Anisotropy in Hydrogen-Passivated and Organically Modified

Jan 7, 2009 - Infrared spectroscopic ellipsometry (IRSE) was applied for characterization of porous silicon (PSi) electrochemically prepared in acidic...
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Langmuir 2009, 25, 1445-1452

1445

Anisotropy in Hydrogen-Passivated and Organically Modified Nanoporous Silicon Surfaces Studied by Polarization Dependent IR Spectroscopy K. Roodenko,*,†,§ J. Rappich,‡ F. Yang,‡ X. Zhang,‡ N. Esser,† and K. Hinrichs*,† ISAS - Institute for Analytical Sciences, Dept. Berlin, Albert-Einstein-Str. 9, 12489 Berlin, Germany, Helmholtz Zentrum Berlin fu¨r Materialien and Energie GmbH, Abteilung Silizium-PhotoVoltaik, Kekule´strasse 5, 12489 Berlin, Germany, and Department of Materials Science and Engineering, UniVersity of Texas at Dallas, Richardson, Texas 75080 ReceiVed August 17, 2008. ReVised Manuscript ReceiVed NoVember 7, 2008 Infrared spectroscopic ellipsometry (IRSE) was applied for characterization of porous silicon (PSi) electrochemically prepared in acidic fluoride solution. When no formation of SiO2 was involved in the preparation, an anisotropic distribution of PSi bonds with the terminating molecules was achieved. On the contrary, oxidation of PSi samples during the preparation led to an isotropic structure. IR spectra obtained from organically functionalized PSi surfaces suggested that the morphology of the organic layer on PSi was anisotropic for electrochemical grafting of methyl but not nitrobenzene. Comparison between the IRSE spectra obtained from PSi and Si(111) surfaces and application of optical models supported these observations.

1. Introduction The discovery of room temperature photoluminescence of porous silicon (PSi)1 has inspired industrial and academic societies to seek the ways of its integration into semiconductor technology. As wide-range applications of PSi have been recognized,2-4 a considerable effort had been taken for understanding and controlling of the pore formation mechanisms.5-7 Furthermore, good electronic passivation of porous silicon should still be achieved, since PSi gets oxidized under atmospheric conditions.8-10 When optical techniques are used for characterization of porous materials, interpretation of the detected signal is hardly straightforward. The challenge lies in describing mathematically interactions between the incoming radiation and the investigated heterogeneous materials. The so-called effective medium theories11-13 are frequently used for simulation of the properties of porous materials. For example, Spanier and Herman14 performed a thorough evaluation of different effective medium models and discussed their performance when modeling infrared reflectance of porous SiC films. More relevant to our studies are the * Correspondingauthors.E-mail:[email protected](K.R.);[email protected] (K.H.). † ISAS - Institute for Analytical Sciences. ‡ Helmholtz Zentrum Berlin fu¨r Materialien and Energie GmbH. § Present address: University of Texas at Dallas.

(1) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (2) Stewart, M. P.; Buriak, J. M. AdV. Mater. 2000, 12, 859. (3) Foell, H.; Christophersen, M.; Carstensen, J.; Hasse, G. Mater. Sci. Eng., R 2002, 39, 93. (4) Yamaguchi, R.; Miyamoto, K.; Ishibashi, K.; Hirano, A.; Said, S. M.; Kimura, Y.; Niwano, M J. Appl. Phys. 2007, 102, 014303. (5) Lehmann, V.; Ro¨nnebeck, S. J. Electrochem. Soc. 1999, 146, 2968. (6) Lehmann, V.; Goesele, U. Appl. Phys. Lett. 1991, 58, 856. (7) Le´vy-Cle´ment, C.; Lust, S.; Mamor, M.; Rappich, J.; Dittrich, Th Phys. Status Solidi A 2005, 202, 1390. (8) Petrova, E. A.; Bogoslovskaya, K. N.; Balagurov, L. A.; Kochoradze, G. I. Mater. Sci. Eng., B 2000, 69, 152. (9) Mawhinney, D. B.; Glass, J. A., Jr.; Yates, J. T., Jr. J. Phys. Chem. B 1997, 101, 1202. (10) Salonen, J.; Lehto, V. P.; Laine, E. Appl. Surf. Sci. 1997, 120, 191. (11) Rossow, U. Phys. Status Solidi A 2001, 184, 51. (12) Looyenga, H. Physica 1965, 31, 401. (13) Granquist, C. G; Hunderi, O. Phys. ReV. B 1978, 18, 2897. (14) Spanier, J. E.; Herman, I. P. Phys. ReV. B. 2000, 61, 10437.

