Controlling the Formation of Metallic Nanoparticles on Functionalized

Article pubs.acs.org/JPCC

Controlling the Formation of Metallic Nanoparticles on Functionalized Silicon Surfaces Kathryn A. Perrine, Jia-Ming Lin, and Andrew V. Teplyakov* Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: With scaling down the size of the features in modern electronic devices, it becomes vital to control surface reactions at the interface for clean deposition on semiconductor substrates. Chemical functionalization of silicon surfaces provides a new approach for tuning the structure and properties of the interfaces formed with this semiconductor. The functionalized surfaces, such as H− Si(100), NH−Si(100), and NH2−Si(100) can be used to prevent surface oxidation at the silicon interface, and OH−Si(100) can be utilized to limit surface diffusion in a reaction with a metal−organic precursor. A model copper metal−organic precursor, copper (hexafluoroacetylacetonato)vinyltrimethylsilane or Cu(hfac)VTMS, was used to grow copper nanostructures by chemical vapor deposition (CVD), as verified by atomic force microscopy (AFM), infrared spectroscopy (MIR-FTIR), X-ray photoelectron spectroscopy (XPS), and temperatureprogrammed desorption (TPD) supported with density functional theory calculations (DFT). These methods help to follow surface reaction products and kinetics of surface processes. The NH2−Si(100) surface yields the largest copper nanostructures at room temperature, and this surface is the most reactive in the CVD process. Understanding the molecular-level mechanisms of the copper deposition onto functionalized surfaces will help to control the nanostructure formation, their properties, and the interface with the solid substrate.

1. INTRODUCTION With the decreasing lateral dimensions of modern microelectronic device components, the thickness of the multiple films making up a device is also becoming crucial. For example, according to the International Technology Roadmap for Semiconductors,1 the barrier film layers that prevent interdiffusion of deposited metals and semiconductor-based support, such as in copper wiring, are quickly approaching the requirement for a few nanometer thickness. As these films and layers become thinner, the corresponding interfaces are becoming especially important.2 To maintain electrical efficiency needed for the optimal operation of interconnects, these interfaces need to be well-defined. To build and maintain them properly, chemical reactions leading to their formation have to be understood, controlled, and tuned on the atomic scale.3−5 Copper is the most common metal used in interconnects. Although there is a multitude of methods to grow or deposit copper on solid supports, including physical vapor deposition (PVD) or electrodeposition, chemical vapor deposition (CVD) approaches provide the most control over the interfacial reactions and produce conformal growth. Although PVD and electrodeposition methods provide relatively straightforward pathways for producing multilayer structures, most of them yield disordered, nonconformal, and ill-defined interfaces. With CVD, chemical control of the surface processes provides a unique handle for tuning the interface properties; however, organic contamination leftover from the ligands, used to give © 2012 American Chemical Society

the metal−organic precursor volatility, can pose a problem. A deposition approach similar to CVD, atomic layer deposition (ALD), uses surface-limiting reactions to control the surface/ interface formation by utilizing surface functionalities as the starting point for film growth.1,3 To date, the most practical and well-characterized silicon substrates for ALD applications are oxide-covered and H-terminated silicon single crystals. Halogenated silicon surfaces are also used, but they are better known as starting points for yielding Si−C bonds at the interface than for production of solid inorganic films.6−10 There have only been limited studies of other types of functionalized surfaces, such as those of ammonia-exposed silicon. These surfaces provide high reactivity toward metal−organic precursors and have been shown to form stable Si−N−metalcontaining interfaces. Ammonia-functionalized silicon surfaces have been demonstrated to reduce interface carbon contamination from dimethylamino ligands used in organometallic precursors to grow titanium nitride films.11−13 Some of the surface reactions involved in production of these interfaces may even be catalytic in nature, similarly to the processes catalyzed by silicon nitride.14−17 The work presented below demonstrates that silicon surfaces functionalized by exposure to ammonia have high reactivity for surface reactions leading to the formation of copper nanostructures, that can in turn be used Received: April 16, 2012 Revised: May 31, 2012 Published: June 1, 2012 14431

dx.doi.org/10.1021/jp3036555 | J. Phys. Chem. C 2012, 116, 14431−14444

The Journal of Physical Chemistry C

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the surface reaction and the diffusion of surface fragments play a major role in the kinetics of deposition and nanostructure formation; however, the overall amount of copper deposited is limited by the amount of reducing agent (Si−H or Si−NHx entities) available on a surface, similarly to an ALD process. In addition, these reactions show that the starting substrate functionalities and a number of defects determine the resulting metal nanostructure growth. These model studies will supplement the spectroscopic investigations of the silicon nitride surfaces24 that can be used to prevent oxygen diffusion into the semiconductor substrate and also provide a number of spectroscopic and microscopic benchmarks for the future investigations of metallic nanostructures on a variety of substrates. The control over the nanostructure shape and size distribution, the oxidation state of the metal in the topmost layers of the metal structures, and ligand removal by chemical processes will be beneficial not only for the technologies directed toward further growth of the metal layer but could also bring forward potential novel applications in heterogeneous catalysis, energy conversion, and plasmonic materials.

for further control of metal deposition or provide a unique and tunable platform for the studies of the nanostructures themselves. This paper focuses on the single crystal silicon surfaces terminated with hydrogen (H−Si(100) or H−Si(111)), NH2 groups (NH2−Si(100)) and hydrogen, NH groups (NH− Si(100)) and hydrogen, and the mixture of the NH2- and NHcontaining functionalities (NHx−Si(100)) and hydrogen. The typical surface reactive sites and the steps for preparation of selected surfaces based on ammonia exposure are presented in Scheme 1. Scheme 1. Schematic Representation of the Surface Reaction on a Clean Si(100) Surface Following Ammonia Exposure

