Molecular Level Insights into Atomic Layer Deposition of CdS by

Sep 15, 2010 - model, calculation, program, method, basis set ..... While our calculations provide evidence for the submonolayer coverage, future ...
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J. Phys. Chem. C 2010, 114, 16618–16624

Molecular Level Insights into Atomic Layer Deposition of CdS by Quantum Chemical Calculations Jukka T. Tanskanen,*,† Jonathan R. Bakke,‡ Stacey F. Bent,‡ and Tapani A. Pakkanen† Department of Chemistry, UniVersity of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland, and Department of Chemical Engineering, Stanford UniVersity, 381 North-South Mall, Stanford, California 94306-5025 ReceiVed: June 26, 2010; ReVised Manuscript ReceiVed: August 9, 2010

Growth characteristics of cadmium sulfide atomic layer deposition (ALD) from dimethylcadmium (DMCd) and hydrogen sulfide have been investigated by hybrid DFT and MP2 calculations. The steady-state film growth during one ALD cycle was modeled by studying dissociative chemisorption of the dimethylcadmium precursor on the sulfur-terminated (111) surface of zincblende CdS and then by investigating the chemisorption of hydrogen sulfide on the surface formed due to the treatment with the Cd reactant. The calculated reaction barriers for the ALD half reactions suggest that elevated temperatures are required for the film growth. Periodic calculations provide evidence for submonolayer growth per ALD cycle and suggest that steric factors prevent full monolayer formation during DMCd exposure, whereas all adsorption sites are likely to react during the hydrogen sulfide pulse. Introduction Cadmium sulfide is a technologically important direct band gap semiconductor which exists in zincblende (cubic) and wurtzite (hexagonal) crystal structures, the wurtzite phase being favored at ambient conditions. Furthermore, thin films of CdS have proven useful in several applications. Most notably CdS serves as ultrathin buffer layers in modern thin film photovoltaic solar cells,1 and they show promise in thin-film transistor devices.2 Thin films of both crystal phases have been prepared by deposition methods such as metal organic chemical vapor deposition (MOCVD),3,4 chemical bath deposition (CBD),5 and atomic layer deposition (ALD).6 ALD is a particularly promising technique for the deposition of CdS films because it enables atomic-level control of the film growth, which is essential for the deposition of ultrathin pinholefree films, and because it can produce films of desired crystal phase through the utilization of appropriate process parameters.7 In addition, the high control over film growth enables the use of small reactant amounts, which reduces the environmental impact of the process. The prospect of a CdS ALD process based on dimethyl cadmium (DMCd) and H2S was demonstrated in the late 90s,8,9 and the first few cycles of CdS growth on ZnSe (100) in ultrahigh vacuum was studied in detail by Luo et al.10–13 This work provided a fundamental understanding of the growth mechanism of CdS on ZnSe, and it was suggested that the DMCd exposure etched the substrate with a concomitant release of DMZn species. In spite of the mechanism studies, the CdS growth characteristics have not been systematically investigated until recently,6 and the mechanistic details of the film growth at steady-state growth have not been reported. The recent work demonstrated that temperature has a strong effect on the film growth, decreasing the growth rate from about 2.0 Å/cycle at * To whom correspondence should be addressed. E-mail: jukka.tanskanen@ uef.fi. † University of Eastern Finland. ‡ Stanford University.

100 °C down to 0.7 Å/cycle at 300 °C. In addition, films with cubic zincblende crystal structure were formed at low temperatures, whereas wurtzite phase content in the films increased with the deposition temperature. The described growth characteristics originate from the not yet fully understood surface chemistries of the precursor molecules; however, the following half reactions have been suggested to be relevant during CdS ALD on the basis of TPD experiments10

DMCd pulse: SH*(s) + Cd(CH3)2(g) f S-Cd-CH3*(s) + CH4(g) (1) H2S pulse: S-Cd-CH3*(s) + H2S(g) f S-Cd-SH*(s) + CH4(g) (2) where “*” refers to a surface species. A detailed understanding of the CdS growth mechanisms would provide insight into the experimental findings and would facilitate ALD process development for the deposition of ultrathin films for various applications.14,15 Generally, an understanding of the ALD surface chemistry is not easily obtainable by experimental methods, and this is particularly true for processes that are sensitive to the process parameters and reactor setup. Accordingly, a computational approach provides an appealing alternative to experimental investigation of the ALD growth characteristics. Here, we determine the growth characteristics of CdS thin films deposited from DMCd and H2S by quantum chemical hybrid DFT and ab initio MP2 methods utilizing both periodic slab models and finite clusters. The main factors influencing CdS growth are determined, and the DMCd is found to limit film growth in terms of material deposited per ALD cycle mainly due to steric effects. Computational Details On the basis of the reported experimental findings for the CdS films deposited from DMCd and H2S using the ALD