publications by the groups of Timoshenko et al.,15,16 Foell et al.,3 Chazalviel et al.,17 and others.18,19 These publications reported observation of anisotropy in PSi which was formed by electrochemical etching. For example, anisotropy on PSi(100) surfaces could be explained by the assumption that the Si micro/ nanocrystallites in normal PSi layers on (100) wafers are preferentially interconnected in the direction perpendicular to the sample surface.18 Recently, Efimova et al.20 showed that the anisotropic optical properties of PSi are also manifested by absorption on the local vibrational modes of silicon-hydrogen (SiHx) groups present on the surface of nanocrystals. They used transmission absorption IR spectroscopy and deconvolution methods to study anisotropy of porous silicon and to relate the absorption bands of SiHx groups to their orientation in the material. In this article, we explore anisotropy related to IR absorption vibrational bands of differently prepared and modified PSi layers. Furthermore, we show that functionalization of PSi by electrochemical grafting of organic molecules preserves the anisotropy when no side reactions, such as polymerization of the radicals in electrolytes or oxidation of PSi, take place. We applied infrared reflection absorption spectroscopy (IRRAS) and IR spectroscopic ellipsometry (IRSE) as characterization techniques. IRSE can be seen as an extension of IRRAS technique and allows one to record additionally the polarization degree and the phase shift between the complex reflection coefficients of light polarized parallel and perpendicular with respect to the plane of incidence21 (see Experimental Section for definition of ellipsometric parameters). (15) Golovan’, L. A.; Kashkarov, P. K.; Timoshenko, V. Yu. Crystallogr. Rep. 2007, 52, 697. (16) Timoshenko, V. Yu.; Osminkina, L. A.; Efimova, A. I.; Golovan, L. A.; Kashkarov, P. K.; Kovalev, D.; Kuenzner, N.; Gross, E.; Diener, J.; Koch, F. Phys. ReV. B 2003, 67, 113405. (17) Outemzabet, R.; Cherkaoui, M.; Gabouze, N.; Ozanam, F.; Kesri, N.; Chazalviel, J.-N. J. Electrochem. Soc. 2006, 153, C108. (18) Koyama, H. J. Appl. Phys. 2004, 96, 3716. (19) Kovalev, D.; Ben Chorin, M.; Diener, J.; Koch, F.; Efros, A. L.; Rosen, M.; Gippius, N. A.; Tikhodeev, S. G. Appl. Phys. Lett. 1995, 67, 1585. (20) Efimova, A. I.; Krutkova, E. Yu.; Golovan’, L. A.; Fomenko, M. A.; Kashkarov, P. K.; Timoshenko, V. Yu. J. Exp. Theor. Phys. 2007, 105, 599. (21) Hinrichs, K.; Gensch, M.; Esser, N. Appl. Spectrsoc. 2005, 59, 272A.

10.1021/la802685m CCC: $40.75  2009 American Chemical Society Published on Web 01/07/2009

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Roodenko et al.

Table 1. Sample Names, Description, and Preparation Techniques As Used in This Paper sample name

sample description

H/Si(111)

H-terminated Si(111)

HPSi1

porous silicon on p-Si(001) surface

HPSi2

porous silicon on p-Si(001) surface

HPSi3

porous silicon on p-Si(001) surface

NBPSi

NB grafted on HPSi3 surface

NB/Si(111)

NB grafted on Si(111) surface

MePSi

methyl (CH3) grafted on HPSi3 surface

The advantage of these techniques is that they are nondestructive and provide data which allow conclusions on chemical structure and composition of the investigated surfaces.21,22 This is especially useful for organically modified surfaces, where the identification of the adsorbates can help in optimization of the preparation techniques and elimination of the side reactions that may take place in electrolytes.23,24 The interpretation of our results is based on the simulations of the line shapes of the IR absorption bands. It was necessary to perform simulations of the obtained spectra in order to understand the optical response of isotropic/anisotropic samples. The structure of this paper is as follows: First, we introduce the experimental techniques for preparation and characterization of the samples. Next, we discuss IRSE spectra obtained from various surfaces: H-passivated Si(111) and PSi. We present the simulated results along with the measured ones and discuss anisotropy on both flat and PSi surfaces. Finally, we discuss spectra of organically modified Si surfaces. Specifically, we refer to methyl-terminated and nitrobenzene-terminated PSi and Si(111) surfaces. Comparison between the spectra from different substrates is presented, and the conclusion on anisotropy of different Si surfaces is drawn. The Supporting Information supplied with this article includes additional comments on structural properties and porosity of the material derived from the effective medium approach. In addition, it shows how the shape of the absorption bands varies when layer properties (dielectric function, layer thickness) or experimental conditions (angle of incidence) change.

2. Experimental Section All samples studied in this manuscript, their abbreviations as used in this paper, and their preparation techniques are summarized in Table 1. 2.1. H-Passivation. H-passivated p-type Si(111) surfaces (2-5 Ω cm) were prepared by etching in 40% NH4F solution.25 Porous silicon was prepared from (100)-oriented p-type Si surfaces in an electrochemical cell where two procedures were applied: (1) Hydrogen-passivated Si(100) was immersed in 48% HF and ethanol (1:1), and a two-electrode method was applied (Au wire as counter electrode, H:Si(100) as working electrode) for 600 s with a constant current density (see Table 1 for details). We refer to the samples (22) Tompkins, H. G., Irene, E. A., Eds. Handbook of Ellipsometry; William Andrew Publishing - Springer: New York, 2005. (23) Rappich, J.; Merson, A.; Roodenko, K.; Dittrich, T.; Gensch, M.; Hinrichs, K.; Shapira, Y. J. Phys. Chem. B 2006, 110, 1332. (24) Roodenko, K.; Rappich, J.; Gensch, M.; Esser, N.; Hinrichs, K. Appl. Phys. A: Mater. Sci. Process. 2008, 90, 175. (25) Allongue, P.; de Villeneuve, C.-H.; Morin, S.; Boukherroub, R.; Wayner, D. D. M. Electrochim. Acta 2000, 45, 4591. (26) Dubois, T.; Ozanam, F.; Chazalviel, J.-N. Electroschem. Soc. Proc. 1997, 97-7, 296.

sample preparation H-passivated Si(111) surfaces (2-5 Ω cm) were prepared by etching in 40% NH4F solution chronopotentiometry of H-passivated Si(100) in 48% HF/ethanol (1:1) at a current density of 1.4 mA/cm2 during 600 s anodic treatment of H-passivated Si(001) in 5% HF/H2O in potentiostatic mode at 0.85 V for 600 s chronopotentiometry of H-passivated Si(001) in 48% HF/ethanol (1:1) at the current density of 0.5 mA/cm2 for 600 s grafting at -1.2 V for 520 s in a solution of 0.01 M H2SO4, 1% HF, and 7.5 mM 4-NBDT grafted on H/Si (111) surface at the potential of -1.2V in 0.01 M H2SO4 and 5 mM 4-NBDT grafting at galvanostatic conditions using 3 M methylmagnesium bromide in diethylether (Sigma-Aldrich) at a current density of 0.5 mA/cm2 for 5 min

