Grain Boundaries In Si Research Articles

Influence of laser power on atom probe tomographic analysis of boron distribution in silicon

The relationship between the laser power and the three-dimensional distribution of boron (B) in silicon (Si) measured by laser-assisted atom probe tomography (APT) is investigated. The ultraviolet laser employed in this study has a fixed wavelength of 355nm. The measured distributions are almost uniform and homogeneous when using low laser power, while clear B accumulation at the low-index pole of single-crystalline Si and segregation along the grain boundaries in polycrystalline Si are observed when using high laser power (100pJ). These effects are thought to be caused by the surface migration of atoms, which is promoted by high laser power. Therefore, for ensuring a high-fidelity APT measurement of the B distribution in Si, high laser power is not recommended.

Effect of grain boundary grooves at the crystal/melt interface on impurity accumulation during the unidirectional growth of multicrystalline silicon

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.

Na-induced Structural Unit in Σ3 [-110]/(-1-11) Grain Boundaries of Si

Na-induced Structural Unit in Σ3 [-110]/(-1-11) Grain Boundaries of Si

Removal of Impurities from Metallurgical Grade Silicon During Ga-Si Solvent Refining

Purification of metallurgical grade silicon (MG-Si), using gallium as the impurity getter has been investigated. The technique involves growing Si dendrites from an alloy of MG-Si with Ga, followed by their separation by acid leaching. The morphologies of impurity phases in the MG-Si and the Ga-Si alloy were investigated during the solvent refining process. Effective segregation ratios of B and P in the Ga-Si system were calculated. Most metallic impurities formed silicides, such as Si-Fe-Ga-Mn or Si-Fe-Ga impurity phases, which segregated to the grain boundaries of Si or into the Ga phase during the Ga-Si solvent refining process. After purification, the refined Si is plate-like with crystallographic orientation, and the removal fraction of B and P was 83.28 % and 14.84 % respectively when the Si proportion was 25 % in the Ga-Si alloy. The segregation ratios of B and P were determined to be 0.15 and 0.83 when the solid fraction of Si was 0.25. The effective removal of B and P by a solidification refining process with a Ga-Si melt is clarified.

Boron Diffusion in Silicon in the Presence of Grain Boundaries and Voids

We have investigated the behavior of one B impurity in the presence of dislocations, nanovoids and grain boundaries (GBs) in Si. Our study has been motivated by recent experiments on highly B-doped polycrystalline Si where a very significant thermoelectric power factor enhancement was observed. We study the influence of nanovoids on B diffusivity and activation in Si. We compare with the grain-boundary of pure tilt Si[001](120) poly-Si, where the presence of dislocations is dominant, similarly as in the case of nanovoids in c-Si. We performed DFT calculations to investigate minimum energy configurations, energy barriers and activation or deactivation of one B impurity in the proximity of the above structures. We found that, B preferably occupies substitutional lattice sites at the edge of the cavities. Thus the presence of small voids reduces its diffusivity and mobility, but the electrical activation of B is not significantly influenced. In the case of Si[001](120) GB, B is found to form Boron interstitials clusters in the close vicinity of a GB. The formation energy of those clusters is lower when the cluster is closer to center of the GB. Our results are in agreement with previously reported experimental data. It is provided insight to the effect of structural inhomogeneity on the dopant properties in Si structures that are currently of interest for thermoelectric applications.

Potential variation around grain boundaries in BaSi2 films grown on multicrystalline silicon evaluated using Kelvin probe force microscopy

