Optical properties of ZnO/Al/ZnO multilayer films for large area transparent electrodes
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Benjamin Agyei-Tuffour Department of Materials Science and Engineering, African University of Science and Technology (AUST), PMB 681, Garki, Abuja, Nigeria
Martiale Gaetan Zebaze Kana Physics Advanced Laboratory, Sheda Science and Technology Complex (SHESTCO), Abuja, Nigeria; and Department of Materials Science and Engineering, Kwara State University, PMB 1531, Malete, Nigeria
Omololu Akin-Ojo Department of Theoretical and Applied Physics, African University of Science and Technology (AUST), PMB 681, Garki, Abuja, Nigeria
Winston Oluwole Soboyejoa) Department of Materials Science and Engineering, African University of Science and Technology (AUST), PMB 681, Garki, Abuja, Nigeria; and Department of Mechanical and Aerospace Engineering & the Princeton Institute of Science and Technology of Materials, Princeton University, New Jersey 08544, USA (Received 22 May 2014; accepted 3 October 2014)
This study presents the optical properties of layered ZnO/Al/ZnO composite thin films that are being explored for potential applications in solar cells and light emitting devices. The composite thin films are explored as alternatives to ZnO thin films. They are produced via radio frequency magnetron sputtering. The study clarifies the role of the aluminum mid-layer in a ZnO (25 nm)/Al/ZnO (25 nm) film structure. Multilayers with low resistivity ;362 lX cm and average transmittances between ;85 and 90% (in the visible region of the solar spectrum) are produced. The highest Haacke figure of merit of 4.72 10 3 X1 was obtained in a multilayer with mid-layer Al thickness of 8 nm. The combined optical band gap energy of the multilayered films increased by ;0.60 eV for mid-layer Al thicknesses between ;1 and 10 nm. The observed shifts in the optical absorption edges to shorter wave lengths of the spectrum are shown to be in agreement with the Moss–Burstein effect.
I. INTRODUCTION
Transparent conducting oxides (TCOs), such as the predominant thin films of indium-doped tin oxide (ITO), are being used for transparent electrodes (TEs) and anodes in various electronic and optoelectronic devices.1 These include applications in layered structures, such as memory devices,2 solar cells, light emitting diodes, heat mirrors, electroluminescent, display technologies,3 and other devices.1,2 Zinc oxide (ZnO) is one of the promising transparent semiconducting oxides that is being explored for TE applications in passive and active devices.4 In its pure form, ZnO usually exhibits n-type conductivity.1,5,6 It has a high melting point of ;1975 °C7 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.298 2912
J. Mater. Res., Vol. 29, No. 24, Dec 28, 2014
http://journals.cambridge.org
Downloaded: 16 Apr 2015
and a large intrinsic band gap energy between ;3.2 and 3.4 eV3,5,6 at room temperature. This leaves it with high optical transmission in the visible region of the solar spectrum. ZnO also has a high breakdown voltage that enables it to sustain large electric fields, high power, and high temperature operations.7
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