Interstitial iron concentrations across multicrystalline silicon wafers via photoluminescence imaging

We present high‐resolution images of the lateral distribution of interstitial iron across wafers from various positions along the length of a directionally solidified multicrystalline silicon ingot. Iron images were taken on wafers in the as‐cut state and also after two different phosphorus gettering steps performed at 845°C for 30 min, one with an additional anneal at 600°C for 5 h (referred to as extended gettering). The iron images were obtained by taking calibrated photoluminescence (PL) images of the low injection carrier lifetimes, before and after dissociation of iron–boron pairs via strong illumination. The iron images clearly reveal the internal gettering of iron during ingot cooling to grain boundaries and dislocation clusters, resulting in much lower dissolved iron concentrations at those features. In contrast, the PL images of gettered wafers exhibit a reversed distribution of dissolved iron compared to the as‐cut wafers, in other words, with higher interstitial iron concentrations at the grain boundaries than within the grains, most probably owing to the precipitated iron at the grain boundaries partly dissolving during the high‐temperature gettering process. Phosphorus gettering was found to result in a significant reduction of interstitial iron both inside the grains and at grain boundaries. The extended gettering resulted in a further significant reduction in all parts of the wafers and along all sections of the ingot. Copyright © 2011 John Wiley & Sons, Ltd.     

硅中Mo-B络合物性质的理论研究(英文)

Recently, considerabl experimental studies have become available for the 3d transition metal-group ⅢA acceptor complex pair in silicon. Our experimental investigations on 4d transition metal impurities in silicon showed that the interstitial 4d atoms are also easy to move in silicon and to form complexes with other point defects. We have studied the electronic states of the substitutional boron-interstitial molybdenum complex in silicon. The self consistent field calculations were performed by using the SW-Xα theory within the framework of the Watson-sphere-terminated molecular-cluster method. Based on the obtained charge distribution of the system, there is no clear indication to show a transfer of one electron from Mo to B as assumed in the ionic model which has been generally accepted in describing the interaction between a substitutional ⅢA group acceptor atom and an interstitial TM atom.     

Grain boundary segregation in multicrystalline silicon: correlative characterization by EBSD,EBIC, and atom probe tomography

This study aims to better understand the influence of crystallographic structure and impurity decoration on the recombination activity at grain boundaries in multicrystalline silicon. A sample of the upper part of a multicrystalline silicon ingot with intentional addition of iron and copper has been investigated. Correlative electron‐beam‐induced current, electron backscatter diffraction, and atom probe tomography data for different types of grain boundaries are presented. For a symmetric coherent Σ3 twin boundary, with very low recombination activity, no impurities are detected. In case of a non‐coherent (random) high‐angle grain boundary and higher order twins with pronounced recombination activity, carbon and oxygen impurities are observed to decorate the interface. Copper contamination is detected for the boundary with the highest recombination activity in this study, a random high‐angle grain boundary located in the vicinity of a triple junction. The 3D atom probe tomography study presented here is the first direct atomic scale identification and quantification of impurities decorating grain boundaries in multicrystalline silicon. The observed deviations in chemical decoration and induced current could be directly linked with different crystallographic structures of silicon grain boundaries. Hence, the current work establishes a direct correlation between grain boundary structure, atomic scale segregation information, and electrical activity. It can help to identify interface–property relationships for silicon interfaces that enable grain boundary engineering in multicrystalline silicon. Copyright © 2015 John Wiley & Sons, Ltd.     

The properties of polycrystalline silicon solar cells with controlled titanium additions

By coupling the results of electrical measurements, such as spectral response, lighted and dark I-V determinations, and deep-level-transient spectroscopy with optical and laser scan photomicroscopy, we have evaluated the effects of grain boundaries and impurities on silicon solar cells. Titanium which produces two deep levels,E_{v} + 0.30andE_{c} - 0.26eV, in silicon, degrades cell performance by reducing bulk lifetime and thus cell short-circuit current. Electrically active grain boundaries induce carrier recombination in the bulk as well as in the depletion region of the solar cell. Experimental data imply a small but measurable segregation of titanium into some grain boundaries of the polycrystalline silicon containing high Ti concentration (2 × 10¹⁴cm^-3). However for the titanium-contaminated polycrystalline material used in this study, solar cell performance is dominated by the electrically active titanium concentration in the grains. Microstructural impacts on the devices are of secondary importance.     

