Soonshin Kwon

Tunable electronic transport characteristics of surface-architecture-controlled ZnO nanowire field effect transistors.

Surface-architecture-controlled ZnO nanowires were grown using a vapor transport method on various ZnO buffer film coated c-plane sapphire substrates with or without Au catalysts. The ZnO nanowires that were grown showed two different types of geometric properties: corrugated ZnO nanowires having a relatively smaller diameter and a strong deep-level emission photoluminescence (PL) peak and smooth ZnO nanowires having a relatively larger diameter and a weak deep-level emission PL peak. The surface morphology and size-dependent tunable electronic transport properties of the ZnO nanowires were characterized using a nanowire field effect transistor (FET) device structure. The FETs made from smooth ZnO nanowires with a larger diameter exhibited negative threshold voltages, indicating n-channel depletion-mode behavior, whereas those made from corrugated ZnO nanowires with a smaller diameter had positive threshold voltages, indicating n-channel enhancement-mode behavior.

Passivation effects on ZnO nanowire field effect transistors under oxygen, ambient, and vacuum environments

We investigated the passivation effects on the electrical characteristics of ZnO nanowire field effect transistors (FETs) under the various oxygen environments of ambient air, dry O2, and vacuum. When the ZnO nanowire FET was exposed to more oxygen, the current decreased and the threshold voltage shifted to the positive gate bias direction, due to electrons trapping to the oxygen molecules at the nanowire surface. On the contrary, the electrical properties of the nanowire FET remained unchanged under different environments with passivation by a polymethyl methacrylate layer, which demonstrates the importance of surface passivation for ZnO nanowire-based electronic device applications.

Surface relief gratings on poly(3-hexylthiophene) and fullerene blends for efficient organic solar cells

The use of periodic submicrometer structures as an efficient light-trapping scheme was investigated for high performance organic solar cells (OSCs) based on poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61. The gratings on an active layer are achieved by a soft lithographic approach using photoinduced surface-relief gratings (SRGs) on azo polymer films and poly(dimethylsiloxane) as a master and stamp, respectively. Incident photon to current conversion efficiency and the power conversion efficiency of OSC with gratings increased primarily due to enhanced short circuit current density, indicating that SRGs induce further photon absorption in active layers by increasing the optical path length and light trapping.

Sub-amorphous Thermal Conductivity in Ultrathin Crystalline Silicon Nanotubes

Thermal transport behavior in nanostructures has become increasingly important for understanding and designing next generation electronic and energy devices. This has fueled vibrant research targeting both the causes and ability to induce extraordinary reductions of thermal conductivity in crystalline materials, which has predominantly been achieved by understanding that the phonon mean free path (MFP) is limited by the characteristic size of crystalline nanostructures, known as the boundary scattering or Casimir limit. Herein, by using a highly sensitive measurement system, we show that crystalline Si (c-Si) nanotubes (NTs) with shell thickness as thin as ∼5 nm exhibit a low thermal conductivity of ∼1.1 W m(-1) K(-1). Importantly, this value is lower than the apparent boundary scattering limit and is even about 30% lower than the measured value for amorphous Si (a-Si) NTs with similar geometries. This finding diverges from the prevailing general notion that amorphous materials represent the lower limit of thermal transport but can be explained by the strong elastic softening effect observed in the c-Si NTs, measured as a 6-fold reduction in Youngs modulus compared to bulk Si and nearly half that of the a-Si NTs. These results illustrate the potent prospect of employing the elastic softening effect to engineer lower than amorphous, or subamorphous, thermal conductivity in ultrathin crystalline nanostructures.

Electrical Properties of Surface-Tailored ZnO Nanowire Field-Effect Transistors

A review on the tunable electrical properties of ZnO nanowire field-effect transistors (FETs) is presented. The FETs made from surface-tailored ZnO nanowire exhibit two different types of operation modes, which are distinguished as depletion and enhancement modes in terms of the polarity of the threshold voltage. We demonstrate that the transport properties of ZnO nanowire FETs are associated with the influence of nanowire size and surface roughness associated with the presence of surface trap states at the interfaces as well as the surface chemistry in environments.

Effect of gate bias sweep rate on the electronic properties of ZnO nanowire field-effect transistors under different environments

We report the effects of gate bias sweep rate on the electronic characteristics of ZnO nanowire field-effect transistors (FETs) under different environments. As the device was swept at slower gate bias sweep rates, the current decreased and threshold voltage shifted to a positive gate bias direction. These phenomena are attributed to increased adsorption of oxygen on the nanowire surface by the longer gate biasing time. Adsorbed oxygens capture electrons and cause a surface depletion in the nanowire channel. Different electrical trends were observed for ZnO nanowire FETs under different oxygen environments of ambient air, N2, and passivation.

