Ungkil Kim

Synthesis of Si Nanosheets by a Chemical Vapor Deposition Process and Their Blue Emissions

We synthesized free-standing Si nanosheets (NSs) with a thickness of about <2 nm using a chemical vapor deposition process and studied their optical properties. The Si NSs were formed by the formation of frameworks first along six different <110> directions normal to [111], its zone axis, and then by filling the spaces between the frameworks along the <112> directions under high flow rate of processing gas. The Si NSs showed blue emission at 435 nm, and absorbance and photoluminescence (PL) excitation measurements indicate that enhanced direct band transition attributes to the emission. Time-resolved PL measurement, which showed PL emission at 435 nm and a radiative lifetime of 1.346 ns, also indicates the enhanced direct band gap transition in these Si NSs. These outcomes indicate that dimensionality of Si nanostructures may affect the band gap transition and, in turn, the optical properties.

Nondestructive propagation loss and facet reflectance measurements of GaAs/AlGaAs strip‐loaded waveguides

A modified Fabry–Perot resonance technique using a single cavity was proposed to obtain the propagation loss of the optical waveguide. The propagation loss as well as the facet reflectance were measured without sequential cleavage for a GaAs/AlGaAs strip‐loaded waveguide based on the contrast ratios of the reflected and transmitted interference patterns.

DILUTED MAGNETIC SEMICONDUCTOR NANOWIRES

An idea for simultaneously manipulating spin and charge in a single semiconductor medium has resulted in the development of diluted magnetic semiconductors (DMSs), which exhibits surprisingly room temperature ferromagnetic signatures despite having controversial ferromagnetic origin. However, achievement of truly room temperature ferromagnetism by carrier mediation is still the subject of intense research to develop the practical spin-based devices. Nanowires with one-dimensional nanostructure, which offers thermodynamically stable features and typically single crystalline and defect free, have a number of advantages over thin films with respect to studying ferromagnetism in DMSs. This review focuses primarily on our works on GaN-based DMS nanowires, i.e., Mn-doped GaN, Mn-doped AlGaN and Cu-doped GaN nanowires. These DMS nanowires have room temperature ferromagnetism by the local magnetic moment of doping elements that are in a divalent state and in tetrahedral coordination, thus substituting Ga in the wurtzite-type network structure of host materials. Importantly, our evidences indicate that the magnetism is originated from the ferromagnetic interaction driven by the carrier. These outcomes suggest that nanowires are ideal building blocks to address the magnetism in DMS due to their thermodynamic stability, single crystallinity, free of defects and free standing nature from substrate. Nanowires themselves are ideal building blocks for nanodevices and, thus, it would also be helpful in developing DMS-based spin devices.

Magnetic anisotropy in vertically aligned diluted magnetic Mn:Ge semiconductor nanowires

Semiconductors doped with magnetic ion, the so-called diluted magnetic semiconductors, are promising candidates for spintronics. Herein, we report on magnetic anisotropy in Mn:Ge diluted magnetic semiconductor nanowires. We grew single crystal Mn:Ge nanowires vertically on a Ge substrate and found the anisotropy in ratios of orbital to spin magnetic moments in the angle-dependent x-ray magnetic circular dichroism measurements. Our further characterization indicates that this anisotropy comes from the unique characteristics of nanowires, i.e., very high aspect ratio in their shape and tensile stress along the longitudinal direction, which confine the spins along the longitudinal direction and make an easy axis in that direction.

Fabrication of Coaxial Si1−xGex Heterostructure Nanowires by O2 Flow-Induced Bifurcate Reactions

We report on bifurcate reactions on the surface of well-aligned Si1−xGex nanowires that enable fabrication of two different coaxial heterostructure nanowires. The Si1−xGex nanowires were grown in a chemical vapor transport process using SiCl4 gas and Ge powder as a source. After the growth of nanowires, SiCl4 flow was terminated while O2 gas flow was introduced under vacuum. On the surface of nanowires was deposited Ge by the vapor from the Ge powder or oxidized into SiO2 by the O2 gas. The transition from deposition to oxidation occurred abruptly at 2 torr of O2 pressure without any intermediate region and enables selectively fabricated Ge/Si1−xGex or SiO2/Si1−xGex coaxial heterostructure nanowires. The rate of deposition and oxidation was dominated by interfacial reaction and diffusion of oxygen through the oxide layer, respectively.

Vertical growth of Mn:Ge nanowires and their magnetic properties

We report on synthesis of Mn-doped Ge nanowires and their magnetic and electrical properties. The vertically aligned Mn:Ge nanowires wer grown on the germanium substrate by vapor-liquid-solid mechanism using Au as catalyst and GeCl4 and MnCl2 as precursor. Anomalous X-ray scattering measurement makes it clear that Mn atoms are substitutionally incorporated with the diamond network of host Ge sites. Also X-ray magnetic circular dichroism spectra at Mn L2,3-edges showed that doped Mn has local spin moment with the 3d5 electronic configuration above room temperature, meaning that the ferromagnetism originates from doped Mn2+ ions.

Optically activated Si nanowires and nanoribbons as a platform for Si-based photonics

This paper reports on optical activation of Si nanowires and Si nanoribbons by coupling Er. Single crystalline Si nanowires with diameters 100 nm and with length exceeding mum, and Si nanoribbons with thickness of 5-10 nm and with length of a few hundred of mum were grown by vapor-liquid-solid (VLS) growth mechanism using metal catalysts. Such Er-activated Si nanowires display strong Er3+ luminescence, excited via carriers in Si nanowires, yet comparable to pure silica in luminescence efficiency, showing promise of becoming a new material platform for Si-based photonics.