2. METHODS 2.1. Surface Preparation in Ultrahigh Vacuum and Multiple Internal Reflection−Fourier Transform Infrared Spectroscopy (MIR-FTIR). An ultrahigh vacuum (UHV) chamber, held at a base pressure lower than 1 × 10−9 Torr, was used to produce the functionalized silicon surfaces and react them with the Cu(hfac)VTMS precursor. This chamber is equipped with an Auger electron spectrometer, a setup for low energy electron diffraction, SRS mass spectrometer for verifying the characteristics and purity of the compounds dosed, and an ion gun for surface cleaning. It is also coupled with a Nicolet Magna 560 infrared spectrometer. A Si(100) trapezoidal crystal with 45° beveled edges (wafers from University Wafer, cut at Hickory Hill Designs) was set up to allow for multiple internal reflection. The clean Si(100)-2 × 1 surface was prepared by cycles of argon sputtering at 1 keV at an argon pressure of 3.5 × 10−5 Torr for 1 h. Between sputtering cycles the silicon surface was annealed to approximately 900−1000 K using an e-beam heater setup described in detail previously.21 During the last annealing cycle, the surface was annealed to temperatures around 1100 K for 20 min for a (2 × 1) reconstruction to form. To functionalize the silicon surfaces, molecular species were dosed in the UHV chamber. For H-termination, the Si(100)-2 × 1 surface was preheated to 650 K before a 5000 L exposure to molecular hydrogen (research purity, Matheson) in the presence of a hot tungsten filament for the MIR-FTIR studies. For NH2−Si(100) surface termination, a clean, freshly prepared Si(100)-2 × 1 was exposed to 100 L of ammonia (Matheson purity, Matheson) in a UHV chamber at room temperature. For NH−Si(100) surface termination, the NH2− Si(100) surface was heated to 650 K for 1 min and recooled before a spectrum was recorded.13,25,26 A mixture of NHx functionalities were created by preheating the Si(100)-2 × 1 surface to 650 K and exposing it to 200 L of ammonia at this temperature. For MIR-FTIR studies, a background spectrum of the clean Si(100)-2 × 1 surface was collected before functionalizing it with other molecular species. After functionalization, a sample spectrum of the Si−H stretch region was taken to confirm the type of functionalization. This resulting functionalized surface was then recorded as a second background spectrum, used for

The structures of these surfaces and the long-range arrangement of the surface functionalities have been discussed extensively over the last few decades, and most of the findings are summarized in several reviews.6,18,19 Reactivity of these surfaces with respect to the copper deposition precursor, copper (hexafluoroacetylacetonato)vinyltrimethylsilane (([(CF3(CO)CH(CO)CF3)]Cu[CH2CHSi(CH3)3] or Cu(hfac)VTMS), and resulting nanostructure growth are compared here with those on silicon oxide-covered silicon single crystals. The chemical reactivity of these functionalized silicon surfaces is investigated by multiple internal reflection− Fourier transform infrared spectroscopy (MIR-FTIR), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and density functional theory (DFT). Cu(hfac)VTMS is a commercially available metal−organic precursor for copper deposition. This volatile precursor has two organic ligands, vinyltrimethylsilane (VTMS) and hexafluoroacetylacetonate (hfac), and can be dosed into a controlled environment setup using a regular leak valve at room temperature. It has been previously shown that the VTMS ligand is cleanly removed upon Cu(hfac)VTMS adsorption20,21 on a clean Si(100)-2 × 1 surface or on a surface of a thin TiCN barrier film22 and does not pose significant contamination problems. The remaining hfac ligand can potentially lead to C, O, and F contamination.23 That is why reliable methods of removal of the hfac ligand are needed. Cu(hfac)VTMS is also an excellent model precursor for these studies due to its reactivity. As mentioned above, the ligands that may remain on the surface following the deposition process pose a contamination problem in a practical application, particularly fluorine contamination; however, they also provide reliable and clearly identifiable spectroscopic markers. Atomic force microscopy (AFM) studies of copper nanoparticles that are formed on the functionalized surfaces show that the ordering and type of initial functionality on the surface will control the particle growth. Results indicate that the ammonia-functionalized silicon surfaces are the most reactive toward copper nanoparticle growth and possess unique chemical properties for molecular-level manipulation of surface reactions. The deposition process is actually a combination of a CVD-like surface process with an overall limiting role of a functionalized surface as a chemical reducing reagent. That is, 14432



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dwell time. The CASA software was used to perform peak fittings. 2.5. Atomic Force Microscopy (AFM). AFM images were collected on a Nanoscope V (Veeco) using tapping mode at 512 lines per scan in the Keck Electron Microscopy Facility at the University of Delaware. Aluminum-coated silicon nitride tips, resonating at 300 kHz (Budget Sensors), were used to probe the resulting copper nanostructures on the functionalized silicon surfaces. 2.6. Density Functional Theory Studies. Cluster model computations were performed using Gaussian 09 suite of programs.30 A Si15H16 cluster, representing a two-dimer cluster model with hydrogen atoms saturating the subsurface silicon atoms to maintain appropriate hybridization, was designed and optimized using the B3LYP/LANL2DZ level of theory.31,32 Once the geometry of this structure was optimized, the functionalities, such as NH2 and H, were added to each silicon dimer in an alternate configuration to model the NH2−Si(100) surface and then reoptimized at the same level of theory. Similar analysis was performed for OH and for NH functionalities on silicon, where NH bridged the Si dimer and H terminated each Si atom of that same dimer. The silicon dimers of the H−Si surfaces were terminated either with one hydrogen atom per silicon or two hydrogen atoms per silicon atom to simulate the mono- and dihydride surface species. The H−Si(111) surface was simulated using a Si17H23 cluster model. Once the Si15H16 and Si17H23 clusters were optimized, a Cu(hfac) fragment was attached and the resulting structures were reoptimized with the same level of theory. Transition states were determined using the synchronous transit-guided quasi-Newton (STQN) method. The presence of a single negative eigenvector in these calculations confirms the convergence to a transition state.

comparison with the same surface reacted with Cu(hfac)VTMS. A resolution of 4.0 cm−1 and 2048 scans were used to collect each spectrum, recorded using an external liquid nitrogen-cooled MCT detector. All spectra were recorded at room temperature. 2.2. Ex Situ Surface Preparation. For all other Hterminated Si(100) and Si(111) surfaces, the modified RCA cleaning and etching procedure,27 as described in detail elsewhere,28 was used to produce H−Si(100) and H−Si(111) surfaces. To produce the OH-terminated silicon surfaces, the SC2 step of the modified RCA cleaning procedure, using a 4:1:1 of MilliQ water:hydrochloric acid:hydrogen peroxide, was utilized. This preparation yields a silicon oxide surface that is known to have OH termination.29 2.3. Temperature-Programmed Desorption (TPD). TPD experiments were performed in a separate UHV chamber with a base pressure of 5 × 10−10 Torr. This setup is equipped with an SRS mass spectrometer covered with a differentially pumped shield. For TPD experiments, the sample, a Si(100) crystal (approximately 10 mm in diameter and 0.5 mm thickness, Wafer World) fastened to a button heater (Heat Wave) by a tantalum holder was positioned approximately 2 mm from the 2 mm aperture of the mass spectrometer shield. This UHV chamber is also equipped with an AES spectrometer, ion gun for surface cleaning, and a low energy electron diffraction setup. To yield the functionalized silicon surface, first, a clean and well-ordered Si(100)-2 × 1 surface was prepared in situ using cycles of argon sputtering at a pressure of 4 × 10−5 Torr at a 1 keV energy for 1 h and annealing to temperatures up to 1000 K. For the final cycle, the Si(100) surface was annealed above 1000 K to allow for surface reconstruction. Surface cleanliness was confirmed by Auger electron spectroscopy. The clean Si(100)-2 × 1 surface was then exposed to 100 L of isotopically labeled ammonia (ND3, ISOTEC, 99% D purity) at room temperature to produce the ND2−Si(100) surface and then subsequently heated to 650 K for 1 min to produce the ND−Si(100) surface.25,26 Cu(hfac)VTMS was then dosed on the functionalized surfaces. The exposures are reported in Langmuirs, where 1 Langmuir = 1 × 10−6 Torr s. The sample was then heated at a linear rate of 2 K/s in front of an RGA mass spectrometer to collect mass fragments desorbing from the surface. 2.4. X-ray Photoelectron Spectroscopy (XPS). The samples prepared as described above were transferred ex situ to XPS instruments to obtain elemental binding energies and Auger signatures. Measurements were performed on a PHI 5600 system with a base pressure of 1 × 10−9 Torr using either an Al Kα monochromatic X-ray source (ℏν = 1486.6 eV) or Mg Kα anode (ℏν =1253.6 eV) to probe the prepared surfaces. The analyzer was placed 45° offset from the sample. Survey spectra were collected at a 187.85 eV pass energy, scanning from 0 to 1200 eV using a 1.6 eV/step rate and 200 ms/step dwell time. High resolution spectra were collected at a 46.95 eV pass energy using a 0.1 eV/step and 200 ms/step dwell time. A VG ESCAlab 220i-XL electron spectrometer (VG Scienta, UK), at a base pressure of 10−8 Torr, in the Surface Analysis Facility in the Department of Chemistry and Biochemistry was also used to collect selected data. Spectra were collected with both Al Kα X-rays and Mg Kα X-rays operated at 15 kV, 8.9 mA, and 120 W using a spot size of 400 μm. Survey spectra were recorded from 0 to 1200 eV using a 100 eV pass energy. High resolution spectra used a 20 eV pass energy and a 100 ms

