Tunable colorimetric thin-film structures based on phase change materials

Phase change materials (PCMs) are materials whose resistive and/or optical properties can be tuned via a phase shift triggered by an external excitation. Within this class of materials, Ge2Sb2Te5 (GST) is the most well-known and widely used PCM, and is currently employed in applications including read/write phase-change memories. Recently, it and similar materials have also drawn interest for use in photonics applications, due to the high optical contrast upon the phase change. However, for applications in the visible range (380-780 nm), GST and many related materials (such as GeTe and GeSnSbTe) exhibit large absorbance, restricting their use to layers of a few nm thick and limiting the achievable phase shift. One promising alternative PCM for photonics applications is Sb2S3. Despite its previous characterization as a "write once/read many times" material, recent work has shown it well-suited to repeated switching, with an activation energy comparable to that of GST. In addition, it exhibits much lower absorbance in the visible range than GST, with strong optical contrast between its crystalline and amorphous phases; and may be thermally, electrically or optically switched, with a switching time on the nanosecond scale. In this paper we present a comparison of tunable color coating designs which exploit the phase changes of Sb2S3 and GST. We show that Sb2S3 offers superior color contrast between its phases, can be used in thicknesses of up to several hundred nm while still realising saturated color, and that a large color shift on switching of up Delta E=122.4 is obtained.