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Dec 16, 2013 - Nitroxidation of H‑Terminated Si(111) Surfaces with Nitrobenzene and Nitrosobenzene. Fangyuan Tian, Yuexing Cui, and Andrew V...
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Nitroxidation of H‑Terminated Si(111) Surfaces with Nitrobenzene and Nitrosobenzene Fangyuan Tian, Yuexing Cui, and Andrew V. Teplyakov* Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: Ultrathin silicon oxynitride films have attracted substantial attention as gate dielectrics. In this work, we investigate a wet-chemistry approach to introduce a monolayer silicon oxynitride film by reacting Hterminated Si(111) surface with nitro- or nitrosobenzene. The bifunctional aromatic molecules serve as a source of oxygen and nitrogen, while phenyl ring remains intact after the reaction and can be used for further modifications or as a resist. Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to confirm surface reaction and to quantify surface coverage. Density functional theory (DFT) cluster calculations were employed to explore feasible reaction pathways, predict the vibrational spectra of possible reaction products, and compare the observed XPS binding energies with calculated N 1s core level energies. Substantial differences in reactions of these two molecules on silicon provide the opportunity to tune the nitroxidation process to achieve the desired levels of oxygen and nitrogen by chemical means at relatively mild conditions. nitrogen-containing compounds, such as nitrosobenzene,9 diazo-compounds,10 and azides,11,12 undergo cycloaddition reactions on Si(100) surface, ultimately leading to the formation of the substituted nitrenes at elevated temperatures. Similar cycloaddition reactions were demonstrated on other surfaces, such as TiO2(110).13 However, these reactions can only be initiated in ultrahigh vacuum (UHV) conditions and a mixture of products is obtained following thermal annealing. Alternatively, our group has previously shown that the reactions of nitrobenzene with H-terminated silicon Si(100) and Si(111) surfaces at room temperature proceed via dehydrative cyclocondensation process that leads to the formation of a mixture of phenylnitrene and nitrosoadducts and the removal of a water molecule.5 It has to be emphasized that in these reactions water is the only byproduct; hydrogen comes from H-terminated silicon, and oxygen is from nitrobenzene. Since the amount of water corresponds to a monolayer of hydrogen atoms on silicon, this tiny amount does not affect the reaction of cyclocondensation and does not lead to the additional oxidation of the silicon substrate. In order to understand the mechanism of nitrobenzene reaction with H-covered silicon, our group also investigated [2 + 2] cycloaddition reaction of nitrosobenzene with a clean Si(100) surface at room temperature,9 because the product of this cycloaddition is essentially identical to the main product of nitrobenzene cyclocondensation with H-terminated silicon. In this work, we investigate a wet-chemistry approach to modify H-terminated Si(111) surface with bifunctional

1. INTRODUCTION Studies of silicon oxynitride films have attracted substantial attention in recent years due to their high thermal stability and low leakage current. These properties suggest that silicon oxynitride films can be used as gate dielectrics and compete with silicon oxide and silicon nitride in multiple applications. Some of the previous studies targeted the reactions of NO and O2 on clean Si(100) to produce an ultrathin SiOxNy film at 800 °C.1 Alternatively, nitromethane and nitroethane were studied as a potential source of oxygen and nitrogen for silicon nitroxidation.2,3 The latter studies lead to carbon contamination of the films produced; however, they also inspired further exploration of bifunctional molecules and their reactions on silicon surfaces to produce oxynitride films with tunable chemical and electronic properties. For example, nitrobenzene readily reacts with a clean Si(100) surface at room temperature4 and above.5 In such a process, aromatic ring is introduced into the system, which can allow tuning dielectric gate current by applying a magnetic field affecting the π-electrons of the aromatic ring. In addition, phenyl ring as a functional group provides further opportunities to chemically modify silicon surface. Both theoretical calculations and experimental studies have already confirmed the reaction of nitrobenzene on a clean Si(100)4,6 surface through 1,3-dipolar cycloaddition that leads to the formation of a Si−O−N−O−Si five-member-ring. This initial attachment is followed by oxygen atom insertion into the Si−Si backbond and the formation of a Si−O−N−Si nitrosoadduct. Following this reaction step, the oxygen of the nitrosoadduct can also move subsurface to form a Si−N−Si phenylnitrene adduct. Similarly, other nitro- compounds,3,7,8 including nitromethane and nitroethane, as well as alternative © 2013 American Chemical Society

Received: October 16, 2013 Revised: December 13, 2013 Published: December 16, 2013 502

