Tetrakis(ethylmethylamido) Hafnium Adsorption and Reaction on

ARTICLE pubs.acs.org/JPCC

Tetrakis(ethylmethylamido) Hafnium Adsorption and Reaction on Hydrogen-Terminated Si(100) Surfaces Kejing Li,†,‡ Shenggang Li,‡ Ning Li,† Tonya M. Klein,*,† and David A. Dixon*,‡ †

Chemical and Biological Engineering Department, The University of Alabama, Box 870203, Tuscaloosa, Alabama 35487-0203, United States ‡ Chemistry Department, Shelby Hall, The University of Alabama, Box 870336, Tuscaloosa, Alabama 35487-0336, United States

bS Supporting Information ABSTRACT: The surface chemistry of the hafnium-based atomic layer deposition precursor, tetrakis(ethylmethylamido) hafnium (TEMAH), on hydrogen-terminated Si(100) was studied for comparison with the previously reported tetrakis(dimethylamido) hafnium (TDMAH) results by in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and in vacuo X-ray photoelectron spectroscopy (XPS). The adsorption and reaction experiments were investigated at substrate temperatures between 25 and 400 °C. Gas-phase products were detected by in situ transmission FTIR. Density functional theory (DFT) calculations were performed for gas-phase decomposition as well as the adsorption and reaction of TEMAH onto a hydrogen-terminated model of a Si(100) surface. The formation of cyclo species containing alkyl C on Hf formed by a intramolecular insertion reaction and Hf H species formed by β-hydride elimination on the surface were identified by XPS at temperatures > 270 °C and a partial pressure of about 0.02 Torr for the former and ATR-FTIR at 100 °C and 0.1 Torr, for the latter. The latter was confirmed by a TEMAH/D2O reaction experiment, supporting the previous observations of Hf H species for TDMAH. The observation of an alkyl C on Hf as a proposed cyclo species is sensitive to experimental conditions, such as residence time, pressure, and temperature. Decompositions at higher pressures had an increased reaction rate. Transmission FTIR detected the formation of the gas-phase products ethylmethylamine and methylethyleneimine or ethylmethyleneimine. DFT calculations predicted the interface between the adsorbed molecules and H-Si(100) to involve Hf Si, Hf N Si, and/or Hf N C Si bonds. The formation of bridged Si N interfacial bonds provides a thermodynamically allowable way to generate these surface species and is supported by the observation of a N 1s XPS peak associated with Si N.

’ INTRODUCTION Atomic layer deposition (ALD) is a widely used technique for growing atomically flat and highly uniform ultrathin films as well as high-aspect-ratio structures. The overall process is separated into two steps: (1) chemisorption of the metal precursor and (2) reaction of the remaining ligands with oxidizing or reducing agents, leaving a functionalized surface for another chemisorption step. Ideally, each step is self-limiting; that is, the precursor coverage saturates after a certain dose within an optimum temperature and pressure process “window”; otherwise, continuous chemical vapor deposition occurs. The use of organometallic precursors as the metal source can invoke additional reaction pathways when the molecules enter the near surface layer, where the decomposition temperature is usually higher than their vapor temperature. These include decomposition, chemisorption, physisorption, diffusion, and selfoligomerization.1 The adsorption process may result in multiple interfacial bond types, and side surface reactions may also occur. Therefore, it is important to understand the gas-phase and surface chemistry to minimize the incorporation of contaminants that form charge traps in ALD high-k materials. r 2011 American Chemical Society

Tetrakis(ethylmethylamido) hafnium (Hf[N(CH3)(C2H5)]4, TEMAH) has been used as the metal precursor in atomic layer deposition (ALD) of HfO2 and HfxNy.2 6 TEMAH is a liquid at room temperature with a vapor pressure of ∼0.3 Torr at 70 °C.7,8 Other alkylamido metal precursors, such as the tetrakis(diethylamido) group IVB metals (Ti, Hf, Zr), have also been studied for ALD applications due to a relatively weak metal N bond conducive to ligand removal.9 11 An improved understanding of the thermal chemistry of these compounds in the gas phase and on the surface is important to improve the synthesis of the desired thin films. In addition, the interfacial structure is of great importance to device performance affecting such properties as electron mobility and threshold voltage in field-effect transistors. With an improved understanding of the underlying chemistry, the interfacial structure can be engineered for desired properties by judicially selecting metal organic precursors with suitable ligands,

Received: December 6, 2010 Revised: August 9, 2011 Published: August 10, 2011 18560

dx.doi.org/10.1021/jp111600v | J. Phys. Chem. C 2011, 115, 18560–18571

The Journal of Physical Chemistry C functionalizing the surface with certain reactive groups, or even using a third interfacial material.12,13 A number of groups have studied the role of gas-phase and surface reactions as well as the role of surface functional groups during ALD.6,14,15 As an example, Engstrom and co-workers12 studied the reactions of Ti(N(CH3)2)4 on hydrogen-terminated silicon surfaces covered with self-assembled monolayers (SAMs) with a CH3 termination, which showed that the initial reactions were with the terminal groups on the SAMs. Previously, we studied the reactions of the precursor tetrakis(dimethylamido) hafnium (Hf[N(CH3)2]4, TDMAH) on H-Si(100) surfaces and identified Hf H species from β-hydride elimination on the surface and Si N bonds at the interface.16 Beta hydride elimination is commonly observed for several metal organic precursors, leading to formation of the metal hydride and alkyleneimine.9,14,17,18 However, other elimination reactions and heterogeneous reactions are possible. For example, a proposed decomposition mechanism of tetrakis(diethylamido) metal precursors (Ta, Nb, Ti) involves an insertion reaction to form an intermediate three- or fourmembered metallacycle species.9,19 In our previous study of TDMAH, the β-hydride elimination product with an Hf H bond was observed on the surface.16 In this work, the reactions of TEMAH were investigated in a combined experimental and computational study. The gasphase reaction of TEMAH and its surface adsorption and reaction on hydrogen-terminated Si(100) (Hx-Si(100)) and/ or Ge were monitored using in situ transmission infrared spectroscopy (IR) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), respectively. In vacuo X-ray photoelectron spectroscopy (XPS) was also performed for the deposited layer in an ALD reactor. Both the ATR-FTIR and the XPS techniques are nondestructive and surface sensitive measurements that can provide qualitative and/or quantitative information, such as composition, chemical states of the elements, and bonding states in the surface layers. The thermochemistry of the gas-phase and surface reactions was studied using electronic structure calculations to help provide a molecular-level understanding of the process and help interpret the experimental IR spectra. The goal of the effort is to better understand which species with what types of bonding are formed in the initial ALD steps.

