Color Purity Enhancement Through Effective Index Modulation of GST for Applications in Reconfigurable Displays

Phase-change materials (PCMs) have been emerged owing to their reconfigurable physical properties such as refractive index, light absorption, electrical conductivity, and Young's modulus by switching between amorphous and crystalline phases [1]. Ge₂Sb₂Te₅ (GST), which consists of germanium, antimony, and tellurium, is one of the representative PCM materials and is widely studied for a dynamic color display or phase change memory by utilizing its great tunability in both optical and electrical properties. Due to the nature of the refractive index variation depending on the phase of GST, there have been tremendous suggestions for reconfigurable photonic structures, including photonic memory, thermal radiation modulator, and tunable structural color [2]. Typically, optical coating of dielectric thin films is based on Fabry-Perot or thin film interference. Recently, high-absorption dielectric films have been studied as optical coatings, and when metals are combined with high-absorption dielectrics, strong optical interference can be obtained with a small phase change of the reflected wave, which can significantly reduce the thickness compared to conventional coating layers. Resonance behavior impacting the structural color can be observed in structures consisting of a metal-coated substrate with an ultra-thin absorbing layer. The high color purity and feasible tuning functions of structural coloration has attracted attention due to its promising applicability as a next generation color display. Despite its great potential application, low chromaticity and limited intermediate step over phase change have been treated as a barrier to the efficient operation as a color display [3].<br/>In this study, we present an ultrathin tunable color filter comprising GST. We modulate the effective index of GST by applying porosity (<i>P_r</i>) with anisotropic geometry, resulting in color purity enhancement and enhanced controllability based on polarization-driven modulation. Specifically, we deposit GST on top of an Au film using glancing angle deposition (GLAD) to manufacture self-aligned porous nanocolumns (PNCs) with varying <i>P_r</i> (0% 40%, 75%). In the amorphous GST/Au reflection spectrum, dip intensity decreases as <i>P_r</i> increases, and we find that color purity can be enhanced by controlling the <i>P_r</i> and phase. Particularly, the amorphous phase of GST exhibits a higher color purity and a wider color range than the crystalline phase. Additionally, mitigate the risk of surface oxidation and potential damage when depositing GST on an Au film exposed to ambient air, we conducted simulations to explore the effect of incorporating an oxide layer as a film passivation strategy. This additional dielectric layer serves noticeable shift in the dip position of the reflectance spectrum and changes in the phase condition, the color response can be dynamically adjusted, offering a wide range of color variations depending on the thickness of the oxide layer. Furthermore, GLAD technique induces atomic shadowing and creates inclined columnar structures on the substrate, resulting in an increased porosity. Due to the shadowing effect, the slanting plane direction of the PNCs has a higher <i>P_r</i> and varies with the polarization angle, causing a structural color change.<br/>We confirm that the color purity can be improved with the <i>P_r</i>, phase, thickness of GST, and polarization angle. And the incorporation of the additional dielectric layer provides both protective benefits and enhanced control over color changes, presenting exciting possibilities for diverse applications. Our suggestions can be used as a way to improve optical properties in applications such as optical coating, optoelectronic displays, and optical storage devices.<br/><br/>References<br/>[1] P. Hosseini, C.D. Wright and H. Bhaskaran, <i>Nature.</i> 511, 206–211 <b>(2014).</b><br/>[2] J. H. Ko, D. H. Kim, S. H. Hong, S. K. Kim and Y. M. Song, <i>iScience.</i> 26, 105780 <b>(2023)</b><br/>[3] J. H. Ko, Y. J. Yoo, Y. Lee, H. H. Jeong and Y. M. Song, <i>iScience.</i> 25, 104727 <b>(2022)</b>