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We present device performance of solar cells based on multicrystalline SiGe (mc-SiGe) bulk crystal with microscopic compositional distribution grown by the casting method. The average Ge composition was systematically changed in the range between 0% and 10%. A small addition of Ge to multicrystalline Si (mc-Si) was found to be very effective to increase the short-circuit current density without affecting the open-circuit voltage. As a consequence, the overall efficiency of a solar cell based on mc-SiGe was improved compared with that based on mc-Si. This result demonstrates that mc-SiGe is a promising candidate to replace mc-Si since it could achieve higher conversion efficiency without drastic increase of the production cost. [ABSTRACT FROM AUTHOR]

Twin boundary formation at grain boundaries in multicrystalline Si during directional solidification was investigated by in situ observation of the crystal/melt interface. It was clearly shown that a twin boundary was formed on the {111} facet of grain-boundary groove at the crystal/melt interface. The large amount of undercooling in the melt at grain-boundary grooves promoted rapid crystallization and twin boundary formation. [ABSTRACT FROM AUTHOR]

Impurity accumulation at the grain boundaries in multicrystalline Si (mc-Si) was investigated by in situ observation of the crystal/melt interface, analysis of the grain boundary characteristics, and measurement of impurity concentrations. The impurity concentration was higher at grain boundaries that formed a groove at the crystal/melt interface than that at regions which did not form a groove at the crystal/melt interface. We conclude that groove formation at the crystal/melt interface is the cause of impurity accumulation at the grain boundary. [ABSTRACT FROM AUTHOR]

The growth of a multicrystalline silicon (mc-Si) ingot for solar cell applications was attempted using a Liquinert quartz crucible. A mc-Si ingot was also grown in a quartz crucible coated with Si3N4 powder for comparison with that from the Liquinert quartz crucible. The mc-Si ingot grown in the Liquinert quartz crucible had a shinier surface which has few impurity particles and higher minority carrier lifetime than the mc-Si ingot grown in a quartz crucible coated with Si3N4 powder. These results indicate that contamination with impurities was reduced with the Liquinert quartz crucible; therefore, this crucible has the potential to be a powerful tool for the production of high-quality mc-Si ingots for solar cell applications. [ABSTRACT FROM AUTHOR]

The effect of grain boundary characteristics on the crystal/melt interface morphology during the unidirectional solidification of multicrystalline Si was studied by in situ observations. It was shown that sharp and smooth grooves were formed at the crystal/melt interfaces at Σ27 and random grain boundaries, whereas no grooves were formed at Σ3 {111} twin boundaries. We explain the reasons why the grooves form at grain boundaries and discuss the impurity segregation at the grain boundary grooves on the crystal/melt interface. [Copyright &y& Elsevier]

The formation mechanism of a cellular structure during the growth of Si-rich SiGe crystals was studied by in situ observation. We directly observed the morphological transformation of the crystal-melt interface during the unidirectional growth of Si-rich SiGe. It was found that the morphology of the interface transformed from a planar to a zigzag facets to a faceted cellular interface with increasing growth rate. It is clarified that Ge segregation at valleys of zigzag facets leads to the formation of a cellular structure in Si-rich SiGe crystals. [ABSTRACT FROM AUTHOR]

It is imperative to improve the crystal quality of Si multicrystal ingots grown by casting because they are widely used for solar cells in the present and will probably expand their use in the future. Fine control of macro- and microstructures, grain size, grain orientation, grain boundaries, dislocation/subgrain boundaries, and impurities, in a Si multicrystal ingot, is therefore necessary. Understanding crystal growth mechanisms in melt growth processes is thus crucial for developing a good technology for producing high-quality Si multicrystal ingots for solar cells. In this review, crystal growth mechanisms involving the morphological transformation of the crystal-melt interface, grain boundary formation, parallel-twin formation, and faceted dendrite growth are discussed on the basis of the experimental results of in situ observations. [ABSTRACT FROM AUTHOR]

