The effect of substrate roughness on the properties of RF sputtered AZO thin film
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esearch Letter
The effect of substrate roughness on the properties of RF sputtered AZO thin film Kairi Hamada, Takaya Ogawa, Hideyuki Okumura, and Keiichi N. Ishihara, Graduate School of Energy Science, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto 6068501, Japan Address all correspondence to Keiichi N. Ishihara at [email protected] (Received 24 January 2019; accepted 14 May 2019)
Abstract Aluminum-doped zinc oxide (AZO) is one of the most promising transparent conductive oxide materials for a front electrode in solar cells. In this work, we roughened substrate surface and sputtered AZO films, where the effect of roughness on various AZO properties was investigated. The haze values were largely enhanced, retaining other important properties such as conductivity and transparency. The optical band gap exhibits a clear blue shift because of the roughness. The possible cause of this shift may be variation in the Al content due to the different deposition and post-annealing mechanisms of AZO films on the roughened surface.
Introduction Aluminum-doped zinc oxide (AZO) thin film is one of the most promising transparent conductive oxide (TCO) materials for a front electrode in a solar cell.[1] An AZO layer must have both properties of high conductivity and high transparency. In addition, to improve the efficiency of the device, the AZO front layer requires optical properties of light scattering, which is obtained by making a rough surface on the film or the substrate.[2] Some previous studies focused on the roughness of the TCO surface, which was made by wet-chemical etching using weak or diluted acids, such as hydrochloric acid.[3–6] Other previous studies focused on the roughness of the substrate surface, which was produced by etching with hydrofluoric acid or a standard reactive ion etching process.[7,8] These studies report that the haze value is increased by the formation of a rough surface; however, no studies have investigated the effect of surface roughness on other film properties. In this work, we focus on the effect of rough surface on other characteristics such as optical band gap properties, as it is one of the most important features for solar cell applications. We prepared roughened substrates by abrasive papers on a rotary polishing machine. AZO thin film was prepared by film deposition on the substrates with radio frequency (RF) magnetron sputtering, followed by post-annealing. The effect of roughness on various AZO properties has been investigated, compared with the sample on a not-roughened substrate. In addition, we employed various different substrates other than glass, including quartz plate, and investigated the possible cause of the AZO properties change.
Experimental We prepared various types of substrates with different coefficients of thermal expansion (CTE), including glass slide
(87 × 10−7/K), quartz (5.5 × 10−7/K), soda lime (87 × 10−7/K), and white plate (100 × 10−7/K). The roughened substrates were prepared by roughening the surface with two kinds of abrasive papers using a rotary p
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