Electrical properties of purified solar grade silicon substrates using a combination of porous silicon and SiCl4

This work investigates the photo-thermal treatment of solar grade (SG) silicon to reduce impurities to a low level suitable for high efficiency low-cost solar cells application. It describes experiment carried out by using a tungsten lamps furnace (rapid thermal processing, RTP) to purify solar grade silicon wafers using a combination of porous silicon (PS) and silicon tetrachloride. This process enables to attract the impurities towards the porous layer where they react with SiCl₄ to form metallic chlorides. The gettering effect was studied using the Hall Effect and the Van Der Pauw methods to measure the resistivity, the majority carrier concentration and mobility. We have obtained a significant improvement of the majority carrier mobility after such thermo-chemical treatment. The gettering efficiency is also evaluated by the relative increase of the minority carrier diffusion length L, measured by the light beam induced current (LBIC) technique.     

Improving the material quality of silicon ingots by aluminum gettering during crystal growth

We present a method for the purification of silicon ingots during the crystallization process that reduces significantly the width of the low charge carrier lifetime region at the ingot top. The back‐diffusion of impurities from the ingot top is suppressed by adding a small amount of pure aluminum into the silicon melt right at the end of the solidification. We study the aluminum gettering effect by instrumental neutron activation analysis (INAA) and Fe_i imaging. Furthermore, we present a model for aluminum gettering of Fe in the silicon ingot that is in agreement with literature data for aluminum gettering at lower temperature. The distribution of iron in the ingots with and without aluminum is fairly well predicted by a combination of this model with a model for Fe contamination from the crucible system. A simulation with varying Al content exhibits further potential for an increased yield of silicon wafers with high charge carrier lifetime. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

低温退火磷吸杂工艺对低少子寿命铸造多晶硅电性能的影响

本文针对低少子寿命铸造多晶硅片进行试验, 通过一种将多温度梯度磷扩散吸杂工艺与低温退火工艺结合的新型低温退火吸杂工艺, 去除低少子寿命多晶硅片中影响其电性能的Fe杂质及部分晶体缺陷, 提高低少子寿命多晶硅所生产的太阳电池各项电性能. 通过低温退火磷扩散吸杂工艺与其他磷扩散吸杂工艺的比较, 证明了低温退火吸杂工艺具有更好的磷吸杂和修复晶体缺陷的作用. IV-measurement发现经过低温退火工艺处理后的低少子寿命多晶硅, 制备的太阳电池光电转换效率比其他实验组高0.2%, 表明该工艺能有效地提高低少子寿命多晶硅太阳电池各项电性能参数及电池质量. 本研究结果表明新型低温退火磷吸杂工艺可将低少子寿命硅片应用于大规模太阳电池生产中, 提高铸造多晶硅材料在太阳能领域的利用率, 节约铸造多晶硅的生产成本. 关键词： 低温退火 磷吸杂 低少子寿命多晶硅 太阳电池     

Gettering of metallic impurities in photovoltaic silicon

 This work addresses the issue of structural defect-metallic impurity interactions in photovoltaic silicon and their effect on minority carrier diffusion length values. Aluminium and phosphorus segregation gettering studies were performed on photovoltaic silicon in order to gain insight into these interactions and quantify the effect of gettering on solar cell performance. Integrated circuit grade silicon was also studied for comparative purposes. Additionally, a novel rapid thermal annealing technique, designed to dissolve metallic impurity precipitates, and Deep Level Transient Spectroscopy were utilized to determine the as-grown impurity concentration in both grades of materials. Significant differences in gettering responses between the two grades of silicon are observed. Gettering treatments greatly improve I.C. grade silicon with a specific gettering temperature providing the optimal response. Photovoltaic grade silicon does not respond as well to the gettering treatments and, in some cases, the material degrades at higher gettering temperatures. The degradation is primarily observed in dislocated regions of multicrystalline photovoltaic silicon. Additionally, these dislocated regions were found to possess the highest as-grown metallic impurity concentration of all the materials studied. The dislocation-free photovoltaic silicon has a higher diffusion length relative to dislocated silicon but could not be improved by the gettering methods employed in this study. A model is presented to describe these phenomena where the high concentration of metallic impurities at dislocations produce relatively low minority carrier diffusion lengths as well as the degrading response with higher gettering temperatures while microdefects create an upper limit to the photovoltaic grade material’s diffusion length. Received: 21 June 1996/Accepted: 2 September 1996     

