Insights into the Surface Chemistry of Tin Oxide Atomic Layer

Article pubs.acs.org/JPCC

Insights into the Surface Chemistry of Tin Oxide Atomic Layer Deposition from Quantum Chemical Calculations Jukka T. Tanskanen*,†,‡ and Stacey F. Bent† †

Department of Chemical Engineering, Stanford University, 381 North-South Mall, Stanford, California 94306-5025, United States Department of Chemistry, University of Eastern Finland, PO Box 111, FI-80101 Joensuu, Finland

‡

ABSTRACT: The surface chemistry and growth characteristics of tin oxide atomic layer deposition (ALD) from tetrakis(dimethylamino)tin (TDMASn) and H2O have been investigated by density functional theory and second-order Møller−Plesset perturbation theory calculations. The determined reaction pathways provide the basis for a mechanistic understanding of SnOx ALD on OH-terminated silicon and facilitate analysis of previous experimental observations. In particular, the calculations provide insight into the origin of the experimentally observed increase in the growth rate of SnOx at temperatures below 100 °C, for which the role of physisorbed H2O has been suggested, by demonstrating that the energetics for the TDMASn reaction with surface OH groups are similar to that with H2O molecules. Also, the reaction barrier for the water half reaction is shown to depend on the presence of hydroxyl groups adjacent to O2SnN(CH3)2* reaction sites on the growth surface, and this plays a role in the SnOx ALD at low deposition temperatures. The surface chemistry understanding developed here for SnOx is expected to be of relevance for the ALD of other technologically promising materials such as zinc tin oxide.

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temperature range of 30−300 °C).1,9 In contrast, the growth of crystalline SnOx, with x = 1.6 − 2.2 and the crystal structure of rutile SnO2, from N2,N3-di-tert-butyl-butane-2,3-diamido-tin(II) and H2O2 (investigated temperature range of 50−250 °C) was reported by Gordon et al.,10 and this work was followed by a study reporting ALD of nanocrystalline films close to SnO2 in composition by the same authors (investigated temperature range of 130−250 °C).11 It should be noted that the previous studies of SnOx ALD suggest the deposition temperature is of importance in the process, with the growth characteristics and material properties of the SnOx films deposited at high and low temperatures differing significantly. Surface chemistry studies provide a means to understand the origins of the above-mentioned findings, and in this context in situ experiments can be useful for investigating the ALD surface chemistry. For example, in situ quartz crystal microbalance (QCM) and Fourier transform infrared (FTIR) spectroscopy techniques provided valuable insight into the mechanism of SnOx ALD from SnCl4 and H2O2.8 In addition to the experimental techniques, computational approaches such as density functional theory (DFT) simulations are helpful in the investigation of the ALD surface chemistry by providing information on reaction pathways for material growth. Unfortunately, these mechanistic details have been explored computationally for only a small fraction of all the reported ALD processes.

INTRODUCTION Metal oxide thin films are essential components in a range of technological applications such as solar cells, catalysis, and integrated circuits. A representative of such a technologically important material is tin oxide, which shows promise, for instance, in transparent conductive oxides (TCOs) in displays and photovoltaics1 and as thin coatings in functional glasses.2 For many modern technologies, ultrathin, pinhole- and impurity-free, and conformal tin oxide films are necessary. In addition, a certain crystallographic phase and composition may be needed, providing challenges for the tin oxide deposition method. For example, more than two crystal structures and compositions ranging from Sn(II)O to Sn(IV)O2 are known for tin oxide. As a consequence, tin oxide deposition has been demonstrated using a number of methods, including chemical vapor deposition (CVD), atomic layer deposition (ALD),3 spray-pyrolysis, physical vapor deposition,4 and sputtering.5 The ALD technique is promising for the deposition of high quality and ultrathin SnOx films because of the method’s ability to grow material in an atomically controlled fashion.6 In an ALD process, film growth characteristics and the resulting material properties originate from complex surface chemistries of gaseous precursor molecules,7 meaning that different precursors may give rise to growth of films with different structures, compositions, and material properties. This has been observed for SnOx ALD: X-ray amorphous SnOx with x = 1.1− 1.5 has been grown from SnCl4 and H2O2 (under an investigated temperature range of 150−450 °C) 8 and amorphous oxygen-deficient SnO2 has been grown from tetrakis(dimethylamino)tin (TDMASn) with water, ozone, and hydrogen peroxide as the O source (under an investigated © 2013 American Chemical Society

