Toward Atomic-Scale Patterned Atomic Layer Deposition: Reactions of

Article pubs.acs.org/JPCC

Toward Atomic-Scale Patterned Atomic Layer Deposition: Reactions of Al2O3 Precursors on a Si(001) Surface with Mixed Functionalizations R. C. Longo,*,† J. H. G. Owen,*,‡ S. McDonnell,† D. Dick,† J. B. Ballard,‡ J. N. Randall,*,‡ R. M. Wallace,† Y. J. Chabal,† and K. Cho*,† †

Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States Zyvex Laboratories, LLC., 1301 North Plano Road, Richardson, Texas 75081, United States

‡

S Supporting Information *

ABSTRACT: In this paper, we use density functional theory (DFT) calculations to investigate the initial surface reactions involved in the atomic layer deposition (ALD) of Al2O3 from H2O and Al(CH3)3 (trimethylaluminum, TMA) molecular precursors on the Si(001)-(2×1) reconstructed surface with different chemical terminations. Our results for the kinetic barriers and adsorption energies of both Al and oxygen precursors along different reaction pathways show the dependence of the ALD nucleation rate on the surface defects (Si dangling bonds or dimer trench) and how it can be modified with suitable p-doping. Finally, our ab initio thermodynamics study clearly determines the relation between typical ALD working conditions and the different chemical functionalizations of the Si(001) surface with the growth properties of Al2O3 nanofilms.

1. INTRODUCTION The continuous scaling of metal-oxide-semiconductor field effect transistors (MOSFETs) in reducing the standard gate dielectric (SiO2) layer thickness to the leakage current limit (due to quantum tunneling effects), appears to be a critical problem.1,2 Also, the increasing demand for more compact memory storage devices from a variety of electronic applications motivates the necessity to develop high-κ dielectric materials to replace SiO2. To improve the deposition techniques in order to afford high surface uniformity, and control of the growth with atomic layer precision are the two most critical issues that the semiconductor industry is currently facing (see, e.g., ref 3). Among all the possible deposition and fabrication technologies, atomic layer deposition (ALD) has proven to be an ideal candidate.4 During the ALD process, precursor vapors (oxygen precursor in first place, followed by an exposure to the metal precursor) are alternately pulsed onto the surface of a substrate, with a purge cycle of an inert gas between each precursor pulse. These surface reactions are complementary and self-limiting, enabling the deposition of a material through highly uniform growth, with thickness control at the atomic level, ideally resulting in one monolayer growth after each exposure of the surface to the gas-phase precursors.4 Moreover, one means of transferring atomically precise patterns into 3D nanostructures is patterned ALD of oxide films into chemically active patterns created by selectively removing H atoms from a H-terminated Si(001) surface using a scanning tunneling microscope (STM) tip. In this situation, the ALD precursors encounter a variety of different surface terminations: © XXXX American Chemical Society

bare Si dimers within the pattern, while the background comprises a majority of fully passivated Si:H dimers, with a small density of single dangling bonds resulting from missing H atoms. Thus, for selective nucleation of oxide only within the pattern, the background sites must not be reactive with the chosen ALD precursors. Al2O3 has been extensively studied as a high-κ dielectric material because of its interesting properties:4−7 it has a wide band gap (9 eV), a large dielectric constant of 9.8 (κSiO2 = 3.9), and it is thermodynamically stable in contact with Si.8 However, its use as a hard mask in patterned ALD of oxide films has not been studied in detail yet.9 In previous ALD experiments, trimethylaluminum (TMA, Al(CH3)3) was often used as gasphase precursor for Al, due to its thermal stability, high vapor pressure and highly exothermic reaction with H2O, which is in turn the most common precursor for oxygen.10−14 Despite its importance in ALD, the atomistic interactions of TMA with Si surfaces are not yet fully understood. Although the TMA molecule shows interesting properties for the layer by layer growth of Al2O3 oxides,11−14 its degree of selectivity for patterned deposition applications is relatively low. A deep understanding of the TMA reactivity with the Si substrate in the early stages of deposition is fundamental to achieve atomically precise structural growth.15−17 To date, several theoretical studies of the chemical interaction of TMA with Si Received: September 16, 2015 Revised: January 14, 2016

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mechanisms proposed for oxygen insertion on subsurface positions of the Si(001):H surface but, on the other side, the reactivity of the TMA molecule is much higher and, consequently, much less selective than H2O. While a few years ago it was only possible to imagine it, atomic-scale manipulation of surface chemical properties by incorporating doping atoms into the surface or subsurface is now a reality.16,17 A detailed understanding of the role of dopants at the atomic scale will contribute significantly to improve the ALD process and to construct the desired interface with the required properties for chemical or electronic applications. Boron is the most widely used p-type impurity in Si, thus we have chosen to investigate whether B subsurface impurities and the change in the surface charge state induced by the impurity can affect the TMA reactivity and, thus, the rate of the ALD nucleation process. Finally, in order to set up a complete picture of the ALD process, by means of ab initio thermodynamics we intend to establish a relationship between typical working conditions and the different chemical functionalizations of the Si surface with the growth properties of Al2O3 nanofilms. The computational methods are described in detail in section 2, our results are presented and discussed in section 3, and in section 4, we summarize the main conclusions. Section 3 is divided into four subsections, describing H2O adsorption (section 3.1), ALD TMA deposition (section 3.2), the effects of B p-doping on ALD of Al2O3 (section 3.3) and ab initio thermodynamics of the ALD of Al2O3 nanofilms (section 3.4), respectively.

surfaces have appeared in the literature. Most of them use a cluster model to represent the Si substrate: for instance, Widjaja and Musgrave performed density-functional theory (DFT) studies of the mechanisms and thermochemistry of TMA and H2O half-reactions with small gas-phase Al(OH)n(CH3)m clusters, taken to represent the Al2O3 surface;18 Halls and Raghavachari studied the initial deposition mechanisms of both H2O and TMA on a fully passivated or OH-terminated Si cluster, representing the Si(100) surface.19−21 Hu and Turner reported a new mechanism for the incorporation of oxygen from H2O to the Si(100) surface,22 due to the fact that TMA lowers the adsorption barrier; another reaction mechanisms for the initial deposition of TMA and H2O on Si substrates were reported in refs 23−25. More recently, Lin et al. studied the ligand-exchange reactions of TMA with bare and hydroxylterminated Si(001) surfaces, also represented using a cluster model.26 Conversely to TMA deposition on Si substrates, the reaction of H 2 O with Si(001) surfaces has been investigated exhaustively, both experimentally and theoretically.27−51 For hydrogen-free Si (001) surfaces, there is an agreement in the literature that most of the surface species consist (after interaction with H2O) of very stable H:Si−Si:OH groups, keeping the dimer-row pattern intact.30−43,51,52 The result is different for hydrogen-passivated Si(001):H surfaces, where the monohydride reconstruction is very stable and the surface has shown to be more resistant to oxidative damage.36−38,53 There are fewer theoretical and experimental studies on wet oxidation of hydrogen-passivated Si(001) surfaces: a recent study combining experiments and cluster model ab initio calculations36−38 reported that, in the initial stages of oxidation, insertion of the H2O molecule occurs as dissociation into a silanol (Si:OH) group and dimer-bond breaking (H:Si:H and H:Si:OH). At advanced stages of wet oxidation, they also showed the presence of IR peaks attributed to back-bond oxygen structures, thus suggesting that the back-bond is the main following mechanism (after the silanol precursor) for oxygen chemisorption. More recently, a DFT study51 provided a very detailed description of the initial mechanisms for H2O adsorption on a hydrogen-passivated Si(001) surface: it was shown that the kinetic energy barrier from silanol adsorbate species to back-bond subsurface oxidation is relatively high, implying that the silanol is not a good effective precursor for inserting oxygen into subsurface sites. Instead of that, the authors proposed two novel mechanisms for on-dimer and subsurface oxidation, in both cases with the loss of a H2 molecule. However, a complete picture of the initial stages of H2O and TMA deposition, together with further evolution mechanisms for ALD growth of Al2O3 monolayers on Si substrates are still missing. Our aim in this work is 2-fold: it is well-known from the experimental evidence that, although after a H2 exposure of the clean Si(001)-(2×1) surface the vast majority of the Si dimer atoms are almost fully passivated, there always remains some density of dangling bonds. The goal of this study is to perform a theoretical investigation of the kinetic energy barriers, transition states and reaction pathways of the first stages of H2O and TMA insertion in the Si(001)-(2×1):H surface, in order to study how the kinetic processes, transition states, and reaction products are affected by the concentration of dangling bonds in the Si dimers of the (2 × 1) reconstruction of the Si(001):H surface. Our second goal is the following: as stated previously, there are several