which were prepared in this way as HPSi1 and HPSi3 hereafter. (2) The second procedure was done using potentiostatic mode in a threeelectrode setup: Si surface as working electrode, Au as counter electrode, and Ag/AgCl (saturated KCl) as reference electrode. The hydrogen-passivated Si(100) was polarized in 5% HF/H2O solution at 0.85 V for 600 s. This sample will be referred to as HPSi2 throughout the manuscript. 2.2. Methyl Termination. Methyl termination on porous silicon (HPSi3) was performed in 3 M methylmagnesium bromide in diethylether from Sigma-Aldrich under galvanostatic conditions, in a two-electrode cell equipped with a Pt plate as a counter electrode. The reaction mechanism can be found, for example, in ref 26. The sample is referred to as MePSi hereafter. 2.3. Nitrobenzene Deposition. Nitrobenzene (NB) was grafted on Si(111) and HPSi1 from 4-nitrobenzene diazonium tetrafluoroborate (4-NBDT). The grafting process and the mechanism of formation of thin organic layers from diazonium salts on various surfaces are described in detail elsewhere.27,28 Here, the electrochemical preparation of the front H/Si(111) was performed in a single-compartment Teflon cell, using a three-electrode configuration (sample as working electrode, Au ring as counter electrode, and Au wire as reference electrode). The grafting was performed under potentiostatic conditions, where the potential was switched to -1.2 V and 4-NBDT was injected into the cell. Electrochemical grafting on PSi was performed in a thee-electrode setup with PSi surface as working electrode, Au as counter electrode, and Ag/AgCl (saturated KCl) as reference electrode at a constant potential of -1.2 V for 520 s. The NB-modified porous Si sample will be referred to as NBPSi, while the NB modified Si(111) will be referred to as NB/Si(111). 2.4. SEM Measurements. The samples HPSi1 and HPSi2 were investigated by scanning electron microscopy (SEM) using a Hitachi S-4100 system with a cold-field cathode. 2.5. Infrared Spectroscopic Ellipsometry. IRSE measurements were performed using a photometric ellipsometer attached to a Bruker IFS 55v/S Fourier transform interferometer.29,30 Ellipsometry measures the change of the polarization state upon reflection or transmission from the sample surface. In our case, the measurements were performed at a 65° angle of incidence, and the polarization state of the reflected radiation was analyzed using the setup as described in refs 29-31. The measured ellipsometric parameters tan Ψ and ∆ represent the amplitude ratio and phase difference, respectively, through the following relation: tan Ψ ei∆ ) rp/rs, where rs and rp are the complex reflection coefficients oriented perpendicular and parallel with respect to the plane of incidence. The spectral (27) Allongue, P.; de Villeneuve, C. H.; Cherouvrier, G.; Cortes, R.; Bernard, M.- C. J. Electrochem. Soc. 2003, 550, 161. (28) Wang, D.; Buriak, J. M. Langmuir 2006, 22, 6214. (29) Roeseler, A. In Handbook of Ellipsometry; Tompkins, H. G., Irene, E. A., Eds.; William Andrew publishing - Springer: New York, 2005; pp 763-798. (30) Roeseler, A.; Korte, E. H. In Handbook of Vibrational Spectroscopy; Chalmers, J., Griffith, P., Eds.; Wiley: Chichester, U. K., 2001; Vol 2. (31) Roeseler, A. Infrared Spectroscopic Ellipsometry; Akademie Verlag: Berlin, 1990.

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Figure 1. Illustration of the model used in simulations of the spectra. The model consists of the isotropic Si substrate, the anisotropic PSi layer with thickness d, and the ambient surrounding with n ) 1. A harmonic oscillator model was used to model absorption bands in the obtained spectra. Each molecular vibration was modeled by specific parameters of the oscillator strengths F (in-plane Fxy and out of plane Fz in case of anisotropic layers), frequency ν0, and damping constant Γ. Transition dipole moments of the molecular vibrations are pictured as arrows (with various lengths, to illustrate molecular vibrations with various parameters of the oscillator strengths Fxyz). The Cartesian (x,y,z) system coordinate as used throughout the manuscript is also shown.

resolution was 4 cm-1. A mercury cadmium telluride (MCT) detector from Kolmar Technologies was used for all of the reported measurements, unless otherwise specifically stated. In this case, the liquid He cooled bolometer from Infrared Laboratories was used. 2.6. Simulation of Spectra. The simulation of IRSE spectra was performed using special solutions of the 4 × 4 matrix formalism for anisotropic films.32-34 The vibrational properties were simulated using the Lorenz oscillator model30,35 through the following relation for the complex dielectric function εˆ (ν):

εˆ (ν) ) ε∞,x,y,z +

j Fx,y,z

∑ (νj )2 - ν2 + iνΓj j

0

Figure 2. IRSE tan Ψ (a) and ∆ (b) spectra obtained from H-passivated Si(111) surface. Black dotted line, measured data; solid line, simulated results. The spectra were referenced to oxidized Si(111) surfaces. The subscript “S” on the ordinate stands for “bare substrate”, while “F” stands for “film-modified substrate”. δ(Si-H) (bending) and δ(Si-H) (stretching) absorption bands are marked in gray. The Si-H layer thickness was taken as the length of the Si-H bond of 1.5 Å.37 Spectra below 660 cm-1 were obtained using a liquid He-cooled bolometer detector. Table 2. H-Si(111) absorption bands’ positions (Fig. 2) and their assignments. ν: stretching; δ: bending vibrations. absorption peak position (cm-1) (this work, Figure 2) 626 2082

