Surface Transamination Reaction for Tetrakis(dimethylamido)titanium

Oct 5, 2007 - Juan Carlos F. Rodríguez-Reyes, and Andrew V. Teplyakov*. Department of Chemistry and Biochemistry, University of Delaware, Newark, ...
0 downloads 0 Views 366KB Size
Download PDF
16498

J. Phys. Chem. C 2007, 111, 16498-16505

Surface Transamination Reaction for Tetrakis(dimethylamido)titanium with NHX-Terminated Si(100) Surfaces Juan Carlos F. Rodrı´guez-Reyes and Andrew V. Teplyakov* Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: June 15, 2007; In Final Form: August 14, 2007

The adsorption of tetrakis(dimethylamido)titanium, TDMAT, on ammonia-saturated Si(100) surfaces at 300 K is followed by infrared (IR) spectroscopy, temperature-programmed desorption (TPD), and density functional calculations. Experimental observations suggest the occurrence of a surface transamination reaction, where an approaching TDMAT molecule reacts with a surface NHX site, eliminating dimethylamine and attaching Ti to the surface N atom. Density functional calculations show that the reaction is thermodynamically possible, and the comparison of predicted vibrational frequencies to spectroscopic features further supports a surface transamination process. The reaction of TDMAT with Si-H surface sites is not found experimentally in agreement with calculations predicting a process less favorable thermodynamically. Since the transamination reaction investigated here can be viewed as the first step in the atomic layer deposition of TiN on silicon, the conditions required for this first step toward the formation of a well-defined Si/TiN interface are discussed.

1. Introduction Understanding surface reactions is enhancing our ability to control the formation of surface structures at a molecular level, which is crucial for developing applications such as molecular electronics and surface self-assembly.1,2 However, other more traditional fields of surface science, such as thin film growth, have reached a critical point where molecular-level control is also desired.3-7 This is particularly necessary for the microelectronics industry, where ultrathin films are currently required to have a thickness of only a few nanometers and the interfaces created often define the properties of the entire structures. One of the most promising techniques to meet this requirement is atomic layer deposition (ALD).3-5 In ALD, the film growth takes place by alternating doses of two compounds and, since each dose is self-limiting, a layer-by-layer growth is possible, enhancing the possibility of controlling the thickness at a molecular level. Although ALD is a promising deposition technique, its success depends on the complete understanding of surface reaction mechanisms for the compounds involved. One of the most important groups of ultrathin film materials is metal nitrides, and thus the deposition of these materials has received substantial attention recently.8 In particular, nitrides of hafnium, zirconium, tantalum, tungsten, and titanium have been the focus of extensive research because of their imminent use in a new generation of integrated circuits.3,9,10 These metal nitrides can be deposited using metal alkylamines and ammonia (NH3) as precursors.11-23 For the deposition of titanium nitride, for example, a common precursor is tetrakis(dimethylamido)titanium, TDMAT.24-36 Figure 1 schematically shows the ALD growth using TDMAT and NH3, which is a process that has the elimination of one ligand as dimethylamine and insertion of ammonia as NH2 as its main characteristic.27 This insertionelimination reaction, called transamination, consists of the attack of the lone pair of a nitrogen atom of an amine, R2N-Y (where R is an alkyl group and Y represents the leaving group for the * To whom correspondence should be addressed. Phone: (302) 8311969; fax: (302) 831-6335; e-mail: [email protected].

Figure 1. Schematic view of an ideal cycle during film growth by atomic layer deposition, ALD. The substrate is successively exposed to NH3 and a titanium-containing compound, TiL4. In particular, when the ligand L is N(CH3)2, the scheme shows an ALD cycle using TDMAT as metallorganic precursor.

reaction given below), onto the metallic center of a metallorganic compound of a general formula ML4:

R2N-Y + L3M-L f L3M-NR2 + L-Y

(1)

In solution, this reaction has been known for a long time37-39 and is still receiving attention for the preparation of novel metallorganic compounds.40,41 In the gas phase, transamination reaction between TDMAT and ammonia

[(CH3)2N]4Ti + NH3 f [(CH3)2N]3Ti-NH2 + (CH3)2NH (2)

10.1021/jp074656r CCC: $37.00 © 2007 American Chemical Society Published on Web 10/05/2007

Surface Transamination Reaction has been observed at room temperature under ambient conditions,25,35,42-44 but under ultrahigh vacuum (UHV, P < 10-6 Torr) conditions, the reaction has been found to be negligible.24-26 The possibility of surface transamination has been considered previously, and the reaction was observed to be hindered because of NH3 desorption, which is a process that is favored kinetically on certain surfaces, such as titanium nitride, TiN.24-28 This fact explains the need of temperatures in the range of 450-650 K during the ALD film growth of TiN with TDMAT and NH314,20,31,33,45 (notice that the growth of TiN by ALD shown in Figure 1 is accomplished by successive transamination reactions similar to the ones shown in eqs 1 and 2). Elimination of ligands has also been explored through the reaction of metal alkylamides with a hydrogen-terminated Si(100) surface.46 This surface proves to be unreactive toward metal alkylamides, as expected from theoretical calculations, and large exposures (about 104 L and above, where 1 L ) 1 Langmuir ) 10-6 Torr· s) were needed to obtain evidence of a surface reaction at room temperature. However, the reaction of TDMAT with an OHterminated silica surface at 300 K has been observed to result in the attachment of Ti to the substrate.47 In addition, it has been shown recently that TDMAT reacts with self-assembled monolayers having NH2 and OH terminations in the 223-373 K range.48-50 Transamination reactions of alkylamides with a NH-terminated silicon surface have been studied theoretically by Xu and Musgrave, finding that the reactions are slightly favored thermodynamically, although the predicted kinetic barrier (approximately 63 kJ/mol) might represent a considerable requirement.51 Similarly, the barrier corresponding to the reaction of TDMAT with a NH2-terminated self-assembled monolayer has been predicted to be 41.8 kJ/mol.49 To gain more insight into the first steps of titanium nitride deposition, we have previously followed the mechanisms of adsorption and decomposition for both TDMAT and NH3 on the Si(100) surface.52,53 The (100) crystallographic orientation of the silicon surface is a commonly used semiconductor substrate that can be described as rows of silicon dimers resulting from a stable 2 × 1 reconstruction. Buckling of these dimers further stabilizes the surface and confers them a zwitterionic character because of the charge redestribution.54 Our investigations suggest that TDMAT adsorbs molecularly on the Si(100) surface through the formation of a dative NfSi bond, but this structure further dissociates to allow the N atom to regain its tricoordination. Figure 2 shows the structures corresponding to the molecularly adsorbed state, A1, and the possible dissociation products, D1 and D2. It can also be observed that the dissociation through scission of the metalligand bond (formation of D1) does not have significant kinetic requirements, and therefore it is expected to take place at low temperatures. Dissociation of the N-C bond (formation of D2), on the other hand, produces highly stable structures, but this pathway is expected only when temperatures are sufficiently high to overcome the substantial kinetic barrier. The possibility for dissociative adsorption through the scission of the C-H bond is not expected to be a substantial contributor to the surface processes at low temperatures.26 In the case of ammonia, the thermal decomposition of (Si)NH2 species, known to be present upon room temperature adsorption,55-61 was investigated.53 We found that annealing the surface in the range of 500-600 K leads to decomposition of the adsorbed species to (Si)2NH, which further decomposes to form (Si)3N structures. Figure 3 shows the possible structures that can be formed during the first step of decomposition: (Si)2NH structures can be produced upon

