Electronic states at dislocations and metal silicide precipitates in crystalline silicon and their role in solar cell materials
Electronic states at dislocations and metal silicide precipitates in crystalline silicon and their role in solar cell materials
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DOI：
10.1007/s00339-008-5027-8
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发表时间：
2009-07
期刊：
影响因子：
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通讯作者：
M. Seibt;R. Khalil;V. Kveder;W. Schröter
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文献类型：
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作者：
M. Seibt;R. Khalil;V. Kveder;W. Schröter
太阳能硅中的主要位错类型被分解为具有重构核的 30° 和 90° 部分。除了位于应变场中的浅一维能带和连接两个部分的堆垛层错处的准二维能带之外，电子自旋共振还证明了存在几个具有深层能级的本征核心缺陷。大多数核心缺陷发生在非平衡情况下，除了分配给重建缺陷的小 EPR 信号外，在 800°C 以上仔细退火后消失。现在有充分的证据表明，在位错硅中观察到的部分深层能级与杂质有关，尤其是与过渡金属杂质有关。位错处的电子空穴对复合主要通过其浅能带进行，并且通过结合到其核心或应变场中的杂质而强烈增加。通过吸杂过程可以将这些杂质的浓度降低到如此低的水平，使得位错处的辐射复合在室温下产生0.1%的发光效率。对于硅中的金属杂质沉淀，已经出现了一幅相当一致的图景。无缺陷硅沉淀的早期阶段的特征是由于沉淀的大化学驱动力而形成动力学选择的亚稳态缺陷。此类缺陷与深能级光谱相关，深能级光谱显示了扩展多电子缺陷的特性。系统向能量上更有利的构型的演化通过普通颗粒粗化进行，但也通过内部成熟进行，这一过程让人想起上述亚稳态缺陷。从电子学角度来看，这些缺陷演变成类金属夹杂物，它们通常似乎充当少数载流子的强复合中心。在存在位错的情况下，亚稳态缺陷在沉淀过程中很快转变为平衡结构，或者根本不形成。在存在几种金属杂质的情况下，观察到硅化物沉淀物，其可以被描述为各个金属原子的固溶体，这至少在定性上与三元相图一致。与单金属硅化物沉淀一样，强少数载流子复合对于那些多金属硅化物颗粒也是典型的。
Predominant dislocation types in solar silicon are dissociated into 30°- and 90°-partials with reconstructed cores. Besides shallow 1D-band localized in their strain field and a quasi-2D band at the stacking fault connecting the two partials, the existence of several intrinsic core defects with deep lying levels has been demonstrated by electron spin resonance. The majority of core defects occur in nonequilibrium situations and, with the exception of a small EPR-signal assigned to a reconstruction defect, vanish after careful annealing above 800°C. There is good evidence now that part of deep levels observed in dislocated silicon is associated with impurities, especially with transition metal impurities. Electron-hole-pair recombination at a dislocation mainly runs via its shallow bands and is strongly increased by impurities bound to its core or in the strain field. The concentration of these impurities can be reduced by gettering processes to such a low level that radiative recombination at dislocations yields a luminescence efficiency of 0.1% at room temperature.A quite coherent picture has emerged for metal impurity precipitation in silicon. Early stages of precipitation in defect-free silicon are characterised by kinetically selected metastable defects forming as a result of large chemical driving forces for precipitation. Such defects are associated with deep level spectra which show the properties of extended multielectron defects. The evolution of the system to energetically more favourable configurations proceeds via ordinary particle coarsening but also via internal ripening, a process reminiscent of the above-mentioned metastable defects. Electronically, the defects evolve into metal-like inclusions which in general seem to act as strong recombination centers for minority carriers. In the presence of dislocations metastable defects quickly transform into equilibrium structures in the course of precipitation or do not form at all. In the presence of several metal impurities silicide precipitates which can be described as solid solutions of the respective metal atoms are observed, which is at least qualitatively in accord with ternary phase diagrams. Like single-metal silicide precipitates, strong minority carrier recombination is also typical for those multi-metal silicide particles.