Over the last years, the interest in the field of transparent conductive oxides (TCOs) has grown dramatically due to their wide applicability and improved properties that may be reached when incorporating these materials into devices. TCOs are mainly used in the industry of low-emissivity windows, flat panel displays, light emitting diodes and photovoltaics [1]. For photovoltaic applications, the main purpose of TCOs is to let light enter into the solar cell and to extract the electric charges allowing them to be drifted towards the electric contacts. Therefore, it is necessary for these materials to be as transparent and as conductive as possible [2]. Ideally, TCOs should be indium-free, as indium is scarce and hence expensive [3]. The goal is therefore to optimize a material that is earth-abundant, low-cost and with good electrical and optical properties. As many steps in photovoltaic device fabrication require a high temperature, a crucial requisite for TCOs is also thermal stability.

Based on these criteria, an amorphous compound of Zn-Sn-O (ZTO) deposited by sputtering was selected for the present study [4]. The microstructure of ZTO is known to strongly influence its electrical and optical properties, as well as its thermal stability. In that regard, transmission electron microscopy (TEM), in situ X-ray diffraction (XRD) experiments and conventional electrical and optical characterization were performed to assess the links between annealing treatments, ZTO microstructure and optical and electrical properties. 

First, samples were annealed in air, in an oven up to 150 and 500 °C and then investigated by transmission electron microscopy. While electrical and optical properties were measured to change significantly upon annealing, no major microstructural change was observed in TEM images. In situ theta-2theta XRD experiments were then performed by increasing the temperature up to 1000-1200°C in air and vacuum. Substrates resistant to these temperatures were employed, namely fused silica and sapphire. Different heating rates were used, ranging from 3°C/min up to 10°C/min. The XRD results (Fig.1) demonstrate that the amorphous phase is stable up to >500 °C when annealed in air and > 900 °C when annealed in 10^-4 mbar, hence highlighting a strong influence of the annealing atmosphere on the crystallisation temperature. Rutile SnO₂ is the first phase to crystallize and remains the main crystal structure observed throughout the whole process, with Al₂ZnO₄ forming at higher temperatures as a result of an interaction between the TCO layer and the sapphire substrate. Electrical properties were measured to decrease after annealing, with TEM measurements demonstrating that Zn migration at high temperature leads to the formation of a defective crystalline structure (Fig.2). This effect is more severe when annealing in air when compared to vacuum conditions. Indeed, the presence of oxygen in the surrounding atmosphere facilitates the formation of crystalline SnO₂, a process that repeals  Zn atoms to grain boundaries and surfaces of the TCO layer (Fig.3). On the other hand, the formation of crystalline SnO₂ and the release of zinc are both delayed when annealing in vacuum. In general, crystallisation and Zn evaporation are observed to be detrimental to the electrical properties as it leads to the formation of voids in the structure. On a technological level, the high thermal stability of the defect-free amorphous ZTO microstructure in oxygen-poor atmospheres may enable its application in high efficiency photovoltaic architectures. 

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EMC Abstracts - https://emc-proceedings.com/abstract/the-microstructure-of-znsno-and-its-correlation-to-electrical-and-optical-properties/