Effects of phosphorus diffusion gettering on minority carrier lifetimes of single-crystalline,multi-crystalline and UMG silicon wafer

Phosphorus diffusion gettering, which can effectively reduce the transition-metal impurities in the bulk of Si wafer and enhance the minority carrier lifetime (MCLT), is a well-known process to improve the performances of solar cells. Especially, the appropriate gettering process is further required for manufacturing solar cells using an upgraded metallurgical-grade silicon (UMG Si) wafer. In this work, an improvement in the MCLT of the UMG Si wafer including the single-crystalline and multi-crystalline Si wafer after phosphorus diffusion gettering was confirmed by using the quasi-steady state photo-conductivity (QSSPC) measurement and the microwave photo-conductance decay (μW-PCD) method. The experimental results were compared with the MCLT variations calculated through the simulation of the Fe distributions in the Si wafers. It was also observed that the efficiency of the UMG Si solar cell increased by 0.53% due to the two-step gettering process.     

Photoluminescence activity of Yang and Secco etched multicrystalline silicon material

Ultraviolet and blue-green photoluminescence (PL) was investigated on multicrystalline silicon (mc-Si) samples chemically etched by Secco and Yang solutions. The samples were characterized by dislocation density (10⁵-10⁶ cm⁻²). The form of etched pits is triangular with Yang etch and like a honeycomb with Secco etch as observed with a scanning electron microscope (SEM). These textures of mc-Si wafers give a PL activity similar to that obtained with nanostructures of porous silicon (PS) as reported in the literature. The ultraviolet PL spectra observed with Yang etch shift to the blue-green spectrum range when applying Secco etch. In our experiments we have observed 3-5 μm diameter macro pores separated by a high density of nanowalls. These observations suggest that the origin of the PL activity are quantum dots resulting from the silicon nanocrystallites obtained after few minutes of chemical etching.     

Observation of an enhanced gettering effect in silicon under germanium molecular ion implantation

Germanium atomic (Ge₁ ⁺) and molecular ions (Ge₂ ⁺) of equal energy per atom are implanted in silicon at an elevated temperature. The ion induced damage has been monitored by following the intensity variation of the LO Raman peak of Si. The germanium implanted samples have been labeled with 10 keV Au ions. The gettering of gold has been observed by Rutherford backscattering spectrometry in the post-annealed samples. This paper reports a first time observation of an enhanced gettering of gold in silicon implanted with molecular ions. PACS 61.72.Ji; 61.80.Lj; 61.80.Jh; 61.72.Yx     

Application of the chemical vapor-etching in polycrystalline silicon solar cells

This paper reports a study of the application of chemical vapor-etching (CVE) for the rear surface and in the emitter of polycrystalline silicon (pc-Si) solar cells. The CVE technique consists of exposing pc-Si wafers to a mixture of HF/HNO₃. This technique is used to groove the rear surface of the pc-Si wafers for acid vapors rich in HNO₃ (HNO₃/HF > 1/4), in order to realize rear-buried metallic contacts (RBMC) and the formation of a porous silicon (PS) layer on the frontal surface of the cell for volume ratio of HNO₃/HF = 1/7. A significant increase of the spectral response in the long wavelength range was observed when a RBMC is formed. This increase was attributed to the reduction of the effective thickness of the base of the cells and grain boundary Al gettering. The achievement of a PS layer on the emitter of the pc-Si cells passivates the surface and reduces the reflectivity. The dark I-V characteristics of pc-Si cells with emitter-based PS show an important reduction of the reverse current together with an improvement of the rectifying behaviour. The I-V characteristic under AM1.5 illumination shows an enhancement of both short circuit current density and fill factor. The internal quantum efficiency is improved, particularly in the short wavelengths region.     

