The Case of Zinc Tin Oxide - American Chemical Society

Apr 4, 2014 - and Stacey F. Bent. †. †. Stanford University, Department of Chemical Engineering, 381 North-South Mall, Stanford, California 94305-...
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Correlating Growth Characteristics in Atomic Layer Deposition with Precursor Molecular Structure: The Case of Zinc Tin Oxide Jukka T. Tanskanen,*,†,‡ Carl Hag̈ glund,† and Stacey F. Bent† †

Stanford University, Department of Chemical Engineering, 381 North-South Mall, Stanford, California 94305-5025, United States University of Eastern Finland, Department of Chemistry, P.O. Box 111, FI-80101 Joensuu, Finland

‡

S Supporting Information *

ABSTRACT: The growth characteristics in atomic layer deposition (ALD) of mixed oxide thin films have been investigated by DFT calculations and in situ quadrupole mass spectrometry (QMS) using zinc tin oxide (ZTO) ALD from diethylzinc (DEZn), tetrakis(dimethylamido)tin (TDMASn), and H2O as a case study. The DFT-calculated Gibbs free energies of reaction for binding TDMASn on OH-terminated ZnO surfaces demonstrate the reaction to be feasible and provide evidence for a reduction in surface reaction site density upon mixing a small number of SnOx cycles into the ZnO ALD process. The in situ QMS experiments verify the reduction in surface reaction site density during the SnOx cycle, and demonstrate restoration of reaction site density during the subsequent ZnO cycles. The reduction in reaction site density, which is a consequence of the four exchangeable ligands of the TDMASn precursor, is shown to provide an atomic-level explanation for experimentally observed ZTO ALD growth characteristics. The correlation between precursor molecular structure and material growth established here for ZTO ALD applies to other ALD processes where precursors follow ligand-exchange surface chemistries, and thus, it provides general guiding principles useful in understanding and developing ALD processes.

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INTRODUCTION Atomic layer deposition (ALD) is a chemical vapor deposition (CVD) technique that is based on self-limiting material deposition achieved by exposing the sample to two or more separate gaseous reactant (precursor) pulses. The precursors react with the sample by gas−solid reactions forming up to a monolayer of the deposited material per ALD cycle.1 These surface reactions, which are governed by the interplay between the reaction sites at the growth surface and the precursor molecules, control the material deposition and give rise to the observed crystallographic structures and compositions, and hence the material properties. The atomically controlled growth in ALD finds use in applications such as thin film photovoltaics and microelectronics, where deposition of high quality, ultrathin films is necessary.2,3 Despite the development of numerous ALD processes for depositing elemental, binary, and ternary films as well as films with even higher compositions,1 the atomic-level details of growth are lacking for most ALD processes, and particularly so for ternaries and materials of higher compositions. This lack in understanding of ALD growth originates from the complex surface chemistries of the precursors, since a number of atomiclevel details that are not easy to obtain need to be determined in order to develop a mechanistic description of film growth. The atomic-level details include features such as overall reaction stoichiometry, possible stepwise surface decomposition of precursors, and changes in metal oxidation state, growth surface structure, and reaction sites. Furthermore, the growth mechanisms may differ between the initial nucleation on the substrate and the steady-state ALD growth.3 As such, a © 2014 American Chemical Society

combination of information from a number of experimental techniques and theoretical approaches is useful to obtain an atomic-level understanding of material growth via ALD. While the bulk of the reported ALD processes have been developed for elemental films and binary materials (most commonly oxides), an increasing number of studies on the growth of films of ternary and higher compositions have been reported. Examples of materials deposited by the latter ALD processes are LaNiO3,4 which is the first ternary material deposited by ALD, Al2O3/ZnO (AZO) and ZnO/SnOx (ZTO) mixed films for use as transparent conductive oxides (TCOs),5−9 and copper zinc tin sulfide (CZTS) for solar absorber layers in thin film photovoltaics.10,11 The ALD characteristics of ternary oxides may differ significantly from those of the constituent binaries, and reduced growth rates are commonly observed relative to the binaries. A nonlinear correlation between precursor cycle ratios and film composition has been demonstrated for AZO,10,12,13 ZTO,14 gallium zinc oxide (GZO),15 Zn1−xMgxO,16 and (Sn,Al)Ox.17 This characteristic has generally been attributed to slower growth rates of the binaries on each other rather than on themselves due to reduced reaction site densities and/or less reactive surface sites, and also to surface etching reactions. However, the roles of the precursor molecular structures and surface chemistries, and the interplay between the precursors in Received: November 27, 2013 Revised: March 14, 2014 Published: April 4, 2014 2795

