RGB Tunable Color Filters Using Germanium Telluride

This paper introduces tunable color reflectors for three primary colors (red, blue, and green) for use in low power display systems employing a phase change material (PCM) using interference resonance in an optical cavity. Optical index tunability of the PCM sitting on top of the cavity, results in tuning the device reflection spectrum and thus permits vivid color tuning. The phase change material used to achieve these results was Germanium Telluride (GeTe) due to its high stability. Specifically, ultra-thin films of GeTe was grown on top of a SiO2 cavity with a bottom palladium reflector in a Fabery-Perot type design. This enhances the color tuning when a double-layer anti-reflection coating with high and low refractive indexes are used on top of the GeTe film. Low sensitivity to incident light angle and polarization without the need of sub-micron lithography, provide the potential for this device to be very useful for portable device applications. The devices with different thickness of GeTe were fabricated to demonstrate green, red, blue colors. Electrical pulses with different periods and duty cycles were used to switch the phase of the GeTe locally using joule heating method for several cycles. After transition, darker green, blue, and purple colors were shown for devices with 8 to 20 nm thickness of GeTe films.