’ EXPERIMENTAL AND COMPUTATIONAL METHODS Experimental Methods. The adsorption experiments were performed under vacuum in two setups. The in situ FTIR experimental setup (base pressure: 10 3 Torr) is described elsewhere.16 The in vacuo ALD-XPS schematic setup is shown in Figure 1. Liquid TEMAH (99.99%, Strem Chemical Inc.) was kept in a quartz bubbler connected to the tubing via Swagelock VCR fittings, at a preset temperature between 60 and 80 °C and stabilized for half an hour before each experiment. The manufacturer specified that the TEMAH vapor pressure is 0.1 Torr at 60 °C. All the mass flow controllers (MFC) and stainless steel tubing were connected via Swagelock VCR or VCO fittings. Before introducing the precursor, the gas tubing and reactor chambers were heated to above 80 °C to prevent precursor condensation. In the ATR-FTIR flow cell, a second precursor line with D2O or H2O in a stainless steel container at room temperature was used. The base pressure of the flow cell was 10 3 Torr obtained by using a rotary vane pump. The pressure in the ATR-FTIR flow cell was 0.1 0.3 Torr for the TEMAH cycle

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Figure 1. Schematic of the in vacuo ALD-XPS setup.

and 3.6 Torr for the D2O/H2O cycle, without a carrier gas. The ATR-FTIR cell is purged between experiments with dry nitrogen to minimize contaminants. In the cylindrical ALD chamber with a diameter of 12 in. and a length of 12 in., the base pressure was 10 8 Torr pumped by a 200 L/s turbo molecular pump. The pressure in the ALD chamber was in the range of 0.002 0.076 Torr using Ar carrier gas (99.99%). The pressure was modulated by a MKS 153 throttle valve and a Baratron capacitance manometer gauge that is coupled with a MKS 146C Cluster Gauge Measurement & Control System. Two different FTIR accessories from Harrick Scientific Inc., designed to fit in the Nicolet 4700 FTIR instrument, were used. One is a temperature-controlled stainless steel gas cell with intrinsic Si windows for transmission FTIR. The other is an ATR flow-through cell. Both cells were modified to have compatible vacuum fittings. The Si windows and the internal reflectance elements (IRE, 50 10 2 mm Si(100) and Ge(100) crystals with two 45° beveled ends) were sequentially treated with acetone to remove organic contaminants, a 1:1:5 NH4OH/H2O2/H2O solution at 80 °C for 10 min to oxidize the surface, a 1:1:100 HF/NH4F/H2O buffered oxide etch solution with a pH ≈ 9 for 3 min to remove the surface oxide, a deionized water rinse, and were dried using ultra-high-purity N2 via a blow gun. The wet chemistry treatment was used due to its facility and popularity used in the industry for Si(100). This results in 3 1, 2 1, or 1 1 H-Si(100) surfaces.20 22 It is unlikely that this surface is atomically smooth, and there may be a range of steps, edges, and defect sites, including the possibility of facets of other crystal orientations, such as the (111). The Ge IRE with a wider transmittance range than Si (down to 700 cm 1) was briefly treated in the buffered oxide etch solution for 1 min before each adsorption experiment. Before introducing the precursor, background spectra were taken at each temperature. After TEMAH vapor was introduced, time-resolved spectra were taken at room temperature and 100 °C at a resolution of 4 cm 1 (an average of 32 scans). Unless otherwise noted, all spectra were collected in the flow mode. In the closed system mode, the inlet and outlet valves of the gas cell were both closed after introducing a desired dose into the gas cell. The pressure was recorded as a function of time in the closed mode. 18561

dx.doi.org/10.1021/jp111600v |J. Phys. Chem. C 2011, 115, 18560–18571

The Journal of Physical Chemistry C In vacuo ALD-XPS adsorption and characterization were carried out in a home-built system. The substrates used were cut from 4 in. Si(100) wafers (Silicon Quest International, ( 0.5°, 0.01 0.018 ppm boron-doped), which have a native oxide layer of ∼7 Å as measured by a variable-angle spectroscopic ellipsometer from J. A. Woollam Co. Prior to the experiment, the substrates were cleaned and prepared as Hx-Si(100) as described above. The substrate was heated using the radiation from six J-type tungsten halogen lamps (500 W each) underneath, and the experimental temperatures vary from room temperature to 400 °C. The precursor and substrate were allowed to stabilize for 20 min after reaching the preset temperature. The chamber was evacuated and prepurged with 5 sccm research grade (99.999%) N2. Introduction of the purge gas and precursors were controlled by two-way and/or three-way pneumatic valves. The flow rate of the Ar carrier gas was set to 2.5 sccm. After the adsorption of TEMAH for 20 min, the sample was cooled to room temperature at the process base pressure and transferred in vacuo to the XPS chamber for chemical analysis. XPS measurements were performed in a Physical Electronics APEX system, which is equipped with a UHV analysis chamber with a base pressure of ∼10 10 Torr and a dual anode X-ray source of Mg KR (1253.6 eV) and Al KR (1486.6 eV). A 125 mm mean radius electrostatic hemispherical analyzer and a sevenchannel electron multiplier were utilized in this system to analyze and detect the photoelectron signals. In our case, the analyzer was operated in the constant analyzer energy (CAE) scan mode to keep the pass energy constant at 20 eV. By adjusting the entrance and exit aperture of the analyzer, an analysis area of around 6 mm in diameter was selected. The energy scanning step is 0.2 eV and the dwell time is 20 ms, which results in an analyzer resolution of ∼0.6 eV. Energies for the peaks were shifted with respect to the Si 2p peak at 99.8 eV as measured for a clean H-Si(100) surface to compensate for charging effects. Angleresolved XPS (ARXPS) taken at an angle of 45 90° was used to detect the composition of the layers. Deconvolution of the XPS peaks was performed using CasaXPS, Version 2.3.15.23 Computational Methods. Electronic structure calculations were performed using the Gaussian 09 software suite to help interpret the experimental observations.24 The gas-phase decomposition of TEMAH and its adsorption and reactions on a H-Si(100) surface (represented by a cluster model25 of Si9H14) were calculated. The boundary Si atoms were terminated with H atoms along the cleaved Si Si directions to avoid unrealistic charges. We studied the monohydride (single hydrogen on the Si) as it is unlikely that the use of a dihydride species (due to reconstruction of a 100 surface) will significantly affect the thermodynamics for the addition of various Hf species to the surface as the H is only slightly more electronegative than a SiH3 group. This is consistent with earlier computational studies.10 The cluster geometry is fully relaxed during the optimization. The calculations were performed at the density functional theory (DFT) level with the hybrid gradient-corrected B3LYP exchange-correlation functional.26,27 Transition states were obtained using the STQN method.28,29 The geometry optimizations and frequency calculations were done with the 6-31+G(d,p) basis set for the first row atoms and the LanL2DZ relativistic effective core potential (RECP) and associated basis set for the Hf atom, which will be collectively denoted as BS1. Single-point energies were calculated with the aug-cc-pVDZ basis set30 for the first row atoms and the aug-ccpVDZ-PP basis set31 for the Hf atom, collectively denoted as