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min in 40% ammonia fluoride solution to form a well-ordered hydrogen-terminated Si(111) surface. 2.2.2. Preparation of Nitro- and Nitrosobenzene-Functionalized Si(111) Surface. Nitrobenzene. The freshly prepared hydrogen-terminated Si(111) sample was immediately transferred into a three-neck round-bottom flask with 5 mL of neat nitrobenzene. The flask was outfitted with a condenser and an argon flow needle. Nitrobenzene was warmed to room temperature prior to placing into the reaction flask to prevent water condensation from ambient, as the sample was stored in a refrigerator. Once in the flask, nitrobenzene was bubbled with Ar through a needle dipped under the liquid surface to remove air for at least 30 min. The needle was moved to above the solution level after H−Si(111) was transferred to the flask. Then, the flask was immersed into a preheated oil bath and the mixture was refluxed for 2 h under N2 atmosphere at the temperature of the oil bath of 110 °C. After the reaction, the sample was thoroughly rinsed with THF and this step was immediately followed by an appropriate analytical characterization. Nitrosobenzene. A volume of 5 mL of 0.6 M nitrosobenzene solution in distilled THF was prepared in a 25 mL three-neck round-bottom flask fitted with a condenser. Nitrosobenzene is a colorless solid powder at room temperature, and its melting point is 65−69 °C, which indicates it would be very difficult to use pure liquid nitrosobenzene for the reaction with a silicon surface. Thus, THF was used as an organic solvent to dissolve nitrosobenzene powder. THF was previously shown not to affect nitrogen nucleophilic reactions on silicon surfaces in similar processes.15 The freshly prepared nitrosobenzene/THF solution gives a peacock green color; however, the solution may change to a darker green color after thermal reaction if the concentration of nitrosobenzene is too high. Using LC/MS, we determined that the darker color was caused by nitrosobenzene self-reaction. Following a series of concentration dependent experiments, the concentration of nitrosobenzene in THF was chosen to be 0.6 M to prevent self-reaction during the course of surface modification process. The experimental setup used for nitrosobenzene reaction with H-terminated silicon was the same as that for nitrobenzene described above. The freshly prepared H-terminated Si(111) wafer was transferred into this nitrosobenzene/THF solution, then the solution was refluxed on an oil bath at 55−60 °C for 2 h under Ar flow. After reaction, the sample was rinsed with dry THF and this was immediately followed by an appropriate analytical characterization. 2.3. Characterization Techniques. 2.3.1. Fourier-Transform Infrared (FTIR) Spectroscopy. Single beam spectra were collected using a Nicolet Magna-IR 560 spectrometer with a liquid-nitrogen-cooled external MCT detector. The FTIR spectra were collected in the range of 4000−650 cm−1 with a 60° angle with respect to the incoming infrared beam with splitter aperture of 100. A total of 512 scans per spectrum and a resolution of 8 cm−1 were used to collect all the spectra. The native-oxide covered or hydrogen terminated Si(111) wafers were used to collect background spectra, as indicated below. 2.3.2. X-ray Photoelectron Spectroscopy (XPS). The XPS spectra were collected on a PHI-5600 instrument with a monochromatic Al Kα anode at energy of 1486.6 eV. The measurements were performed in a vacuum chamber with a base pressure of 1 × 10−9 Torr. The takeoff angle was 45° with respect to the analyzer. The survey spectra were collected over the energy range of 0−1000 eV. The high-resolution spectra for

aromatic molecules, nitrobenzene and nitrosobenzene, and to compare the mechanisms of these interactions. H-terminated Si(111) surface presents a unique opportunity for this comparison because, unlike the H-terminated Si(100) prepared by wet chemistry methods, H−Si(111) can be prepared to have only one type of surface species, monohydride. Thus, these reactive sites are the only ones participating in surface reactions and that can be modeled by density functional theory (DFT) calculations to help understand the reactions of this surface with nitro- and nitroso-compounds. Several challenges in controlling the reactions of nitrobenzene and nitrosobenzene with H-terminated Si(111) surface will be addressed. First is the effect of water as a byproduct. It was demonstrated to be minimal in the previous studies of nitrobenzene but has to be taken into account when a new process, the reaction of nitrosobenzene, is analyzed. Second, the fact that the phenyl ring remains intact following the condensation process has to be confirmed. Finally, we will demonstrate that the differences in the reactions of nitro- and nitrosobenzene with H-terminated Si(111) surface will allow for tuning the amount of oxygen and nitrogen in the target ultrathin oxynitride films. Spectroscopic studies with infrared spectroscopy and XPS will be utilized to investigate the chemical identities of surface functionalities formed and modified, and also to quantify the amount of nitrogen and oxygen introduced by cyclocondensation process. DFT calculations will be used to interrogate the mechanistic differences between the reactions of these two functional aromatic compounds with H-terminated Si(111) surface to offer the roadmaps for successful silicon oxynitridation with nitro- and nitroso-compounds.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Materials. The n-type double-side polished Si(111) wafers (>0.1 Ω-cm resistance, 500 μm thickness) were obtained from Virginia Semiconductor. All chemicals were reagent grade or better and used as received: hydrogen peroxide (Fisher, 30% certified ACS grade), ammonium hydroxide (Fisher, 29% certified ACS plus grade), buffer-HF improved (Transene Company, INC.), ammonia fluoride (Fluka, Sigma Aldrich, 40% in H2O), hydrochloric acid (Fisher, 37.3% certified ACS grade), nitrobenzene (Acros, 99+% certified ACS grade), nitrosobenzene (Fluka, 97.5+% certified ACS grade), methanol (Fisher, 99.9%), and tetrahydrofuran (THF; Fisher, distilled from Na/benzophenone). The deionized water with 18 MΩ· cm resistivity used to rinse the surfaces and containers was from a first generation Milli-Q water system (Millipore). 2.2. Sample Preparation. 2.2.1. Preparation of Hydrogen-Terminated Si(111) Surface. The hydrogen-terminated Si(111) surface was prepared by a modified RCA cleaning procedure.14 The Teflon beakers were cleaned in a SC1 solution which contains 4:1:1 Milli-Q water, hydrogen peroxide, and ammonium hydroxide for 30 min with N2 bubbling through in an 80 °C water bath. Then, the precut Si(111) samples were placed into precleaned beakers and covered with SC1 solution for 10 min on the same water bath. After rinsing with Milli-Q water, the clean wafers were etched in HF buffer solution for 2 min and rinsed again with Milli-Q water. After that, the wafers were placed in a freshly prepared SC2 solution of 4:1:1 Milli-Q water, hydrogen peroxide, and hydrochloric acid on an 80 °C water bath for 10 min to grow a silicon oxide layer. Following this step, the rinsed wafers were etched in an HF buffer solution again for 1 min, followed by 6 503