Received: June 26, 2013 Revised: August 22, 2013 Published: August 26, 2013 19056

dx.doi.org/10.1021/jp4063324 | J. Phys. Chem. C 2013, 117, 19056−19062

The Journal of Physical Chemistry C

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Here, we determine reaction pathways for SnOx ALD on a hydroxylated silicon substrate by both periodic and molecular calculations at the DFT and second-order Møller−Plesset perturbation theory (MP2) levels of theory. We focus on TDMASn, which has recently been utilized for SnOx ALD by several groups, as the Sn source and H2O as the oxidant counter-reactant. We provide an atomic-level understanding for this ALD process by comparing the computational results with previous experimental findings. In addition, we provide insight into the origin of the increase in SnOx growth rate at low deposition temperatures.9 This understanding enables us to make suggestions for ALD process development for applications in technologies such as thin film photovoltaics.

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COMPUTATIONAL DETAILS Computations of the surface chemistry of SnOx ALD on partially OH-terminated Si were carried out by CRYSTAL09,12 Gaussian09,13 and Vienna Ab Initio Simulation Package14,15 (VASP) programs. For the bulk of the calculations, a PBE016,17 hybrid functional with standard split-valence + polarization (SVP) basis sets and two modified SVP basis sets was used.18 Dispersion-corrected DFT method (PBE-D), as implemented in CRYSTAL09, was utilized to determine adsorption energies for physisorbed HDMA species. The following basis sets were applied in the PBE0 and PBE-D calculations: for carbon, hydrogen, nitrogen, and oxygen, standard all-electron SVP basis;19,20 for silicon, a modified all-electron def2-SVP basis;18 and for tin, a modified SVP basis derived from the molecular def2-SVP basis set,18 together with 28-electron scalar-relativistic effective core potentials. Reaction pathways for the ALD growth were determined by performing full PBE0 optimizations on the investigated systems and frequency calculations on the optimized structures, giving access to zero-point energy (ZPE) corrected reaction energies (ΔE) and Gibbs free reaction energies (ΔG) at 298 K. Transition state (TS) structures were identified by frequency calculations. A reconstructed Si(100)2×1 surface with an OH coverage of 50% was adopted to represent surface OH species reacting with TDMASn during the Sn half-cycle. The surface was modeled by a periodic slab with a unit cell composition of Si32O4H16 and with the outermost surface being composed of Si(OH)-SiH dimers (Figure 1). The nonreactive side of the slab is terminated with hydrogens. Notably, structurally analogous surface models for hydroxylated silicon oxide have been previously utilized in a number of studies.21−23 For the evaluation of the Coulomb and exchange integrals (TOLINTEG in CRYSTAL09 input), tight tolerance factors of 8, 8, 8, 8, and 16 were used. Default optimization convergence thresholds, 34 k-points in the irreducible Brillouin zone, which were generated by the Monkhorst−Pack method,24 and an extra large integration grid were adopted in the calculations. Calibration calculations on the pristine slab confirmed the applied computational approach to yield well-converged results. Molecular PBE0 calculations with the standard SVP basis were performed to simulate the TDMASn reaction with molecular water on the growth surface at low deposition temperatures. Here, a polarizable continuum model (PCM) for water as implemented in Gaussian09 was utilized to take into account the presence of neighboring waters. Single-point MP2-energies using the def-TZVP25 basis set were calculated on the PBE0optimized molecular structures to verify the energetics. Additional cluster calculations (PBE0) were performed for orbital analysis of Sn(DMA)OH* and Sn(DMA)2* surface

Figure 1. Top and side views of the Si(100)-2×1 slab with an OH coverage of 50% on the reactive side and the nonreactive side terminated with hydrogens as utilized in the periodic calculations. The unit cell, with an atomic composition of Si32O4H16, is highlighted.

species. Here, the surface was modeled by a H-terminated cluster with two Si(OH)-SiH dimers and a composition of Si15H18(OH)2. DFT molecular dynamics (DFT-MD) simulations were performed by a PBE16 functional, without empirical dispersion correction, coupled with projector augmented wave (PAW) pseudopotentials,26 as implemented in VASP, to study the release of nonchemically bound HDMA from the growth surface. The simulations were performed at the Γ point using a time step of 1 fs, Nosé thermostat, default accuracy parameters for plane wave kinetic energy cutoff, fast Fourier transform (FFT) grid, and real space projectors (PREC=NORMAL in VASP input).