Potential variations across the grain boundaries (GBs) in a 100 nm thick undoped n-BaSi2 film on a cast-grown multicrystalline Si (mc-Si) substrate are evaluated using Kelvin probe force microscopy (KFM). The θ-2θ X-ray diffraction pattern reveals diffraction peaks, such as (201), (301), (410), and (411) of BaSi2. Local-area electron backscatter diffraction reveals that the a-axis of BaSi2 is tilted slightly from the surface normal, depending on the local crystal plane of the mc-Si. KFM measurements show that the potentials are not significantly disordered in the grown BaSi2, even around the GBs of mc-Si. The potentials are higher at GBs of BaSi2 around Si GBs that are formed by grains with a Si(111) face and those with faces that deviate slightly from Si(111). Thus, downward band bending occurs at these BaSi2 GBs. Minority carriers (holes) undergo a repelling force near the GBs, which may suppress recombination as in the case of undoped n-BaSi2 epitaxial films on a single crystal Si(111) substrate. The barrier height for hole transport across the GBs varies in the range from 10 to 55 meV. The potentials are also higher at the BaSi2 GBs grown around Si GBs composed of grains with Si(001) and Si(111) faces. The barrier height for hole transport ranges from 5 to 55 meV. These results indicate that BaSi2 GBs formed on (111)-dominant Si surfaces do not have a negative influence on the minority-carrier properties, and thus BaSi2 formed on underlayers, such as (111)-oriented Si or Ge and on (111)-oriented mc-Si, can be utilized as a solar cell active layer.

Polarized photoluminescence imaging analysis around small-angle grain boundaries in multicrystalline silicon wafers for solar cells

We have shown the effectiveness of polarized photoluminescence imaging for analyzing the structural and spectroscopic properties of small-angle grain boundaries (SA-GBs) in multicrystalline Si. The dislocation-related deep-level emission band at approximately 0.79 eV at room temperature was found to be polarized, whereas the band-edge emission did not show the polarization effect. The anisotropy of the 0.79 eV band was classified into two groups depending on the tilt and twist characteristics of SA-GBs determined by the electron backscatter diffraction measurement.

Photoluminescence Analysis of Oxygen Precipitation around Small-Angle Grain Boundaries in Multicrystalline Silicon Wafers

We have investigated the correlation between deep-level photoluminescence and the density of small-angle grain boundaries in multicrystalline Si. A deep-level photoluminescence component around 0.87 eV, which we previously ascribed to oxygen precipitates, became lower and higher in the region with high and low density of small-angle grain boundaries, respectively. This can be explained by the di erences in the availability of oxygen atoms around respective small-angle grain boundaries. We performed focused ion beam time-ofight secondary ion mass spectroscopy on special points emitting extremely strong 0.87 eV emission, and detected a clustered area of 16O−. This is strong evidence for the idea that the 0.87 eV band is due to oxygen precipitates.

How to best measure atomic segregation to grain boundaries by analytical transmission electron microscopy

This study provides an overview of the recent experiments employing methods that analyse, systematically, series of analytical spectra acquired either in nanobeam mode in a transmission electron microscope or using elemental mapping in a scanning transmission electron microscope. A general framework is presented that describes how best to analyse series of such spectra to quantify the areal density of atoms contained within a very thin layer of a matrix material, as, for example, appropriate to measure grain boundary segregation. We show that a systematic quantification of spectra as a function of area size illuminated by the electron beam eliminates the large systematic errors inherent in simpler approaches based on spatial difference methods, integration of compositional profiles acquired with highly focused nanoprobes or simple repeats of such measurements. Our method has been successfully applied to study dopant segregation to inversion domain boundaries in ZnO, to quantify the thicknesses of sub-nm thin layers during epitaxial growth by molecular beam epitaxy of (In)GaAs and to prove the absence of gettering of dopants at Σ = 3{111} grain boundaries in Si, with a precision <1 atom/nm2 in all these cases.

Dependence of electrical and structural properties on the thickness of n-type μc-Si thin-film silicon solar cells grown by linear facing target sputtering

In this work, the construction of an (Al+Ag)/n+-Si/p-Si/Al thin-film silicon (TF-Si) solar cell is presented. The maximum achievable current density generated by a planar solar cell, with an optimum antireflection coating, for different values of the cell thickness are analysed. Both electrical and optical properties of (Al+Ag)/n+-Si/p-Si/Al TF-Si solar cell have been studied, which assumes the generation of one electron–hole pair per photon and a collection efficiency of unity. A reduction in thickness can lead to an increase in VOC, it can have the opposite effect if surface recombination is not reduced simultaneously. Thin-film solar cell has an absorbed photon flux density about three times higher at each interface, implying that carrier generation at the interfaces of the thinner cell is about three times higher. Finally, for larger grain size, the recombination at the grain boundaries (GBs) of Si will mainly degrade VOC, and not JSC. From the obtained results, it is evident that the larger the grain size, the better the performance of the device. However, the interface recombination has a strong influence on each parameter.

