Bum-Kyu Kim

Electrical and Thermoelectric Transport by Variable Range Hopping in Thin Black Phosphorus Devices

The moderate band gap of black phosphorus (BP) in the range of 0.3-2 eV, along a high mobility of a few hundred cm(2) V(-1) s(-1) provides a bridge between the gapless graphene and relatively low-mobility transition metal dichalcogenides. Here, we study the mechanism of electrical and thermoelectric transport in 10-30 nm thick BP devices by measurements of electrical conductance and thermopower (S) with various temperatures (T) and gate-electric fields. The T dependences of S and the sheet conductance (σ□) of the BP devices show behaviors of T(1/3) and exp[-(1/T)(1/3)], respectively, where S reaches ∼0.4 mV/K near room T. This result indicates that two-dimensional (2D) Motts variable range hopping (VRH) is a dominant mechanism in the thermoelectric and electrical transport in our examined thin BP devices. We consider the origin of the 2D Motts VRH transport in our BPs as trapped charges at the surface of the underlying SiO2 based on the analysis with observed multiple quantum dots.

Influence of metal work function on the position of the Dirac point of graphene field-effect transistors

We studied the effect of metal contact on the position of the Dirac point by means of transport measurements. To determine the sole effect of metal contact, we prepared more than 100 graphene devices following the same fabrication procedure and with a device layout that differed only in the type of metal electrode used. By measuring the peak position of the resistance, the Dirac points (VgDirac) were recorded in the gate response. The work function of metal-graphene complex was found to be a fair phenomenological indicator of the location of VgDirac in the transfer response.

Low-temperature formation of epitaxial graphene on 6H-SiC induced by continuous electron beam irradiation

It is observed that epitaxial graphene forms on the surface of a 6H-SiC substrate by irradiating electron beam directly on the sample surface in high vacuum at relatively low temperature (∼670 °C). The symmetric shape and full width at half maximum of 2D peak in the Raman spectra indicate that the formed epitaxial graphene is turbostratic. The gradual change of the Raman spectra with electron beam irradiation time increasing suggests that randomly distributed small grains of epitaxial graphene form first and grow laterally to cover the entire irradiated area. The sheet resistance of epitaxial graphene film is measured to be ∼6.7 kΩ/sq.

Positioning of the Fermi level in graphene devices with asymmetric metal electrodes

To elucidate the effect of the work function on the position of the Dirac point, we fabricated graphene devices with asymmetric metal contacts. By measuring the peak position of the resistance for each pair of metal electrodes, we obtained the voltage of the Dirac point VgDirac (V) from the gate response. We found that the position of VgDirac (V) in the hybrid devices was significantly influenced by the type of metal electrode. The measured shifts in VgDirac (V) were closely related to the modified work functions of the metal-graphene complexes. Within a certain bias range, the Fermi level of one of the contacts aligned with the electron band and that of the other contact aligned with the hole band.

Tunable double and triple quantum dots in carbon nanotube with local side gates

We demonstrate a simple but efficient design for forming tunable single, double and triple quantum dots (QDs) in a sub-μm-long carbon nanotube (CNT) with two major features that distinguish this design from that of traditional CNT QDs: the use of i) Al2Ox tunnelling barriers between the CNT and metal contacts and ii) local side gates for controlling both the height of the potential barrier and the electron-confining potential profile to define multiple QDs. In a serial triple QD, in particular, we find that a stable molecular coupling state exists between two distant outer QDs. This state manifests in anti-crossing charging lines that correspond to electron and hole triple points for the outer QDs. The observed results are also reproduced in calculations based on a capacitive interaction model with reasonable configurations of electrons in the QDs. Our design using artificial tunnel contacts and local side gates provides a simple means of creating multiple QDs in CNTs for future quantum-engineering applications.

Macroscopic Quantum Tunneling in Superconducting Junctions of β-Ag2Se Topological Insulator Nanowire

We report on the fabrication and electrical transport properties of superconducting junctions made of β-Ag2Se topological insulator (TI) nanowires in contact with Al superconducting electrodes. The temperature dependence of the critical current indicates that the superconducting junction belongs to a short and diffusive junction regime. As a characteristic feature of the narrow junction, the critical current decreases monotonously with increasing magnetic field. The stochastic distribution of the switching current exhibits the macroscopic quantum tunneling behavior, which is robust up to T = 0.8 K. Our observations indicate that the TI nanowire-based Josephson junctions can be a promising building block for the development of nanohybrid superconducting quantum bits.