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Figure 2. IR spectrum of TEMAH. Top: liquid drop of TEMAH on Ge IRE. Bottom: calculated TEMAH IR spectrum at the B3LYP/BS1 level, with a scaling factor of 0.9398 applied for the 3100 2700 cm 1 region.

BS2. All energies were corrected with the zero-point energies (ZPEs).

’ RESULTS As a comparison to our previous study on TDMAH, gas-phase FTIR analysis in the transmission cell and surface adsorption analysis by ATR-FTIR were performed.16 Because of a more complicated IR spectrum for TEMAH and overlapping of its peaks with peaks of certain products, interpretation of the TEMAH spectrum is more difficult. In this work, in vacuo XPS was also performed as a supplementary method to detect the surface products. The results are given in the sequence from gas phase to surface, and from experimental (FTIR and XPS) to computational (DFT). The focus of the FTIR and XPS results is more of a qualitative rather than a quantitative analysis. Figure 2 shows the IR spectrum of liquid TEMAH on a Ge IRE crystal, together with the calculated IR spectrum within the normal mode and harmonic approximations. Because of anharmonic effects, especially for the C H stretches, a scaling factor of 0.9398 was applied to the C H stretches for comparison with the experimental spectrum. The scaling factor was calculated as the ratio of the frequency of the most intense C H band in the liquid drop spectrum and its corresponding calculated value. Peak assignments are given in Table 1. The low-frequency C H peak at 2777 cm 1 may be a Bohlmann-type band,32 which corresponds to the C H stretch antiparallel to the lone pair on N. The calculated spectrum below 1600 cm 1 (unscaled) matched well with the experimental spectrum with an error of ∼20 cm 1. The peaks at 874, 977, and 1208 cm 1 were assigned to the vibrations of TEMAH on the basis of analysis of our calculations. (These bands are in the typical infrared fingerprint region.) These bands can be assigned to combinations of the Hf N stretches, a methyl rocking, and a C N stretch. They were chosen as they indicate the presence of a free molecular species containing Hf. Gas-Phase FTIR Analysis. The gas-phase IR spectra of TEMAH in the flow mode and the closed mode at a gas cell temperature of 100 ( 5 °C are shown in Figure 3. The integrated absorbance of νC H from 3030 to 2730 cm 1 is linearly proportional to the pressure, suggesting that the IR signal is 18562


The Journal of Physical Chemistry C

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Table 1. Band Positions of the Experimental IR Spectra in cm 1 for the Liquid Drop of TEMAH and the Calculated Values at the B3LYP/BS1 Level Without the Scaling Factor for TEMAH and EMA TEMAH (exptl)

TEMAH (calcd)

EMA (calcd)

assignmentsa

2961

3101

3121, 3109

νas(C

2927

3066

3117

νas(C

2847

3040

3042

νs(C

2818

3007, 2967

3077

νas(C

2777

2963

3065

νs(C

H)Et H)Me H)Et H)Me H)Me

νas(C H)CH2, νs(C H)CH2

2946, 2935 2937, 2936

ν(C

H)CH2,

ν(C

H)Me

1467, 1449

1519, 1502

1525

γ(CH2)

1415

1425

1415

δumbrella(CH3)Et

1366

1396, 1381

1392

ω(CH2)

1344, 1306

1336

1327

τ(CH2)

1208

1246

1152

1176, 1137

1181

νas(C

1112, 1081,

1167, 1073,

1050

νs(C

1053 977

F(CH3)Me, νas(N νs(N C)

Hf),

N) N)

1045 F(CH3), νas(N

988

νas(N 874

892

791

799

C),

F(CH3), νas(N 809

νs(N F(CH2)

759

δN

C),

Hf),

νs(C

N)

Hf),

νs(C

N)

H o./p.

Abbreviations: ν, stretches (subscripts: as, asymmetric; s, symmetric); δ, bending (o./p., out of plane: ω, wagging; τ, twisting; i./p., in plane: γ, scissoring; F, rocking). The assignments are considered to be only approximate. a

from the gas phase rather than from adsorbed species on the windows. Within a minute after being introduced into the heated gas cell, no peaks were observed at 874 or 977 cm 1, which are the characteristic peaks of TEMAH, as discussed above. The gasphase spectra indicate that some volatile products were formed and released into the gas phase, which did not have observable intact TEMAH molecules that can adsorb on Si windows or tubing walls. Besides, the concentration of surface species adsorbed on the window was low, with one single pass of the infrared beam, which may result in the signal being below the detection limit. Our recorded gas-phase spectra are similar to those from a previous study at ∼70 °C.33 By comparison to a reference gaseous ethylmethylamine (EMA) IR spectrum (Figure 3, spectrum f), we can conclude that EMA is the major product in gas phase. No peaks above 3000 cm 1 were observed, and this can be attributed to the intrinsically weak intensity of the νN H stretch at ∼3200 cm 1. In addition to the peaks for EMA, a NdC stretch at 1664 cm 1 in ethylmethyleneimine (EMI) or methylethyleneimine (MEI) with a full width at half-maximum (fwhm) of 20 cm 1 is also present in these spectra, together with the broad peak around 982 cm 1 for δCH2 and νN CH3. EMI or MEI may come from βhydride elimination that forms 1a and EMI or MEI (Figure 4),

Figure 3. Gas-phase IR spectra measured at 100 °C after introducing TEMAH into the gas cell. (a d) Using Si windows and (e) using KBr windows. (a c) In the flow mode for 1, 5, and 10 min, (d) under a closed system mode for 5 min, and (e) in the flow mode for 1 min. (f) Calculated IR spectrum for ethylmethylamine with a scaling factor of 0.9557 applied in the C H stretch region of 3400 2600 cm 1 and with an fwhm of 10 cm 1. No IR peaks above 3000 cm 1 were observed.