3. RESULTS 3.1. Chemistry of Copper Deposition Process onto Functionalized Silicon Surfaces. The copper precursor reacts with the functionalized Si(100) surfaces at room temperature, as supported by the MIR-FTIR spectra in Figure 1. All the spectra presented clearly show the vibrational signatures of the Cu(hfac) fragment, specifically, the growth of the intensity corresponding to νC−F and νCC/CO. The VTMS ligand is not observed following the reaction of Cu(hfac)VTMS, because no additional C−H stretching modes are recorded following the reaction (not shown). The elimination of VTMS upon adsorption is expected and has been previously documented in detailed spectroscopic studies of the reactivity of Cu(hfac)VTMS on several clean, hydrogen-terminated and thin film-covered surfaces.20−22,28,33 The examination of the Si−H spectral region of these infrared spectra suggests that each functionalized surface (used as a background for the studies of Cu(hfac)VTMS reaction) is unique. The shape and position of the Si−H stretch vary from substrate to substrate, as they are very sensitive to the specific chemical environment of the Si−H bond.25,34−36 Once a functionalized surface is exposed to a saturation dose of Cu(hfac)VTMS, a loss in the intensity of the Si−H stretch region bands is observed, suggesting that surface hydrogen participates in the reaction with Cu(hfac)VTMS. The NH2−Si(100), NH−Si(100) and the NHx−Si(100) surfaces initially exhibit different Si−H spectral signatures that can distinguish them spectroscopically, as was reported previously.25 The NH2−Si(100) surface shows an absorption band at 2058 cm−1, indicative of a surface 14433



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hydrogen atom adjacent to a surface NH2 functional group. When this surface is heated, the NH2 group transforms into NH leaving surface hydrogen adsorbed on either end of a silicon dimer of the Si(100) substrate, thus blue-shifting the observed band and splitting it into two peaks, at 2099 cm−1 and 2106 cm−1.25,26 A mixture of these two functionalities is created by preheating the silicon surface before exposure to ammonia, leaving a less well-defined NHx−Si(100) surface, with indicative absorption bands at 2081 cm−1 and 2121 cm−1. The Si−H vibrational signatures of these ammonia-treated surfaces are different from those of the monohydride H-terminated Si(100) surface that exhibits absorption bands at 2089 cm−1 and 2098 cm−1.13,25,26 When the functionalized surfaces are reacted with Cu(hfac)VTMS, there is an apparent decrease in the intensity of the Si− H frequencies and the specific absorption changes depend on the starting surface. For example, the 2091 cm−1 loss following Cu(hfac)VTMS reaction is observed to be predominant on both the NH−Si(100) and NHx−Si(100) surfaces. The only surface that exhibits a uniform loss of the vibrational Si−H signature following its reaction with Cu(hfac)VTMS is monohydride H−Si(100). This observation may imply that the reaction occurs only on specific surface sites, that the adsorbed Cu(hfac) fragment affects the dipole moment of the resulting Si−H stretch, and of course that the surface hydrogen is removed in the reaction of Cu(hfac)VTMS with the functionalized−Si(100) surfaces. Qualitatively, the infrared spectroscopy results summarized in Figure 1 confirm the presence of hfac ligand and the removal of surface hydrogen following Cu(hfac)VTMS reactions with the functionalized silicon surfaces; however, the exact quantification of these processes is complicated for a single spectroscopic technique and will be addressed further by comparing these data with other approaches.

Figure 1. MIR-FTIR studies of the room temperature reaction of Cu(hfac)VTMS with functionalized silicon surfaces, as indicated. The plots are offset for clarity. The labels above the plots show the spectral regions corresponding to the stretch vibrations of C−F, coupled CC/CO, and Si−H surface groups. A clean Si(100)-2 × 1 surface is used as a background for the functionalized surfaces (shown in black), which in turn are used as background spectra for the surfaces reacted with Cu(hfac)VTMS (shown in gray). Two spectra are given as an example for Cu(hfac)VTMS dosed onto a NH2−Si(100) surface at low and saturation exposures.