2. COMPUTATIONAL METHODS Ab initio calculations were performed using DFT with plane wave basis sets and projector augmented wave (PAW) pseudopotentials, as implemented in the VASP code.54,55 The electronic wave functions were represented by plane wave basis with a cutoff energy of 500 eV. We include the exchange correlation interactions by using the semilocal Perdew−Burke− Ernzerhof (PBE) functional of the generalized gradient approximation (GGA).56 The unit cell of the ideal Si(001)-(2×1):H is made of two Si atoms per layer, with the surface dimers passivated with hydrogen, forming H:Si−Si:H groups. In our simulations, we modeled the Si(001)-(2×1):H surface with a periodic slab containing six Si atomic layers, with only the bottom Si layer passivated by two H atoms per Si atom (See Supporting Information for entire pictures of the supercell). The thickness of the vacuum region in the direction perpendicular to the slab is 15 Å. The H2O and TMA molecular precursors were initially adsorbed on the dimer-reconstructed side of the slab. To make their interaction sufficiently weak, we employed a (2 × 4) unit cell that includes four Si dimers along the dimer row and a trench between each one of the two dimer rows. To test the accuracy of our calculations, in some specific cases we extended the simulations to (4 × 4) surface unit cells, thus increasing the lateral periodicity. All the atoms, except the bottom two layers of Si and the bottom hydrogen-termination layer, were allowed to relax without any constraint. We started our calculations with the experimental lattice constant of 5.431 Å for Si.57 Upon relaxation, our obtained GGA value was 5.47 Å. The initial configurations for gas phase (desorbed) H2O and TMA molecular precursors were taken from the available experimental results.4 The kinetic barriers, transition states and reaction paths were obtained using the climbing image−nudged elastic band B

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The Journal of Physical Chemistry C method (CI−NEB).58−60 This method allows us to obtain the minimum energy path (MEP) between a set of two different states. To do that, the reaction path is divided into a set of images “connected with a spring”. During the relaxation, the initial and final states are kept frozen while the images move according to the constraint of the “elastic band”. The MEP is then found when the components of the forces perpendicular to the “elastic band” vanish, the relative positions of the images and the barrier being determined by the parallel components of the forces.59,60 Some of the limitations of the NEB method include the possibility of having a lower barrier if the initial orientation of the molecule with respect to the surface is not properly chosen. In this work, we account for that possibility by optimizing the initial geometry of the TMA desorbed molecule (for H2O the problem is practically irrelevant) with respect to both the initial and final configurations. That obviously excludes certain cases where the interaction of the TMA with the functional groups play a fundamental role, such as OH terminations in neighboring dimers. In all cases, we used a (3 × 6 × 1) k-point mesh (this sampling is equivalent to the one obtained with (12 × 12 × 1) grids for the primitive cell of the surface) within the Monkhorst−Pack61 scheme to ensure a convergence of 10 meV/unit cell. We performed structural relaxations without including any type of symmetry, to a tolerance of 10−4 eV in the total energy and 0.01 eV/Å in the forces on every atom, for both standard and CI−NEB structural minimizations. The adsorption energies at T = 0 K were obtained with the usual formula: ΔE = ET − Esurface − NmolEmol

⎡⎛ 2πmkT ⎞3/2 ⎤ ⎟ kT /p⎥ Δμ(T , p) = −kT ⎢⎜ ⎣⎝ h2 ⎠ ⎦ − kT[

+ kT ∑ ∑ ln[1 − exp( −hωi /kT )] i

μ(mol)(T , p) = μref (T , p0 ) + kT ln p /p0

Δμ(mol)(T , p0 ) = ΔH(mol)(T , p0 ) − T ΔS(mol)(T , p0 ) (9)

allows us to use tabulated enthalpies and entropies at a reference pressure65 to obtain the change in the chemical potential with respect to the T = 0 K calculated values. Combining these last two equations, we can write

(1)

Δμ(mol)(T , p) = ΔH(mol)(T , p0 ) − T ΔS(mol)(T , p0 ) + kT ln p /p0

ΔG = Nmol[ΔE − Δμ(mol)(T , p)]

1 (ztranszrot zvibzel)N N!

3. RESULTS AND DISCUSSION 3.1. H2O Adsorption on the Si(001)-(2×1) Surface. First, we will discuss the H2O adsorption on the Si(001):H surface, and the effect of surface defects (Si dangling bonds) on the kinetic energy barriers and reaction energies. As was previously mentioned, a very recent work by Sousa and Caldas51 described in detail the main reaction mechanisms for H2O adsorption on a fully passivated Si(001):H surface. Sousa and Caldas51 obtained a kinetic barrier for the CSil formation on the H-passivated surface of 1.12 eV, which is in agreement with the well-known experimental evidence that the hydrogen passivation hinders the oxidation of Si surfaces. Our own calculations of H2O adsorption on a fully passivated Si(001) surface yielded a kinetic barrier for CSil formation of 1.26 eV. This slight difference (coming from the different pseudopotentialsultrasoft vs PAWand exchange-correlation functionalsPW91 vs PBEused in both calculations) makes necessary to include a small correction in the kinetic energy barriers when comparing our results for the Si(001):H surface with dangling bonds with those obtained for the fully passivated Si(001):H surface. For the Si(001):H surface with one dangling bond, we considered two different initial configurations of the H2O

(2)

(3)

(4)

(5)

Then, we have μ(mol)(T , p) = μref (mol) + Δμ(mol)(T , p)

(11)

where Δμ(mol)(T,p) can be estimated by eqs 7 and 10 for TMA and H2O, respectively.

where Z is the total partition function of an ideal gas of N indistinguishable molecules with translational, rotational, vibrational and electronic levels: Z=

(10)

Finally, eq 2 reads as

where E is the internal energy and Fvib the vibrational free energy. The pV term can be neglected at typical ALD pressures.64 The chemical potential of the molecular precursors is defined as μ(mol)(T , p) = ( −kT ln Z + pV )/N

(8)

gives us the temperature and pressure dependence of μ(mol)(T,p) once its temperature dependence at a reference p0 is known, and

For the condensed phases, the Gibbs free energy can be written as

G = E + Fvib + pV

(7)

contains the remaining translational, rotational and vibrational terms. In the previous equation, σ is the symmetry number, IA, IB, and IC the principal moments of inertia and ωi the vibrational frequencies of the molecule. The vibrational term for the TMA molecular precursor (0.1 eV) is smaller than the rotational (0.4 eV) and translational (0.9 eV) terms63 and can thus be neglected. We have used the experimental data to obtain the chemical potential of H2O. The well-known following equation:

where ET is the total energy of the system (with adsorbed molecular precursors), Esurface is the energy of the Si slab, and Emol (H2O or TMA) is the energy of the molecule in the corresponding gas phase. To take into account the effect of temperature and pressure in the ALD process, we followed the ab initio thermodynamics approach.62 Following the methodology shown in ref63., the Gibbs free adsorption energy for Nmol molecular precursors on a slab, ΔG, is defined as ΔG = GT − Gsurface − Nmolμmol

8π 2 (2πkT )3/2 (IAIBIC)1/2 ] σh3

(6)

where μref(mol) is the total energy of the molecular precursor at T = 0 K, and C

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physisorption, but these relatively low values make it difficult to state which configuration is more stable. Upon chemisorption, the final configurations considered in this work are also shown in Figure 1 (the Section S1 of the Supporting Information shows the entire pathways of the H2O adsorption) and their adsorption energies are also listed in Table 1. Our calculations show that the on-dimer configuration (COD) is the most stable of all the structures considered in this work, and that the silanol (CSil) is the least stable chemisorbed configuration. We now discuss in detail the reactions leading to all the configurations shown in Figure 1. For the reaction path followed by the H2O molecule adsorbed over the dimer rows, Figure 1 shows that there is no physisorbed state along this path, similarly to the result obtained for the hydrogen passivated Si(001) surface. The kinetic barrier for H2O dissociation and formation of the CSil configuration is slightly lower than that of the fully passivated Si(001):H surface (0.92 and 1.26 eV respectively), and the energy gain is also similar (0.49 and 0.35 eV respectively). By comparison, the H2O dissociation on the clean Si(001) surface is very exothermic (2.52 eV of energy gain) and a barrierless process. The reason for this difference is that the two Si surface atoms do not need to break their bond, due to the existence of the dangling bond in the Si atom to which the H2O molecule is initially physisorbed before the dissociation. In both cases, the desorption kinetic barrier is relatively high, 1.61 and 1.41 eV. Once formed, the CSil will not easily desorb from the Si(001) surface, thus forming the initial oxidation step of the Si substrate. Thus, in summary, the single dangling bond case is much more like the fully passivated surface than the clean surface. Figure 1 also shows the MEP for the reaction leading to the oxygen back-bond (CBB) in the Si(001):H surface. The overall result shows that the energy kinetic barrier is very large, even with a dangling bond present in the hydrogen-passivated Si(001) surface. The insertion of the oxygen into the back-bond of the surface, starting from the CSil configuration, needs to overcome two different kinetic energy barriers: the first one corresponds to the breaking of the OH bond of the silanol unit and the adsorption of the H in the Si surface atom, this value is 2.15 eV. The barrier for the reverse desorption process is only 0.63 eV, whereas the barrier for the second step of the backbond formation, i.e., the stretching of the Si−Si bond to insert the O atom into the back-bond position, is of only 0.15 eV (see Figure 1). Once the back-bond is formed, the energy gain with respect to the CSil configuration is only 0.08 eV. As a consequence of the high kinetic barrier and the small energy difference, the oxygen back-bond formation does not seem to be the most likely mechanism for wet oxidation of the Si(001):H surface with surface defects-dangling bonds. The other reaction pathways investigated involve the release of a H2 molecule. Starting from the H2O over the valley (dimer trench), we found two different pathways for oxygen back-bond formation (see the lower panel of Figure 1). In the first one, there is a relatively high kinetic barrier (1.71 eV), in order to release the H2 molecule. No physisorbed state was obtained in our calculations, due to the promptness in losing the H2 molecule. Interestingly, if the oxygen atom is adsorbed in the valley, the obtained configuration is slightly more stable than the oxygen back-bond (0.06 eV). A small kinetic barrier of 0.09 eV should be overcome in order to jump from the valley to a back-bond or vice versa. However, whichever configuration the oxygen atom is chemisorbed, a kinetic barrier of 2.25 eV

molecule: over the dimer row or the trench between both dimer rows. Figure 1 shows these two initial configurations and

