7668
J. Phys. Chem. B 2000, 104, 7668-7676
Second Harmonic Generation at Chemically Modified Si(111) Surfaces† S. A. Mitchell,* R. Boukherroub, and S. Anderson Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex DriVe, Ottawa, Ontario, Canada K1A 0R6 ReceiVed: February 3, 2000; In Final Form: April 25, 2000
Si(111) surfaces with covalently attached monolayers have been investigated by optical second harmonic generation (SHG). The surfaces are modified by wet chemical procedures and studied by SHG in air or in a liquid immersion cell, using fundamental wavelengths λ ) 775 and 830 nm. Results are reported for monolayers including -H, -C10H21 (decyl), -O-C10H21 (decyloxy) and -Cl, and for a Si(111) surface with a native oxide film. The rotational anisotropy of the SHG efficiency has been studied and estimates have been made of the relative values of nonlinear susceptibilities for the surface and bulk response. The mechanisms that contribute to SHG are identified. The application of SHG for in situ monitoring of chemical reactions of Si(111)-H is demonstrated, and results are presented for the formation of Si(111)-Cl in a UV photoinduced reaction in liquid CCl4.
1. Introduction Optical second harmonic generation (SHG) has been shown to be a sensitive probe of the surface and interfacial structure of silicon crystals.1 SHG is specifically sensitive to interfaces of centrosymmetric crystals because the second-order nonlinear response vanishes in the bulk of such crystals in the electricdipole approximation, but is allowed at interfaces where the symmetry of the bulk is necessarily broken.2 Extensive SHG studies have been reported on the technologically important buried interface between silicon and silicon dioxide, for both Si(001) and Si(111) crystal faces.1,3,4 The phenomenological theory of SHG at such interfaces has been described in detail,5,6 but the microscopic origin of the nonlinear response is not well understood. Several mechanisms can contribute to the nonlinearity, including local (i.e., electric-dipole) effects due to the presence of noncentrosymmetric chemical bonds at the interface, and higher order effects that depend on the spatial variation of the electric field at the fundamental frequency in the interfacial region. For Si/SiO2, interfacial strain has been recognized as a mechanism that can break the inversion symmetry of Si-Si bonds.7 Distortion of the bonds may occur because of the volume mismatch of the unit cells of silicon and silicon dioxide, or it could arise from partial charge transfer from silicon to oxygen atoms at the interface.8 These two effects are difficult to separate because the atomic and electronic structure of Si/SiO2 interfaces is complex and subject to variability depending on processing conditions. Recently it was shown how part of the nonlinear response of silicon surfaces can be attributed to the presence of unbalanced microscopic fields that arise due to induced polarization of Si-Si bonds on only one side of the interface.9 Such a local field effect is intrinsic to all interfaces. To clarify the various contributions to the nonlinear response it is desirable to systematically modify the properties of the interface. This can be done by chemical modification of the dielectric layer that serves to terminate the dangling bonds of silicon and passivate the surface.10 †
Issued as NRCC 43825.
It is well-known that an ideal termination of dangling bonds on Si(111) surfaces can be achieved by wet chemical etching of an oxidized surface in a buffered ammonium fluoride solution.11,12 Such H-terminated surfaces (Si(111)-H) have broad terraces on which the normally oriented dangling bonds of the topmost Si atoms are capped with H atoms. The H-terminated surfaces are remarkably stable and can be handled in air for several hours without appreciable oxidation. Following earlier reports by the groups of Chidsey13 and Lewis,14 recent work in this laboratory has shown that Si(111)-H surfaces can be further modified by wet chemical procedures to produce densely packed monolayers of saturated hydrocarbons and related organic molecules.15,16 These monolayers are linked to the silicon surface by robust C-Si or O-Si covalent bonds, and are stable in air for extended periods. Recently, monolayers have been prepared with terminal ester groups, which enables further modification by standard organic chemical reactions to produce extended organic molecular structures on the surface.17 Such chemically modified silicon surfaces are interesting for a number of applications, in the area of molecular electronics and for chemical and biochemical sensors. The present work was undertaken to explore the application of SHG for monitoring chemical reactions of Si(111)-H surfaces. In this paper we describe the variation of the SHG response for different chemical modifications of Si(111). The results provide some new insights into the origins of the SHG response for passivated Si(111) surfaces. Further work and improved understanding are needed to fully exploit the surface sensitivity of SHG for characterizing these systems. The present work shows that SHG can be used as a qualitative probe of the chemical state of Si(111) surfaces, useful for in situ monitoring of the progress of surface chemical reactions. 2. Experimental Section 2.1. Second Harmonic Generation. Second harmonic generation (SHG) was monitored in reflection from samples cut from Si(111) wafers and chemically modified according to the procedures described below. Wafers of n-type (F ) 2 Ω cm)
