Sunglyul Maeng

Mechanism and observation of Mott transition in VO2-based two- and three-terminal devices

An abrupt Mott metal-insulator transition (MIT) rather than the continuous Hubbard MIT near a critical on-site Coulomb energy U/U_c=1 is observed for the first time in VO_2, a strongly correlated material, by inducing holes of about 0.018% into the conduction band. As a result, a discontinuous jump of the density of states on the Fermi surface is observed and inhomogeneity inevitably occurs. The gate effect in fabricated transistors is clear evidence that the abrupt MIT is induced by the excitation of holes.When holes of about 0.018% are induced into a conduction band (breakdown of critical on-site Coulomb energy), an abrupt first-order Mott metal–insulator transition (MIT) rather than a continuous Hubbard MIT near a critical on-site Coulomb energy U/Uc=1, where U is on-site Coulomb energy between electrons, is observed on an inhomogeneous VO2 film, a strongly correlated Mott insulator. As a result, discontinuous jumps of the density of states on the Fermi surface are observed and inhomogeneity inevitably occurs. The off-current and temperature dependences of the abrupt MIT in a two-terminal device and the gate effect in a three-terminal device are clear evidence that the abrupt Mott MIT was induced by the excitation of holes. Raman spectra measured by a micro-Raman system show an MIT without the structural phase transition. Moreover, the magnitude of the observed jumps ΔJobserved at the abrupt MIT is an average over an inhomogeneous measurement region of the maximum true jump, ΔJtrue, deduced from the Brinkman–Rice picture. A brief discussion of whether VO2 is a Mott insulator or a Peierls insulator is presented.

Epitaxial growth of ZnO nanowall networks on GaN/sapphire substrates

Heteroepitaxy of vertically well-aligned ZnO nanowall networks with a honeycomblike pattern on GaN∕c-Al2O3 substrates by the help of a Au catalyst was realized. The ZnO nanowall networks with wall thicknesses of 80–140nm and an average height of about 2μm were grown on a self-formed ZnO thin film during the growth on the GaN∕c-Al2O3 substrates. It was found that both single-crystalline ZnO nanowalls and catalytic Au have an epitaxial relation to the GaN thin film in synchrotron x-ray scattering experiments. Hydrogen-sensing properties of the ZnO nanowall networks have also been investigated.

ZnO film thickness effect on surface acoustic wave modes and acoustic streaming

Surface acoustic wave(SAW) devices were fabricated on ZnO thin films deposited on Si substrates. Effects of ZnOfilm thickness on the wave mode and resonant frequency of the SAWs have been investigated. Rayleigh and Sezawa waves were detected, and their resonant frequencies decrease with increase in film thickness. The Sezawa wave has much higher acoustic velocity and larger signal amplitude than those of Rayleigh mode wave.Acoustic streaming for mixing has been realized in piezoelectric thin filmSAWs. The Sezawa wave has a much better efficiency in streaming, and thus is very promising for application in microfluidics.

White-light emitting surface-functionalized ZnSe quantum dots: europium complex-capped hybrid nanocrystal

A color-tunable emitter comprising Eu complex-capped ZnSe quantum dot (QD) organic–inorganic hybrid nanocrystals (NCs) was simply synthesized by a hot-injection method, plus the addition of an Eu precursor. Hybrid NCs have the emission of both Eu complexes and ZnSe QDs, and they show bluish white light. In the case of composite NCs (Eu/Zn = 1.0), the emission increases up to 174% compared with that of pristine ZnSe QDs. It is due to the sensitization of the Eu complex acting as an antenna, so the energy obtained by the Eu complexes transfers to the ZnSe QDs. In addition, the NCs have a strong excitation band in the near-UV region, which gives them an advantage over wavelength converters for white light-emitting diodes (LEDs). The expected structure of the hybrid NCs was verified by TEM, XRD, and XPS. It features a zinc blende crystal structure identical to the ZnSe QD, along with Eu-based complexes that can be coordinated with the Se ion on the surface of ZnSe QDs. Therefore, new organic–inorganic hybrid luminescent material using the emission of both QDs and lanthanide (Ln) complexes can potentially serve as a light source in white LEDs.