Figure 1. Dissociation paths of the H2O molecule adsorption on the Si(001)-(2×1):H with one dangling bond per dimer row, showing the different structures obtained (CSil, CBB, and COD correspond to the silanol, oxygen back-bond and on-dimer configurations, respectively) and the respective kinetic barriers (see text for details and Supporting Information for the complete pathways), when the H2O molecule impinges on the Si surface over the dimer row (upper panel) or over the dimer trench (lower panel). Yellow spheres represent Si atoms; white, H; and red, O; respectively.

Table 1 shows their adsorption energies (with respect to the energies of the Si(001):H surface with one dangling bond and the H2O gas phase), kinetic energy barriers and the distance between the oxygen atom and the Si surface atom with the dangling bond. Our results predict in both cases a weak Table 1. Adsorption Energies, Kinetic Energy Barriers, and Si−O Distance of the Different Configurations Obtained for H2O Adsorption and Dissociation on the Si(001)-(2×1):H Surface with One Dangling Bonda configuration

adsorption energy (eV)

dimer valley silanol (CSil) back bond (CBB) back bond + H2 on dimer (COD) + H2

0.00 −0.01 −0.49 −0.57 −0.48 −1.01

kinetic barrier (eV)

dSi−O (Å)

0.92 2.15 1.71/0.67 1.44

3.72 4.21 1.67 1.65 1.66 1.67

a The sign of the adsorption energies identifies the character of the reaction (exo- and endothermic correspond to minus and plus signs, respectively).

D

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than that of the Si(001)-(2×1) surface and the H2O gas phase. To conclude, we can say that the oxidation of the Si(001)(2×1) with available Si dimers with dangling bonds takes place via a surface on-dimer mechanism, rather than oxygen backbond formation. In order to test the clean surface limit, we also performed calculations with three out of the four Si atoms of the dimer row with available dangling bonds (see Figure S4 of the Supporting Information to see the entire set of adsorption energies and kinetic barriers). Very briefly, the main conclusions that can be extracted from our results are the following: First, the kinetic barrier for CSil formation is also negligible (0.03 eV), i.e., only one Si dimer with dangling bonds per dimer row is needed to spontaneously form the CSil defect, which is always a transition state for every surface oxidation pathway. Second, all the oxidation mechanisms considered in our work are possible under these conditions: oxygen backbond and on-dimer oxidation show kinetic barriers as low as 0.20 and 0.27 eV, respectively, but both pathways entail the passivation of the Si(001)-(2×1) surface with the two hydrogen atoms of the H2O molecule. The release of the hydrogen would enlarge the kinetic barriers up to 2.14 and 1.38 eV for the ondimer and oxygen back-bond oxidation mechanisms, respectively. 3.2. ALD TMA Deposition on the Si(001)-(2×1) Surface. In a typical ALD process, a thin solid film is deposited through several cycles of self-terminated surface reactions. For binary oxides like Al2O3, a complete growth cycle consists of two-half reaction exposures, usually first exposing the surface to the oxygen precursor and then to the metal (Al in this case) precursor, both exposures separated by a purge period. In the previous section we investigated in detail the initial steps for surface oxidation, and all of the processes studied entail the formation of a OH (CSil) group on the surface as an intermediate step for every oxidation mechanism. In this section we will analyze in detail the early stages of TMA deposition on the Si(001)-(2×1) surface with different chemical functional groups, i.e., Si(001)-(2×1) clean surface (dimer-reconstructed), hydrogen passivated (with and without dangling bonds) and OH-terminated (with different OH concentrations), in order to study the ALD second halfreaction (after an initial exposure to the oxygen precursor) or TMA deposition with no previous H2O ALD pulse. Finally, we will also outline the second cycle of the ALD process, i.e., a second exposure to the oxygen precursor after the initial deposition of H2O and TMA. 3.2.1. TMA Adsorption on the Si(001)-(2×1) Surface with No Previous H2O ALD Pulse. First, we consider the TMA deposition on a clean, dimer-reconstructed Si(001)-(2×1) surface. There are three possible reactions of Al(CH3)3 with the Si(001)-(2×1) surface:

prevents any possible H2 dissociation or release of the H2O molecule. More interesting is the second path found in our calculations. This path comprises two different steps: the first one is the formation of a CSil unit, but with the release of a H2 molecule. The barrier for this step is relatively low, 0.67 eV, and the reaction is exothermic (0.31 eV with respect to the H2O gas phase). Before the chemisorption of the CSil unit, in this first step we obtained a physisorbed state with an energy of 0.06 eV (due to a weak Coulombic interaction between the positively charged H atoms in the H2O molecule and the negatively charged H:Si−Si:H surface units). The second step consists of the oxygen back-bond formation. The kinetic barrier is 0.52 eV and the final energy gain is 0.48 eV (with respect to the initial surface and H2O gas phase states). This pathway is only possible due to the availability of the Si dangling bond, which facilitates the loss of the H2 molecule in the first step. Consequently, the formation of the oxygen back-bond with the release of a H2 molecule and CSil formation as an intermediate step is a very likely oxidation path of the defective Si(001):H surface, with one dangling bond per dimer row. Finally, we examined the possibility of on-dimer surface oxidation by breaking the Si−Si surface dimer bond. The upper panel of Figure 1 shows the obtained MEP, which again goes through two different transition states. Starting from the CSil configuration, the first kinetic barrier (1.44 eV) corresponds to the loss of a H2 molecule, and the second one (0.25 eV with respect to the intermediate stable configuration) to the oxygen insertion after the breakage of the Si surface dimer bond. Although this is the most exothermic reaction among all the processes studied in this work (1.01 eV with respect to the Si surface and H2O gas phase), the kinetic barrier is much higher than that of the oxygen back-bond process. In our surface model, the Si surface dangling bond increases the stability of the Si:OH unit (with respect to the fully passivated Si(001):H surface), thus making the on-dimer surface oxidation mechanism less likely than oxidation through the oxygen back-bond pathway. If we have an additional dangling bond in the dimer row, the results are slightly different (see Figure S3 of the Supporting Information for all the adsorption energies and kinetic barriers obtained in our calculations). First, the availability of an extra Si atom without hydrogen passivation makes the kinetic barrier for CSil formation almost negligible (0.07 eV) and, moreover, the process is continuously exothermic, since the physisorption energy is relatively high, 0.46 eV. Consequently, the sticking coefficient for CSil formation must be considerably larger and the desorption barrier of the H2O molecule (2.44 eV) must be unreachable at practical temperatures. The rest of the mechanisms considered show similar kinetic barriers. For instance, the oxygen back-bond formation must overcome two different barriers, 2.29 and 1.20 eV, respectively, as this process is hardly affected by the presence of the additional Si dangling bond. However, if the oxygen back-bond process entails the release of a H2 molecule, the kinetic barrier is much lower, 1.43 eV, although it is still similar to the result obtained with only one dangling bond on the Si(001)-(2×1):H surface. Finally, for the on-dimer oxidation of the surface, there are two different pathways: the release of a H2 molecule shows a large kinetic barrier (2.72 eV), but the full passivation of the Si(001)-(2×1) surface with hydrogen atoms (there are two Si atoms with dangling bonds available) lowers the barrier to only 0.87 eV and, moreover, it shows the largest exothermicity of all the pathways considered, with a final reaction energy 3.22 eV lower

Si + Al(CH3)3 → Si−CH3 + Si−Al(CH3)2

(12)

Si + Al(CH3)3 → Si−CH−Al(CH3)2 + H 2

(13)

Si + Al(CH3)3 → Si−Al−CH 2−CH3 + CH4

(14)