10.1021/jp000450d CCC: $19.00 Published 2000 by the American Chemical Society Published on Web 07/20/2000
SHG at Chemically Modified Si(111) Surfaces Cz-silicon with miscut less than 0.5° were purchased from Virginia Semiconductor. In most experiments the sample was exposed to air during the measurements. The wafer fragment was mounted in such a way that it could be rotated about its surface normal, with the laser beam incident at 45° in a horizontal plane. The zero of the azimuthal rotation angle was such that a 〈011〉 direction was perpendicular to the plane of incidence. The particular 〈011〉 direction was selected with reference to the primary flat on the single batch of wafers. SHG was studied for fundamental wavelengths λ ) 775 and 830 nm. These wavelengths were selected in part for reasons of convenience, 830 nm being readily accessible in our laboratory. 775 nm was chosen because previous work8 has shown that SHG of oxidized silicon surfaces is resonantly enhanced at this wavelength. Laser pulses with central wavelength 830 nm and duration ∼80 fs were from the output of a 1 kHz Ti:sapphire regenerative amplifier (Positive Light) seeded with the output a solid-state pumped femtosecond Ti:sapphire oscillator (Spectra-Physics). For the fundamental wavelength 775 nm, laser pulses with duration ∼60 fs were obtained by frequency doubling the output of a femtosecond optical parametric oscillator (Quantronix) pumped by the output of the regenerative amplifier. Dielectric mirrors and a colored glass filter were used to isolate the frequency doubled output at 775 nm. Spectra of the laser pulses were nearly Gaussian with full width at half-maximum ∼13 nm. The laser beam with pulse energy ∼3 µJ at 1 kHz repetition rate was focused on the sample by using a 100 cm focal length lens. A Berek’s polarization compensator and a Glan-laser polarizer were used to select the plane of polarization of the incident laser beam, and a second polarizer was used to select the polarization of the reflected second harmonic light. The second harmonic was separated from reflected fundamental light by using a Pellin-Broca prism and colored glass filters, and detected with a photomultiplier tube (PMT). A glass filter was placed just before the sample to remove second harmonic radiation from the incident laser beam. The signal from the PMT was averaged by boxcar integration and displayed with a personal computer. A small fraction of the incident laser beam was split off and used for SHG by transmission through a quartz waveplate. This reference signal was recorded simultaneously with the SHG signal from the sample and used for normalization against variations in the energy or duration of the laser pulses. For comparison of the SHG response at the fundamental wavelengths λ ) 775 and 830 nm, the signals were normalized to the reflected SHG signal that was measured when a z-cut quartz wedge was substituted for the sample. 2.2. Liquid Immersion Cell. SHG at Si(111)/CCl4 interfaces was studied by using a liquid immersion cell made from Teflon. The 2 in. diameter cell had the silicon sample and one fused silica window mounted parallel at a distance of 0.5 in., and sealed on the cell with Teflon O-rings. Following transfer of CCl4 through Teflon tubing under pressure of argon, the cell was sealed with compact Teflon valves. The cell was mounted in such a way that it could be rotated through 360°, with the laser beam incident on the fused silica window at 45°. The angle of incidence on the Si(111)/CCl4 interface was 29°.18 2.3. Preparation of Samples. Native oxide samples were cut from as-received wafers and sonicated in Milli-Q water for 5 min. Excess water was removed with pressurized nitrogen and the samples were dried in air. Hydrogen terminated Si(111) was prepared by first cleaning a native oxide sample in 3:1 concentrated H2SO4/30% H2O2 at 100 °C for 20 min and thoroughly rinsing with Milli-Q water. The sample was
J. Phys. Chem. B, Vol. 104, No. 32, 2000 7669 then etched in clean room grade 40% aqueous deoxygenated NH4F for 15 min.19 For the preparation of a decyl monolayer on Si(111), the Si(111)-H sample was transferred, without rinsing, into a Schlenk tube containing 10 mL of deoxygenated decylmagnesium bromide (1.0 M solution in diethyl ether) and warmed to 85 °C for 16 h.15 The surface was then rinsed at room temperature with 1% CF3COOH solution in THF, Milli-Q water and finally trichloroethane. The decyloxy monolayer on Si(111) was prepared by reaction of Si(111)-H in deoxygenated neat decanol for 16 h at 85 °C.16 All cleaning and etching reagents were clean room grade and were supplied by Amplex. All other reagents were obtained from Aldrich and were the highest purity available. The characterization of the modified surfaces by attenuated total internal reflectance FT-IR spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy has been described in detail elsewhere.15,16 3. Background Theory The phenomenological theory of SHG in reflection from lowindex planes of silicon crystals has been described in detail in the literature.6 The efficiency of SHG is governed by the secondorder nonlinear susceptibility tensor χ(2), through the relation P(2)(2ω) ) χ(2) E(ω) E(ω). For clarity in the discussion the detailed form of χ(2) for the case of SHG at a Si(111) surface is shown in eq 1, where the P(2) i (2ω) are Cartesian components of the nonlinear polarization at the second harmonic frequency, and the Ei(ω) are components of the electric field at the fundamental frequency on the silicon side of the interface.
