Yoshitaro Nose

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.

Impact of Defect Density in Si Bulk Multicrystals on Gettering Effect of Impurities

We investigated the impact of defect density in Si multicrystals on the efficiency of gettering of impurities. Samples with low defect density were prepared by controlling crystal growth and compared with those grown by a conventional cast method with high defect density. As a result, increment in minority carrier diffusion length after gettering can be enlarged by decreasing defect density. Therefore, control of microstructures in Si multicrystals by controlling crystal growth is concluded to be beneficial not only for improvement of macroscopic properties of as-grown samples but for realization of high-performance solar cells.

Realization of Bulk Multicrystalline Silicon with Controlled Grain Boundaries by Utilizing Spontaneous Modification of Grain Boundary Configuration

We succeeded in the realization of bulk multicrystalline silicon (mc-Si) with electrically inactive grain boundaries. A group of single-crystal Si wafers was used as an artificial multicrystalline seed with random grain boundaries. The crystal growth was carried out by the floating zone technique under ultrahigh vacuum at a growth rate of 1.0 mm/min. Under these conditions, most of the grain boundaries were spontaneously modified to Σ3. In contrast, random grain boundaries remained even after 40-mm growth when the growth rate was decreased to 0.2 mm/min. From these results, we suggest that the control of both the initial grain boundary configuration and the growth conditions is important to realize mc-Si with electrically inactive grain boundaries, which is a promising material for solar cell applications.

First-principles study of valence band offsets at ZnSnP2/CdS, ZnSnP2/ZnS, and related chalcopyrite/zincblende heterointerfaces

The valence band offsets of chalcopyrite ZnSnP2 (ZSP), CdSnP2 (CSP), CuInSe2 (CIS), and CuGaSe2 (CGS) against zincblende CdS and ZnS are obtained using first-principles calculations based on hybrid density functional theory. The ZSP-CSP (ZCSP) alloy is isostructural to the CIS-CGS (CIGS) alloy and is known for its potential usage in photovoltaic applications. Therefore, the band offsets with other semiconductors, such as CdS and ZnS, are important. The calculated valence band offsets are ∼1.0u2009eV for ZSP/CdS and CSP/CdS, ∼1.2u2009eV for ZSP/ZnS and CSP/ZnS, ∼1.2u2009eV for CIS/CdS and CGS/CdS, and ∼1.3u2009eV for CIS/ZnS and CGS/ZnS. The CdS/ZnS valence band offset is within 0.1u2009eV. Transitivity of natural valence band offsets in the investigated semiconductors holds within ∼0.1u2009eV, which is smaller than the error in band alignment of ∼0.2u2009eV when ionization potential differences are used. The ZSP-CSP and CIS-CGS systems have similar valence and conduction band positions, which is an important piece of information for...

Improvement in the conversion efficiency of single-junction SiGe solar cells by intentional introduction of the compositional distribution

We attempted to clarify the impact of the compositional distribution on recently reported improvement in the conversion efficiency of solar cells based on bulk multicrystalline SiGe. For this purpose, Si1−xGex/Si1−yGey multiple quantum well structures on heavily doped Si-on-insulator were employed as model crystals. The combination of x and y, the width of each layer, and the number of repetitions were systematically changed to study the influence of the introduction of Ge on photocarrier generation and carrier transport while keeping the average Ge composition as 0.03. Spatial modulation of the band structure leads to formation of quantum wells for holes and gives negative impact especially in the photocarrier collection from the n-type region. When the depth of wells was designed to be constant, short-circuit carrier density was found to show a maximum at appropriate compositional distribution due to the competition between the increase in the photocarrier generation and the decrease in the minority car...