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the molecular SVP basis set to avoid diffuse functions that are detrimental for the performance of periodic calculations.24 The diffuse outermost s-exponents in the Zn basis were increased from 0.042 and 0.119 to 0.16 and 0.40, respectively. Guided by previous work on crystallinity and growth direction of ZTO films deposited by ALD and on computational investigation of ZnO surfaces,14,25,26 periodic OH-terminated ZnO (100), (101), and (002) surface models were developed from wurtzite ZnO to represent the growth surfaces for the TDMASn half-reaction. The bulk ZnO was fully optimized at the 186 (P63mc) space group symmetry and verified as the true minimum energy structure by vibrational analysis, and the OH terminations were generated on both reactive and nonreactive surfaces of the slabs by saturating surface O atoms with hydrogens and unsaturated surface Zn atoms by OH groups. This is to avoid unrealistic slab dipole effects, and to provide surface models without partially occupied bands and with wide bandgaps, in analogy with bulk ZnO. The OH-terminated slabs were then fully optimized without symmetry constraints. Simulation of the TDMASn half-reaction on three ZnO surfaces, rather than focusing on the one corresponding to the preferred growth orientation, enabled comparison of the halfreaction energetics on different surface atomic arrangements. The unit cell atomic compositions utilized were Zn24O32H16 for the (100) and (101) slabs, and Zn18O27H18 for the (002) slab, and the surface models used are illustrated in Figure 1. The utilized slab

determining the growth characteristics of these mixed ALD processes are less explored. In the study reported herein, we use density functional theory (DFT) and in situ quadrupole mass spectrometry (QMS) experiments to extract atomic-level information on the precursor surface chemistries for a mixed oxide process, namely ZTO ALD by means of diethylzinc (DEZn), tetrakis(dimethylamido)tin (TDMASn), and H2O. Modulation of surface reaction site density is shown to play an important role in ZTO ALD growth, causing reduced Zn growth as compared to the binary growth rates and resulting in excess Sn in the films as compared to the compositions expected from the binaries. The reaction site density modulation in ZTO ALD is a consequence of the TDMASn precursor having four exchangeable ligands (with DEZn having two exchangeable ligands), and the understanding of this correlation between the precursor’s molecular structure and the ALD growth characteristics is of general importance and provides guiding principles useful in understanding and developing ALD processes beyond mixed oxides.

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EXPERIMENTAL AND COMPUTATIONAL DETAILS

The ALD was carried out in a custom-built, viscous-flow, hot-wall reactor previously described.18 Diethyl zinc (DEZn; Sigma-Aldrich) and tetrakis(dimethylamido)tin (TDMASn; Strem, >99% purity) were used as metal precursors with deionized water as the oxygen source. The precursors were dosed from 10 cm3 stainless steel sample vials using computer controlled air-actuated valves. The water and DEZn sample vials were kept at room temperature, whereas the TDMASn vial was heated to ∼45 °C during the ALD experiments. Precursor condensation was prevented by maintaining a positive temperature gradient toward the reaction zone. About 1 cm2 sized n-type Si(100) substrates with phosphorus dopant and a resistivity of 1.0−5.0 Ω-cm (WRS Materials) were utilized, and the substrates were cleaned using UV-ozone (PSD Benchtop UV-Ozone Cleaner) to remove organic contamination, leaving behind a Si oxide surface. During ALD, any excess precursor was purged with nitrogen for separation of halfreactions. The ZTO was deposited at 150 °C using an ALD supercycle composed of one SnOx ALD cycle followed by 5 ZnO ALD cycles, i.e. using a SnOx:ZnO cycle ratio of 1:5 with a bilayer period (the number of ALD cycles in a supercycle) of 6. The ZTO ALD was monitored in situ by a quadrupole mass spectrometer (SRS RGA200). Namely, we monitored signals at mass to charge (m/z) ratios of 18, 29, and 44 originating from water, ethane, and dimethylamine, respectively. Focus on these species was based on the previously proposed ALD halfreactions for SnOx ALD and ZnO ALD (see reference 14 and the citations therein). For these m/z signatures, there are no important contributions from the cracking patterns of the other molecules. Specifically, there is no contribution at m/z 29 from dimethylamine, but a small contribution at m/z 18 which may be due to a small amount of residual water in the reactant. The latter is, on the other hand, typically much weaker than the peak due to water, and is therefore neglected in our analysis. Conversely, there is no visible contribution from the ethane groups at mass 18 or 44. Precursor pulsing times of 1 s were utilized based on previous work on SnOx and ZnO ALD,14 and relatively long N2 purges of 60 s were adopted. These precursor pulses and purge steps were sufficient for the precursor concentrations to return close to their baseline values as monitored by reactor pressure and in situ mass spectrometry. Thus, physisorbed species, which are removed with 60 s purges, are not an issue in practical ALD processes. Computations of the surface chemistry of ZTO ALD were carried out by the CRYSTAL0919 software. A PBE020,21 hybrid functional with standard split-valence plus polarization (SVP) basis sets and two modified SVP basis sets was utilized. The following basis sets were applied in the calculations: for C, H, N, and O, standard all electron SVP basis;22,23 for Sn and Zn, modified SVP basis sets derived from