Impurities Removal from Metallurgical-Grade Silicon by Combined Sn-Si and Al-Si Refining Processes

The purification of metallurgical-grade silicon (MG-Si) by combined solvent refining processes has been studied. The final high-purity silicon was recovered through Sn-Si refining and Al-Si refining processes in sequence after acid leaching, and the removal mechanism of impurities was explored. Inductively coupled plasma (ICP) chemical analysis revealed the concentrations of main impurities including B and P, and typical metallic impurities except for solvents Sn and Al were reduced to below 1 ppmw. The final removal efficiencies of B and P were 97.7 pct and 99.8 pct, respectively, and those of most metallic impurities were above 99.9 pct. SEM analysis showed that P-containing phases (Al-Ca-Mg-Si-P and Al-Si-P) formed on the surface of refined Si after Sn-Si refining and Al-Si refining, which was confirmed to be the main approach for P removal. It was also found that the formation of binary silicide such as Fe3Si7 and Mn11Si19 or multicomponent phases such as Ca-Mg-Si phase occurred during the solvent refining process, and they segregated on the grain boundaries in Si or attached to the surface of Si, which led to high removal efficiency of metallic impurities by the solvent refining process.

Single Grain Boundary Modeling and Design of Microcrystalline Si Solar Cells

For photovoltaic applications, microcrystalline silicon has a lot of advantages, such as the ability to absorb the near-infrared part of the solar spectrum. However, there are many dangling bonds at the grain boundary in microcrystalline Si. These dangling bonds would lead to the recombination of photo-generated carriers and decrease the conversion efficiency. Therefore, we included the grain boundary in the numerical study in order to simulate a microcrystalline Si solar cell accurately, designing new three-terminal microcrystalline Si solar cells. The 3-μm-thick three-terminal cell achieved a conversion efficiency of 10.8%, while the efficiency of a typical two-terminal cell is 9.7%. The three-terminal structure increased the JSC but decreased the VOC, and such phenomena are discussed. High-efficiency and low-cost Si-based thin film solar cells can now be designed based on the information provided in this paper.

The role of extended defects in device degradation

AbstractGrain boundaries and dislocations are well known to cause trouble in electronic devices, but in most cases they can be avoided by using single crystal films and eliminating or suppressing dislocation densities. Hetero‐interfaces, on the other hand, are essential features of devices. This papers reviews several topics from the author's published work, combining theoretical calculations with experimental data (microscopy, electrical measurements) to illustrate the interplay of impurities with extended defects, especially interfaces: dopant segregation in grain boundaries of polycrystalline Si, the role of hydrogen in the degradation of the Si–SiO2 interface in Si‐based metal‐oxide‐semiconductor field‐effect transistors (MOSFETs), the role of carbon, nitrogen, and hydrogen in the quality and degradation of the SiC–SiO2 interface in SiC‐based MOSFETs, and the role of vacancies in room‐temperature degradation of III–V high‐mobility electron transistors by the formation of microvoids.

Electrical/Optical Activities of Grain Boundaries in Multicrystalline Si

Removed due to authors request.

Carrier mobility measurement across a single grain boundary in polycrystalline silicon using an organic gate thin-film transistor

In this study, we developed a measurement method for field-effect-carrier mobility across a single grain boundary in polycrystalline Si (poly Si) used for solar cell production by using an organic gate field-effect transistor (FET). To prevent precipitation and the diffusion of impurities affecting the electronic characteristics of the grain boundary, all the processing temperatures during FET fabrication were held below 150 °C. From the grain boundary, the field-effect mobility was measured at around 21.4 cm2/Vs at 297 K, and the temperature dependence of the field-effect mobility suggested the presence of a potential barrier of 0.22 eV at the boundary. The technique presented here is applicable for the monitoring of carrier conduction characteristics at the grain boundary in poly Si used for the production of solar cells.

