Takashi Sameshima

Evaluation of defects generation in crystalline silicon ingot grown by cast technique with seed crystal for solar cells

Although crystalline silicon is widely used as substrate material for solar cell, many defects occur during crystal growth. In this study, the generation of crystalline defects in silicon substrates was evaluated. The distributions of small-angle grain boundaries were observed in substrates sliced parallel to the growth direction. Many precipitates consisting of light elemental impurities and small-angle grain boundaries were confirmed to propagate. The precipitates mainly consisted of Si, C, and N atoms. The small-angle grain boundaries were distributed after the precipitation density increased. Then, precipitates appeared at the small-angle grain boundaries. We consider that the origin of the small-angle grain boundaries was lattice mismatch and/or strain caused by the high-density precipitation.

Interaction between Metal Impurities and Small-Angle Grain Boundaries on Recombination Properties in Multicrystalline Silicon for Solar Cells

Recombination properties at small-angle grain boundaries (SA-GBs) in multicrystalline silicon were evaluated. After Fe contamination, the electron-beam-induced current (EBIC) contrast at most SA-GBs became stronger, especially at >1.5°. After Al gettering, EBIC contrast of most 1.5° SA-GBs showed comparable contrast before gettering. In addition, there were SA-GBs which have different recombination properties even with the same misorientation angle. Between these SA-GBs, there were differences in the rotation axis, boundary direction, and existence of defects. The associativity of metals might be affected by the existence of defects caused by these differences.

Impact of Light-Element Impurities on Crystalline Defect Generation in Silicon Wafer

In multi-crystalline silicon grown by unidirectional solidification, there are many origins of crystalline defects. In this study, we investigated the effect of light-element impurities on the generation of crystalline imperfections during crystal growth. In order to control the interfusion of impurities, we regulate the Ar gas flow in the atmosphere on the basis of a computer simulation. The etch pit densities in the sample fabricated without and with Ar gas flow control in the atmosphere were 1.5×105–7.0×107 and 5.0×103–4.0×105 cm-2, respectively. In the sample fabricated without Ar gas flow control, the precipitates consisting of light-elements were observed in the region where the etch pit density markedly increased. In the region with the highest etch pit density, there were small-angle grain boundaries consisting of dislocations. We believed that the precipitates consisting of light-element impurities were the potential origins of small-angle grain boundaries. The light-element impurities should affect the crystalline defect generation induced during crystal growth, and thereby should be controlled.

Material Research on High-Quality Passivation Layers with Controlled Fixed Charge for Crystalline Silicon Solar Cells

We evaluated the three types of composition spread passivation layer, i.e., Al2O3–HfO2, HfO2–Y2O3, and Al2O3–Y2O3 systems, by combinatorial pulsed laser deposition to evaluate and control the fixed charge, while interface states were kept constant with a SiO2 interlayer. The flat-band voltage of the capacitance–voltage (C–V) curves was shifted widely from positive to negative by changing the composition. The calculated fixed charge in the Y2O3 was positive while those in the HfO2 and Al2O3 were negative. In the Al2O3–Y2O3 system, the fixed charge was significantly varied between -2.7 and 1.3×1012 cm-2 with composition spread. The maximum negative charge was found in the Al2O3 layer with a slight amount of Y2O3, while the maximum positive charge was realized with almost pure Y2O3. The fixed charge modifications were also found in the Al2O3–HfO2 and HfO2–Y2O3 systems. Additional oxidation after layer deposition also modified the fixed charge properties. The largest negative fixed charge of -3.1×1012 cm-2 was found in approximately HfO2:Y2O3=1:1 after the annealing process, while the largest positive charge of 1.3×1012 cm-2 was found for Y2O3 with Al2O3 incorporation. The passivation layers with controlled fixed charge can be promising materials for the high-quality passivation layer in crystalline silicon solar cells.

Correlation between carbon incorporation and defect formation in quasi-single crystalline silicon

We investigated the correlation between C incorporation and defect generation in quasi-single crystalline silicon ingots. The substitutional carbon concentration and etch pit density in the ingot fabricated with atmosphere control to suppress C incorporation were much lower than those in the ingot fabricated without control. In addition, the precipitates consisted of C, N and Si were confirmed in the ingot fabricated without control. After the precipitation, small-angle grain boundaries (SA-GBs) were generated. We consider that the precipitation were the origin of SA-GBs, therefore the crystalline defect density can be decreased by reducing the incorporation of C impurities during crystal growth.

Behaviors of Fe and Ni at crystal defects in multi-crystalline silicon by intentional contamination and phosphorus gettering

We investigated behaviors of Fe and Ni at crystalline defects in multi-crystalline silicon by intentional contamination and phosphorus (P) gettering processes. After contaminations, EBIC contrasts became stronger at Σ27 and Random grain boundaries (GBs), and at small-grain boundaries (SA-GBs) with >; 1° misorientation angles. After P gettering processes, EBIC contrasts recovered as low as those before metal contamination at most GBs and SA-GBs. However, some Σ27, Random GBs and SA-GBs with >;1° misorientation angle remain high contrast. Defect properties such as boundary orientations or tilt and twist components consisting misorientation angles might affect on the minority carrier recombination.

Evaluation of Silicon Substrates Fabricated by Seeding Cast Technique

We evaluated the properties of crystalline defects in silicon substrate, and clarified the origin of small-angle grain boundaries. In order to eliminate the effects of grain boundaries, the ingot was fabricated by unidirectional solidification technique with seed crystal. In single-crystalline region, Σ3 twin boundaries and SiC precipitates were observed near the seed crystal. No obvious relationship between twin boundaries and precipitates was observed. These defect decreased once and the precipitations appeared again. The density of precipitates increased through the crystal growth procedure. These precipitates were consisted of Si, C, and N. After the precipitation density increased, the small-angle grain boundaries appeared and some precipitates were observed at the boundaries. We considered the precipitations consisted of light element impurities such as C and N were one of the major origins of the small-angle grain boundary generation.

EBIC Study on Metal Contamination at Intra Grain Defects in Multicrystalline Silicon for Solar Cells

Interactions between intra-grain defects and metal impurities in multicrystalline silicon (mc-Si) were evaluated. After metal contaminations, EBIC contrasts at > 1.5o SA-GBs were more enhanced than those at Ni/1000 oC > Ni/600 oC. These results might attribute to Fe atoms form deeper energy levels of recombination centers than Ni atoms and the gettering abilities at SA-GBs depend on the misorientation angles. Many dark spots were observed in EBIC images in the Ni/600 oC. Since the dark spots corresponded to the etch pits, the dark spots might be dislocations decorated with Ni. The gettering abilities of SA-GBs depended on the misorientaion angles, and the recombination properties at SA-GBs and dark spots, such as small defects after metal contamination were different by annealing temperatures and the types of metal impurities.