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each element, Si 2p, O 1s, C 1s, and N 1s, were collected over the range of 20 eV using an X-ray voltage of 13.5 keV at 0.1 eV/ step. The data analysis was performed using Casa software. All peak positions and relative sensitivity factors were calibrated to the main component of the C 1s peak, which was taken to have the binding energy of 284.6 eV. 2.3.3. Computational Details. The optimized structures and corresponding predicted FT-IR spectra and binding energies for comparison with XPS experiments, were computed using DFT with a B3LYP/6-311+G(d,p) approach16−19 utilizing Gaussian 09 suite of programs.20 Hydrogen-terminated Si(111) surface was modeled by a Si17H24 cluster representing two topmost neighboring silicon atoms as reactive sites, in reactions with either a nitrobenzene molecule or a nitrosobenzene molecule. No subsurface atoms were fixed during the majority of the calculations. Several intermediate structures were also examined by fixing the two bottom layers of silicon atoms, as summarized in the Supporting Information section. To confirm the structures of the surface products and to determine the possible surface reaction mechanisms, computational results are used to compare with FTIR and XPS observations. Based on Koopmans’ theorem, N 1s core-level energy is predicted for several possible reaction products, and then calibrated by a correction factor, which was found to be 8.76 eV based on our previous investigations.21 All the N 1s energy predictions from calculations involved in the comparison with the experimental results in this study are calibrated with the same correction factor. Computationally predicted infrared frequencies were scaled by the factor of 0.95 for comparison with experimental data. This scaling factor was obtained from our previous studies of a variety of similar cluster models.22,23 Synchronous transitguided quasi-newton (STQN) method was employed to determine transition states, which was confirmed by the presence of a single negative eigenvector in the vibrational frequency calculations.

Scheme 1. Frontier Orbital Analysis and the Proposed Mechanistic Steps of Dehydrative Cyclocondensation Reaction of Nitrobenzene and Hydrogen-Terminated Si(111) Surfacea

a The initial adsorption is followed by the formation of a Si−N−O−Si bridge structure. Following this step, the phenylnitrene product of dehydrative cyclocondensation if formed upon oxygen insertion into the subsurface silicon layer.

into the highest occupied molecular orbital (HOMO) of nitrobenzene is very substantial. Following the approach described above for nitrobenzene reaction, the mechanism of nitrosobenzene reaction with Hterminated Si(111) surface is proposed as shown in Scheme 2. Unlike nitrobenzene, there is only one oxygen atom in each nitrosobenzene molecule, which may result in a simpler reaction scheme on H-terminated Si(111), since the oxygen atom could be expected to be removed from the surface as a water molecule, leaving the produced surface oxygen-free. However, from our previous study of ammonia reaction on Si(111) surface,15 if the Si−NH−Si structure is produced on this surface as a result of chemical modification, it is highly strained, which is different from similar adducts on Si(100),26 and may lead to very quick surface oxidation upon exposure to ambient conditions. In Scheme 2, nitrosobenzene adsorbs on the H-terminated Si(111) surface via O−H bonding, which requires additional explanation. In the frontier orbital analysis similar to that performed for nitrobenzene, the HOMO is located on the −NO bond. Thus, in principle, it could be expected that not only oxygen atom would interact with the hydrogen atom of the silicon surface, but that the nitrogen atom of the nitroso group could also approach and attack the surface Si−H bond by a nucleophilic substitution. We did consider this possibility; however, the O−H hydrogen bond formed in the case of nitrosobenzene dominated any adsorption process that we modeled and any starting structure ultimately converged into the same adduct, with the oxygen atom of the nitroso group interacting with the hydrogen of the silicon surface. The details of this study will be shown in the next part. After this first step, the reaction scheme proposes the formation of the phenyl nitrene surface adduct and elimination