(1)

x,y,z

where the subscripts x, y, z indicate the components in the respective directions and superscript j indicates the summation components. Here, Γ is the damping constant, ν0 is the resonance oscillator position (in our case, in cm-1), and F is the parameter of the oscillator strength. ε∞ is the high-frequency dielectric constant and ν are the wavenumbers in the spectral range of interest. Figure 1 schematically illustrates the model. To fit the absorption bands of the PSi sample, the following steps were applied: First, the Rs spectrum was fitted with three oscillators oriented along the x and y directions. Next, the Rp spectrum was fitted with six oscillators, where the first three were an input from the Rs fit and were not allowed to vary. The other three included only z-components. They were set initially at the same ν0 frequencies as Rs-obtained oscillators and were varied until the best fit was achieved. This procedure was employed, since the s-polarized reflectance Rs is only dependent on the in-plane components (x or y)32,33 but independent of z-components. The p-polarized reflectance Rp is dependent on the mutually orthogonal in-plane components (x or y) and the z-components. Thus, the x- or y-components were used as an input into Rp from the fit applied on Rs and kept constant; only z-components of the oscillator parameters were then allowed to vary.36 We would like to point out that, for the investigated materials, the wavelength (in IR spectral range) is much larger than the pore sizes, and thus, the radiation probes the average properties of the material (porous Si + air + embedded organics). The model aims to reproduce those average properties as seen by the probing radiation. (32) Dluhy, R. A. J. Phys. Chem. 1986, 90, 1373. (33) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North Holland Publishing Company: Amsterdam, 1977. (34) Chabal, Y. J. Surf. Sci. Rep. 1988, 8, 211. (35) Bianchi, R. F.; Balogh, D. T.; Tinani, M.; Faria, R. M.; Irene, E. A. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 1033. (36) Hinrichs, K.; Tsankov, D.; Korte, E. H.; Roeseler, A.; Sahre, K.; Eichhorn, K.-J. Appl. Spectrosc. 2002, 56, 737.

literaturea (cm-1) 38

39

40

627, 627, 631 2088,41 2084 42

assignment δ(Si-H) ν(Si-H)

a For mono-, di-, and trihydride mode assignments on vicinal Si(111)/H surfaces, see also ref 41.

3. Results and Discussion In the following sections, we present a detailed analysis of the IR absorption bands in polarization dependent spectra. First, we discuss the spectra measured from hydrogen-passivated and next from organically modified PSi. Discussion of the spectra measured from PSi is somewhat complicated due to the complex structure of this material. In order to facilitate the discussion, we perform a comparison of the spectra obtained from PSi to those obtained from well-defined Si(111) surfaces. We apply similar optical models to simulate the spectra measured at PSi and Si(111), and we show how the molecular orientation and the optical effects influence the observed spectra. 3.1. Characterization of H-Terminated Si Surfaces. 3.1.1. H-PassiVated Si(111) Surfaces. Figure 2 shows KramersKronig-related tan Ψ and ∆ spectra obtained from H-passivated Si(111) surfaces. The absorption peaks due to the bending δ(Si-H) and the stretching ν(Si-H) vibrational modes are marked in Figure 2. The assignments are summarized in Table 2. The simulation of the spectra was performed using the Lorenz oscillator model. An important property that we would like to point out is the peak orientation of the absorption bands due to the ν(Si-H) and δ(Si-H) in Si(111) samples. The ν(Si-H) absorption bands were fitted solely with harmonic oscillator oriented perpendicular to the surface (oscillator parameters in x-y directions were Fx ) Fy ) 0). The simulated peak-down (37) Ong, C. K.; Khoo, G. S. J. Phys. C 1986, 20, 419. (38) Caudano, Y.; Thiry, P. A.; Chabal, Y. J. Surf. Sci. 2002, 502-503, 91. (39) Webb, L. J.; Rivillon, S.; Michalak, D. J.; Chabal, Y. J.; Lewis, N. S. J. Phys. Chem. B 2006, 110, 7349. (40) Stuhlmann, Sh.; Bogdanyi, G.; Ibach, H. Phys. ReV. B 1992, 45, 6786. (41) Zhang, X.; Chabal, Y. J.; Christman, S. B.; Chaban, E. E.; Garfunkel, E. J. Vac. Sci. Technol., A 2001, 19, 1725. (42) Chabal, Y. J. Phys. B 1991, 170, 447.

1448 Langmuir, Vol. 25, No. 3, 2009

Roodenko et al. Table 3. Observed Si-H Absorption Peaks Observed for the HPSi1 Sample (Figure 3) and Their Assignmentsa absorption peak position (cm-1) (This work, Figure 3) 674 910 2095 2120 2145 a

Figure 3. IRSE spectra of hydrogenated porous silicon: (a) HPSi1 and (b) HPSi2 obtained at an angle of incidence of 65° (thick dotted line). The fit is shown in the spectral range between 2000 and 2200 cm-1 for ν(SiHx) absorption bands (black solid line). Plots (c) and (d) show zoomin of (a) and (b), respectively, in the spectral range below 1350 cm-1. For assignments of the marked absorption bands, see Table 3. Data used for the fit are given in Table 4.

shape fits well the measured tan Ψ spectrum and the related ∆ parameter. On the contrary, δ(Si-H) at 626 cm-1 in the Si(111) spectrum was fitted with the harmonic oscillator oriented parallel to the sample surface (Fz ) 0), which reproduced well the measured data. This is in agreement with the transition dipole moments for the Si-H monohydrides oriented perpendicular to the Si(111) surface plane: the transition dipole moment due to the δ(Si-H) vibration mode is parallel to the Si(111) surface plane, while the one due to ν(Si-H) is perpendicular to it. In conclusion to this part, it was demonstrated that simulations of the ellipsometric spectra allow one to determine the orientation of the adsorbates on the surfaces. Observation of the sign of the absorption bands may give a good indication of the orientation of the adsorbates. Recalling that tan Ψ ) |rp|/|rs|, the absorption peaks in the tan Ψ spectra of thin films on silicon would appear as peak-down features for the vibrational transition dipole moments oriented perpendicular to the surface, while peak-up features correspond to the vibrational transition dipole moments oriented parallel to the surface. 3.1.2. Porous Si Surfaces. IRSE spectra measured at porous silicon surfaces HPSi1 and HPSi2 are shown in Figure 3. Table 3 gives detailed mode assignments along with the literature references for the HPSi1 sample. For this sample, Figure 3a exhibits three distinctive peaks at 2095, 2120, and 2145 cm-1, which are due to the stretching vibrations of ν(SiHx) with x ) 1, 2, and 3, respectively (see Table 3 for the literature references). The peak at 910 cm-1 is due to the bending δ(SiH2) scissors vibrations, and the peak at 674 cm-1 is due to the δ(SiH2) wagging vibration mode. Figure 3b shows the spectrum obtained from the

