磁控溅射工艺引起硅表面超薄钝化层电子结构变化

Abstract

During fabrication of TCO/SiO x /n-Si (SIS) photovoltaic devices, deposition of TCO films by sputtering damages the electronic structure of the SiO x /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 SiO x (1.5-2.0 nm)/c-Si(150 μm) samples 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 films 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 SiO x /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 SiO is generated at the SiO x /c-Si interface under this annealing condition, which damages the SiO x /c-Si interface and augments the unsaturated bond silicon oxide SiO x (0 x <2), 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.