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Rubrene is a promising and archetypal organic semiconductor owing to its high reported hole mobility. However, this high mobility only exists in its crystalline state, with orders of magnitude reduction in disordered films. Thus, as it pertains to thin film polycrystalline rubrene, it is important to understand structure and the presence of disordered regions at grain boundaries. Here, we use scanning transmission X-ray microscopy (STXM) to investigate polycrystalline rubrene thin films with either platelet or spherulite morphology. The STXM images allow us to distinguish and quantify the arrangement of the local structure in the crystal. The analysis suggests that the platelet film has more oriented molecules in the crystal than in the spherulite phase. Also, at spherulite grain boundaries, we reveal a high number of misaligned molecules compared to the smooth boundary in the platelet case, with grain boundary sinuosity of 0.045 ± 0.002 and 0.139 ± 0.002 μm for the platelet and spherulite cases, respectively, which help to explain the higher mobility in the former case.

We report on our attempt to perform the three-dimensional (3D) visualization of dislocation clusters in multicrystalline silicon (mc-Si) ingot by processing photoluminescence (PL) images and analysis of dislocation clusters in mc-Si. As-sliced wafers prepared using a high-performance (HP) mc-Si ingot were sequentially measured by PL imaging with intentional superposition of reflection. Then, various image processing techniques were applied to all the PL images to extract dark regions, which most likely correspond to dislocation clusters, as well as microstructures. By 3D reconstruction using a large quantity of 2D images, we could successfully visualize the generation, propagation and annihilation of dislocation clusters in HP mc-Si ingot. In addition, relationship between source region of dislocation clusters and crystal orientation were investigated by combining data scientific and experimental approaches. As a result, it was suggested that small angle grain boundaries with angular deviation of less than 10 degrees cause the generation of dislocation clusters.

Analysis of nucleation sites in multicrystalline silicon (mc-Si) grown by directional solidification is required for further grain refinement to reduce dislocation density. In this study, Voronoi diagrams were utilized to analyze nucleation sites of mc-Si grown by single-layer Si beads (SLSB)-seeding method. The grain distribution at the bottom of the ingot was almost reproduced by the weighted Voronoi diagram with a relaxation method to optimize the positions of generating points of the diagram, which correspond to the nucleation sites, and the difference of nucleation timing of each crystal grain. Comparison of the generating points with the optical image indicated that the nucleation started at the remarkably deep portions of the nucleation layer. Further grain refinement by SLSB-seeding method could be achieved by suppressing the formation of the remarkably deep portions of the nucleation layer.

We report on the application of Bayesian optimization (BO), which could accelerate the time-intensive process optimization of many parameters, to fabrication of the high-performance titanium oxide/silicon oxide/crystalline silicon passivating contact. The process contains pre-deposition treatment to form SiOy interlayer, atomic layer deposition (ALD) of TiOx, and hydrogen plasma treatment (HPT) as post-process. We attempted to optimize seven parameters for ALD and HPT by dealing with samples treated by three kinds of chemical solutions in the same batch. This permits to perform BO for each structure at the same time and determine the superior pre-deposition treatment. Consequently, carrier selectivity S10 estimated by independent measurements of the saturation current density and contact resistance was significantly improved by BO of only 12 cycles and 10 initial random experiments. These results certify that BO could efficiently provide experimental conditions in multidimensional parameter space although we need to consider the impact of the metallization process on the passivation performance.

We applied a Bayesian optimization (BO) to optimize the epitaxial growth process of Si thin films. The BO enables us to effectively explore the optimal film growth conditions considering several experimental parameters and their interactions. In this way we reduced the total number of experiments required in the optimization. The epitaxial growth rate was maximized while five quality parameters were maintained within an acceptable range. Additionally, we considered two practical issues: eliminating equipment errors and the time cost of the quality parameter evaluation. To overcome these issues, we adaptively conducted BO with different constraints for different situations. As a result of these optimizations, the crystal growth rate was increased to be approximately twice as high as that under standard conditions, while satisfying the five quality parameter conditions.

Heavily tin (Sn)-doped Si crystals in a concentration up to 4×1019 cm−3 were grown by the Czochralski method. Variation of Sn concentration in the crystals was well expressed by the Pfann equation using a segregation coefficient of k=0.016. From the occurrence of growth interface instability and the appearance of Sn precipitates in the grown crystals, the solubility limit of Sn was considered to be around 5×1019 cm−3. Interstitially dissolved oxygen Oi was presented at a concentration of 8–9×1017 cm−3 in the grown Sn-doped crystals. The FT-IR absorption peak relating to a Si–Oi–Si quasi-molecule at 1106 cm−1 showed preferential occupation of Oi at the bond-centered position of Si–Si. The Oi peak shifted to the lower wave number side with increasing Sn concentration in Si, implying expansion of the Si–Si bond.