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16499

Figure 2. Dissociative adsorption of TDMAT on a clean Si(100) surface represented by the top two layers in the Si15H16 two-dimercluster model. The molecularly adsorbed structure A1 has two pathways for dissociation. D1 is the product of N-Ti scission, while D2 is the product of N-C scission. Energies with respect to the clean surface and TDMAT in gas phase (E) and the kinetic barriers (EB) are expressed in kJ/mol and were calculated at the B3LYP/LANL2DZ level. Black, titanium; light gray, nitrogen; dark gray, carbon; white, silicon. Hydrogen atoms from methyl groups are omitted for clarity. This figure is based on the data from ref 52.

Figure 3. Pathways for decomposition of NH3 on Si(100) represented by the top two layers in the Si9H12 cluster model. The initial structure (Si)NH2 decomposes thermally to produce (Si)2NH structures. The two possible structures, bridge and backbonded, can potentially be present on the surface. White, silicon; light gray, nitrogen; black, hydrogen.

insertion of nitrogen into the Si-Si surface dimer or into the Si-Si backbond. These structures will be referred to as bridge structure and backbonded structure, respectively. In spite of the fact that these two structures are similar energetically and spectroscopically, our previous results suggest that the decomposition of (Si)NH2 occurs preferentially through the formation of the backbonded structure, since the (Si)3N products identified at high temperatures correspond to the subsurface insertion mechanism. We present here an investigation involving the adsorption of TDMAT on a NHX-terminated Si(100) surface. Similar to our previous strategies, we use a combination of infrared (IR) spectroscopy and density functional calculations. In addition, this tandem is complemented by temperature-programmed desorption (TPD) experiments. Our results indicate that surface transamination reactions are possible when TDMAT is adsorbed on the NHX-terminated Si(100) surface at 300 K. In spite of the presence of Si-H groups that could potentially interact with TDMAT, this reaction does not take place under the conditions used in this investigation. In addition, no reaction is observed upon exposure of NH3 to a TDMAT-covered Si(100) surface at room temperature. The significance of our findings toward the control of the first steps of TiN deposition is discussed. 2. Experimental and Theoretical Details Experiments were carried out in two ultrahigh vacuum (UHV) chambers (base pressure of approximately 5 × 10-10 Torr) located at the University of Delaware. One chamber was

16500 J. Phys. Chem. C, Vol. 111, No. 44, 2007 dedicated to infrared (IR) spectroscopy in the multiple internal reflection (MIR) mode, using a Nicolet Magna 560 spectrometer with an external MCT detector. Each spectrum was collected at room temperature with the background corresponding to the clean Si(100) surface. In every case, the collection consisted of 2048 scans at room temperature, with a resolution of 4 cm-1. This IR chamber was equipped with a mass spectrometer and an Auger electron spectrometer, which were used to confirm the purity of the compounds used and the cleanliness of the silicon surface, respectively. The silicon sample was a 25 × 20 × 1 mm trapezoidal Si(100) crystal polished on both sides, with 45° beveled edges (Harrick Scientific), which allowed the use of IR spectroscopy in the multiple internal reflection mode. The sample was mounted on a manipulator capable of heating it to 1150 K using an e-beam heater (McAllister Technical Services) and cooling it with liquid nitrogen. The second chamber was mainly dedicated to temperature-programmed desorption (TPD) mass spectrometry, using a shielded differentially pumped mass spectrometer (Hiden Analytical) with the inlet aperture positioned approximately 2 mm from the silicon surface. The heating rate during TPD experiments was 2 K/s. This chamber is also equipped with an Auger electron spectrometer to confirm the cleanliness of the surface. The silicon sample for TPD studies was cut from a Si(100) wafer, provided by Semiconductor International, and was mounted in a manipulator capable of heating the surface up to 1150 K. For both chambers, the surface was cleaned by argon (Mathesson, 99.999%) sputtering, followed by annealing to 1100 K. These sputtering-annealing cycles were performed until the Auger spectrum showed a clean Si surface. While ammonia was used as provided (Air Products, Anhydrous Grade), TDMAT (Epichem, 99.99%) was cleaned using freeze-pump-thaw cycles before introduction into the UHV chamber. Purity of both compounds was confirmed in situ by mass spectrometry. Doses of the compounds are expressed in Langmuirs (1 Langmuir ) 1 L ) 10-6 Torr·sec). The NHX-terminated surfaces were prepared by exposing the clean Si(100) surface to 100 L NH3, which is sufficient to guarantee the saturation of the surface. Larger doses of ammonia do not change the intensity of the IR spectra, proving that the saturation point has been reached. As explained in detail in the next section, through a 1-min annealing to 600 K, it is possible to convert most surface species from (Si)NH2 to (Si)2NH.53 Hereafter, the surface corresponding to the room-temperature adsorption of NH3 without annealing and after a 1-min annealing to 600 K will be referred to as the NH2-terminated surface and the NH-terminated surface, respectively. Once these surfaces were prepared, they were exposed to 1000 L of TDMAT at room temperature. Density functional calculations were performed at the B3LYP/ LANL2DZ level of theory.62-66 The LANL2DZ basis set has been previously used in our group to investigate the adsorption of TDMAT on the clean Si(100) surface.52 In addition, other metal-containing systems have been successfully investigated in our group using this level of theory.67-71 A Si9H12 cluster model, representing one silicon surface dimer, was used to simulate the Si(100)-2 × 1 surface. To mimic the conditions present in a lattice, only the atoms representing the top two layers of the silicon surface were allowed to relax, while atoms representing deeper layers were held in fixed positions. This procedure, with small variations, has been used successfully by other research groups.72,73 Predicted vibrational frequencies of optimized structures were calculated and compared to experimental spectroscopic features, using a correction factor of 0.933535. This factor was obtained by comparing the predicted