literature (cm-1) 45

9,26

667, 666 916,45 910,26,9 915 9 2087,9 2090 46,9 2108,45 2120,46 2115,9 2113 47 2142,45 2140 45,47,9

assignment δ(SiH2) wag δ(SiH2) “scissors” ν(SiH) ν(SiH2) ν(SiH3)

ν, stretching vibration; δ, bending vibration.

HPSi2 sample. Here, a strong broad band between 990 and 1215 cm-1 can be seen, which arises due to the SiO2 stretching vibrations. This band was not observed in the spectrum from the HPSi1 sample. In addition, spectra from HPSi2 show a broad absorption band between 2060 and 2165 cm-1 due to the ν(SiHx) stretching vibrations. The line shapes of these absorption bands are different from the HPSi1 spectrum, where the individual peaks due to ν(SiHx) (x ) 1, 2, 3) can be distinguished. The broadening in HPSi2 is due to the mixed back-bonding of the Si atoms: some of the Si-Hx groups are back-bonded to oxygen atoms, while the others are not. The back-bonding of the Si-Hx groups to oxygen atoms is known to shift the vibrational frequency of these modes.43 The HPSi2 sample shows additional peaks at 800, 863, 904, and 700 cm-1 superimposed on broad absorption backgrounds. These peaks can be generally assigned to δ(OySi-Hx) absorption modes.9,44 The line shapes of the absorption bands in Figure 3 look different for HPSi1 and HPSi2 samples in the region of ν(Si-H) stretching vibrations (2060-2165 cm-1). The spectrum obtained from HPSi1 sample shows peak-up shaped spectra, while the HPSi2 spectra show lines peaked down. The main reason for such differences is the pore morphology of the samples. SEM micrographs in Figure 4 show that the HPSi1 sample has a more structured pore morphology, while the HPSi2 sample is amorphous. To understand the reason behind the differently oriented absorption bands, simulations and fits of the measured spectra were performed. Fitting of the spectral lines was performed in several stages. First, the thickness and the refractive index were determined by the fit of the interference pattern which was observed in the obtained spectra. The layer model was used as described in the Experimental Section. It should be mentioned that the thicknesses of the porous layers were inhomogeneous, which made it difficult to fit on the measured data. The illuminating radiation spot was around 60 mm2 in area; thus, the measured spectra represent the average thickness over the measured area. However, the thickness parameters were in agreement with those estimated from SEM micrographs (Figure 4). The error in the estimated thicknesses and refractive indices was within 10% of the reported values. The anisotropic values which were obtained for the refractive index parallel to the surface plane (nx, ny) and perpendicular to the surface plane (nz) were in agreement with the conclusions drawn by Timoshenko et al.16 regarding anisotropic optical properties of porous silicon layers prepared on Si(110) wafers. Earlier, Kovalev et al.19 reported on anisotropic linear polarization (43) Lucovsky, G.; Yang, J.; Chao, S. S.; Tyler, J. E.; Czubatyj, W. Phys. ReV. B 1983, 28, 3225. (44) Pi, X. D.; Mangolini, L.; Campbell, S. A.; Kortshagen, U. Phys. ReV. B 2007, 75, 085423. (45) Ogata, Y. H.; Tsuboi, T.; Sakka, T.; Naito, S. J. Porous Mater. 2000, 7, 63. (46) Maruyama, T.; Ohtani, S. Appl. Phys. Lett. 1994, 65, 1346. (47) Hinrichs, K.; Gensch, M.; Esser, N.; Ro¨seler, A. J. Phys.: Condens. Matter 2004, 16, S4335.

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Table 4. Harmonic Oscillator Parameters Obtained for the Absorption Bands of the Investigated Porous Silicon Samplesa vibrational mode ν(Si-H)

in-plane out-of-plane

ν(Si-H2)

in-plane out-of-plane

ν(Si-H3) δ(Si-H2) ν(Si-H) ν(Si-H2) ν(Si-H3)

frequency ν0 (cm-1)

F (cm-2)

Γ (cm-1)

HPSi1 sample: nx ) ny ) 1.33, nz ) 1.27, d ) 1.33 µm 2087 Fz ) 0; Fx ) Fy ) 1940 2080 Fz ) 7660; Fx ) Fy ) 0

Γz ) 0; Γx ) Γy ) 27 Γz ) 60; Γx ) Γy ) 0

2115 2108

Fz ) 0; Fx ) Fy ) 2990 Fz ) 460; Fx ) Fy ) 0

Γz ) 0; Γx ) Γy ) 30 Γz ) 8; Γx ) Γy ) 0

2144 911

Fz ) 35; Fx ) Fy ) 1270 Fz ) 1700; Fx ) Fy ) 4100

Γz ) 7; Γx ) Γy ) 20 Γz ) 22; Γx ) Γy ) 35

HPSi2 sample: n ) 1.30, d ) 165 nm 2094 Fz ) Fx ) Fy ) 2250 2116 Fz ) Fx ) Fy ) 11800 2184 Fz ) Fx ) Fy ) 5420

Γz ) Γx ) Γy ) 42 Γz ) Γx ) Γy ) 65 Γz ) Γx ) Γy ) 150

a The refractive index of the supporting substrate was 3.62. The fits were performed for the angle of incidence of 65°. Please note that the in-plane and out-of-plane assignments refer to the x-, y-, or z-projected components of the transition dipole moment of certain Si-Hx vibrations. The in-plane and out-of-plane assignments do not necessarily imply the existence of two different modes of vibration.