Feasibility of directional solidification of silicon ingot by electromagnetic casting

Electromagnetic casting is a novel technology that combines the advantages of electromagnetic cold crucible and continuous casting. By controlling induction heating and strengthening bottom cooling, multicrystalline silicon ingot with 60 mm×60 mm in cross section was continuously and directionally solidified by this technology. The ingot shows smooth surface and consists of uniform columnar crystals with no precipitate except in the top part. The grain size of the top part is smaller due to higher impurity content and the impurities acting as nucleation sites for grain. The density of dislocations at the bottom and middle parts, about 1×10⁶ cm⁻², is much less than that at the top part.     

Modeling of poly-crystalline silicon ingot crystallization during casting and theoretical suggestion for ingot quality improvement

Finite element (FE) and cellular automaton (CA) models are used in order to predict the microstructure of Poly-Crystalline Silicon (PC-Si) ingots during casting in a directional solidification furnace. During solidification, the 3D-FE model is used to simulate thermal field inside the furnace, while the nucleation and growth of PC-Si are simulated by the modified version of CA model. In this model both nucleation at silicon-crucible interface and nucleation of equiaxed grains on impurities are modeled. The origin of impurity is considered SiC particles when carbon segregates during the solidification and precipitates as SiC particles if carbon solubility limit is reached. The model is evaluated with experimental data for different parameters, such as: the temperature profile at the top and bottom of the PC-Si, grain size, and the height of solidified silicon during solidification. A one-dimensional form of grain growth is observed at the crucible-silicon interface, while a more two-dimensional form of growth is predicted at higher silicon height. The microstructure of PC-Si shows that the grain size increases as a function of PC-Si ingot height. The effects of the cooling rate and competitive growth on the final microstructure of PC-Si are also investigated. The results show that less nucleation sites are formed at the bottom of the crucible with a slower cooling rate, which leads to a larger grain size. Furthermore, the grains at the crucible side walls tend grow inward at a lower cooling rate, which can significantly change the microstructure of PC-Si. This study shows that the competitive growth phenomena plays an important role in the final microstructure of silicon ingots. A simulated model is proposed which shows that by changing the density and location of nucleation sites it is possible to achieve more effective control on final PC-Si micro structure. This simulation helps to explain the crystal behavior observed in ingots cast under different conditions.     

Phosphorus Removal from Silicon by Vacuum Refining and Directional Solidification

Silicon is widely used as a raw material for production of solar cells. As a major impurity in silicon, phosphorus must be removed to 1 × 10^?5 wt.%. In the present study, based on the distribution of phosphorus in a silicon ingot obtained by vacuum refining and directional solidification, the mechanism for removal of phosphorus from silicon is investigated. The results show that the distribution is controlled not only by segregation at the solid–liquid interface but also by evaporation at the gas–liquid interface, showing some deviation from Scheil’s equation. A modified model which considers both segregation and evaporation is used to simulate the distribution, matching quite well with the experimental results. The temperature and solidification rate are two important parameters that affect the overall mass transfer coefficient and the effective segregation coefficient and thus the distribution of phosphorus. A high removal efficiency and a homogeneous distribution can be obtained by adjusting these two parameters.     

纳米硅薄膜的Raman光谱

通过等离子增强化学气相沉积法 ,制备了本征和掺磷的氢化纳米硅薄膜 (nc- Si:H) ,研究了晶粒尺寸和掺杂浓度对纳米硅薄膜喇曼谱的影响 .结果表明晶粒变小和掺杂浓度增加都使纳米晶粒的 TO模峰位逐渐偏离声子限制模型的计算值 .X射线衍射和高分辨电镜像的结果表明晶粒变小导致硅晶粒应力增加 ,而掺杂使晶粒内部杂质和缺陷增多 ,这些因素破坏了晶粒内晶格的平移对称性 ,进一步减小声子的平均自由程 ,导致实验值偏离理论计算值 .晶格平移对称性的破缺还体现在 ,随晶粒尺寸减小或掺杂浓度增加 ,喇曼谱中 TA、LA振动模的相对散射强度增加 .     

