Ultra-Thin Aluminum-Doped Silver for Transmissive Thermally Reconfigurable Visible Photonics

Abstract

Functional materials with high electrical conductivity and optical transmittance are vital for thermally tunable free-space photonic systems. Conventional transparent conductors such as graphene and indium tin oxide are limited by high contact resistance, poor mechanical stability, or complex fabrication. Ultra-thin metals, such as pure silver, have also been explored with limited success due to thermal instability and dewetting. Here, we propose an ultra-thin Al-doped Ag film to tackle these challenges. Aluminum promotes heterogeneous nucleation of silver, enabling the formation of continuous, smooth films that are thermally stable at reduced thicknesses while maintaining excellent electrical conductivity and transparency. We find that a 12 nm Al-doped Ag film exhibits an average transmittance of 80% across the visible range with a sheet resistance of 8.3$\pm$1.16 $Ω$cm$^2$. Moreover, on-chip Al-doped Ag microheaters exhibit uniform, rapid thermal response, and stable electrical performance, maintaining functionality for over $10^7$ ON and OFF cycles at temperatures below 400$°$C. Furthermore, as a benchmark, we demonstrate reversible phase-change switching in Ge$_2$Sb$_2$Se$_4$Te (GSST) and VO$_2$. 30$\times$30 $μ$m$^2$ GSST cells exhibited complete crystallization and amorphization under 2.2 V - 200 ms and 4.1V - 50$μ$s pulses, respectively, resulting in a 40% transmission contrast at 780 nm and a tenfold improvement in power consumption compared to similar devices. Additionally, VO$_2$ films displayed reversible insulator-to-metal transitions near 65°C with reflectance and transmittance modulation in the visible and the near-infrared at frequencies up to 25 Hz with room for improvement. These results establish Al-doped Ag as a robust transparent metallic heater for integration in dynamic metasurfaces, optical coatings, and more.

Figures (8)

• Figure 1: Ultra-thin Al-doped Ag microheaters. (a) Optical image of the 12 nm-thick Al-doped Ag film over a 3 in sapphire wafer.(b) The reflection and (c) transmission images of multiple Al-doped Ag heaters with Ge$_2$Sb$_2$Se$_4$Te cells under an optical microscope with halogen illumination. (d) The refractive index and extinction coefficient from ellipsometry, measured on a 12 nm-thick Al-doped Ag film. (e) Reflectance and transmittance spectra in the visible and near-infrared for 7, 12, and 16 nm-thick Al-doped Ag thin films. (f) Electrical resistance measured on a 35$\times$35 $\mu$m$^2$ bowtie microheater with 7, 12, and 16 nm-thick Al-doped Ag films.
• Figure 2: Thermal performance of Al-doped Ag heater using GSST thermo-optical response. (a) Optical microscope image of a measured 25 $\times$ 25 $\mu$m$^2$ heater with 30 nm-thick GSST. Transient thermoreflectance images at the end of (b) 2.2 V - 200 ms and (c) 3.5 V - 50 $\mu$s pulses. The dashed lines delimit the area considered in the temperature profiles in (d). (d) Average temperature vs. position along the horizontal direction of the microheater bridge. (e) Experimental electrical pulse and real-time temperature at the center of the GSST pattern, marked in (a). (f) Endurance test: number of thermal cycles (black) and the accumulated heating time (red) vs. temperature at the end of a 50 $\mu$s-width pulse
• Figure 3: Reversible switching of GSST by 12 nm Al-doped Ag microheaters. (a) The cross-section schematic of the device with GSST. (b) Transmission and reflection optical images of GSST reversible switching 30 $\times$ 30 $\mu$m$^2$ under halogen illumination for better amorphous-crystalline contrast. (c) Raman spectra measured at the center of the PCM cell as-deposited (black), after a 2.2 V - 200 ms pulse (red), and after a 4.1V - 50 $\mu$s pulse (blue). (d) Applied voltage to the device and its resistance when applying a 50 $\mu$s-width amorphization pulse. (e) The Reflectance and transmittance spectra of GSST on 12 nm Al-doped Ag in amorphous and crystalline states in the visible to NIR range
• Figure 4: Reversible switching of VO$_2$ by 12 nm Al-doped Ag microheaters. Reversible switching of VO$_2$ by 12 nm Al-doped Ag microheaters.(a) The cross-section schematic of the Al-doped Ag microheaters with embedded VO$_2$.(b) Transmission and reflection optical images of 30 nm-thick VO$_2$ at room temperature, and optical images in reflection before (insulator) and after heating (metallic), in the visible and infrared ranges. (c) Transmittance (solid line) and reflectance (dashed line) of the 30 nm VO$_2$ at insulator phase and spectra measured at the center of the VO$_2$ cell at insulator (red) and metallic (blue) phases. The inset optical image shows the device in which this data was collected.
• Figure S1: Simulated temperature profile applied on Al-doped Ag heater. The temperature profile on the top surface of a 30 $\times$ 30 $\mu$m$^2$ Al-doped Ag heater with the GSST pattern on the top. t$_{Heater}$=12 nm, t$_{Electrode}$=80 nm, t$_{AlN}$=5 nm, t$_{SiO_{2}capping}$=50 nm. (a) Square GSST pattern with different pulse widths, W$_{GSST}$=15 $\mu$m. (b) Round GSST pattern with different pulse widths, R$_{GSST}$=7.5 $\mu$m. (c) Round pattern with different radius.