Figure 4. SEM images of HPSi1 and HPSi2 samples. Upper images: magnification ×100. Lower images: zoom ×30 for HPSi1 and ×10 for HPSi2. The tilt angle of the surface was 30°.

of photoluminescence from porous Si prepared on p-type Si(100). This was attributed to anisotropy in the dielectric function, which was supported by theoretical simulations.19 Thus, the observed optical anisotropy of the porous silicon surfaces is not unexpected. As to the values of the obtained refractive indices, similar results were reported in refs 48-50. In the next step of the simulations, the above parameters were used to fit the ν(Si-Hx) absorption bands. The obtained oscillator parameters for this spectral range are given in Table 4. As expected from our earlier discussion for the flat Si(111) surface, the differences in orientation between the ν(Si-Hx)related absorption bands results from the average predominant orientation of the ν(Si-Hx) bonds in the HPSi1 sample. This is discussed in detail in the next section. 3.1.2.1. HPSi1 Sample. The fitting procedure for the spectra obtained from the HPSi1 sample is described in the Experimental Section and also discussed in ref 36. Table 4 summarizes the fitted parameters, and the result of the fit is shown in Figure 5. Table 4 shows that the ν(SiH3)-related absorption peak was fitted only with a single oscillator positioned at 2144 cm-1, with the projected major oscillator components along the x-y plane. The ν(SiH2)-component required two oscillators, which were justified by the separated fits of the Rp- and Rs-components. (48) Setzu, S.; Romestain, R.; Chamard, V. Thin Solid Films 2004, 460, 53. (49) Torres, J.; Castillejo, F.; Alfonso, J. E. Braz. J. Phys. 2006, 36, 1021. (50) Pap, A. E.; Kordas, K.; Vahakangas, J.; Uusimaki, A.; Leppavuori, S.; Pilon, L.; Szatmari, S. Opt. Mater. 2006, 28, 506.

Figure 5. Measured (circles) and fitted (solid lines) data for the HPSi1 sample. Anisotropic fit for an uniaxial symmetry with optically inequivalent x, y, and z directions was applied. The oscillator parameters are given in Table 4.

Intuitively, one could interpret the necessity for these two components as symmetric and asymmetric stretching vibrations of ν(SiH2). However, it was shown that, in amorphous silicon, the split between these modes is very small.51,52 Calculations carried out for porous silicon by Ogata et al.53 favor no split at all for ν(SiH2) stretching vibrations. Moreover, these calculations predicted similar IR intensities for both symmetric and asymmetric stretching vibrations in ν(SiH2) in porous silicon. The obtained Fxy value for ν(SiH2) most probably contains certain percentages of both νas(SiH2) and νss(SiH2), where the transition dipole moments average to yield a major component in the x-y plane. The two different values that were obtained for ν(SiHx) could be either due to different back-bonding of the Si atoms involved in SiH2 stretching vibrations, or due to the surface-specific positions of the SiH2 groups in the PSi sample and their dipole-dipole interactions with the neighboring groups. (51) Brodsky, M. H.; Cardona, M.; Cuomo, J. J. Phys. ReV. B 1977, 16, 3556. (52) Lucovsky, G.; Nemanich, R. J.; Knights, J. C. Phys. ReV. B 1979, 19, 2064. (53) Ogata, Y.; Niki, H.; Sakka, T.; Iwasaki, M. J. Electrochem. Soc. 1995, 142, 195.

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Figure 6. Measured (dotted) and simulated (solid line) data for the scissors δ(SiH2) absorption band at 911 cm-1 for the HPSi1 sample. The differences in the background between the calculated and measured data are due to the thickness deviation in the porous silicon layer.

Also fitting of monohydrides, ν(Si-H), required a similar fitting approach, where the oscillators’ components were fitted separately for Rs and then for the Rp reflectances.36 The higherpositioned component (2087 cm-1) had a lower parameter of the oscillator strength (Fxy ) 1940 cm-2) than the mode at 2080 cm-1 with Fz ) 7660 cm-2. The existence of the Fxy-component in this case can be explained by the contribution of the Si-H bonds which are oriented parallel to the surface plane. The strong Fz-component comes from the contribution of Si-H bonds which are perpendicular to the sample plane and suggests that there is a predominant orientation of Si-H bonds. In contrast to the absorption bands due to the stretching ν(SiHx) vibrations, the absorption bands due to the bending δ(SiH2) vibrational modes have all peak-down orientation in the tan Ψ spectra of the HPSi1 sample (Figure 3a). We simulated the absorption band due to the SiH2 scissors mode at 911 cm-1 (Figure 6). The in-plane (Fxy) component of the parameter of the oscillator strength was higher than the out-of-plane (Fz) component. Geometrically, this means that the inclination angle of the transitional dipole moment due to the scissors δ(SiH2) was (on average) roughly 22° to the sample plane54 (see Table 4 for the Fj values at 911 cm-1 from which this angle was derived). The error in this value depends on the uncertainties in estimation of high-frequency dielectric function, and oscillator parameters. Experimentally, also the angle of incidence has an opening angle of about 3-4° which contributes to the error bars. Taking these into account, we estimate the error to be within 30% of the reported value. In conclusion to this part, simulations of the absorption bands in the spectra obtained from the HPSi1 sample showed certain anisotropy. Due to the lack of possibility to distinguish between the symmetric and asymmetric absorption bands in ν(SiH2) and ν(SiH3), it is difficult to draw a conclusion regarding the orientation of these groups on the sample. Simulations of the scissors δ(SiH2) mode gave a more clear resolution as to the average orientation of the SiH2 groups relative to the sample (54) Scholz, R.; Friedrich, M.; Salvan, G.; Kampen, T. U.; Zahn, D. R. T.; Frauenheim, T. J. Phys.: Condens. Matter 2003, 15, S2645.