Figure 2. XPS binding energies for the N 1s and C 1s regions collected for the functionalized silicon surfaces reacted with Cu(hfac)VTMS. The N 1s region shows that the different ammonia-terminated silicon surfaces have similar binding but different reactivity toward Cu(hfac)VTMS. The spectra are offset for clarity. N 1s spectral regions are not shown for H-terminated silicon surface or silicon oxide surface since no nitrogen is present on these samples. Computational studies for selected surface adsorbates depicted in this figure are shown as solid lines underneath the spectra. 14434



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spectroscopically observed difference between the surfaces is not substantial. Nevertheless, the starting NH−Si(100) and NHx−Si(100) surfaces that are less ordered compared to NH2−Si(100) seem to result in lower extent of the changes of the nitrogen environment over the course of the reaction with Cu(hfac). As will be shown below, this agrees well with the proposed reaction mechanisms despite the fact that the XPS studies shown in Figure 2 were performed following a brief exposure of the samples to ambient and the surface oxidation may have played some role in the ultimate distribution of surface species. The C 1s region shown in the right panel of Figure 2 indicates that several different types of carbon species are present on the surfaces from the hfac ligand following the reaction. Since the samples were exposed to ambient during the transfer, this is expected. Some of the species likely correspond to the products of copper precursor reaction and are consistent with the previous XPS studies of hfac ligands on surfaces.45−47 The most intense experimental C 1s peak at 284.5 eV overlaps with that attributed to hydrocarbons or “free carbon” from ambient exposure.48 According to the computational prediction, for the simple models considered, the expected positions for the appearance of the C 1s XPS features are shown as solid lines. Quantitative interpretation of these features is hindered by the fact that the samples may have different reactivity with respect to the impurities present at ambient conditions; however, again a simple general observation suggests that the surfaces containing nitrogen have qualitatively different types of surface species present following exposure to Cu(hfac)VTMS. Specifically, the H-terminated silicon and ammonia-terminated silicon surfaces all exhibit similar C 1s spectral features except for a peak at 282.9 eV, characteristic only of N-containing functionalities. There is a possibility that this peak may correspond to the formation of silicon-carbide-like species because the position of this peak is rather unique.48−50 The reduction process on ammonia-exposed silicon surfaces may result in different types of products compared to hydrogenterminated silicon, as it may follow a different reduction pathway. It should be pointed out that the C 1s spectrum of the oxidecovered silicon surface following its interaction with Cu(hfac)VTMS seems to indicate that there are no CF-containing groups present on this surface following the reaction with Cu(hfac)VTMS. Although it is indeed possible, unfortunately, only oxidized copper is present on the surface after this reaction, making the process very different from the copper deposition on reducing ammonia- or hydrogen-terminated silicon, as will be further explained below.48 Additional XPS spectra corresponding to F 1s, O 1s, and Si 2p regions are presented in Supporting Information. To summarize, these spectral regions confirm that there is surface oxidation from functionalization treatments and ambient exposure during sample transfer, as observed in the Si 2p region. Copper silicide formation is evident on the H−Si(100) but not on the amine-functionalized silicon surfaces (which is a subject of a separate detailed study of the interfaces produced). The O 1s region suggests multiple surface oxide species from either the presence of the hfac ligand or ambient oxidation. The F 1s region shows similar CF-containing species for all functionalized surfaces. Further details are addressed in the discussion section. A combination of the Cu 2p XPS spectra and Cu LMM Auger analysis was used to determine the binding energy and

Additional understanding of the chemical nature of the surface species on functionalized silicon surfaces can be obtained by XPS. As shown in Figure 2, the Cu(hfac) fragment reacts differently on distinct ammonia-functionalized Si(100) surfaces and this reaction can be distinguished from that on the H-terminated Si(100) surface. The N 1s region shown in the left panel of Figure 2 follows the surface products of the reaction of Cu(hfac)VTMS with the NH2−Si(100), NH− Si(100), and NHx−Si(100) surfaces. These results suggest that copper reacts with the ammonia-functionalized silicon surfaces, as confirmed by the observed N 1s binding energies within the range of 397.9−397.3 eV, in agreement with recent studies of CVD grown copper nitride, specifically the surface layer of copper nitride, at 397 eV.37−40 Notably, the N 1s binding energy of ammonia exposed to Si(001)-2 × 1, the NH2− Si(100) surface, at room temperature is 398.95 eV;41 however, for the NH−Si(100) and NHx−Si(100) surfaces, XPS literature comparisons are not currently available. Despite the fact that these data were collected ex situ, after a brief exposure of the samples to the ambient conditions upon transfer, several important observations can be made if the interpretation of these spectra is aided by DFT calculations. The interaction of the Cu(hfac) fragment with the functionalized silicon surfaces can be represented by multiple cluster models. Two of them are presented in Figure 2, describing the interaction of these fragments with surface NH2 and NH species. Of course, the realistic surface deposits may be substantially different; however, qualitatively, these computational models can help one understand the observed spectra. The predicted N 1s binding energies for surface species before and after reacting with Cu(hfac) are presented below the experimental spectra as solid lines. The calibration for these predictions is performed according to our previous work.42 Predicted DFT binding energies were computed using the B3LYP/LANL2DZ level of theory and the Koopman’s theorem approximation as described elsewhere.42 The most important conclusion that can be inferred is that at this level of theory, the spectral difference between surface NH2 and NH species is not significant and that the reaction with the copper precursor likely shifts the expected XPS peaks in this region by less than 2 eV. More rigorous calculations with multiple copper atoms involved in the proposed surface species and a better level of theory (which was previously mostly applied to the organic functionalities on silicon crystals42) will be required for more quantitative interpretation of these results. It should be emphasized again that the exact positions of the N 1s peaks in the products of copper deposition are not known because the structure of these products could be much more complex compared to the ones depicted in Figure 2. In addition, a slightly higher full width at half maximum value for the NH2−Si(100) and NH−Si(100) compared to the NHx−Si(100) may suggest that at least two types of qualitatively different species are formed on those surfaces. Another very important piece of information that can be obtained based on these spectra is virtual absence of the oxidation products that involve surface nitrogen. It would be expected, according to several previous studies42−44 that surface oxidation processes involving nitrogen would shift the observed N 1s peak energy to above 400 eV, which is the region with no observed experimental features in Figure 2. It is indeed possible that multiple surface species exist following the reaction with copper deposition precursor; however, the relatively narrow observed peaks in this spectral region suggest that the 14435