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RESULTS Following a proposed mechanism for SnOx ALD from TDMASn and H2O2,1 we focused on investigating an analogous ALD overall reaction with H2O as a counter-reactant, and this reaction can be expressed as (OH)*x + Sn(DMA)4 → Ox Sn(DMA)4 − x * + x HDMA↑ (1)

Ox Sn(DMA)4 − x * + 2H 2O → (OH)x * + SnO2 + (4 − x)HDMA↑

(2)

The * in the equations refers to surface species, and x defines the extent to which TDMASn reacts with surface OH groups by releasing its DMA ligands as HDMA (reaction 1). Notably, recent work based on in situ QCM and in situ mass spectrometry monitoring of SnOx ALD provided evidence for the release of HDMA during both half reactions.1,9 The extent of conversion, x, also determines the number of DMA ligands remaining on the surface for the subsequent water half reaction. Elam et al.1 demonstrated that three DMA ligands are lost with H2O2 as a counter-reactant, whereas x is lower when using H2O 19057



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(a value of 2.5 was reported). Note that a value of x between 2 and 3 suggests a distribution of surface species corresponding to x = 2, O2Sn(DMA)2*, and x = 3, O3Sn(DMA)*, is formed during the TDMASn pulse, but to limit the computational effort for providing the basic surface chemistry understanding for the SnOx ALD process, we focused on reactions with x = 2, i.e., on the following ALD half reactions

Table 1. PBE0-Calculated Energies (ΔE) and Gibbs free energies (ΔG) for the ALD Half Reactions (OH)2* + Sn(DMA)4 → O2Sn(DMA)2* + 2 HDMA and O2Sn(DMA)2* + 2 H2O → (OH)2* + SnO2 + 2 HDMAa state on a reaction pathwayb TDMASn pulse Si(100)-2×1 surface 1. adsorption state 2. transition state 3. bound state 4. 1 × HDMA released 5. transition state 6. bound state 7. 2 × HDMA released H2O pulse O2Sn(DMA)2* surface 1. adsorption state 2. transition state 3. bound state 4. 1 × HDMA released 5. adsorption state 6. transition state 7. bound state 8. 2 × HDMA released

(OH)2 * + Sn(DMA)4 → O2 Sn(DMA)2 * + 2HDMA↑ (3)

O2 Sn(DMA)2 * + 2H 2O → (OH)*2 + SnO2 + 2HDMA↑

(4)