Study of SiC and Si3N4 inclusions in industrial multicrystalline silicon ingots grown by directional solidification method

In directional solidification of multicrystalline silicon ingots for solar cells, the concentration of C and N impurities in the silicon melts increases with progression of solidification due to their relatively low segregation coefficients. In the case of supersaturation of C and N in silicon melts, SiC and Si₃N₄ inclusions are formed. In this work, a piece of multicrystalline silicon was selected from a central block, which was cut out from an industrial multicrystalline silicon ingot grown by directional solidification method. The distribution of SiC and Si₃N₄ inclusions from the top to bottom regions was systematically studied. It was found that majority of SiC and Si₃N₄ inclusions are present in the top region and the amount of inclusions decreases exponentially from the top surface down to the bulk of the ingot. Morphologies and characteristics of the SiC and Si₃N₄ inclusions were investigated. The presence of SiC and Si₃N₄ inclusions generates high density of dislocations in multicrystalline silicon, and sometimes can also introduce pores into multicrystalline silicon. The results of this work will be of practical interest to the photovoltaic industry.     

Impact of Metal Contamination in Silicon Solar Cells

The impact of the transition metals iron, chromium, nickel, titanium and copper on solar‐cell performance is investigated. Each impurity is intentionally added to the silicon feedstock used to grow p‐type, directionally solidified, multicrystalline silicon ingots. A state‐of‐the‐art screen‐print solar‐cell process is applied to this material. Impurities like iron, chromium and titanium cause a reduction in the diffusion length. Nickel does not reduce the diffusion length significantly, but strongly affects the emitter recombination, reducing the solar‐cell performance significantly. Copper has the peculiarity of impacting both base‐bulk recombination as well as emitter recombination. Two models based on the Scheil distribution of impurities are derived to fit the degradation along the ingot. Solar‐cell performances are modelled as a function of base‐bulk recombination and emitter‐bulk recombination. The model fits the experimental data very well and is also successfully validated. Unexpectedly, the distribution of impurities along the ingot, due to segregation phenomena (Scheil distribution), leaves its finger‐print even at the end of the solar‐cell process. A measure of impurity impact is defined as the level of impurity that causes a degradation in cell performance of less than 2% up to 90% of the ingot height. The advantage of this impurity‐impact metric is that it comprises the different impurities’ physical characters in one single parameter, which is easy to compare.     

Characterization of p-type multicrystalline silicon prepared by cold crucible continuous melting and directional solidification

A solar-grade boron doped silicon ingot with the cross section of 62 mm× 62 mm was cast by cold crucible continuous melting and directional solidification (CCDS). The characterization of this p-type multicrystalline silicon (mc-Si) was measured and evaluated. The results indicate that the ingot mainly consists of uniform columnar grains preferentially aligned parallel to the ingot growth direction. The average density of dislocations (N_dis) in the center area varies from 4 × 10⁴ cm⁻² to 4 × 10⁵ cm⁻², and it is much lower than that in the peripheral area. Comparing with the raw material, the oxygen concentration in the cast ingot is much lower while the carbon holds the same level. The electrical resistivity distributes uniformly and its average value is same as that of the raw material. The minority carrier lifetime (MCLT) is higher than that of the raw material and no region with obvious low MCLT is observed. CCDS has shown to be a potential process to produce mc-Si for solar cells with no crucible contamination and consumption, high production efficiency and uniform quality.     