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dependent plots of the loss in the infrared absorbance within the Si−H stretch region for each functionalized surface studied to estimate the amount of hydrogen consumed during the surface reaction, similarly to our previous studies of the reactivity of H-covered silicon surfaces.28 The kinetics of the initial reaction with Cu(hfac)VTMS was investigated, and the results suggest that the hydrogen on the surface with the NH2 functionalities will be consumed faster in a reaction with Cu(hfac)VTMS compared to the surfaces with the NH or NHx functionalities. We considered the surface reactions of first order with respect to the incoming reactive gas and first or second order in the concentration of surface reactive sites. Thus, a single molecule of Cu(hfac)VTMS is proposed to require either a single surface hydrogen or two hydrogen atoms to complete the process. The realistic surface reaction may have a more complex mechanism, but considering the initial steps of the reaction and analyzing the apparent reaction order available from the experimental measurements may shed the light on the overall process. The coverage profiles were fit to first- or second-order surface kinetics using the equations ln(θ/θmax) = −kx and (θ/θmax)−1 − 1 = kx, respectively, derived from the ideal gas law and surface coverage obtained based on the FTIR absorption peak intensity measurements. Here θ is the surface hydrogen coverage based on the integrated intensity of the Si− H stretching vibrations. The relative coverage is obtained from the ratio of the peak area corresponding to the absolute signal of the Si−H stretch to that on the same surface before the reaction. In the equations above, k is the rate constant and x is Cu(hfac)VTMS exposure in Langmuirs.53 Note that the general rate laws with respect to first or second order in the concentration of surface reactive sites are dθ/dt = keffPθ and dθ/dt = keffPθ2, respectively, and because the relative coverage is unitless, the units of rate constant for both rate laws are torr−1·s−1 which in equations above can be replaced by L−1 (inverse Langmuir). These studies and the analysis of the corresponding R2 value fits were used to determine first- or second-order kinetics with respect to the hydrogen removal from the functionalized silicon surface. Upon analysis of the reaction with NH2−Si(100) surfaces, both first and second regressions show a possible change of reaction mechanism around 400 L exposure of the Cu precursor, yet the same trend is absent in the reaction with the NH−Si(100) surface. As shown in Table 1, the reaction rates obtained suggest that the reaction with the H−Si(111) surface likely follows first-order kinetics, but the reaction orders could not be verified for all the other surfaces studied here, since their R2 values show no substantial differences for the studies presented. Although the H−Si(111) surface exhibits the fastest reaction with the copper metal−organic precursor, of the ammonia-prepared surfaces, the NH2−Si(100) surface has the faster initial reaction rate constant. However, it should be pointed out that this approach should be treated as only semiquantitative, because it assumes that the νSi−H vibrational frequencies for all the possible surface species have the same intensity and are unaffected by the presence of a copper deposition precursor, which may not be the case in these experiments. One of the possible reasons for intensity variations in infrared spectroscopy measurements may be the topographical changes in surface structure following chemical modification that can be determined by AFM. AFM investigation suggests that the reaction of Cu(hfac)VTMS with functionalized silicon surfaces leads to the formation of nanostructures. We have previously shown that

chemical state of copper on the functionalized surfaces following Cu(hfac)VTMS reactions. As was described in detail previously for copper deposition onto H-terminated silicon substrates,28 one must use a combination of these techniques to undoubtedly identify the presence of metallic copper. XPS analysis normally allows one to distinguish Cu2+ from Cu1+ or Cu0; however, Cu1+ (that could be present either as a low oxide or as copper bonded to the remaining hfac ligands) and Cu0 (metallic copper) can only be distinguished by the analysis of the corresponding Auger lines. The XPS Cu 2p binding energies and the corresponding observed Auger peak energies are plotted with respect to each other in Figure 3.51 This

Figure 3. Cu 2p XPS binding energies and Cu Auger energies recorded following a saturation Cu(hfac)VTMS exposure onto the indicated silicon-functionalized surfaces. The surface prepared by Cu(hfac)VTMS exposure to silicon oxide and a metallic copper standard are also shown for comparison. In situ vs ex situ refers to the sample preparation procedure (with UHV corresponding to the full UHV preparation and HV denoting silicon samples prepared by wet chemistry methods and exposed to the deposition precursor in HV).

summary compares the state of copper following Cu(hfac)VTMS exposure to all the functionalized silicon surfaces studied here, H−Si(111), H−Si(100), NH2−Si(100), NH− Si(100), NHx−Si(100), OH−Si (from silicon oxide), and copper standards. This analysis suggests that the majority of the copper deposited on the functionalized Si(100) surfaces is metallic, as contrasted with the copper standards and copper oxide samples.28,52 It should again be emphasized that the presence of CuO as a majority species could be ruled out based on the XPS spectra alone. The diagram presented in Figure 3 helps to rule out Cu1+ species as a majority as well. 3.2. Surface Reactivity and Its Role in Nanostructure Formation. Understanding the initial reaction of Cu(hfac)VTMS with the functionalized silicon surfaces should yield some insight into the role of surface functionality in the surface reactivity. Figure 4 shows the Cu(hfac)VTMS exposure14436



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Figure 4. Left: room temperature MIR-FTIR studies of the loss of the relative intensity of the νSiH absorption bands with Cu(hfac)VTMS exposures on the functionalized silicon surfaces indicated. Dashed lines are provided to guide the eye without any specific fit. Right: kinetic studies of the reaction with NH2−Si(100) and NH−Si(100) surfaces.

Table 1. Estimated Kinetic Parameters Obtained Assuming First- and Second-Order Reactions (for surface species) Based on Initial Reaction Velocities of Cu(hfac)VTMS Deposition on Functionalized Silicon Surfaces Calculated from the Integrated Absorption Features Corresponding to the Si−H Lossa first-order rate constant (L−1) surfaces NH2−Si(100) NH−Si(100) NHx−Si(100) H−Si(100) H−Si(111) a

second-order rate constant (L−1)

400 L

400 L

(R = 1.000) 1.1 × 10−4 (R2 = 0.989) 1.2 × 10−4 (R2 = 0.957) 2.2 × 10−4 (R2 = 0.894) 5.5 × 10−4 (R2 = 0.889) 3.59 × 10−3 (R2 = 0.732) 2

The quality of the fits is estimated by the R2 value.

copper nanoparticle growth was initiated at surface defects, resulting in a narrow size distribution of copper nanoparticles on the H−Si(100) surface and substantially wider size distribution on H−Si(111).28 Similar observations can be made based on the summary of the microscopy studies shown in Figure 5. The AFM analysis of the surface nanostructure formation following Cu(hfac)VTMS reaction with functionalized silicon surfaces supports the initial reaction rate analysis for the functionalized surfaces with copper based on the results summarized in Figure 4 above. Copper nanostructures are formed by dosing the same saturation exposure (3000 Langmuirs) of Cu(hfac)VTMS in UHV to the functionalized silicon surfaces at room temperature. This exposure is sufficient to saturate the surface sites according to the Si−H stretch region in MIR-FTIR and no further changes at higher

exposures are observed either spectroscopically or in microscopy studies. Because the reaction studied is self-limiting, this behavior is expected, and the reaction rate decreases as the surface runs out of the reactant, in agreement with the approximate kinetics data in Figure 4 and Table 1. The reaction of Cu(hfac)VTMS with the H-terminated silicon surfaces have been suggested previously to initiate at surface defects.28 It is also known from detailed spectroscopic and microscopic studies27,54−56 that the H−Si(100) surface prepared by a modified RCA procedure is atomically rough, leaving many surface defects available for reaction; thus, following the reaction of this surface with Cu(hfac)VTMS, nanoparticles with a narrow size distribution are formed, as shown in Figure 5. The H−Si(111) surface, when etched with ammonium fluoride, is atomically flat, with a minimum of surface defects; 14437



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Figure 5. AFM images and size distributions of copper nanostructures formed on the functionalized silicon surfaces indicating that the order and functionality of the surface will determine the nanostructure growth. The corresponding average particle heights and rms values are shown in Table 2 below. The asterisk (*) denotes that in the case of the Cu/NH2/Si(100) system, the particle height distribution was difficult to estimate because of high surface roughness that may hide the possibility of film formation instead of particle growth.