Roodenko et al.

Figure 7. Measured (dotted) and fitted (solid line) data for the HPSi2 sample. The isotropic fit with equivalent x, y, and z directions was applied. The oscillator parameters are given in Table 4.

plane (roughly 22°). For monohydrides, absorptions at two different frequencies were obtained from the fit. This finding could point out that different groups/clusters of monohydrides or monohydrides residing at crystallographically different sites exist in the PSi layer. 3.2.1.2. HPSi2 Sample. In contrary to the HPSi1 tan Ψ spectrum, all absorption bands in the tan Ψ spectrum of the HPSi2 sample are oriented downward (Figure 3b). The simulated results for HPSi2 sample are shown in Figure 7. The fit was obtained with isotropic oscillator parameters, which result in a peak-down shape of the broad absorption band. We believe that the oxide formation during the preparation of this porous silicon layer is responsible for isotropic distribution of SiHx bands in the HPSi2 sample. In conclusion to this section, we showed that differently prepared porous silicon surfaces can possess differently oriented Si-Hx bonds. For the HPSi2 sample, where a great amount of SiO2 was present (Figure 3), no anisotropy was found. On the contrary, the HPSi1 sample showed no SiO2-related absorption bands, and at the same time, through the simulation of the measured IRSE parameters, we determined that ν(Si-Hx) bonds have certain anisotropy which arises most likely due to certain order in the porous structure. 3.2. Characterization of Functionalized Surfaces. 3.2.1. Methyl-Terminated Porous Silicon. Rp, Rs, and tan Ψ spectra obtained from the methylated PSi (MePSi) sample are shown in Figure 8a,c,d, along with the spectra of hydrogenterminated HPSi3 (Figure 8b) for comparison. The spectra from the methylated PSi exhibit absorption bands due to the CH3 termination, as summarized in Table 5. There appear also absorption bands due to SiHx groups. Figure 8c shows the tan Ψ spectrum of MePSi divided by the tan Ψ spectrum of unmodified HPSi3, whereby the CH2-related absorption band due to contaminations at 2930 cm-1 disappears, leaving only the true bands originating from the grafted CH3 groups. Figure 8d shows a tan Ψ spectrum of MePSi in the extended spectral range. It can be seen that no SiO2-related absorption (55) Gurtner, C.; Wun, A. W.; Sailor, M. J. Angew. Chem., Int. Ed. 1999, 38, 1966. (56) Ferguson, G. A.; Raghavachari, K. J. Chem. Phys. 2006, 125, 154708.

Hydrogen-PassiVated, Organically Modified PSi

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Figure 9. Ellipsometric parameters tan Ψ (a) and ∆ (b) of NB/PSi in the range between 470 and 2350 cm-1. Peaks 1-5 are assigned in Table 6. Regions I-III are described in the text.

Figure 8. Ellipsometric parameters tan Ψ and reflectances Rp and Rs obtained from MePSi (a) and HPSi3 (b) samples at a 65° angle of incidence. The background of Rp was shifted in respect to that of Rs by +0.482 in (a) and by +0.534 in (b) for visual convenience. Plot (c) shows the referenced tan Ψ MePSi spectrum in respect with the HPSi3 tan Ψ spectrum. Upon referencing, only the bands due to the grafted CH3 groups remain in the spectrum. Plot (d) shows the MePSi tan Ψ spectrum in the extended spectral range. The arrows in (d) emphasize the directions of the observed absorption bands. ν, stretching; F, rocking; δ, bending vibrational modes. Please pay attention to the different scales of the ordinates between the right and left sides of the plots in (a) and (b). Table 5. Observed Si-CH3 Absorption Peaks (Figure 8) and Their Assignmentsa absorption peak position [cm-1] (as observed in Figure 8)

assignment

reported absorption peaks [cm-1]b

773 2904 2967

F(C-H)CH3 νss(C-H)CH3 νas(C-H)CH3

75739 2909,39 290026 2965,39,26295955

a ν, stretching; F, rocking. b For theoretical calculations of methylterminated Si(111), see also ref 56 and references therein.

bands between 1000 and 1250 cm-1 were observed on the spectra of MePSi. It is interesting to note two types of absorption bands related to the CH3 molecular vibrations: the bands shaped as peak-down (F(CH3) at about 773 cm-1) and the bands shaped as peak-up (ν(CH3) in the range between 2900 and 2970 cm-1). In the lower spectral range, also absorption bands due to the bending vibrations of SiH2 groups can be observed. Interestingly, a typically strong absorption band at 1253 cm-1 due to the CH3 umbrella mode,39 which is present in all our tan Ψ spectra obtained from CH3-terminated Si(111) surfaces, was absent from our