Figure 1. Top (left) and side (right) views of the OH-terminated ZnO (100), (101), and (002) slabs used as growth surfaces for the TDMASn half-reaction. H, O, and Zn atoms are shown in white, red, and cyan, respectively. thicknesses were 5 atomic layers for the (002) slab and 6 for the other two slabs, and three slab thicknesses were tested for each surface, namely 3, 5, and 6 for (002) and 4, 6, and 8 for (100) and (101). The utilized slab thicknesses were sufficient for bandgap convergence and stable structures, as verified by frequency calculations. Gibbs free reaction energies per precursor molecule at 150 °C for binding TDMASn on ZnO were determined by performing full PBE0 optimizations on the investigated slab systems, and on the free reactant and reaction product molecules in the gas phase, followed by performing frequency calculations on the optimized structures to get 2796

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access to Gibbs-corrected total energies. Thus, entropies were computed on all systems. 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 generated by the Monkhorst−Pack method,27 and an extra-large integration grid were adopted in the calculations. Anderson’s method, as implemented in CRYSTAL09, for accelerating convergence in the electronic optimization was utilized and calibration calculations on the TDMASn half-reaction energetics verified convergence of the results with this computational approach.

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RESULTS Here, we focus on a ZTO ALD process from TDMASn, DEZn, and H2O, where the material is deposited using a SnOx:ZnO cycle ratio of 1:5, i.e. by using an ALD supercycle composed of one SnOx cycle (TDMASn/H2O) followed by five cycles of ZnO (DEZn/H2O). This ZTO ALD process provides a good starting point for understanding the growth characteristics of mixed oxides because it has been thoroughly characterized experimentally and shown to produce crystalline films,14 which facilitates surface model preparation for simulating the process computationally. Note, however, that computational results from a crystalline surface model may not be applicable to amorphous films due to possibly different reactivities and reaction site densities on the crystalline and amorphous surfaces. The ZTO deposited by this SnOx:ZnO 1:5 process has a preferential growth direction along the wurtzite (100), a growth rate of 1.36 Å/cycle, which is reduced by 18% as compared to the growth rate estimated from the binary ALD growth rates, and a composition rich in Sn as compared to the composition expected from the densities and growth rates of the binaries.14 As suggested by the SnOx:ZnO cycle ratio of 1:5, these characteristics originate from treating a ZnO surface produced by the 5 ZnO cycles with TDMASn and H2O (the SnOx cycle). Notably, previous analysis of the ZTO ALD growth rate provided an estimate for the restoration of ZnOlike growth on SnOx after three ZnO ALD cycles,14 providing support for the formation of a pure ZnO surface by the fifth ZnO ALD cycle. As discussed above, ALD of ZTO can be achieved by mixing ZnO and SnOx ALD cycles, and the proposed surface chemistries can be summarized with the following half-reactions 1 and 2 for the ZnO28 cycle and 3 and 4 for the SnOx18 cycle:

Figure 2. Reaction schemes for (a) ZnO half-reactions and for (b) SnOx half-reactions. The OH-terminated surface, on which DEZn and TDMASn chemisorb, is represented by the gray slab.