Abstract: Si and Ge hemispherical concave wafers can be prepared by plastic deformation using Si and Ge single- and polycrystal wafers. Deformation regions in which such Si and Ge hemispherical wafers can be obtained by high-temperature plastic deformation were systematically studied. The deformation regions in which well-shaped Si concave wafers can be obtained were studied for (100), (111), and polycrystal Si. It was found that Si (111) crystal wafers can be more easily deformed to a perfect hemispherical shape than (100) wafers because of the crystallographic symmetry. Si hemispherical wafers with a small radius of 25mm can be perfectly deformed when 0.5-mm-thick Si (111) crystal wafers are used. Ge hemispherical wafers with a radius of 100mm can be perfectly deformed when 0.5-mm-thick Ge crystal wafers are used. Ge hemispherical concave wafers with a perfect shape can be more easily obtained using Ge (111) wafers than (100) wafers. According to these results, the deformation behavior of Ge wafers is very similar to that of Si wafers at a normalized pressing temperature. As both the radius and the load on Si and Ge hemispherical wafers increase, thicker Si and Ge wafers can be used to obtain hemispherical wafers with a perfect shape. The dislocation density in the shaped wafers markedly decreases as the pressing temperature and hemisphere radius increase. Thus, it is suggested that Si and Ge shaped wafers are of sufficient quality and have the high potential for use as several types of lens such as those in Si and Ge X-ray monochromators. [Copyright &y& Elsevier]

Abstract: We directly observed the transition of crystal growth behavior of Si in a low undercooling region. We succeeded in observing the initiation of faceted dendrite growth from a portion of parallel twins with increasing degrees of undercooling. The critical undercooling for growing a faceted dendrite was experimentally determined to be ΔT =10K. We also confirmed that parallel twins associated with faceted dendrite growth were formed between grain boundaries and not at grain boundaries during melt growth. The parallel-twin formation was explained in terms of a model of twin formation on the {111} facet plane at the growth interface. [Copyright &y& Elsevier]

Abstract: We propose a new concept of growing a polycrystalline Si ingot suitable for solar cells by casting based on the directional growth behavior of polycrystalline Si investigated using an in situ observation system. The cooling conditions for the Si melt were crucial for controlling growth in the initial stage. At a high cooling rate, dendrite growth occurred along the crucible wall. This growth mechanism was found to be useful for obtaining a polycrystalline structure with large oriented grains. In fact, using the dendrites grown in the initial stage of casting, a polycrystalline Si ingot with large oriented grains was obtained. The solar cell properties of such structure-controlled polycrystalline Si were as good as those of single-crystalline Si. [Copyright &y& Elsevier]

Abstract: We investigated structural properties of polycrystalline SiGe grown by directional growth method for solar cells. The average Ge composition was systematically changed in the range between 0 and 10at%. Distributions of concentration and crystallographic orientation in the SiGe crystal were measured and the preferential growth orientation was found to change from (111) to (110) with increasing average Ge composition. Misfit dislocation was observed using transmission electron microscope. There was few dislocations in SiGe crystals when the average Ge composition was between 0 and 5at% though a lot of dislocations existed at the average Ge composition of 10at%. We concluded that the optimum Ge composition is around 5at% for solar cells. [Copyright &y& Elsevier]

Plastic deformation is an unlikely process by which to mould pristine silicon wafers into three-dimensional shapes owing to the inevitable detrimental impact that the resulting mechanically induced defects would have on their electrical properties. However, if one were to find a way of doing so without substantial degradation of these properties, a range of new applications might be opened up. Here we report on the successful plastic deformation of silicon crystal wafers for the preparation of wafers with various shapes. A silicon wafer was set between dies and pressed at high temperatures. One application of shaped wafers is as well-shaped concave silicon crystal lenses or mirrors. The lattice plane of such a crystal lens has a curvature exactly along the surface. A concave spheroidal X-ray lens, in the form of two-dimensional Johann and Johansson's monochromators, is proposed for an X-ray optical component system. We propose and demonstrate a new solar cell system with the concave silicon crystal mirror used as both a solar cell and a focused mirror. This system can make use of the reflected photons from solar cells. [ABSTRACT FROM AUTHOR]

We investigated the grain growth behaviors of polycrystalline silicon during directional growth from melt. Two types of grain growth behaviors were directly observed using an in situ monitoring system. In the first, when the moving velocity of solid–liquid growth interface is slow and the interfacial morphology is flat, a grain with a plane of lower surface energy with respect to the growth direction expands to lateral direction. In the second, when the interface moves fast and it has an irregular shape because of the differences of the growth rate among grains, a faster growing grain competitively expands to lateral direction covering the slower one. We suggested that the undercooling at the growth front is the key parameter to divide those growth behaviors. [Copyright &y& Elsevier]

Melt growth behavior of polycrystalline silicon was observed using a newly developed in-situ monitoring system consisting of a furnace and a microscope. It is possible to perform a directional growth from seed crystal or crucible wall and to observe the growth interface by using this system. Differences of growth rate or interfacial morphology among each grain of polycrystalline silicon were observed during melt growth. The Mullins–Sekerka instability of growth interface was also observed in a silicon melt for the first time. This observation system is confirmed to become a useful tool for revealing the growth mechanism of polycrystalline silicon. [Copyright &y& Elsevier]