X-ray beam induced current method at the laboratory x-ray source

The x-ray beam induced current method (XBIC) is realized on the laboratory x-ray source using the polycapillary x-ray optics. It is shown that rather good images of grain boundaries in Si can be obtained by this method. The parameters of x-ray beam are estimated by the simulation of Schottky diode image. A good correlation between the experimental and calculated grain boundary XBIC contrast is obtained. The possibilities of laboratory source based XBIC method are estimated.

XBIC Investigation of the Grain Boundaries in Multicrystalline Si on the Laboratory X-Ray Source

It is shown that the X-ray beam induced current method (XBIC) can be realized at the laboratory X-ray source using the polycapillary x-ray optics. The images of iron contaminated grain boundaries in multicrystalline Si are obtained. It is shown that the grain boundary XBIC contrast is 2-3 times smaller than the EBIC one. A simulation of XBIC and EBIC contrast values for two-dimensional defects is carried out and a good correlation between the experimental and calculated values is obtained. The dependence of grain boundary XBIC contrast on the X-ray beam width is calculated.

Electrical and optical activities of small angle grain boundaries in multicrystalline Si

AbstractWe have characterized the electrical and optical activities of small grain boundaries (SA‐GBs) by using electron‐beam‐induced current (EBIC) and cathodoluminescence (CL). EBIC observation at room temperature indicates that the SA‐GBs are grouped into two categories. SA‐GBs with misorientation angle lower than 1° shows lower EBIC contrast like &lt;10%, named as “general”. Those with 2‐3° have higher contrast more than 20%, named as “special”. At 100 K, all the SA‐GBs become visible and their contrast increases. On the other hand, CL observation at 15 K shows that general SA‐GBs show D3 and D4, while special ones show D1 and D2. We explained these properties of SA‐GBs in terms of dislocation model. General SA‐GBs are regarded as the ensemble of individual dislocations. Special SA‐GBs are composed of the densely aligned dislocations in which dislocation interaction takes place (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Structure of an incommensurate 90° Si grain boundary resolved with the help of a Cs-corrector for illumination

The atomic structure of an incommensurate (001)/(110) Si grain boundary (GB) or 90° Si GB has been studied by transmission electron microscopy (TEM) and refined by atomistic simulations (Stillinger-Weber potential). Samples were made by bonding one (001) Si wafer with one (110) Si wafer and carefully orienting the 2 wafers in order that they have a common [11̄0] direction. In the interfacial direction perpendicular to [11̄0], the [110]I direction of grain I is parallel to the [001]II direction of grain II and, as the ratio of these 2 vectors is sqrt2, it is impossible to find 2 integers n and m such that n[110]I=m[001]II. The structure is incommensurate in this direction. Z-contrast images obtained in an FEI-Titan microscope equipped with a probe Cs-corrector easily resolve the Si dumb-bells in the two grains and allow us to determine the complex atomic structures of the interface. On the other hand, near on-axis high resolution TEM images obtained in a JEOL 4000EX microscope are very efficient to analyse the long range order of the interface.

Structure and effects of vacancies in Σ3 (112) grain boundaries in Si

Using the first-principle density-functional theory, we study the structure and effects of vacancies in Σ3 (112) grain boundary with the coincident-site lattice structure in Si. We find that the formation energy for a Si vacancy in the grain boundary is significantly lower than that in Si perfect region, indicating strong segregation of Si vacancy in grain boundary regions. The formation of Si vacancies in grain boundaries either cleans up the deep levels or facilitates complete passivation by H atoms. Our results suggest that vacancies in grain boundaries may play important role in determining grain boundary physics and passivation behavior.