mechanistic details of the half-reactions 1 and 2 are not available. Nonetheless, it has been well established by FTIR spectroscopy measurements that the surface ethyl (−CH2CH3) and OH species oscillate versus DEZn and H2O exposures during ZnO ALD,28,31 providing support for the reactions 1 and 2. Also, ethane is observed during DEZn pulses by in situ QMS (vide infra), providing additional support for the ligandexchange reaction chemistries in eqs 1 and 2. For comparison, TDMASn decomposition is unlikely at the deposition temperature of 150 °C utilized in this work due to the decomposition taking place at temperatures around 350 °C.32 In addition, no CVD component was observed for the TDMASn/H2O ALD process in a recent work.18 DEZn can in principle react on the growth surface by releasing one or both of its ethyl ligands, but it is generally accepted that most DEZn binds to the surface according to Reaction 1.33 On the other hand, TDMASn may release 1 to 3 of its DMA ligands (release of all four DMA ligands being chemically unreasonable), and it has been proposed based on in situ quartz crystal microbalance (QCM) and QMS experiments that on the average x = 3

OH* + Zn(CH 2CH3)2 → O−Zn(CH 2CH3)* + C2H6↑ (1)

O−Zn(CH 2CH3)* + H 2O → O−ZnOH* + C2H6↑ (2)

(OH)x * + Sn(DMA)4 → (O)x Sn(DMA)4 − x * + x HDMA↑

(3)

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

(4)

where “*” refers to surface species and the arrows to desorbed products. The OH, O−Zn(CH 2 CH 3 )*, and (O) x Sn(DMA)4−x* groups are referred to as reaction sites for the half-reactions illustrated in Figure 2, whereas the term “reaction site density” is used to discuss the relative amounts of the reaction sites on the growth surfaces. Note that DEZn is likely to decompose at elevated temperatures,29,30 and the full 2797

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during SnOx ALD when using H2O2 as the counter-reactant,32 while an average value of x ∼2.5 was reported in the same study for SnOx ALD from TDMASn and H2O. Note that the number of available surface sites for the next half-reaction is modulated by x, that is by the surface chemical behavior of TDMASn, as illustrated in Figure 2b. Assuming all OH* groups react during the TDMASn pulse, then x = 1 would triple the reaction site density, x = 2 would have no effect on the reaction site density, x = 3 would reduce the reaction site density to 1/3 of the original density, and x = 4 would result in complete removal of the reaction sites. Notably, ZnO surfaces hydroxylate easily in the presence of water,34 resulting in high surface OH density. For example, our surface models for ZnO have surface OH densities ranging from 11.1 OH/nm2 for ZnO (100) to 12.0 OH/nm2 for ZnO (101) (see Experimental and Computational Details section). To verify the feasibility of the TDMASn half-reaction on ZnO and to predict the extent of surface reaction site density modulation in ZTO ALD (see Figure 2), Gibbs free energies of reaction at 150 °C for TDMASn binding on the experimentally observed ZnO (100), (101), and (002) surface planes, which are fully OH-terminated, were determined by PBE0-calculations. The use of Gibbs free energies was necessary to take into account entropy contributions to the energetics associated with formation of gas phase species. For comparison, the Gibbs free energies of reaction for DEZn binding on the ZnO surface were also determined and the results are summarized in Figure 3. The optimized structures are illustrated in Figure 4. Precursor physisorption, which precedes the half-reactions,

was not investigated since previous studies have shown that both TDMASn and DEZn readily physisorb on OH-terminated surfaces and that the physisorbed species are removed by even short ALD purges around 5 s, as evidenced by self-limiting ALD growth of the binaries.10,32,35,36 Furthermore, the mechanistic details of the TDMASn half-reaction, such as possible multiple proton diffusion or surface densification effects resulting from the different TDMASn-derived surface species as recently demonstrated for TDMAHf during HfO2 ALD,37 were not investigated here although they provide an interesting target for future theoretical investigations. Our focus on the main ALD half-reactions enabled a systematic evaluation of the extent of ligand conversion (x in reactions 3 and 4) on the ZTO ALD growth. Note that the extent of ligand conversion is of general importance in ALD processes from precursors with more than one exchangeable ligand. In general, the energies show that the formation of TDMASn-derived surface species with x = 1 to x = 3 is thermodynamically favorable, with the exception of binding TDMASn on the (002) surface with x = 3. For comparison, DEZn binding on the ZnO surfaces according to the generally accepted half-reaction (see Reaction 1; this is equivalent to x = 1 using the TDMASn reaction nomenclature) is also thermodynamically favorable. However, a fraction of the DEZn may bind to the surfaces also by releasing both ethyl ligands (equivalent to x = 2) due to the calculated negative Gibbs free energies of reaction. It is noted here that DEZn should release both of its ligands in irreversible reactions typical for ALD processes due to the favorable Gibbs free energies of reaction, whereas in case of reversible reactions the structure with the most favorable Gibbs free energy of reaction would be favored. In analogy, TDMASn should favor the reaction corresponding to x = 3. The release of both ethyl ligands in DEZn is, however, unlikely due to a large activation barrier associated with the release of the second ethyl, as suggested by a previous computational study on DEZn reacting with two OH groups, for which an energy barrier of about 40 kcal/mol was reported.36 Furthermore, surface ethyls are experimentally observed after DEZn pulses.28 In agreement with experimental observations, the TDMASn binding is most favorable on ZnO (100), which is the preferred growth orientation for this ZTO ALD process.14 Structural analysis of the (O)3Sn-DMA* surface species provides insight into this preference: the averaged O−Sn−N angles of the species are 114°, and 118°, and 119° on (100), (101), and (002) surfaces, respectively. In TDMASn, the averaged N−Sn− N angle in TDMASn is 110°, close to the angle of 114° on (100). Thus, the atomic arrangement of the (001) surface is structurally well-suited for TDMASn binding, resulting in the favorable energetics. Furthermore, the results suggest that TDMASn reacts on the investigated ZnO surfaces, with the exception of the (002) surface, according to (OH)3* + Sn(DMA)4 → (O)3Sn(DMA) * + 3 HDMA ↑ with x = 3, thus reducing the surface reaction site density. In practice, TDMASn-derived surface species corresponding to x = 1 and 2 may coexist with the (O)3Sn(DMA) species due to competition of the surface OH groups by TDMASn molecules resulting in limited availability of OH groups for some precursor molecules. Therefore, the calculated Gibbs energies of reaction suggest x to be close but not larger than 3 on average, which will result in reduced surface reaction site density. The latter should reflect in the ZTO growth characteristics. For comparison, Elam et

