Phase shift optimization of III/V-on-bulk-Si DFB LD for single-mode stability.

A III/V-on-Bulk-Si DFB laser with a long phase shift section optimized for single-mode stability is presented. The optimized phase shift allows stable single-mode operations up to 20 times a threshold current. This mode stability is achieved by a gain difference between fundamental and higher modes maximized by sub-wavelength-scale tuning of the phase shift section. In SMSR-based yield analyses, the long-phase-shifted DFB laser showed superior performance compared to the conventional λ/4-phase-shifted ones.

Fabrication of thin silicon sheets for solar cells using the spin casting method.

Fabrication of thin silicon sheets via spin casting is investigated for its potential use in a kerf-free wafer production process. To reduce silicon usage and understand the melt spreading process, numerical simulation of spin casting was performed. The simulation showed a specific initial droplet velocity is required to spread small amounts of liquid, and subsequent experiments confirmed this prediction. The increase in initial droplet velocity enabled the reduction in the spreading time and allowed the liquid melt to be spread fully before solidification began. Using a rotating graphite mold, silicon sheets of 100 microm thickness can be produced under optimized experimental conditions.

Refining behavior of aluminum alloyed metallurgical-grade silicon during fractional melting process.

In order to overcome the limitations in refining metallurgical-grade silicon (MG-Si) by fractional melting (FM), an alloying process was adopted. Aluminum was selected as the alloying element as it has no intermetallic compound and the lowest eutectic point (577 degrees C). A 2% Al-Si sample was made and purified by FM. The removal of 99.8% of impurities was obtained by FM via aluminum alloying with MG-Si. The 2% Al-Si sample was also effective in boron removal (99% refining ratio).

Numerical simulation of solid liquid interface behavior during continuous strip casting process.

A new metal-strip-casting process called continuous strip-casting (CSC) has been developed for making thin metal strips. A numerical simulation model to help understand solid-liquid interface behavior during CSC has been developed and used to identify the solidification morphologies of the strips and to determine the optimum processing conditions. In this study, we used a modified level contour reconstruction method (LCRM) and the sharp interface method to modify interface tracking, and performed a simulation analysis of the CSC process. The effects of process parameters such as heat-transfer coefficient and extrusion velocity on the behavior of the solid-liquid interface were estimated and used to improve the apparatus. A Sn (Tin) plate of dimensions 200 x 50 x 1 mm3 was successfully produced by CSC for a heat-transfer coefficient of 104 W/m2 K and an extrusion velocity of 0.2 m/s.

Numerical simulation and fabrication of silicon sheet via spin casting.

A spin-casting process for fabricating polycrystalline silicon sheets for use as solar cell wafers is proposed, and the parameters that control the sheet thickness are investigated. A numerical study of the fluidity of molten silicon indicates that the formation of thin silicon sheets without a mold and via spin casting is feasible. The faster the rotation speed of graphite mold, the thinner the thickness of sheet. After the spread of the molten silicon to cover the graphite mold with rotation speed of above 500 rpm, the solidification has to start. Silicon sheets can be produced by using the centrifugal force under appropriate experimental conditions. The spin-cast sheet had a vertical columnar microstructure due to the normal heat extraction to the substrate, and the sheet lifetime varied from 0.1 microS to 0.3 microS measured by using the microwave photoconductance decay (MW-PCD) to confirm that the spin-cast silicon sheet is applicable to photovoltaics.

Study of the microstructures and lifetime of spin-cast silicon sheets for photovoltaics.

Silicon sheets were fabricated by a new fabricating method, spin casting with various rotation speeds of the graphite mold. The microstructure of spin-cast silicon sheets were investigated using an electron probe microanalyzer (EPMA) and scanning electron microscope/electron backscatter diffraction/orientation image micrograph, and the lifetime of the sheets was mapped using microwave photoconductance decay. The silicon sheets were vertically aligned, with sizes ranging from tens of microns to one hundred microns. The as-grown lifetime was measured and found to range from 0.049 micros to 0.250 micros. The ASTM number was plotted against the lifetime using ASTM E112 to estimate the grain size. Approximately half of the grain boundaries seemed electrically inactive with meaning of no recombination center since the grains were growth directionally, especially in a longitudinal aligned. It was confirmed that the lifetime of spin-cast sheets makes them suitably applicable for photovoltaics compared to those produced by alternative ribbon-producing methods.

Behavior of the impurity-rich phase in metallurgical grade silicon during fractional melting.

A new fractional melting (FM) process that uses centrifugal force to separate the liquid from the cake (liquid + solid) was developed for refining metallurgical grade Si. The behavior of the solute and the refining mechanism during the FM process were studied using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). During the FM, the solutes migrated very quickly and aggregated at seemingly disordered locations, where they subsequently melted before the silicon bulk matrix melted.

Formation of silicon sheet on a rotating substrate.

A spin casting process to fabricate polycrystalline silicon sheets for use as solar cell wafers is presented and the parameters that control the sheet thickness are investigated. The computational model for the spin casting is proposed in order to understand the melt flow and solidification behaviors in the mold. The effect of the rotating speed of the mold and substrate morphology on the silicon sheets is studied via computer simulations, and the simulation results are compared with the experimental results. The numerical study of the fluidity and solidification behavior of the silicon predicted that the formation of rectangular sheets via spin casting is feasible, and the subsequent experiment confirmed this prediction. Using a square mold, rectangular silicon sheets can be produced under appropriate experimental conditions. Microstructural analyses verified the presence of long columnar structures on the sheets.