Wonjae Kim

Rapid large-area multiphoton microscopy for characterization of graphene

Single- and few-layer graphene was studied with simultaneous third-harmonic and multiphoton-absorption-excited fluorescence microscopy using a compact 1.55 μm mode-locked fiber laser source. Strong third-harmonic generation (THG) and multiphoton-absorption-excited fluorescence (MAEF) signals were observed with high contrast over the signal from the substrate. High contrast was also achieved between single- and bilayer graphene. The measurement is straightforward and very fast compared to typical Raman mapping, which is the conventional method for characterization of graphene. Multiphoton microscopy is also proved to be an extremely efficient method for detecting certain structural features in few-layer graphene. The accuracy and speed of multiphoton microscopy make it a very promising characterization technique for fundamental research as well as large-scale fabrication of graphene. To our knowledge, this is the first time simultaneous THG and MAEF microscopy has been utilized in the characterization of graphene. This is also the first THG microscopy study on graphene using the excitation wavelength of 1.55 μm, which is significant in telecommunications and signal processing.

Tunable Graphene–GaSe Dual Heterojunction Device

A field-effect device based on dual graphene-GaSe heterojunctions is demonstrated. Monolayer graphene is used as electrodes on a GaSe channel to form two opposing Schottky diodes controllable by local top gates. The device exhibits strong rectification with tunable threshold voltage. Detailed theoretical modeling is used to explain the device operation and to distinguish the differences compared to a single diode.

Nonlinear behavior of three terminal graphene junctions at room temperature

We demonstrate nonlinear behavior in three-terminal T-branch graphene devices at room temperature. A rectified nonlinear output at the center branch is observed when the device is biased by a push-pull configuration. Nonlinearity is assumed to arise from a difference in charge transfer through the metal–graphene contact barrier between two contacts. The sign of the rectification can be altered by changing the carrier type using the back-gate voltage.

A physics-based model of gate-tunable metal–graphene contact resistance benchmarked against experimental data

The metal-graphene contact resistance is a technological bottleneck for the realization of viable graphene based electronics. We report a useful model to find the gate tunable components of this resistance determined by the sequential tunneling of carriers between the 3D-metal and 2D-graphene underneath followed by Klein tunneling to the graphene in the channel. This model quantifies the intrinsic factors that control that resistance, including the effect of unintended chemical doping. Our results agree with experimental results for several metals.

Highly tunable local gate controlled complementary graphene device performing as inverter and voltage controlled resistor

Using single-layer CVD graphene, a complementary field effect transistor (FET) device is fabricated on the top of separated back-gates. The local back-gate control of the transistors, which operate with low bias at room temperature, enables highly tunable device characteristics due to separate control over electrostatic doping of the channels. Local back-gating allows control of the doping level independently of the supply voltage, which enables device operation with very low VDD. Controllable characteristics also allow the compensation of variation in the unintentional doping typically observed in CVD graphene. Moreover, both p-n and n-p configurations of FETs can be achieved by electrostatic doping using the local back-gate. Therefore, the device operation can also be switched from inverter to voltage controlled resistor, opening new possibilities in using graphene in logic circuitry.

All-Graphene Three-Terminal Junction Field-Effect Devices as Rectifiers and Inverters

We present prominent tunable and switchable room-temperature rectification performed at 100 kHz ac input utilizing micrometer-scale three-terminal junction field-effect devices. Monolayer CVD graphene is used as both a channel and a gate electrode to achieve all-graphene thin-film structure. Instead of ballistic theory, we explain the rectification characteristics through an electric-field capacitive model based on self-gating in the high source-drain bias regime. Previously, nanoscale graphene three-terminal junctions with the ballistic (or quasi-ballistic) operation have shown rectifications with relatively low efficiency. Compared to strict nanoscale requirements of ballistic devices, diffusive operation gives more freedom in design and fabrication, which we have exploited in the cascading device architecture. This is a significant step for all-graphene thin-film devices for integrated monolithic graphene circuits.

Active synchronization and modulation of fiber lasers with a graphene electro-optic modulator

We report the synchronization of two actively Q-switched fiber lasers operating at 1.5 μm and 2 μm with a shared broadband graphene electro-optic modulator. Two graphene monolayer sheets separated with a high-kHfO2 dielectric layer are configured to enable broadband light modulation. The graphene electro-optic modulator is shared by two optical fiber laser cavities (i.e., an erbium-doped fiber laser cavity and a thulium/holmium-codoped fiber laser cavity) to actively Q-switch the two lasers, resulting in stable synchronized pulses at 1.5 μm and 2 μm with a repetition rate ranging from 46 kHz to 56 kHz.

Scaling of graphene field-effect transistors supported on hexagonal boron nitride: Radio-frequency stability as a limiting factor

The quality of graphene in nanodevices has increased hugely thanks to the use of hexagonal boron nitride as a supporting layer. This paper studies to which extent hBN together with channel length scaling can be exploited in graphene field-effect transistors (GFETs) to get a competitive radio-frequency (RF) performance. Carrier mobility and saturation velocity were obtained from an ensemble Monte Carlo simulator that accounted for the relevant scattering mechanisms (intrinsic phonons, scattering with impurities and defects, etc). This information is fed into a self-consistent simulator, which solves the drift-diffusion equation coupled with the two-dimensional Poissons equation to take full account of short channel effects. Simulated GFET characteristics were benchmarked against experimental data from our fabricated devices. Our simulations show that scalability is supposed to bring to RF performance an improvement that is, however, highly limited by instability. Despite the possibility of a lower performance, a careful choice of the bias point can avoid instability. Nevertheless, maximum oscillation frequencies are still achievable in the THz region for channel lengths of a few hundreds of nanometers.

Third-harmonic and multiphoton excitation fluorescence microscopy of single and few layer graphene

Single- and few-layer graphene was studied with simultaneous third-harmonic and multiphoton-induced fluorescence microscopy. Both CVD grown and exfoliated graphene were studied [1,2]. The photo-thermal CVD growth was carried out on copper foils in a methane/hydrogen atmosphere, followed by a transfer process. Oxidized silicon substrates were used as the final substrate in both cases. The oxide thickness was 300 nm to achieve maximal contrast in optical microscope. Single- and few-layer graphene areas were identified using optical microscopy and Raman spectroscopy.

Electrical properties of CVD-graphene FETs

Graphene field-effect transistors (GFET) were first presented in 2004, and quickly became an interesting electronics research topic due to the many promises that the material holds. We have fabricated GFETs using an IC-compatible chemical vapour deposition (CVD) process. This paper presents experimental results of graphene field-effect transistors with CVD grown graphene layer. In addition, this paper reviews briefly the state-of-the-art GFETs and device physics.