NHx−Si(100) surface. These particle size distribution patterns can be compared to those for copper nanostructures formed on hydrogen-terminated silicon surfaces. On H−Si(111), particle growth is initiated at a small number of surface defects, and nanostructures large in size are formed as described previously,28 similarly to the particles on NH−Si(100). The NHx−Si(100) surface, likely the more defective surface, has copper particle size distribution similar to that of the nanostructures on the H−Si(100) surface suggesting that the reaction on this surface also begins at defects, resulting in a narrower size distribution.28 All these are compared to copper deposited on a grown silicon oxide, which is known to be terminated with disordered surface hydroxyl groups.56,58 However, it is questionable whether the copper precursor reacts with oxygen vacancies or OH groups. Note that this

thus, larger copper particles are formed with a wide size distribution. The copper nanostructures produced on the other functionalized surfaces in Figure 5 are compared to understand the copper nanoparticle growth on these surfaces. The NH2− Si(100) surface functionality25,26,57 exhibits the formation of relatively large particles. Unlike the NH2−Si(100) surface, the NH−Si(100) substrate shows the formation of a combination of large and small copper particle growth regimes with an overall narrow size distribution centered around 3 nm in height, reflective of the starting nonuniform surface containing either bridged NH species between each Si(100)-2 × 1 dimer or NH bonded to a surface silicon atom and a subsurface silicon atom.25,26,57 An even tighter copper nanostructure size distribution centered at 2 nm in height is observed on the 14438



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formed on the H−Si(100), NHx−Si(100), and H−Si(111) surfaces. Overall, AFM investigation of the copper nanostructure formation on functionalized silicon surfaces suggests that the NH2−Si surface produces the largest particles. On the basis of the FTIR studies, the same surface exhibits the fastest initial deposition reaction rate among the ammonia-prepared silicon substrates. Similarly to the previous studies, we assume that the copper particles have to nucleate on a surface reactive site, such as a N- or O-containing functional group or a defect (for example, an unoccupied dangling bond).28 Thus, the surface with the largest number of reactive sites per unit area should result in the fastest initial deposition rate. At the same time, particle nucleation competes with the growth process, governed both by the chemistry of the deposition and by the surface diffusion processes, making the analysis of the overall process on the functionalized surface complicated. Nevertheless, several conclusions about the deposition and growth process for copper on functionalized silicon surfaces can be reached. For example, the H−Si(100) surface, which has a large number of defects, following copper deposition, shows the highest similarity to the Cu/NHx/Si(100) system. On the other hand, the largest nanoparticles within the size distribution observed are formed on H−Si(111) and even more so on the NH2−Si(100) system. This may suggest that NH2-functionalized silicon actually provides a better venue for surface diffusion of copper-containing fragments that can in turn form larger nanoparticles. Thus, a process that is expected to be very different from that on the nearly defect-free H−Si(111) surface leads to a similar result. Of course, these results are not identical: H−Si(111) seems to simply yield a small number of large nanoparticles (where the total amount of surface copper is

oxide was prepared ex situ using the RCA cleaning procedure, which results in a 2 nm thick oxide film. The copper nanostructure growth on this surface results in the widest particle size distribution, as compared to the ammonia-prepared or H-terminated silicon surfaces. The average particle heights, based on the statistics, and rms values are compared for the copper nanostructure formation on the functionalized silicon surfaces in Table 2. The ratios of the Table 2. Average Nanoparticle Heights and RMS Values for the AFM Images Shown in Figure 5 a surface supporting Cu nanoparticles

average height (nm)

average rms

rms ratio

NH2−Si(100) NH−Si(100) NHx−Si(100) H−Si(100) H−Si(111) SiO2

17.31* 3.17 1.91 0.54 2.89 3.48

9.30 5.28 1.75 0.32 2.40 1.50

29.46 16.73 5.55 1.00 7.62 4.76

a These averaged RMS values are normalized to the H−Si(100) to compare particle growth on the functionalized silicon surfaces. The asterisk (*) denotes the data for the average nanostructure height determined for the NH2−Si(100) system, where the average particle size determination should be treated with caution, as the structures formed lead to high roughness indicated by the RMS data.

averaged rms values for each surface are normalized to the H− Si(100) surface to compare the roughness of the nanostructures. The largest nanostructures are present on the NH2− Si(100) surface. The NH−Si(100) and SiO2 surface yield similar average particle heights. The smaller particles are

Figure 6. Reactivity of ammonia-functionalized silicon surfaces using the TPD method. (a) Exposure profile of deuterium desorption from the ND2−Si(100) surface after a series doses of Cu(hfac)VTMS. (b) The loss of deuterium on the silicon surface in percentage. (c and d) Plots that estimate rate constants and reaction order parameters. 14439



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Table 3. Rate Constants from TPD Kinetic Studies Based on Deuterium Loss from ND2−Si- and ND−Si-Functionalized Surfaces first order (L−1)

second order (L−1)

surfaces

500 L

500 L

ND2−Si(100) ND−Si(100)

1.7 × 10−3 (R2 = 0.985) 1.84 × 10−3 (R2 = 0.9999)

2.5 × 10−4 (R2 = 0.915) 0

2.74 × 10−3 (R2 = 0.949) 3.01 × 10−3 (R2 = 0.993)

7.2 × 10−4 (R2 = 0.912) 0

Figure 7. DFT barrier predictions for Cu(hfac) to abstract hydrogen from the surface by either (A) a hydrogen from the surface functionality or (B) hydrogen from the silicon surface. The B3LYP/LANL2DZ level of theory was used to compute adsorption energies and transitions states. Note that for the H-terminated silicon surfaces, the RHx represents a dangling bond. Both dihydride and monohydride surface species are considered for the H−Si(100) surface, as summarized in Table 4.