The first reaction is just the dissociation of the TMA molecule into a methyl group and a DMA (dimethylaluminum) molecule. The reaction path (shown in Figure 2a and Figure S5 of the Supporting Information) shows a moderate kinetic barrier of 0.75 eV (see Table 2). Besides, the reaction is strongly exothermic: the dissociation of the TMA on the clean E

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with the TMA bound to the Si surface through a CH group. This reaction has a larger kinetic barrier than that of the dissociation of the TMA molecule on the surface, 1.88 eV (see Figure 2a). The corresponding transition state is just the chemisorption of the TMA molecule to the Si(001)-(2×1) surface and the weakening of the C−H bond. The final Si−C distance is 1.80 Å (cf. with 1.96 Å for Al−C bond distance). The reaction is endothermic, with a final energy of 1.56 eV. The small desorption energy barrier obtained (0.22 eV) results in a high possibility of readsorption of the released H2 molecule and, consequently, the desorption of the chemisorbed TMA molecule. In the last reaction, a weak Al−Si bond is formed (the bond distance is 2.50 Å), resulting in the loss of a methane CH4 molecule and the deposition of Al−CH2−CH3 surface species. Figure 2a also shows the final state of this reaction (the entire energy pathway can be found in the Supporting Information). When the Al−Si distance is ≈4.6 Å, the TMA molecule dissociates into CH4 (taking one hydrogen atom from one of the CH3) and Al−CH2−CH3. Then, the molecule is chemisorbed to the Si surface. The kinetic barrier is as high as 2.88 eV, and the reaction is also endothermic (1.37 eV). The desorption barrier would then be 1.51 eV, indicating that, if for some reason the Si−Al bond can be thermodynamically formed, the desorption of the TMA will be an unlikely process. Our calculations show that, at a shorter TMA-surface distance, the dissociation kinetic barrier will be even higher (more than 4 eV), making the Al deposition on a dimer-reconstructed Si(001)-(2×1) surface through the release of a CH4 molecule not very plausible at low-medium temperatures. Finally, our results also show that TMA can not be molecularly chemisorbed on a buckled-up Si atom, since Si can not donate electron density to the unsaturated Al atom as efficiently as O, as we will show later. The final Si−Al distance obtained in our calculations is 4.12 Å, regardless of the initial position of the TMA molecule. If the Si(001)-(2×1) surface is hydrogen-passivated, the reaction mechanisms are similar to those identified for the clean, dimer-reconstructed Si surface, but the kinetic properties differ significantly. Figure 2b shows the pathways for the three reactions (see Figure S6 of the Supporting Information for the whole trajectories and Table 2 for the reaction energies and kinetic barriers). If the Si(001) surface is hydrogen-passivated, there is a weak Coulomb attraction between the partially filled p orbitals of the Al atom of the TMA molecule and the (slightly) negatively charged surface hydrogen atom. This attraction weakens the Al−C bonds and makes possible the different reactions considered. The kinetic barrier for the first reaction (the exchange of a CH3 ligand with a surface hydrogen) is as high as 2.63 eV, and the reaction is slightly endothermic (the

Figure 2. Dissociation paths (methyl adsorption, black curve; H2 release, red curve; CH4 formation, blue curve; respectively) of the TMA molecule adsorption on the Si(001)-(2×1) with different chemical functionalizations: (a) clean surface, (b) hydrogen-passivated surface and (c) hydrogen-passivated surface with one dangling bond per dimer row (see Supporting Information for the complete pathways). Yellow spheres represent Si atoms; white, H; blue, C; and pink, Al; respectively.

Si(001)-(2×1) surface is 1.98 eV more stable than the TMA molecule in the gas phase and the Si surface (cf. with the value of 2.48 eV obtained by Lin et al. using a surface cluster model26). The second reaction entails the loss of a H2 molecule

Table 2. Adsorption Energies and Kinetic Barriers (in parentheses) of the Three Reactions Considered in This Work of TMA Adsorption on the Si(001)-(2×1) Surface with Different Chemical Functionalizationsa Si(001) funct. clean surface H-passivated 1 dangling bond 1 OH group intradimer OH interdimer OH

methyl adsorption −1.98 +0.04 +0.38 +1.04 +1.03 +0.87

(0.75) (2.63) (0.46) (2.54) (2.09) (3.89)

H2 release +1.56 +0.54 +1.16 +1.84 +1.77 +1.72

CH4 release

(1.88) (3.90) (1.57) (2.78) (4.40) (2.73)

+1.37 −0.47 −0.39 −1.74 −1.75 −1.79

(2.88) (1.36) (1.51) (0.38) (0.90) (0.05)

ring-closing

2nd H2O adsorption

−0.74 (1.13) −1.21 (0.01)

−1.49 (0.13) −1.17 (2.03) −1.43 (−)

a

The energies are given in eV. The sign of the adsorption energies identifies the character of the reaction (exo- and endothermic correspond to minus and plus signs, respectively). F

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reaction pathways obtained in our calculations can be found in the Figure S8 of the Supporting Information. All of them are slight variations of the reactions aforementioned. The first reaction shows the chemisorption of a methyl CH3 group on one of the dangling bonds, detached from the TMA molecule. Our obtained kinetic barrier is 0.97 eV, and the reaction is 0.86 eV endothermic. The second reaction is again the loss of a H2 molecule, with a kinetic barrier (1.77 eV) and endothermic character (1.45 eV) very similar to those of the clean Si(001) surface, because the dimer with two dangling bonds shows similar features (one Si atom is buckled-up and the other buckled-down) to the clean Si surface. The other three reactions considered in our work are, respectively, the dissociation of the TMA molecule into a methyl CH3 group and DMA surface species over the two surface dangling bonds (with a kinetic barrier of 0.95 eV and a strong exothermic character of 2.08 eV); starting from the same transition state of this last reaction, the loss of a H2 molecule (kinetic barrier of 0.73 eV and endothermic energy of 1.39 eV) and, finally, the release of a methane CH4 molecule. Similarly to the clean Si(001)-(2×1) surface, the kinetic barrier is relatively high (1.98 eV, the additional hydrogen atom for the methane formation is taken from the neighboring Si dimer), but the reaction is slightly exothermic, 0.28 eV. These results seem to indicate that direct deposition of Al onto the Si(001)-(2×1) without a previous ALD oxygen precursor cycle is hindered by the high kinetic barriers needed to overcome the TMA dissociation and Al−(CH3)x deposition. The only (relative) exception is given when the surface is fully hydrogen-passivated, and the release of a CH4 molecule (taking away one of the surface hydrogen atoms) is a thermodynamically favored process (in part due to the large formation energy of the methane molecule) and needs to overcome a relatively low kinetic energy barrier of 1.36 eV. 3.2.2. TMA Adsorption on the Si(001)-(2×1) Surface after a Previous H2O ALD Pulse. The situation is completely different if we have OH (CSil) surface species from a previous oxygen molecular precursor ALD cycle (H2O in this work). The OH (CSil) surface species obviously provide a more reactive environment than the clean or hydrogen-passivated Si(001)-(2×1) surface. In our study, we have considered several possibilities, regarding the concentration and distribution of CSil surface species, in order to see how they affect the kinetics and the initial deposition rates at the atomic level. The first scenario considered in our study is a Si(001)(2×1):H surface with one OH group replacing one of the hydrogen atoms, i.e., a previous ALD oxygen precursor cycle. Subsequent alternating exposures of those Si:OH surface species to TMA and H2O will result in the growth of Al2O3 nanofilms. Similarly to the hydrogen-passivated Si surface (with or without dangling bonds), we considered three different reactions: proton exchange (methyl formation), release of a H2 molecule and formation of CH4 with an O−Al bond. According to our results, there is no TMA molecular adsorption on the OH lone pair, although the final H−Al physisorption distance obtained in our calculations is 2.95 Å, much smaller than the Si−Al (surface-TMA molecule) distance obtained in the TMA deposition on the clean Si surface. Figure 3 (and the Figure S9 of the Supporting Information) shows the reaction paths of the three different reactions. The main difference with respect to the clean or hydrogen-passivated Si(001)-(2×1) surface is that, as the TMA molecule impinges on the Si:OH reaction site, there is a strong interaction between the charged O2− and one

