Deposition of microcrystalline intrinsic silicon by the Electrical Asymmetry Effect technique

Abstract

Depositing microcrystalline intrinsic silicon films is an important step for the production of thin silicon tandem junction solar cells. Due to the high cost of capital equipment, it is becoming increasingly important to improve the processing speed of thin silicon films for continued commercial viability. In this work, a combination of the excitation frequencies 13.56MHz + 27.12 MHz was used for thin silicon film deposition. According to the electrical asymmetry, the DC self bias on the RF electrode was varied by adjusting the phase between the two applied frequencies. A single junction microcrystalline cell with above 5.5% efficiency was deposited in a Gen5 PECVD process using the Electrical Asymmetry Effect (EAE). The deposition rate was higher than 0.8 nm/s. A similar increase of the deposition rate in a pure 13.56 MHz discharge led to a strong degradation of the μc-Si:H quality and the single junction cell performance fell to 4% efficiency. It was found that layers deposited using the EAE have a better uniformity compared to layers deposited in a pure 27.12 MHz discharge. In comparison to traditional RF-PECVD processes, electrically asymmetric discharges allow to achieve a regime of plasma conditions with low ion energies and high electron densities.