Jee-Eun Yang

Shape-controlled growth of single-crystalline Ge nanostructures

We report the shape-controlled growth of single-crystalline germanium nanostructures by Au catalyst-assisted chemical-vapor syntheses using GeH4 as a precursor. By independently controlling the axial- and the radial-growth kinetics near the eutectic temperature of Au and Ge, we reproducibly direct the shape of Ge nanostructures from nanowires to nanocones with various aspect ratios. Based on our observation of the shape variation we discuss a phenomenological model of the growth of Ge nanostructures that goes beyond the conventional vapor-liquid-solid growth mechanism. The precise control of the shape in semiconductor nanostructures in our study suggests the implication of various applications into electronic and optical devices.

On-Nanowire Band-Graded Si:Ge Photodetectors

We report an on-nanowire (NW) band-graded photodetector, which pertains to the on-NW composition gradation from pure Si to pure Ge, Si<inf>1−x</inf>Ge<inf>x</inf> (0 ≤ x ≤ 1). By spatially and spectrally resolved photocurrent imaging on individual Si:Ge NWs, we demonstrated that the spectral on-set of interband photocarrier generation and its photocurrent amplitude is on-NW demultiflexed with respect to the relative Si:Ge composition in an individually addressable manner. It is also found that the on-NW variation in photoconductivity and gain by up to two orders of magnitude upon the visible light incidence. We attribute our observations to the photocarrier density modulation arising from both the continuously varying energy band-gap and surface trap-state density in the range of 1.5×10¹¹ cm⁻² and 4.3 × 10¹² cm⁻². Our on-NW band-modulated photodetectors suggest general implications for the heteroepitaxial integration of the broadband Si nanophotonic components.

Fabrication of Si1−xGex alloy nanowire field-effect transistors

The authors present the demonstration of nanowire field-effect transistors incorporating group IV alloy nanowires, Si1−xGex. Single-crystalline Si1−xGex alloy nanowires were grown by a Au catalyst-assisted chemical vapor synthesis using SiH4 and GeH4 precursors, and the alloy composition was reproducibly controlled in the whole composition range by controlling the kinetics of catalytic decomposition of precursors. Complementary in situ doping of Si1−xGex nanowires was achieved by PH3 and B2H6 incorporation during the synthesis for n- and p-type field-effect transistors. The availability of both n- and p-type Si1−xGex nanowire circuit components suggests implications for group IV semiconductor nanowire electronics and optoelectronics.

Axially graded heteroepitaxy and Raman spectroscopic characterizations of Si1−xGex nanowires

We report the axially graded heteroepitaxy of Si1−xGex nanowires, by the kinetic controls of the Au-catalytic decomposition of precursors during chemical vapor syntheses. Transmission electron microscope studies demonstrate that the relative composition of Si and Ge is continuously graded along the uniformly thick nanowires, sharing the same crystal structures with the continuously varying lattices. We also employed a confocal Raman scattering imaging technique, and showed that the local variations in Raman phonon bands, specific to Si and Ge alloying (νSi–Si, νSi–Ge, and νGe–Ge), can be spatially and spectrally resolved along the individual nanowires, within the spatial resolution of ∼500nm.

Large-scale assembly of highly flexible low-noise devices based on silicon nanowires

Recently, integrated flexible devices based on silicon nanowires (Si-NWs) have received significant attention as high performance flexible devices. However, most previous assembly methods can generate only specifically-shaped devices and require unconventional facilities, which has been a major hurdle for industrial applications. Herein, we report a simple but very efficient method for assembling Si-NWs into virtually generally-shape patterns on flexible substrates using only conventional microfabrication facilities, allowing us to mass-produce highly flexible low-noise devices. As proof of this method, we demonstrated the fabrication of highly bendable top-gate transistors based on Si-NWs. These devices showed typical n-type semiconductor behaviors, and exhibited a much lower noise level compared to previous flexible devices based on organic conductors or other nanowires. In addition, the gating behaviors and low-noise characteristics of our devices were maintained, even under highly bent conditions.

On-nanowire band-graded Si:Ge photodetectors

We report an on-nanowire (NW) band-graded photodetector, which pertains to the on-NW composition gradation from pure Si to pure Ge, Si 1−x Ge x (0 ≤ x ≤ 1). By spatially and spectrally resolved photocurrent imaging on individual Si:Ge NWs, we demonstrated that the spectral on-set of interband photocarrier generation and its photocurrent amplitude is on-NW demultiflexed with respect to the relative Si:Ge composition in an individually addressable manner. It is also found that the on-NW variation in photoconductivity and gain by up to two orders of magnitude upon the visible light incidence. We attribute our observations to the photocarrier density modulation arising from both the continuously varying energy band-gap and surface trap-state density in the range of 1.5×1011 cm−2 and 4.3 × 1012 cm−2. Our on-NW band-modulated photodetectors suggest general implications for the heteroepitaxial integration of the broadband Si nanophotonic components.

Computer calculation of the Lorentz microscopy image and magnetic domain structure of weakly ordered FePt:C thin film

A computer calculation technique of Lorentz microscopy image was developed and was applied to the analysis of the Lorentz microscopy image and magnetic domain structure of a weakly ordered FePt:C thin film for recording media. The magnetic domain structure was, at the as-deposited (random) state, a vortex network structure and evolved into the islandlike elongated reverse domain structure at the dc-demagnetization state, via a featherlike ripple structure at the remanent state. The magnetization reversal occurred not by the domain wall motion but by the local magnetization rotation to form a series of local vortices leading to the formation of reverse domains.

Growth and band-gap modulation in single-crystalline Si 1-x Ge x nanowires for nanophotonics applications

We report growth of single crystalline Si1-xGex nanowires, whose relative composition is controllably tuned over the entire composition range by Au-catalyst assisted chemical vapor syntheses. We also present experimental demonstration of the band-gap modulation from near infrared to ultraviolet regions with alloying of Si and Ge, and their spatial confinement at the nanometer scale. Our finding demonstrates that the energy band-gap of Si1-xGex nanowires can be modulated in a wider energy range, and suggests implications for group IV semiconductor nanophotonics.