Oxidation Effects on Graded Porous Silicon Anti-Reflection Coatings

Abstract

Efficient anti-reflection coatings (ARC) improve the light collection and\nthereby increase the current output of solar cells. By simple electrochemical\netching of the Si wafer, porous silicon (PS) layers with excellent broadband\nanti-reflection properties can be fabricated. In this work, ageing of graded PS\nhas been studied using Spectroscopic Ellipsometry, Transmission Electron\nMicroscopy and X-ray Photoelectron Spectroscopy. During oxidation of PS\nelements such as pure Si (Si$^0$), Si$_2$O (Si$^+$), SiO (Si$^{2+}$),\nSi$_2$O$_3$ (Si$^{3+}$), and SiO$_2$ (Si$^{4+}$) are present. In addition both\nhydrogen and carbon is introduced to the PS in the form of Si$_3$SiH and CO.\nThe oxide grows almost linearly with time when exposed to oxygen, from an\naverage thickness of 0 - 3.8 nm for the surface PS. The oxidation is then\ncorrelated to the optical stability of multi-layered PS ARCs. It is found that\neven after extensive oxidation, the changes in the optical properties of the PS\nstructures are small.\n