铸造多晶硅晶界的EBSD和EBIC研究

利用电子背散射衍射(electron back-scattered diffraction,EBSD)和电子束诱生电流(electron beam induced cur-rent,EBIC)技术对铸造多晶硅的晶界类型和晶界复合特性进行了研究。EBSD分析显示铸造多晶硅中的大部分晶界为大角度晶界(tθ>10°),且以特殊晶界Σ3和普通晶界为主,同时还存在少量小角度晶界(tθ>10°)。EBIC观察发现洁净晶界(包括大角度和小角度晶界)在300K下的复合能力很弱,晶界类型对其复合特性没有明显影响。洁净的小角度晶界本征上在100K下具有强复合特性,而大角度晶界则不具有;在引入金属沾污后,小角度晶界对金属杂质的吸杂能力最强。小角度晶界的复合特性可能与其界面特殊的位错结构有关。     

铸造多晶硅铝吸杂的EBIC研究

该文利用电子束诱生电流(EBIC)对太阳电池用铸造多晶硅材料铝扩散吸杂进行了研究.通过对铸造多晶硅中晶界、位错等缺陷不同温度EBIC图像的分析,确定了复合类型,分析了铝扩散的作用.此外,还发现奇怪的位错明衬度缺陷,分析了其产生原因.     

铸造多晶硅中氧与碳在连续冷却中的沉淀研究

实验结果显示,铸造多晶硅经1350℃加热1h后在高达10℃/s的速率下冷却仍会产生氧沉淀和热施主,生成氧沉淀的量和热施主的浓度在0.017～10℃/s范围内随冷却速率的增大而减少;与等温过程相比,连续冷却中的氧沉淀和热施主形成速率明显提高;碳在低至0.017℃/s冷却速率下也基本不发生沉淀析出。多晶硅片的少子寿命随冷却速率的增大而减小。基于硅中氧、碳的结构状态与扩散性质对上述沉淀动力学特征及其对多晶硅电学性能的影响进行了讨论。     

外延晶体硅薄膜电池的EBIC与SR-LBIC特性分析

电子束诱导电流(EBICElectron Beam Induced Current)是研究晶体缺陷(如晶界、位错、沉淀等)复合特性的一种有力工具.该文对以颗粒硅带为衬底的晶体硅薄膜电池表面及电池截面的晶体缺陷、特别是对晶界的复合行为进行了研究.电池表面EBIC照片表明复合中心位于晶界处,在小颗粒集中区域复合越强.截面的EBIC结果表明在颗粒晶界处分别有着强弱复合,与晶界处强的复合行为相比,颗粒内部没有或仅有比较弱的复合行为发生.靠近电池表面处的颗粒晶界和颗粒内部复合行为由于H钝化得到减弱,少数载流子扩散长度随深度的增加而降低.光谱决定的光束诱导电流(SR-LBIC)表明扩散长度在整个电池表面是不均匀的,最大扩散长度与外延层厚度相当.     

铸造多晶硅中杂质对少子寿命的影响

应用微波光电导衰减仪(μ-PCD)测得了铸造多晶硅硅锭沿生长方向少子寿命的分布图。结果显示:距离硅锭底部4~5cm以及顶部约2cm的范围内均存在一个少子寿命值过低的区域,而硅锭中间区域的少子寿命值较高且分布均匀。通过将样品在200℃下热处理10min,根据处理前后少子寿命值的变化,获得了间隙铁浓度沿硅锭方向的一维线性分布曲线。从曲线中可以发现铁在硅锭两端浓度较高,这与硅锭冷却过程中铁从坩埚向硅锭底部发生的固相扩散以及铁的分凝特性有关。另外通过傅立叶变换红外光谱仪(FTIR)测试发现间隙氧浓度在硅锭底部较高,呈现从硅锭底部向顶部逐渐降低的趋势。研究结果表明硅锭中存在的高浓度的氧、铁等杂质为影响其少子寿命值的关键因素。     

快速热处理工艺下金属杂质对铸造多晶硅少子寿命的影响

利用离子注入技术在铸造多晶硅中分别引入铜、铁、镍杂质玷污,用微波光电导衰减(μ-PCD)仪测试技术研究了铜、铁、镍杂质对硅片少子寿命的影响.研究发现:原生铸造多晶硅片和离子注入铜、铁、镍杂质的多晶硅片经1000℃常规热处理2h后少子寿命值都会降低,下降幅度基本一致.原生硅片在低中温条件下,少子寿命值呈现先下降后上升的趋势;当硅片经高温RIP后,其少子寿命值得到明显改善.低中温RTP条件下,经铜、镍杂质玷污的硅片随着退火温度的升高,少子寿命变化不大.高温RTP条件下,经铜、铁、镍杂质玷污后硅片的少子寿命迅速下降.实验结果表明高温RTP能够提高杂质含量较低硅片的少子寿命,而对杂质含量较高硅片的少子寿命有负面影响.     