Crystal growth behavior of silicon melt was observed using a confocal scanning laser microscope with an infrared image furnace. The morphology of the growth interface changed from planar to facet with increasing growth rate. The facet vanishing process was also observed. It was shown the (1 1 1) facet formed at a steady state. The undercooling in front of the facet interface was measured by an infrared camera and found to be almost 7°. The growth behavior of silicon melt was explained by an analytical expression based on a two-dimensional nucleation model. [Copyright &y& Elsevier]

We propose a new method for determination of interdiffusion coefficient in high temperature solutions. This method can be characterized by the three following facts: the flux of diffusion is temporarily measured by in situ measurement technique for solution concentration in a diffusion cell; the intrinsic diffusion coefficient is derived from the flux on a basis of Fick''s first law; the interdiffusion coefficients are evaluated from the intrinsic diffusion coefficients. In practice, the interdiffusion coefficient of GaZn binary system was successfully determined using the present method. [Copyright &y& Elsevier]

The diffusion behavior of Al and Ni in Ni3Al with the L12-type crystal structure is examined by measuring intrinsic diffusion coefficients. First, interdiffusion coefficients are determined using Ni/NiAl diffusion couples forming a Ni3Al phase at the interface. The intrinsic diffusion coefficients are then determined from the interdiffusion coefficients and velocities of the Kirkendall interface with respect to the Matano plane through the Darken equation. It is shown that the intrinsic diffusion coefficients of Al and Ni in Ni3Al are of almost equal magnitude in the investigated temperature range (1423–1523 K). This conclusion is evaluated within the experimental error involved in the determination of the Matano plane. The self-diffusion coefficient of Al in Ni3Al is derived from the intrinsic diffusion coefficients of Al and Ni in Ni3Al and is compared with the estimation reported in the literature. It is shown that the present results support the diffusion mechanisms considering anti-site defects. [ABSTRACT FROM AUTHOR]

The growth velocity of silicon <110> faceted dendrites as a function of the twin spacing was investigated by in situ observation. As the twin spacing increases, <110> dendrite growth velocity nonlinearly decreases. The theoretical <110> and <112> dendrite growth velocities were calculated on the basis of the modified re-entrant corner model, and the theoretical growth velocity of <112> dendrites was found to be slower than that of <110> dendrites under the same twin spacing. The theoretical <110> dendrite growth velocity fits quite well with the experimental results. [ABSTRACT FROM AUTHOR]

The < 110 > directional solidification of silicon under varying overall temperature gradients was investigated using an in situ observation system. The growth velocity of an atomically rough interface was found to decrease with increasing temperature gradient. A theoretical model of the thermal field taking undercooling into account was developed to describe this phenomenon and was demonstrated to be valid. The results of this work indicate that the reported linear relationship between growth velocity (V) and undercooling (ΔT), given by V (mm s−1) = 120ΔT (K), is most accurate in the case of a rough interface. In the case that the overall temperature gradient is small, the melting point isotherm moves rapidly such that it becomes more difficult for the interface to keep pace with the isotherm compared with a large temperature gradient. This effect leads to increased undercooling at the interface and consequently a rapid growth velocity. Thermal field calculations confirm that a rapid increase in the ratio of the temperature gradient in the crystal to that in the melt should increase the latent heat release, again providing a more rapid growth velocity. [ABSTRACT FROM AUTHOR]

We investigated the effects of B/Ge codoping on the minority carrier lifetime in gallium (Ga)-doped Czochralski-silicon (CZ-Si) crystals. Minority carrier lifetime decreased from 28 to 0.1 μs when the B concentration was increased from 1×1015 to 1×1017 cm-3 in Ga/B codoped CZ-Si crystals. The minority carrier lifetime increased from 30 to 76 μs with increasing Ge concentration from 1×1017 to 2×1020 cm-3 in Ga/Ge codoped CZ-Si crystals. Light-induced degradation experiments showed that Ga/B codoped CZ-Si degraded rapidly, while Ga/Ge codoped CZ-Si showed no degradation. Moreover, the flow pattern defect density related to grown-in microdefects in as-grown Ga/Ge codoped CZ-Si decreased with increasing Ge concentration. The experimental results are explained using a defect reaction model based on the formation of Ge-vacancy-oxygen dimer complexes in the CZ-Si crystal during postgrowth cooling. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report on growth and characterizations of Si/multicrystalline-SiGe (mc-SiGe) heterostructure as a promising candidate to surpass mc-Si solar cells. Spatial distribution of the status of strain in Si was investigated using microscopic Raman spectroscopy. The strain was found to be strongly influenced by the composition and microstructure of underlying mc-SiGe. Spatial variation of the strain as well as strain relaxation was found to be suppressed by decreasing average Ge composition of underlying SiGe. [ABSTRACT FROM AUTHOR]