Rodrı´guez-Reyes and Teplyakov

Figure 4. Infrared spectra corresponding to NHX-terminated Si(100) surfaces before (black) and after (gray) exposure of TDMAT at room temperature. Spectra a1 and b1 are obtained upon adsorption of NH3 at room temperature and further annealing of this surface to 600 K, respectively. Adsorption of TDMAT on these two surfaces is characterized by spectra a2 (on NH2-terminated surface) and b2 (on NHterminated surfaces).

vibrational frequencies of a single TDMAT molecule in the gas phase with the experimental vibrational spectra of a condensed multilayer of TDMAT.52 Calculations were performed using the Gaussian 03 suite of programs.74 3. Results 3.1. Preparation of NHX-Terminated Si(100) Surfaces. The adsorption of NH3 at 300 K is known to be dissociative, forming (Si)NH2 and (Si)H surface species (Figure 3).55-61 Figure 4 shows the spectrum corresponding to this NH2-covered surface, labeled as a1. The appearance of a Si-H stretch signal at 2072 cm-1 in the IR spectrum (Figure 4) confirms the formation of a NH2-terminated Si(100) surface and is consistent with theoretical predictions.53 As indicated in the previous section, an NH3 dose sufficient to saturate the surface (100 L) was used. Thermal decomposition of surface species is deduced from the blue shift of the Si-H stretch mode, which is accompanied by an increase in the intensity of the absorption. The spectrum corresponding to the flash annealing to 600 K features two components at 2072 cm-1 and 2102 cm-1, indicating the presence of (Si)NH2 and (Si)2NH species on the surface, respectively (annealing to higher temperatures produces a continuous blue shift to 2108 cm-1, which is attributed to the formation of (Si)3N species).53 The 1-min annealing to 600 K leads to the formation of (Si)2NH structures on the surface almost exclusively, as confirmed by the dominance of the 2102 cm-1 component in spectrum b1 in Figure 4. Since annealing was performed below 650 K, temperature at which ammonia recombination is observed as a minor channel,55,75 the surface is expected to remain almost completely covered by adsorbed species. Two possible (Si)2NH species are the bridge structure and the backbonded structure, as indicated in Figure 3. Although our theoretical investigation suggested that the backbonded structure is likely to be more abundant, the mechanistic studies presented here will consider both (Si)2NH structures. A detailed discussion of experimental and theoretical results for the thermal decomposition of NH3 is available in the literature.53

Surface Transamination Reaction

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16501

Figure 5. Temperature-programmed desorption of TDMAT adsorbed on clean and NHX-terminated Si(100) surfaces. The most representative spectra are shown corresponding to m/z ) 15, 27, and 44.

3.2. Adsorption of TDMAT on NHX-Terminated Si(100) Surfaces. Vibrational spectra corresponding to the NHXterminated Si(100) surfaces change drastically upon exposure of 1000 L TDMAT to these modified substrates, as shown in Figure 4 (spectra a2 and b2). An increase of the C-H stretch intensity, corresponding to the presence of dimethylamino groups on the surface, is observed in the spectral range of 27003000 cm-1. At the same time, the Si-H signature is red-shifted from 2072 to 2057 cm-1 for NH2-terminated surfaces and from 2102 to 2067 cm-1 for NH-terminated surfaces. Although this indicates a chemical change on the surface, it could be possible that TDMAT was only weakly adsorbed through interactions with the NHX surface species. To gain more insight into the nature of the surface reaction of TDMAT with these modified surfaces, we followed the thermal desorption of the surface species by TPD, comparing them to the desorption of TDMAT from a clean Si(100) surface. Despite the different structures of the surfaces analyzed, in all cases the most representative spectra correspond to the mass/charge (m/z) ratio of 15, 27, and 44 (Figure 5). The fact that no high m/z fragments were observed in our experiments indicates that TDMAT decomposition is the primary process. Signals corresponding to the m/z ratio of 15 and 27 have been previously associated with methyl-containing products and hydrogen cyanide, respectively.76,77 The m/z ) 44 is the most prominent fragment in the mass spectrum of dimethylamine, and the evolution of this compound is confirmed by following other characteristic m/z traces (m/z ) 43 and m/z ) 45, not shown). The different stages of decomposition are the subject of ongoing research in our group and will not be discussed in detail here. For our purpose, it is sufficient to notice that chemical species corresponding to TDMAT adsorption on NHX-terminated surfaces exhibit a strong interaction with the substrate, since high temperatures are needed to induce the observed desorption processes. This demonstrates that in fact there is a reaction taking place on the surface and that the products of this reaction are tightly bound to the surface. Confirming this observation, vibrational frequencies corresponding to C-H modes obtained upon room-temperature adsorption (Figure 4) are detected even after annealing the surface to temperatures up to 600 K (not shown). Previous desorption studies on different surfaces found that a multilayer of condensed TDMAT desorbs from the surface at ∼220-230 K,24,26 and