Modeling of Cu gettering in p- and n-type silicon and in poly-silicon

Based on experimental findings we set up calculations of numerical modeling of gettering efficiencies for Cu in various silicon wafers. Gettering efficiencies for Cu were measured by applying a reproducible spin-on contamination in the 10¹² atoms/cm² range, followed by a thermal treatment to redistribute the metallic impurity. Subsequently, the wafers were analyzed by a novel wet chemical layer-by-layer etching technique in combination with inductively coupled plasma mass spectrometry. We investigated p/p+ and n/n+ epitaxial wafers with different doping levels and different substrate-doping species. We have also investigated gettering efficiencies of phosphorus-diffused p- and n-type wafers. Heavilyboron doped silicon exhibited a gettering efficiency of ∼100%, while gettering by n+ silicon occurred for doping levels >3×10¹⁹ atoms/cm³ only. In another set of experiments we investigated the dependence of the gettering efficiency of p-type wafers with poly-silicon back sides for different cooling rates and Cu spiking levels. A strong dependence on both parameters was found. Cu gettering in p/p+ epitaxial wafers was modeled by calculating the increased solubility of Cu in p+ silicon compared to non-doped silicon taking into account the Fermi-level effect, which stabilizes donors in p+ silicon, and the pairing reaction between Cu and B. Calculated gettering efficiencies were in very good agreement with experimental results. Gettering in n+ silicon was similarly modeled in terms of pairing reactions and the Fermi-level effect. But, for n-type silicon, many experimental uncertainties existed; thus, we applied our expressions to solubility data of Hall and Racette to obtain the unknown parameters. The empirical calculations were in good agreement even with results on n/n+ wafers. For phosphorus-diffused wafers we had to consider an excess vacancy concentration of 1.2–5.5 times the equilibrium concentration to explain the experimental findings by the model. Gettering by poly-silicon back sides was simulated by solving the time-dependent diffusion equation with boundary conditions that take into account different surface reaction rates of silicon point defects. Using this advanced model, the experimentally measured gettering efficiencies were reproduced within the uncertainty of the measurement. Received: 3 September 2001 / Accepted: 4 September 2001 / Published online: 20 December 2001     

Scanning infrared microscopy investigation of copper precipitation in cast multicrystalline silicon

The behavior of copper precipitation in cast multicrystalline silicon (mc-Si) annealed at different temperatures under air cooling (30 K/s) or slow cooling (0.3 K/s) was investigated by scanning infrared microscopy (SIRM). Comparing to Czochralski-grown silicon (Cz-Si), copper precipitated more easily in mc-Si, and the lowest temperature of copper precipitation in mc-Si was about 700 °C, lower than that in Cz-Si. It was also observed that copper preferably precipitated on grain boundaries so that near the grain boundaries the denuded zone formed. The results indicate that the defects including dislocations, grain boundaries and microdefects, as the heteronucleation sites, enhanced copper precipitation. Moreover, cooling rates had a great influence on the copper precipitation, especially at lower annealing temperatures. Generally air cooling led to the formation of high density of copper-precipitate colonies.     

Low-porosity porous silicon nanostructures on monocrystalline silicon solar cells

In this work we present a study of low-porosity porous silicon (PS) nanostructures stain etched on monocrystalline silicon solar cells. The PS layers reduce the reflectance, improve the diffusion of dopants by rapid thermal processes, and increase the homogeneity of the sheet resistance. Some samples were subjected to chemical oxidation in HNO₃ to reduce the porosity of the surface layer. After the diffusion process, deposition of a SiN_x antireflection layer, and screen printing of the samples, an efficiency of 15.5% is obtained for low-porosity PS solar cells, compared with an efficiency of 10.0% for standard PS cells and 14.9% for the reference Cz cells.     

单晶硅表面均匀小尺寸金字塔制备及其特性研究

表面织构是一种通过有效的光俘获增加短路电流从而提高太阳电池效率的主要途径之一.在加入间隙式超声和NaClO添加剂的碱性四甲基氢氧化铵(TMAH)溶液中对单晶硅表面进行织构化处理,研究超声与NaClO在织构过程中对金字塔成核和生长的影响,以及金字塔大小对高温工艺之后的单晶硅少子寿命的影响.研究表明,通过在织构溶液中加入间隙式超声控制气泡停留在硅片表面的时间和脱离硅片表面速度,增强了小尺寸金字塔的均匀分布.织构之后硅片在AM1.5G光谱下的加权平均反射率能够达到12.4%,在高温扩散和氧化之后少子寿命的大小与金字塔大小之间存在近似于指数衰减函数的关系. 关键词： 表面织构化 反射率 少子寿命 单晶硅太阳电池     

Hydrogen passivation of multi—crystalline silicon solar cells

The effects of hydrogen passivation on multi-crystalline silicon (mc-Si) solar cells are reported in this paper. Hydrogen plasma was generated by means of ac glow discharge in a hydrogen atmosphere. Hydrogen passivation was carried out with three different groups of mc-Si solar cells after finishing contacts. The experimental results demonstrated that the photovoltaic performances of the solar cell samples have been improved after hydrogen plasma treatment, with a relative increase in conversion efficiency up to 10.6\%. A calculation modelling has been performed to interpret the experimental results using the model for analysis of microelectronic and photonic structures developed at Pennsylvania State University.     