due to a surface reaction as the elimination reaction is not thermodynamically favorable in the gas phase (see detailed discussion later). A new peak at 1595 cm 1 appears after 10 min, which is an unknown peak prevalent in the decomposition residual of TEMAH left on the IR windows.33 There are many possibilities for assigning this frequency: NH2 scissoring in methylamine or a surface NH2 species,34 H O H scissoring in water for Hf silicates prepared in solution,35 the CdC stretch in methoxyethylene containing O CdC,34 and a Hf H stretch in a dative bonded R3(H)Hf 3 3 3 MMI species.16 We tentatively assign the peak at 1595 cm 1 to γNH2. The delayed appearance of the species with the peak for γNH2 as compared to the peaks for EMI indicates that the former is most likely due to a surface species and that the EMI is more readily released into the gas phase due to its higher volatility. In the closed cell mode, the pressure was recorded as a function of time and a pressure drop was observed at various temperatures, indicating that EMA can adsorb on the walls of the tubing (Supporting Information, Figure S1). Surface ATR-FTIR Analysis. Low-temperature adsorption of TEMAH was performed on Hx-Si(100) and Ge IRE’s. The calculated vibrational frequencies for possible surface reaction products and respective observed experimental peak locations are listed in Table 2. At a substrate temperature range of 25 250 °C and a pressure of ∼0.7 Torr with the use of Ar as a carrier gas, nucleation occurred immediately upon opening the precursor inlet valve in the time scale of the measurement based on the measured intensity of the νC H stretching region. At a lower pressure of 0.1 Torr obtained by adjusting the valve on the bubbler, it took a longer time to observe a signal consistent with the decreased impinging flux due to the lower pressure. Figure 5 shows the adsorption of a dose of 4 108 langmuir of TEMAH at 0.1 Torr on Hx-Si(100) and a following cycle of D2O. This very weak peak is consistent with a variety of surface silicon hydride species, but we note that it is very small. Therefore, we cannot precisely determine the exact nature of the hydride species. A negative amplification region around 2100 cm 1 (shown as an inset in Figure 5) indicated the loss of surface hydrogen from the 18563



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Figure 4. Schematic for the gas-phase decomposition reactions of TEMAH. ΔHcomplex and ΔHproducts are the reaction energies to form the product complex and the separated products, respectively. E‡ is the reaction barrier. All energies are calculated at the B3LYP/BS2//B3LYP/BS1 level at 0 K and are given in kcal/mol.

Table 2. Experimental Peak Positions (cm 1) and Their Assignments Based on the Calculated Harmonic Frequencies (cm 1) and IR Intensities (km/mol) at the B3LYP/BS1 Level assignmenta,b

exptl

calcd freq.

calcd intensity

1664 (m) (Figure 3)

νNdC in EMI, MEI

1750, 1694

78

1640 (m) (Figure 5)

νHf

1648

288

1596 (m) (Figure 5)

δNH2 on surface δCH2 in 4d

1665 1331

19 79

in 4a

H

ν(Hf)N

C

in 4d

1234

185

ν(Hf)N

C

in 4b

1253

383

977 (s) (Figure 2)

νHf

νs(C

988

113

893 (m) (Figure 8, on Ge)

νSi

N

727 (s) (Figures 5, 8, 11)

δN

H o./p.

N,

N)

in TEMAH

in 4a in EMA

782 (s), 753 (as)

265, 99

759

114

a Abbreviations: ν, stretching; δ, bending. b See Figure 7. Subscripts m and s in the parentheses for peak location (1st column) denote the intensity observed experimentally. Subscripts in the calculated frequency (3rd column) denote the symmetric and asymmetric vibrational modes.

Si. The characteristic peaks of the adsorbed spectrum in the ranges of 3048 2730, 1676 1615, 1615 1569, and 1378 1333 cm 1 are integrated and plotted as a function of time in Figure 6. The range of 3048 2730 cm 1 is the νC H region. It took about 25 min before a detectable peak for νC H was observed, as shown in Figure 6. The adsorption of TEMAH on Ge shows similar trends (10 min before a detectable peak for νC H (Supporting Information, Figure S2). On the basis of the rates of the intensity changes of these peaks, the integrated intensity for the νC H region grows continuously with time, indicating non-self-limiting adsorption. In addition, the spectrum of the adsorbed TEMAH is very similar to that of liquid TEMAH on Ge at room temperature, indicating the formation of multilayers of TEMAH. Thus, the adsorption of TEMAH is not a self-limiting process at 100 °C with a partial pressure of 0.1 Torr.

The peaks at 1640 and 1596 cm 1 were observed on both H-Si(100) and Ge surfaces after the adsorption of TEMAH. In our previous work, a peak at 1650 cm 1 was assigned as the Hf H stretch on the Si surface.16 IR data have been reported for several metal hydrides. For example, silica supported [Zr]s-H synthesized from hydrogenolysis of a tris(neopentyl)zirconium surface complex has a peak centered at 1635 cm 1.36 The asymmetric Hf H stretch in HfH4 was measured to be 1678 cm 1,37 and the Hf H stretch in HHfO was observed at 1626 cm 1.38 D2O or H2O may react with the Hf H species to generate H2 and the loss of this peak, so we can assign the peak at 1640 cm 1 to an Hf H stretch.36 The calculated vibrational frequency of the Hf H stretch in the proposed surface reaction product 4a (Figure 7) is 1648 cm 1, only a few wavenumbers higher than the experimentally observed value. 18564



Figure 5. IR spectra of TEMAH/D2O on hydrogen-terminated Si(100) at 100 °C. Bottom: 4.0 108 langmuir dose of TEMAH. Top: 2.0 108 langmuir of D2O after the TEMAH cycle.