Dislocations were introduced in wurtzite zinc oxide single crystals by Vickers indentations on { 2 1 ¯ 1 ¯ 0 } and { 10 1 ¯ 0 } surfaces at elevated temperatures, and their slip systems were determined by transmission electron microscopy combined with etch pit observations. The observed system for { 2 1 ¯ 1 ¯ 0 } indentations was { 10 1 ¯ 1 ¯ } 〈 1 ¯ 2 1 ¯ 0 〉 or (0001) 〈 1 ¯ 2 1 ¯ 0 〉 , while that for { 10 1 ¯ 0 } indentations was { 10 1 ¯ 0 } 〈 1 ¯ 2 1 ¯ 0 〉 . Activation of the slip systems was briefly discussed in terms of the Peierls stress and resolved shear stress acting on the slip systems.

Heavily indium (In)-doped Si crystals were grown by the Czochralski method under a consideration of the effects of co-doping of electrically neutral group-IV elements (C, Ge or Sn). The In concentration in In-doped Si increased with the amount of In charged into the crucible and reached 3.5×1017 cm−3. The carrier concentration was at most 6×1016 cm−3, limited by the low ionization ratio of ~20% of In. Co-doping of C and Ge effectively enhanced the In concentration while Sn did not, which was examined in terms of the atomistic size, lattice parameter change, mutual bonding energy and solubility of group-IV elements in Si. However, no sufficient increase in carrier concentrations was detected in Si by the co-doping, and formation of some clusters or complexes was suggested.

A noncontact crucible method using conventional silica crucibles for reducing stress in Si bulk crystals is proposed. In this method, the Si melt used has a low-temperature region in its upper central part so that natural Si crystal growth occurs inside it. Compared with the conventional growth method, the present method has several merits such as the convex shape of the interface in the growth direction, the possibility of obtaining large ingots even with the use of a small crucible because of the growth in the large low-temperature region, and the small convection in the Si melt due to the existence of the low-temperature region. When using crucibles without Si 3 N 4 coating, p-type Si single bulk crystals can grow inside the Si melt without touching the crucible wall. The single bulk crystals grown have low dislocation densities (on the order of 10 3 cm −2 ). The diameters of the ingots obtained using a crucible with 30 or 33 cm diameter are 21−22 cm. The surface orientation of the cross section is (100). An n-type ingot with Σ3 twin grain boundaries is grown using a crucible without Si 3 N 4 coating. The average minority carrier lifetime of a cross section is 82.8 μs for the passivated surface of an n-type wafer, which is higher than those (7.3–16.0 μs) in the case of p-type wafers. A larger temperature reduction is required for the growth using crucibles without Si 3 N 4 coating than that for the growth using crucibles with Si 3 N 4 coating to obtain ingots with the same diameter. A crystal diameter, as large as 72% of the crucible diameter is obtained for the p-type single bulk crystal grown using crucibles without Si 3 N 4 coating.

Conventional crystal growth methods using silica crucibles cannot control the stress caused by expansion due to the solidification of the Si melt because crucible walls made of silica have insufficient flexibility to reduce the stress. In the case of crystal growth using a silicon nitride crucible, it was reported that an ingot can be more easily released from the crucible. A noncontact crucible method was proposed using conventional silica crucibles that reduces the stress and number of dislocations in Si multicrystalline ingots. We used the present method to grow wafers with only twin boundaries. An ingot with several grains and twin boundaries was realized using a crucible that had not been coated with Si3N4 particles. Several important characteristics were reported such as the presence of a low-temperature region in the Si melt, the possibility of growing large ingots with a diameter close to the crucible diameter, the minority carrier lifetime, the distribution of dislocations, the O concentration and the effect of Si3N4 particles on the crystal structure. Dislocations were almost undetectable in a large area of the cross section when a necking technique was used for the seed growth. The O concentration in ingots grown using crucibles coated with Si3N4 particles was lower than that in an ingot grown using a crucible without a coating of Si3N4 particles. A large ingot with a diameter of 25 cm was obtained using a crucible with a diameter of 33 cm.

Stress control is necessary when preparing high-quality multicrystalline Si ingots using crucibles because crystal defects such as dislocations are mainly generated by stress in the ingots. Conventional crystal growth methods using crucibles cannot control the stress caused by expansion due to the solidification of the Si melt. We proposed a noncontact crucible method using conventional crucibles that reduces the stress in Si multicrystalline ingots. In this method, nucleation occurs on the surface of a Si melt using seed crystals, and crystals grow inside the Si melt without touching the crucible walls. Then, the ingots continue to grow while being slowly pulled upward to ensure that the crystal growth remains in the low-temperature region. The diameter and solidification ratio of the ingots can be controlled by reducing the melt temperature in the low-temperature region and by varying the product of the temperature reduction from the melting point of Si and the total growth time, respectively. A Si ingot with a diameter of 21 cm and a solidification ratio of 83% was obtained in a crucible with a diameter of 30 cm. We have confirmed that ingot growth in a crucible is feasible, during which the ingot does not come in contact with the crucible walls.

