Seung-Jae Moon

Excimer laser-induced temperature field in melting and resolidification of silicon thin films

The liquid/solid interface motion and temperature history during excimer laser annealing of 50-nm-thick Si films on fused quartz substrates are investigated by in situ nanosecond time-resolved electrical conductance, optical reflectance, and transmittance at visible and near-IR wavelengths, combined with thermal emission measurements. The temperature response, melt propagation and evolution of the recrystallization process are fundamentally different in the partial-melting and the complete-melting regimes. Because it is necessary to balance the latent heat across the propagating phase-change interface, the maximum induced temperature in the partial-melting regime remains close to the melting point of amorphous Si. The peak temperature rises in the complete-melting regime, but the nonparticipating nature of the liquid Si/fused quartz interface allows substantial supercooling (>200 K), followed by spontaneous nucleation into fine-grained material. These phase transformations are consistent with the recrysta...

Relationship between fluence gradient and lateral grain growth in spatially controlled excimer laser crystallization of amorphous silicon films

In order to clarify the relationship between excimer laser fluence gradient and the length of lateral grain growth, the excimer laser fluence used for crystallization is modulated by a beam mask. The crystallized 50-nm-thick a-Si films are secco etched in order to observe the lateral grain growth by scanning electron microscope. The fluence distribution across the pattern is measured by a negative photoresist that has a linear relationship between laser fluence and resist thickness after development. This mapped fluence distribution is utilized to deduce the fluence gradient for each laser energy output. It is shown that lateral growth length increases and the directionality of the grains improves as the fluence gradient increases. Lateral growth length as long as 1.5 μm can be driven in a 50-nm-thick a-Si film by a single excimer laser pulse without any substrate heating. Electrical conductance measurement is used to probe the solidification dynamics. The lateral solidification velocity is estimated to b...

Thermal conductivity of amorphous silicon thin films

The thermal conductivity of amorphous silicon thin films is determined by using the non-intrusive, in situ optical transmission measurement as well as by the 3ω method. The temperature dependence of the film complex refractive index is determined by spectroscopic ellipsometry. The acquired transmission signal is fitted with predictions obtained by coupling conductive heat transfer with multi-layer thin film optics in the optical transmission measurement. The results of the two independent methods are in close agreement.

Catalytic conversion of 1,2-dichlorobenzene using V2O5/TiO2 catalysts by a thermal decomposition process

This study examined the catalytic oxidation of 1,2-dichlorobenzene on V(2)O(5)/TiO(2) nanoparticles. The V(2)O(5)/TiO(2) nanoparticles were synthesized by the thermal decomposition of vanadium oxytripropoxide and titanium tetraisopropoxide. The effects of the synthesis conditions, such as the synthesis temperature and precursor heating temperature, were investigated. The specific surface areas of V(2)O(5)/TiO(2) nanoparticles increased with increasing synthesis temperature and decreasing precursor heating temperature. The catalytic oxidation rate of the V(2)O(5)/TiO(2) catalyst formed by thermal decomposition process at a catalytic reaction temperature of 150 and 200 degrees C was 46% and 95%, respectively. As a result, it was concluded that the V(2)O(5)/TiO(2) catalysts synthesized by a thermal decomposition process showed good performance for 1,2-DCB decomposition at a lower temperature.

In situ and ex situ diagnostics on melting and resolidification dynamics of amorphous and polycrystalline silicon thin films during excimer laser annealing

The liquid/solid interface motion and the temperature history of Si films during excimer laser annealing are observed by in situ experiments combining time-resolved (∼1 ns) electrical conductance, optical reflectivity/transmissivity at visible and near-infra-red wavelength, and thermal emission measurements. The existence of partial and complete melting regimes are detected. In the partial melting regime, the maximum temperature remains close to the melting point of amorphous Si (a-Si), since the laser energy is consumed by the latent heat of phase-change. In the complete melting regime, substantial supercooling, followed by spontaneous nucleation is observed. These phase transformations are consistent with the recrystallized poly-Si morphologies. It is also found that phase change temperature of poly-Si films is about 140 K higher than that of the a-Si films.