[ ][ P(2) x (2ω)
P(2) y (2ω) P(2) z (2ω)
∂11 -∂11 0 0 ∂15 0 0 0 ∂15 0 -∂11 ) 0 ∂31 ∂31 ∂33 0 0 0
[ ]
]
Ex(ω)2 Ey(ω)2 Ez(ω)2 2Ey(ω)Ez(ω) 2Ex(ω)Ez(ω) 2Ex(ω)Ey(ω)
(1)
Sipe et al.6 have described in detail the relationship between the nonlinear polarization and the intensity of second harmonic radiation. χ(2) has four independent nonzero elements, of which only ∂11 contributes to the anisotropic part of the SHG response, i.e., the response that depends on the orientation of the crystal axes relative to the plane of incidence of the laser beam. The elements ∂15, ∂31, and ∂33 describe the isotropic part of the SHG response. Note that the Cartesian coordinates of eq 1 are fixed to the crystal axes, with z normal to the surface and y perpendicular to a mirror plane of the unreconstructed (111) surface. In addition to the susceptibility elements in eq 1 that describe the electric-dipole response of the surface, the total nonlinear response has electric-quadrupole contributions from the bulk of the crystal as well. The isotropic and anisotropic contributions from the bulk are described by the susceptibilities γ and ζ, respectively. It has been shown that the surface- and bulk-derived contributions can be combined in an effective χ(2) that has the same form as eq 1.6 In the following we refer to the response of the surface to include both of these contributions. There are thus six, in general complex susceptibilities that are needed to fully describe the SHG response for the case of a Si(111) surface. SHG efficiencies for p- and s-polarized second harmonic radiation generated by p- or s-polarized incident radiation are defined in terms of the intensities of the fundamental and second harmonic radiation as shown in eq 2 for the (s, p) case, where
7670 J. Phys. Chem. B, Vol. 104, No. 32, 2000
Mitchell et al.
the first index refers to the fundamental frequency and the second to the second harmonic frequency.
Rsp )
Ip(2ω)
(2)
Is(ω)2
The rotational anisotropy of the SHG efficiency is described by isotropic |A| and anisotropic |B| parameters as shown in eq 3,
Rsp ) |Asp + Bsp cos(3φ)|2 ) |Asp|2 + |Bsp|2 cos2(3φ) + 2|Asp||Bsp| cos(3φ) cos(θsp) (3) where φ is an angle of rotation of the crystal about its surface normal. φ is defined as the angle between the plane of incidence and the [21h1h] direction on the (111) surface. (Note that φ is indistinguishable from φ (120°, due to the 3-fold symmetry of the surface.) In eq 3 |Asp| and |Bsp| are absolute magnitudes of the complex valued isotropic and anisotropic parameters, and θsp is their relative phase (in the range 0-180°), as shown in eq 4. In practice one uses eq 3 to obtain the relative phase and
Asp |Asp| ) exp(iθsp) Bsp |Bsp|
(4)
quantities that are proportional to the absolute magnitudes. Similar expressions hold for the SHG efficiencies Rpp, Rps, and Rss, except that the isotropic response is not present for s-polarized second harmonic radiation, as shown in eqs 5-7.
Rpp ) |App + Bpp cos(3φ)|2
(5)
Rps ) |Bps sin(3φ)|2
(6)
Rss ) |Bss sin(3φ)|
(7)
2
s- and p-polarized components of the electric field at the fundamental frequency are equal in magnitude on the silicon side of the interface.20,21 In this case the SHG efficiency for s-polarized second harmonic radiation is given by eq 14, and the corresponding isotropic and anisotropic parameters are related to the susceptibilities as shown in eqs 15 and 16.
Rqs ) |Aqs + Bqs cos(3φ)|2
(14)
Aqs ) a9ζ + a10∂15
(15)
Bqs ) b9ζ + b10∂11
(16)
It can be seen from the above equations that the anisotropic (B) parameters for Si(111) samples always depend on both surface and bulk susceptibilities, ∂11 and ζ, respectively. This is not the case for Si(001) samples, for which the surface-derived response does not include an anisotropic component. Thus, the SHG efficiency Rps for (001) samples has the form given by eq 17, and the anisotropic parameter is described by eq 18.
