Supporting information Infrared Characterization of Interfacial Si-O Bond Formation on Silanized Flat SiO 2 /Si Surfaces Ruhai Tian,, Oliver Seitz, Meng Li, Wenchuang (Walter) Hu, Yves Chabal, Jinming Gao,* Department of Chemistry, Department of Electrical Engineering, Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854 *Corresponding author, E-mail: jinming.gao@utdallas.edu Materials. Triethoxysilyl undecanal (TESU, >90%), triethoxysilyl butyraldehyde (TESBA, >90%), n-octadecyltrimethoxysilane (OTMS, >90%), Carboxyethylsilanetriol, Sodium (CSS 25% in water) were purchased from Gelest; 4A molecular sieves (1.6 mm pellets), anhydrous chlorobenzene (>99.8%), anhydrous toluene (>99.8%), anhydrous 3-aminopropyltriethoxysilane (APTES, >98%), trichlorosilane (TCS, >99%), and anhydrous ethanol (>99.8%) were purchased from Sigma-Aldrich and used without further purification. FTIR and Ellipsometry measurements. FTIR spectra were collected with a Thermo 6700 FTIR spectrometer in transmission mode under dry N 2 purge conditions with 4 cm -1 resolution and a DTGS detector. To detect both TO and LO phonon absorption, an incident angle of 74º was used. Typical data acquisition involved a series of 500 scans. H-terminated silicon wafers were used as reference to study the SiO 2 spectral region (900-1300 cm -1 ). To distinguish the spectrum of the silanized surface from that of the oxidized substrate, spectra taken just before the silanization process were used as references. Ellipsometry measurements were performed with a Sentech 800 instrument. The wave-length range 330-830 nm (3.76-1.49 ev) was used in the film-thickness measurement. Preparation and characterization of thermal and chemical SiO 2. Double sided, p-doped silicon (100) wafers (Fz, resistivity>10 Ω cm, Soitec USA Inc.) were first cleaned by immersing in 85
piranha solution (H 2 O 2 :H 2 SO 4 =1:3) for at least 20 minutes, and then SC-1 solution (1:1:5 volume ratio of 30% H 2 O 2, 28% NH 4 OH and H 2 O) and SC-2 solution (1:1:5 of 30% H 2 O 2, 37% HCl and H 2 O). Subsequently, the native SiO 2 was removed using a 30% HF solution for 30s to obtain hydrogen terminated silicon surfaces which were used as references for the IR spectra of SiO 2. Both thermal and chemical SiO 2 were grown on hydrogen terminated silicon. Thermal oxide layers with thicknesses of 3.4 and 4.3 nm were grown with a conventional oxidation furnace. Temperature was controlled at 850 and the mixture gas was 10 % O 2 in N 2. Thermal SiO 2 with 0.8-2.7 nm thickness was obtained by etching 3.4 nm SiO 2 wafer with a 0.05% HF solution for different times, and then rinsed with DI water. The SiO 2 thicknesses were determined from the integrated area of the TO peak in the normal incidence FT-IR spectrum (referenced to hydrogen terminated silicon surfaces), normalized to the ellipsometrically determined thickness of 21 Å as standard. 1 Fresh chemical SiO 2 layers were grown in 80 Piranha solution for different times. The SiO 2 thicknesses were measured by ellipsometry with a refractive index of 1.465 for SiO 2. All the samples were immediately transferred into the FTIR chamber with nitrogen flowing after the surfaces were blow dried with pure nitrogen. The LO, TO peak areas were integrated with PEAK 4.0 software and OMNIC. Silanization of chemical SiO 2. Each SiO 2 sample was cut into rectangles (2 cm 3 cm) before surface modification. APTES and TESBA solutions used in the self-assembly process were 1% in ethanol. Before SiO 2 wafer immersion, the molecules were mixed in the solvent for 10 minutes. After 40 min, the modified surfaces were rinsed with ethanol for 15 seconds. The self-assembly of CSS on SiO 2 surface was achieved by immersion of a SiO 2 sample in a 0.5% CSS solution in water at ph 4.0 (HCl adjusted). The modified surfaces were rinsed by DI water for 15 seconds after 40 minutes reaction. Silanization of silicon oxide surfaces with TCS was carried out in a N 2 purged glove-box. The sample was kept inside the 0.1% silane solution for ~20 hours, rinsed thoroughly with toluene and ultrasonicated for 5 minutes in toluene. The self-assembly of TESU and OTMS were carried out in anhydrous chlorobenzene with 1% silane for at least 1 hour. The modified surfaces were rinsed with plenty of chlorobenzene and ethanol. The positions of initial oxide LO peak and corresponding newly formed LO peak are listed in Table S1.