energy difference with respect to the surface and the TMA gas phase is only 0.04 eV). The reaction energy agrees with previous results,18−21 but the kinetic barrier is considerably higher, as the previous results were only estimations based on energetic calculations, without considering the kinetics of the process. The proton−ligand exchange will then be thermodynamically favorable but kinetically unlikely. The second reaction is the loss of a H2 molecule and the chemisorption of the TMA molecule to the surface through a CH2−Si bond. The barrier is even higher than that of the clean Si(001)-(2×1) surface, 3.90 eV, and the reaction is again endothermic, with an adsorption energy of 0.54 eV. If the reaction is thermodynamically forced, the large desorption barrier (3.36 eV) makes the reverse process (readsorption of the H2 molecule and desorption of the TMA molecule) also very unlikely. The Si−C bond distance is now 1.90 Å, slightly larger than that for the clean Si(001)-(2×1) surface (the bond is now Si−CH2, instead of Si−CH). Finally, as shown in Figure 2b, the third reaction is the loss of a methane CH4 molecule and the deposition of Al−(CH3)2 (DMA) surface species. The reaction proceeds through a transition state where the surface loses the hydrogen (when the Si−Al distance is 2.94 Å) and it is bound to the Al atom. The kinetic barrier is moderately low, 1.36 eV (slightly larger than in a previous study191.14 eV but using different methodology), and the overall reaction is exothermic, with an adsorption energy with respect to the Si surface and TMA gas phase of 0.47 eV. The bond distance between Al and the surface Si atom is 2.48 Å. The desorption energy barrier is then 1.96 eV, which makes the TMA desorption very unlikely once the CH4 has been released. Now, we want to investigate how the presence of dangling bonds (as the most common surface defects) affects the kinetics and energetics of the TMA adsorption pathways considered. Figure 2c and Table 2 (and Figure S7 of the Supporting Information) show the reaction paths and the adsorption energies and kinetic barriers of the same three reactions considered above, but now with a dangling bond in the Si(001)-(2×1):H surface. The first adsorption pathway consists in the loss of one of the methyl CH3 groups from the TMA molecule, which is chemisorbed to the surface dangling bond. As Figure 2c shows, although the reaction is slightly endothermic (0.38 eV), the kinetic barrier for this process is very low (0.46 eV). This might be problematic for the ALD growth process, as it would introduce ligand C−H surface species (although it could also be seen as an effective way to passivate the dangling bond). The second reaction (Figure 2c) is the loss of a H2 molecule and the deposition of Si−CH−Al− (CH3)2 surface species, similarly to the process already shown for the clean Si(001)-(2×1) surface. The kinetic barrier is slightly lower than that of the clean surface, 1.57 eV, and the reaction is also less endothermic (1.16 eV). The reason for this result is that the Si surface dimers are not buckled, as in the clean Si surface, because there is only one dangling bond and the remaining Si surface atoms are hydrogen-passivated. The Si−C bond distance is the same as for the clean surface, 1.80 Å. Finally, the release of a CH4 molecule (see Figure 2c for the reaction path) shows a slightly larger kinetic barrier (1.51 eV) and less exothermic character (0.39 eV) than that of the Si(001)-(2×1):H surface, because the additional H atom to form the methane molecule is taken from the neighboring Si atom of the same dimer. For completeness, we also studied the case of the Si(001) surface with two dangling bonds per dimer row. The main G

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reaction (i.e., the formation of a double O−Al−O bond). However, further exposure to the oxygen precursor results in additional growth of the Al2O3 layer, by releasing CH4, with a large sticking coefficient (see the right-hand side of the blue curve of Figure 3). Our calculations show that the most favorable path for H2O molecular adsorption is an Al−(CH3)2 surface site. The DMA molecule has a planar configuration, where one Al 3s and two Al 3p orbitals hybridize to form sp2 orbitals, bound to the OH and CH3 groups. This leaves an empty p orbital, that interacts with the incoming H2O molecule. The adsorption is then followed by CH4 formation and desorption, with an almost negligible kinetic barrier of 0.13 eV and an exothermic reaction energy of 1.49 eV. Subsequent exposure to H2O results in the loss of the last CH4 methane molecule with similar kinetic barrier and reaction energy, and the formation of Al−(OH)2 surface species, ready for the next TMA exposure of the ALD process. If the number of OH-terminated Si dimers increases (due to missing H atoms), the kinetic barriers for the ALD process become slightly different, due to the van der Waals interaction between the CH3 ligands and the neighboring OH groups. We first consider the intradimer configuration, with two OH groups adsorbed on the same dimer of the Si(001)-(2×1):H surface (replacing two surface hydrogen atoms). Figure 4 (upper panel) and Table 2 (and Figure S10 of the Supporting Information) show the three pathways corresponding to similar reactions as those considered previously, together with the kinetic barriers and adsorption energies. The first reaction is the proton-methyl ligand exchange. The kinetic barrier (2.09 eV) and endothermic reaction energy (1.03 eV) are only slightly lower than that of the Si(001)-(2×1):H surface with one OH group, because of the long O−O distance (3.77 Å). The release of a H2 molecule, however, shows a high kinetic barrier (4.40 eV) and strong endothermic character (1.77 eV), making this reaction very unlikely. The reason for that, as can be seen in Figure 4, is the Coulomb repulsion between one of the CH3 groups of the TMA and the surface OH species, due to the planar configuration of the TMA. Any other possibility can be ruled out, because if the TMA impinges the Si surface across the trench between both Si dimer rows, the kinetic barrier would be even larger. Finally, the release of the CH4 methane molecule shows a similar exothermic reaction energy (1.75 eV) but the kinetic barrier is 0.90 eV, larger than that of the surface with only one OH group in the Si dimer. The transition state shows a Al−O distance of 2.67 Å, whereas the distance of the CH3 methyl group closest to the neighboring surface OH is only 2.36 Å. The final O−Al bond distance is 1.72 Å. Previous works found a much lower kinetic barrier (0.50 eV with respect to the TMA physisorbed configuration) for this pathway using a fully passivated Si(001):OH surface model24 or no kinetic barrier using cluster-constrained models.18,21 Because of the neighboring surface OH species, the chemisorbed DMA molecule can undergo a ring closing reaction, by forming other Al−O bond and releasing another CH4 methane molecule. The reaction path is also shown in the upper panel of Figure 4. The reaction energy is −0.74 eV (exothermic) and it shows a moderately low kinetic barrier of 1.13 eV, in contrast to previous studies.24 Although full monolayer deposition can be difficult to achieve due to effects like steric repulsions between adsorbates at adjacent reaction sites (as stated previously), this finding shows the importance of the neighboring OH group to create uniform Si−Al2O3 junctions by saturating all the surface bonding sites, in contrast

Figure 3. Dissociation paths (methyl adsorption, black curve; H2 release, red curve; and CH4 formation, blue curve; respectively) of the TMA molecule adsorption on the Si(001)-(2×1):OH surface (see Supporting Information for the entire pathways). Yellow spheres represent Si atoms; white, H; blue, C; red, O; and pink, Al; respectively. For the CH4 formation reaction (blue curve), the dissociation path of the 2nd step of the ALD process is also shown.

of the empty p orbitals of the Al atom. As a result, a Si−OHTMA physisorbed surface complex can be formed, although this fact does not imply that all the reactions considered in our study are going to be kinetically favored. That would depend on the relative energy of the transition and final states. Indeed, Figure 3 and Table 2 show that the proton-methyl group ligand exchange reaction is slightly endothermic but it has to overcome a relatively high kinetic barrier of 2.54 eV. The release of a H2 molecule (see Figure 3) is an even less favorable process, with an endothermic reaction energy of 1.84 eV and a kinetic barrier of 2.78 eV. Both reactions entail the chemisorption of C and H species. Thus, we can assert that there is no favorable mechanism for such processes, both kinetically and thermodynamically. The last pathway considered is the release of a methane CH4 molecule and the chemisorption of a DMA molecule through the formation of an O−Al bond (see the blue curve of Figure 3). This mechanism should be the most favorable one, in terms of metal-oxide ALD deposition. Our results (Table 2) show that this process is strongly exothermic (the adsorption energy of the final state is 1.74 eV lower than that of the Si(001):OH surface and TMA gas phase) and that the obtained kinetic barrier is moderately low (0.38 eV), as compared to the clean or hydrogen-passivated Si(001) surface. Similar kinetic barriers (0.55 eV) have been obtained using surface cluster models.26 This finding allows to conclude that, with a slow rate of oxygen precursor exposure, the TMA chemisorption and growth rate of Al2O3 nanofilms would be kinetically and thermodynamically favored. On the contrary, if the previous ALD cycle results in the oxygen adsorbed on a back-bond site (CBB), that still would leave a highly polarized dangling bond on the Si surface. Our results show (see Figure S9e of the Supporting Information) that the kinetic barrier and the reaction energy of the TMA dissociative adsorption (with the release of CH4) on that site are relatively similar to the results obtained for TMA adsorption on the Si(001)-(2×1) surface with a single dangling bond (see Figure 2), thus remarking the need to form CSil surface species to promote the growth of Al2O3. As there are no additional OH groups on the Si(001)-(2×1) surface, the chemisorbed TMA can not undergo a ring closing H