We proposed a simple lattice model to describe a solid–liquid interface of silicon based on experimental facts and molecular dynamics simulation results, and evaluated the relationship between the interface structure and the interfacial tension by comparing the model with experimental values. As a result, the entropy was found to give a major contribution to the interfacial tension, and it was revealed that the difference of entropy due to lattice disorder of bulk liquid and interface structure is the dominant factor of the entropy contribution. Moreover, the solid–liquid bond energy, which is crucial to estimate the contribution of the enthalpy, was successfully derived. The present model can be also applied to be the semiconductor material which has a diamond structure or a zinc blende structure. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A current-modulating four-point-probe method (CMR) is proposed as a method to determine the conversion efficiency of crystalline Si solar cells. This technique was implemented based on the comprehensive consideration of multiple problems or factors that are present in crystalline Si wafers and the subsequent thermal processes during solar cell fabrication, rather than employ conventional lifetime mapping or diffusion length mapping. A single parameter measurement of Ith-Is from the CMR method reflects the effective resistivity or minority carriers of an entire wafer, to quickly characterize the quality or conversion efficiency of all kinds of crystalline Si. [ABSTRACT FROM AUTHOR]

Abstract: The use of SiGe binary multicrystals with microscopic compositional distribution was propose as a solution to obtain new SiGe solar cells with higher conversion efficiency using the casting method. The SiGe multicrystals have many Ge-rich regions in Si-rich matrix, and this structure causes large increase of the photo-current for such SiGe multicrystal solar cells in the longer-wavelength region comparing to the Si multicrystal solar cells. The microscopic compositional distribution in SiGe multicrystals and wavelength dependence of absorption coefficient can be freely controlled. SiGe multicrystal solar cells with microscopic compositional distribution have higher efficiency than Si multicrystal solar cells mainly due to the increase of the total photo-current by a factor of 1.2, and the smaller decrease of open-circuit voltage within the average Ge composition of lower than 5%. The efficiency of SiGe multicrystal solar cells has a maximum value at the average Ge composition around 5%. [Copyright &y& Elsevier]

The effect of point defects on the Curie temperature ( Tc) of LiNbO3 ( LN) was investigated by combining Tc measurements with an analysis of the defect structures of LN doped with impurities having various valences. The data show that Tc of congruent LN increases with the impurity concentration up to around 3 and 2 mol% for divalent and trivalent impurities, respectively, whereas it decreases continuously with increased concentrations of tetravalent impurities. These Tc variations were examined with respect to the defect structures of impurity-doped LN, which are expressed in the form chemical formulae using Kröger-Vink notation. The defect structures of divalent and trivalent impurity-doped LN are [ABSTRACT FROM AUTHOR]

The segregation of Ge in B and Ge codoped Czochralski (CZ)-Si crystal growth was investigated. The concentration of Ge in heavily Ge codoped CZ-Si was measured by electron probe micro analysis (EPMA) and X-ray fluorescence spectroscopy. The effective segregation coefficient of Ge ( k eff ) was calculated by fitting the EPMA data to the normal freezing equation, and by taking the logarithmic ratio of the Ge concentrations at the seed and tail of the ingots (top to bottom approach). The k eff of Ge increased from 0.30 to 0.55, when the initial Ge concentration in the Si melt (C L(o) Ge ) was increased from 3 × 10 19 to 3 × 10 21 cm −3 . To avoid cellular growth, the crystal pulling rate was decreased for heavily Ge codoped crystal growth (C L(o) Ge > 3 × 10 20 cm −3 ). The equilibrium segregation coefficient ( k 0 ) of Ge was calculated by partitioning theory, and was smaller than the experimentally estimated k eff . The variation of k eff from k 0 was discussed based on Ge clustering in the heavily Ge codoped crystal, which led to changes in the bonding and strain energies caused by the incorporation of Ge into Si. [ABSTRACT FROM AUTHOR]