To verify the feasibility of this reaction mechanism and to provide the basic surface chemistry understanding for SnOx ALD, we determined the reaction pathways for a cycle of SnOx ALD by periodic PBE0 calculations, and these are supported by calculations utilizing DFT molecular dynamics simulations and the MP2 method. Furthermore, Gibbs free energies calculated for reactions 3 and 4 suggest that the reactions are spontaneous. A Si(100)-2×1 slab composed of Si(OH)-SiH dimers presented surface OH species for the first half reaction (Figure 1). This surface model, representing 50% OH coverage, had an OH surface density of 3.5 OH/nm2, in agreement with typical surface OH densities ranging from about 2.5 to 4.6 OH/nm2 on SiO2.27 Studying a surface with this OH coverage also provided the practical advantage of limiting the number of potential pathways because both geometric constraints of the TDMASn molecule (diameter ∼0.7 nm) and OH−OH distances of about ∼0.5 nm on the surface make only two OH groups accessible to TDMASn. The partial H-termination was included to saturate dangling bonds on the surface in order to avoid unrealistic charge transfer effects. The calculated energetics are summarized in Table 1 and discussed in detail in the following sections. Unless otherwise noted, we utilize the ZPE-corrected energies (ΔE) in the analysis of the results (see Computational Details). Pathway for TDMASn Half Reaction. The calculated reaction pathway for TDMASn chemisorption on the OHterminated growth surface is illustrated in Figure 2. TDMASn is adsorbed strongly to the surface via two hydrogen bonds formed between its N atoms and the surface OH* species of neighboring SiH-Si(OH) dimers, yielding a ZPE-corrected interaction energy of −11.5 kcal/mol (state 1 in Figure 2). TDMASn chemisorption proceeds via H-transfer from a surface OH to a DMA ligand with a moderate energy barrier of ∼17.8 kcal/mol from state 1, followed by Sn−O bond formation (state 3) and concomitant release of HDMA to the gas phase (state 4, HDMA not shown), resulting in an overall reaction energetically favored by 11.4 kcal/mol with respect to the reactants. Notably, the HDMA species in state 3 is calculated to be adsorbed to the surface by 3.1 kcal/mol, suggesting it may remain on the growth surface during ALD, in particular at low deposition temperatures. Furthermore, because dispersion interactions are not properly described by standard DFT functionals, we calculated the energy for the release of physisorbed HDMA from the surface by dispersion-corrected PBE-D calculations as the energy difference between states 3 and 4. The PBE-D-calculated energy is 8.7 kcal/mol, providing additional evidence for possible residual HDMA remaining on the surface during the growth process, as has been previously

ΔE

ΔG

0.0 −11.5 6.3 −14.5 −11.4 10.4 −2.1 1.4

0.0 − − − −6.7 − − −5.9

0.0 −7.3 9.5 −12.6 −5.2 −17.5 −11.7 −13.4 −6.6

0.0 − − − −7.5 − − − −11.8

a

The energies are given in kcal/mol with respect to the Si(100)-2×1 slab with 50% OH coverage for the TDMASn half reaction and with respect to the O2Sn(DMA)2*-terminated Si(100)-2×1 slab for the H2O half reaction. Gibbs free energies (ΔG, T = 298 K) are given for the half reactions to estimate the effects of thermodynamics on the energetics. bThe labels 1−8 refer to specific states on the reaction pathways illustrated in Figures 2 and 4.

Figure 2. PBE0-calculated reaction pathway from local optimizations for TDMASn chemisorption on half-OH terminated Si(100)-2×1 surface according to an overall reaction (OH)*2 + Sn(DMA)4 → O2Sn(DMA)2* + 2HDMA↑. Si, H, O, N, C, and Sn atoms are shown in yellow, white, red, blue, gray, and green, respectively.

suggested for SnOx ALD at deposition temperatures less than 150 °C.1 To estimate the deposition temperature necessary for releasing HDMA, we performed DFT-MD simulations at 50, 100, and 150 °C using the PBE0-optimized system as an initial structure and by following the dynamics of the physisorbed HDMA as a function of time. It should be noted that the simulations provide a lower estimate for the temperature necessary to release HDMA because weak interactions are not fully captured by DFT; in addition, the HDMA desorption behavior may be different during steady-state SnOx growth 19058



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kcal/mol with respect to the reactants. Note that the product of the TDMASn half reaction (state 7) formed according to (OH)*x + Sn(DMA)4 → OxSn(DMA)4‑x* + x HDMA↑, where x = 2, is energetically unfavorable as compared to the state 4 structure (x = 1). Nevertheless, the calculated Gibbs free energies of reaction are practically equal for these reactions (Table 1), and previous studies suggest x to be around 2.5,1 providing evidence for conversion from state 4 to state 7 during the TDMASn exposure. Experimentally, it is known that the energy barriers for the TDMASn half reaction can be overcome as evidenced by both the ALD growth demonstrated at low temperatures and by the increasing SnOx growth rate when lowering the deposition temperature from 200 °C to 30 °C.1,9 The growth at low temperatures may be rationalized if surface OH* groups on the deposited SnOx surface are more reactive than those on the initial SiO2 substrate, thus lowering the energy barriers with respect to the energy barriers reported here for SiOH surface species. However, this does not explain the increasing growth with decreasing temperature, which has been suggested to result from water and water-derived species remaining on the surface during the growth process.9 These water species would contribute to the accelerated growth by serving as additional OH* reaction sites during TDMASn exposure and by potentially being more reactive toward TDMASn. To provide insight into the origin of the increased growth at low temperatures, we simulated the reaction between TDMASn and H2O by molecular PBE0 calculations using a PCM water solvent model to estimate the influence of neighboring water molecules. The PBE0 energy trends were verified by MP2 calculations, and the resulting reaction pathways are illustrated in Figure 3. The reaction proceeds via H-transfer from H2O to DMA in an analogous fashion with the TDMASn chemisorption on silicon (Figure 2). In particular, the PBE0calculated reaction barrier here is 21.0 kcal/mol, which is close