As a comparison to our previous TDMAH experiment, a control experiment was performed to indentify the peak at 1640 cm 1. The IR spectra after a dose of 4 108 langmuir of TEMAH on Si and on Ge at 100 °C are shown in Figures 5 and 8, respectively. The long dose time was needed to observe the small features on ATR IRE.16 The IR spectra of TEMAH on H-Si(100) and Ge were similar except for the limit for transmittance (down to 1200 cm 1 for Si and 700 cm 1 for Ge). After the D2O/H2O cycle, the peak at 1640 cm 1 decreased to half its intensity or disappeared while the peak at 1596 cm 1 persisted. One possible assignment for the 1640 cm 1 peak is νNdC in EMI, but this peak is at 1664 cm 1 in the gas-phase spectra. Therefore, this may not be an appropriate assignment. A more likely possibility is νHf H (4a or 4c in Figure 7), with a calculated value of 1651 and 1649 cm 1 at the B3LYP/BS1 level, respectively. The peak at 1596 cm 1 is possibly due to the NH2 bending, as discussed above. The peaks at 1640 and 1600 cm 1 are almost saturated after 6 min once the precursor started nucleation. There was a slight decrease of about 4 10 6 absorption units/sec for the slope of the peak at 1640 cm 1 and a small increase of about 8 10 6 absorption units/sec for the peak at 1600 cm 1 (Figure 6). On Ge, the decrease in the slope was 9 10 6 absorption units/sec for the peak at 1640 cm 1 and a 1 10 5 absorption units/sec increase for the peak at 1596 cm 1 (Supporting Information, Figure S2). There is a correlation between the two species Hf H and N H2, but we do not have enough data to determine which, if any, reactions are coupling them. The different time dependence of the 1640 and 1596 cm 1 peaks as compared to that for the TEMAH C H stretch and each other indicates that these two peaks are due to different species and are not from intact TEMAH. The saturation of the 1640 cm 1 peak after a specific reaction time without continuous growth, as found for the C H stretching region, indicates that the Hf H species may be located at the interface in a sublayer under physisorbed TEMAH. Further study is necessary to fully explain the above observations. Besides the Hf H species from the β-hydride elimination, an insertion reaction was predicted for the hafnium and titanium amido precursors on Si surfaces.9,16 Because of the possible overlap of the peaks for TEMAH and the peaks for the N C, Hf N, and Hf C stretches in the cyclo species, for example, 4b or 4d in Figure 7 (vibrational modes calculated to be at 1234 or 1253 cm 1, Table 2) and the δ(C C H), νas(N Hf stretch), and νs(N C stretch) modes in TEMAH at ∼1208 cm 1, and the possible interfacial Si N stretch overlapping the C N

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Figure 6. Integrated peak area for the C H stretch region (3048 2730 cm 1, the left axis) and 1640, 1595, and 1370 cm 1 (the right axis), as a function of exposure time of TEMAH on H-Si(100) at 100 °C and 0.1 Torr.

stretch at 874 cm 1, the detection of the insertion reaction product and interfacial bonding states was not feasible by ATRFTIR as was done for TDMAH.16 As a summary for the TEMAH adsorption on hydrogenterminated Si(100) detected by ATR-FTIR, the adsorption is a non-self-limiting process and Hf H-containing surface species from β-hydride elimination was observed as one decomposition product, with a Hf H stretch mode located at 1640 cm 1. In Vacuo ALD-XPS Analysis. XPS spectra of the N 1s, C 1s, Si 2p, and Hf 4f peaks are shown in Figure 9. Because of different sensitivity factors, the Si and Hf spectra have better signal-tonoise ratios than do the N and C spectra. It has been reported that the intrinsic Si0 2p3/2 has a BE of 99.0 eV, while the 2p1/2 component has a BE of 99.6 eV.39 Si 2p peaks of the native oxide, sputtered (to remove adventitious C), and HF-treated Si (forming the H-Si(100) surface) are given in the Supporting Information (Figure S3). For the H-Si(100) surface, the Si 2p peak was located at 99.8 eV, and this value was used to shift peaks formed after TEMAH exposure to compensate for charging effects. The Si 2p peak detected after adsorption changed by less than 0.2 eV from 99.8 eV. There may be a very weak peak near 102 eV that would be assigned to a Si2+ species, but, if present, it is very small. As XPS can be used to detect the reduced carbon state in the Hf C bond, as reported earlier,9 the C 1s peak in Figure 9b can be deconvoluted into four peaks for four different chemical states each with an fwhm of 1.9 eV (referencing an fwhm of 1.9 eV and the intensity for the adventitious C located at 285.2 eV on a clean H-Si(100), as shown in the inset of Figure S3 (Supporting Information)). On the basis of the DFT calculations, and comparison with the undecomposed TEMAH C 1s peak (Figure S4, Supporting Information), the peaks at 286.5 and 285.2 eV are assigned to C bonded to N and C bonded to C in TEMAH, respectively (for example, C (1) and C (2) in 4b, Figure 7). The 285.2 eV peak may also have a contribution from the adventitious C 1s peak that we have observed on hydrogenterminated Si. The calculated C 1s orbital energies for TEMAH and the reaction products show that there is ∼1 eV difference between the organic C atom and the one bonded to Hf in the cyclo structure after the insertion reaction. A possible assignment for the lower BE at 283.8 eV in Figure 9 is for a C bonded to Hf after the insertion reaction (C (3) in 4b, Figure 7). The C 1s peak in amorphous silicon carbon nitride (a-SiCN) thin film was reported to be 283.2 eV for C Si, 284.9 eV for C C, and 287.0 eV for C Nx.40 Thus, the 283.8 eV peak could also be the C 1s of 18565



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Figure 7. Schematic for the reactions of TEMAH with the model H-Si(100) surface. All energies are calculated at the B3LYP/BS2//B3LYP/BS1 level at 0 K and given in kcal/mol. The reaction energies are given on the arrows, with selected reaction barriers followed by ts. For product 4b, the reaction energy for forming the product complex is 26.6 kcal/mol.

Figure 8. IR spectra of TEMAH/D2O or H2O on Ge at 100 °C: (a) 4.0 108 langmuir dose of TEMAH, (b) 2.0 108 langmuir of D2O after the TEMAH cycle, (c) 2.0 108 langmuir of H2O after the TEMAH cycle.