• Small-angle grain boundaries (SAGB) form spontaneously by dislocation aggregation during solidification of Si at a high dislocation density. • The critical density for SAGB formation was found as ca. 107 cm-2. • SAGBs tend to coalesce with other SAGBs to leave the surviving SAGBs with higher misorientation. • SAGBs are able to spontaneously increase their misorientation by absorption of individual lattice dislocations during solidification. In situ observation of the solidification process of Si combined with ex situ characterization revealed that small-angle grain boundaries (SAGBs) increase misorientation through two paths. First, coalescence with other SAGBs leads to a boost in misorientation and SAGBs tend to coalesce during solidification. Second, the absorption of individual lattice dislocations also increases the misorientation of an SAGB. Three possible mechanisms behind dislocation absorption by SAGBs have been proposed and are discussed in this paper. Among these mechanisms, the incorporation of same-sign lattice dislocations is regarded as the prominent mechanism when misorientation increases during solidification. [ABSTRACT FROM AUTHOR]

A simple, low-cost, and non-vacuum epitaxial growth method to realize large-area semiconductors on crystalline silicon will become the game-changer for various applications. For example, we can expect the disruptive effect on the cost of large-scale III–V multi-junction solar cells if we could replace the high-cost germanium substrate with silicon–germanium (SiGe) on Si. For SiGe epitaxial growth, we attempted to develop a process using original Al–Ge pastes for screen printing and subsequent annealing. We compare two pastes including Al–Ge alloyed pastes with compositional uniformity in each particle and Al–Ge mixed pastes. We revealed that Al–Ge alloyed paste could form flatter SiGe film with much less residual pastes, supported by in-situ observations. The uniform and sufficient dissolution of the alloyed paste is responsible for these and led to higher average Ge-composition by annealing at 500 °C. The composition in SiGe was vertically graded up to ~ 90% at the topmost surface. These results show that printing and firing of Al–Ge alloyed paste on Si is the desirable, simple, and high-speed process for epitaxial growth of SiGe, which could be potentially used as the lattice-matched virtual substrate with III–V semiconductors. [ABSTRACT FROM AUTHOR]

The effect of Ge codoping on the light induced degradation (LID) of B doped Czochralski-silicon (CZ-Si) was investigated. The rate of degradation under illumination is relatively suppressed in B and Ge codoped Czochralski-silicon (CZ-Si) compared to B-doped CZ-Si. The Oi concentrations measured by FTIR spectroscopy was decreased as the Ge concentration increased in the Si crystal. The formation of Ge-VO complex in silicon lattice is considered as a possible reason for the variation in Oi. Moreover, the compressive strain field around the Ge-VO complex may increase the barrier for O diffusion which limits the formation of fast diffusing O dimmers. As a consequence, the B-O dimmer (O2i) related defects were relatively suppressed, which causes the suppression of LID effect in B and Ge codoped CZ-Si. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The minority carrier lifetime (MCL)/flow pattern defect of B and Ge co-doped Czochralski silicon (CZ-Si) was increased/decreased with the initial Ge concentration up to 3×1020 cm−3 and decreased/increased at higher Ge concentrations. Secco etch pit defects were increased in heavily (⩾3×1020 cm−3) Ge co-doped CZ-Si (grown at low growth rate), which resulted in a low MCL. The influence of Ge on microdefect variation was relatively lower than the crystal growth rate in heavily Ge co-doped CZ-Si. [ABSTRACT FROM AUTHOR]

Abstract: The intensity of the infrared absorption band at 1107cm−1, related to interstitial oxygen (O i ) concentration, decreased as the Ge concentration increased in Ga and Ge codoped CZ–Si crystals. In contrast, the number of precipitates observed on the etched surfaces of CZ–Si wafers increased as the Ge concentration increased. From an energy dispersive X-ray (EDX) analysis, O was observed to be one of the major components of the precipitates. Moreover, Ge was found as one of the components in the precipitate observed on the heavily Ge (>1×1018 cm−3) codoped CZ–Si wafers. These results suggest that the grown-in O precipitates increase as the O i concentration decreases when the Ge concentration increases in the Si crystal. The Ge-vacancy (V) complex in the Si lattice probably acted as a heterogeneous nucleation center and may enhanced the grown-in O precipitates thereby reducing the dissolved O i concentration in the Si lattice. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The segregation of Ga during the growth of Czochralski-Si crystals with Ge codoping was investigated. The effective segregation coefficient of Ga in Ga/Ge-codoped Si crystal growth was nearly constant over a wide Ge concentration range, even at high Ge concentrations of about 1021 cm−3. In contrast, the effective segregation coefficient increased at high B concentrations in Ga/B-codoped CZ-Si crystal growth. The segregation behavior of Ga in Ga/Ge- and Ga/B-codoped CZ-Si crystal growth was theoretically compared. The difference in the segregation coefficients of Ga as a function of the codoped impurity (Ge or B) between the two Si crystals was attributed to a difference in the excess enthalpy due to impurity incorporation into the Si crystal between Ga–Ge pairs and Ga–B pairs The effect of Ge codoping on the minority carrier lifetime in Ga/Ge-codoped CZ-Si crystals was also investigated. The minority carrier lifetime increased with increasing Ge concentration. The higher minority carrier lifetime was associated with a decrease in interstitial oxygen related to D-defects in the Si crystal. [ABSTRACT FROM AUTHOR]