Figure 6. Models for the adsorption of TDMAT on NHX-terminated Si(100) surfaces represented by the top two layers in the Si9H12 cluster model. For each potential surface (center), the reaction with TDMAT can take place through the scission of the N-H bond (right) or through the scission of the Si-H bond (left). As indicated in the text, there are two possibilities for the reaction of TDMAT with a Si-H bond for the backbonded (Si)2NH structure but, since the stability of the possible products is very similar, we include only one for visualization purposes. Predicted thermodynamic stabilities (B3LYP/LANL2DZ) are shown for each structure. Black, titanium; white, silicon; light gray, nitrogen; dark gray, carbon; black sticks, hydrogen from ammonia. Hydrogen atoms from TDMAT are omitted for visualization purposes.

TABLE 1: Predicted Energies (kJ/mol, B3LYP/LANL2DZ) for the Possible Structures Resulting from the Adsorption of TDMAT on NHX-Terminated Surfacesa NHX species on Si(100) (Si)NH2 (Si)2NH bridge (Si)2NH back

TDMAT(gas) + product of Si-H product of N-H NHX-Si(100) scission scission -237.2 -254.0 -243.0

-197.8 -195.4 -184.2

-237.0 -254.6 -244.6

a Values with respect to the energies of the free Si H cluster, NH 9 12 3 (gas phase), and TDMAT (gas phase).

therefore the possibility of physisorption at room temperature can be essentially ruled out. The titanium atom of the TDMAT molecule can form a chemical bond with surface entities only if one of its ligands is eliminated, so that after reaction its stable tetracoordination is maintained. Such reaction is possible with the participation of surface hydrogen, since in this case a dimethylamino ligand can be eliminated as dimethylamine, (CH3)2NH. The reaction of TDMAT with a NHX-terminated surface suggests two possible sources of hydrogen: N-H species or Si-H species. Models corresponding to the reaction of TDMAT with both species present in NHX-terminated surfaces are shown in Figure 6, and the predicted thermodynamic stabilities (at the B3LYP/ LANL2DZ level of theory) are summarized in Table 1. When the reaction of TDMAT with Si-H species from backbonded (Si)2NH surface structures is considered, there are two different types of surface hydrogen atoms that can be involved in such a process. Optimization and frequency calculations were

16502 J. Phys. Chem. C, Vol. 111, No. 44, 2007

Rodrı´guez-Reyes and Teplyakov

Figure 7. C-H stretch region corresponding (from top to bottom) to condensed TDMAT on Si(100), TDMAT chemisorbed on Si(100), TDMAT adsorbed on NH2-terminated Si(100), and TDMAT adsorbed on NH-terminated Si(100). In all cases, the surface was exposed to 1000 L TDMAT at temperatures indicated. The position of the Bohlmann band is indicated for all spectra.

performed to investigate both possibilities, finding that the difference in stability is only 5.0 kJ/mol and that the C-H region of the spectra (and particularly the position of the Bohlmann band) was practically the same for both cases. For didactic purposes, we present in this paper the results corresponding to the reaction with the H atom closer to the NH moiety. For the three NHX-terminated surfaces considered here, the attachment of TDMAT to the surface through scission of the Si-H bond produces surface structures that are less stable than the products of the reaction of TDMAT with N-H species, which is an example of surface transamination. Even though our calculations show that surface structures produced by transamination do not increase their stability significantly with respect to the NHXterminated surface and TDMAT(gas), there is no thermodynamic hindrance for this reaction to occur, as opposed to the result of the reaction of TDMAT with Si-H bonds, where the surface products are predicted to be approximately 40-60 kJ/mol less stable than reactants. Experimental evidence for the selectivity of this reaction can be obtained from the fact that the Si-H vibrational feature is not observed to decrease in intensity upon TDMAT adsorption (Figure 4). The comparison of the vibrational signatures in the C-H stretch region can offer additional insight into the mechanism of reactions occurring on the surface. Spectra corresponding to the room-temperature adsorption of TDMAT on NHX-terminated surfaces are shown in Figure 7. In addition, spectra corresponding to adsorption of TDMAT on a clean Si(100) at 100 and 300 K are shown for comparison. All these spectra have a common prominent absorption signal below 2800 cm-1. This feature, known as Bohlmann band, is originated from a C-H bond that is in a position anti-periplanar to the nitrogen lone pair. Our previous investigation of the adsorption of TDMAT on clean Si(100) resulted in the prediction that TDMAT would adsorb dissociatively on the surface at room temperature through the scission of the N-Ti bond, as indicated in Figure 2.52 This prediction was supported by the analysis of the position and width of the Bohlmann band in the experimental spectra and its comparison with predicted vibrations in the same spectral region. For a TDMAT multilayer condensed at 100 K, it is

Figure 8. Predicted infrared frequencies for possible products of TDMAT adsorption on NHX-terminated Si(100) surfaces represented by the top two layers in the Si9H12 cluster model as shown in Figure 6. Structures representing the product of surface transamination are put in squares, and their predicted frequencies are in black. As indicated in the text, there are two possibilities for the reaction of TDMAT with a Si-H bond for the backbonded (Si)2NH structure but, since their vibrational modes are very similar, only one is included for visualization purposes. Vibrational frequencies are calculated at the B3LYP/ LANL2DZ level of theory and are scaled by 0.933535 as indicated in the text. Black, titanium; white, silicon; light gray, nitrogen; dark gray, carbon; black sticks, hydrogen from ammonia. Hydrogen atoms from TDMAT are omitted for visualization purposes.