退火对冶金法太阳能级多晶硅中缺陷影响的研究

研究了不同条件下退火对冶金法太阳能级多晶硅锭不同部位的位错、晶界及择优生长取向的影响。利用金相显微镜,电子背散射衍射和X射线衍射仪分别对退火前后多晶硅锭的位错、晶界和择优生长取向的变化规律进行表征,结果表明:退火前后多晶硅中的位错密度大小始终是中部<底部<顶部。1100℃下随着保温时间的延长,多晶硅中的位错密度逐渐减小,到5.0h后达到回复的极限程度;小角度晶界比例不断减小,直至消失;大角度晶界中R型晶界比例先增加后减小,CSL晶界比例缓慢增加,孪晶晶界Σ3比例呈稳定地增加,到退火5.0h后,其比例约占30%;主峰〈111〉织构峰强一直增加,〈311〉织构峰强则是逐渐减小至零,而〈511〉、〈531〉、〈620〉织构强度则是先减小、后增大、最后再减小,退火保温5.0h后择优生长取向达到高度一致。另外,还得到了不同温度下的极限回复程度。     

多晶硅锭高氧浓度与少子寿命的研究

在多晶硅定向凝固高少子、低氧区域通入含氧气体,研究氧浓度对多晶硅锭少子寿命的影响程度。研究结果表明,当间隙氧浓度低于4.5×10~(17)cm~(-3)时,对于铸造多晶硅少子寿命的影响非常小;当间隙氧浓度为4.5×10~(17)~7.0×10~(17)cm~(-3)时,降低平均少子寿命0.50μs;当间隙氧浓度高于7.0×10~(17)cm~(-3)时,硅锭少子寿命受到极大影响,平均少子寿命降低到1.50μs,形成红区。因此,只有当铸造多晶硅尾部间隙氧浓度达到7.0×10~(17)cm~(-3)时,才可能形成尾部红区。     

垃圾焚烧飞灰玻璃化的控制参数

通过模拟飞灰残渣的方法,研究了垃圾焚烧飞灰玻璃化技术中玻璃体的形成条件,实验发现1,350,℃下 Cl和 S 不影响玻璃体的形成;相比于在炉内慢速冷却,熔体经空气中自然冷却更容易形成玻璃体;空气自然冷却时,高 Ca 组分比高 Na 组分更难形成玻璃体,而炉内慢速冷却时,高 Na 组分更难形成玻璃体;Al 对玻璃体的形成具有促进和抑制的双重作用;B2O3可以代替 SiO2组建网络结构形成玻璃体.定义 O/F,给出计算公式中各系数的经验值,得出玻璃体的形成条件为炉内慢速冷却时,O/F<3.2;空气自然冷却时 O/F<3.4.利用深圳市生活垃圾焚烧飞灰与添加剂 SiO2和 B2O3的玻璃化实验进一步验证了玻璃体的形成条件     

铸造多晶硅中热施主形成规律

采用四探针电阻率测试仪和傅立叶红外光谱测试仪，研究了铸造多晶硅中热施主的形成规律。实验发现，初始氧浓度高、位错密度高、碳含量低的样品中，所形成的热施主浓度较高。这表明：碳对热施主的形成有抑制作用；位错对热施主的形成有促进作用；晶界对热施主的形成没有明显地影响，而且碳和初始氧浓度的影响最大。实验还发现，在650℃,0.5h退火处理可以消除部分原生热施主，原生热施主对于随后的热施主的形成规律没有明显影响。     

热退火对太阳电池用多晶硅特性的影响

对多晶硅片进行三步退火处理,用傅里叶红外光谱仪(FTIR)和准稳态光电导衰减法(QSSPCD)测硅片退火前后氧碳含量及少子寿命,并对单晶硅片做同样处理进行比较。实验发现:经三步退火后,多晶硅比单晶硅氧碳含量下降的幅度大,这表明多晶硅内部形成的氧沉淀多,其体内的高密度缺陷如晶界、位错等对氧沉淀的形成有促进作用。多晶硅与单晶硅少子寿命大大提高,可能是由于高温退火后晶体内部形成氧沉淀及缺陷的络和物可以作为电活性杂质的吸除中心,从而减少了分散的载流子复合中心,提高了硅片的少子寿命。变化趋势的不同与晶体内部结构有关。     

