Doo-Hyeb Youn

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.

Raman study of electric-field-induced first-order metal-insulator transition in VO2-based devices

An abrupt first-order metal-insulator transition (MIT) without structural phase transition is first observed by current-voltage measurements and micro-Raman scattering experiments, when a DC electric field is applied to a Mott insulator VO_2 based two-terminal device. An abrupt current jump is measured at a critical electric field. The Raman-shift frequency and the bandwidth of the most predominant Raman-active A_g mode, excited by the electric field, do not change through the abrupt MIT, while, they, excited by temperature, pronouncedly soften and damp (structural MIT), respectively. This structural MIT is found to occur secondarily.An abrupt first-order metal-insulator transition (MIT) as a current jump has been observed by applying a dc electric field to Mott insulator VO2-based two-terminal devices. The size of the jumps was measured to be asymmetrical depending on the direction of the applied voltage due to heating effects. The structure of VO2 is investigated by micro-Raman scattering experiments. An analysis of the Raman-active Ag modes at 195 and 222cm−1, explained by pairing and tilting of V cations, and 622cm−1, shows that the modes below a low compliance (restricted) current do not change when the MIT occurs, whereas a structural phase transition above the low compliance current is found to occur secondarily, due to heating effects in the device induced by the MIT. The MIT has applications in the development of high-speed and high-gain switching devices.

Flexible and Transparent Gas Molecule Sensor Integrated with Sensing and Heating Graphene Layers

Graphene leading to high surface-to-volume ratio and outstanding conductivity is applied for gas molecule sensing with fully utilizing its unique transparent and flexible functionalities which cannot be expected from solid-state gas sensors. In order to attain a fast response and rapid recovering time, the flexible sensors also require integrated flexible and transparent heaters. Here, large-scale flexible and transparent gas molecule sensor devices, integrated with a graphene sensing channel and a graphene transparent heater for fast recovering operation, are demonstrated. This combined all-graphene device structure enables an overall device optical transmittance that exceeds 90% and reliable sensing performance with a bending strain of less than 1.4%. In particular, it is possible to classify the fast (≈14 s) and slow (≈95 s) response due to sp(2) -carbon bonding and disorders on graphene and the self-integrated graphene heater leads to the rapid recovery (≈11 s) of a 2 cm × 2 cm sized sensor with reproducible sensing cycles, including full recovery steps without significant signal degradation under exposure to NO2 gas.

Highly Oriented VO2 Thin Films Prepared by Sol-Gel Deposition

Highly oriented VO2 thin films were grown on sapphire substrates by the sol-gel method that includes a low pressure annealing in an oxygen atmosphere. This reduction process effectively promotes the formation of the VO2 phase over a relatively wide range of pressures below 100 mTorr and temperatures above 400oC. X-ray diffraction analysis showed that as-deposited films crystallize directly to the VO2 phase without passing through intermediate phases. VO2 films have been found to be with [100]- and [010]-preferred orientations on Al2O3(1012) and Al2O3(1010) substrates, respectively. Both films undergo a metal-insulator transition with an abrupt change in resistance, with different transition behaviors observed for the differently oriented films. For the [010]-oriented VO2 films a larger change in resistance of 1.2x10^4 and a lower transition temperature are found compared to the values obtained for the [100]-oriented films.

Graphene transparent electrode for enhanced optical power and thermal stability in GaN light-emitting diodes.

We report an improvement of the optical power and thermal stability of GaN LEDs using a chemically doped graphene transparent conducting layer (TCL) and a low-resistance contact structure. In order to obtain low contact resistance between the TCL and p-GaN surface, a patterned graphene TCL with Cr/Au electrodes is suggested. A bi-layer patterning method of a graphene TCL was utilized to prevent the graphene from peeling off the p-GaN surface. To improve the work function and the sheet resistance of graphene, CVD (chemical vapor deposition) graphene was doped by a chemical treatment using a HNO(3) solution. The effect of the contact resistance on the power degradation of LEDs at a high injection current level was investigated. In addition, the enhancement of the optical power via an increase in the current spreading and a decrease in the potential barrier of the graphene TCL was investigated.

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.

Phase and structural characterization of vanadium oxide films grown on amorphous SiO2/Si substrates

The VO2 multiphases such as V2O5, VO2, and V2O3 are confirmed and the correlations between structural characteristics and growth conditions was investigated using the scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). Also, the electrical characteristics of VO2-based three terminal devices, attributed to structural and phase changes, are discussed. The spectra of VO2 have three peaks composed of VO2 at binding energy (BE)=516.2 eV, V2O3 at BE=515.6 eV, and V2O5 at BE=517.0 eV. With increase in the growth temperature, crystal quality of VO2 films improves and approaches single phase of VO2, then the peak position shifts to the spectra of oxygen-poor phase (V2O3). With increase in the O2 flow, the peak position shifts to the spectra of oxygen-rich phase (V2O5). VO2 films grown at optimal growth conditions have a change in resistivity of the order of 102 near a critical temperature, Tc=340 K.

The effects of isoelectronic Al doping and process optimization for the fabrication of high-power AlGaN-GaN HEMTs

In order to improve the electrical characteristics of AlGaN-GaN heterostructures for applications in high electron mobility transistors (HEMTs), high-quality AlGaN-GaN was grown by way of metal-organic chemical vapor deposition on sapphire. We applied isoelectronic Al doping into the GaN-channel layers of modified AlGaN-Al-doped GaN channel-GaN heterostructures. We then compared the electrical performance of the fabricated heterostructures with those of conventional AlGaN-GaN heterostructures. The AlGaN-GaN HEMTs that were fabricated achieved power densities of up to 4.2 W/mm, some of the highest values ever reported for 0.25-/spl mu/m gate length AlGaN-GaN HEMTs. These devices exhibited a maximum drain current density of 1370 mA/mm, a high transconductance of 230 mS/mm, a short-circuit current gain cutoff frequency (f/sub T/) of 67 GHz, and a maximum frequency of oscillation (f/sub max/) of 102 GHz.

Effects of post-growth annealing on the structure and electro-optical properties of low-temperature grown GaAs

This paper investigates how postgrowth annealing affects the structure and the electro-optical properties of low-temperature grown GaAs (LT-GaAs). A systematic study of as-grown and annealed LT-GaAs revealed that the carrier lifetime is directly related to the density of the An duster and distance between As clusters. The Ga/As compositional ratio and the crystal structure of As clusters were observed for the first time. The As/Ga ratio of the As clusters is higher than that obtained from the LT-GaAs. The carrier lifetime of the annealed LT-GaAs increases as the annealing temperature increases from 400 (less than 30 fs) to 800 °C (824 fs). Under the annealing temperatures ranging from 600 to 700 °C for 90 s, we observed the emission of terahertz radiation using the annealed LT-GaAs grown at temperatures ranging from 260 to 320 °C.

Preparation and Characteristics of PMMA Microlens Array for a BLU Application by An Inkjet Printing Method

In this study, poly(methyl methacrylate)(PMMA) microlens array for a back light unit(BLU) application was fabricated by an inkjet printer, which loaded piezoelectric microchannel mounted 30 µm nozzles to deposit polymer droplets via drop-on-demand fashion. Before the inkjet printing, the surface of PMMA substrates was modified chemically by a phase separation technique. The surface topologies and aspect ratios of microlens were measured. A 5.2 × 7 cm2 BLU plate with microlens arrays printed by the inkjet method was fabricated for the application of a mobile phone.