SUPPLEMENTARY INFORMATION
Chemical Properties of Oxidized Silicon Carbide Surfaces
upon Etching in Hydrofluoric Acid
Sarit Dhar1#, Oliver Seitz2, Mathew D. Halls2,3, Sungho Choi1,5, Yves J. Chabal2,4 and Leonard C. Feldman1,4
1
2
Department of Physics and Astronomy, Vanderbilt University, Nashville, TN
Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX
3
4
Institute for Advanced Materials and Devices and Nanotechnology, Rutgers University, Piscataway, NJ
5
#
Materials Science Division, Accelrys, Inc., San Diego, CA
Device Materials Research Center, Korea Research Institute of Chemical Technology, Rep. of Korea
Present address: Cree Inc., Durham, NC
Corresponding author: [email protected]
Nuclear Reaction Analysis
The chemical termination of the surface was investigated first with nuclear reaction analysis using the
2
H (3He,p) 4He reaction for deuterium detection and the 18O(p,α)15N reaction for oxygen(18) detection,
which offer high sensitivity (~2x1013 cm-2) in each case. To first order, no difference in chemistry is
expected between D and H. A 700 keV 3He+ ion beam was used as the probe and the emitted protons
were detected in the case of deuterium, and a 730KeV proton beam with α detection was used for 18O
determination. In both cases, the intensity of the detected particles is directly proportional to the areal
density of D/O on the surface of the sample. The measured intensity was converted to an absolute surface
coverage (cm-2) through the measurement of a standard deuterium implanted Si substrate containing
5×1015 atoms/cm2. Oxygen analysis was carried in a similar fashion using the 18O(p,α)15N reaction with
an energetic proton beam of 730 KeV and a natural standard. Typical D areal densities measured on the
crystal faces of 4H-SiC and the (111) Si surface after etching in various solutions of D2O: HF volume
ratio are shown in Table 1S. Surprisingly, the deuterium coverage is apparently a strong function of the
SiC crystal face. The highest D coverage is observed for the carbon face, ideally 100% carbon,
corresponding to about 37% surface coverage. The Si-face, which ideally contains no carbon on the
surface plane, gives no finite deuterium at the level of the detection limits. Note that the etching may
result in surfaces with both H and D containing species as both isotopes are available in the etching
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solution. The nuclear reaction used however, is sensitive only to D and not to H. Both surfaces show a
substantial amount of 18O (~1 monolayer) associated with the enriched oxide.
Atomic density
SURFACES
Si(111)
SiC (C-face)
SiC (Si-face)
Surface Atomic Density
(atoms/cm2)
7.8x1014
1.2x1015
1.2x1015
4.5x1014
< 2.5x1013
(sensitivity limit)
< 2.5x1013
< 2.5x1013
7.0x1014
1.0x1015
D coverage
16
Si O2/substrate
D2O/HF+H216O (95-5)
7.8x1014
Oxygen 18 coverage
Si16O2/substrate
HF+H218O (50-50)
< 2.5x1013
(sensitivity
limit)
Oxygen 18 coverage
Si18O2/substrate
HF+H216O (50-50)
< 2.5x1013
100% H
~38 % H
~ No H
Residual surface after
No oxygen
~60% oxygen
~ 100% oxygen
HF etching
18
Table 1S: Measured atomic densities of D and O from Nuclear Reaction Analysis: The last
row describes the final surface assuming the measured deuterium coverage also represents the
hydrogen coverage in standard HF processing.
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XPS data and analysis
Elements
C1s
C1s
C1s
Si2p
Si2p
Si2p
O1s
B.E. (eV)
on C-term
282.3
283.7
284.9
100.1
101.1
--
531.7
% on C-term
44.0
8.9
2.9
29.0
3.6
--
11.6
B.E. (eV)
on Si-term
282.6
284.0
285.1
100.5
101.5
103.1
532.0
% on Si-term
37.2
4.5
3.2
32.8
3.3
0.9
18.1
Table 2S: Peak assignments and surface concentrations of carbon, silicon and oxygen derived
from XPS measurements performed at 45 degree on C- and Si-face SiC surfaces and corrected
for the XPS cross-sections.
Peak Identification:
Based on the variation of intensity with angle the different components are identified as:
C face: BE=282.3 eV-bulk; 283.7 eV -surface carbon; 284.9 eV -surface contamination.
