Ellipsometry as a Real-Time Optical tool for Monitoring
and Understanding Graphene Growth on Metals
SUPPORTING INFORMATION
Maria Losurdo*, Maria M Giangregorio, Pio Capezzuto and Giovanni Bruno
Institute of Inorganic Methodologies and of Plasmas, National Council of Research, IMIP-CNR,
via Orabona 4, 70126 Bari, Italy
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SUPPORTING INFORMATION PARAGRAPH
Sample Preparation and Experimental setup
Graphene was grown by chemical vapor deposition (CVD) from a mixture of CH4 : H2 = 100:50 sccm gases at a temperature of 900 °C and at a total pressure of 4 Torr in a stainless-steel CVD reactor.
Growth times between 2 and 15 min were used to vary the thickness of the few-layer graphene (FLG) between approximately 3 Å and 25 Å (this is an optical equivalent thickness, corresponding to Raman analysis showing between 1-8 layers). The sample is then cooled at a rate of ~2 °C/min in 1 Torr of H2.
300nm Ni/300nm SiO2/Si obtained by Ni sputtering was used as substrate. It was pre-annealed at 400°C in UHV for nickel oxide desorption for a time of 5-15min and then heated to 900 °C in 1 Torr of H2 over a period of ~40-60 before addition of methane.
The graphene growth process was monitored in real-time by an in-situ phase modulated spectroscopic ellipsometer (UVISEL, Horiba Jobin-Yvon) integrated on the CVD reactor as shown in Figure S1.
Spectroscopic ellipsometer- Source head
CVD reactor
Spectroscopic ellipsometer- Analyzer
Heater
Ellipsometry light spot probing during graphene deposition
Ni substrate during graphene deposition
Figure S1. The UVISEL phase-modulated spectroscopic ellipsometer integrated on the CVD reactor to monitor graphene formation on nickel . The inset shows the white light spot on the nickel samples at 900°C during graphene growth 2
Real Time Spectroscopic Ellipsometry.
Ellipsometry is based on measurements of the changes in light polarization upon reflection from a sample surface [M. Losurdo, M. Bergmair, G. Bruno, D. Cattelan, C. Cobet, A. de Martino, K. Fleischer, Z. Dohcevic-Mitrovic, N. Esser, M. Galliet, R. Gajic, D. Hemzal, K. Hingerl, J. Humlicek, R. Ossikovski, Z.V. Popovic, O. Saxl J. Nanopart. Res. 2009, 11, 1521]. Linearly polarized light with known polarization impinges on the samples at an angle of incidence (70° in the present case), becoming elliptically polarized upon reflection depending on the optical properties of the surface. Indeed, the two components of the electromagnetic field-in the plane of incidence (p), and perpendicular
(s) to the plane of incidence-experience different attenuation and phase shifts at the reflection (see
Fig. S2). Those variations are described by tanΨ that represents the absolute value , while the phase change between the two polarization is related to cos related to the material refractive index through Fresnel’s law, being ρ the complex reflection coefficient for the parallel, p, and perpendicular, s, polarizations, defined as:
j∆ ρ = tanΨe = rp/rs
Because rp and rs are linked, the material’s optical properties can be derived as a function of the recorded wavelengths
Light source Erp Eip
Ers Eis
N0
N1
Figure S2. Scheme of the working principle of ellipsometry: linearly polarized light impinging on a surface and becoming elliptically polarized upon reflection.
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Therefore, spectroscopic ellipsometry monitored in real-time the growth by directly recording the pseudodielectric function <ε>=<ε1>+i<ε2>, which is related to the graphene pseudoextinction coefficient
2 (1− ρ) 2 i 2 +=〉〈+〉〈=〉〈 2 φφεεε ikn 〉+〈= 1 2 tan1sin 2 () ()1+ ρ where φ is the angle of incidence fixed at 70°
Kinetic ellipsometric data were acquired every 1 s using a phase-modulated spectroscopic ellipsometer
(UVISEL, Horiba Jobin Yvon) in the 1.5-5.5 eV spectral range with a 0.01 eV resolution.