Table S1. LO peak frequency of initial chemical SiO 2 and the newly formed silane-sio 2 interface Silane molecules APTES TESBA TESU CSS TCS OTES SiO 2 LO frequency (cm -1 ) 1212 1211 1219 1206 1211 1215 New LO frequency (cm -1 ) 1225 1219 1229 1221 1223 1222 Immobilization of polycondensed silane molecules on SiO 2 surfaces. The deposition of polycondensed TCS on SiO 2 surface was carried out by fast immersion of the SiO 2 /Si (100) wafer into a mixture of TCS with excess water (1/10 or 1/5 v/v). The formation of polycondensed TESU on SiO 2 surface was achieved by immersion of the SiO 2 /Si wafer into anhydrous chlorobenzene containing 0.5% TESU for 48 hours. The chlorobenzene solution was kept open so that the silane molecules could react with water molecules from the atmosphere. After the immobilization of polycondensed silane, a white thin layer was observed with the naked eye. Figure S1 shows that a polycondensed TESU and TCS on SiO 2 /Si surfaces do not have a detectable LO absorption peak in the 1200-1260 cm -1 region. Instead, the polymerized films are characterized by two broad bands centered at 1050 and 1125-1160 cm -1, corresponding to TO- and LO-like modes of the polycondensed Si-O-Si bonds. a) Poly-HSiCl3 Si-O-Si H-Si 5-3 b) Poly-HSiCl3 1225 cm -1 10-3 500 1000 1500 2000 2500 3000 1230 cm -1 Poly-(C2H5O)3Si(CH2)10CHO 10-2 Si-O-Si C=O CH2 Poly-(C2H5O)3Si(CH2)10CHO 500 1000 1500 2000 2500 3000 1000 1100 1200 1300 Figure S1. Differential IR spectra of a) Poly-HSiCl 3 and Poly-(C 2 H 5 O) 3 Si(CH 2 ) 10 CHO on SiO 2. b) Comparison of Si-O absorption between the initial oxide (dash) and polycondensed silane (solid). The SiO 2 spectra are referenced to H-Si surfaces and the polycondensed silane spectra are referenced to SiO 2 /Si surfaces. The LO peaks of initial SiO 2 /Si are marked with arrows.
Silanization of different thicknesses of thermal SiO 2 by TESU. Preparation of thermal SiO 2 with different thicknesses (0.8-3.4 nm) was described previously. Before surface silanization, the samples were rinsed sequentially with SC-1 and SC-2 solutions for 1 min, followed by a piranha treatment for 1 min. The self-assembly of TESU was carried out in chlorobenzene with 1% TESU for 1 hr. The modified surfaces were then rinsed with plenty of chlorobenzene and ethanol. Figure S2 shows the LO peak positions of the initial SiO 2 and TESU-silanized SiO 2 as a function of the thermal SiO 2 thickness. As expected, LO peaks blue shift in the silanized samples from initial SiO 2 at all thickness values. The magnitude of LO shift decreases with increasing thickness of SiO 2. More specifically, the LO frequency shifts ( LO = LO TESU-SiO2 - LO SiO2 ) are 11, 10, 3 and 1 cm -1 for SiO 2 with 0.8, 0.9, 1.8 and 3.4 nm thickness, respectively. This behavior is consistent with the fact that, to a first approximation, the magnitude of the shift is proportional to the derivative of the initial thickness. When the LO peak position increases linearly with oxide thickness, as is the case for the ultra-thin chemical oxides, the blue shift is constant (~12 cm -1 ). For the denser SiO 2 films, the LO peak position is not linear and increases less rapidly, leading to smaller and decreasing magnitude of the blue shift. a) 5-3 3.4 nm 1.6 nm 0.9 nm 0.8 nm 900 1000 1100 1200 1300 1400 b) LO peak position (cm -1 ) 1260 1255 1250 1245 1240 1235 1230 1225 1220 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Thickness of SiO2 (nm) Figure S2. a) Differential IR spectra of TESU-silanized thermal SiO 2 with different initial SiO 2 thicknesses. The IR spectra of initial thermal SiO 2 were shown as the dashed lines. Filled and open circles represent the LO peaks in the silanized and initial SiO 2, respectively. b) LO peak frequency (cm -1 ) as a function of thermal SiO 2 film thickness for the TESU-silanized (open squares) and initial SiO 2 (filled squares).
Kinetic analysis of TESU silanization on SiO 2. The vapor deposition of TESU on SiO 2 surface was carried out in a conical flask under dry conditions. The conical flask and the homemade sample holder were dried in an oven at 180 for 24 hours before use. An anhydrous chlorobenzene solution with 0.5% silane molecule was used as a vapor source. Chlorobenzene was chosen as solvent because its boiling point (131 ) is very close to the boiling point of TESU (130 ). The chlorobenzene was dried with 4Å molecule sieve before use. After introduction of the silane/ chlorobenzene solution into the conical flask, the system was sealed promptly and purged with dry nitrogen. The flask was warmed on a hotplate to keep the chlorobenzene temperature at 130 for 3 hours. After the vapor deposition process, the sample was rinsed with chlorobenzene and blow dried with nitrogen. The sample was immersed in phosphate buffer solution (ph=7.4, 50 mm) to induce the surface silanization and self-assembly process. The sample was then removed from the solution at selected times (30, 90, 180, 390 min), and rinsed with chlorobenzene and ethanol before FTIR analysis. Reference (1) Queeney, K.; Weldon, M.; Chang, J.; Chabal, Y.; Gurevich, A.; Sapjeta, J.; Opila, R. J. Appl. Phys., 2000, 87, 1322-1330.