Effect of porous silicon on the performances of silicon solar cells during the porous silicon-based gettering procedure

In this work we analyse the effect of porous silicon on the performances of multicrystalline silicon (mc-Si) solar cells during the porous silicon-based gettering procedure. This procedure consists of forming PS layers on both front and back sides of the mc-Si wafers followed by an annealing in an infrared furnace under a controlled atmosphere at different temperatures. Three sets of samples (A, B and C) have been prepared; for samples A and B, the PS films were removed before and after annealing, respectively. In order to optimize the annealing temperature, we measure the defect density at a selected grain boundary (GB) using the dark current–voltage (I–V) characteristics across the GB itself. The annealing temperature was optimized to 1000 °C. The effect of these treatments on the performances of mc-Si solar cells was studied by means of the current–voltage characteristic (at AM 1.5) and the internal quantum efficiency (IQE). The results obtained for cell A and cell B were compared to those obtained on a reference cell (C).     

p型铸造单晶硅光注入再生后LID与LeTID的机制分析

光致衰减（LID）与热辅助光诱导衰减（LeTID）是晶硅太阳电池的2种主要衰减,其典型光注入条件分别为25 ℃、1 sun（LID环境）和75 ℃、1 sun（LeTID环境）。从有效少子寿命的角度研究p型铸造单晶硅的衰减机制。200 ℃、7 suns光注入处理过程中样品少子寿命先下降后恢复,这一过程称为光注入再生处理。在LID环境下,无光注入再生处理的样品具有快速与慢速2个衰减阶段,光注入再生处理的样品只有快速衰减阶段。计算两组样品带隙中央附近的缺陷电子/空穴俘获截面比k约为7,表明其中缺陷与直拉单晶硅（Cz-Si）中的BO缺陷相同。对光注入再生处理的样品,在LeTID环境下的衰减阶段计算出k约为35,此数值与多晶硅（mc-Si）中LeTID缺陷的一致。     

Characterisation of direct epitaxial silicon thin film solar cells on a low-cost substrate

Direct epitaxial crystalline silicon thin film (CSiTF) solar cells on low-cost silicon sheets from powder (SSP) ribbons have been prepared using rapid thermal chemical vapour deposition (CVD) growth. The characterisation of CSiTF solar cells was investigated by electron and spectrally resolved light beam induced current (EBIC and SR-LBIC, respectively). All EBIC measurements were performed on both the front-side surface as well as on the cross-section of CSiTF solar cells. The electrical recombination was detected by EBIC and compared with their morphologies. The results of EBIC scan show that recombination centres are situated at grain boundaries (GBs); higher the density of grain, higher the recombination activities (higher contrast). Recombination of different intensity (strong and weak) takes place at vertical GBs. Compared with the high recombination at GBs, the contrast in intragrain is low. The dark contrast of the GBs and intragrain defects is clearly reduced near the surface due to the passivation by hydrogen, which indicates that the minority carrier diffusion length decreases gradually with the depth perpendicular to the surface. The diffusion length was determined by SR-LBIC. The results show that the diffusion length distribution is quite inhomogeneous over the whole cell area. A maximum L_eff of about 25 μm and mean values around 15 μm are calculated for the best solar cell.     

Microscopic homogeneity of emitters formed on textured silicon using in-line diffusion and phosphoric acid as the dopant source

An important point of comparison between POCl₃ emitter diffusion in a quartz tube furnace and in-line diffusion using sprayed phosphoric acid is the microscopic homogeneity of the diffusion, i.e. the homogeneity along the texture of a silicon surface. Two characterization methods were used. In each case, the cross-section of cleaved mc-Si and Cz-Si textured samples was observed in a scanning electron microscope (SEM). First, the thickness of the phosphosilicate glass (PSG) was measured. Second, the emitter was observed on SEM images which showed the n-type silicon as a darker region. The results show comparable homogeneity for in-line and POCl₃ diffusion.     

SEM investigation of structural defect hydrogen passivation in silicon

The results of the investigation of the structural defect passivation in solar cells made from both shaped polycrystalline and monocrystalline CZ-silicon by means of scanning electron microscope (SEM) in the electron beam induced current mode (EBIC) are reported. Atomic hydrogen effectively passivates the different types of grain boundaries in the shaped polysilicon as well as the microdefects in the CZ-silicon. The changes of the hydrogenated sample electrophysical properties are observed during the electron beam irradiation in SEM (experiments “in situ”). These changes indicate a hydrogen state variation in silicon crystal. The character of these changes depends on the sample nature. In the shaped polysilicon the irradiation leads to both the increase of the GB recombination activity and the local decrease of EBIC, but in the CZ-silicon samples with microdefects it leads to the formation of fields with an increased EBIC value. The mechanism of the observed effect is discussed.     

