Manil Kang

Metal-insulator transition without structural phase transition in V2O5 film

Spectroscopic ellipsometry and x-ray diffraction are used to investigate the metal-insulator transition in V2O5 films. Below and above Tc no significant change in the (001) peak is observed, but both n and k spectra undergo remarkable changes over the entire photon energy range. The SE and XRD results indicate that the V2O5 film undergoes a MIT without a structural phase transition near 280 °C. Further the MIT is confirmed by measurement of the resistance with temperature.

Optical properties for the Mott transition in VO2

The relationship between the first-order metal-insulator transition (MIT) and the structural phase transition (SPT) in VO2 film is analyzed by dielectric function, optical conductivity, plasma energy, and electrical conductivity. The MIT and SPT temperatures in VO2 films were approximately 68 and 75 °C, respectively, with an intermediate phase existing between 68 and 75 °C. The optical and electrical results indicate that the first-order MIT in VO2 films is not driven by the SPT.

Temperature dependence of the interband transition in a V2O5 film

The temperature dependence of the interband transition in an α-V2O5 film was investigated using absorption and photoluminescence spectral measurements at a temperature range of 10–300 K. Transmission measurements in the experimental temperature range indicate that the α-V2O5 film has two distinct interband transitions, implying indirect and direct transitions. This result was confirmed by spectroscopic ellipsometry. The blue shift of both the transitions in the α-V2O5 film with decreasing temperature was explained by a reduction in the lattice-dilatation effect and the electron-phonon interaction. The PL measurements in the experimental temperature range showed that the emission near 530 nm is due to the indirect transition in the α-V2O5 film.

Analysis of diverging effective mass extracted from thermoelectric power across the metal–insulator transition in VO2

In order to reveal the electron-electron correlation (interaction) effect in a Mott insulator VO2, we measured the temperature dependence of the thermopower, Hall effect, and Raman spectra in VO2 films and extracted the diverging effective mass from the thermoelectric behavior in the metal-insulator transition (MIT) region, using a combination of the thermopower formula for two-dimensional electron system and the extended Brinkman-Rice (EBR) model explaining the correlation effect. The temperature dependence of the measured diverging thermoelectric power is closely fitted by the band-filling ( ρ) dependent effective mass m*/m=1/(1−ρ4) given in the EBR model. The diverging behavior is the effect of measurement and analyzed as the percolation phenomenon increasing the metal region with increasing temperature. The true effective mass in the metal region is regarded as constant regardless of ρ. The true correlation strength κBR in the BR model deduced by extrapolation near ρ=1 is evaluated as κBR > 0.92. The ...

Optical Properties of Sputtered Ta 2 O 5 Thin Films Using Spectroscopic Ellipsometty

thin films were deposited by RF magnetron sputtering method under various RF power, substrates and oxygen partial pressure. Elliptic constants were measured by using a phase modulated spectroscopic ellipsometer and analyzed with the Tauc-Lorentz dispersion formula and best fit method in the range of 3101239 nm. Also, transmittance spectra of the films were measured by UV -Vis spectrophotometer in the range of 3001000 nm. From these data, thickness of and surface layer were analyzed and changes of magnitude and shape of dispersion of optical constants according to fabricated conditions were measured. Also, to evaluate thickness and optical constants data analyzed by Tauc-Lorentz dispersion formula, the measured and analyzed transmittance spectra were compared. In result of the comparison, two spectra were in good agreement each other. Accordingly, it indicates that our ellipsometric analysis is valid.

Confocal laser scanning microscopy measurement of the morphology of vanadium pentoxide nanorods grown by electron beam irradiation or thermal oxidation

Abstract. In order to observe the morphology of nanostructures at the submicroscale, we use a confocal laser scanning (CLS) microscope built in our laboratory. The theoretical resolution of the hand-made CLS microscope is 150 nm and the performance of the microscope is evaluated by observing a USAF target. Vanadium pentoxide nanorods grown by electron beam irradiation and thermal oxidation methods are used as nanostructures and the morphologies of the nanorods observed by confocal laser scanning microscopy (CLSM) are compared with those obtained by scanning electron microscopy. The magnification and resolution of the CLSM were estimated to be approximately 1500 and 800 nm, respectively. From the results, we confirm that the CLSM can be used to measure nanostructures at the sub-micro-scale without a preconditioning process.

Growth Properties of Sputtered ZnO Thin Films Affected by Oxygen Partial Pressure Ratio

ZnO thin films were grown on a glass by RF sputtering system with RF power 100W and oxygen partial pressure of . Elliptic constants were measured by using a phase modulated spectroscopic ellipsometer and analyzed with the Tauc-Lorentz dispersion formula and best fit method in the range of 1.5 to 3.8eV. Also, scanning electron microscope(SEM) was used for the analysis of surface crystallization condition. From elliptic constants spectra, optical constants, thickness and roughness of ZnO films were evaluated. Total thickness of ZnO films obtained by ellipsometry showed good agreement with SEM data. It was found that the grain size of the films were getting smaller with increasing oxygen partial pressure. Band-gap of ZnO films increase with the oxygen partial pressure. These findings clearly indicate that optical properties of ZnO films are strongly dependent on the oxygen partial pressure. It could be explained that increasing the oxygen partial pressure induced high crystalline imperfection in the ZnO films.

Investigation of Growth Properties of Sputtered V 2 O 5 Thin Films Using Spectroscopic Ellipsometry

Optical structure of thin films were analyzed and confirmed, the films were deposited in oxygen partial pressure 0% and 10% by RF magnetron sputtering system. Measurements of the elliptic constants were made in the range of by using phase modulated spectroscopic ellipsometer. The elliptic constants of the thin films were analyze by Double Amorphous dispersion relation. The calculated n, k spectra of layer were obtained over the range of photon energy. SEM and XRD measurements were also made to validate the ellipsometric analysis and they give good agreement with the structural properties of the films. It was found that optical structure of the layer has a 3 phase(roughness/film/substrate) and optical absorption properties are greatly depend on the partial pressure of the oxygen.

Electron beam-formed ferromagnetic defects on MoS2 surface along 1 T phase transition.

1 T phase incorporation into 2H-MoS2 via an optimal electron irradiation leads to induce a weak ferromagnetic state at room temperature, together with the improved transport property. In addition to the 1T-like defects, the electron irradiation on the cleaved MoS2 surface forms the concentric circle-type defects that are caused by the 2 H/1 T phase transition and the vacancies of the nearby S atoms of the Mo atoms. The electron irradiation-reduced bandgap is promising in vanishing the Schottky barrier to attaining spintronics device. The simple method to control and improve the magnetic and electrical properties on the MoS2 surface provides suitable ways for the low-dimensional device applications.

Temperature-dependent photoluminescence properties of amorphous and crystalline V 2 O 5 films

In order to investigate the photoluminescence (PL) properties of V O films, amorphous and crystalline films were prepared by using RF sputtering system, and the PL spectra of the films were measured at the temperatures ranging from 300 K to 10 K. In the amorphous V O film grown at room temperature, a PL peak centered at ~505 nm was only observed, and in the crystalline V O film, two peaks centered at ~505 nm and ~695 nm, which is known to correspond to oxygen defects, were revealed. The position of PL peak centered at 505 nm for both the amorphous and crystalline V O films showed a strong dependence on temperature, and the positions were 2.45 eV at 300 K and 2.35 eV at 10 K, respectively. The PL at 505 nm was due to the band energy transition in V O , and also, the reduction of the peak position energy with decreasing temperature was caused by a decrement of the lattice dilatation effect with reducing electron-phonon interaction.