3. RESULTS AND DISCUSSION 3.1. General Consideration of the Reaction Mechanisms. Scheme 1 shows the general cyclocondensation reaction for nitrobenzene on H-terminated Si(111) surface. Basically, the nitro-functional group will interact with the hydrogen atom from the silicon substrate to form a water molecule, and the remaining oxygen will eventually migrate into the subsurface. From the frontier orbital analysis, it can be proposed that oxygen or nitrogen from nitrobenzene molecule can engage the hydrogen atom of the Si(111) surface. The average Si−H bond dissociation enthalpy is 318 kJ/mol, while the formation of Si−O gains 452 kJ/mol and for Si−N this number is approximately 355 kJ/mol.24 Thus, although the thermodynamics seems to yield preferentially the Si−Ocontaining surface adducts, both Si−O and Si−N formation is feasible. The detailed DFT computational studies of both reaction pathways are important and are presented below. Following the initial attachment, the cyclocondensation process is expected to produce a water molecule and a nitrosoadduct with a Si−N−O−Si ring structure. Finally, similarly to the previous studies of nitrobenzene and nitrosobenzene on a clean silicon,5,9,25 a phenylnitrene adduct with a Si−N−Si structure is proposed to form, as the remaining oxygen atom inserts into the subsurface silicon layer. Thus, a monolayer thick silicon oxynitride functionalized by phenyl rings is expected to form. The fact that the phenyl ring remains intact has to be proven experimentally since the involvement of this chemical group 504

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of a single water molecule, with an additional hydrogen atom provided by a neighboring Si−H surface site. This chemical process is expected to produce a layer of silicon nitride-like structures that is expected to be converted to silicon oxynitride following exposure to ambient conditions. 3.2. DFT Computational Studies of Possible Reactions of Nitro- and Nitrosobenzene on H−Si(111). In order to explore the feasibility of surface reaction mechanisms proposed above, DFT computational studies of the reactions of nitro- and nitrosobenzene with H-terminated Si(111) surface were performed. The condensation reaction pathway of nitrobenzene on a hydrogen-terminated Si(111) surface modeled by a Si17H24 cluster is shown in Figure 1. Nitrobenzene initially adsorbs on the H−Si(111) surface with a stabilization energy of 4.0 kJ/mol below the energy level of the reactants. A 143.8 kJ/ mol energy barrier is overcome for a hydrogen atom to be removed from the surface and to attach to one of the oxygen atoms of the nitro-group. This step can potentially lead to the formation of either a N−Si bond or an O−Si bond, as discussed above. However, the process of N−Si bond formation in this particular case is endothermic. Not only that, but any further transformations of the proposed product would require substantial barriers as illustrated in Figure 1. Thus, it is expected that surface transformations of the initial weakly adsorbed nitrobenzene molecule lead to the formation of the Si−O bond shown in structure A of Figure 1. Following this step, the oxygen of the OH group can pick up a hydrogen atom over a very modest barrier from a neighboring Si−H site to form a Si−O−N−Si bridge structure shown as structure B in Figure 1.

Scheme 2. Frontier Orbital Analysis and the Proposed Mechanistic Steps of Dehydrative Cyclocondensation Reaction of Nitrosobenzene on a Hydrogen-Terminated Si(111) Surfacea

a

Following the adsorption of nitrosobenzene on a hydrogenterminated Si(111) surface, the water molecule is eliminated and a phenylnitrene adduct is formed.

Figure 1. Comparison of DFT-predicted reaction pathways for the reaction of nitrobenzene with a H−Si(111) surface: silicon (gray), nitrogen (blue), oxygen (red), and hydrogen (white). Structures in dashed frames correspond to the transition states. Reaction pathway shown as a bold solid line represents the most likely reaction mechanism. Hydrogen atoms terminating the silicon atoms representing subsurface layers are omitted for clarity. The positions of silicon atoms representing subsurface were not restricted in the cluster calculations. 505

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Figure 2. DFT predicted energy diagram of nitrosobenzene on H-terminated Si(111) surface represented by a Si17H24 cluster with two Si−H reactive sites: silicon (gray), nitrogen (blue), oxygen (red), and hydrogen (white). Structures in dashed frames correspond to the transition states. Hydrogen atoms terminating the silicon atoms representing subsurface layers are omitted for clarity. The positions of silicon atoms representing subsurface were not restricted in the cluster calculations.

Finally, a very stable final product (structure C in Figure 1) is formed by overcoming a 105.2 kJ/mol energy barrier for oxygen to insert into the Si−Si back-bond. Other oxygen insertion positions were also taken into consideration, for example, O insertion to the silicon sublayer involved in the phenylnitrene adduct, where structures of similar stability can be formed. It is important to emphasize here that the exact final structures may depend on the extensive surface reconstructions and are not amenable to the cluster calculations used in this study. Nevertheless, they are expected to be stable as additional Si−O bonds are formed as a result of those reactions. One more point worth mentioning is that this is the only step (and the only type of product) of nitrobenzene dehydrative cyclocondensation process that causes substantial distortion of the subsurface silicon atom positions, making it difficult to pinpoint the specific resulting product without substantially more sophisticated computational approaches. Similarly, DFT calculations have also been performed for the reaction of nitrosobenzene with a H-terminated Si(111) surface, and the predicted energy diagram is shown in Figure 2. The oxygen atom of the molecular nitrosobenzene weakly adsorbed on H-terminated Si(111) (structure D) picks up a surface hydrogen to form a transition state. This process requires overcoming an energy barrier of 146.6 kJ/mol. We also considered a nucleophilic attack of the nitrogen atom of the nitroso-group onto a surface hydrogen; however, all our attempts converged into structure E via an intermediate. Once structure E is formed, the −OH moiety of the surface adduct can attract another H from the neighboring surface site forming one molecule of water. With molecular water leaving the surface, a stable phenylnitrene adduct of structure F is formed. Similarly to the case of nitrobenzene described above and to the previously reported studies of nitrosobenzene reaction on a clean Si(100) surface,9 the exact structural arrangement of the phenylnitrene product within surface reconstruction would require further studies. The predicted

