work function of the metal in a graphene channel. The feasibility of a ternary inverter indicates that a general ternary logic architecture can be realized using complementary TGFETs. This breakthrough will provide a key stepping-stone for an extreme-low-power computing technology.