The formation mechanism of twin boundaries in multicrystalline Si was studied using an in situ observation technique. We directly observed the growing interface and analyzed change in the growth rate. We found that the formation of twin boundaries in crystal grains was always accompanied by a marked increase in the growth rate and they were rarely formed when the growth rate was constant at a high value. The formation mechanism is discussed from the viewpoint of driving force.

Dislocations in Si multicrystals strongly affect the efficiency of solar cells, and are usually generated from random grain boundaries during crystal growth. The low density of random grain boundaries and the coherency of random grain boundaries are very important in suppressing dislocations. Controlling arrangement of dendrite crystals grown along the bottom of ingots is effective for decreasing the density of random grain boundaries and for improving the coherency of random grain boundaries. A method of controlling thermal conductivity under crucibles to control the arrangement of dendrite crystals was proposed. Graphite plates with different thermal conductivities were used all over the bottom surface of crucibles. Two types of graphite plates, one with a line-shaped highly cooled part and the other with a ring-shaped one, were used. Using the graphite plates, the distribution of dendrite crystals was well arranged, and dendrite crystals were controlled to be fairly parallel to each other.

We investigated the generation mechanism of dislocations by comparing dislocation occurrence in multicrystalline silicon with calculated results of the shear stress on the slip plane by finite element analysis. To mimic the multicrystalline Si and to observe structural modification around grain boundaries a model crystal growth set-up was applied using artificially designed seed. We found that the dislocations occur at grain boundary and propagate as crystal growth proceeds. The generation of dislocations was not spatially uniform but often localized in one of the grains. The calculated stress distribution, which depends on crystallographic orientation, implies that the shear stress on the slip plane around the grain boundary is likely to cause occurrence of dislocations.

The growth behavior of faceted Si crystals in grain boundary formation during crystallization was studied by the in situ observation technique. We directly observed the transition of the shape of the growing interface just before the impingement of two crystals. It is found that when a crystal with a zigzag-shaped faceted interface encounters another crystal, {1 1 1} facet facing on the growing interface gradually disappears with the transition of the interfacial shape from zigzag to linear. This shape transition of the growing interface determines the grain boundary shape and grain boundary character.

We carried out a systematic investigation of microstructures in a particular zone of Si multicrystals which shows a strong local variation in minority carrier diffusion length (MCDL). It was found that three typical regions with distinct microstructures correspond to the microscopic origins of the local MCDL difference. The region with perfect twin structure (Σ3 boundaries) has a high MCDL, while the regions with either high-angle grain boundaries (Σ9 and Σ27 boundaries) or sub-grain boundaries (high density dislocation) exhibit lower minority carrier diffusion. The relationships between the microstructures and the corresponding MCDLs are briefly discussed. This study has implications for developing improved Si multicrystals with appropriate microstructure for application in solar cells.

This work deals with liquid phase epitaxy (LPE) from pure Si melt, which has been proposed as a method to grow Si thin layer crystals without suffering from incorporation of solvents in contrast to conventional LPE. Dropping-type LPE from pure Si melt was employed to independently control substrate temperature and cooling rate. We found that the Si epitaxial layer could be obtained in a wide temperature window even at the temperatures of 200 °C lower than the Si melting point, which demonstrates the feasibility of the proposed method. With the increase of substrate temperature, the etch pit density of epitaxial layer decreased and minority carrier diffusion length increased. Furthermore, the decrease in the cooling rate after the LPE growth was found to improve the quality of Si epitaxial layer. This shows that substrate temperature and cooling rate after the LPE growth are the key growth-controlling parameters.

We report on a simple approach for fabrication of SiGe-on-insulator (SGOI), which contains the growth of Ge on a Si-on-insulator (SOI) substrate and following high-temperature annealing. We discuss the guiding principle for choice of the processing parameters to obtain high-quality SGOI. Furthermore, SGOI was utilized as a substrate for subsequent epitaxial growth of a strained-Si layer. As a result, fluctuation of the strain in the Si film on SGOI was found to be suppressed compared with that on a conventional SiGe virtual substrate, which shows that SGOI grown by our method is promising for the substrate to grow high-mobility strained-Si channel layers.

We report on fabrication of SiGe single crystal film on insulator by a simple approach including a growth of a thin Ge film on a commercially available SOI substrate, formation of a SiO2 protective layer, and rapid thermal annealing (RTA) in an Ar atmosphere. Homogeneity of the local Si fraction in SiGe-on-insulator (SGOI) was found to be closely connected with the SiGe phase diagram, and RTA below the solidus line is required to obtain homogeneous SGOI. In spite of the high annealing temperature beyond the melting point of Ge, obtained SGOI was revealed to be single crystalline as evidenced by electron back scattering pattern analysis.