Si and Ge are widely used as analyzing crystals for x-rays. Drastic and accurate shaping of Si or Ge gives significant advance in the x-ray field, although covalently bonded Si or Ge crystals have long been believed to be not deformable to various shapes. Recently, we developed a deformation technique for obtaining strongly and accurately shaped Si or Ge wafers of high crystal quality, and the use of the deformed wafer made it possible to produce fine-focused x-rays. In the present study, we prepared a cylindrical Ge wafer with a radius of curvature of 50 mm, and acquired fluorescent x-rays simultaneously from four elements by combining the cylindrical Ge wafer with a position-sensitive detector. The energy resolution of the x-ray fluorescence spectrum was as good as that obtained using a flat single crystal, and its gain was over 100. The demonstration of the simultaneous acquisition of high-resolution x-ray fluorescence spectra indicated various possibilities of x-ray spectrometry, such as one-shot x-ray spectroscopy and highly efficient wave-dispersive x-ray spectrometers. [ABSTRACT FROM AUTHOR]

Abstract: Theoretical investigation on the photovoltaic power conversion of SiGe solar cells was carried out focusing on the impact of the compositional distribution. The absorption coefficient and the intrinsic carrier concentration were assumed to be macroscopically uniform parameters controlled by the compositional distribution. Under an assumption with finite minority carrier diffusion length, solar cell based on SiGe was revealed to have power conversion superior to that based on Si in a limited compositional window on the Si-rich side. In the window, the increase in the photocurrent density overcompensates the decrease in the voltage, and controls the overall conversion efficiency. The width of the window was greatly affected by the compositional distribution in SiGe. A comparison was made with experiments to support the existence of such a compositional window. [Copyright &y& Elsevier]

Abstract: We demonstrated a solar cell system comprising a concave-Si-mirror solar cell and a conventional small Si solar cell set at the focal point of the concave-mirror solar cell. The concave-Si-crystal mirror was prepared by mechanically polishing a single-crystal Si wafer. It was used both as a solar cell and mirror. This system can make effective use of the photons reflected from the concave-mirror solar cell because the number of total photons is the sum of photons from both the mirror solar cell and the small solar cell set at the focal point. When the conventional small solar cell with an efficiency of 13.0% was used, the total conversion efficiency of the present system increased to 12.2% compared with the conversion efficiency of 11.5% for only the concave-Si-mirror solar cell. These results show that the present solar cell system has the capability to achieve high efficiency through the effective use of the photons reflected from conventional solar cells. [Copyright &y& Elsevier]

Abstract: Effects of additives, Cu and Ni, on intermetallic compound formation at the interface between Sn–Zn-based solder and Cu substrate have been studied. (Sn91Zn9)99.98−x M x Al0.02 (wt.%) solder (M=Cu or Ni) alloys were prepared. The microstructures of the samples after annealing at 120°C for 100h were investigated using a scanning electron microscope, and the melting temperatures of the alloys were measured with a differential scanning calorimeter for different Cu or Ni contents. Although the melting (liquidus) temperature increases up to about 210–220°C by alloying Cu or Ni of 2.6–2.7wt.% to the solder, the well-known formation of intermetallic compounds, Cu5Zn8 and Sn6Cu5, are found to be significantly restrained. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

The growth technique and physical properties of SiGe multicrystals with microscopic compositional distribution are demonstrated for new high-efficiency solar cells in which the wavelength dependence of the absorption coefficient can be freely designed by controlling the compositional distribution in the SiGe multicrystals. This growth technique is suitable for the practical casting method, and it is made up of melt growth of SiGe multicrystals with wide and microscopic distribution of the composition from Si to Ge all over the crystals. It is studied how much widely the microscopic compositional distribution in SiGe multicrystals grown from binary Si–Ge melts can be controlled by the melt composition and the cooling process. The range of the microscopic compositional distribution becomes wider as the starting Si concentration in the growth melt becomes larger. SiGe multicrystals with various microscopic compositional distribution can be freely controlled by optimizing the melt composition and the cooling process. The wavelength dependence of the absorption coefficient of such SiGe multicrystals can also be freely designed. Using the experimentally determined absorption coefficient of a SiGe crystal with microscopic compositional distribution, the short circuit photo-current of solar cells was calculated and it is demonstrated that the short circuit photo-current can be much larger for SiGe with microscopic compositional distribution than for SiGe with uniform composition. Si thin film can be easily grown on such a SiGe multicrystal and the Si/SiGe heterostructure can be obtained. These results show that SiGe multicrystals with microscopic compositional distribution are hopeful for new high-efficiency solar cell applications by using the practical casting method. [Copyright &y& Elsevier]

