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Supplementary Materials for Ultrathin phase-change coatings on metals for electrothermally-tunable colors Gokhan Bakan1,2,*, Sencer Ayas2,3, Tohir Saidzoda1, Kemal Celebi2, Aykutlu Dana2 1 Deparment of Electrical and Electronics Engineering, Antalya International University, Antalya 07190, Turkey 2 UNAM Institute of Materials Science and Nanotechnology, Bilkent University, Ankara 06800, Turkey 3 Current address: Bio-Acoustic-MEMS in Medicine (BAMM) Laboratory, Department of Radiology, Canary Center at Stanford for Cancer Early Detection, Stanford University School of Medicine, Electrical Engineering Department (by courtesy), Stanford University, Palo Alto, CA 94304, USA Methods: Fabrication: Al, Ag and Au are deposited on Si substrates following deposition of 3 nm Ge adhesion layer in a thermal evaporation system under ~10-6 Torr pressure. The deposition rates are ~6 nm/min. The GST films are deposited in a sputtering system at a rate of 10 nm/min using a single target from ACI alloys. The sputter’s chamber pressure is first brought to ~5x10-6 Torr, then GST deposition is performed under 10 mTorr pressure with a constant Ar flow. 25 W DC power is used for sputtering. The color gradient on Al, Ag and Au is achieved by tilting the samples 90° with respect to the substrate holder. The HfO2 films are deposited in the same sputtering system at a rate of 2.8 nm/min using a single target from Kurt J. Lesker Company. Sputtering conditions are 20 mTorr pressure, 150 W RF power, and constant Ar flow. Film thicknesses and optical properties (n, k vs. λ) of thin GST and HfO2 films on Si wafers are characterized by spectroscopic ellipsometer measurements. Deposition rates are calculated using these film thicknesses and assumed to be the same for all depositions. The as-deposited aGST films are crystallized on a hot plate with a heating rate of ~1 K/s. The surface temperature of the hot plate is measured using a K-type thermocouple. Optical measurements: The reflectance spectra of the surfaces are measured at a low angle of incidence using a visible, near infrared ellipsometer system (J. A. Woollam). The angle and polarization dependent reflectance spectra of the 30 nm aGST film on Al in Figure 3 in the main text are measured using the same tool. The photographs are taken using a Samsung Galaxy S3 phone’s integrated camera. Simulations: A commercial FDTD package (Lumerical) is used for the optical simulations. 2D geometries with symmetric boundary conditions along the x-axis are used. Perfectly matched layer (PML) boundary conditions are used in the z-axis. A broadband plane wave (0.3–2 µm) is used to calculate the reflection spectrum. The mesh size is 2.5× 1 nm2. The dielectric functions used in the simulations are from “A. D. Rakić, A. B. Djurišic, J. M. Elazar, and M. L. Majewski, Appl. Opt. 37, 5271 (1998)” for metals and from our measurements for GST and HfO2. These values as a function of wavelength are shown in Figure S1. COMSOL Multiphysics’s Joule heating module is used for the electrothermal simulations. A 2D structure is used with boundary conditions as shown in Figure S9. The same figure shows the temperature dependent electrothermal properties of GST. The electrothermal properties of GST used in the simulations are obtained from “A. Faraclas, G. Bakan, L. Adnane, F. Dirisaglik, N.E. Williams, A. Gokirmak, and H. Silva, IEEE Trans. Electron Devices 61, 372 (2014).” A metastable model is used for the electrical and thermal conductivities of GST. Most of the electrical current pass through the metal layer, since its electrical conductivity is 2-3 orders of magnitude larger than that of hot GST (T~900 K). Latent heat of fusion is incorporated in the heat capacity function as a peak at the melting temperature. The electrothermal module solves for the following equations on the whole geometry simultaneously: V dT (T ) J 2 where, J is the current density, ρ is the dt electrical resistivity, V is the electrical potential, d is the mass density, C is the heat capacity, T is the temperature, κ is the thermal conductivity. Thermal boundary resistance is used between low and high J ( )0 dC thermal conductivity materials, i.e., GST-Au and SiO2-Au following the equation: Q T / Rthermal , where Q is the heat flux (W/m2), ΔT is the temperature difference (K) across a boundary and Rthermal is the thermal boundary resistance (Km2/W). 3 7 Al 6 cGST Ag 2 4 aGST Au n n 5 1 3 2 0 1 250 500 750 1000 1250 1500 5 250 500 750 1000 1250 1500 14 cGST 4 12 10 3 k k 8 2 aGST 1 0 6 Al 4 Ag 2 Au 0 250 500 750 1000 λ (nm) 1250 1500 250 500 750 1000 λ (nm) 1250 1500 Figure S1. Refractive index (n) and extinction coefficient (k) of GST, Al, Ag, and Au used for the optical simulations. The values for Al, Ag, and Au are obtained from “A. D. Rakić, A. B. Djurišic, J. M. Elazar, and M. L. Majewski, Appl. Opt. 37, 5271 (1998)”. The GST values are characterized using spectroscopic ellipsometer measurements of 10 nm GST films on Si wafers at various angles of incidence. Tauc-Lorentz and TaucLorentz+Drude models are used for aGST and cGST, respectively. Figure S2. (a) Photographs of GST films with indicated thicknesses on 80 nm Ag layers. (b) Corresponding measured reflection spectra: black symbols (aGST), blue symbols (cGST). λ for reflection minimum (nm) 3500 3000 cGST 2500 2000 1500 aGST 1000 500 0 0 50 100 GST thickness (nm) 150 Figure S3. Wavelength for the reflection minimum as a function of the GST thickness. 5 % thickness reduction upon crystallization is taken into account. Figure S4. Measured reflectance spectra of GST (80-140 nm) coated Al surfaces as measured by a visible, near IR ellipsometer and Fourier Transform Infrared Spectroscopy (FTIR) tool. Figure S5. Simulated reflection spectra of GST films on Al showing tuning of the reflection spectra with increasing GST thickness (0-80 nm) and with phase-change. Figure S6. Photograph of GST coated Al foil. The GST thickness is increasing from center to corner. Shiny side of a piece of Al foil is used as the substrate. Figure S7. Photographs of 10 nm aGST and cGST films on 150 nm HfO 2 on Al taken at low and high viewing angles. Figure S8. (a) Cartoon illustrations of aGST film (i) on Al and (ii) in between Al layer and glass substrate. Light shines through the glass substrate for condition (ii). (b) Measured reflectance spectra for both conditions using 10, 20 and 60 nm aGST films. Measuring reflection spectra through the glass substrate slightly red-shifts the reflection spectrum and filters out the UV wavelengths. Figure S9. Electrothermal parameters, resistivity (ρ), thermal conductivity (k), heat capacity (C) of GST and 2D structure used for the simulations. Electrothermal parameters of Au and SiO 2 are obtained from the COMSOL Multiphysics’s library. Thicknesses of the top SiO2, GST and Au heater layers are 100 nm, 20 nm and 80 nm, respectively. Red lines on the 2D structures indicate boundaries at which the thermal boundary resistance of 20 Km2/GW is applied. Blue lines indicate the electrical boundaries (Pulse and 0 V) and boundaries at which temperature is set to 300 K. (b) (a) (c) Al 10 µm 500 K (d) (e) 450 T( ) Al 400 wire 350 -3 300 K -2 -1 0 1 x ( m) 2 3 Figure S10. Optical microscope images of an Al wire, delineated with dashed line, (a) before and (b) after 20 nm aGST deposition, and (c) after annealing the GST film. (d) Simulated temperature distribution and (e) temperature profile along the GST layer for the electrical current levels reached during annealing.