Letter pubs.acs.org/JPCL
Electronic Properties of Si−Hx Vibrational Modes at Si Waveguide Interface Muhammad Y. Bashouti,*,† Peyman Yousefi,†,‡ Jürgen Ristein,‡ and Silke H. Christiansen†,§ †
Max-Planck Institute for the Science of Light, Günther-Scharowsky-Str. 1, Erlangen D-91058, Germany Universität Erlangen-Nürnberg, Department of Physics, Chair of Laser Physics, Staudtstr. 1, D-91058 Erlangen, Germany § Institute of Nanoarchitectures for Energy Conversion, Helmholtz-Zentrum für Materialien und Energie Berlin (HZB), Hahn-Meitnerplatz 1, D-14109 Berlin, Germany ‡
S Supporting Information *
ABSTRACT: Attenuated total reflectance (ATR) and X-ray photoelectron spectroscopy in suite with Kelvin probe were conjugated to explore the electronic properties of Si−Hx vibrational modes by developing Si waveguide with large dynamic detection range compared with conventional IR. The Si 2p emission and work-function related to the formation and elimination of Si−Hx bonds at Si surfaces are monitored based on the detection of vibrational mode frequencies. A transition between various Si−Hx bonds and thus related vibrational modes is monitored for which effective momentum transfer could be demonstrated. The combination of the aforementioned methods provides for results that permit a model for the kinetics of hydrogen termination of Si surfaces with time and advanced surface characterizing of hybrid-terminated semiconducting solids.
I
optical properties with their electronic properties;5,6 however, in situ measurements of KP in XPS for hybrid devices combined with ATR measurements provide added value relative to individual measurements, which eliminate the following: (i) the impact of carbon contaminations that change the work function of the surface on a scale of ∼0 to 1 eV and screen the total Si−Hx vibrational modes, and thus correlation between emission, vibrational modes, and work-function would be useless, (ii) the inhomogeneous variation between pristine samples, and (iii) the inhomogeneously molecule-terminated samples. Combining the in situ techniques broadens the perspective to understand the growth and decay mechanisms of Si−Hx modes under varied hydrogenation and annealing (temperatures, times, atmospheres) conditions. We combine the two aforementioned surface analysis techniques (ATR and XPS) to investigate vibrational modes related to Si−Hx bonds with their electronic properties when exposing the Si waveguide surface to hydrogenation. By merging the Si 2p emission with the ATR frequency, we could reveal the electronic properties (such as Fermi level, work function, electron affinity, and surface dipoles) of the vibrational Si−Hx modes. To this end, we observe a transition state between the different vibrational oxide modes with an effective momentum transfer. Hydrogen removal and oxygen diffusion mechanisms are clearly identified and distinguished.
nfrared (IR) and X-ray photoelectron spectroscopies (XPS) are powerful surface analytical techniques available for the detection of surface functionalities.1,2 IR spectra are mainly used for molecular spectroscopy, while XP spectra are usually utilized to detect electronic properties of materials’ surfaces. For the detection of Si−Hx bonds at silicon (Si) surfaces, IR spectroscopy could be used; however, the detection of vibrational modes is sometimes limited by strong absorption of the incident IR light at the Si surface. In particular, at low concentrations of Si−Hx bonds at the Si surface, absorption of incident IR light further reduces the detection limit. However, Si−Hx bonds cannot be distinguished by XPS either because the hydrogen bonds with their 1s orbital cannot be resolved.2 Because of these limitations in the availability of quantitative measurement techniques for Si−Hx bonds, the investigation of the electronic properties related to these surface functional groups is still limited to date. This is intolerable because Si−Hx groups constitute important surface functional groups as they stabilize the Si surface against unwanted and uncontrolled oxidation for a certain time and can be exchanged by a large body of different chemical or biological moieties, thereby constituting an entrance card for further Si surface functionalization.3 In addition, hydrogen-terminated Si surfaces show a lower surface states density and surface recombination velocities than Si surfaces with native oxide, which may be useful for version device applications.3,4 In this context, attenuated total internal reflectance (ATR) spectroscopy to be carried out on Si waveguides, which based on internal total reflection, operated with XP spectra, offer a useful alternative to correlate the vibrational modes, that is, the © 2015 American Chemical Society
Received: September 2, 2015 Accepted: September 22, 2015 Published: September 22, 2015 3988
DOI: 10.1021/acs.jpclett.5b01918 J. Phys. Chem. Lett. 2015, 6, 3988−3993
Letter
The Journal of Physical Chemistry Letters
Figure 1. (a) Top and side view SEM images of the internal reflections within the polished Si waveguide slab with a beveled angle of 45° on both sides of the slab. (b) Schematic drawing of the Si waveguide with “hot spots”, where different Si−Hx bonds have formed at the Si surface such as, for example, Si−H, Si−H2, and Si−H3.
Figure 2. ATR (left) spectroscopy and XPS (right) of the Si−Hx modes (oxide and oxide-free modes) on a Si waveguide after a two-step hydrogenation/annealing procedure, that is, using HF/NH4F treatment and subsequent annealing in water vapor (30 ± 10%) at different temperatures: (a) 100, (b) 200, (c) 300, (d) 400, and (e) 500 °C.
