Kentaro Kutsukake

Relationship between grain boundary structures in Si multicrystals and generation of dislocations during crystal growth

We attempted to clarify relationship between grain boundary structures in Si multicrystals and generation of dislocations during crystal growth. Systematic variation of grain boundary structures was realized by employing dendritic nucleation at the initial stage of crystal growth. Etch-pit observation revealed that the contact angle of adjacent dendrite crystals to form a grain boundary affects generation of dislocations. Experimentally observed dislocation density was found to be well correlated with shear stress around the grain boundary calculated by finite element analysis.

Control of Grain Boundary Propagation in Mono-Like Si : Utilization of Functional Grain Boundaries

We attempted to control grain boundary propagation in mono-like Si utilizing functional grain boundaries introduced by multi-seeds with specific configurations, and demonstrated that functional grain boundaries can suppress multi-crystallization during growth. The suppression mechanism was studied on the basis of grain boundary character. The grain boundaries nucleate at the crucible surface, which generally leads to multi-crystallization, interact with the functional grain boundaries, resulting in a change of their character and configuration. This would increase the yield of the mono-like wafers in an ingot and would enable a large reduction of cost per watt of solar cells.

Quantitative analysis of subgrain boundaries in Si multicrystals and their impact on electrical properties and solar cell performance

We implement spatially resolved x-ray rocking curve for quantitative analysis of subgrain boundary (sub-GB) density in Si multicrystals with controlled microstructures by “dendritic casting method.” The oriented crystal grains made it possible to analyze a large area over different crystal grains by using the same optical configuration. Sub-GBs were found to be spatially localized in a particular region and spread in the growth direction of the ingot. By combining sub-GB density with a separate measurement of minority carrier diffusion length, carrier recombination velocity at sub-GBs was revealed to be at the same order as random GBs. Furthermore, by analyzing electroluminescence images of a solar cell with different detection wavelengths, sub-GBs were shown to behave as shunts with stronger activity than random GBs.

On the origin of strain fluctuation in strained-Si grown on SiGe-on-insulator and SiGe virtual substrates

We investigated the microscopic strain fluctuation in strained-Si grown on SiGe-on-insulator (SGOI) and SiGe virtual substrates, and clarified the origins of the strain fluctuation in the strained-Si film. A periodic strain fluctuation, which reflects a cross-hatch pattern of the substrate, was observed in the sample on the virtual substrate. On the other hand, a featureless strain fluctuation with suppressed amplitude was observed in the sample on SGOI substrate. By analyzing the correlation of the Raman peak positions of the Si–Si modes in strained-Si and SiGe, the dominant mechanism of the strain fluctuation in the strained Si film was found to be the compositional fluctuation in underlying SiGe for the sample on SGOI, and the strain fluctuation reflecting the cross-hatch pattern for the sample on the virtual substrate, respectively.

Implementation of faceted dendrite growth on floating cast method to realize high-quality multicrsytalline Si ingot for solar cells

We carried out small-scale crystal growth experiment to apply faceted dendrite growth at the top of the melt to floating cast method in order to grow high-quality multicrystalline Si (mc-Si) ingot for solar cells. By appropriate cooling at the initial stage, the most part of the top surface was covered by a single faceted dendrite crystal followed by directional solidification from the top to the bottom. As a consequence, the cross-section of the ingot was dominated by a single crystal grain. The crystal grain was found to be almost free from subgrain boundaries as evidenced by spatially resolved single-peak x-ray rocking curve profiles. The minority carrier diffusion length was found to be much longer than that in a control sample grown from the bottom to the top presumably due to the minimized contact with the crucible wall. This shows that the floating cast method combined with formation of faceted dendrite crystals at the initial stage is a promising route to realize high-quality mc-Si ingot for high-...