Infrared Measurement of the Pseudogap of P-Doped and Co-Doped High-Temperature BaFe2As2 Superconductors

We report on infrared studies of charge dynamics in a prototypical pnictide system: the BaFe2As2 family. Our experiments have identified hallmarks of the pseudogap state in the BaFe2As2 system that mirror the spectroscopic manifestations of the pseudogap in the cuprates. The magnitude of the infrared pseudogap is in accord with that of the spin-density-wave gap of the parent compound. By monitoring the superconducting gap of both P- and Co-doped compounds, we find that the infrared pseudogap is unrelated to superconductivity. The appearance of the pseudogap is found to correlate with the evolution of the antiferromagnetic fluctuations associated with the spin-density-wave instability. The strong-coupling analysis of infrared data further reveals the interdependence between the magnetism and the pseudogap in the iron pnictides.

Heat Transfer and Phase Transformations in Laser Annealing of Thin Si Films

Recrystallization of thin amorphous silicon (a-Si) films can yield polysilicon (p-Si) material with functional properties suitable for fabrication of electronic devices, including high definition large area active matrix liquid crystal displays. Pulsed laser-effected melting and recrystallization is exceptionally effective since it avoids damage to the underlying insulator structure. The ensuing phase transformations and ultimately the quality of the produced p-Si material strongly depend on the temperature history. This article presents a review of research aiming to understand the complex nucleation, resolidification and crystal growth phenomena that evolve under severely non-equilibrium conditions. It is shown that elucidation of the fundamental thermodynamic processes enables conception of novel practical thin film crystal growth techniques.

Laser sintering of inkjet-printed silver nanoparticles on glass and PET substrates

Laser sintering of inkjet-printed silver nanoparicles was studied. Conducting lines were printed on glass and PET substrates of various thicknesses by a drop-n-demand (DOD) inkjet printer and silver nanoparticle ink. Selective sintering of the printed silver nanopartilces was then performed using laser beam to enhance electrical conductivity. Focused laser beam was irradiated normally to the silver layer printed on the transparent substrates with various beam intensity and various irradiation time. Conductivity and morphology of the laser sintered silver patterns were measured and compared with the results of oven-heated sintering. Temperature elevation by laser irradiation was also calculated based on laser beam intensity, irradiation time, surface reflectivity, and thermophysical property of a substrate to estimate enhancement of the electrical conductivity by laser sintering.

INTERPRETATION OF OPTICAL DIAGNOSTICS FOR THE ANALYSIS OF LASER CRYSTALLIZATION OF AMORPHOUS SILICON FILMS

The focus of this investigation is on optical diagnostics of melting and solidification during excimer laser crystallization of a-Si. Although optical techniques are effective for probing the laser annealing process, the analysis of the results is not straightforward due to temperature variation of the optical properties, phase change, and interference effects. These effects are clarified by analyzing the front and back, s- and p-polarized reflectivity traces at the lambda=633-nm and 1,520-nm laser wavelengths. Ellipsometry was utilized to measure the temperature-dependent spectral optical properties of the deposited 50-nm a-Si films. Spectral optical properties of laser-annealed poly-silicon were also measured at room temperature. These optical properties were used in the optical models constructed for the melting and solidification processes. Comparison with the experimental results unveiled evidence of spontaneous volumetric nucleation for laser fluences sufficiently high to impart complete melting of ...The focus of this investigation is on optical diagnostics of melting and solidification during excimer laser crystallization of a-Si. Although optical techniques are effective for probing the laser annealing process, the analysis of the results is not straightforward due to temperature variation of the optical properties, phase change, and interference effects. These effects are clarified by analyzing the front and back, s- and p-polarized reflectivity traces at the lambda=633-nm and 1,520-nm laser wavelengths. Ellipsometry was utilized to measure the temperature-dependent spectral optical properties of the deposited 50-nm a-Si films. Spectral optical properties of laser-annealed poly-silicon were also measured at room temperature. These optical properties were used in the optical models constructed for the melting and solidification processes. Comparison with the experimental results unveiled evidence of spontaneous volumetric nucleation for laser fluences sufficiently high to impart complete melting of the a-Si film.

Optimal Design of a Parallel-Flow Heat Exchanger Using a Response Surface Methodology

ABSTRACT The heat and flow characteristics in a single-phase parallel-flow heat exchanger were examined numerically to obtain its optimal shape. A response surface methodology was introduced to predict the performance of the heat exchanger with respect to selected design parameters over the design domain. The design parameters were the inflow and outflow angles of the working fluid and the horizontal and vertical locations of the inlet and outlet. The relative priority of the design parameters was evaluated to identify the most important parameters, and these were then optimized using a response surface methodology. The JF factor was chosen as the evaluation characteristic value since it can consider the heat transfer and pressure drop simultaneously. The JF factor of the optimum model was 5.3% greater than that of the reference model.