Supawan Joonwichien

Effects of crystal defects and their interactions with impurities on electrical properties of multicrystalline Si

We investigated the effects of different crystal defects and their interactions with impurities on the electrical properties of multicrystalline Si (mc-Si) using samples with unique defect patterns and impurities. By using the floating cast method, a single grain boundary (GB), identified as a Σ27 boundary, was first formed with a high density of impurities from atmosphere, leading to an inefficient external gettering of impurities during phosphorus (P) diffusion. During crystal growth, the Σ27 GB splits into the Σ3 and Σ9 GBs with accompanying generation of dislocations and reduction in the density of impurities. The external gettering of impurities became efficient for removing impurities as evidenced by an increase in average minority carrier lifetime. At the final stage of crystal growth, the decrease in minority carrier lifetime was significant, which could not be improved by phosphorus diffusion because of the high densities of segregated impurities and crystal defects originating from the strong co...

Seed manipulation for artificially controlled defect technique in new growth method for quasi-monocrystalline Si ingot based on casting

We propose a new growth method for quasi-monocrystalline Si that achieves high-quality ingots and a high yield ratio. This method induces defect regions with compose dislocations surrounded by grain boundaries. These functional defects benefit the crystalline quality through impurity gettering, dislocation-propagation blocking, and stress relaxation with plastic deformation. Functional defect regions were grown from designed seeds and introduced to ingot edges where crystalline Si was disposed because of contamination. Preliminary experimentation demonstrated that functional defects could effectively form from the manipulated seeds. We call this method the Seed Manipulation for Artificially Controlled Defect Technique.

Relationship between dislocation density and contact angle of dendrite crystals in practical size silicon ingot

We suggested the possibility to suppress dislocation generation by controlling the microstructure of dendrite crystals in practical size Si wafers grown by the floating cast method. With the floating cast method, the contact angle between adjacent dendrite crystals can be used as a structural parameter to define grain boundaries (GBs). We fabricated a practical size silicon ingot fully covered with dendrite crystals and investigated dislocation density near the GBs as a function of the contact angle. The dislocation density was found to decrease with decreasing contact angle. This result can be explained by differences in shear stress on {111} slip surface around the GBs, as supported by numerical calculations considering various structural parameters in multicrystalline Si. These results confirm our previous results with laboratory-scale ingots, and we believe this concept can be applied to commercial growth processes.

Magnetic field effects on photodecomposition of methylene blue over ZnO particles

The magnetic field effects (MFEs) on the photocatalytic degradation of methylene blue (MB) solution over ZnO particles have been studied. A positive MFE was clearly confirmed, while the MFE is decreased with increasing the interval time (settling time) between the preparation of MB solution and the start of photodegradation. It is suggested that one of the key factors of the MFEs is the amount of dissolved oxygen in the MB solution.

Comparison of phosphorus gettering effect in faceted dendrite and small grain of multicrystalline silicon wafers grown by floating cast method

We attempt to clarify the effect of phosphorus diffusion gettering (PDG) on the differences in microstructures of multicrystalline silicon (mc-Si) wafers. From the results of the floating cast method, the obtained microstructure of mc-Si was determined to contain unique microstructures, consisting of large faceted dendrites and small grains with low and high dislocation densities. The PDG efficiency was evaluated through the change in minority carrier lifetime. As a result, a stronger positive PDG effect was found for large faceted dendrites with low dislocation density, attributed to a small number of defects that can act as recombination centers of carriers. Lifetime improvement was also found in dislocation regions possibly owing to the redistribution of interstitial metals. These results suggest the importance of controlling the microstructure of mc-Si, which could strongly affect the PDG efficiency and existence of incorporated impurities.

Utilization of Magnetic Field for Photocatalytic Decomposition of Organic Dye with ZnO Powders

The discovery of magnetic field effects (MFEs) on homogeneous chemical reactions have led to leads many reports on these effect, however a few have been applied on heterogeneous systems. The aim of this work, therefore, is to investigate the MFEs on photocatalytic decomposition of methylene blue (MB) using ZnO particles. The UV–VIS–NIR spectrometer was used to monitor the MB concentrations. The dependence of the reaction rate under UV irradiation on light intensity, the kept time between preparation of MB solution and before dispersing catalyst powders, so called settling duration, are studied. It is clear that the magnetic field enhances the decomposition rate of MB using ZnO. In case of zero magnetic field, the results show that the photocatalytic decomposition rate followed a first-order model. On the other hand, the decomposition rate of MB using ZnO did not follow first-order reaction under magnetic field. Furthermore, base on the results it is suggested that the condition of the MB solution is an important factor for MFEs on the photodegradation rate.

Structural control of dendrite crystals in practical size silicon ingots grown by floating cast method

We report on successful control of the growth of dendrite crystals to suppress dislocation generation in practical size multicrystalline silicon ingots by modifying the growth condition. Local temperature distributions and gas flow above silicon melt surface were controlled by utilizing additional heat insulators and carbon components in a furnace. The size, positions and shapes of the insulators and the components were optimized by the aid of numerical calculations on heat transfer and radiation. As a result, we succeeded in controlling the nucleation of dendrite crystals and realized directional growth on the melt surface to form multicrystalline structure. This structural control leads to suppression of dislocation generation around grain boundaries which are formed by contacts of dendrite crystals. This shows that the structural control by manipulating dendrite crystal growth by the floating cast method is one of the promising approaches to realize high quality Si ingot based on a casting method.

Improvement of annealing procedure to suppress defect generation during impurity gettering in multicrystalline silicon for solar cells

We demonstrate an improved annealing procedure to suppress defect generation during impurity gettering process. A multiple cycled annealing and cooling for impurity gettering provides higher carrier lifetime in the wafers compared with continuously annealed wafers (conventional method). Microscopic photoluminescence images revealed that dislocation propagation from grown-in dislocations and defect generation in intra grain were suppressed in samples with the multiple cycled annealing. Therefore the multiple cycled annealing procedure is concluded to be a promising technique to improve electrical property of multicrystalline silicon for solar cells.

Contact Angles of Adjacent Dendrite Crystals to Form Grain Boundary and Iron Precipitation in Multicrystalline Silicon Ingot Grown by Floating Cast Method

We report on the impact of contact angles of adjacent dendrite crystals on the interstitial iron concentrations in multicrystalline silicon (mc-Si) ingot grown by floating cast method. Different contact angles of <10°, 60°-70°, and 115° were selected from the upper surface of the ingot. It was found that the contact angles of adjacent dendrite crystals to form grain boundary (GB) affect the iron precipitation at GB. When the two dendrite crystals arranged in parallel to each other, the density of interstitial iron was revealed to be less than 10 atoms/cm. On the other hand, contact angles of 60°-70° and 115° resulted in high densities of interstitial iron. Photoluminescence imaging technique was used to confirm that the dislocations cluster can be generated at GB when the contact angle is large. This suggests that parallel arrangement of the dendrite crystals is preferable to decrease density of dislocations. Increase of contact angles resulted in large shear stress around GB, causing the generation of dislocations, which may act as a sink of metallic impurities.