because of different chemical environments on the silicon substrate and on the growing film. As summarized in Table 2, Table 2. Summary of DFT-Molecular Dynamics Simulations Performed on the OH-Terminated Si(100)-2×1 Surface with Chemisorbed TDMASn and Physisorbed HDMA to Estimate the Release of HDMA to the Gas Phase simulation temperature (°C)

simulation length (ps)a

behavior of initially physisorbed HDMA (remains bound to surface or is released into gas phase)b

50 100 150

10 5 5

bound to surface release to gas phase after 1.2 ps release to gas phase after 1.0 ps

a

A time step of 1 fs was utilized in the simulations. bHDMA was considered released from the surface once it was located above the plane formed by the outermost atoms of chemisorbed TDMASn and was moving away from the surface.

we observed the release of HDMA at and above 100 °C after simulating the system for about 1 ps, whereas at 50 °C HDMA remained on the surface coordinated to a surface OH* group even after simulating the system for 10 ps. For comparison, using the Arrhenius equation for reaction rates and the HDMA adsorption energy of 3.1 kcal/mol, the release of HDMA is about 2 and 3 times faster at 100 and 150 °C, respectively, as compared to the reaction at 50 °C. Overall, DFT-MD suggests deposition temperatures around 100 °C are sufficient to avoid HDMA contamination if the chemical environment during the steady-state SnOx ALD is similar to the one simulated here. It should be noted, however, that neither carbon nor nitrogen is observed in the films by X-ray photoelectron spectroscopy (after removal of adventitious carbon) deposited at temperatures between 30 and 200 °C,9 suggesting the physisorbed HDMA species are removed from the surface during the water pulse. The energy barrier of 17.8 kcal/mol for the H-transfer step (from states 1 to 3) originates from the breaking of O−H and Sn−N bonds, and the overall reaction is energetically favorable because of the formation of new N−H and Sn−O bonds. Notably, bond strengths of 110, 95, and 126 kcal/mol have been reported for O−H in SiOH,28 N−H in HDMA, and Sn− O in tin oxide, respectively.29 Using these values and assuming the calculated overall reaction energies originate from the bonds being formed and cleaved, the Sn−N bond strength is calculated to be around 100 kcal/mol. This suggests the Sn−O bond being formed is stronger than the Sn−N bond being cleaved, which can be understood by the fact that the electronegativity of O is higher than that of N, giving rise to additional stabilization due to electrostatic attractive interactions between O and Sn. The presence of a neighboring surface OH* enables the release of a second HDMA in a fashion analogous to the release of the first DMA ligand (states 5, 6, and 7). Here the energy barrier is calculated to be around 21.8 kcal/mol (energy difference between states 4 and 5), which is slightly higher than the barrier of 17.8 kcal/mol for the release of the first ligand. On the basis of analysis of the PBE0-optimized structures, this higher barrier originates from strong distortions in the orientation of the two Si−O bonds during the formation of the O−Sn−O linkage. In particular, the Si−Si−O angles range from 103° to 131° in the state 6 structure, whereas the same angles associated with surface OH* groups range from 110° to 116°. The overall reaction is energetically unfavored by 1.4

Figure 3. PBE0- and MP2-calculated reaction pathways for the reaction Sn(DMA)4(aq) + H2O(aq) → Sn(DMA)3OH(aq) + HDMA(aq) using a PCM solvent model for water (solid line). The dashed line refers to the reaction pathway determined from MP2 single-point energies calculated on the PBE0-optimized structures. H, O, N, C, and Sn atoms are shown in white, red, blue, gray, and green, respectively. 19059