多晶硅片少子寿命的影响因素研究与分析

为了提高多晶硅片的转换效率,提高多晶硅片少子寿命是一个重要的方法和途径,然而在生产过程中影响多晶硅片少子寿命的因素有很多,主要有杂质含量,硅片厚度及晶粒尺寸均匀性等。通过对分凝原理的研究,利用微波光电导测试原理对多晶硅锭少子寿命的分布做了分析,并对硅片厚度及晶粒尺寸进行研究,经过研究发现,多晶硅片少子寿命主要受原料金属杂质含量的大小,硅片厚度及晶粒尺寸均匀性等因素的影响。     

A model for conduction in polycrystalline silicon—Part I: Theory

A new phenomenological model for the electrical conduction in polycrystalline silicon is developed. The combined mechanisms of dopant segregation, carrier trapping, and carrier reflection at grain boundaries are proposed to explain the electrical conduction in polycrystalline silicon. The grain boundaries are assumed to behave as an intrinsic wide-band-gap semiconductor forming a heterojunction with the grains. Thermionic emission over the potential barriers created within the grains due to carrier trapping at the grain boundaries and then tunneling through the grain boundaries is proposed as the carrier transport mechanism. A generalized current-voltage relationship is developed which shows that the electrical properties of polycrystalline silicon depend on the properties of the grain boundaries.     

Impurity redistribution during epitaxial growth

The distribution of impurities incorporated in epitaxial layers during their growth is determined by diffusion due to concentration gradients and by drift in the built-in electric field. This problem has been previously treated in the approximation that the impurities reach their equilibrium distribution during growth. We have extended this calculation to treat impurity drift and diffusion under non-equilibrium conditions, a situation much more characteristic of most realistic growth condi-tions. In the new calculation, the solution of the Shockley-Poisson problem is exact and the boundary con-dition at the growth interface is an approximation based on the electrostatics of surface states. The new calcu-lation also permits consideration of outdiffusion from the substrate, a phenomenon of technological significance. It is also capable of modeling variations of source im-purity concentration and growth rate during epitaxial growth. Calculations modeling n-type GaAs epitaxy of practical interest are presented. Supported by ONR Contract N00014-79-C-0840 and ONR Contract N00014-80-C-0762.     

Electrical resistivity distribution of silicon ingot grown by cold crucible continuous melting and directional solidification

A multicrystalline silicon ingot was grown by cold crucible continuous melting and directional solidification. The electrical resistivity, shallow level impurities׳ concentrations and microstructure of the ingot were measured, and their relationships were studied and discussed. The results show that in the vertical direction the electrical resistivity gets its maximum value at the height of 90 mm and then decreases toward both sides. In the horizontal direction, it is distributed uniformly in the inner area and increases slightly in the peripheral area. The electrical resistivity of the silicon ingot is affected by its shallow level impurities׳ concentrations and its microstructure. Among these impurities the effect of Al is less than those of B and P, since Al tends to form complex precipitates with other elements.     

Evaluation of Mn Uniformity in CdMnTe Crystal Grown by the Vertical Bridgman Method

Cd_1−x Mn _x Te is a typical diluted magnetic semiconductor, as well as substrate for the epitaxial growth of Hg_1−x Cd _x Te. In this paper, the homogeneity of a Cd_1−x Mn _x Te (x = 0.2) single-crystal ingot grown by the vertical Bridgman method was studied. The crystal structure and quality of the as-grown ingot were evaluated. Near-infrared (NIR) transmission spectroscopy was adopted to develop a simple optical determination of the Mn concentration in the as-grown ingot. A correlation equation between cut-off wavelength λ _co from NIR transmission spectra and Mn concentration by inductively coupled plasma atomic emission spectrometry (ICP-AES) was established. Using this equation, we investigated the Mn concentration distribution in both the axial and radial directions of the ingot. It was found that the segregation coefficient of Mn in the axial direction of the ingot was 0.95, which is close to unity. The Mn concentration variation in the wafers from the middle part of the ingot was 0.001 mole fraction. All these results proved that homogeneous Cd_0.8Mn_0.2Te crystals can be grown from the vertical Bridgman method.     