Figure 3. Calculated Gibbs free energies of reaction (ΔG, T = 150 °C) for DEZn and TDMASn binding on ZnO (101), (002), and (100) surfaces according to reactions 1 and 3, respectively. The extent of ligand conversion (x) is varied. The zero energy corresponds to the sum of the energies of the isolated surface and the gas phase precursor. Red, green, and blue colors refer to the ZnO (002), (101), and (100) surfaces, respectively. Schematic TDMASn-derived surface species corresponding to the different values of x are shown on top. 2798

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Figure 4. Schematic and PBE0-optimized structures of the surface species of TDMASn (left) and DEZn (right) on ZnO (002), (101), and (100) surface slabs. H, O, Zn, and Sn atoms are shown in white, red, cyan, and green, respectively. x refers to the extent of ligand conversion in the TDMASn half-reaction.

al.32 reported a value of x ∼2.5 for SnOx ALD from TDMASn and H2O. By recognizing that the number of surface reaction sites correlates with the number of water molecules reacted during each water pulse in ZTO ALD (see Figure 2 and Reactions 2 and 4), changes in surface reaction site density can be estimated by monitoring the amount of adsorbed water during ZTO ALD. We measured the amount of reacted water during each water pulse using in situ QMS by monitoring the signal at m/z = 18, and example QMS results during four supercycles of ZTO are illustrated in Figure 5a. The large signals occur during the H2O pulse and the small shoulders arise during the Sn or Zn metal precursor pulses due to water desorbing from the reactor walls. The water signals behave systematically in that they are large during SnOx cycles (i.e., during the H2O pulse directly following the TDMASn pulse, red peaks in Figure 5a) and decrease during the subsequent five ZnO cycles (black peaks). A large peak during a SnOx cycle means that less water is bound to the surface as compared to the smaller peaks during the ZnO cycles. Note that the binding of increasing amounts of water during the ZnO cycles (reflected in decreasing H2O peaks in Figure 4) correlates with the observation of generally increasing ethane signals at m/z = 29 during the water pulses of the ZnO cycles, because a H2O molecule releases an ethane molecule

from the surface according to Reaction 2 (see Figure S1 in Supporting Information). The QMS data was converted to fractional amounts of adsorbed water during a ZTO supercycle by averaging the intensities over 20 supercycles of steady-state ZTO ALD and by referencing to the average from 80 non-ALD water pulses preceding the ZTO supercycles (i.e., initially pulsing only water into the reactor without the metal precursor and taking the average of the measured water signals), providing the results illustrated in Figure 5b. Standard deviations for the peak intensities were about 6−8% and the QMS data are given in full in Supporting Information (Figure S1). The systematic behavior of the water signal intensities during the experiment suggests that drift of the water signal baseline does not significantly bias the results. The amount of water bound during a SnOx ALD cycle is about 30% less as compared to the amount of water bound during the last ZnO cycle in a supercycle. Since the amount of water bound is linearly correlated with the number of reaction sites (see discussion above), the TDMASn treatment of the ZnO surface reduces the surface reaction site density. This is likely to be caused by the formation of (O)3Sn(DMA)* surface species on the surface during the TDMASn pulse, as suggested by the Gibbs free energies for binding TDMASn on ZnO (see 2799