Three-dimensional distribution of arsenic (As) dopants at Σ3{111}, Σ9{221}, Σ9{114}, and Σ9{111}/{115} grain boundaries (GBs) in silicon (Si) is examined by correlative analytical methods using atom probe tomography (APT) combined with low-temperature focused ion beam (LT-FIB), scanning transmission electron microscopy, and ab initio calculations. Σ3{111} GBs, consisting of only 6-membered rings with small bond distortions, do not exhibit an apparent As segregation. Meanwhile, it is hypothesized that As atoms would segregate at 5-membered rings in the other GBs via anisotropic bond distortions spontaneously introduced so as to lower the donor level, as Jahn-Teller distortions. In addition, APT combined with LT-FIB suggests preferential As segregation around stretched 1 1 ‾ 0 bonds reconstructed in Σ9{114} and Σ9{111}/{115} GBs, that are inevitably introduced in the 1 1 ‾ 0 tilt GBs with the tilt angle larger than 70.5°. It is hypothesized that As atoms would form As dimers at stretched 1 1 ‾ 0 bonds and the adjacent ⟨ 111 ⟩ bonds, due to the tendency of As with five valence electrons to form a three-coordinated configuration, which is efficiently attained by an As dimer of a long length. This work provides important insights into As segregation at GBs; it is mainly determined by electronic interactions depending on the characteristics of valence electrons of As atoms, as well as on local bond distortions at GBs, via anisotropic bond distortions and dimerization. [ABSTRACT FROM AUTHOR]

• Observing the growth behavior of a grain boundary with variated groove in mc-Si. • Proposing a geometric model with consideration of different kinetic models of facets. • Reproducing the observed results by our model with a comparable results. • Suggesting that vicinal growth could be one of groove facet kinetics in Si. • Perturbations may come from parallel dislocations. The morphology of the solid–liquid interface and the GB development during directional solidification process of multi-crystalline silicon (mc-Si) has been studied with an in situ observation system. A small angle grain boundary was observed to have a gradually increasing faceted groove, finally reaching a steady state. A geometric model is used to simulate the growth of groove facets with consideration of different facet growth kinetics. It appears that the kinetics of a face vicinal to a {1 1 1} plane is most likely to reproduce the observed behavior. Later on, variations of groove size and grain boundary direction occurred. The geometric simulation results show that such perturbations could be produced by the interaction of single dislocations with one of the vicinal facets. [ABSTRACT FROM AUTHOR]

The solid-liquid interfaces of silicon grains have been observed by an in-situ system. Two silicon seeds having different crystallographic orientations (as ⟨100⟩, ⟨110⟩, and ⟨111⟩) along the growth direction were placed side by side in a silica crucible for the direct comparison of interface behavior during the melt-growth process. Experimental evidence proved the existence of flat {111} surfaces at different grain orientations during crystallization. The difference in the interface positions of the two grains was used to calculate the undercooling of the {111} surfaces. The growth rate-undercooling relationship was linear in all experiments, showing that the observed flat surfaces were planes vicinal to {111} facets, growing through a step flow mode. The terrace length was constant but different from one experiment to another. Therefore, this work indicates that it is crucial to consider the vicinal surface kinetics in the analysis of Si facet planes and grooves during the melt-growth process. [Display omitted] [ABSTRACT FROM AUTHOR]