Thermal transport in amorphous materials: a review

Thermal transport plays a crucial role in performance and reliability of semiconductor electronic devices, where heat is mainly carried by phonons. Phonon transport in crystalline semiconductor materials, such as Si, Ge, GaAs, GaN, etc, has been extensively studied over the past two decades. In fact, study of phonon physics in crystalline semiconductor materials in both bulk and nanostructure forms has been the cornerstone of the emerging field of ‘nanoscale heat transfer’. On the contrary, thermal properties of amorphous materials have been relatively less explored. Recently, however, a growing number of studies have re-examined the thermal properties of amorphous semiconductors, such as amorphous Si. These studies, which included both computational and experimental work, have revealed that phonon transport in amorphous materials is perhaps more complicated than previously thought. For instance, depending on the type of amorphous materials, thermal transport occurs via three types of vibrations: propagons, diffusons, and locons, corresponding to the propagating, diffusion, and localized modes, respectively. The relative contribution of each of these modes dictates the thermal conductivity of the material, including its magnitude and its dependence on sample size and temperature. In this article, we will review the fundamental principles and recent development regarding thermal transport in amorphous semiconductors.

Three-terminal nanoelectromechanical field effect transistor with abrupt subthreshold slope.

We report the first experimental demonstration of a three-terminal nanoelectromechanical field effect transistor (NEMFET) with measurable subthreshold slope as small as 6 mV/dec at room temperature and a switching voltage window of under 2 V. The device operates by modulating drain current through a suspended nanowire channel via an insulated gate electrode, thus eliminating the need for a conducting moving electrode, and yields devices that reliably switch on/off for up to 130 cycles. Radio-frequency measurements have confirmed operation at 125 MHz. Our measurements and simulations suggest that the NEMFET design is scalable toward sub-1 V ultrahigh-frequency operation for future low-power computing systems.

Misfit-Guided Self-Organization of Anticorrelated Ge Quantum Dot Arrays on Si Nanowires

Misfit-strain guided growth of periodic quantum dot (QD) arrays in planar thin film epitaxy has been a popular nanostructure fabrication method. Engineering misfit-guided QD growth on a nanoscale substrate such as the small curvature surface of a nanowire represents a new approach to self-organized nanostructure preparation. Perhaps more profoundly, the periodic stress underlying each QD and the resulting modulation of electro-optical properties inside the nanowire backbone promise to provide a new platform for novel mechano-electronic, thermoelectronic, and optoelectronic devices. Herein, we report a first experimental demonstration of self-organized and self-limited growth of coherent, periodic Ge QDs on a one-dimensional Si nanowire substrate. Systematic characterizations reveal several distinctively different modes of Ge QD ordering on the Si nanowire substrate depending on the core diameter. In particular, Ge QD arrays on Si nanowires of around 20 nm diameter predominantly exhibit an anticorrelated pattern whose wavelength agrees with theoretical predictions. The correlated pattern can be attributed to propagation and correlation of misfit strain across the diameter of the thin nanowire substrate. The QD array growth is self-limited as the wavelength of the QDs remains unchanged even after prolonged Ge deposition. Furthermore, we demonstrate a direct kinetic transformation from a uniform Ge shell layer to discrete QD arrays by a postgrowth annealing process.

High-Performance Screen-Printed Thermoelectric Films on Fabrics

Printing techniques could offer a scalable approach to fabricate thermoelectric (TE) devices on flexible substrates for power generation used in wearable devices and personalized thermo-regulation. However, typical printing processes need a large concentration of binder additives, which often render a detrimental effect on electrical transport of the printed TE layers. Here, we report scalable screen-printing of TE layers on flexible fiber glass fabrics, by rationally optimizing the printing inks consisting of TE particles (p-type Bi0.5Sb1.5Te3 or n-type Bi2Te2.7Se0.3), binders, and organic solvents. We identified a suitable binder additive, methyl cellulose, which offers suitable viscosity for printability at a very small concentration (0.45–0.60 wt.%), thus minimizing its negative impact on electrical transport. Following printing, the binders were subsequently burnt off via sintering and hot pressing. We found that the nanoscale defects left behind after the binder burnt off became effective phonon scattering centers, leading to low lattice thermal conductivity in the printed n-type material. With the high electrical conductivity and low thermal conductivity, the screen-printed TE layers showed high room-temperature ZT values of 0.65 and 0.81 for p-type and n-type, respectively.