The Optical Properties Characterization of Hydrogenated Silicon by Spectroscopic Ellipsometry for Solar Cell Applications

Araya Mungchamnankit; Puenisara Limnonthakul

doi:10.59796/jcst.V14N1.2024.11

Authors

Araya Mungchamnankit Department of Physics, Faculty of Science, Rangsit University, Pathum Thani, 12000, Thailand
Puenisara Limnonthakul Department of Physics, Faculty of Science, Srinakharinwirot University, Bangkok, 10110, Thailand

DOI:

https://doi.org/10.59796/jcst.V14N1.2024.11

Keywords:

spectroscopic ellipsometry, physical model, Tauc-Lorentz oscillator, very high-frequency plasma enhanced chemical vapor deposition

Abstract

Amorphous and microcrystalline silicon play a critical role as layers in solar cell design, specifically in the absorption process layer which is crucial for the efficiency of sunlight conversion. This study explores the creation of hydrogenated amorphous silicon films (a-Si:H) via very high-frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) and analyzes their optical properties and crystal structure. We determine the optical band gap of amorphous silicon using a Tauc plot and Spectroscopic Ellipsometry (SE) analysis of transmittance results. By comparing the data, we gain insights into the material's optical properties, aiding our understanding of the findings and enabling a swift evaluation of its optical band gap. Furthermore, we characterize the film's crystal structure using grazing X-ray diffraction. Our results reveal that a-Si:H on glass, produced by VHF-PECVD at a substrate temperature of 200°C with various hydrogen dilutions (R_H=H₂/SiH₄) ranging from 1.0 to 5.0, remains in the amorphous phase. The SE modeling provides the optical band gap of the a-Si:H film, with the lowest and highest optical band gap values occurring at RH 1.0 (1.79 eV) and R_H 3.0 (1.84 eV), respectively. We employed the Tauc-Lorentz model with five fitting parameters to extract optical properties and the band gap of amorphous silicon, including the real part of the dielectric function. This model incorporates three layers: the interface layer between the glass substrate and a-Si:H, the a-Si:H film, and surface roughness, yielding the most accurate model. This comprehensive analysis unveils the exact optical properties of the fabricated films, offering valuable insights for solar cell design and manufacturing. This research confirms that Spectroscopic Ellipsometry, a rapid and non-destructive method for measuring the optical band gap, can significantly benefit the solar cell fabrication industry.

References

Cody, G. D. (1984). The Optical Absorption Edge of a-Si: H. Semiconductors and Semimetals, 21, Part B, 11-82. https://doi.org/10.1016/S0080-8784(08)62910-5

Dorostghol, Z., & Kosarian, A. (2021). Improving structural, optical, and electrical properties of μc-Si: H using non-uniform hydrogen dilution treatment on RF-PECVD prepared layers. Materials Science in Semiconductor Processing, 129, Article 105790. https://doi.org/10.1016/j.mssp.2021.105790

Jellison Jr, G. E., & Modine, F. A. (1996). Parameterization of the optical functions of amorphous materials in the interband region. Applied Physics Letters, 69(3), 371-373. https://doi.org/10.1063/1.118064

Juneja, S., Sudhakar, S., Gope, J., & Kumar, S. (2015). Mixed phase silicon thin films grown at high rate using 60 MHz assisted VHF-PECVD technique. Materials Science in Semiconductor Processing, 40, 11-19. https://doi.org/10.1016/j.mssp.2015.06.046

Lin, Y., Feng, M., Wang, Z., Zeng, Y., Liao, M., Gong, L., ... & Ye, J. (2020). Excellent passivation with implied open-circuit voltage of 710 mV for p-type multi-crystalline black silicon using PECVD grown a-Si: H passivation layer. Solar Energy, 211, 753-758. https://doi.org/10.1016/j.solener.2020.10.023

McGahan, W. A., Makovicka, T., Hale, J., & Woollam, J. A. (1994). Modified Forouhi and Bloomer dispersion model for the optical constants of amorphous hydrogenated carbon thin films. Thin solid films, 253(1-2), 57-61. https://doi.org/10.1016/0040-6090(94)90294-1

Panyanon, C., Dungkaew, W., & Chainok, K. (2021). Synthesis and structural characterization of (Na6F(H2O)18[(VO4)2]•2H3O•2HF. Journal of Current Science and Technology, 11(1), 32-39. https://doi.org/10.59796/jcst.V13N2.2023.1752

Phae-ngam, W., Kamoldilok, S., Rattana, T., Lertvanithphol, T., & Mungchamnankit, A. (2023). Preparation and Characterization of Low-Emissivity AlN/Ag/AlN Films by Magnetron Cosputtering Method. Journal of Current Science and Technology, 13(3), 533-541. https://doi.org/10.1002/pssa.200777876

Podraza, N., Li, J., Wronski, C. R., Horn, M. W., Dickey, E. C., & Collins, R. W. (2008). Analysis of compositionally and structurally graded Si: H and Si1− xGex: H thin films by real time spectroscopic ellipsometry. Physica Status Solidi(A), 205(5), 892-895. https://doi.org/10.59796/jcst.V13N3.2023.498

Shin, C., Park, J., Jung, J., Bong, S., Kim, S., Lee, Y. J., & Yi, J. (2014). Control of micro void fraction and optical band gap in intrinsic amorphous silicon thin films (VHF-PECVD) for thin film solar cell application. Materials Research Bulletin, 60, 895-899. https://doi.org/10.1016/j.materresbull.2014.09.019

Takatsuka, H., Noda, M., Yonekura, Y., Takeuchi, Y., & Yamauchi, Y. (2004). Development of high efficiency large area silicon thin film modules using VHF-PECVD. Solar Energy, 77(6), 951-960. https://doi.org/10.1016/j.solener.2004.06.007

Tjaden, B., Cooper, S. J., Brett, D. J., Kramer, D., & Shearing, P. R. (2016). On the origin and application of the Bruggeman correlation for analysing transport phenomena in electrochemical systems. Current opinion in chemical engineering, 12, 44-51. https://doi.org/10.1016/j.coche.2016.02.006

Zeng, Y., Ma, D., Liu, Z., Liao, M., Xiao, M., Xing, H., ... & Ye, J. (2022). Effects of PECVD preparation conditions and microstructures of boron-doped polysilicon films on surface passivation of p-type tunnel oxide passivated contacts. Materials Science in Semiconductor Processing, 150, Article 106966. https://doi.org/10.1016/j.mssp.2022.106966