Photoluminescence enhancement and stabilisation of porous silicon passivated by iron

Iron is incorporated in porous silicon (PS) by impregnation method using Fe(NO₃)₃ aqueous solution. The presence of iron in PS matrix is shown from energy-dispersive X-ray (EDX) analysis and Fourier transform infrared (FTIR) measurements. The optical properties of PS and PS-doped iron are studied by photoluminescence (PL). The iron deposited in PS quenched the silicon dangling bonds then increased the PL intensity. The PL peak intensity of impregnated PS is seven times stronger than that in normal PS. Upon exposing iron-PS sample to ambient air, there is no significant change in peak position but the PL intensity increases during the first 3 weeks and then stabilises. The stability is attributed to passivation of the Si nanocrystallites by iron.     

Fabrication of black multicrystalline silicon surface by nanosecond laser ablation

The optimal regimes for uniform texturing of a multicrystalline silicon (mc-Si) surface by pulsed laser radiation have been determined. The morphology and reflectance spectra of the texturized mc-Si have been studied. The laser-texturized mc-Si samples with reflectance of 2?C3% over a wide spectral region have been produced. The influence of subsequent chemical etching on the reflective properties of the texturized surface has been analyzed.     

Enhanced physical properties of porous silicon for improved hydrogen gas sensing

In the current communication, porous silicon samples were prepared by pulsed photoelectrochemical etching using a hydrofluoric acid-based solution. The structural and gas-sensing properties of the samples were studied. Apart from the cycle time T and pause time T_off of the pulsed current, a novel parameter, in the shape of the current named ‘delay time T_d’ was introduced. Our results showed that by optimization of delay time, the porosity of samples can be controlled due to the chemical preparation of silicon surface prior to electrochemical anodization. The fourier-transform infrared measurements of porous silicon (PS) layers on Si substrate showed that the typical PS surface was characterized by chemical species like Si–H and Si–O–Si terminations. The two-minute delay before applying electrical current was considered sufficient for the fabrication of higher porosity (83%), more uniform, and more stable structures. The photoluminescence (PL) peak of the optimized sample showed higher intensity than the other samples. An obvious PL blue shift also revealed a change in the crystallographic characteristics of silicon due to quantum confinement effects. Metal–semiconductor–metal diodes with Schottky contacts of nickel were fabricated on PS samples and the potential application of optimized substrates for the improved sensitivity, stability, response time and recovery time of hydrogen gas sensors was subsequently studied.     

X-ray topography study of monocrystalline silicon wafers diffused with phosphorus by different methods

To reduce the cost of the emitter diffusion process, there has been increasing interest to substitute the standard process of batch POCl₃ emitter diffusion used in the silicon solar-cell manufacturing industry with in-line diffusion processes such as the spray-on and screen-printing process. For this reason, it is essential to study and compare the processes of different diffusion methods from the point of view of the crystalline quality of the final wafers. X-ray transmission topography was employed to characterize the possible precipitates and other microdefects generated in Czochralski-grown silicon (Cz Si) during the emitter diffusion process carried out by screen-printing, spray-on and the standard process, in which the emitter was provided by a liquid (POCl₃) source. The results indicate that the phosphorus diffusion process influences the crystalline quality of the wafers and the efficiency of the external gettering process that takes place during phosphorus diffusion depends on the diffusion method employed.     

Microstructure and optical properties of ZnO/porous silicon nanocomposite films

In this study, porous silicon (PS) templates were formed by electrochemical anodization on p-type (100) silicon wafer and ZnO films were deposited on PS substrates using radio frequency (RF) reactive magnetron sputtering technique. The effects of oxygen partial pressures of growth ZnO films and annealing ambience on the microstructure and photoluminescence (PL) of the ZnO/PS nanocomposite films were systematically investigated by X-ray diffraction and fluorescence spectrophotometry. The results indicated that all ZnO/PS nanocomposite films were polycrystalline in nature with a hexagonal wurtzite structure and the (002) oriented ZnO films had the best crystal quality under O₂:Ar ratio of 10:10 sccm and annealing in vacuum. PL measurements at room temperature revealed that ZnO/PS nanocomposite systems formed a broad PL band including the blue and green emissions from ZnO and red-orange emission from the PS. The mechanism and interpretation of broadband PL of the nanocomposites were discussed in detail using an oxygen-bonding model in PS and a native defects model in ZnO.