stability of the specific phenylnitrene products similar to structure F in Figure 2 can vary depending on the specific computational model and approach (especially that without accounting for the surface reconstruction and without restricting the positions of the cluster atoms representing the surface, predicting the stability of the produced phenylnitrene surface species is not reliable). Nevertheless, there is no doubt that the produced species are thermodynamically stable and their formation does not require substantial barriers. Following the feasible reaction pathways outlined above, spectroscopic studies were set up to confirm these predictions. 3.3. Experimental Confirmation of the Proposed Mechanisms of the Reactions of Nitro- and Nitrosobenzene on H−Si(111). Before proceeding with investigating surface reaction mechanisms for nitro- and nitrosobenzene interaction with H-terminated Si(111), the quality of the starting surface was addressed and the fact of surface reaction was confirmed by infrared spectroscopic studies of the Si−H stretching region summarized in Figure 3. According to the detailed previous studies, monohydrogen-covered Si(111) produces a single sharp absorption band at 2083 cm−1 in the infrared spectrum of well-prepared H-terminated Si(111) surface.14,27,28 The position of this peak is very sensitive to the surface quality and can be used to assess surface cleanliness.29,30 As illustrated in Figure 3a, the absorption feature observed at 2083.1 cm−1 in our studies confirms high quality of the H-terminated surface produced. Following nitrobenzene or nitrosobenzene modification, this Si−H feature decreased dramatically, shown as negative peaks in Figure 3b and c, where the surface presented in spectrum (a) is taken as a background. The decrease of the vibrational absorption feature does not always linearly correlate with the concentration of the corresponding species on a surface. However, it is clear from the results in Figure 3 that the decrease of the Si−H peaks following nitrobenzene (b) and nitrosobenzene (c) reactions may not account for a complete removal of hydrogen from H506

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Figure 4. High resolution C 1s XPS spectra of nitrobenzene (a) and nitrosobenzene (b) reacted with H-terminated Si(111) surface. Figure 3. Transmission infrared studies of the Si−H stretching region for the Si(111) modification: (a) H-termination by modified RCA method with thermally grown silicon oxide as a background; (b) nitrosobenzene reaction with H−Si(111) with a single beam spectrum collected for spectrum (a) as a background; (c) nitrobenzene reaction with H−Si(111) with a single beam spectrum collected for spectrum (a) as a background.

at 284.6 eV, which was used to calibrate the positions of all other peaks based on previous investigations.15 In addition, four more features can be observed within the range of 286−293 eV, with all the full width at half-maximum (fwhm) set within 2 eV during peak-fitting procedure. The peak at 286.1 eV is mostly a contribution from the C−H features of the phenyl ring.5,11 The peak at 287.3 eV corresponds to C−N and C−N−O spicies.9 These features confirm the surface reaction that involves carbon, nitrogen, and oxygen in both cases. The signature at 289.0 eV is most likely due to the adventitious CO containing species adsorbed on samples during transfer process. Finally, the peaks around 292 eV are a signature of a shakeup feature of the phenyl ring, which also indicates the successful modification of a silicon surface by nitro- and nitrosobenzene preserving the aromaticity of phenyl substituent in both cases. In Figure 5, high resolution N 1s XPS spectra were compared with DFT-predicted core-level energies based on Koopman’s theorem21 computed for key structures shown in Figures 1 and 2. The XPS spectra were collected immediately following sample preparation after a very brief (less than 10 min) exposure to ambient conditions. Thus, they are expected to provide accurate chemical information about the surface caused by the nitrobenzene and nitrosobenzene treatment. N 1s spectrum of silicon surface that is modified with nitrobenzene is shown in Figure 5a. The spectrum clearly consists of at least three features. They were fitted with the same fwhm value

covered Si(111) surface following modification. For nitrosobenzene, a quick analysis of the integrated absorption feature would correspond to over 80% of surface hydrogen removal, while for nitrobenzene the decrease of the Si−H peak area corresponds to less than 70% of hydrogen removal. The examination of the spectrum in Figure 3c corresponding to the nitrobenzene reaction also reveals an additional absorption feature observed at 2078 cm−1 likely suggesting the existence of less well-defined Si−H-containing species on this silicon surface after its modification with nitrobenzene. Overall, these studies verify the quality of the starting H−Si(111) surface and confirm the removal of surface hydrogen as a result of a reaction with either nitro- or nitrosobenzene. To understand the chemical environment of surface elements and to quantify their concentrations, reactions of nitrobenzene and nitrosobenzene on H-terminated Si(111) surface by wetchemistry methods were investigated by XPS. Survey and high resolution C 1s, N 1s, Si 2p, and O 1s spectra were collected. C 1s spectra are shown in Figure 4. For both nitrobenzene and nitrosobenzene reactions, C 1s spectra exhibit the main feature 507