C in the C Si interfacial bonds. However, because of a high barrier for methyl or hydrogen removal from TEMAH, and the lack of a strong IR peak for νC H2 above 3000 cm 1 or δCH3 on Si at 1270 cm 1,41 we assign the peak to C bonded to Hf after the insertion reaction. Although a BE of 283.8 eV is higher than that

observed by Musher and Gordon at 282.0 eV,9 it is reasonable to assign this peak to C on Hf since the observation for 282.0 eV was for C on Ti for a sputtered sample, which may cause chemical shifts to a lower BE.42 Others have noted that the 282.0 eV C 1s peak is for a reduced state of C and assigned it as “carbidic” C, whereas a BE of 283.0 eV for the C 1s peak was assigned to a “metallacycle” for a C bonded to a transition metal.43,44 Because of the difficulty in avoiding the presence of adventitious C and the possibility of a number of possible assignments, it is difficult to confirm our assignment of the 283.8 eV peak. The N 1s peak can be deconvoluted into three components, as shown in Figure 9a, using a fixed fwhm of 2.2 eV. The peaks at 400.2 and 401.7 eV are assigned to organic N as they are also observed for adsorbed TEMAH on Hx-Si(100) at 200 °C (Supporting Information, Figure S4). The apparent shoulder peak at 398.0 eV can be assigned to N on Si45 47 and is evidence of the formation of N Si bonds at the interface, concurrent with the formation of a 283.8 eV C 1s peak for the alkyl C on Hf, as discussed above. In addition, the calculated N 1s orbital energies for TEMAH and for N bonded to Si at the interface in structures 4b or 4d (Figure 7) are predicted to have an ∼0.6 eV difference. It would be surprising if the N 1s has two components for an intact TEMAH molecule. The presence of the higher binding energy peak at 401.7 eV could be assigned to a nitrogen species formed by a surface reaction, such as oxidation, but this is unlikely because it is observed in the adsorbed species and there is a lack 18566



Figure 9. XPS spectra for TEMAH adsorption on H-Si(100) at 320 °C with a partial pressure of 0.02 Torr: (a) N 1s, (b) C 1s, (c) Si 2p, and (d) Hf 4f. The peak location and fwhm (in parentheses) of each deconvoluted peak component in (a) and (b) are given for each deconvoluted peak.

of oxygen in the system. There is controversy about assigning the N 1s in the literature. Some relevant examples observed at a similar position include molecularly chemisorbed N2,48,49 residual N H bonded to carbonyl,50 N C with sp3 hybridization in carbon nitride films,51 and [N-(Si)4]+ in silicon oxynitride films.52 The assignment to a N 1s in Hf N H2, as discussed earlier, was considered as one possibility. However, as the 401.4 eV peak also existed at 200 °C, as shown in Figure S4 (Supporting Information), a possible assignment for the 401.7 eV peak in our environment is N attached to some siloxo substrate sites, which may induce the chemical shift, or it may represent the N in a decomposed ligand. As a short summary for the XPS observed TEMAH adsorption species, alkyl C on Hf from the insertion reaction was assigned to the C 1s at 283.8 eV. Interfacial bonding of N Si was possible, evidenced by the N 1s at 398.0 eV. However, the assignments for N 1s at 401.7 eV and its formation mechanism need further study. Calculations of Gas-Phase Reaction Energies. Figure 4 shows the reaction energetics and barriers for several possible decomposition mechanisms for TEMAH in the gas phase at the B3LYP/BS2//B3LYP/BS1 level. The reaction energies calculated at the B3LYP/BS2//B3LYP/BS1 level are within (2 kcal/mol from the B3LYP/BS1 values (Supporting Information, Figure S5). The β-hydride elimination reaction leads to the Hf H species 1a, and the intramolecular insertion reactions lead to a three-membered ring structure 1h and fourmembered ring structure 1b, which contain the alkyl C on Hf, as discussed above. However, all of the above three reactions to generate separated products are endothermic by 25 52 kcal/mol. The reaction barriers to form the product complexes are on the order of 35 53 kcal/mol, and most of the reaction barrier height is due to the reaction endothermicity. In reaction 1h (Figure 4), a product complex is formed with a barrier height of 36 kcal/mol from the reactants. The complex

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then dissociates to form the products, so the best estimate of the barrier height is the endothermicity of the reaction. The unimolecular decomposition reactions of TEMAH leading to the formation of methane, ethane, and ethylene (1c 1g) have reaction energies of 5 to 12 kcal/mol with much higher reaction barriers of 60 100 kcal/mol. The above calculated results for the decomposition reactions of TEMAH are similar to those for TDMAH16 as expected. For example, the reaction energies and activation barriers for the β-hydride elimination and insertion reactions differ by less than 2 kcal/mol. Calculations of Surface Reaction Energies. The reaction of TEMAH with H-Si(100) has been predicted by Chen et al. to form the Hf Si bond and EMA, with an energy barrier of 17.8 kcal/mol for the first step.25 To better account for the complexity of a real hydrogen-terminated Si surface, the energetics for the first step adsorption of TEMAH with a dihydrogen Si site on cluster models of 2 1 H Si and 3 1 H-Si(100) (Figure S7, Supporting Information) are calculated using B3LYP/BS1 for comparison with the H-Si(100) cluster shown in Figure S8 (Supporting Information). Figure 7 shows the surface reactions of TEMAH onto the model H-Si(100) surface (Si9H14) at the B3LYP/BS2//B3LYP/BS1 level. The energies at the B3LYP/ BS1 level are given in the Supporting Information (Figure S6). According to the data presented here, Hf Si are the least likely bonds to form at the interface. The bond dissociation energies for the different interfacial bonds follow the order of Si N > Si C > Si Hf,16 and thus the formation of Si C and Si N are thermodynamically favorable, consistent with our XPS observations. The comparison of the energies for the H-Si(100), 2 1 H-Si(100), and 3 1 H-Si(100) cluster models shows that the form of the cluster model does not qualitatively change the energies and quantitatively the energies are within about 3 kcal/ mol for the different models. The β-hydride elimination and insertion reactions are consistent with the results from our previous studies on TDMAH.16 Another path starting from the gas-phase reaction product 1b reacting with the model H-Si(100) surface to form the Hf Si bond or the C Si bond has a smaller reaction barrier (Supporting Information, Figure S9). However, as shown in Figure 4, the gas-phase decomposition reaction leading to 1b is predicted to be endothermic by ∼35 kcal/mol with a reaction barrier of ∼37 kcal/mol mostly due to the endothermicity. Thus, this reaction would need to be coupled with an exothermic reaction to reduce the endothermic behavior. The calculated dissociative chemisorption energies are in the range of 20 kcal/mol (Figure 7), which is consistent with the observed value for forming 2b or 2c. There is not much of a Me/ Et substituent effect on this energy. 2b and 2c can further react to form bridged species with an additional 20 kcal/mol of exothermicity. The bridged species can eliminate a variety of small organic species, such as those observed in the gas-phase IR spectra described above, with barriers of about 36 46 kcal/mol. The barrier heights are consistent with the value of 39.6 ( 2.8 kcal/mol measured for film growth, to be discussed later. This type of process would eliminate steric constraints on the forming film and enable additional TEMAH molecules to react on the surface, leading to film formation. These activation energies are mostly due to the endothermicity of the reaction. If the initial chemisorption energies are coupled to the final endothermic reaction energies, then the overall energy from precursor to the elimination of the small molecules with the formation of a surface species is about thermoneutral. This would require that not all of the energy released on chemisorption be dissipated into the 18567