limited by the amount of available surface hydrogen), while Cu/NH2/Si(100) likely forms a more complex structure. In fact, the Cu/NH2/Si(100) system may stimulate surface diffusion of the chemical fragments that can ultimately lead to the formation of the copper films. To quantify the extent of surface chemical reaction leading to copper deposition and to explain the differences observed on differently functionalized surfaces, TPD and computational analyses presented below were used. TPD of Cu(hfac)VTMS on two of the ammonia-functionalized silicon surfaces should aid the quantitative analysis of the reaction extent and its kinetics. It is important to emphasize again that the reaction of Cu(hfac)VTMS at room temperature with any of the surfaces functionalized with ammonia should stop once the surface completely runs out of the reducing agent, likely hydrogen atoms that could either be delivered directly from Si−H surface sites or from the NH or NH2 surface functionality. Isotopically labeled ammonia ND3 was used to create the ND2−Si(100) and ND−Si(100) surfaces before exposure to Cu(hfac)VTMS. Following the reaction, thermal desorption traces were collected, with special emphasis on masses 2 and 4. Because the reaction with Cu(hfac)VTMS would deplete the surface deuterium, regardless of exactly which surface functional group would participate in this

reaction, the TPD study following this process should reveal the amount of deuterium left behind. It has been shown in multiple previous studies that hydrogen desorbs from silicon single crystals around 800 K,59,60 and its amount can be easily estimated based on the area of the TPD peak. The use of ND3 instead of NH3 makes such an analysis straightforward, because there is no m/z = 4 (corresponding to D2) in the background of the UHV chamber within the sensitivity range of the mass spectrometer used. Figure 6 shows that the normalized peak area of the deuterium desorption peak of mass 4 around 800 K drops to approximately one-third of its initial value and comes to a saturation plateau on both functionalized surfaces. The TPD studies suggest possible mechanism changes for both ND2−Si(100) and ND−Si(100) surfaces above Cu(hfac)VTMS exposure of 500 L. FTIR studies shown above demonstrated that such changes are observed for the NH2− Si(100) surface but are not recorded for the NH−Si surface. It should be noted that the absolute exposures in the TPD and FTIR instruments may differ substantially because of the different design of the instruments and dosing arrangements. Mechanistic analysis based on the DFT investigations presented below will address these differences. The initial reaction rate constants appear identical on both surfaces, which is different from the FTIR kinetic predictions 14440



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containing functional groups. In reality, this initial defect site on H-terminated silicon could also be one of the functionalities, such as OH or NH, present as a result of surface contamination. Once the Cu(hfac) fragment reacts with a defect on Hcovered silicon or with the surface NH2, NH, or OH functionality, one possible mechanism to reduce Cu+ to Cu0 is by oxidizing the hfac ligand to hfacH via hydrogen abstraction from the surface. Hydrogen can either be abstracted from the functionality itself or from the silicon surface hydrogen adsorbed near the functional group. These predictions, illustrated in Figure 7, suggest that hydrogen abstraction from the H−Si(100) surface and the OH−Si (oxide) surface is more favorable than from the NH2−Si (100) surface and NH−Si(100) surface. The barriers for hydrogen abstraction are listed in Table 4. The energy barriers for

and what has been observed previously on H-terminated silicon surfaces.12−17,61 Despite this fact, only 2/3 of the surface hydrogen is consumed on both surfaces at saturation exposure. This suggests that if one Cu(hfac) fragment reacts with a single surface functionality per silicon dimer (for example, NH2 group), only two out of three hydrogen atoms of the initial NH3 molecule are consumed in the reaction, leaving the remaining hydrogen atom on the surface to desorb at 800 K. This also means that the nitrogen atom from this functionality remains on the silicon surface and may be a factor in defining the interactions between the silicon and deposited copper. Further studies of this aspect of the proposed chemistry is a subject of additional research, but this may suggest further uniqueness of the ammonia functionalities.62 In addition, this initial reaction of Cu(hfac)VTMS with the ammonia functionalities is the start of metal nucleation for subsequent reaction with incoming Cu(hfac)VTMS molecules to begin the process of film growth.

Table 4. Predicted DFT Barrier Heights (in kJ/mol) of Hydrogen Abstraction from Either the Silicon Surface or the Functionality Itself

4. DISCUSSION OF POSSIBLE REACTION MECHANISMS The nature of the copper nanoparticle formation on functionalized silicon surfaces clearly depends on the specific substrate properties, including the type of functionality, its basicity, the ordering of the functionality on the surface, and the density of surface defects. Some of these issues have been previously described to play a substantial role in the formation of metallic nanoparticles on metallic and metal oxide surfaces pertinent to catalysis;63−65 however, in the present study, the self-limiting reaction of the surface itself provides an instrument for understanding and manipulating metallic nanostructure formation on f unctionalized semiconductor surfaces. Here the reconstructed Si(100)-2 × 1 surface is used to produce different types of surface functionalities, as highlighted in a recent review,6 before reacting with a metalorganic precursor. The starting functional groups provide control over the surface reactivity and thus over the nanostructure formation. For example, the surfaces presented here provide a range of different functional groups with different basicity (propensity to provide electrons in a chemical reaction involving a nucleophilic addition to a metal center of a CVD precursor molecule).66 These differences are discussed below. 4.1. Effects of Surface Chemical Functionality on Reactivity and Nanostructure Growth. To understand the effect of chemical functionality on the mechanism of the initial stages of the reaction, DFT was used to predict the reaction of the Cu(hfac) fragment with the functionalized silicon surfaces. The VTMS ligand was not considered because the FTIR results confirmed that it is eliminated upon initial adsorption. First the direct adsorption (addition) of Cu(hfac) fragment onto the functionalized silicon surfaces is analyzed computationally in Figure 7. The predicted reaction of the Cu(hfac) fragment with the NH2−Si(100) surface is thermodynamically favorable by 147.1 kJ/mol as compared to the NH−Si(100) surface at 125.6 kJ/mol and the OH−Si(100) surface at 135.7 kJ/mol. The H− Si(100) surface at approximately 110 kJ/mol for either dihydride or monohydride models seems to produce the least favorable adducts. However, it should be noted that this prediction for H-terminated silicon only considered a possible reaction of the Cu(hfac) fragment with a single, open dangling bond present on the surface. The ammonia-functionalized surface models considered did not have a dangling bond, but instead the adsorption was described for the available N- or O-

surface

H from functionality (kJ/mol)

H from silicon (kJ/mol)

NH2−Si(100) NH−Si(100) H−Si(111) di-H−Si(100) mono-H−Si(100) OH−Si(100)

223.3 192.4 − − − 151.8

172.4 219.1 120.7 127.3 144.1 140.8

hydrogen abstraction from the surface Si−H f unctionality located next to the O- or N-containing functional groups increase in the following order: OH−Si < NH2−Si < NH−Si, which is in accordance with the FTIR kinetic studies shown in Figure 4 for ammonia-prepared silicon surfaces. However, the predicted barriers for hydrogen removal from the N-containing functional groups (NH2−Si(100) and NH−Si(100)) show the opposite trend, suggesting that the barrier for H removal from the NH−Si functionality is smaller than the barrier for the hydrogen removal from NH2−Si species. These opposite trends are the origins of the experimentally observed difference between the reaction rate constants obtained by TPD and by FTIR for NH2−Si(100) and NH−Si(100) functionalities. In addition, the results in Figure 7 suggest that hydrogen is more easily abstracted from silicon near the dangling bond or surface OH species compared to the NH2−Si(100) or NH−Si(100) functionalities. An alternative way to rationalize the reactivity differences for functionalized silicon surfaces is by analyzing their respective basicities.66 Because the reaction of the Cu(hfac) fragment with the surface functionality involves a reduction step for producing metallic copper, the first step of this reaction can be viewed as a nucleophilic attack of the surface functionality onto the copper center of the Cu(hfac) fragment. Thus, the kinetics of this step should be determined by the reactivity of the surface functional group in donating its electron to copper. This rationale is supported by the above DFT predictions that the initial reaction of the Cu(hfac) fragment with the surface functionalities is more favorable on the more basic NH2−Si(100) surface as compared to the less basic H−Si(100) surface. In agreement with these predictions, AFM studies suggest that the larger nanostructures are formed on the NH2−Si(100) surface, compared to, for example, the hydrogen-covered silicon. A similar rationale for the reactivity of surface amino groups with 14441