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to the ∼20 kcal/mol barriers calculated for TDMASn chemisorption on silicon. These similar energetics suggest that the increased SnOx growth at low deposition temperatures does not originate from a facile reaction between TDMASn and physisorbed water. Instead, it likely originates from an increased number of OH* reaction sites introduced by the adsorbed water. It should be noted that in the presence of additional neighboring OH* groups, the O2Sn(DMA)2* surface species may react to release its third DMA, which would correspond to an extent of conversion x = 3 (see reaction 1), but after this third ligand loss, establishing a close proximity for the reaction between the remaining fourth DMA and a surface OH* (x = 4 in Reaction 1) is unlikely because Sn has already reacted with three neighboring surface OH* species. Investigation of TDMASn behavior on Si with different OH* surface coverages is an interesting target for future theoretical investigations, but because our focus here is on determining the basic surface chemistry of the precursors, we turn to the O2Sn(DMA)2*terminated surface (see Figure 2, state 7) which serves as the growth surface for the water half reaction. Pathway for Water Half Reaction. The calculated reaction pathway for two water molecules reacting in a stepwise fashion with a O2Sn(DMA)2* surface species is illustrated in Figure 4. Initially, one water molecule forms a hydrogen bond

mol with respect to reactants (Table 1, state 8). This can be explained by the Sn−O bond formed by the first water reaction disturbing the neighboring Sn−N bond, giving rise to the smaller energy barrier which facilitates the H2O half reaction, analogous to ZnO ALD from DEZn on H2O.21 To provide a deeper understanding of this reactivity change between the two water reactions, we performed orbital analysis on the O2Sn(DMA)2* and O2Sn(DMA)OH* surface species by cluster calculations, and their highest occupied molecular orbitals (HOMO) are illustrated in Figure 5. The HOMO

Figure 5. PBE0-determined highest occupied molecular orbitals shown in orange and purple for (a) O2Sn(DMA)2* and (b) O2Sn(DMA)OH* surface species. Si, H, O, N, C, and Sn atoms are shown in yellow, white, red, blue, gray, and green, respectively. An isovalue of 0.03 was used to visualize the orbitals.

delocalization over both Sn−N bonds in O2Sn(DMA)2* stabilizes this species and results in the observed activation barrier. In contrast, the HOMO is localized on the N atom in O2Sn(DMA)OH*, which makes the HOMO electrons more available for a reaction with H2O as compared to the reaction between O2Sn(DMA)2* and H2O and brings down the energy barrier. Overall, this half reaction facilitated by neighboring hydroxyls is likely to play a role in enabling ALD growth of SnOx at low deposition temperatures. Note that the OH groups of the O2Sn(OH)2* surface species (state 8 in Figure 4) would serve as reaction sites for the following TDMASn half reaction. Overall, both TDMASn and H2O half reactions have been shown to take place through moderate activation barriers, and the overall reactions are thermodynamically favored, which demonstrates their feasibility at typical elevated temperatures of the experiments. Moreover, it is shown that a neighboring OH group strongly facilitates the H2O half reaction. Our calculations also suggest that the experimentally observed increasing growth rate at low deposition temperatures, which has been attributed to the existence of molecular water species on the growth surface during ALD,9 is not due to an especially facile reaction between TDMASn and water. Instead, it is likely caused by an increased density of OH reactions sites due to the physisorbed water species, and future investigations are necessary for understanding the mechanistic details of TDMASn behavior on surfaces that contain both chemisorbed and physisorbed OH groups.

Figure 4. PBE0-calculated reaction pathway from local optimizations for H2O chemisorption on O2Sn(DMA)2*-terminated Si(100)-2×1 surface according to an overall reaction O2Sn(DMA)2* + 2 H2O → (OH)2* + SnO2 + 2 HDMA↑. Si, H, O, N, C, and Sn atoms are shown in yellow, white, red, blue, gray, and green, respectively.

