Effect of roughness and nanoporosity on optical properties of black and reflective Al films prepared by magnetron sputtering

• Published in: Journal of alloys and compounds 2021-08, Vol.872, p.159744, Article 159744
• Main Authors: Hruška, Petr, More-Chevalier, Joris, Novotný, Michal, Čížek, Jakub, Melikhova, Oksana, Fekete, Ladislav, Poupon, Morgane, Bulíř, Jiří, Volfová, Lenka, Butterling, Maik, Liedke, Maciej Oskar, Wagner, Andreas, Fitl, Přemysl
• Format: Article
• Language: English
• Subjects: Aluminum; Atomic force microscopy; Black aluminum; Diameters; Incident light; Light scattering; Magnetic properties; Magnetron sputtering; Microstructure; Morphology; Optical properties; Photovoltaic cells; Positron annihilation; Positron annihilation spectroscopy; Positronium; Roughness; Spectrophotometric reflectance; Thickness
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2021.159744

•Black Al films were deposited by magnetron sputtering using a N2/Ar gas mixture of ~6.5%.•Microstructure, optical properties, and defect structure of black and classic reflective Al films were investigated.•Black Al films exhibit a moth-eye-like surface capable of> 97% absorption in the wide wavelength range of 190–1200 nm.•The nanoporosity, observed by positron annihilation spectroscopy, is the key property of black films. In this work, a comparison of the microstructure of black and classic reflective aluminum films is provided. The N2 concentration during the magnetron sputtering deposition has a key impact on the growth process and final moth-eye-like morphology of black Al films. The study of films with thickness ~1.5 µm and ~8 µm and fully developed microstructure enabled us to clarify the origin of different optical properties of black and reflective Al. Atomic force microscopy measurements showed high roughnesses for both types of films leading to light scattering from their surface. In the case of black Al, the incident light is absorbed in a fractal-like nanoporous surface. Less than 3% of the intensity in the wavelength range from 190 nm to 1200 nm is reflected. Positronium formation in columnar nanopores with a diameter of 4 – 5 Å was observed by positron annihilation lifetime spectroscopy. The nanoporosity rather than the roughness is the key feature of black films compared to reflective ones.