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pected to be the most stable structure. However, our experimental results show that the final product on surface from the reaction of nitrobenzene with H-terminated Si(111) is a mixture of three different types of species. Each of these species may be affected by the neighboring ones, by the overall surface reconstruction, and by surface oxidation caused by the exposure of the samples to ambient conditions that can influence the kinetic barriers involved in the process. Further studies of temperature-dependent behavior are needed to explore the kinetic barriers involved in the overall reaction. However, it is important to emphasize that the surface structures observed can be quantified and all seem to maintain the integrity of the phenyl ring. In Figure 5b, the N 1s XPS spectrum following the reaction of nitrosobenzene with H-terminated Si(111) is compared with our DFT predictions. The constituent features were fitted with peaks constrained to the fwhm value of 2.0 eV. The peak assignments can be aided by our previous studies of nitrosobenzene on a clean Si(100) surface.9 Similarly to the case of nitrobenzene described in detail above, the peaks at 398.18 and 399.97 eV correspond to phenylnitrene and nitroso adducts. The peak of the nitroso adduct is observed at 0.2 eV lower binding energy level compared to that for nitrobenzene N 1s spectrum, which may reflect subtle differences in reactions and surface reconstruction caused by these two different compounds, as well as by a different level of surface oxidation following brief exposure to ambient. The small peak (corresponding to less than 10% of the overall area) at 401.84 eV seems to coincide with the molecular nitrosobenzene weakly adsorbed on a surface; however, this weakly bound state is not expected to be observed at room temperature. Most likely, this feature indicates the reaction at surface defect sites or may be caused by surface oxidation during sample transfer. Thus, the comparison between experimental XPS spectrum and DFT computational results confirmed that the resulting surface has at least two different types of products: nitrosoadduct and phenylnitrene. It is interesting to note that the ratio of two most prominent features (B and C in the case of nitrobenzene and E and F in the case of nitrosobenzene) is similar in both cases, 15:11 and 24:21, respectively. This similarity should be expected based on the similarity of structures C and F, as well as B and E. The differences in the exact peak location are most likely caused by different amounts of subsurface oxygen in the reactions of nitrobenzene compared to nitrosobenzene. Based on the presence of nitrogen in the proposed structures, we quantified surface adsorbate coverage using our XPS data with the overlayer-substrate model first proposed by Briggs and Seah32 and studied in detail on silicon substrates.15,33,34 The coverage of surface species containing a nitrogen atom was calculated by the following equation:

Figure 5. High resolution N 1s XPS spectrum of nitrobenzene (a) and nitrosobenzene (b) reacted with H-terminated Si(111) surface by wet chemistry. Experimental results are compared with DFT-predicted N 1s core-level energies shown as solid bars below the experimental spectra, corresponding to computational structures (A−C) shown in Figure 1 and structures (D−F) shown in Figure 2.

limited at 2.0 eV. The specific peak assignments were based on previously reported data for in situ nitrobenzene5 and nitromethane2 reactions on a clean Si(100) surface and on our DFT calculations. The peak at 398.18 eV should correspond to the aromatic amine and the position of this feature is fully consistent with that predicted for the phenylnitrene adduct (structure C). The most intense feature at 400.00 eV was assigned to a nitroso adduct,5 which is also fully consistent with our computational result for structure B, as shown by a corresponding bar in Figure 5. The area ratio of this main peak to the area of the peaks in the entire N 1s region is 58.1%, which indicates that the majority of the surface products correspond to the nitroso adduct structure, with one oxygen atom connected to a nitrogen. It should be pointed out that the peak positions for N 1s features have been studied in detail previously in our group15,21,23,31 and they are very sensitive to the number of oxygen atoms connected to a nitrogen atom. Thus, this spectral region is probably the most reliable in assessing the chemical environment and success of the surface modification with nitro- and nitroso-compounds. Finally, the peak at 401.80 eV, the smallest of the three features, could correspond to the intermediate structure A. The position of this peak is fully consistent with our predictions and also with the previous studies nitro-compounds adsorbed on a clean silicon surface, where the formation of the chemically similar Si−O− N−O−Si structures was observed.3,5−7 It is also possible that partially this feature corresponds to the reaction on defect sites, since it will be shown below to also be observed for nitrosobenzene reaction. As suggested above by the DFT investigations, thermodynamically, the phenylnitrene is ex-

⎤ ⎡⎛ λ sin Θ ⎞⎛ SFSi ⎞⎛ ρSi ⎞⎛ IOv ⎞⎥ ΦOv = ⎢⎜ ⎟⎜ ⎟⎜⎜ ⎟⎟⎜ ⎟ ⎢⎣⎝ aOv ⎠⎝ SFOv ⎠⎝ ρOv ⎠⎝ ISi ⎠⎥⎦