possible to observe a narrow feature located at 2764 cm-1, while for TDMAT chemisorbed at room temperature, this feature is broader and blueshifts to 2778 cm-1. This experimental observation was explained in terms of the formation of two different sets of features: a minor component around 2763 cm-1, corresponding to the Si-N(CH3)2 moiety, and a greater component around 2790 cm-1, which corresponds to Si-Ti[N(CH3)2]3 species.52 While the presence of these two components explained the broadening of the Bohlmann band, the fact that the greater of them is located around 2090 cm-1 explains the blueshift of the signal observed experimentally. Predicted C-H stretches corresponding to the possible products of the reaction of TDMAT with the NHX-terminated surfaces are shown in Figure 8, and the average positions of the Bohlmann band for all the aforementioned structures are summarized in Table 2. Different from the system TDMAT/Si(100), where the position of the Bohlmann band is determined by the overlap of two sets of features, for the products of transamination the Bohlmann band

Surface Transamination Reaction

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16503

TABLE 2: Predicted Average Position of the Bohlmann Band (cm-1, B3LYP/LANL2DZ) for Possible Structures from the Adsorption of TDMAT on NHX-Terminated Surfacesa NHX species on Si(100)

product of Si-H scission

product of N-H scission

(Si)NH2 (Si)2NH bridge (Si)2NH back

2791 2793 2792

2774 2779 2778

a

A correction factor of 0.933535 was used.

is predicted to have a vibrational component centered around 2775-2780 cm-1, matching perfectly with the experimental observations. The other possibility for TDMAT adsorption, the elimination of dimethylamine through the scission of the Si-H bond, predicts a Bohlmann band around 2790 cm-1, which is not observed experimentally. Thus, through the thorough analysis of the spectra, we have found additional support for the occurrence and selectivity of the surface transamination reaction. The ratio of the integrated peak areas corresponding to the C-H stretch spectral region for TDMAT adsorption on clean, NH2-terminated and NH-terminated Si(100) surfaces is approximately 10:7:8, indicating that fewer C-H-containing ligands are present on the NHX-terminated surfaces as compared to the clean silicon. In agreement with this observation, TPD experiments summarized in Figure 5 show a decrease in the amount of TDMAT fragments desorbing from the surface when it is NHX-terminated. These concentrational differences can also potentially be caused by the different saturation of clean and NHX-terminated surfaces by TDMAT. This is particularly important for NH-terminated surfaces, since in this case because of the geometrical arrangement of NH species, steric hindrance could result in a lower amount of TDMAT adsorbed, and thus more exact quantification of the observed differences is difficult. In addition, the IR intensities are dependent not only on the number of C-H bonds present on the surface but also on the dipole moments of these bonds and their orientation with respect to the surface; however, these differences are expected to be negligibly small for the C-H stretch components of similar surface adsorbates with nearly random geometrical distribution of their C-H bonds. A final piece of information can be deduced from the frequency shift observed experimentally for the Si-H stretch signal upon TDMAT adsorption, as summarized in Figure 4 for both modified surfaces. This red shift is indicative of the weakening of Si-H bonds possibly originated from the interaction of nitrogen lone pairs (present in surface-bound species produced by transamination processes) with H atoms of the surface Si-H entities. The use of the B3LYP/LANL2DZ level of theory did not allow us to obtain an accurate prediction of the frequency shifts of the surface Si-H stretches, and the use of better computational methods is likely needed to predict these rather weak but noticeable interactions. 3.3. Adsorption of NH3 on a TDMAT-Covered Si(100) Surface. The possibility of surface transamination during the exposure of NH3 to a TDMAT-covered surface was also considered in this investigation. The clean Si(100) surface was exposed to 1000 L of TDMAT at room temperature, which was found sufficient to saturate the surface.52 In addition to the prominent C-H stretch signal shown in Figure 8, a small Si-H feature, indicative of a minor channel of adsorption, is found in the IR spectrum of this surface. Exposure of NH3 (doses in the range of 100-1000 L) to this TDMAT-covered surface at room temperature did not introduce any noticeable change in

the IR spectrum. Thus, our experimental results indicate that no reaction takes place under these conditions. 4. Discussion The reaction observed in the present work constitutes the first step for the ALD deposition of TiN on Si(100) using NH3 and TDMAT as precursors and therefore is crucial toward the formation of a sharp Si/TiN interface. This process has been observed to take place at room temperature, showing that the temperatures needed for this process are rather low. In contrast, typical ALD processes are carried out using temperatures higher than 450 K.14,20,31,33,45 This discussion will be focused on establishing the requirements for different thermal conditions needed for these two similar processes and on analyzing the impact of temperature on the interfacial properties. 4.1. Requirements for Surface Transamination. The possibility of surface transamination has been investigated previously on TiN surfaces finding that this process is hindered under UHV conditions in the range of 300-500 K.24-28 In the studies of coadsorbed TDMAT and NH3, the inefficiency of the transamination process was attributed to the kinetically favored ease of NH3 desorption from this substrate.24 A similar inertness for surface transamination on the Si(100) surface is observed in this investigation but only when a TDMAT-covered surface is exposed to NH3. In our previous work, we have suggested that the chemisorption of TDMAT leading to the formation of the dissociatively adsorbed structure D1 (Figure 2) changes the reactivity of the metal-ligand bonds. Since in this structure the Ti atom is bonded to Si (an element that is more electropositive than nitrogen), there is an imbalance of electronic density in titanium as compared to the TDMAT molecule in the gas phase. Under this circumstance, nitrogen atoms in the three ligands confer more electronic density to the metal center, increasing the strength of the three Ti-N bonds and shifting the Bohlmann band toward higher wavenumbers, as observed experimentally.52 The presence of stronger Ti-N bonds suggests that the surface structure D1 is less reactive toward scission of the Ti-N bond as compared to the molecular TDMAT. Another factor that determines the possibility of surface transamination is the reactivity of the particular NHX surface species. The TPD results displayed in Figure 5 show different desorption temperatures for the observed terminal products (675 and 720 K for NH2- and NH-terminated surfaces, respectively). This observation suggests a difference in reactivity of (Si)NH2 and (Si)2NH species. In addition, NH3 seems to have lower reactivity toward transamination compared to these NHX species. This can be estimated by comparing the prompt reaction observed when TDMAT is adsorbed on ammonia-covered surfaces (where NHX surface species are present) to the inertness observed when the silicon surface reacted with TDMAT is then exposed to NH3. Two previously published observations may support this difference in reactivity of ammonia-related species. First, if the ammonia itself does not dissociate on a specific substrate, such as TiN, this inertness can be overcome by using hydrazine (N2H4) instead of NH3. This simple substitution leads to the presence of surface NH2 species that are active in further surface modification processes, including transamination.26 Second, recent investigations have proposed that NH2 and NH sites present in silicon nitride have an increased reactivity and, specifically, catalytic properties have been attributed to NH surface species.78-80 Therefore, it is apparent that the transamination reaction of TDMAT with NHX-terminated surfaces occurs because of the reactivity of the NHX surface species, while the absence of a reaction when NH3 is exposed to a TDMAT-