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Figure 10. Schematic for MEI chemisorption onto H-Si(100). Reaction energies and reaction calculated at the B3LYP/BS1 level at 0 K are given in kcal/mol.

surface and that the various steps occur rapidly and are coupled. This is consistent with the concurrent observations of alkyl C on Hf and the N Si interfacial bonds by XPS, as previously discussed. In addition, the computational results reported for the addition of Hf(N(CH3)2)4 to a cluster model of a Si surface terminated with hydrogen and with the presence of one OH group differ from our results as the formation of the Hf O Si moiety leads to loss of HN(CH3)2 in an exothermic process, although some of the energetics are similar.10 The product EMI or MEI formed from the β-hydride elimination reaction of TEMAH, which was detected by transmission FTIR spectroscopy of the gas phase, may also react with the H-Si(100) surface. Figure 10 shows the reaction of MEI with H-Si(100) to form a N Si or C Si bond. Both reactions are exothermic by 9 to 20 kcal/mol with reaction barriers of ∼35 46 kcal/mol. No complex was predicted between MEI and H-Si(100), consistent with it being observed in the gas phase in our IR experiment.

’ DISCUSSION The decomposition reaction rate was determined as a function of temperature and pressure. As impinging flux is linearly proportional to the pressure according to the Knudsen equation,1 higher pressures and higher temperatures increase the reaction rate. The XPS studies enabled the detection of decomposition products, such as the cyclo species 4b, 4d, or 4e (Figure 7), as a function of pressure and temperature. Compared to the ATRFTIR flow-through cell conditions, the pressures of 0.002 0.02 Torr during exposure for the XPS experiments are 1 or 2 orders of magnitude lower, which reduces the impinging flux of the gas molecules to the surface. A TEMAH adsorption dose of at least 20 min was required to generate a detectable XPS signal due to the inert nature of the hydrogen-terminated Si surface and the UHV chamber, which will favor desorption. A series of XPS experiments were performed from room temperature to 400 °C. Assuming that the above assignments are correct, a TEMAH insertion reaction was not observed under the XPS measurement conditions until 270 °C with a partial pressure of 0.02 Torr (for example, see the 250 °C values in Figure 11). This suggests that the insertion reaction has to overcome a relatively high reaction barrier. Although a peak for the Hf H stretch was observed in the ATR-FTIR experiments at a temperature of 100 °C, the ATR cell had a higher experimental pressure. The initial decomposition temperature for TEMAH was reported to be ∼100 °C as determined by thermogravimetric analysis53 and 140 °C as determined by discoloration of the precursor.11 There may be competing steps in the TEMAH decomposition and other decompositions on the surface. Figure 11, spectra a d, shows

Figure 11. XPS spectra for TEMAH adsorption on H-Si(100) at different temperatures and pressures: (a) 350 °C, 0.02 Torr; (b) 350 °C, 0.002 Torr; (c) 300 °C, 0.02 Torr; (d) 300 °C, 0.002 Torr; (e) 250 °C, 0.02 Torr.

Figure 12. Intensity ratio of alkyl C on Hf (283.8 eV) to organic C on N (286.2 eV) as a function of substrate temperature, at takeoff angles of 90° and 45° at a pressure of 0.02 Torr.

that the cyclo species (4b, 4d, or 4e in Figure 7) depend not only on the temperature but also on the pressure. The position of the C 1s and N 1s peaks depends on the temperature and the pressure. We take our baseline spectra to be the one at 250 °C and a pressure of 10 2 Torr. Increasing the temperature by 50 and 100 °C and lowering the pressure by an order of magnitude does not lead to any substantial change in the position of the peak. If the pressure remains the same at 10 2 Torr and the temperature is increased, the C 1s peak clearly broadens and shifts to a lower BE. The peaks were deconvoluted, and the amount of the peak due to the change in temperature is shown as a shaded area. We tentatively assign this new region to the alkyl C bonded to Hf. A similar behavior is observed for the N 1s peaks, and the shaded portion (Figure 11) is tentatively assigned to N Si. These results suggest that an increase in temperature at a higher pressure leads to an increase in the insertion reaction rates, leading to cyclo species formation. In addition, bond cleavage of N C and Si H lead to N Si interfacial bond formation. 18568



Figure 13. Adlayer thickness in angstroms for the chemisorption of TEMAH at 0.02 Torr measured by Hf and N calibrations as a function of substrate temperature shown as a plot of ln(thickness) versus 1/T. Activation energies and adsorption energies derived from the linear fit are given in kcal/mol for both calibrations.

ARXPS was used to detect the composition distribution in adsorbed surface layers as a function of the takeoff angle, as shown in Figure 12. The C 1s peak at 283.2 eV for alkyl C on Hf versus organic carbon at 286.2 eV increases with increasing temperature, indicating more decomposition. As a comparison, the ratio is less than (0.02 different than the reported “carbide C” versus organic carbon as reported for tetrakis(diethylamido) titanium of 0.083 at 280 °C and 0.235 at 420 °C.9 At a normal takeoff angle, the alkyl carbon on Hfto-organic carbon ratio is generally larger than that at the 45° takeoff angle. This indicates that the insertion reaction occurs more easily at the interface between the adlayer and the Si substrate. However, the kinetic information drawn from the data shows a nonlinear kinetic behavior, indicating that there may be more than one rate-limiting step during the adsorption and decomposition processes. The ratio between the signal for the adlayer and the substrate for varying takeoff angles was used to estimate the thickness of the adlayer. The resulting natural log of thickness as a function of inverse temperature is given in Figure 13. Different models can be used to analyze the thickness measurements, assuming that the adlayer has a uniform composition, density, and spatial distribution. The thickness data (d/λ) is obtained using the overlayer model for dielectric thin film measurements on Si,47 from the slope of the plot of ln(IA 3 S.F.(B)/S.F.(A) 3 IB) versus sin(θ), where d is the thickness of the adlayer, λ is the attenuation length of the photoelectron in the substrate and the adlayer, θ is the takeoff angle, S.F. is the atomic sensitivity factor, I is the integrated intensity, and subscripts A and B represent the element, such as Hf or N, in the adlayer and substrate, respectively.54 The attenuation length of the photoelectrons in the dielectric adlayer is based on a semiempirical estimate of 17 Å.55 The error in the predicted average thickness is large due to our analysis approach as well as experimental variables, including the cooling rate and sample transfer time of each experiment. Original thickness data are shown in Figure S10 (Supporting Information). The thickness decreases with increasing temperature between 200 and 300 °C and reaches a minimum of about 7.6 Å at 300 °C, indicating an accumulation of multilayers due to physisorption. Above 300 °C, there was more decomposition and the process started to behave like chemical vapor deposition, leading to the growth of thin films. The ALD temperature window for the TEMAH self-limiting behavior was reported to be below 40011 and 370 °C,2 in which cases, the dose was 1 order of magnitude less than the dose in