to N with an interaction energy of −7.3 kcal/mol (state 1 in Figure 4), followed by H-transfer with an energy barrier of 16.8 kcal/mol (energy difference between states 1 and 2) to form physisorbed HDMA and chemisorbed O2Sn(DMA)OH* species (state 3). Release of the HDMA from the surface to the gas phase (state 4) results in an overall reaction energetically favored by about 5.2 kcal/mol with respect to the reactants (Table 1, state 4). The values of the calculated energy barrier and the overall reaction energy are similar to those of the TDMASn half reaction because of similar O−H and Sn−N bonds being cleaved and N−H and Sn−O bonds being formed during the reaction. The second water molecule reacts in an analogous fashion with the O2Sn(DMA)OH* species, but here the activation barrier is only 5.8 kcal/mol (energy difference between states 5 and 6), with the formation of the second Sn−OH* being energetically favored by 6.6 kcal/ 19060



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(5) Isidorsson, J.; Granqvist, C. Electrochromism of Li−Intercalated Sn Oxide Films Made by Sputtering. Sol. Energy Mater. Sol. Cells 1996, 44, 375−381. (6) Puurunen, R. Surface Chemistry of Atomic Layer Deposition: A Case Study for the Trimethylaluminum/Water Process. J. Appl. Phys. 2005, 97, 121301. (7) Zaera, F. The Surface Chemistry of Atomic Layer Depositions of Solid Thin Films. J. Phys. Chem. Lett. 2012, 3, 1301−1309. (8) Du, X.; Du, Y.; George, S. M. In Situ Examination of Tin Oxide Atomic Layer Deposition using Quartz Crystal Microbalance and Fourier Transform Infrared Techniques. J. Vac. Sci. Technol., A 2005, 23, 581−588. (9) Mullings, M. N.; Hägglund, C.; Bent, S. F. Tin Oxide Atomic Layer Deposition from Tetrakis(dimethylamino)tin and Water. J. Vac. Sci. Technol., A 2013, 31, 061503. (10) Heo, J.; Hock, A. S.; Gordon, R. G. Low Temperature Atomic Layer Deposition of Tin Oxide. Chem. Mater. 2010, 22, 4964−4973. (11) Heo, J.; Kim, S. B.; Gordon, R. G. Atomic Layer Deposition of Tin Oxide with Nitric Oxide as an Oxidant Gas. J. Mater. Chem. 2012, 22, 4599−4602. (12) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; Zicovich− Wilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco, P.; Llunell, M. CRYSTAL09 User’s Manual; University of Torino: Torino, Italy, 2009. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (14) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (15) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (16) Perdew, J.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (17) Adamo, C.; Barone, V. Toward Reliable Density Functional Methods without Adjustable Parameters: The PBE0 Model. J. Chem. Phys. 1999, 110, 6158−6170. (18) Karttunen, A. J.; Fassler, T. F.; Linnolahti, M.; Pakkanen, T. A. Structural Principles of Semiconducting Group 14 Clathrate Frameworks. Inorg. Chem. 2011, 50, 1733−1742. (19) Schafer, A.; Horn, H.; Ahlrichs, R. Fully Optimized Contracted Gaussian-Basis Sets for Atoms Li to Kr. J. Chem. Phys. 1992, 97, 2571− 2577. (20) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (21) Ren, J. Initial Growth Mechanism of Atomic Layer Deposition of ZnO on the Hydroxylated Si(100)-2×1: A Density Functional Theory Study. Appl. Surf. Sci. 2009, 255, 5742−5745. (22) Ardalan, P.; Dupont, G.; Musgrave, C. B. Reactions of Amino Acids on the Si(100)-2×1 Surface. J. Phys. Chem. C 2011, 115, 7477− 7486. (23) Kelly, M. J.; Han, J. H.; Musgrave, C. B.; Parsons, G. N. In-Situ Infrared Spectroscopy and Density Functional Theory Modeling of Hafnium Alkylamine Adsorption on SiOH and SiH Surfaces. Chem. Mater. 2005, 17, 5305−5314. (24) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (25) Schafer, A.; Huber, C.; Ahlrichs, R. Fully Optimized Contracted Gaussian Basis Sets of Triple Zeta Valence Quality for Atoms Li to Kr. J. Chem. Phys. 1994, 100, 5829−5835. (26) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758− 1775.