number of dangling Si bonds (0−100%) at the Si surface that are replaced by Si−H x bonds of the various mode configurations can be varied, and the replacement of Si−Si− Hx bonds to O−Si−Hx bonds with the oxidation time can be analyzed. Nevertheless, post-oxide treatment time of 20 min with different temperatures was selected herein consistent with previous studies (Figure 2).7,8 To correlate optical ATR modes with their electronic properties, such as Fermi level and work function, we identified their Si 2p emission in situ with Kelvin probe (KP) measurements inside a vacuum chamber. The Si−Hx modes (of oxide-free Si−Si−Hx and ultimately oxide mode type O− Si−Hx) were distinguished after deconvolution of the ATR spectra and the XPS studies of the Si 2p emission (cf. Figure 4s).9,10 The oxide-free modes with their respective energy in ATR are as follows: Si−H (2080 cm−1), Si−H2 (2110 cm−1), and Si−H3 (2133 cm−1), while the oxide-related modes are SiH2(O2) at 2200 cm−1 and SiH(O3) at 2250 cm−1.11−13 XP spectra of these modes reveal binding energies as follows: Si 2p3/2 (99.79 ± 0.30 eV), Si 2p1/2 (100.39 ± 0.30 eV), Si2O (100.48 ± 0.02 eV), SiO (101.25 ± 0.02 eV), Si2O3 (102.03 ±
Achievable sensitivity of Si−Hx modes in ATR is largely determined and tailored by the Si waveguide properties. The Si waveguide used in this study was realized by using a polishing procedure (cf. Figure 1S). We made an ATR element out of Si(100) with dimensions of 49 × 19 × 0.525 mm3 with 45° beveled edges that allowed 93 total reflections of the IR beam. (More information can be found in the Supporting Information “SI” and Figures 2S and 3S.) Scanning electron micrographs (SEM) from the top and the side of the Si waveguide slap are shown in Figure 1a. The general principle of ATR spectroscopy is schematically drawn and illustrated in the hot spots in Figure 1b. To validate the feasibility of ATR spectroscopy with a Si waveguide for enhanced sensitivity of, for example, Si surface functionalization through Si−Hx bonds, vibrational modes are detected in ATR spectroscopy. Different hydrogenation and subsequent oxidation treatments were used such as a short dip of the Si waveguide in a solution of HF/NH 4F for hydrogenation and oxidation in water vapor (30 ± 10% humidity). NOTE: Different humidity may change the hydrogen removal and oxygen diffusion kinetics. With the combination of these treatments with varied times, the overall 3989
DOI: 10.1021/acs.jpclett.5b01918 J. Phys. Chem. Lett. 2015, 6, 3988−3993
Letter
The Journal of Physical Chemistry Letters
Figure 3. (a) Normalized intensity of the oxide and oxide-free modes as obtained from XP spectra. (b) Normalized intensity of the oxide and oxidefree modes frequencies as obtained by ATR spectroscopy.
Figure 4. (a) Normalized intensity of the H removal and O diffusion at different temperatures (100−500°) and times (0−60 min). (b) Vibrational frequency of each mode (the Si−H frontbonds) as a function of the electronegativity backbonds (Si or O backbonds).
0.4 eV), and SiO2 (103.49 ± 0.02 eV). The Si 2p3/2 and Si 2p1/2 emissions represent the oxide-free emission, that is, Si−Si−Hx bonds, while the other emissions, that is, Si2O, SiO, Si2O3,and SiO2, represent the oxide emissions, that is, O−Si−Hx (cf. Figure 2).14 By comparing the Si 2p emission with the corresponding modes in ATR, three main consequences are noted: (i) Oxidefree emissions of Si 2p peaks in XPS are attributed to Si−H, Si−H2, and Si−H3 modes in ATR spectroscopy; (ii) the Si 2p spectra related to SiO2 formation show a complete detachment of hydrogen bonds from the Si surface; that is, no Si−Hx modes remain at the surface, which explains the quick decay of the oxide-free modes in ATR when the SiO2 emission becomes visible in XP spectra (Figure 2c−e); (iii) the oxide modes SiH2(O2) and SiH(O3) in ATR are related to Si2+ and Si3+ in XP spectra. Interestingly, the formation of different oxide modes is not identical: The SiH(O3) grows gradually, while SiH2(O2) mode grows much faster and eventually disappears (cf. Figure 2b−e), a fact that may be attributed to a transition between those modes during the oxygen diffusion, as will be shown later in more detail. Table 1S shows the structure and the relevant bond configurations with optical and electronic properties of the Si−Hx modes, as observed by both techniques. As it can be seen, the emission and the vibrational modes of a central Si atom, which attached H as a frontbond and Si or O as a backbond, can be identified based on wavenumber and binding energy. The intensities of vibrational modes related to oxide and oxide-free bond modes are shown as a function of annealing temperature in Figure 3. The mode intensity (Imode) can be expressed by the ratio of the integrated area under the
considered peak with respect to the integrated area under all peaks related to vibrational modes of oxide and oxide-free modes (cf. Figure S4). The intensity of the oxide and oxide-free vibrational modes as a function of annealing time and temperature is shown in Figures 5S and 6S.15,16 Figure 3a shows the normalized intensity of vibrational modes in a bond configuration with the central Si atom being connected to O and Si (i.e., O−Si and Si−Si backbonds) as a function of annealing temperature. For O−Si backbonds, which are responsible for so-called oxide modes in the ATR spectra, 20% of the normalized intensity was achieved after annealing at 200 °C. Higher annealing temperatures, up to 500 °C, increased the O−Si intensity by a factor of 15 ± 5% only. 80% of the normalized ATR peak intensity remains after annealing at 200 °C. Higher annealing temperatures, up to 500 °C, decrease the Si−Si mode intensity by 15 ± 5%. It is inferred that oxide modes appear only after oxygen diffusion into the Si−Si backbonds, which polarizes the backbonds and shifts the binding energy to higher values.7 Figure 3b shows the normalized intensity of the frontbonds of the central Si atom connected to H as a function of the annealing temperature. In this case, nonsymmetrical kinetics for the oxide modes and the oxide-free modes were obtained. For the oxide-free modes, 75% of the normalized XPS intensity was reached after short annealing times of a few minutes (
