磁控溅射工艺引起硅表面超薄钝化层电子结构变化

Changes of electronic structure of ultra thin passivation layer on silicon surface caused by magnetron sputtering

通过真空热退火、有效少子寿命（πeff）的测量（利用微波光电导衰减μ-PCD法）和表面-界面光电子能谱分析（X-ray Photoemission Spectroscopy,XPS）等方法,研究了磁控溅射沉积ITO（Indium Tin Oxide）薄膜过程中,等离子体中载能粒子束（原子/离子和紫外辉光）对超薄Si Ox（1.5~2.0 nm）/c-Si（150μm）样品界面区的原子成键和电子态的损伤问题,并就ITO薄膜的硅表面电子态有效钝化功能进行了研究.结果表明,溅射沉积ITO薄膜材料后该样品的πeff衰减了90%以上,从105μs减少到5μs.但是,适当退火条件可以恢复少子寿命到30μs,表明Si Ox/c-Si之间界面态的降低有助于改善氧化层的钝化效果.ITO薄膜和c-Si之间Si Ox薄层的形成和它的结构随退火温度的变化,是导致界面态、少子寿命变化的主要原因,且得到了XPS深度剖析分析的确认.

During fabrication of TCO/Si Ox/n-Si（SIS） photovoltaic devices, deposition of TCO films by sputtering damages the electronic structure of the Si Ox/c-Si interface, leading to higher interface recombination rate and lower open circuit voltage of the obtained photovoltaic device. Enhancing the interface recombination rate is equivalent to decreasing the diffusion length in the bulk and reducing the short circuit current. It is obvious that damage to the interface can seriously affect the overall performance of the device. Usually, researchers are only concerned with the effect of passivation before TCO film deposition by sputtering, or with the overall performance of the fabricated device; thus, the damage incurred by sputtering during device fabrication is often overlooked. The damage incurred by sputtering on the efficient silicon-based heterojunction solar cells, especially the SIS devices, is rarely considered. In this letter, we report the investigations of the damage incurred by the energetic particle beams in the plasma（atoms/ions and UV glow） on the atomic bonding and electronic states in both ultra-thin silicon oxides and Si Ox（1.5–2.0 nm）/c-Si（150 μm） sample＇s interface. The analysis was performed by using vacuum thermal annealing, effective minority carrier lifetime measurements（by using μ-PCD）, and surface X-ray photoemission spectroscopy（XPS）. The damage was observed in the process of magnetron sputtering deposition of the ITO thin film. We also investigated the effective passivation function of the ITO thin film＇s silicon surface. The results show that the samples＇ lifetime was reduced by more than 90%（from 105 μs to 5 μs） after the sputtering deposition of the ITO thin film. However, vacuum annealing at 100–400°C for 30 min partially eliminated the damage to the Si Ox/c-Si interface and improved the passivation effect of the thin oxide layer on the crystalline silicon. Vacuum annealing at 200°C for 30 min restored the lifetime to nearly 30 μs; this annealing temperature corresponds to the typical temperature for the preparation processes of most efficient crystalline silicon heterojunction photovoltaic devices. However, vacuum annealing at 500–700°C for 30 min yielded a lifetime of about 3 μs. The analysis performed by comparing the results of XPS depth profiling revealed that the gaseous unstable material Si O is generated at the Si Ox/c-Si interface under this annealing condition, which damages the Si Ox/c-Si interface and augments the unsaturated bond silicon oxide Si Ox（0x2）, increasing the density of interface states, resulting in the lifetime attenuation. Therefore, low-temperature vacuum annealing can partially reduce the sputtering-induced structural damage, which prompts studying the effects of magnetron sputtering damage, and is also helpful for constructing the SIS photovoltaic devices and improving their performance.