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O−Al bond shows the most favorable characteristics for ALD Al2O3 nanofilm growth of all the reaction paths considered: the reaction is strongly exothermic (1.79 eV), with an almost negligible kinetic barrier (0.05 eV with respect to the TMA gas phase) and an initial physisorption energy of the TMA molecule on the Si(001)-(2×1):OH surface of 0.15 eV. Although the bond distances in the transition state are rather similar to the previous surface configuration (both OH groups in the same Si dimer) the TMA molecule adopts a more energetically favorable nonplanar configuration with a weak Coulomb interaction between the lower CH3 groups of the TMA molecule and the neighboring OH group and surface hydrogen, respectively. This result agrees with previous studies which found no kinetic barrier for this process, but considering an unrealistic situation of a fully OH-passivated Si(001) surface.24 Another work, which considered only one OH termination per Si dimer (similar to our model) found a kinetic barrier of 0.62 eV,25 but with respect to the TMA physisorbed state (0.20 eV in our calculations). It is also important to note that, by comparison to a real space study of ammonia deposition on Si surfaces,66 the surface is likely to include a mixture of linear and zigzag arrangements of OH groups. Furthermore, it might be possible to engineer a dominance of linear patterns, perhaps by varying the adsorption temperature, in order to maximize the nucleation efficiency. The ring closing reaction (Figure 4) to form the second O−Al bond and release the second methane molecule is a barrierless and also exothermic (1.21 eV) process. The O−Al bond distance is 1.73 Å (practically the same1.75 Åas for the intradimer O−Al−O bond formation). Another exposure to the oxygen molecular precursor (H2O) shows (see the lower panel of Figure 4 and Table 2) no kinetic barrier for the release of the last CH4 molecule and the reconstruction of the OH surface termination. The reaction is also exothermic, with a reaction energy of 1.43 eV. In our calculations, we have considered the most favorable pathway, where the H2O molecule impinges the surface over the trench between Si dimer rows. Other choices would recover the large kinetic barrier obtained in the previous situation, with both OH groups adsorbed in the same Si dimer. In summary, among all the pathways considered, the last one described (H2O dissociation on the same dimer followed by TMA chemisorption and formation of a double O−Al−O bond) is the lowest barrier nucleation route for the ALD growth of Al2O3 nanofilms on the Si(001) surface. 3.3. Effect of p-Doping on the TMA and H2O Chemisorption on the Si(001)-(2×1) Surface. As was previously stated, chemical doping is widely employed to modify the surface properties of semiconductors. Then, it is possible to envisage atomic-scale manipulation of surface chemical properties by incorporating dopant atoms into the surface or subsurface. Boron is the most widely used p-type impurity in Si, and it is a well-known property that B atoms remain in subsurface lattice sites, rather than staying at the surface.67 A B atom at the subsurface alters the local atomic structure, because of its small atomic radius, and also modifies the charge polarization of B-connected Si dimers. However, regarding H2O adsorption and dissociation, subsurface incorporated B atoms are likely to insignificantly affect adsorption and dissociation kinetic barriers, and only small changes in the adsorption energy (of 0.15 eV), as a consequence of the change in the surface polarization, will be noticeable.67 The situation is remarkably different in the presence of surface defects like Si dangling bonds. Indeed,

Figure 4. Dissociation paths (methyl adsorption, black curve; H2 release, red curve; and CH4 formation, blue curve; respectively) of the TMA molecule adsorption on the Si(001)-(2×1):OH surface through: (a) intradimer (upper panel) and (b) interdimer mechanisms (lower panel, see Supporting Information for the entire pathways). Yellow spheres represent Si atoms; white, H; blue, C; red, O; and pink, Al; respectively. For the CH4 formation reaction (blue curve), the ringclosing reaction pathway and the 2nd step of the ALD process are also shown.

to the hydrogen-passivated Si(001) surface. However, subsequent exposure to the oxygen molecular precursor (H2O) shows (see Figure 4) that, although the reaction is strongly exothermic (1.17 eV), the kinetic barrier to release the last CH4 molecule from the TMA and reconstruct the OH surface termination is relatively large, 2.03 eV. This might be another rate-limiting step for the ALD growth of Al2O3 uniform films on the Si(001) surface. Finally, the last configuration considered in our study is a Si(001)-(2×1):H with two OH CSil groups replacing two hydrogen atoms but in an interdimer configuration, i.e., at adjacent Si dimers (it would be unrealistic to consider a higher concentration of OH surface species, as they come from H2O dissociation). Figure 4 (lower panel) and Table 2 (and Figure S11 of the Supporting Information) show the same three similar reaction pathways considered, together with the kinetic barriers and adsorption energies. Again, the first two pathways (proton-methyl ligand exchange and H2 release) are very unlikely, both kinetically and thermodynamically (3.89 and 0.87 eV for proton-methyl exchange and 2.73 and 1.72 eV for H2 release, respectively, see Figure 4 and Table 2). However, the release of a CH4 methane molecule and the formation of an I

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The Journal of Physical Chemistry C Figure 5a shows the reaction pathway to form CSil species on the B-doped (in the subsurface) Si(001) surface. The kinetic

Figure 6. Reaction pathways of TMA adsorption on the B-doped Si(001)-(2×1) surface with different chemical functionalizations: (a) clean surface, (b) hydrogen-passivated surface, (c) hydrogenpassivated surface with one dangling bond per dimer row, and (d) OH-terminated surface through the intradimer mechanism (after a previous H2O ALD cycle). The insets show the corresponding initial and final states. Yellow spheres represent Si atoms; white, H; green, B; blue, C; pink, Al; and red, O; respectively (for comparison, the dashed blue lines show the results for the corresponding undoped surface).

Figure 5. (a) Reaction pathway to form silanol (CSil) surface species from H2O adsorption on the B-doped Si(001)-(2×1):H surface with one dangling bond per dimer row. Yellow spheres represent Si atoms; white, H; green, B; and red, O; respectively (for comparison, the dashed blue lines show the results for the corresponding undoped surface). (b) Local density of states (LDOS) of the Si dangling bond without (left panel) and with B doping (right panel). The inset shows the charge difference between both configurations, with the electron density (blue) located at the B site. Black and red lines correspond to the 3s and 3p orbitals of Si, respectively.

1e− (according to our Bader charge calculations68) is transferred from the two Si dimers to the subsurface B impurity. Figure 6a shows the pathway to release one methane CH4 molecule and adsorb the Al−CH3−CH2 species in the clean Si(001) surface. Several considerations are in order. First, the two valleys shown in the picture are caused by the B subsurface impurity. Indeed, and similar to the result obtained for the clean Si(001) surface, the chemisorption of the Al− CH3−CH2 group in the buckled-down Si atom (the most energetically favorable adsorption site, according to our calculations) buckles the Si dimer in the opposite direction (without this charge transfer from the buckled-up Si atom to the otherwise empty B-connected Si atoms, the TMA adsorption would be unfeasible), and finally, the two Si dimers are alternately buckled, as in the clean surface but with a subsurface B impurity. The strain to change the buckling during the NEB calculation significantly reduces the energy of two of the transition states, giving rise to the energy profile shown in Figure 6a. The kinetic barrier to release the CH4 molecule, despite the charge transfer to the B impurity and from the now buckled-up Si atom to the positively charged Al atom, does not differ significantly from that of the clean Si(001) surface, 2.55 eV (slightly lower than that of the clean surface, 2.88 eV) and the process is also endothermic, with a reaction energy of 1.19 eV, also slightly lower than that of the clean Si(001) surface (1.37 eV). If the Si surface is hydrogen-passivated (with or without dangling bonds), the B subsurface impurity increases the kinetic barriers, thus indicating that p-doping may be a good way to increase the degree of selectivity of the TMA Al precursor, depending on the type of application pursued with the ALD growth process. In a hydrogen-passivated Si(001) surface, the charge transfer from the B-connected Si dimers is larger than that of the clean Si(001) surface, around 2e−, due to the hydrogen atoms that passivate the surface dangling bonds, thus creating Si−H+ bonds with electron-poor surface Si atoms,

barrier (0.41 eV) and reaction energy (−0.99 eV) are almost half and twice the values of the undoped surface, respectively. Obviously, this result is not unexpected. As can be seen in Figure 5b, which compares the local density of states (LDOS) of the Si dangling bond with and without B doping, the charge transfer from the Si atom to the B impurity makes the dangling bond empty, thus facilitating the dissociation of the H2O molecule and the chemisorption of the CSil species, via a dative bond between the Si surface atom and the H2O molecule, similarly to the water molecular adsorption on a buckled-down Si atom of the clean surface. We also want to investigate the effect of B-subsurface doping on the second half of the ALD reaction, i.e., the adsorption of the TMA molecule. To do that, we restricted our study to the most favorable pathways described in the previous section. More specifically, to those reactions involving the release of a CH4 methane molecule, which are the most likely to promote the ALD growth of Al2O3 nanofilms. Figure 6a−d shows the reaction pathways of the four different situations considered (clean, hydrogen-passivated, hydrogen-passivated with one dangling bond and OH-terminated Si(001) surfaces). B incorporation into the subsurface shortens the bond distance with the nearest surface Si atoms (2.02 Å), as compared to the Si−Si bond distance in the clean surface (2.40 Å). The first consequence is that the two B-connected Si atoms are buckled-down, i.e., the two adjacent Si dimers are oriented in the same direction, contrary to what happens in the clean surface. The Si−Si bond length of these two dimers is 2.35 Å. The B impurity also changes the charge polarization and almost J