CONCLUSIONS The ALD of tin oxide from TDMASn and H2O on an OHterminated silicon substrate has been investigated by DFT and MP2 methods. Pathways for both half reactions have been determined by periodic calculations at the PBE0 level of theory to provide an atomic-level understanding of the surface chemistry of the precursors during SnOx ALD. The computational results are analyzed by making comparisons to experimental findings reported for SnOx ALD. The largest PBE0-calculated reaction barriers for both half reactions are close to 20 kcal/mol. Experiments show that these can be overcome even at low deposition temperatures, as demonstrated by accelerated ALD growth of SnOx as a function of decreasing the deposition temperature from 200 °C to well below 100 °C.1,9 The role of physisorbed water on the growth surface has been suggested as an explanation for ALD at these low temperatures. Although our calculations do not support a facile reaction between TDMASn and molecular water, our results suggest that the accelerated growth may originate instead from increased reaction site density due to physisorbed precursors. Also, the reaction barrier for the water half reaction was shown to be strongly lowered by an OH species bound to a O2SnN(CH3)2* surface group. This behavior also contributes to the feasibility of SnOx ALD at temperatures close to room temperature. Overall, we have provided atomic-level insights into SnOx ALD by quantum chemical calculations. This ALD surface chemistry understanding is helpful in SnOx ALD process development for the deposition of thin films for various applications, and it may prove useful in the understanding of the growth of other technologically promising materials such as SnSx for photovoltaics and zinc tin oxide for TCOs.30,31

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The work was supported by the Department of Energy under Award DE-SC0004782. J.T.T. gratefully acknowledges the Academy of Finland (Grant 256800/2012) and the Finnish Cultural Foundation for financial support.

(1) Elam, J. W.; Baker, D. A.; Hryn, A. J.; Martinson, A. B. F.; Pellin, M. J.; Hupp, J. T. Atomic Layer Deposition of Tin Oxide Films Using Tetrakis(Dimethylamino) Tin. J. Vac. Sci. Technol., A 2008, 26, 244− 252. (2) Vergöhl, M.; Malkomes, N.; Matthée, T.; Bräuer, G.; Richter, U.; Nickol, F.; Bruch, J. In Situ Monitoring of Optical Coatings on Architectural Glass and Comparison of the Accuracy of the Layer Thickness Attainable with Ellipsometry and Photometry. Thin Solid Films 2001, 392, 258−264. (3) Harsta, A.; Sundqvist, J.; Jun, L.; Ottosson, M. Growth of SnO2 Thin Films by Atomic Layer Deposition and Chemical Vapour Deposition: A Comparative Study. Thin Solid Films 2006, 514 (63), 63−6868−68. (4) Park, S.; Son, Y.; Willis, W.; Suib, S.; Creasy, K. Tin Oxide Films Made by Physical Vapor Deposition-Thermal Oxidation and Spray Pyrolysis. Chem. Mater. 1998, 10, 2389−2398. 19061



Article

(27) Mueller, R.; Kammler, H. K.; Wegner, K.; Pratsinis, S. E. OH Surface Density of SiO2 and TiO2 by Thermogravimetric Analysis. Langmuir 2003, 19, 160−165. (28) McCarthy, M. C.; Tamassia, F.; Woon, D. E.; Thaddeus, P. A Laboratory and Theoretical Study of Silicon Hydroxide SiOH. J. Chem. Phys. 2008, 129, 184301. (29) Luo, Y. R. In Comprehensive Handbook of Chemical Bond Energies; CRC Press: Boca Raton, FL, 2007. (30) Heo, J.; Kim, S. B.; Gordon, R. G. Atomic Layer Deposited Zinc Tin Oxide Channel for Amorphous Oxide Thin Film Transistors. Appl. Phys. Lett. 2012, 101, 113507. (31) Lee, Y. S.; Heo, J.; Siah, S. C.; Mailoa, J. P.; Brandt, R. E.; Kim, S. B.; Gordon, R. G.; Buonassisi, T. Ultrathin Amorphous Zinc−Tin− Oxide Buffer Layer for Enhancing Heterojunction Interface Quality in Metal−Oxide Solar Cells. Energy Environ. Sci. 2013, 6, 2112−2118.

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Insights into the Surface Chemistry of Tin Oxide Atomic Layer

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