(1)

where ΦOv is the overlayer surface coverage, penetration depth λ = 2.0 nm at takeoff angle of 45° (Θ), aOv is the atomic diameter of the overlayer element, in our case nitrogen. aN = 0.26, SF is the sensitivity factor, 0.90 for Si 2p and 1.69 for N 1s, atomic density ρ equals to 5.41 × 1022 and 5.0 × 1022 atom/ cm3 for N and Si, respectively. I is the observed peak intensity from XPS experimental results calibrated by CASA. We have previously applied the same approach in our study of ammonia reaction on Cl-terminated Si(111) surface.15 In that study we 508

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found that if the starting Cl-terminated Si surface is wellprepared and nearly 100% Cl-terminated, the resulting ammonia-modified silicon surface yields approximately 55% nitrogen coverage, which indicates that on average one nitrogen atom (replacing two chlorine atoms) is connected to two surface silicon atoms, corresponding to the formation of a secondary amine as the main reaction product. In the present study we used the same scheme to determine that the surface N coverage following nitrosobenzene reaction with the H− Si(111) surface is 40.2 ± 1.5%, which is fully consistent with the infrared investigation result described in Figure 3 suggesting that over 80% of surface hydrogen is removed in a reaction with nitrosobenzene. The reaction of nitrobenzene and Hterminated Si(111) surface results in a much lower N coverage, at approximately 17.0 ± 7.0%. It is possible that the approach for surface coverage estimation of nitrogen suffers from the fact that the overlayer model assumes a submonolayer of atomic species located directly on the solid substrate, while in our case surface oxygen-containing species may affect this assumption; however, then the systematic errors would be expected to be similar in the cases of nitro- and nitrosobenzene. Another explanation is the difference in surface reactions between the two compounds, leading to different rates of oxygen incorporation into the silicon substrate and thus affecting the energy landscape of the subsequent reactions in a way that the computational models used in this work do not describe correctly. One clear conclusion based on the surface coverage estimate is that the reaction of nitrosobenzene is much more efficient than that of nitrobenzene, and also that the amount of nitrogen incorporated depends on the chemical pathway chosen, allowing for varying the composition of the surface nitroxide layer produced. It should be noted that this experimental comparison is for a reaction of neat nitrobenzene vs 0.6 M solution of nitrosobenzene in THF. However, to make the comparison more equivalent, we have also performed a reaction with 0.6 M solution of nitrobenzene instead of neat nitrobenzene and the reaction efficiency based on the obtained nitrogen coverage is essentially the same, yielding only about 20% nitrogen coverage following the reaction. Thus, the solvent used does not appear to have any substantial effect in determining the efficiency of the reaction of H-terminated Si(111) surface with nitrobenzene within the range of conditions tested here. The assessment based on the nitrogen coverage can be aided by examining the Si 2p spectral region following the condensation processes. The Si 2p region, shown in Figure 6, exhibits two main features for both nitrobenzene and nitrosobenzene reactions. The bulk silicon features are centered at approximately 99 eV, and another small peak is centered around 102.6 eV for nitrosobenzene and at 102.8 eV for nitrobenzene. This small peak corresponds to the mixture of silicon oxide (normally at ∼103 eV35) and silicon nitride (usually at ∼102 eV36). It is difficult to quantify the contribution of oxide and nitride into this feature, especially that surface defect sites may play a substantial role in its appearance; however, it is clear that in case of nitrobenzene this feature is more prominent and shifted to higher energy, suggesting higher degree of surface oxidation as expected based on the N 1s spectral region. This result is also consistent with the O 1s spectra presented in Figure S1 in the Supporting Information. There is no observed shift in the position of the single feature presented there depending on the reaction with nitro- or nitrosobenzene; however there is a clear increase in

Figure 6. Si 2p XPS spectra of nitrobenzene (a) and nitrosobenzene (b) reaction with H-terminated Si(111) surface by wet chemistry.

the amount of oxygen upon nitrobenzene reaction. This ∼12% increase can only be treated semiquantitatively because the surface can indeed be oxidized upon transfer into the XPS instrument and no additional treatment of the samples was performed following their loading. Nevertheless, the observation is consistent with higher concentration of surface oxygen following the reaction of nitrobenzene with H-terminated Si(111). It must be emphasized that the XPS spectra were recorded following a brief exposure of the samples to ambient conditions and at least some of the oxidation could be induced by this exposure. Thus, further spectroscopic methods should be used to identify the nature of surface species present following cyclocondensation reactions on H-terminated silicon. One of the most informative methods that can also be supported by computational investigations is vibrational spectroscopy, as described below. XPS spectroscopic studies and DFT calculations indicated that both cyclocondensation reactions on H-terminated Si(111) surfaces resulted in multiple products. Although the phenylnitrene adduct is the most thermodynamically stable structure predicted in both cases, nitroso-type adduct is also present in both cases at a noticeable concentration. Infrared spectroscopy can help us understand how the surfaces look like from a molecular perspective. Figure 7 shows infrared spectra of nitrobenzene (a) and nitrosobenzene (f) reacted with hydrogen-terminated Si(111). The spectra were compared with vibrational frequencies predicted by DFT calculations based on structures A−H in Figures 1 and 2. The comparison between spectroscopic results and predicted IR spectra can help us 509