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The Journal of Physical Chemistry C which slightly increases the Coulomb repulsion with the Al p empty orbitals. The kinetic barrier shown in Figure 6b, 3.08 eV, is substantially higher than that of the Si(001) surface without B impurities (1.36 eV). The main reason, as can be noticed in the final state shown in the picture, is that the loss of the surface H (to form the released CH4 molecule) momentarily bucklesdown the Si dimer, creating an almost empty Si dangling bond (due to charge transfer to the B subsurface impurity). A weak Coulomb interaction with the Al−(CH3)2 molecule, which again makes both Si dimers symmetrical, is responsible for the large kinetic barrier obtained. The reaction is slightly exothermic, 0.41 eV (very similar to the 0.47 eV result obtained for the hydrogen-passivated Si(001) without B impurities), indicating that the process could be thermodynamically, but not kinetically, controlled. If the Si(001) surface shows surface defects like dangling bonds (see Figure 6c), the almost empty Si dimer with the dangling bond is buckled-down, similarly to the clean Si surface. The charge transfer to the B impurity is then slightly less than for the fully hydrogen-passivated Si(001) surface, 1.5e−. The most favorable path to release the methane CH4 molecule involves the loss of a H atom from the neighboring Si atom of the same dimer, thus forming a clean dimer that, after the chemisorption of the DMA molecule, is buckled-up with respect to the subsurface B impurity (i.e., there is some charge transfer to the B-connected Si atom, in order to allow the chemisorption of the DMA). The obtained kinetic barrier is 2.61 eV, almost 1 eV higher than that of the same system without p-doping, and the reaction is also slightly less exothermic (cf. 0.21 vs 0.39 eV). Finally, we considered an OH-terminated Si(001) surface via the intradimer mechanism (there is practically no kinetic barrier for the release of CH4 and the adsorption of Al−(CH3)2 in a OH-terminated Si(001) surface via the interdimer mechanism, see the lower panel of Figure 4). Interestingly, the reaction energy is not affected by the subsurface B impurity (1.75 eV) but the obtained kinetic barrier is substantially lower, 0.26 eV (cf. with 0.90 eV for the same system without B doping, see Figure 6d and upper panel of Figure 4). While there is no charge transfer from the adsorbed oxygen ions (with or without B impurities), the B-connected Si dimers lose almost 2.5e−, increasing the O−Si dipole moment and facilitating the loss of the surface hydrogen and the subsequent formation of the Si− O−Al bond. 3.4. Ab Initio Thermodynamics of the ALD of Al2O3 Nanofilms. The chemisorption of both TMA and H2O during the ALD of Al2O3 strongly depends on the temperature of the surface and the partial pressure of the molecular precursors in contact with it. The adsorption energies and kinetic barriers reported so far have been obtained at zero temperature and pressure. To estimate the influence of real ALD conditions on the obtained adsorption energies, we plot in Figure 7 the Gibbs free adsorption energies of H2O on the Si(001)-(2×1) surface with different functionalizations as a function of T, at the typical ALD pressure of p = 1 Torr. Each line corresponds to a different termination of the Si surface, and the zero of energy marks the desorption value of the surface. It can be seen in the picture that, below the typical ALD temperature of 423 K, the system is completely dehydrated if the Si surface is completely hydrogen-passivated. If a dangling bond defect is present, the surface will be partially hydrated around the defect and, with a B subsurface impurity, the water adsorption rate will be much higher. Finally, for a larger concentration of dangling bonds, the

Figure 7. Gibbs free adsorption energies of H2O on the Si(001)-(2×1) surface with different chemical functionalizations, as a function of T and at the typical ALD pressure of p = 1 Torr.

H2O sticking coefficient and adsorption rate increases drastically, approaching the clean surface limit. This finding is important, because it implies that, at the typical ALD temperature of 423 K, water molecules will be desorbed from the Si surface if it is completely hydrogen passivated. On the contrary, a relatively small number of dangling bond defects and/or B subsurface impurities (p-doping) would suffice to completely hydrate the Si surface. If not, the second halfreaction of the ALD process, i.e., the adsorption of TMA will take place on a hydrogen-passivated Si surface. As has been previously shown, the hydroxylated Si surface deeply affects the subsequent half-reaction of the first ALD cycle, both kinetically and thermodynamically (via reaction energies). Figure 8 shows the Gibbs free energies of the two

Figure 8. Gibbs free energies of the two most favorable TMA adsorption reactions (proton-ligand exchange and CH4 release) considered in this work, obtained at typical ALD working conditions (T = 423 K, p = 1 Torr).

most favorable TMA adsorption reactions (proton-ligand exchange and CH4 release) considered in this work (we do not include the B-doped Si surface, as the reaction energies are relatively similar to those of the surface without p-impurities, as was shown in the previous section), obtained at typical ALD working conditions (T = 423 K, p = 1 Torr). The picture shows that the TMA adsorption through a dissociation process (DMA chemisorption and CH4 release) is always an exothermic reaction with similar Gibbs free energies, regardless of the surface termination. Moreover, the OH-termination is not necessarily the most favorable surface functionalization for K

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urations. Besides, a small amount of CSil will be formed on isolated dangling bonds in the background of the pattern. The first TMA pulse will then react with the OH dimers in the pattern, particularly at the intradimer configuration. Besides, some additional dangling bonds of the background can be passivated with CH3 (with DMA floating off into vacuum or reacting further with the surface), becoming inactive or, if they formed CSil during the first H2O pulse, become terminated with DMA (and releasing CH4). Finally, further H2O and TMA pulses will result in the nucleation and growth of Al2O3 from the DMA species in the pattern and some possible nucleation from background DMA on isolated OH groups. Therefore, to get the best selectivity, it will be necessary to prevent OH adsorption in the background of the pattern. As mentioned previously, increasing the working temperature up to 600 K (at the limit of the H stability at the background), should avoid H2O adsorption at the background, while still allowing reaction in the pattern.

TMA adsorption (that would correspond to a Si surface with high concentration of dangling bond defects). In subsequent steps, more methane molecules can be released, leading to an Al−O reconstruction of the surface and the growth of the Al2O3 nanofilm. These follow-up reactions are also exothermic and, as shown previously, they can be of two different types: intrinsic (ring-closing reaction and methane release if there are enough OH groups in neighboring dimers) or extrinsic (release of a methane molecule after another water pulse). The extrinsic reaction is slightly more favorable than the intrinsic one (0.33 and 0.84 eV with respect to the interdimer and intradimer OH termination, respectively), as can be noticed in Figure 8, comparing the Gibbs free energies of the second CH4 release step. This finding implies that the TMA molecule is relatively reactive and that its deposition will provide an acceptable Al2O3 uniform film growth rate. In contrast, Figure 8 also shows that the proton-ligand (CH3 for the TMA molecule) exchange reaction always shows negative Gibbs free energies for all the surface functionalizations considered (except the clean Si surface), which means that there will always be some Si−C− H or Si−O−C−H species present during the ALD growth of Al2O3. At this point, we must turn again into the kinetic features of the TMA deposition reactions, as the Gibbs free energy only provides a necessary but not sufficient condition for TMA dissociation and subsequent Al2O3 growth. The kinetic barriers for CH3 desorption range from ≈0.1 (surface with dangling bond defects) to 2.5 eV (clean surface) without a previous water pulse and from ≈1 to ≈1.5 eV (depending on the OH concentration on the surface) with a previous H2O cycle of the ALD process. Together, these findings help to understand the growth mechanism of the Al2O3 ALD on Si(001)-(2×1) with different initial conditions and surface functionalizations: on a clean Si surface, the H2O dissociates and chemisorbs with no kinetic barrier but, in the second halfreaction, the presence of C impurities will be almost impossible to remove at typical ALD working conditions. If the Si(001) surface is fully passivated, at ALD temperature and pressure conditions the surface will be completely dehydrated. Although possible, TMA dissociation (DMA deposition and CH4 release) should overcome a moderately low kinetic barrier of 1.36 eV. This Si-DMA surface species can then act as nucleation centers for further H2O pulses and Al2O3 growth, with the subsequent reactions showing negative Gibbs free energies and very low kinetic barriers. Possible problems to obtain high-quality uniform Al2O3 nanofilms may stem from Si−C surface species, difficult to remove at ALD working conditions. Finally, the presence of a moderate number of dangling bond defects will facilitate the hydration of the surface and the desorption of the Si−C and Si−O−C surface species, thus helping the growth of a highly uniform Al2O3 oxide. A different question would be if those dangling bonds can act as nucleation centers to promote patterned epitaxial growth, similarly to other oxides like TiO2.17,69 The negative Gibbs free energies obtained for all the growth-related deposition reactions and the similar trends of the kinetic barriers do not allow us to draw any definitive conclusion at typical ALD working conditions. However, by increasing the working temperature (up to 550−600 K) one could have positive Gibbs free energies on single dangling bonds, whereas the free energy would still be negative for H2O adsorption on clean Si dimers. Such an ALD temperature rise could promote selective adsorption and subsequent patterned Al2O3 growth. Indeed, the first H2O pulse can form OH dimers in the pattern, in intradimer, interdimer or zigzag config-