16504 J. Phys. Chem. C, Vol. 111, No. 44, 2007 covered surface is explained by both the stronger character of metal-ligand bonds in surface species and the lower reactivity of ammonia as compared to the reactivity of (Si)NH2 and (Si)2NH surface species. Surface transamination has been observed to take place at 300 K in this investigation, suggesting that the kinetic requirements are not high. A previous computational study considering the adsorption of tetrakis(diethylamino)hafnium on (Si)2NH bridge sites predicted a kinetic barrier of approximately 63 kJ/mol for the transamination reaction.51 Although this barrier is considerable, the stability of the surface structures shown in Table 1 indicates that it is quite small as compared to the stability of the TDMAT molecule in gas phase and the NH2terminated surface cluster model, -237.2 kJ/mol. Thus, kinetic hindrance might not be a problem during the room-temperature reaction, as is observed experimentally. The same investigation found the reaction to be slightly exothermic, with the transamination product being more stable than the reactants by approximately 5 kJ/mol,51 while our results show a negligible energetic variation upon transamination. Nevertheless, both results predict that the reaction is favored thermodynamically. In opposition to this pathway, dimethylamine elimination through the reaction with the surface Si-H species is found to be less advantageous thermodynamically by approximately 4060 kJ/mol, depending on the initial NHX surface considered (at the B3LYP/LANL2DZ level of theory), as observed by comparing columns 3 and 4 of Table 1. This, in turn, indicates that the H-terminated silicon surface would be relatively inert toward a reaction with metal alkylamides, in agreement with the experimental and theoretical investigation by Kelly et al.46 4.2. Effects of Temperature on the Formation of the Si/ TiN Interface. The successive exposures of NH3 and TDMAT to the silicon surface at room temperature suggests that temperatures needed for the very first cycle of TiN growth by ALD on silicon are rather low. In our previous studies, we have found significant evidence that the occurrence of this first step at temperatures typical for ALD (450-650 K) introduces major problems toward the formation of the interface. On one hand, the direct interaction of NH3 with a clean Si surface will result in the decomposition and insertion of nitrogen, probably forming backbonded (Si)2NH structures and decomposed (Si)3N structures, while the surface silicon atoms are passivated with hydrogen. In addition, if temperatures are sufficiently high, remaining NH2 species can recombine with surface hydrogen and desorb, leaving silicon sites available for other processes. On the other hand, the reaction of TDMAT with a clean Si(100) surface has been found to produce surface species with remarkable stability if the N-C bond is broken to form Si-C structures (Figure 2). While this reaction has been observed to be kinetically hindered at room temperature, it is likely that it is the major pathway if temperatures are sufficiently high to overcome the kinetic barrier. Therefore, the beginning of an ALD process for nitride deposition on silicon at high temperatures can potentially produce (1) subsurface N species, (2) Si-H surface species, (3) unreacted Si surface sites, and (4) Si-C surface species. Because of the presence of subsurface N structures, not easily available for a reaction with an upcoming TDMAT molecule, it is possible that at these high-temperature conditions the initial reaction on the surface may involve the Si-H bonds. The reaction with Si-H bonds might be eased at higher temperatures or larger doses, as observed in a previous investigation,46 but it is overall a process not favored thermodynamically, as can be observed from the computational results shown in this work. Thus, the first TDMAT exposure would