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FTIR spectrum of cyclocondensation reaction of nitrosobenzene on H-terminated Si(111) is somewhat different from nitrobenzene. The spectrum shown as (f) in Figure 7 fits quite well with our predicted frequencies for structure F in Figure 2, which is thermodymically most stable final product with pheylnitrene structure. Two peaks were observed above 3000 cm−1 corresponding to symmetric and asymmetric C−H stretching peaks from a phenyl ring. Only the spectrum predicted for structure H suggested that the C−H stretch frequencies could exhibit the split similar to that. At lower wavenumbers, the CC stretching mode manifests itself as a broad peak around 1550 cm−1; however, we cannot rule out the possibility that other similar structures are also present on the surface. Similarly to the case of nitrobenzene described above, no aliphatic C−H stretch vibrations are observed following nitrosobenzene reaction with H−Si(111), assuring that the only carbon-containing surface species are the ones with phenyl ring and that carbon is not directly incorporated into the nitroxide layer. Thus, the IR spectra in both cases are consistent with the predictions based on XPS and DFT studies and represent an overlap of vibrational signatures of several stable phenylcontaining structures.

4. CONCLUSIONS Both nitrobenzene and nitrosobenzene were found to react with H-terminated Si(111) surface through cyclocondensation reaction by wet-chemistry methods. Following the reactions, the produced surface species were examined spectroscopically and the findings were corroborated by DFT calculations. Both reagents lead to the formation of a mixture of surface species but all the species can be identified and the approximate quantification can be made based on the XPS assignments. The common feature for all the species produced is that the phenyl ring is retained. At the same time, the efficiency of a reaction is substantially higher for nitrosobenzene compared to nitrobenzene. As expected based on the predicted reaction mechanisms, oxygen incorporation into the surface layer of oxynitride is higher in the case of nitrobenzene compared to that of nitrosobenzene. Although it is indeed difficult to control oxygen incorporation into the films produced by wet chemistry with nitro- and nitrosobenzene on H-terminated Si(111) because of ambient exposure, it is definitely possible to tune the amount of nitrogen in the produced single layers of oxynitride functionalized with phenyl rings. Thus, cyclocondensation reactions between nitrobenzene or nitrosobenzene and hydrogen-terminated Si(111) surface are relatively simple and effective ways to produce mixed monolayer oxynitride films on silicon and to vary surface concentrations of oxygen and nitrogen for potential applications. These oxynitride layers are chemically functionalized by phenyl groups that can be used for further treatment of the samples.

Figure 7. Infrared spectroscopy studies of cyclocondensation processes on H−Si(111): (a) cyclocondensation reaction completed by heating a neat nitrobenzene at 110 °C for 2 h with H−Si(111) surface prepared with RCA procedure; (b−d) DFT-predicted FTIR frequencies for structures A−C in Figure 1; (e) DFT-predicted spectrum for a single nitrobenzene molecule; (f) cyclocondensation reaction of nitrosobenzene with H−Si(111) surface; (g, h) DFTpredicted FTIR frequencies for structures E and F in Figure 2; (i) DFT-predicted spectrum for a single nitrosobenzene molecule. All the computationally predicted FTIR results are scaled by 0.95 to account for systematic errors in the vibrational frequency calculations.

pinpoint specific surface species. The C−H stretching peaks above 3000 cm−1, as well as the CC bending and stretching features around 1600 and 1480 cm−1 confirm that the aromatic ring remains intact after thermal reactions in both cases. The absence of aliphatic C−H stretching features (which are normally much more intense than the aromatic C−H signatures37) confirms that the aromatic ring remains intact and does not directly interact with the surface, which is one of the most important practical requirements for the proposed silicon nitroxidation scheme. In other words, carbon-containing fragments present on the surface are not a part of the silicon oxynitride layer produced but can indeed be used for either chemical modification or as a resist. The peaks assignments are also fully consistent with our previous studies of nitrosobenzene reaction with a clean and hydrogen-terminated Si(100) surfaces. From the XPS results, we know that nitroso adduct (structure B in Figure 1) is one of the dominant surface species following nitrobenzene cyclocondesation reaction on H-terminated Si(111), which is also confirmed by our IR results showing the presence of the N−O stretching feature in Figure 7 spectrum (a), but quantifying this feature is complicated by its overlap with the C−H bending modes.

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ASSOCIATED CONTENT

S Supporting Information *

summary of density functional theory computations of the proposed structures, O 1s XPS spectra of modified silicon surface, and complete refs 1 and 20. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 302-831-1969. E-mail: [email protected]. 510

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CHE 1057374). We would like to thank Prof. Robert L. Opila and his group in the Department of Materials Science, University of Delaware for the ex situ XPS data collection and useful discussions. YC acknowledges Mr. David Plastino and Chemistry Alumni Science Scholar Award.

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