) |B(001) sin(4φ)|2 R(001) ps ps
(17)
B(001) ) b11ζ ps
(18)
Equation 18 shows that the bulk anisotropic response ζ is isolated by a measurement of |B(001) ps |. A comparison of the (p, s) SHG efficiencies for Si(001) and Si(111) samples, which gives the ratio |Bps|/|B(001) ps |, therefore provides a means of separating the surface and bulk contributions to the anisotropic response of Si(111).20,22 4. Results and Discussion
The isotropic and anisotropic parameters in eqs 3-7 are related to the surface and bulk nonlinear susceptibilities of Si(111) as shown in eqs 8-13. In these and the following similar equations
App ) a1ζ + a2(γ/2 + ∂31) + a3(∂33 - ∂31) + a4∂15 (8) Bpp ) b1ζ + b2∂11
(9)
Asp ) a5ζ + a6(γ/2 + ∂31)
(10)
Bsp ) b3ζ + b4∂11
(11)
Bps ) b5ζ + a6∂11
(12)
Bss ) b7ζ + b8∂11
(13)
the a and b coefficients are known complex constants that can be evaluated from the angle of incidence and linear optical constants of the interface, as described in detail in ref 6. 2 in eqs 8 and 10 is the dielectric constant of silicon at the second harmonic frequency. The experimental quantities |A|, |B| and θ for s- and p-polarized fundamental and second harmonic radiation provide only limited information on the relative magnitudes of the nonlinear susceptibilities. Additional information can be obtained by considering the SHG efficiency for mixed s- and p-polarized fundamental radiation. In the q-polarization configuration, the
4.1. Rotational Anisotropy of SHG for Chemically Modified Si(111) Surfaces in Air. In Figure 1 are shown plots of the rotational anisotropy of the SHG efficiency Rpp for the (p, p) polarization combination for a fundamental wavelength λ ) 830 nm, for Si(111) samples in air including the native oxide and surfaces terminated with hydrogen (-H), decyl (-C10H21) and decyloxy (-O-C10H21) monolayers. These plots show the variation of the SHG efficiency with chemical modification of the surface, with all data normalized with respect to a reference SHG signal from a quartz crystal. The chemically modified samples were found to be stable in air over a period of at least 1 h, as judged by the repeatability of SHG measurements during this period. Several measurements were undertaken with samples mounted in a dry nitrogen purge box, and the results were indistinguishable from those obtained without using the purge box. These results are consistent with literature reports13-16 and unpublished work from this laboratory that indicates that the chemically modified samples are stable in air for many hours. In all cases it was confirmed that the SHG signal was quadratic in the laser fluence. The fluence on the samples (∼10 mJ cm-2) was well below the damage threshold of silicon, and there were no indications that the SHG measurements produced irreversible changes in the samples. The solid lines in Figure 1 show fits to the data using eq 5. The parameters obtained from the fits are given in Table 1, which also shows the results obtained for the polarization combinations (s, p), (q, s) and (p, s), with all data referenced to the same incident fluence. The results in Table 1 were found to be reproducible in the range (20%, for at least three repeat measurements with different samples.
SHG at Chemically Modified Si(111) Surfaces
J. Phys. Chem. B, Vol. 104, No. 32, 2000 7671
Figure 2. SHG parameters for the indicated Si(111) surfaces in air, with λ ) 830 and 775 nm. The vertical scales in arbitrary units are the same for each parameter. The solid and broken lines show the magnitude of the contribution of the bulk anisotropic response for 830 and 775 nm, respectively.
TABLE 2: Rotational Anisotropy of SHG Efficiency for Chemically Modified Si(111) Surfaces in Air, with λ ) 775 nma Si(111) sample
Figure 1. Rotational anisotropy of SHG efficiency Rpp with λ ) 830 nm, for the indicated chemically modified Si(111) surfaces and Si(111) with a native oxide film, in air. The SHG efficiency in arbitrary units is shown on the same scale in each plot, with all data normalized to the same incident fluence. The abscissa is at the zero of SHG efficiency in each plot.
TABLE 1: Rotational Anisotropy of SHG Efficiency for Chemically Modified Si(111) Surfaces in Air, with λ ) 830 nma Si(111) sample parameter
Si-H
Si-decyl
Si-O-decyl
native oxide
|App| |Bpp| θpp |Asp| |Bsp| θsp |Aqs| |Bqs| θqs |Bps|
0.079 0.37 154