Rodrı´guez-Reyes and Teplyakov be characterized by a low amount of Ti deposited, which leads to the formation of a low-density interface. In addition, adsorption of TDMAT at high temperatures can potentially lead to ligand decomposition through N-C bond scission (a thermodynamically favored process as shown in Figure 2), which would introduce carbon into the interface. Thus, the first ALD cycle for TiN deposition on silicon at high temperatures can potentially produce an interface with low density and high levels of carbon, which decreases the performance of the film as a diffusion barrier. In this investigation, we have shown that this first ALD cycle is a process that occurs at room temperature and, in a contrast to the possible scenarios at elevated temperatures often used in ALD, surface transamination is the only reaction taking place on the surface, ensuring a high control of the process. This suggests that the formation of sharp interfaces free of contaminants can be obtained by using deposition temperatures lower than the ones employed for nitride film growth. In addition, the passivation of Si surface sites with hydrogen atoms decreases the possibility of Si-C bond formation. The control of subsequent steps, particularly the reaction of an upcoming NH3 molecule with the transaminated Ti surface structure and the elimination of surface hydrogen, is required for the formation of a dense, sharp Si/TiN interface. 5. Conclusions The reaction of TDMAT with NHX-terminated surfaces, which constitutes the first step for the growth of TiN on Si(100) by ALD, has been observed and characterized at room temperature. Our results indicate that adsorption of TDMAT occurs through a surface transamination mechanism, where the metallorganic molecule reacts specifically with amino groups available on the surface. The presence of NHX and H surface species blocks the direct interaction of TDMAT with silicon atoms, and the reaction of TDMAT with Si-H surface species is not observed along this study, leading to a high degree of control of the surface reactions involved under these thermal conditions. The elucidation of this mechanism opens the possibility for an atomic-level control of interface formation. Acknowledgment. This work was supported by the National Science Foundation (CHE-0313803 and CHE-0650123). GridChem (http://www.gridchem.org)81,82 is acknowledged for computational resources and services for the selected results used in this publication. J.C.F.R.-R. would like to acknowledge the helpful assistance of Dr. Sudhakar Pamidighantam (National Center for Supercomputing Applications, NCSA) and Dr. Stelios Kyriacou (Ohio Supercomputer Center, OSC) with the use of Gridchem. Supporting Information Available: Graphics of the structures discussed in the text, atomic coordinates, energies, and selected vibrational frequencies predicted using B3LYP/ LANL2DZ. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wolkow, R. A. Annu. ReV. Phys. Chem. 1999, 50, 413. (2) Bent, S. F. J. Phys. Chem. B 2002, 106, 2830. (3) Kim, H. J. Vac. Sci. Technol., B 2003, 21, 2231. (4) Lim, B. S.; Rahtu, A.; Gordon, R. G. Nat. Mater. 2003, 2, 749. (5) Puurunen, R. L. J. Appl. Phys. 2005, 97, 121301. (6) Niinisto, L.; Paivasaari, J.; Niinisto, M.; Putkonen, M.; Nieminen, M. Phys. Status Solidi A 2004, 201, 1443. (7) Wong, H.; Iwai, H. Microelectron. Eng. 2006, 83, 1867. (8) International Technology Roadmap for Semiconductors, 2005 ed.; Semiconductor Industry Association (SIA).

Surface Transamination Reaction (9) Kaloyeros, A. E.; Eisenbraun, E. Annu. ReV. Mater. Sci. 2000, 30, 363. (10) van der Straten, O.; Zhu, Y.; Dunn, K.; Eisenbraun, E.; Kaloyeros, A. E. J. Mater. Res. 2004, 19, 447. (11) Fix, R. M.; Gordon, R. G.; Hoffman, D. M. Chem. Mater. 1990, 2, 235. (12) Hoffman, D. M. Polyhedron 1994, 13, 1169. (13) Shin, H.-K.; Shin, H.-J.; Lee, J.-G.; Kang, S.-W.; Ahn, B.-T. Chem. Mater. 1997, 9, 76. (14) Min, J.-S.; Son, Y.-W.; Kang, W.-G.; Chun, S.-S.; Kang, S.-W. Jpn. J. Appl. Phys. 1998, 37, 4999. (15) Lee, J.; Kim, J.; Shin, H. Thin Solid Films 1998, 320, 15. (16) Kim, D.-H.; Lim, G. T.; Kim, S.-K. J. Vac. Sci. Technol., B 1999, 17, 2197. (17) Panda, S.; Kim, J.; Weiller, B. H.; Economou, D. J.; Hoffman, D. M. Thin Solid Films 1999, 357, 125. (18) Yun, J.-Y.; Park, M.-Y.; Rhee, S.-W. J. Electrochem. Soc. 1999, 146, 1804. (19) Kim, D.-J.; Jung, Y.-B.; Lee, M.-B.; Lee, Y.-H.; Lee, J.-H.; Lee, J.-H. Thin Solid Films 2000, 372, 276. (20) Kim, J.-Y.; Choi, G. H.; Kim, Y. D.; Kim, Y.; Jeon, H. Jpn. J. Appl. Phys. 2003, 42, 4245. (21) Becker, J.; Gordon, R. G. Appl. Phys. Lett. 2003, 82, 2239. (22) Becker, J.; Kim, E.; Gordon, R. G. Chem. Mater. 2004, 16, 3497. (23) Park, S.-G.; Kim, D.-H. Jpn. J. Appl. Phys. 2004, 43, 303. (24) Corneille, J. S.; Chen, P. J.; Truong, C. M.; Oh, W. S.; Goodman, D. W. J. Vac. Sci. Technol., A 1995, 13, 1116. (25) Dubois, L. H.; Zegarski, B. R. J. Electrochem. Soc. 1992, 139, 3603. (26) Truong, C. M.; Chen, P. J.; Corneille, J. S.; Oh, W. S.; Goodman, D. W. J. Phys. Chem. 1995, 99, 8831. (27) Elam, J. W.; Schuisky, M.; Ferguson, J. D.; George, S. M. Thin Solid Films 2003, 436, 145. (28) Okada, L. A.; George, S. M. Appl. Surf. Sci. 1999, 137, 113. (29) Berry, A.; Mowery, R.; Turner, N. H.; Seitzman, L.; Dunn, D.; Ladouceur, H. Thin Solid Films 1998, 323, 10. (30) Dubois, L. H. Polyhedron 1994, 13, 1329. (31) Kim, J.-Y.; Seo, S.; Kim, D.-Y.; Jeon, H. J. Vac. Sci. Technol., A 2004, 22. (32) Kim, J.-Y.; Kim, D.-Y.; Park, H.-O.; Jeon, H. J. Electrochem. Soc. 2005, 152, G29. (33) Min, J.-S.; park, H.-S.; Kang, S.-W. Appl. Phys. Lett. 1999, 75, 1521. (34) Musher, J. N.; Gordon, R. G. J. Electrochem. Soc. 1996, 143, 736. (35) Prybyla, J. A.; Chiang, C.-M.; Dubois, L. H. J. Electrochem. Soc. 1993, 140, 2695. (36) Ruhl, G.; Rehmet, M.; Knizova, M.; Merica, R.; Veprek, S. Chem. Mater. 1996, 8, 2712. (37) Bradley, D. C.; Torrible, E. G. Can. J. Chem. 1963, 41, 134. (38) Brown, G. M.; Maya, L. J. Am. Ceram. Soc. 1988, 71, 78. (39) Koyama, S.; Iizuka, D.; Sugahara, Y.; Kuroda, K. Appl. Organomet. Chem. 1998, 12, 787. (40) Fan, M.; Duesler, E. N.; Janik, J. F.; Paine, R. T. J. Inorg. Organomet. Polym. Mater. 2007, 17, 423. (41) Jackson, A. W.; Hector, A. L. J. Mater. Chem. 2007, 17, 1016. (42) Prybyla, J. A.; Chiang, C.-M.; Dubois, L. H. Mater. Res. Soc. Symp. Proc. 1993, 282, 287. (43) Cundari, T. R.; Morse, J. M., Jr. Chem. Mater. 1996, 8, 189. (44) Amato-Wierda, C. C.; Norton, E. T., Jr.; Wierda, D. E. Chem. Mater. 1999, 11, 2775. (45) Yun, J.-H.; Choi, E.-S.; Jang, C.-M.; Lee, C.-S. Jpn. J. Appl. Phys. 2002, 41, L418. (46) Kelly, M. J.; Han, J. H.; Musgrave, C. B.; Parsons, G. N. Chem. Mater. 2005, 17, 5305. (47) Beaudoin, M.; Scott, S. L. Organometallics 2001, 20, 237. (48) Killampalli, A. S.; Ma, P. F.; Engstrom, J. R. J. Am. Chem. Soc. 2005, 127, 6300. (49) Haran, M.; Engstrom, J. R.; Clancy, P. J. Am. Chem. Soc. 2006, 128, 836.