Mono-Like Silicon Growth Using Functional Grain Boundaries to Limit Area of Multicrystalline Grains

We propose a new growth method for mono-like silicon (Si): the suppression of multicrystallization using functional grain boundaries artificially formed by multiseed crystals. In our previous study, we demonstrated such suppression in an ingot 30 mm in diameter. In this paper, we grew mono-like Si ingots of 100 and 400 mm on a side. Functional grain boundaries successfully suppressed the increase in the area of multicrystalline grains nucleated on crucible side walls, which indicates a large volume of quasi-monocrystalline Si up to the top of the ingots. This enables a large increase in the yield of quasi-monocrystalline wafers in an ingot and would lead to a reduction in the cost of the solar cells.

Influence of structural imperfection of Σ5 grain boundaries in bulk multicrystalline Si on their electrical activities

Bulk multicrystalline Si with {310} Σ5 grain boundaries (GBs) was grown by Bridgman growth method using seed crystals with artificially controlled configuration. The structure of the GBs was preserved in the epitaxial growth region without formation of more GBs. However, the GBs were revealed to contain small-angle deviation of ∼5o from the perfect Σ5 relative crystal orientation both in tilt and twist components due to the inaccuracy of the seed crystal arrangement. Such an unintentional misalignment was utilized to investigate the relationship between electrical activity and the deviation angle of {310} Σ5 GB. Electron beam-induced current measurement clarified that carrier recombination velocity at the {310} Σ5 GBs decreases with decreasing deviation angle and shows a minimum at the perfect Σ5 relationship. This tendency suggests that {310} Σ5 GB is electrically inactive, as well as Σ3 and Σ9 GBs.

Structural Origin of a Cluster of Bright Spots in Reverse Bias Electroluminescence Image of Solar Cells Based on Si Multicrystals

Electroluminescence (EL) imaging was utilized to investigate spatial distribution of electric properties of solar cells based on Si multicrystals with different base resistivity. A part of electrically active defects in solar cells, observed as dense dark lines in the EL image under the forward bias, was found to be detected as a cluster of bright spots in the EL image under the reverse bias especially when the base resistivity is low. This observation was accompanied with decrease in filling factor of solar cells. Local structural analysis clarified that sub-grain boundaries are responsible for the defects.

Fabrication of SiGe-on-insulator through thermal diffusion of Ge on Si-on-insulator substrate

We report on the fabrication of a homogeneous SiGe-on-insulator as a substrate for strained Si-on-insulator (SOI) metal-oxide-semiconductor field-effect-transistors. The fabrication process includes the growth of a thin Ge film on a commercially available SOI substrate at 100°C using a molecular beam epitaxy system, the formation of a SiO2 cap layer by radio-frequency sputtering, and rapid thermal annealing (RTA) in an Ar atmosphere. After RTA at an appropriate temperature, the SiGe-on-insulator with a laterally homogeneous Si fraction was successfully obtained by the formation of epitaxial SiGe on a thin SOI as a seed and interdiffusion of Ge and Si atoms. However, inhomogeneous SiGe films were obtained when the annealing temperature was very high. The conditions for the realization of SiGe with a homogeneous Si fraction were found to be closely related to the phase diagram of the Si–Ge binary alloy.

Modification of local structures in multicrystals revealed by spatially resolved x-ray rocking curve analysis

We show that spatially resolved x-ray rocking curve analysis can be a useful technique to investigate local structures in bulk multicrystals and their modification during crystal growth when combined with appropriate samples. For this purpose, a model crystal growth experiment was carried out using a Si seed crystal with artificially controlled ⟨110⟩ tilted grain boundaries. The growth orientation was chosen as the scan axis for rocking curve measurements in different crystal grains. Thanks to the superior angle resolution, the gradual structural changes can be monitored by the changes in the rocking curve profile such as the peak shift and the peak splitting. The amount as well as the sign of the peak shift was found to be strongly dependent on the initial grain boundary structure. Furthermore, the technique was applied to investigate local structures of Si multicrystals with controlled grain orientation grown without any seeds.