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Implications of the Reduction in Reaction Site Density on ZTO Growth. The reduction in reaction site density by the SnOx cycle, as implied by the DFT calculations and confirmed by in situ QMS, agrees with the growth rate and film composition observed for this ZTO ALD process:14 the ZTO growth rate is about 1.36 Å/cycle, which is reduced by 18% as compared to the growth rate of 1.66 Å/cycle expected from the growth rates of the binaries. By linearly correlating the ZnO growth rates with the fractions of the reaction sites, i.e. by scaling the ZnO growth rates in a supercycle by the fractions shown in Figure 5b, a ZTO growth rate of 1.44 Å/cycle results. This differs by only 6% from the experimentally observed ZTO growth rate, providing additional support for the reaction site density reduction in particular because such errors are typical for experimentally determined growth rates. Further, the ZTO deposited by this ALD process is Sn-rich, as measured by inductively coupled plasma optical emission spectroscopy (ICP-OES), with respect to the composition expected from the growth rates and densities of the binaries.14 This finding agrees with the reaction site density reduction, which reduces ZnO growth as discussed above, and results in deposition of less Zn, making the material Sn-rich. Note, however, that while the above qualitative analysis provides strong support for a direct link between precursor molecular structure and the ZTO growth, the ALD growth of several other ternary oxides has been successfully modeled analytically, e.g., by Elliot et al., by considering factors such as precursor surface coverage and metal charge.38 Thus, for a more quantitative analysis of the ZTO growth, the factors utilized in the work by Elliot et al. should be considered. Nevertheless, incorporating the reaction site density modulation into the existing analytical models on ALD growth provides the possibility for accuracy improvements and applicability beyond mixed metal oxides. Overall, our results from DFT calculations and in situ QMS experiments suggest that the reduction in reaction site density caused by mixing SnOx ALD cycles to ZnO ALD plays a major role in ZTO ALD growth. Furthermore, the reaction site density reduction provides an explanation for the experimentally observed ZTO ALD growth characteristics. The reaction site density modulation is a consequence of TDMASn having four ligands available for ligand-exchange surface reactions, and hence our work reveals a correlation between precursor molecular structure and ALD growth characteristics. Furthermore, the number of ligands available for ligand-exchange reactions originates from the oxidation state (or valence) of the metal precursor, and thus the reaction site density modulation may be linked with the difference in the oxidation states of the metal centers in the precursors on a more general level. The reaction site density modulation is expected to play a role in other ALD processes from precursors with more than one exchangeable ligand, and it is expected to be a useful parameter for further developing existing analytical ALD growth models. Further, our work implies that the reduced growth rates of mixed oxides may be avoided by using precursors without the potential to reduce reaction site density.

Figure 5. (a) Water signal at mass to charge (m/z) ratio of 18 measured in situ by QMS during four supercycles of ZTO ALD from DEZn, TDMASn, and H2O using a SnOx:ZnO ALD cycle ratio of 1:5. (b) Relative fractions of reacted water determined by averaging the QMS data over 20 ZTO ALD supercycles and referencing 80 signals from non-ALD water pulses. In (a) and (b), water signals/fractions during the SnOx ALD cycle are shown in red. Films were deposited on Si(100) at 150 °C using 1 s reactant pulses and 60 s N2 purges.

Figure 3). Assuming consumption of all surface OH sites and the concomitant formation of only (O)2Sn(DMA)2* and (O)3Sn(DMA)* surface species during the TDMASn pulse, binding of every third TDMASn to the surface through formation of (O)3Sn(DMA)* would result in a reaction site density reduction by about 30%. The tabulation is based on recognizing that each (O)3Sn(DMA)* species reduces the number of surface OH groups by 2, while the formation of (O)2Sn(DMA)2* species does not change the number of surface OH groups. The details of the calculation are given in the Supporting Information. The reaction site density reduction by about 30% corresponds to the extent of ligand conversion being around 2.3 in ZTO ALD using the SnOx:ZnO cycle ratio of 1:5, which is close to the value of ∼2.5 reported for pure SnOx ALD.32 As illustrated in Figure 5b, an increasing number of water molecules is reacting on the surface during the ZnO cycles following the SnOx cycle, suggesting restoration of the reaction sites during the ZnO cycles. While the mechanistic details of the restoration are not available, it is likely that the reaction site density is increased by hydroxylation of the as-deposited ZnO particles/layers (i.e., OH groups are generated) during the water pulses, because ZnO surfaces are known to easily reconstruct and hydroxylate in the presence of water.32