spectra. This is most likely an effect of the specific average orientation of the transitional dipole moment related to this molecular vibration. The line shapes of the absorption bands in tan Ψ spectra for MePSi qualitatively follow the same pattern as that for the structured HPSi sample: absorption bands due to the stretching vibrations appear as peak-up features, while the absorption bands due to the bending vibrations appear as peak-down features. Thus, we conclude that grafting of the CH3 groups follows the structure of the porous silicon layer and exhibits anisotropy. 3.2.2. Nitrobenzene-Terminated Silicon. IRSE spectra obtained from nitrobenzene-terminated PSi samples are shown in Figure 9. Table 6 summarizes the peak assignment (peaks 1-5 as shown in Figure 9). For the weak peak (1) at 752 cm-1, the assignment is unclear. In addition to peaks 1-5, Figure 9 also shows regions I-III, where broad absorption peaks were observed. Region I (800-960 cm-1) belongs to δ(-OySi-Hx) bending modes.9,45,57 Region II (1000-1280 cm-1) is due to the Si-O-Si-related vibrations.9,45,57 Finally, region III (2130-2300 cm-1) is due to the ν(-OySi-Hx) stretching modes.9,45,57 The formation of the SiOx groups occurs due to the grafting in aqueous electrolyte, in accordance to the earlier reported side reactions.23,24 Figure 10 shows for comparison spectra obtained from NB/ Si(111) in the spectral range where the main absorption bands due to νas(NO2), νss(NO2), and ν(C-C)ring occur. These peaks are marked as 3-5, respectively, and they correspond to the respective peaks found for NB on PSi in Figure 9. It is evident from Figures 9 and 10 that all of the peaks in both NB-modified Si(111) and PSi are aligned downward. Figure 11 shows simulated spectra for NB on a flat Si surface.62-64 Comparison of the simulated line shapes to those obtained experimentally leads to the conclusion that on both flat and porous (57) Borghesi, A.; Sassella, A.; Pivac, B.; Pavesi, L. Solid State Commun. 1993, 87, 1. (58) Chen, Q.; Haq, S.; Frederick, B. G.; Richardson, N. V. Surf. Sci. 1996, 368, 310. (59) Bocharov, S.; Teplyakov, A. V. Surf. Sci. 2004, 573, 403. (60) Clarkson, J.; Smith, W. E. J. Mol. Struct. 2003, 655, 413. (61) Syomin, D.; Wang, J.; Koel, B. E. Surf. Sci. 2001, 495, L827. (62) Gensch, M.; Roodenko, K.; Hinrichs, K.; Hunger, R.; Guell, A. G.; Merson, A.; Schade, U.; Shapira, Y.; Dittrich, T.; Rappich, J.; Esser, N. J. Vac. Sci. Technol., B 2005, 23, 1838. (63) Gensch, M. Infrared ellipsometry for the investigation of interfacial layers and thin organic films on silicon. Ph.D. Thesis, Mensch & Buch Verlag, Berlin, 2005. (64) Gensch, M.; Hinrichs, K.; Ro¨seler, A.; Korte, E. H.; Angelova, P.; Tsankov, D. Bulg. Chem. Commun. 2005, 37, 350.

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Table 6. Observed NB/Si Absorption Peaks (Figures 9 and 10) and Their Assignments

a

peak no. (as observed in Figure 9)

absorption peak position [cm-1]

assignment

reported absorption peaks [cm-1]a

1 2 3 4 5

752 1112 1350 1523 1600

? ν(C-N) + ν(C-C)ring νss(NO2) νas(NO2) ν(C-C)ring

see text 1106,58 1107,59 1108 60 1350,61 134858-60 1523,58-60 1528,61 1603,58 1606,59,60 1588 61

ref 58, NB/Cu(110); ref 59, NB/Si(100); ref 61, NB/Au(111); ref 60, liquid NB.

procedure, which may lead to disordered structures even at flat surfaces.65,66 In conclusion to this section, we showed examples for two types of organic modification: the one which preserves the anisotropy when binding to PSi and the one that does not. In the case of nitrobenzene, we found that NB/PSi forms an isotropic layer. This was achieved by polymerization of nitrobenzene units (due to radical side reactions) and due to the oxidation of the surface during grafting.23,24

4. Conclusions

Figure 10. Ellipsometric parameters tan Ψ (a) and ∆ (b) of NB/Si (111) in the range between 1300 and 1630 cm-1. Peaks 3-5 correspond to those shown in Figure 9 for NB/PSi and are assigned in Table 6.

Figure 11. Simulated tan Ψ and ∆ spectra for NB on flat Si surfaces for (a) molecules oriented perpendicular to the surface, (b) molecules oriented parallel to the surface, and (c) isotropic distribution of molecules on the surface. The spectra were referenced to the simulated bare-substrate spectra. Molecular transition dipole moments due to the symmetric and asymmetric stretching vibrations in the NO2 group are shown to the right of the plots. Simulations were performed for a 65° angle of incidence, and n∞ of NB was taken as 1.46.

silicon surfaces nitrobenzene has no preferential orientation. The lack of the orientation of nitrobenzene on porous silicon comes from two reasons: (1) formation of silicon oxide during grafting, which on the example of HPSi2 sample we have learned that it leads to isotropic distribution of Si-Hx bonds; and (2) possible polymerization of the nitrobenzene molecules during the grafting

In this paper, we presented line shape analysis of vibrational bands in IRSE and IR reflectance spectra for porous and flat Si surfaces. Simulations of the absorption band line shapes were performed on both flat and porous silicon surfaces, for comparison. Simulations of H/Si(111) showed anisotropic distribution of SiHx bonds. Porous silicon studies presented in this paper revealed two different types of porous layers: a structured one, which showed a certain anisotropic distribution of SiHx bonds, and an unstructured one, which involved formation of silicon oxide during preparation. Spectra obtained from methylated porous silicon suggest that the formed organic layer preserved anisotropy of the porous layer, following the morphology of the substrate (initially hydrogen-passivated). On the contrary, grafting of nitrobenzene leads to isotropic distribution of material, which we attributed to two reasons: one is the formation of silicon oxide during grafting,23,24 and the second is the polymerization of nitrobenzene radicals in electrolyte.65,66 In comparison, also on flat silicon surfaces, the nitrobenzene layer did not present any preferential structure. Acknowledgment. K.R. acknowledges the financial support by the Minerva foundation. We thank Ilona Fischer for the technical support and Dr. Michael Gensch for valuable discussions. The financial support by the Senatsverwaltung fu¨r Wissenschaft, Forschung and Kultur des Landes Berlin and by the Bundesministerium fu¨r Bildung and Forschung is acknowledged. Supporting Information Available: Additional comments on structural properties, and discussion of the dependence of absorption bands on the angle of incidence and layer properties. This material is available free of charge via the Internet at http://pubs.acs.org. LA802685M (65) Laforgue, A.; Addou, T.; Belanger, D. Langmuir 2005, 21, 6855. (66) Pinson, J.; Podvorica, F. Chem. Soc. ReV. 2005, 34, 429.