Directed crystallization of multicrystalline silicon under weak melt convection and gas exchange

For the first time, the multicrystalline silicon is grown via directed crystallization using a heater submerged into the melt. The interaction of the heater casing material with molten silicon is studied on a model heater in the form of a graphite plate coated with a special structure SiC protective layer. During the crystallization, the plate has been kept on the melt surface and has almost completely overlaid the melt mirror, which favored a significant decrease in the rate of gas exchange between the melt and the atmosphere in the furnace. Marangoni convection was not observed in the absence of the free surface of the melt and the crystal was grown on the reduced melt convection, especially at the final stages of crystallization, when the thickness of the melt layer was inferior to the cross size of the crucible. The crystal is established to have a strongly pronounced columnar structure. The measured specific resistance varies over the ingot height from 1 to 1.3 Ω cm, and the lifetime of minority carriers is about 3.7 μs. The carbon and oxygen content in the ingot has been measured via FTIR spectroscopy, and the carbon concentration through the ingot height is shown to strongly differ from the linear dependence typical of directed crystallization.     

Impurity segregation in horizontal bridgman grown cadmium zinc telluride

Segregation of impurities that cause low infrared (IR) transmission in horizontal Bridgman (HB) grown cadmium zinc telluride (CdZnTe) has been investigated. This segregation was characterized using IR transmission, glow discharge mass spectrometry (GDMS), and IR microscopy measurements. In the studied HB CdZnTe ingots, impurity segregation causes the formation of a small volume in the last-to-freeze portion of the ingot that has high impurity concentration and low IR transmission. Outside this region the concentration of impurities is low and the material shows high IR transmission. The region is visibly observable on CdZnTe slices and appears as a dark area with a sharp boundary. Free carrier absorption within the region causes a decrease in IR transmission with an increase in wavelength and correlates with the concentration of lithium and sodium impurities. Impurity segregation in HB ingots is described and explains the location of the high impurity region. The location of the visible boundary correlates with the first measurable change in IR transmission as compared to the high IR transmission of the surrounding material and supports the hypothesis that the darkening of the region is due to a reduction of the reflectivity due to free carrier absorption. With a properly controlled cool-down recipe, the impurities segregated in the last-to-freeze section of the ingots can remain localized, thereby improving the purity of the remaining bulk of the material.     

Laser‐induced breakdown spectroscopy for photovoltaic silicon wafer analysis

The principal subject matter of this work is the application of laser‐induced breakdown spectroscopy for the multi‐elemental analytical characterization of different qualities of solid silicon. The physical process upon which the technique is based is the temporally resolved observation of emission spectra emitted by a micro‐plasma generated by a laser focused on the surface of a given sample. The optimal environmental parameters such as the composition of the buffering gas for the identification and measurement of several metallic, non‐metallic, and dopant impurities were determined. Particular attention was given to boron. A detection limit of 2.10⁻⁴ mg/g of boron was found using a calibration curve, which was made in the range of 1 to 100 ppmw. Silicon samples from different production techniques (4C and directional solidification), which permit the segregation of different impurities along the length of the silicon ingot were analyzed using laser‐induced breakdown spectroscopy. Copyright © 2011 John Wiley & Sons, Ltd.     

太阳电池用多晶硅锭红外检测中阴影的表征分析

研究了多晶硅铸锭过程中,硅锭内部红外探伤测试中显示黑斑位置晶体缺陷的杂质组成,并根据杂质成分推导了此种缺陷的形成机理和条件.采用光致发光(PL)技术、扫描电子显微镜(SEM)和X射线能谱仪(EDS)对杂质进行了表征与分析.结果显示,形成阴影的夹杂在晶界中存在的形态主要为针状或薄片状,其组成成分主要为C,N和Si元素.而Si3N4的出现可能有两个原因:一是Si3N4涂层脱落而沉积在晶界中;二是溶解在液相中的N局部过饱和.此外,结晶过程中,SiC也随之成核并生长,在晶界上形成夹杂物,同时伴随着微缺陷的增加.据此提出了去除多晶硅锭内部阴影的几点措施.