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CONCLUSIONS A combined theoretical−experimental approach based on DFT computations and in situ QMS experiments was successfully utilized to provide insight into the growth characteristics of mixed oxide ALD processes using zinc tin oxide ALD as a case study. In particular, the consequences of incorporating SnOx 2800

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support. We would like to thank Dr. Antti Karttunen for providing the basis set for Zn.

ALD cycles (TDMASn/H2O) into ZnO ALD (DEZn/H2O) using a SnOx:ZnO cycle ratio of 1:5 was investigated in detail. Gibbs free energies of reaction calculated at the DFT level of theory for binding TDMASn on ZnO surfaces according to (OH)x* + Sn(DMA)4 → (O)xSn(DMA)4−x* + xHDMA↑ demonstrate that the reactions are spontaneous up to a ligand conversion of x = 3 and that the reactions are most favorable on ZnO(100), which is the preferred growth direction for the investigated ZTO ALD process. According to the energetics, the most likely TDMASn-derived surface species are (O)2Sn(DMA)2* and (O)3Sn(DMA)*, with the latter species having the potential to reduce surface reaction site density during ZTO ALD. An approach based on in situ QMS monitoring of water during the ZTO ALD was successfully utilized to probe changes in reaction site density. A single TDMASn treatment of ZnO deposited by five ZnO ALD cycles in a supercycle was shown to significantly reduce the reaction site density, which slowly increased during subsequent ZnO ALD cycles. The reaction site density reduction, which originates from TDMASn having four ligands available for ligand-exchange surface reactions, provides an explanation for previous experimental findings on ZTO growth rate and film compositions. Consequently, our work reveals an important correlation between precursor molecular structure and ALD growth characteristics. The different ALD growth characteristics of mixed oxides as compared to their constituent binary ALD processes are typically attributed to less reactive reaction sites, changes in reaction site density, or etching reactions during the mixed oxide ALD. Here we showed that the reaction site density reduction resulting from TDMASn treatment of ZnO plays a major role in ZTO ALD and it is expected to be important in the ALD of other mixed oxides, such as AZO and GZO, from precursors with more than one exchangeable ligand. Thus, the atomic-level ALD understanding developed here is expected to be useful in developing novel ALD processes including, but not limited to, mixed oxides.

