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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.