Barbara Yuri Choi

Trap-mediated electronic transport properties of gate-tunable pentacene/MoS2 p-n heterojunction diodes

We investigated the trap-mediated electronic transport properties of pentacene/molybdenum disulphide (MoS2) p-n heterojunction devices. We observed that the hybrid p-n heterojunctions were gate-tunable and were strongly affected by trap-assisted tunnelling through the van der Waals gap at the heterojunction interfaces between MoS2 and pentacene. The pentacene/MoS2 p-n heterojunction diodes had gate-tunable high ideality factor, which resulted from trap-mediated conduction nature of devices. From the temperature-variable current-voltage measurement, a space-charge-limited conduction and a variable range hopping conduction at a low temperature were suggested as the gate-tunable charge transport characteristics of these hybrid p-n heterojunctions. Our study provides a better understanding of the trap-mediated electronic transport properties in organic/2-dimensional material hybrid heterojunction devices.

Photoelectron spectroscopy study of Fe-diluted Au–Fe alloys

The electronic structure of Fe-diluted Au–Fe alloys has been studied by taking core-level and valence-band spectra using x-ray photoemission spectroscopy and synchrotron radiation. From the core-level spectroscopy, we found that the Fe 2p spectrum is composed of d6 and d7 multiplets from Fe impurity atoms. This behaviour is qualitatively discussed within the context of electron–electron interaction. In order to explore the electron-correlation effects in the valence band, we obtained Fe 3d partial spectral weights by taking advantage of the Cooper-minimum phenomenon of an Au 5d photoionization cross section. It was found that the spin-down states have an appreciable amount of spectral weights throughout the host Au 5d band, contrary to previous one-electron calculations predicting two-peak structure of the Fe 3d states. We suggest that this discrepancy results from the correlation effect of the Fe 3d electrons.

Contact‐Engineered Electrical Properties of MoS2 Field‐Effect Transistors via Selectively Deposited Thiol‐Molecules

Although 2D molybdenum disulfide (MoS2 ) has gained much attention due to its unique electrical and optical properties, the limited electrical contact to 2D semiconductors still impedes the realization of high-performance 2D MoS2 -based devices. In this regard, many studies have been conducted to improve the carrier-injection properties by inserting functional paths, such as graphene or hexagonal boron nitride, between the electrodes and 2D semiconductors. The reported strategies, however, require relatively time-consuming and low-yield transfer processes on sub-micrometer MoS2 flakes. Here, a simple contact-engineering method is suggested, introducing chemically adsorbed thiol-molecules as thin tunneling barriers between the metal electrodes and MoS2 channels. The selectively deposited thiol-molecules via the vapor-deposition process provide additional tunneling paths at the contact regions, improving the carrier-injection properties with lower activation energies in MoS2 field-effect transistors. Additionally, by inserting thiol-molecules at the only one contact region, asymmetric carrier-injection is feasible depending on the temperature and gate bias.

Analysis of noise generation and electric conduction at grain boundaries in CVD-grown MoS2 field effect transistors

Grain boundaries in a chemical vapour deposition (CVD)-grown monolayer of MoS2 induce significant effects on the electrical and low frequency noise characteristics of the MoS2. Here, we investigated the electrical properties and noise characteristics of MoS2 field effect transistors (FETs) made with CVD-grown monolayer MoS2. The electrical and noise characteristics of MoS2 FETs were analysed and compared for the MoS2 channel layers with and without grain boundaries. The grain boundary in the CVD-grown MoS2 FETs can be the dominant noise source, and dependence of the extracted Hooge parameters on the gate voltage indicated the domination of the correlated number-mobility fluctuation at the grain boundaries. The percolative noise characteristics of the single grain regions of MoS2 were concealed by the noise generated at the grain boundary. This study can enhance understanding of the electrical transport hindrance and significant noise generation by trapped charges at grain boundaries of the CVD-grown MoS2 devices.

Two-Dimensional Thickness-Dependent Avalanche Breakdown Phenomena in MoS2 Field-Effect Transistors under High Electric Fields

As two-dimensional (2D) transition metal dichalcogenides electronic devices are scaled down to the sub-micrometer regime, the active layers of these materials are exposed to high lateral electric fields, resulting in electrical breakdown. In this regard, understanding the intrinsic nature in layer-stacked 2D semiconducting materials under high lateral electric fields is necessary for the reliable applications of their field-effect transistors. Here, we explore the electrical breakdown phenomena originating from avalanche multiplication in MoS2 field-effect transistors with different layer thicknesses and channel lengths. Modulating the band structure and bandgap energy in MoS2 allows the avalanche multiplication to be controlled by adjusting the number of stacking layers. This phenomenon could be observed in transition metal dichalcogenide semiconducting systems due to its quantum confinement effect on the band structure. The relationship between the critical electric field for avalanche breakdown and bandgap energy is well fitted to a power law curve in both monolayer and multilayer MoS2.

Tailoring the electrical properties of MoS2 field effect transistors by depositing Au nanoparticles and alkanethiol molecules.

We fabricated and characterized MoS2 field effect transistors. First, we measured the electrical properties of MoS2 field effect transistors (FETs) that were made with mechanically exfoliated MoS2 flakes. Then, we deposited Au nanoparticles on the MoS2 channel and measured the electrical properties. We observed whether the source-drain current increased or decreased after the Au particles were deposited. The deposited Au particles either formed an extra current path and increased the current or behaved as charge-withdrawing sites and decreased the current. Next, we deposited alkanethiol molecules on the Au particles to reduce the work function of the Au. Alkanethiol molecules are known to form a self-assembled monolayer on the Au surface, and the electric dipole moment of the molecules causes the work function of the Au to decrease. Au particles can capture electrons from the MoS2 channel due to their high work function. However, the decreased work function of the Au particles subjected to alkanethiol treatment could cause captured electrons to be released from the Au particles to MoS2. Therefore, the current increased after alkanethiol treatment. This study may provide useful methods to utilize surface treatments with particles and molecules to tailor the electrical properties of MoS2-based FETs.