The defect structures of impurity-doped congruent lithium niobates (c-LN) were determined for impurities with various valences, including divalent, trivalent, and tetravalent impurities, in a concentration range where antisite niobium (NbLi) exists. On the basis of the "Li site vacancy model," six kinds of defect structures in impurity-doped c-LN are possible. Using thermodynamic constraints, these can be narrowed down to two kinds. The first structure is that in which impurities, vacancies and Nb exist as defects in the Li site and no defects exist in the Nb site (structure A), described as {[LiLi]?1-5x-jy[NbLi]x[MLi]y[VLi]4x+(j-1)y}[NbNb][OO]3 (V: vacancy, M: impurity, j: valence of impurity, x, y: compositional variable (≠0), Li/Nb?=?congruent ratio). {[Li×Li]1-5x-2y[Nb∙∙∙∙Li]x[M∙Li]y[V'Li]4x+y}[Nb×Nb][O×O]3 is an example by the Kröger-Vink notation for divalent M. In the second structure, vacancies and Nb exist as defects in the Li site and impurities exist as defects in the Nb site (structure B), described as {[LiLi]1-5x-(j-5)y[NbLi]x[VLi]4x+(j-5)y}{[NbNb]1-y[MNb]y}[OO]3. {[Li×Li]1-5x+y[Nb∙∙∙∙Li]x[V'Li]4x-y}{[Nb×Nb]1-y[M'Nb]y}[O×O]3 is an example for tetravalent M. Since the relationship between impurity concentration and lattice constants for structures A and B differs, the defect structures can be differentiated by analyzing lattice constant variations as a function of impurity concentration. The results show that the defect structure of divalent and trivalent impurity-doped c-LN is structure A and that of tetravalent impurity-doped c-LN is structure B. The NbLi concentration increased with increasing tetravalent impurity concentration. In contrast, the NbLi concentration decreased with increasing divalent and trivalent impurities, leading to suppression of optical damage. The valence of an impurity determines whether the impurity is located in the Li site or Nb site in c-LN, consequently determining whether NbLi decreases or increases when the population of the impurity changes. [ABSTRACT FROM AUTHOR]

Higher minority carrier lifetimes (76 μs) were measured in Ga and Ge codoped Czochralski-silicon (CZ-Si) crystal compared to Ga-doped CZ-Si (28 μs). The minority carrier lifetime increase was associated with a decrease in etch pit density related to grown-in microdefects (GMD) in as-grown CZ-Si with increasing Ge concentration. Furthermore, oxygen (O) precipitation was enhanced in CZ-Si by increased Ge concentration, but this had a lesser limiting influence on minority carrier lifetime. The mechanism by which Ga and Ge codoping affects GMD suppression and enhances O precipitation in CZ-Si is discussed on the basis of the behavior of Ge-vacancy-O related complexes. [ABSTRACT FROM AUTHOR]

Si (111) wafers deformed at elevated temperatures into cylindrical or hemispherical shapes have been examined by X-ray diffraction. Well defined 333 peaks shifted with in-plane distance from the centre of the wafer as determined by ω scans. The shift of the peak position agreed with the curvature of the Si wafer, suggesting that the Si (111) lattice plane can be designed on an arbitrarily curved surface by this method. These crystals have potential for innovative applications in X-ray instrumentation, for both diffraction/scattering and spectroscopy. [ABSTRACT FROM AUTHOR]

The effect of Ge codoping on minority carrier lifetime in boron (B)-doped Czochralski-silicon (CZ-Si) crystals was investigated. The minority carrier lifetime increased from 110 to 176 µs as Ge concentration was increased from zero to 1 × 1020 cm-3 in B/Ge codoped CZ-Si crystals. Light-induced degradation (LID) experiments showed that B-doped CZ-Si degrades rapidly, while B/Ge codoped CZ-Si degrades more slowly. Moreover, the flow pattern defect (FPD) density of grown-in micro-defects (GMD) in as-grown B/Ge codoped CZ-Si decreased with increasing Ge concentration. From the infrared (IR) absorption studies, it was observed that the interstitial oxygen (Oi) concentration decreased as Ge concentration increased in the crystal. The suppressed LID effect in the B/Ge codoped CZ-Si appears to be related to the low concentration of B-O associated defects, possibly because Ge doping retards the Oi diffusion in addition to the low Oi concentration present (evidenced from IR studies). The mechanism by which the Ge concentration influences the reduction of FPDs and Oi concentration is discussed in terms of Ge-vacancy defect formation during post-growth cooling of the ingots. [ABSTRACT FROM AUTHOR]

The generation mechanism of dislocations and their clusters during the two-dimensional growth of multicrystalline Si was studied by in situ observation of a growing interface and subsequent analysis of dislocations. Dislocations were frequently generated at impingement points of the growth of crystal grains where Si melt was enclosed by crystal grains when it solidified. The generation of dislocations was accompanied by the formation of a new twin boundary. On the other hand, no dislocations were observed at impingement points of the growth of crystal grains where Si melt was open when it solidified. We herein present a scheme for dislocation generation with the formation of a new twin boundary on the basis of the results of our former study on dislocation generation in the unidirectional growth of multicrystalline Si ingots. [ABSTRACT FROM AUTHOR]

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-efficiency solar cells. [ABSTRACT FROM AUTHOR]