Kyung Soo Yi

Room-Temperature Quantum Confinement Effects in Transport Properties of Ultrathin Si Nanowire Field-Effect Transistors

Quantum confinement of carriers has a substantial impact on nanoscale device operations. We present electrical transport analysis for lithographically fabricated sub-5 nm thick Si nanowire field-effect transistors and show that confinement-induced quantum oscillations prevail at 300 K. Our results discern the basis of recent observations of performance enhancement in ultrathin Si nanowire field-effect transistors and provide direct experimental evidence for theoretical predictions of enhanced carrier mobility in strongly confined nanowire devices.

Unravelling the switching mechanisms in electric field induced insulator–metal transitions in VO2 nanobeams

We studied insulator–metal transitions in VO2 nanobeams for both abrupt and gradual changes in applied electric fields. Based on the observations, the Poole–Frenkel effect explained the abrupt transition, while the gradual case is found to be dominated by the Joule heating phenomenon. We also carried out power model and finite element method based simulations which supported the Joule heating phenomena for gradual transition. An in-principle demonstration of the Poole–Frenkel effect, performed using a square voltage pulse of 1 µs duration, further confirms the proposed insulator–metal transition mechanism with a switching time in the order of 100 ns. Finally, conductivity variations introduced via rapid thermal annealing at various temperatures validate the roles of both Joule heating and Poole–Frenkel mechanisms in the transitions.

Conductance control in VO2 nanowires by surface doping with gold nanoparticles.

The material properties of semiconductor nanowires are greatly affected by electrical, optical, and chemical processes occurring at their surfaces because of the very large surface-to-volume ratio. Precise control over doping as well as the surface charge properties has been demonstrated in thin films and nanowires for fundamental physics and application-oriented research. However, surface doping behavior is expected to differ markedly from bulk doping in conventional semiconductor materials. Here, we show that placing gold nanoparticles, in controlled manner, on the surface of an insulating vanadium dioxide nanowire introduces local charge carriers in the nanowire, and one could, in principle, completely and continuously alter the material properties of the nanowire and obtain any intermediate level of conductivity. The current in the nanowire increased by nearly 3 times when gold nanoparticles of 10(11) cm(-2) order of density were controllably placed on the nanowire surface. A strong quadratic space-charge limited (SCL) transport behavior was also observed from the conductance curve suggesting the formation of two-dimensional (2D) electron-gas-like confined layer in the nanowire with adsorbed Au NPs. In addition to stimulating scientific interest, such unusual surface doping phenomena may lead to new applications of vanadium dioxide-based electronic, optical, and chemical sensing nanodevices.

Gate-Tunable Hole and Electron Carrier Transport in Atomically Thin Dual-Channel WSe2/MoS2 Heterostructure for Ambipolar Field-Effect Transistors

An ambipolar dual-channel field-effect transistor (FET) with a WSe2 /MoS2 heterostructure formed by separately controlled individual channel layers is demonstrated. The FET shows a switchable ambipolar behavior with independent carrier transport of electrons and holes in the individual layers of MoS2 and WSe2 , respectively. Moreover, the photoresponse is studied at the heterointerface of the WSe2 /MoS2 dual-channel FET.

Post-thermal-Induced Recrystallization in GaAs/Al0.3Ga0.7As Quantum Dots Grown by Droplet Epitaxy with Near-Unity Stoichiometry

Here, we investigate the stoichiometry control of GaAs/Al0.3Ga0.7As droplet epitaxy (DE) quantum dots (QDs). Few tens of core nonstoichiometries in the Ga(As) atomic percent are revealed in as-grown “strain-free” QDs using state-of-the-art atomic-scale energy-dispersive X-ray spectroscopy based on transmission electron microscopy. Precise systematic analyses demonstrate a successful quenching of the nonstoichiometry below 2%. The control of the chemical reactions with well-controlled ex situ annealing sheds light on the engineering of a novel single-photon source of strain-free DE QDs free of defects.

A Hierarchical Study of Even-Denominator Filling Fractions in Quantum Hall Structure

A theoretical framework is presented which provides a unified description of the even-denominator fractional quantum Hall structure. We considered states having a pair of additional gauge fluxes attached to the fundamental even filling factor state. The even-denominator hierarchy trees constructed by our approach satisfy a direct sum rule which is also satisfied by Haldanes 1/3, 1/5 trees. The present model should be tested through further experiments and its accuracy lies in the agreement of its results with experimental results.