4. CONCLUSIONS In summary, in this work, we have studied the adsorption of H2O and TMA molecules on the Si(001)-(2×1) surface, in order to obtain the initial surface reactions involved in the ALD of Al2O3 nanofilms and how the growth rate is affected by surface defects (dangling bonds) and p-doping. Our results show that the presence of dangling bonds in the hydrogenpassivated Si(001) surface facilitate the H2O dissociation and CSil formation, whereas the O−Si back-bond with H2 release is the most favorable oxidation mechanism at this first stage of H2O adsorption. Additional dangling bonds in the Si dimer row make the H2O dissociation and CSil formation an almost barrierless process, and the most favorable oxidation mechanism is the Si−O−Si on-dimer bond. On the contrary, the TMA adsorption on the Si(001)-(2×1) surface shows very different features, depending on the chemical termination of the surface. While the Si−Al bond formation is an endothermic process for the clean surface, it shows exothermic character and moderately low kinetic barriers for the hydrogen-passivated (with or without dangling bonds) Si(001) surface. If the surface is already oxidized as a consequence of a previous ALD exposure to the oxygen precursor, the barrier is strongly affected by the arrangement of the OH groups. For the lowest barrier, a pair of neighboring OH groups in an interdimer configuration is required, suggesting the engineering of linear patterns of OH groups as a way to optimize the nucleation efficiency. Boron p-doping affects the kinetic barriers and adsorption energies of H2O and TMA deposition. While it barely affects H2O adsorption in a fully passivated Si(001) surface, the presence of dangling bonds halves the kinetic barrier and doubles the adsorption energy. For TMA adsorption, B-doping increases the kinetic barriers for Si−Al bond formation if the Si(001) is hydrogen-passivated (with or without dangling bonds), due to larger charge transfer between the Si surface atoms and the subsurface B impurity. Whereas the TMA adsorption on a B-doped clean Si(001) surface shows similar features to the undoped case (due to similar buckling of the Si dimer rows); if the surface is oxidized, the Si−O dipole moment substantially lowers the TMA dissociation kinetic barrier. Finally, our ab initio thermodynamics results show that, at the typical ALD temperature of 423 K, a small amount of surface dangling bond defects or B p-doping at low concentrations L

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Studied by In Situ Transmission FTIR Spectroscopy and Quadrupole Mass Spectrometry. J. Phys. Chem. C 2008, 112, 19530−19539. (11) Grigoras, K.; Sainiemi, L.; Tiilikainen, J.; Säynätjoki, A.; Airaksinen, V. M.; Franssila, S. Application of Ultra-Thin Aluminum Oxide Etch Mask Made by Atomic Layer Deposition Technique. J. Phys.: Conf. Ser. 2007, 61, 369−373. (12) Ha, S. C.; Choi, E.; Kim, S. H.; Sung Roh, J. Influence of Oxidant Source on the Property of Atomic Layer Deposited Al2O3 on Hydrogen-Terminated Si Substrate. Thin Solid Films 2005, 476, 252− 257. (13) Xu, M.; Zhang, C.; Ding, S. J.; Lu, H. L.; Chen, W.; Sun, Q. Q.; Zhang, D. W.; Wang, L. K. Mechanism of Interfacial Layer Suppression After Performing Surface Al(CH3)3 Pretreatment During Atomic Layer Deposition of Al2O3. J. Appl. Phys. 2006, 100, 106101. (14) Frank, M. M.; Chabal, Y. J.; Wilk, G. D. Nucleation and Interface Formation Mechanisms in Atomic Layer Deposition of Gate Oxides. Appl. Phys. Lett. 2003, 82, 4758−4760. (15) Soref, R. A. Silicon-Based Optoelectronics. Proc. IEEE 1993, 81, 1687−1706. (16) Veyan, J. F.; Choi, H.; Huang, M.; Longo, R. C.; Ballard, J. B.; McDonnell, S.; Nadesalingam, M. P.; Dong, H.; Chopra, I. S.; Owen, J. H. G.; et al. Si2H6 Dissociative Chemisorption and Dissociation on Si(100)-(2 × 1) and Ge(100)-(2 × 1). J. Phys. Chem. C 2011, 115, 24534−24548. (17) McDonnell, S.; Longo, R. C.; Seitz, O.; Ballard, J. B.; Mordi, G.; Dick, D.; Owen, J. H. G.; Randall, J. N.; Kim, J.; Chabal, Y. J.; et al. Controlling the Atomic Layer Deposition of Titanium Dioxide on Silicon: Dependence on Surface Termination. J. Phys. Chem. C 2013, 117, 20250−20259. (18) Widjaja, Y.; Musgrave, C. B. Quantum Chemical Study of the Mechanism of Aluminum Oxide Atomic Layer Deposition. Appl. Phys. Lett. 2002, 80, 3304−3306. (19) Halls, M. D.; Raghavachari, K. Atomic Layer Deposition of Al2O3 on H-Passivated Si. I. Initial Surface Reaction Pathways with H/ Si(100)-(2 × 1). J. Chem. Phys. 2003, 118, 10221−10226. (20) Halls, M. D.; Raghavachari, K.; Frank, M. M.; Chabal, Y. J. Atomic Layer Deposition of Al2O3 on H-Passivated Si: Al(CH3)2OH Surface Reactions with H/Si(100)-(2 × 1). Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 161302R. (21) Halls, M. D.; Raghavachari, K. Atomic Layer Deposition Growth Reactions of Al2O3 on Si(100)-2 × 1. J. Phys. Chem. B 2004, 108, 4058−4062. (22) Hu, Z.; Turner, C. H. Atomic Layer Deposition of TiO2 from TiI4 and H2O onto SiO2 Surfaces: Ab Initio Calculations of the Initial Reaction Mechanisms. J. Am. Chem. Soc. 2007, 129, 3863−3878. (23) Ghosh, M. K.; Choi, C. H. Adsorption Reactions of Dimethylaluminum Isopropoxide and Water on the H/Si(100)-2 × 1 Surface: Initial Reactions for Atomic Layer Deposition of Al2O3. J. Phys. Chem. B 2006, 110, 11277−11283. (24) Ghosh, M. K.; Choi, C. H. The Initial Mechanisms of Al2O3 Atomic Layer Deposition on OH/Si(100)-2 × 1 by Tri-Methylaluminum and Water. Chem. Phys. Lett. 2006, 426, 365−369. (25) Kim, D. H.; Baek, S. B.; Kim, Y. C. Energy Barriers for Trimethylaluminum Reaction with Varying Surface Hydroxyl Density. Appl. Surf. Sci. 2011, 258, 225−229. (26) Lin, J. M.; Teplyakov, A. V.; Rodríguez-Reyes, J. C. F. Competing Reactions During Metalorganic Deposition: LigandExchange versus Direct Reaction with the Substrate Surface. J. Vac. Sci. Technol., A 2013, 31, 021401. (27) Incoccia, L.; Balerna, A.; Cramm, S.; Kunz, C.; Senf, F.; Storjohann, I. The Adsorption Site of Oxygen on Si(100) Determined by Sexafs. Surf. Sci. 1987, 189−190, 453−458. (28) Hamers, R. J.; Wang, Y. Atomically-Resolved Studies of the Chemistry and Bonding at Silicon Surfaces. Chem. Rev. 1996, 96, 1261−1290. (29) Chabal, Y. J.; Christman, S. B. Evidence of Dissociation of Water on the Si(100)2 × 1 Surface. Phys. Rev. B: Condens. Matter Mater. Phys. 1984, 29, 6974−6976.

would suffice to completely hydrate the Si surface. On the contrary, Si−Al and Si−C surface bond formation always show negative Gibbs free energies, which makes it necessary to carefully analyze the initial conditions of the Si(001) surface to prevent a nonuniform growth of the Al2O3 nanofilms.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09053. Charts with the entire H2O and TMA adsorption pathways described in this work (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(R.C.L.) E-mail: [email protected]. *(J.H.G.O.) E-mail: [email protected]. *(J.N.R.) E-mail: [email protected]. *(K.C.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Scott Schmucker for careful revision of this manuscript. This work has been supported by the Defense Advanced Research Project Agency (DARPA) and Space and Naval Warfare Center, San Diego (SPAWARSYSCEN-SD), under Contract N66001-08-C-2040. It is also supported by a grant from the Emerging Technology Fund of the State of Texas to the Atomically Precise Manufacturing Consortium. Y.J.C. acknowledges the support of the National Science Foundation through Grant NSF-CHE 1300180. The authors also acknowledge the Texas Advanced Computing Center (TACC) for providing computing resources.

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REFERENCES

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