Influence of the high-temperature 'firing' step on high-rate plasma deposited silicon nitride films used as bulk passivating antireflection coatings on silicon solar cells

摘要

The influence of a short high-temperature step, comparable to the so-called "firing" of the metallization on silicon solar cells, on properties of high-rate (>0.5 nm/s) plasma deposited silicon nitride (a-SiN_x:H) films has been investigated. These a-SiN_x:H films are used as antireflection coating on multicrystalline silicon (mc-Si) solar cells and, after the firing process, they also induce hydrogen bulk passivation in the mc-Si. Three different types of remote plasma deposited a-SiN_x :H films have been investigated: (ⅰ) expanding thermal plasma (ETP) deposited a-SiN_x. :H films from a N_2-SiH_4 gas mixture, (ⅱ) ETP deposited a-SiN_x:H films from a NH_3-SiH_4 mixture, and (ⅲ) microwave plasma deposited a-SiN_x :H films from a NH_3-SiH_4 mixture. The atomic composition and optical and structural properties of the films have been studied before and after the high-temperature step by the combination of elastic recoil detection, spectroscopic ellipsometry, and Fourier transform infrared spectroscopy. It has been observed that the high-temperature step can induce significant changes in hydrogen content, bonding types, mass density, and optical absorption of the films. These thermally induced effects are more enhanced for Si- than for N-rich films, which in some cases have a high thermal stability. Furthermore, the material properties and the influence of the high-temperature step have been related to the bulk passivation properties of the a-SiN_x:H coated mc-Si solar cells. It is found that in particular the density and thermal stability of the a-SiN_x:H films seem to be important for the degree of the bulk passivation obtained.