Characteristics of erbium-silicided n-type Schottky barrier tunnel transistors

The current–voltage characteristics of erbium-silicided n-type Schottky barrier tunnel transistors (SBTTs) are discussed. The n-type SBTTs with 60 nm gate lengths shows typical transistor behaviors in drain current to drain voltage characteristics. The drain current on/off ratio is about 105 at low drain voltage regime in drain current to gate voltage characteristics. However, the on/off ratio tends to decrease as the drain voltage increases. From the numerical simulation results, the increase of off-current is mainly attributed to the thermionic current and the increase of drain current is mainly attributed to the tunneling current, respectively. This phenomenon is explained by using drain induced Schottky barrier thickness thinning effect.

ZnO film for application in surface acoustic wave device

High quality, c-axis oriented zinc oxide (ZnO) thin films were grown on silicon substrate using RF magnetron sputtering. Surface acoustic wave (SAW) devices were fabricated with different thickness of ZnO ranging from 1.2 to 5.5 µm and the frequency responses were characterized using a network analyzer. Thick ZnO films produce the strongest transmission and reflection signals from the SAW devices. The SAW propagation velocity is also strongly dependent on ZnO film thickness. The performance of the ZnO SAW devices could be improved with addition of a SiO 2 layer, in name of reflection signal amplitude and phase velocity of Rayleigh wave.

Surface acoustic wave induced streaming and pumping in 128° Y-cut LiNbO3 for microfluidic applications

This paper provides a detailed study on surface acoustic wave (SAW) induced acoustic streaming and pumping, focusing on the effects of the wave mode and surface modification. SAW devices with wavelengths of 32 and 64 µm were fabricated on 128° Y-cut lithium niobate substrates with aluminium interdigitated transducers. A higher order harmonic mode wave appears in addition to the fundamental Rayleigh wave for samples with metallization ratios less than 0.6. Both waves have demonstrated the ability to induce acoustic streaming and to pump liquid. A high streaming velocity, and hence a high mixing efficiency and a higher acoustic force, can be obtained using the fundamental Rayleigh wave as the high harmonic waves have large propagation losses. A linear relationship between the streaming velocity and RF signal voltage has been obtained, and effective mixing can be achieved. An acoustic wave has also been utilized to manipulate and pump droplets with sizes up to 5 µl, and a moving speed of ~1.4 cm s−1 has been obtained on an octadecyltrichlorosilane-treated SAW device using a signal voltage of 40 V.

Growth optimization and electrical characteristics of VO2 films on amorphous SiO2/Si substrates

In order to fabricate an electronic device, we have optimized growth conditions of VO2 films on amorphous SiO2/Si structure substrates by pulsed laser deposition. The phase of VO2 films observed consisted of multiphases such as VO2, V2O5, and V2O3. The correlation between phase changes and growth conditions of VO2 films was analyzed. Also, the electrical characteristics of VO2-based three terminal devices, attributed to structural and phase changes, are discussed. VO2 films under optimal growth conditions have a change in resistivity of the order of 102 near a critical temperature, Tc=340 K. Furthermore, an abrupt jump in current was observed when an electric field was applied to the three terminal device in the VO2-based SiO2/Si structure. The source–drain voltage, in which the abrupt current jump occurred, changed with application of a gate electric field.

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.

Assembly of thermally reduced graphene oxide nanostructures by alternating current dielectrophoresis as hydrogen-gas sensors

Chemo-resistive hydrogen-gas sensors based on thermally reduced graphene oxide (rGO) have been fabricated on a micro-hotplate by positive ac dielectrophoresis (DEP). The optimized DEP parameters for manipulating rGO nanostructures into Au electrodes for hydrogen sensing are: applied frequency = 1 MHz, peak-to-peak voltage = 5 V, and DEP time = 30 s. The device exhibits good sensitivity (∼6%) with fast response time (∼11 s) and recovery time (∼36 s) for 200 ppm hydrogen gas at room temperature. This result indicates that the DEP process has great potential for assembling rGO for hydrogen-gas sensor in many industrial and scientific applications.