4.3 Results
4.3.2 Cl-Terminated Si(111) Surfaces
4.3.2.1 Experimental Results
phonon absorption such as sample temperature.
-0.005 0 0.005 0.01 0.015 0.02
-0.015 -0.01 -0.005 0 0.005 0.01
400 800
1200 1600
2000 2400
2800 3200
Absorbance Absorbance
wavenumber (cm-1)
ν (Si-H) δ (Si-H)
LO TO 30˚
74˚
Figure 4.2: TIRS and proposed peak assignments of the H-terminated Si(111) surface col- lected at an incident beam angle of 74◦ (bottom) and 30◦ (top) off surface normal. The spectra are on the same scale of absorbance (absorbance units), although they are offset for easier viewing. Data are shown after subtraction of H2O and flattening of the baseline.
-0.006 -0.004 -0.002 0 0.002 0.004
-0.01 -0.008 -0.006 -0.004 -0.002 0 0.002
400 800
1200 1600
2000 2400
2800 3200
Absorbance Absorbance
wavenumber (cm-1) 30˚
74˚
ν δ (Si-Cl)
ν (Si-H)
δ (Si-H)
Figure 4.3: TIRS and proposed peak assignments of the Cl-terminated Si(111) surface collected at an incident beam angle of 74◦ (bottom) and 30◦ (top) off surface normal. The spectra are on the same scale of absorbance (absorbance units), although they are offset for easier viewing. Data are shown after subtraction of H2O and flattening of the baseline.
-0.002 -0.001 0 0.001 0.002 0.003 0.004
-0.004 -0.003 -0.002 -0.001 0 0.001 0.002
500 550
600 650
700
Absorbance Absorbance
wavenumber (cm-1) 30˚
74˚
ν δ
(Si-Cl) δ (Si-H)
Figure 4.4: TIRS and proposed peak assignments of the Cl-terminated Si(111) surface collected at an incident beam angle of 74◦ and 30◦ off surface normal shown in the low wavenumber region. The spectra are on the same scale of absorbance (absorbance units), although they are offset for easier viewing. Data are shown after subtraction of H2O and flattening of the baseline.
of that wafer, so that modes associated with the Cl-terminated surface appear as positive features, while modes present on the H-terminated surface appear as negative features in the spectra. As expected from previous spectra of the freshly etched surface, the Si–H stretching vibrational mode appeared greatly reduced when observed with an IR incident angle of 30◦off surface normal compared to spectra collected at the Brewster angle, while the Si–H bending mode appeared equally strongly negative in both configurations. Two peaks appeared on the Cl-terminated surface, at 583 and 528 cm−1, that were not observed on the H-terminated surface. Polarization-type studies revealed that the strength of the mode at 583 cm−1 was greatly reduced when probed at an incident IR angle of 30◦ rather than at the Brewster angle, indicating that it is a mode that is perpendicular to the surface.
The mode at 528 cm−1 demonstrated similar absorptivity at either collection angle, indi- cating that it is a bending motion that is parallel to the surface. Previous HREELS9, 22 and FTIR17, 23–25 studies have identified the Si–Cl stretching motion between 500–625 cm−1. Given the absence of any other chemical features of this surface, as seen by both survey scan and high-resolution X-ray photoelectron spectroscopy (Chapter 3), these two peaks are assigned to surface Si–Cl vibrational modes, although which particular motions they represent is not immediately clear, and will be discussed further in Section 4.3.2.2. As seen previously on the H-terminated surface, CO2(g) in the sample compartment was observed in a peaks at 2400 cm−1and 667 cm−1, which can be seen in Figure 4.4.
The relative integrated area under each curve was used as a qualitative measurement of the extent of surface coverage. Without separate consideration of the relative oscillator strengths and cross-sections of each IR absorption feature this technique cannot be used to identify quantitatively the absolute monolayer coverage of each chemical species involved in a surface vibrational motion, but it is a useful tool for comparison between surfaces. Peak areas were estimated with the Omnic software package used to collect and analyze all TIRS data, and are given in Table 4.1. The ratio of the area under the observed stretching mode to bending mode is also given as a useful aid for quickly comparing vibrational features on different surfaces. On the freshly etched H-terminated surface examined at an incident IR beam angle of 74◦, the area of the Si–H stretching mode at 2083 cm−1 was 1.48 x 10−2
±0.03 x 10−2, while the area of the bending mode at 627 cm−1 was 3.31 x 10−2 ± 0.08
TIRS peak area
-R θ ν(Si–H) δ(Si–H) ν:δ(Si–H)a
-H 74◦ (1.48±0.03)x10−2 (3.31±0.08)x10−2 0.45±0.02 30◦ (1.36±0.1)x10−3 (4.2±0.2)x10−2 0.033±0.002 -Cl 74◦b (-1.45±0.07)x10−2 (-3.7±0.3)x10−2 0.39±0.05
30◦b (-1.35±0.03)x10−3 (-4.4±0.6)x10−2 0.031±0.005
Table 4.1: Integrated areas of TIRS peaks on H- and Cl-terminated Si(111) surfaces as- signed to Si–H and Si–Cl modes. a Ratio of the integrated area of the stretching modeν to the bending modeδ. b Negative areas identify features on the H-terminated surface that appear as negative peaks in the difference spectrum of the Cl-terminated surface.
x 10−2. The ratio of the area under the observed stretching mode to the observed bending mode was 0.45 ±0.02. When this surface was chlorinated, the area of the negative Si–H stretching peak in the difference spectrum attributed to hydrogen that had been removed from the surface was -1.45 x 10−2 ±0.07 x 10−2. The corresponding values for the Si–H bending motion at 627 cm−1 on this surface was -3.7 x 10−2 ± 0.3 x 10−2. The overall ratio of the Si–H stretching to Si–H bending mode on the Cl-terminated Si(111) surface was 0.39±0.05. Within the error of the experiment and the area determination technique, the relative amount of Si–H present on the freshly etched surface that then disappeared on the Cl-terminated surface was the same, supporting the hypothesis that hydrogen atoms on the freshly etched surface are quantitatively replaced by chlorine atoms when the surface is exposed to the PCl5solution. The integrated areas of both Si–H vibrational modes when scanned at an incident IR beam angle of 30◦, given in Table 4.1, confirm this conclusion.
In a separate study, Dr. Sandrine Rivillon of the Chabal laboratory at Rutgers University compared three methods of Si(111) surface chlorination: exposing the H-Si(111) surface to PCl5, to Cl2(g) illuminated by UV light to form a Cl·radical, and to Cl2(g) heated to 95◦C to generate the Cl·radical. Comparing the two Si–Cl modes on these three surfaces led to the conclusion that there were no substantive chemical differences in these Cl-terminated surfaces prepared through different functionalization methods.18