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(1) Miikkulainen, V.; Leskela, M.; Ritala, M.; Puurunen, R. L. J. Appl. Phys. 2013, 113, 021301. (2) Zaera, F. J. Mater. Chem. 2008, 18, 3521. (3) Zaera, F. J. Phys. Chem. Lett. 2012, 3, 1301. (4) Seim, H.; Molsa, H.; Nieminen, M.; Fjellvag, H.; Niinisto, L. J. Mater. Chem. 1997, 7, 449−455. (5) Elam, J.; Routkevitch, D.; George, S. J. Electrochem. Soc. 2003, 150, G339. (6) Choi, W. S. J. Korean Phys. Soc. 2010, 57, 1472. (7) Hultqvist, A.; Platzer-Bjorkman, C.; Zimmermann, U.; Edoff, M.; Torndahl, T. Prog. Photovoltaics 2012, 20, 883. (8) Lee, Y. S.; Heo, J.; Siah, S. C.; Mailoa, J. P.; Brandt, R. E.; Kim, S. B.; Gordon, R. G.; Buonassisi, T. Energy Environ. Sci. 2013, 6, 2112. (9) Dhakal, T.; Vanhart, D.; Christian, R.; Nandur, A.; Sharma, A.; Westgate, C. R. J. Vac. Sci. Technol. A 2012, 30, 021202. (10) Elam, J. W.; George, S. M. Chem. Mater. 2003, 15, 1020. (11) Thimsen, E.; Riha, S. C.; Baryshev, S. V.; Martinson, A. B. F.; Elam, J. W.; Pellin, M. J. Chem. Mater. 2012, 24, 3188. (12) Gong, S. C.; Choi, Y.; Kim, H.; Park, C.; Park, H.; Jang, J. G.; Chang, H. J.; Yeom, G. Y. J. Vac. Sci. Technol. A 2013, 31, 01A101. (13) Yuan, H.; Luo, B.; Yu, D.; Cheng, A.; Campbell, S. A.; Gladfelter, W. L. J. Vac. Sci. Technol. A 2012, 30, 01A138. (14) Mullings, M. N.; Hägglund, C.; Tanskanen, J. T.; Yee, Y.; Geyer, S.; Bent, S. F. Thin Solid Films 2014, 556, 186−194. (15) Chalker, P. R.; Marshall, P. A.; Romani, S.; Rosseinsky, M. J.; Rushworth, S.; Williams, P. A.; Buckett, J.; McSporran, N.; Ridealgh, J. MRS Proc. 2011, 1315, 39. (16) Törndahl, T.; Platzer-Björkman, C.; Kessler, J.; Edoff, M. Prog. Photovoltaics Res. Appl. 2007, 15, 225. (17) Heo, J.; Liu, Y.; Sinsermsuksakul, P.; Li, Z.; Sun, L.; Noh, W.; Gordon, R. G. J. Phys. Chem. C 2011, 115, 10277. (18) Mullings, M. N.; Hägglund, C.; Bent, S. F. J. Vac. Sci. Technol. A 2013, 31, 061503. (19) Dovesi, R.; Saunders, V. R.; Roetti, C.;Orlando, R.; ZicovichWilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco, P.; Llunell, M. CRYSTAL09 User’s Manual; Torino, Italy, 2009. (20) Perdew, J.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (21) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158. (22) Schafer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571. (23) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297. (24) Karttunen, A. J.; Fassler, T. F.; Linnolahti, M.; Pakkanen, T. A. Inorg. Chem. 2011, 50, 1733. (25) Cooke, D. J.; Marmier, A.; Parker, S. C. J. Phys. Chem. B 2006, 110, 7985. (26) Li, H.; Schirra, L. K.; Shim, J.; Cheun, H.; Kippelen, B.; Monti, O. L. A.; Bredas, J. Chem. Mater. 2012, 24, 3044. (27) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188. (28) Ferguson, J. D.; Weimer, A. W.; George, S. M. J. Vac. Sci. Technol. A 2005, 23, 118. (29) Lee, B. H.; Hwang, J. K.; Nam, J. W.; Lee, S. U.; Kim, J. T.; Koo, S.; Baunemann, A.; Fischer, R. A.; Sung, M. M. Angew. Chem. 2009, 121, 4606. (30) Vidjayacoumar, B.; Emslie, D. J. H.; Clendenning, S. B.; Blackwell, J. M.; Britten, J. F.; Rheingold, A. Chem. Mater. 2010, 22, 4844. (31) Yousfi, E. B.; Fouache, J.; Lincot, D. Appl. Surf. Sci. 2000, 153, 223. (32) Elam, J. W.; Baker, D. A.; Hryn, A. J.; Martinson, A. B. F.; Pellin, M. J.; Hupp, J. T. J. Vac. Sci. Technol. A 2008, 26, 244. (33) Knapas, K.; Ritala, M. Crit. Rev. Solid State Mater. Sci. 2013, 38, 167. (34) Wöll, C. Prog. Surf. Sci. 2007, 82, 55. (35) Tanskanen, J. T.; Bent, S. F. J. Phys. Chem. C 2013, 117, 19056.

ASSOCIATED CONTENT

S Supporting Information *

Basis set for Zn utilized in the DFT calculations, in situ QMS data for ZTO ALD from DEZn, TDMASn, and H2O using a SnOx/ZnO ALD cycle ratio of 1:5, tabulation of the relative amounts of TDMASn-derived surface species, and PBE0calculated Gibbs free energies of reaction. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Department of Energy under Award DE-SC0004782. The development of the ALD reactor was funded as part of the Center on Nanostructuring for Efficient Energy Conversion (CNEEC) at Stanford University, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001060. J.T.T. gratefully acknowledges the Academy of Finland (Grant 256800/ 2012) and the Finnish Cultural Foundation for financial 2801

dx.doi.org/10.1021/cm403913r | Chem. Mater. 2014, 26, 2795−2802

Chemistry of Materials

Article

(36) Ren, J. Appl. Surf. Sci. 2009, 255, 5742. (37) Shirazi, M.; Elliott, S. D. Chem. Mater. 2013, 25, 878. (38) Elliott, S. D.; Nilsen, O. ECS Trans. 2011, 41, 175.

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dx.doi.org/10.1021/cm403913r | Chem. Mater. 2014, 26, 2795−2802