Band offset at the heterojunction interfaces of CdS/ZnSnP2, ZnS/ZnSnP2, and In2S3/ZnSnP2

Heterojunctions were formed between ZnSnP2 and buffer materials, CdS, ZnS, and In2S3, using chemical bath deposition. The band offset was investigated by X-ray photoelectron spectroscopy based on Kraut method. The conduction band offset, ΔEC, between ZnSnP2 and CdS was estimated to be −1.2 eV, which significantly limits the open circuit voltage, VOC. Conversely, ΔEC at the heterojunction between ZnSnP2 and ZnS was +0.3 eV, which is within the optimal offset range. In the case of In2S3, ΔEC was a relatively small value, −0.2 eV, and In2S3 is potentially useful as a buffer layer in ZnSnP2 solar cells. The J−V characteristics of heterojunction diodes with an Al/sulfides/ZnSnP2 bulk/Mo structure also suggested that ZnS and In2S3 are promising candidates for buffer layers in ZnSnP2 thin film solar cells, and the band alignment is a key factor for the higher efficiency of solar cells with heterojunctions.

Application of Czochralski-grown SiGe bulk crystal as a substrate for luminescent strained quantum wells

Czochralski-grown Si1−xGex bulk crystal (x=0.085) was utilized as a substrate for strained Si-based quantum wells (QWs). The linewidth of the (400) x-ray rocking curve of the SiGe(100) substrate was comparable with that of the Si(100) substrate, and no peak splitting was observed in the line scan all over the substrate. Epitaxial growth of strained QWs designed as a couple of strained Si/strained Ge QWs have been attempted simultaneously on SiGe(100) and Si(100). Photoluminescence measurements revealed that the sample on SiGe(100) exhibits peaks from excitons confined in QWs without any dislocation-related luminescence in contrast to that on Si(100).

Liquid Phase Epitaxial Growth of Si Layers on Si Thin Substrates from Si Pure Melts under Near-Equilibrium Conditions

The growth of Si epitaxial layers on Si substrates from Si pure melts was attempted under near-equilibrium conditions by dipping-type liquid phase epitaxial (LPE) growth, in order to eliminate the doping effect from metal solutions on the purity of the Si epitaxial layers. Si epitaxial layers can be grown on Si substrates from a Si pure melt only when the temperature of the Si growth melt is kept 1–2°C above the melting point of Si (1414°C) and the growth melt is cooled just after the substrate is dipped in the growth melt. The Si substrate, Si epitaxial layers, and Si polycrystals can be clearly distinguished in the electron back-scattering diffraction pattern (EBSP) image of their cross sections. The growth rate of Si LPE layers during cooling clearly increases as the amount of overheating of the growth melt decreases from 2 to 1°C and the cooling rate increases from 0.4 to 1.0 mm/min.

Suppression of structural imperfection in strained Si by utilizing SiGe bulk substrate

We attempted to utilize homemade SiGe bulk crystal as a substrate for epitaxy of strain-controlled heterostructures. X-ray reciprocal space mapping clarified that the growth of a Si thin film on a SiGe bulk substrate leads to reduction in the orientation fluctuation compared with that on a SiGe virtual substrate. Furthermore, analysis of Raman spectra revealed a dramatic decrease of the strain fluctuation in the strained Si film on the SiGe bulk substrate. These results suggest that the SiGe bulk crystal can be utilized as a substrate for various strain-controlled heterostructures for fundamental studies as well as improvement of device performance.

Floating zone growth of si bicrystals using seed crystals with artificially designed grain boundary configuration

We attempted to grow Si bicrystals with a controlled grain boundary configuration using a pair of single-crystal seeds, which was purposely designed to have a specified character. The floating zone technique under an ultrahigh-vacuum environment was exploited to grow bicrystals and the growth rate was found to be important to control the grain boundary configuration. The grown bicrystals are useful for the fundamental study of the role of grain boundaries, which might control the overall properties of multicrystals. As one of the examples, a series of Si bicrystals, which were grown using seed crystals with a specified misorientation from Σ3, was processed in solar cells, and the short-circuit current density was found to be strongly affected by the misorientation given to the seed.