Effect of heat treatment on carbon in multicrystalline silicon

Effect of heat treatment on carbon in cast multicrystalline silicon (mc-Si) has been studied by means of Fourier Transmission Infrared Spectroscopy. Carbon is found to be involved in the formation of as-grown precipitates in mc-Si with higher oxygen content. The experimental results reveal that carbon is difficult to precipitate in mc-Si with lower oxygen or higher nitrogen concentration during annealing in the temperature range from 450°C to 1150°C. Carbon can enhance the nucleation of oxygen precipitates at lower temperature (<850°C). Although carbon does not affect the amount of oxygen precipitates at higher temperature (>950°C), it is suggested that carbon diffuses into oxygen precipitates by the enhancement of silicon self-interstitials. The experiments point out that preannealing at 750°C enhances the decrease of substitute carbon concentration during subsequent annealing at 1050°C. Dislocations and grain boundaries in mc-Si do not affect carbon thermal treatment properties.     

Comparative study of different approaches of multicrystalline silicon texturing for solar cell fabrication

Alkali etchant cannot produce uniformly textured surface to generate satisfactory open circuit voltage as well as the efficiency of the multi-crystalline silicon (mc-Si) solar cell due to the unavoidable grain boundary delineation with higher steps formed between successive grains of different orientations during alkali etching of mc-Si. Acid textured surface formed by using chemicals with HNO₃–HF–CH₃COOH combination generally helps to improve the open circuit voltage but always gives lower short circuit current due to high reflectivity. Texturing mc-Si surface without grain boundary delineation is the present key issue of mc-Si research. We report the isotropic texturing with HF–HNO₃–H₂O solution as an easy and reliable process for mc-Si texturing. Isotropic etching with acidic solution includes the formation of meso- and macro-porous structures on mc-Si that helps to minimize the grain-boundary delineation and also lowers the reflectivity of etched surface. The study of surface morphology and reflectivity of different mc-Si etched surfaces has been discussed in this paper. Using our best chemical recipe, we are able to fabricate mc-Si solar cell of 14% conversion efficiency with PECVD AR coating of silicon nitride film. The isotropic texturing approach can be instrumental to achieve high efficiency in mass production using relatively low-cost silicon wafers as starting material with the proper optimization of the fabrication steps.     

Utilisation of a micro-tip scanning Kelvin probe for non-invasive surface potential mapping of mc-Si solar cells

We have applied a micro-tip Scanning Kelvin Probe to produce high-resolution surface potential maps of silicon nitride (Si₃N₄) coated multi-crystalline Silicon (mc-Si) solar cells in a non-contact, non-invasive fashion. We show this technique highlights two types of defects: localised surface charge and shunts. In the latter case we contrast the non-contact surface potential maps with contact measurements made by the Shuntscan technique.Using a guarded micro-tip with active shield we show for the first time surface potential changes at the mc-Si grain boundaries which are due to different mc-Si polytypes. The high-resolution scanning Kelvin probe (HR-SKP) has a surface potential resolution of <10 mV at a tip diameter <200 μm.     

Correlating internal stresses, electrical activity and defect structure on the micrometer scale in EFG silicon ribbons

In the present paper, we study the influence of defects through their stress fields on the electrical activity and residual stress states of as-grown edge-defined film-feed (EFG) multicrystalline silicon (mc-Si) ribbons. We apply a combination of micro-Raman spectroscopy, electron beam induced current, defect etching and electron backscatter diffraction techniques that enables us to correlate internal stresses, recombination activity and microstructure on the micrometer scale. The stress fields of defect structures are considered to be too small (several tens of MPa) to influence directly the electrical activity, but they can enhance it via stress-induced accumulation of metallic impurities. It is commonly found that not all recombination-active dislocations on grain boundaries (GBs) and within grains are accompanied by internal stresses. The reason for this is that dislocations interact with each other and tend to locally rearrange in configurations of minimum strain energy in which their stress fields can cancel partially, totally or not at all. The outcome is a nonuniform distribution of electrical activity and internal stresses along the same GB, along different GBs of similar character as well as inside the same grain and inside different grains of similar crystallographic orientations. Our work has implications for developing crystal growth procedures that may lead to reduced internal stresses and consequently to improved electrical quality and mechanical stability of mc-Si materials by means of controlled interaction between structural defects.