Analysis of the ellipsometric spectra to extract the optical constant of the graphene layers requires the application of an appropriate optical model to fit experimental data. The model used consisted in a one- layer model (substrate/graphene/air). Fit variables in this model are the thickness and the parameterization for the optical constant of graphene. To improve accuracy on the graphene optical properties, it is important that the substrate properties are also well known. Therefore, for the high- temperature substrate data (at the growth temperature) the in situ spectrum recorded for the Ni/SiO2/Si system just before introducing CH4 into the reactor as been used (spot position is always the same). For the room temperature data, in some experimental runs, the substrate has been cooled down after the cleaning and crystallization process. The temperature effect on the dielectric function and optical properties of polycrystalline Ni are shown in the Figure S3.
The isotropy assumption has been used as already argued and demonstrated in ref. [Nelson, F. J.;
Kamineni, V. K.; Zhang, T.; Comfort, E. S.; Lee, J. U.; Diebold A. C. Appl. Phys. Lett. 2010, 97,
253110] because the difference in path lengths of light traversing in different directions due to birefringence in such thin monolayers is negligible. The isotropy assumption still keeps valid the output for close-loop control of the graphene thickness. The isotropy assumption also applies to the nickel substrate as demonstrated in ref. [P.B. Johnson, R.W. Christy, Phys. Rev. B, 9, 5056 (1994)].
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300nm Ni/300nm SiO2/Si catalyst substrate optical behavior
Sensitivity of the optical spectra to Ni quality. Figure S3 shows the dielectric function spectra acquired for 300nm Ni/300nm SiO2/Si substrates that underwent a slightly different crystallization process. The differences observed show the sensitivity of the optical spectra to the quality of the crystalline nickel. The closest the dielectric function to the reference Ni surface, the better the quality of the crystallization process. The Ni reference is from ref. 16.
2 G23 0 4 -2
-4 3 >
1 G15 Ä_rε -6 k <
Figure S3. Ellipsometric spectra of the real part, <ε1>, of the pseudodielectric function and of the pseudoextinction coefficient,
5 increasing the temperature from room temperature (RT) to 900°C in H2, and the thermal dependence of the crystallized Ni optical properties in the range 900°C-RT. The 900°C optical constants, shown in Fig.
4Sb, have been used for the analysis of the growth kinetic data recorded at 900°C; the RT optical constant are used for analysis of FLG (few-layers graphene) samples at RT.
Ni annealing & Cooling down crystallization Ni thermal variation
RT 900°C RT
2 Ni @900°C 2 Ni @RT 4 2.5
1.5 1.5 n k n 2 3 k
1 1.5 1 (a) (b) 2 0 1.000 2.000 3.000 2 3 4 5 Time (s) Photon Energy (eV)
Figure S4. Variation of the optical properties of the crystallized 300nm Ni/300nm SiO2/Si substrate (a) in kinetic mode cooling down the sample from 900°C to room temperature (RT): thermal variation in the range 900°CRT causes a
Similarly, we have also measured the temperature dependence of the optical properties of
FLG/300nmNi/300nm SiO2/Si samples. Figure S5 compares the spectra of the Ni supported FLG as recorded at 900°C and at RT.
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0 4
-2 3.5
> -4 1 FLG on Ni @900°C Ä_rε 3 k < FLG on Ni @25°C
Figure S5. Ellipsometric spectra of the real part, <ε1>, of the pseudodielectric function and of the pseudoextinction coefficient,
Optical spectra for graphene growth on copper.
The same approach is also applied to monitor the growth of graphene on the copper (Cu) catalyst. The figure below shows the variation of the spectra induced by the deposition of graphene on copper.
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11 -5 -1 FLG-on-Cu 10 Cu -10 Ä_r -2 9 > > 1 Ä_r -15 -3 8 Ä_i2 ε ε < < 3 4 5 7 -20 Photon Energy (eV) Cu 6
-25 5 FLG-on-Cu 4 2 3 4 5 Photon Energy (eV)
Figure S6. Spectra of the real, <ε1>, and imaginary, <ε2>, parts of the pseudodielectric function for a clean and crystallized copper surface and for FLG on copper. FLG formation can be seen in the increase of <ε1> and in the increase of <ε2> proportional to the graphene coverage.
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