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16505 (50) Dube, A.; Chadeayne, A. R.; Sharma, M.; Wolczanski, P. T.; Engstrom, J. R. J. Am. Chem. Soc. 2005, 127, 14299. (51) Xu, Y.; Musgrave, C. B. Appl. Phys. Lett. 2005, 86, 192110. (52) Rodrı´guez-Reyes, J. C. F.; Teplyakov, A. V. J. Phys. Chem. C 2007, 111, 4800. (53) Rodrı´guez-Reyes, J. C. F.; Teplyakov, A. V. Phys. ReV. B 2007, 76, 075348. (54) Jung, Y.; Shao, Y.; Gordon, M. S.; Doren, D. J. J. Chem. Phys. 2003, 119, 10917. (55) Dresser, M. J.; Taylor, P. A.; Wallace, R. M.; Choyke, W. J.; Yates, J. T., Jr. Surf. Sci. 1989, 218, 75. (56) Fujisawa, M.; Taguchi, Y.; Kuwahara, Y.; Onchi, M.; Nishijima, M. Phys. ReV. B 1989, 39, 12918. (57) Larsson, C. U. S.; Flodstrom, A. S. Surf. Sci. 1991, 241, 353. (58) Bischoff, J. L.; Lutz, F.; Bolmont, D.; Kubler, L. Surf. Sci. 1991, 248, L240. (59) Franco, N.; Avila, J.; Davila, M. E.; Asensio, M. C.; Woodruff, D. P.; Schaff, O.; Fernandez, V.; Schindler, K. M.; Bradshaw, A. M. J. Phys. Condens. Matter 1997, 9, 8419. (60) Miotto, R.; Srivastava, G. P.; Ferraz, A. C. Phys. ReV. B 1998, 58, 7944. (61) Widjaja, Y.; Mysinger, M. M.; Musgrave, C. B. J. Phys. Chem. B 2000, 104, 2527. (62) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (63) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (64) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (65) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (66) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (67) Pirolli, L.; Teplyakov, A. V. J. Phys. Chem. B 2006, 110, 4708. (68) Pirolli, L.; Teplyakov, A. V. Surf. Sci. 2006, 600, 3313. (69) Pirolli, L.; Teplyakov, A. V. Surf. Sci. 2007, 601, 155. (70) Watras, M. J.; Teplyakov, A. V. J. Phys. Chem. B 2005, 109, 8928. (71) Dmitrenko, O.; Huang, W.; Polenova, T. E.; Francesconi, L. C.; Wingrave, J. A.; Teplyakov, A. V. J. Phys. Chem. B 2003, 107, 7747. (72) Widjaja, Y.; Musgrave, C. B. Surf. Sci. 2000, 469, 9. (73) Raghavachari, K.; Halls, M. D. Mol. Phys. 2004, 102, 381. (74) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C. 02; Gaussian, Inc.: Wallingford, CT, 2004. (75) Bater, C.; Sanders, M.; Craig, J. H. J. Surf. Interface Anal. 2000, 29, 208. (76) Mulcahy, C. P. A.; Carman, A. J.; Casey, C. M. Surf. Sci. 2000, 459, 1. (77) Carman, A. J.; Zhang, L.; Liswood, J. L.; Casey, S. M. J. Phys. Chem. B 2003, 107, 5491. (78) Xia, Y.; Mokaya, R. Angew. Chem., Int. Ed. 2003, 42, 2639. (79) Xia, Y.; Mokaya, R. J. Mater. Chem. 2004, 14, 2507. (80) Deng, X.; Baker, T. A.; Friend, C. M. Angew. Chem., Int. Ed. 2006, 45, 7075. (81) Dooley, R.; Milfeld, K.; Guiang, K.; Pamidighantam, S.; Allen, G. J. Grid. Comput. 2006, 4, 195. (82) Milfeld, K.; Guiang, C.; Pamidighantam, S.; Giuliani, J. Cluster computing through an Application-Oriented Computational Chemistry Grid. Proceedings of the 2005 Linux Clusters: The HPC reVolution, April 2005.