Paper TF-ThA7
Room Temperature Synthesis of Silica and SiO₂-TiO₂ Composites for use as Barrier and Anti-Reflection Coatings

Thursday, November 12, 2009, 4:00 pm, Room B4

Thin film oxides are ubiquitous in photovoltaics, serving as transparent electrodes, passivation layers, optical coatings, and moisture permeation barriers. Pulsed plasma enhanced chemical vapor deposition (PECVD) was used to deliver digital control of SiO₂, TiO₂, and Si_xTi_yO_z composites at room temperature. Sub-angstrom control of SiO₂ deposition rate was demonstrated by varying the SiCl₄ density at low exposure levels (~250 L). No impurities were detected by XPS or FTIR, and the high film quality was confirmed by etch rate measurements. Crack-free SiO₂ films have been deposited on polymer substrates, and we are currently assessing their barrier performance.

Next, SiO₂-TiO₂ composites were formed by pulsed PECVD using SiCl₄ and TiCl₄ as precursors. The refractive index of the SiO₂-TiO₂ material system spans a large dynamic range (n: 1.4 – 2.4), and as such is of great interest for optical coatings. Alloy formation was investigated by maintaining constant delivery of one precursor while varying the second. Film composition was assessed by spectroscopic ellipsometry, XPS, and FTIR. It is shown that the alloy composition and refractive index can be tuned continuously over this broad range using pulsed PECVD. These two precursors were found to be highly compatible, with the alloy growth rate simply reflecting the sum of the contributions from the two individual precursors. The digital control over both thickness and composition offered by pulsed PECVD was demonstrated through room temperature synthesis of antireflection (AR) coatings for crystalline silicon solar cells. One, two, and three-layer AR coatings based on the range of indices offered by the SiO₂/TiO₂ system were designed and optimized to minimize the reflectance across the visible spectrum. AR coatings based on these designs were then fabricated, and in each case the measured optical performance was found to be in excellent agreement with model predictions. The integrated reflectance across the visible spectrum was reduced from 39% for uncoated wafers to 2.5% for the 3-layer AR coating.