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methods, have different starting structures where H−Si(100) is more atomically rough with many defects and with mono- and dihydride sites present and the H−Si(111) surface is atomically smooth with very few defects. On H−Si(111), the majority of surface hydrogen is consumed in the reaction with Cu(hfac)VTMS, but the process yields a wide size distribution of nanostructures. On a H−Si(100) surface, overall less hydrogen is consumed, but as a large number of nanoparticles nucleate and grow simultaneously, the deposition yields a narrow size distribution of nanoparticles.28 This approach can also be applied to understand the copper nanostructure formation on the ammonia-functionalized Si(100) surfaces: the relatively uniform NH2−Si(100) surface has the fastest initial rate of hydrogen abstraction and nanostructure growth, while the more defective NHx−Si(100) surface has the tighter particle size distribution. Thus, the more ordered the functionalized silicon surfaces (and by extent the less defective), the wider the nanostructure size distribution. Alternatively, the more defective the starting surface, the tighter the nanostructure size distribution.

respect to the Ti-containing deposition precursor molecule was described in detail recently.66 More importantly, however, the rate of the second step of the reaction, the removal of the hfac ligand as hfacH following hydrogen addition, will be determined by the acidity of the surface reactive site neighboring the adsorbed Cu(hfac) fragment. In other words, the easier the transfer of a hydrogen atom from the silicon surface to hfac, the faster the rate of surface reaction. Because the adsorption process (the first step) is expected to have a very low or no barrier, it is likely that the acidity of the surface site neighboring the adsorbed Cu(hfac) species should determine the overall surface reactivity (by affecting the second step of the reaction). Thus, the most acidic Si−OH surface site would be expected to lead to the fastest process, as predicted by a simple computational analysis presented in Figure 7 and Table 4. The comparison between NH2 and NH functionalities is the most intriguing one. Although the stability of the initial adduct produced following the adsorption of the Cu(hfac) fragment onto a nitrogen-containing functionality is similar in both cases, the reactions leading to the removal of the hfac ligand as hfacH are drastically different. It seems that for a Cu(hfac) fragment adsorbed onto an NH2-covered silicon surface, it is substantially easier to abstract a hydrogen atom from a neighboring Si−H species. Conversely, if the Cu(hfac) fragment is initially adsorbed onto a silicon surface with NH functionality, the removal of a hydrogen from this functionality is a preferred reaction pathway. Because the TPD allows us to follow the overall amount of hydrogen left on the surface following its reaction with the incoming copper precursor molecules, both NH2 and NH functional groups seem to lead to the same amount of hydrogen left on the surface at saturation exposure. However, in the case of the infrared spectroscopy studies, we can follow explicitly the amount of hydrogen removed from the Si−H surface species (as opposed to, for example, the Ncontaining functionality), making it possible to discern the mechanistic differences between the reactions of two NH3exposed surfaces with this copper deposition precursor molecule. These differences are explained by DFT investigations comparing the barriers required for all possible surface hydrogen abstraction reactions. It can be added that a recent study of a β-diketonate ligand similar to hfac, acac (acetyl acetate), reacting with a dangling bond in a chain-like reaction on a H−Si(100) surface provided similar reaction barriers by DFT investigation,67 although the numeric value for that ligand is lower compared to those obtained here for hfac that has strongly electron-withdrawing CF3 entities. 4.2. Effects of Surface Order and Defects on Reactivity and Nanostructure Growth. The DFT simulations described in Figure 7 predict barriers and initial reactions of Cu(hfac)VTMS with ordered functionalized silicon surfaces. It is important to realize that the initial addition step likely occurs on either a reactive surface site (OH, NH2, or NH) or on a surface defect (such as, for example, a dangling bond). In fact, in silicon surface passivation, the surface OH or NHx groups are also considered defects for the very reason that they are more reactive in a number of processes compared to H-terminated silicon. This signifies that the kinetics of surface deposition reactions and in turn the nucleation and size distribution of the metallic nanoparticles produced by these processes will depend not only on the chemical reactions per se but also on the order and defect density of the solid substrate, as suggested in some of the analysis of the experimental data above. For example, the H−Si(100) and H−Si(111) surfaces, prepared by HF-etching

5. CONCLUSIONS The functionalization of the starting surface has a tremendous influence on the nanoparticle growth on silicon, similarly to growth processes on metal oxides.63 In a copper deposition process based on Cu(hfac)VTMS, the basicity of the functional group defines the rate of the initial attachment, but it is the acidity of the neighboring surface reactive site, following the adsorption of a Cu(hfac) fragment, that determines the rate of the overall reaction.66,68 Out of all the functionalized surfaces studied here, the NH2−Si(100) surfaces not only possess high reactivity toward metalorganic precursors but are also wellordered from dissociative adsorption of ammonia on the Si(100)-2 × 1 surface, making them ideal for designing interfaces with high reactivity and efficient nanostructure growth. The recent work on producing well-defined structured H-terminated69,70 and NH-terminated71 silicon surfaces by wet chemistry methods should aid in further work on manipulating the formation of nanostructures on silicon. This work suggests that tuning surface properties through chemical functionalization can help design interfaces with unique properties, including nanostructure formation for applications in modern electronics, surface catalysis, and materials chemistry.

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ASSOCIATED CONTENT

S Supporting Information *

Additional XPS data and DFT analysis of proposed structures. 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; fax: (302) 831-6335; e-mail: andrewt@ udel.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by the National Science Foundation (CHE-0650123 and CHE-1057374). K.A.P. acknowledges the University of Delaware Graduate Research Fellowship. We would also like to thank Prof. Thomas P. 14442



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Beebe, Jr., and Dr. Holt Bui (Surface Analysis Facility at the Department of Chemistry and Biochemistry, University of Delaware) for selected ex situ XPS data and Dr. Chaoying Ni (the W. M. Keck Electron Microscopy Facility, University of Delaware) for the use of the AFM instrument. We are grateful to Professor Robert L. Opila and his students Dr. Conan R. Weiland and Dr. Fang Fang for their help with XPS measurements and helpful discussions.

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