Si face: BE=100.1 eV-bulk; 101.1 surface silicon;
Coverage estimation
From the angle dependent measurements, the peaks at 283.7 eV and 101.1 eV are assigned to the
C1s and Si2p core levels of surface C and Si, respectively. Assuming that the last layer of the
SiC sample is composed of these two components (8.9% and 3.6%) the coverage of oxygen can
then be derived from the intensity of the measured O1S core levels as follows:
For C-face SiC : %O1s / (%C1s283.7 + %Si2p101.1) = [11.6%/(8.9%+3.6%)] ≈ 0.93 ±0.1 ML
For Si-face SiC: %O1s / (%C1s284 + %Si2p101.5 + 2 x %Si2p103.1) = [18.1%/(4.5%+3.3%+1.8%)]
≈ 1.9 ±0.1 ML, where we have assumed that the 103.1 peak corresponds to SiO2.
Thus, there is ~ 1 oxygen monolayer on top of the C-face SiC and ~2 oxygen monolayers on the
Si-face SiC.
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Thickness estimation
This calculation is based on Cupson and Seah’s article1:
d = λAL cos
Ln [1 + (IA/SA)/(IS/SS)]
where d is the thickness, λAL the corrected attenuation length of the photoelectron in the film, θ
the emission angle (with an optimum value for the calculation at 45 deg), IA and IS the intensity
of the peak respectively from the film and from the substrate, and SA and SS the sensitivity factor
of the element analyzed. The surface coverage for carbon, silicon and oxygen is calculated using
the escape depth, according to the formula:
where Na=atoms/cm3 of a in the bulk compound (SiC), and “compound IMFP” stands for
inelastic mean free path of the electron in the studied compound
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Summary and comparison of NRA, XPS and surface estimation from XPS analysis
Elements
B.E. (eV)
C-face
% on C-term
(atoms/cm2)
Coverage range*
NRA 18O
measured
B.E. (eV)
Si-face
% on Si-term
(atoms/cm2)
Coverage range*
C1s
C1s
C1s
Si2p
Si2p
Si2p
O1s
282.3
283.7
284.9
100.1
101.1
-
531.7
44.0
8.9
1.20×1015
(0.8-2.0)×1015
2.9
29.0
3.6
0.87×1015
(0.5-1.3)×1015
-
11.6
2.30×1015
(1.3-3.3)×1015
.
0.7 E+15
282.6
284.0
285.1
100.5
101.5
103.1
532.0
37.2
4.5
0.83×1015
(0.5-1.2)×1015
3.2
32.8
3.3
0.73×1015
(0.4-1.1)×1015
0.9
0.27×1015
(0.1-0.9)×1015
18.1
3.73×1015
(2.2-5.3)×1015
NRA 18O
measured
1.0 E+15
*
The range arises from the variety of reported escape depths in the literature. Note that the NRA technique
18
18
measures only O while the XPS value corresponds to the total oxygen composed of the O and additional
oxygen associated with exposure to air (leading to SiO2 formation on Si-SiC for instance).
Table 3S: Peak assignments and surface concentrations from XPS measurement at 45 deg of
carbon, silicon and oxygen on C- and Si-face SiC. Surface coverage of each element resulting
from the XPS measurements and NRA values for 18O. Calculation were done using three
different IMFPs illustrating the large range of possible surface coverage due to the variation in
reported values of this quantity, found in the literature, (a) 2.0 nm, (b) 1.2 nm and (c) 2.9 nm. *
We used for the calculation of the oxygen coverage the average value of the Si and C bulk, 36.5
and 35, for C-face and Si-face respectively.
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Coverage estimation from IR absorption measurements
In order to gain quantitative information from IR absorption measurements using ATR with the
GATR apparatus, it is necessary to calibrate the sensitivity with data taken with other
configurations (e.g. multiple internal reflection within the sample itself or transmission). The
GATR configuration, necessary because SiC substrates are opaque in the IR region (due to free
electron absorption), is most sensitive for the component of dipoles perpendicular to the
surface/interface.2 The sensitivity was calibrated for unpolarized radiation using model systems:
a) ideally H-terminated Si(111) obtained by NH4F etching, for which the Si-H stretch and
bending modes are purely perpendicular and parallel to the surface respectively, b) HF-etched
Si(100) surfaces that feature a distribution of orientations for Si-H stretch modes, and c) oxidized
silicon characterized by two phonon modes, LO and TO modes oriented strictly perpendicular
and parallel to the surface, respectively. Table 4S summarizes the data, showing that for modes
that are not perpendicular to the surface, the sensitivity of this method is similar to a single
transmission measurement (possible on transparent substrates only such as Si).