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Figure 14. Atomic ratios of C/Hf and N/Hf as a function of substrate temperature at takeoff angles of 45° and 90° at 0.02 Torr.

this work. The plot of ln(adlayer thickness) versus 1/T (Figure 13) can be used to estimate the adsorption energy from the positive slope region as 21.2 ( 1.3 kcal/mol between 200 and 300 °C using the van’t Hoff’s equation and averaging the linear fit for both the Hf and the N data.56 The activation energy for thin film growth can be estimated from the negative slope region as 39.6 ( 2.8 kcal/mol above 300 °C using the Arrhenius equation57 and represents an average value from linear fits of both the Hf 4f and the N 1s data. As mentioned earlier, these kinetic data match the DFT predicted energy barriers for the possible insertion reactions of 4b, 4d, and 4e as shown in Figure 7. Figure 14 shows the atomic ratios for C/Hf and N/Hf as a function of substrate temperature at takeoff angles of 45° and 90°. The compositional analysis indicates that the precursor has decomposed at all of these adsorption temperatures. The adlayer contained high levels of carbon with C/Hf ∼ 0.3 1.0, and substoichiometric nitrogen with N/Hf ∼ 0.15 0.5. These results are consistent with the observations for tetrakis(dimethylamido) titanium.58 Both the C/Hf and the N/Hf ratios reach a maximum at around 300 °C because there is a minimal thickness of thin film on the surface and there is a more intense signal from the interfacial sublayer, as shown in Figure 14, which may indicate some C contamination from decomposed byproduct chemisorbed to the surface. Our results are consistent with the conclusions of Chabal and coworkers5,6 that the ALD process on hydrogen-terminated silicon is initiated by chemisorption of the organometallic precursor rather than the water vapor exposure step. In addition, comparison of the results for TEMAH and TDMAH16 suggests that the different compounds can have a different transport to the surface due to different vapor pressures and surface interactions.

’ CONCLUSIONS Our studies show that reactions during the initial steps of HfO2 ALD using TEMAH and H2O/D2O involve complex processes. In this work, experimental methods and theoretical calculations are combined to elucidate how TEMAH initially decomposes and reacts with a hydrogen-terminated Si substrate. Comparison of the FTIR spectral results, XPS peak assignments, and kinetic data shows a number of interesting features. The 18569


The Journal of Physical Chemistry C surface products included Hf H species formed by β-hydride elimination and cyclo species with an alkyl C on Hf formed by insertion reactions. The former was identified by the νHf H peak at 1640 cm 1 from the FTIR spectra, and the latter was identified by the presence in the in vacuo XPS spectra of a C 1s peak at 283.8 eV. Interfacial formation of a N Si bond is exothermic, 21.2 ( 1.3 kcal/mol from analysis of the growth of the thin film and 20 to 18 kcal/mol by our DFT calculations. The insertion reaction forming alkyl C on Hf has a reaction barrier of about 40 ( 3 kcal/mol from our chemical vapor deposition film growth experiments (Figure 13), entirely consistent with values of 36 40 kcal/mol from our DFT calculations. The formation of the alkyl species depends on both the substrate temperature and the precursor partial pressure and was observed at temperatures between 270 and 400 °C with a TEMAH partial pressure of 0.02 Torr. A variety of decomposition byproducts, such as EMI or MEI observed by detection of νCdN at 1664 cm 1 using transmission FTIR and EMA observed by the detection of δN H at 727 cm 1 in the FTIR, were produced. The byproduct EMI or MEI can either undergo a silylation reaction, leading to the incorporation of contaminants, or desorb into the gas phase. The results suggest that substantial care must be taken in the preparation of the surface, the substrate temperature, and the reagent pressure in the ALD of TEMAH if one wants to generate defect-free silicon surfaces with Hf interfaces. The deposition of the Hf will have to be carefully controlled to avoid the introduction of significant carbon impurities into the interface. A number of our experiments were carried out under conditions similar to those used in an actual ALD manufacturing process, and the results show that this can lead to additional complexity in the products that are formed.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figures showing pressure drop versus time for TEMAH gas in the closed gas cell mode at different temperatures; integrated peak area as a function of exposure time of TEMAH to Ge for C H stretch, 1640, and 1600 cm 1; binding energies of Si 2p for H-Si(100), cleaned Si wafer with native oxide, and sputtered Si substrate; binding energies of N 1s and C 1s of TEMAH adsorption on H-Si(100) at 200 °C with a partial pressure at 0.002 Torr; gas-phase TEMAH reaction energies calculated at the B3LYP/BS1 level; surface reaction energies of TEMAH on H-Si(100) at the B3LYP/BS1 level; reaction path of TEI-4 chemisorption onto H-Si(100 calculated at the B3LYP/BS1 level; average thickness of the adlayer as a function of substrate temperature; and Cartesian coordinates for various species for the bond dissociation energies calculated at the B3LYP/BS2 level. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (T.M.K.), [email protected] (D.A.D.).

’ ACKNOWLEDGMENT This work was supported, in part, by NSF CAREER award no. 0239213 to T.M.K. This work was supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, U.S. Department of Energy (DOE), under Grant

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No. DE-FG02-03ER15481 (catalysis center program) and by the National Science Foundation (CTS-0608896), through the NIRT program. D.A.D. also thanks the Robert Ramsay Chair Fund of The University of Alabama for support. The authors greatly appreciate Dr. Shane C. Street for helpful discussions, Dr. Joseph Thrasher and Dr. Alfred Waterfeld for providing window materials for the gas cell, the Alabama Supercomputer Center for the computing resources, and Mr. Mingyang Chen for computational help.

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