Samples
Ge ATR
(cm-1)
Transmission (2 faces)
(cm-1)
Comments
H-Si(111)
Si-H stretch
0.030
0.065
SiO2/Si
SiO2 LO mode
1.66
0.40
Modes // and ⊥
SiO2/Si
(SiO2 LO mode)
1.3
0.21
Only ⊥
SiO2/SiC
(SiO2 LO mode)
8.6
H-Si(1OO)
(Si-H stretch)
0.012
0.020
-1
Table 4S: Integrated intensities (in cm ) measured using the ATR and transmission methods
for the indicated vibrational modes of a variety of surface terminations for Si substrates.
From early FTIR measurements of water dissociation on Si(100) surface using multiple internal
reflection, the ratio of the integrated areas of the Si-H and Si-OH stretching modes provides a
relative measure of OH using the ATR (GATR) method, recognizing that the H and OH
coverages are equal on water-dosed Si(100) surfaces3. The measured ratio is ~ 1± 0.2. Using the
ATR (GATR) method for a fully hydrogenated Si(100) surface (freshly HF etched), the
measured Si-H stretch integrated absorption is 0.012 cm-1. On a freshly HF-etched C-face SiC
surface, the OH integrated absorption is 0.008 cm-1, indicating that the OH coverage is ~ 0.7
monolayer within 20%. Hence the estimated hydroxyl coverage is: 0.7 ±0.2 monolayer.
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Self assembled monolayer (SAM) adsorption on C- and Si-face SiC after HF etching
After HF etching, the C- and Si-face SiC samples are introduced in preheated anhydrous toluene
(70oC) with 0.2% 11-(triethoxysilyl)undecanal (C11-ald) molecules and kept overnight (~15
hours). The molecules are added in the toluene solution just before introducing the sample. After
reaction (i.e. after ~ 15h), the functionalized SiC samples are removed from the solution, rinsed
with fresh toluene, sonicated for 3 min in fresh toluene and dried with nitrogen gas. Fig. 3S
summarizes the IR absorption experiments using the GATR. All the modes associated with the
SAM layer and with its bonds to the SiC substrates (i.e. Si-O-Si) are labeled. The integrated
intensities associated with all the modes characteristic of the SAM molecules are summarized in
Table 5S, and compared with the amount of Si surface atoms measured by XPS. The amount of
molecules attached to the surface is similar for both the Si- and C-terminated SiC surfaces and
scales roughly with the amount of available Si surface atoms. Note that the absolute intensities
measured on SiC cannot be directly compared with SAM grafted on Si surfaces because the Si
surface roughness and index of refraction of Si and SiC are different. However, the measured
amounts are similar (e.g. intensity of CH2 mode ~ 0.054 cm-1 on Si is close to 0.045 and 0.073
cm-1 measured on SiC, see table), indicating that fully or nearly fully packed SAM surfaces can
be prepared on the SiC surfaces. For the C-SiC surface, the density of Si surface atoms is high
enough (see Table 3S) and random enough due to roughness that the anchoring sites for the SAM
layer are sufficient to form a reasonably compact SAM layer. Further work with atomically flat
SiC surfaces would be helpful to quantify these conclusions.
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0.0005
Absorbance
CH2
Mode related
to SAM molecules
Si-O-Si
C=O
Si-face
C-face
3000
2500
2000
1500
1000
-1
Wavenumbers (cm )
Fig. 1S: Infrared absorption spectra of C-face and Si-face 4H-SiC surfaces after HF etching
the oxide overlayer and adsorbing a self assembled monolayer (SAM). The spectra are
referenced to the surface after right after HF etching. Table 5S summarizes the integrated
intensities for both surfaces.
Intensity in XPS
(% atomic)
-1
Modes
CH2
C=O
mode at 2250cm
Si-O
C-face SiC
0.045
0.017
0.014
3.6
Si-face SiC
0.073
0.032
0.017
3.3 +0.9 = 4.2
Table 5S: Integrated intensities (in cm-1) of the CH2, C=O and 2250 cm-1 modes measured
using the ATR method after SAM adsorption and shown in Fig. 3S. The intensities are
compared with the amount of Si-O measured by XPS. In both surfaces, the amount of
molecules attached to the surface is similar and scales with the amount of available Si surface
atoms, as measured by XPS.
Intensity in FTIR (cm-1)
REFERENCES
(1)
(2)
(3)
Cumpson, P. J.; Seah, M. P. Surf. Interface Anal. 1997, 25, 430-446.
Chabal, Y. J. Surf. Sci. Rep. 1988, 8, 211-357.
Chabal, Y. J.; Christman, S. B. Phys. Rev. B 1984, 29, 6974.
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