10.1021/jp105911p  2010 American Chemical Society Published on Web 09/15/2010

Atomic Layer Deposition of CdS

Figure 1. Top and side views of (a) the CdS zincblende (111) growth surface and (b) the cluster model utilized in the calculations. A 2 × 2 slab unit cell with four active SH* surface sites is highlighted.

technique,6,10 we focused on modeling the steady-state film growth along the cubic [111] growth direction according to the hydrogen-transfer half reactions (1) and (2). The active growth surface is hydrogenated S-terminated (111) zincblende CdS or (SH*)∞ slab. The half reactions were investigated by cluster models and optimization calculations to determine the ALD pathways (i.e., the reaction barriers required for the material deposition during ALD). Periodic optimization calculations were carried out to study the influence of coverage effects on the half reactions and to model the actual growth of the films which is not easily simulated within the cluster approach. Transition state optimizations within the periodic approach could not be performed due to limitations in the utilized calculation program, and hence the reported growth mechanism is described using the geometries from local optimizations. Fractional precursor surface coverages, i.e., deposited Cd to surface S* ratios or deposited S to surface Cd* ratios, of 0.25, 0.50, 0.75, and 1.00 were considered in the periodic approach, and the utilized computational models are illustrated in Figure 1. The calculations were performed using hybrid PBE016 and resolution of the identity second-order many-body perturbation theory (RI-MP2)17 methods, and the PBE0 functional was utilized due to its reported applicability for reactions involving late-transition-metals and because it is known to perform well in the solid state.18,19 RI-MP2 single-point energy calculations on the PBE0-optimized structures were performed to verify the DFT energetics. The hybrid PBE0 functional is constructed by a mixing of 25% Fock exchange with 75% of the PBE exchange, and the electron correlation is represented by the correlation part of the PBE density functional.20 The expression for the resulting exchange-correlation part of the functional can be PBE0 PBE PBE HF ) Exc + 1/4(EHF written as Exc x + Ex ), where Ex is the PBE the GGA exchange-correlation, Hartree-Fock exchange, Exc the exchange contribution. The RI approximation and EPBE x means expansion of products of virtual and occupied orbitals by expansions of auxiliary functions, which simplifies the calculation and results in computational savings of RI-MP2 as compared to standard MP2. Within the RI approach, transformation of four-center-two-electron integrals is replaced by that of three-center integrals, resulting in the computational speedup without significant loss in computational accuracy. Within the cluster approach, the PBE0 optimizations were performed with the Gaussian 09 program21 using LANL2TZ22 basis set for Cd and standard 6-31G** basis for the lighter atoms. This basis was feasible for the optimizations and provided results in agreement with periodic calculations (see below). The RI-MP2 calculations were carried out using TURBOMOLE program package23 (version 6.1) and the Karlsruhe triple-ζ valence basis set with polarization functions (def2-TZVP) as implemented in the program. The def2-TZVP basis was utilized because at least triple-ζ valence level basis is required for

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16619 converged results with MP2, whereas the use of larger basis sets significantly increases the computational cost. A hydrogen-terminated cluster cut from zincblende CdS and with a stoichiometry of Cd24S22H46 was utilized in the optimizations (see Figure 1), and the active adsorption site and its nearest neighbors were allowed to relax without symmetry constraints in the calculations. Larger cluster models were computationally impractical, whereas smaller models were not successful in the determination of the transition state (TS) structures, which were identified by frequency calculations and verified by intrinsic reaction coordinate (IRC) calculations. Calculations using periodic boundary conditions were carried out by CRYSTAL200624 quantum chemistry software. Karlsruhe split-valence basis set with polarization functions (def2-SVP)25 was used for C and H, and a standard 6-31G** basis was adopted for S. For Cd a triple valence zeta quality 9-7-6-311d631G basis26,27 with the outermost exponent variationally optimized for bulk CdS (0.1739 f 0.1797) was utilized. This basis was necessary for Cd because standard basis sets originally developed for molecular calculations contain diffuse functions, which are usually unnecessary in periodic calculations and lead to numerical difficulties and/or several degradations of performance.25 This basis reproduced the characteristics of bulk CdS well and as an example, the optimized lattice parameter of zincblende CdS was within 2% of the experimental value. A CdS slab model composed of 4 atomic layers was used to represent the hydrogenated sulfur-terminated (111) surface of zincblende CdS, and the outermost atomic layer of the slab was allowed to relax in the optimizations. The convergence of results using the 4 layer slab was verified by performing calculations for DMCd chemisorption (1 ML coverage) on 6 and 8 layer slabs. The relative reaction energies were practically identical regardless of the utilized slab model. Hydrogen termination of the lower Zn-terminated surface of the slab stabilized the surface Zn species by restricting them close to their positions in the corresponding bulk crystals, suggesting the surface dangling bonds to be well saturated. The number of the possible molecular arrangements on the (111) surface depends on the size of the slab unit cell (i.e., on the number of active sites on the surface), and a 2 × 2 cell was necessary for the investigation of the different precursor coverages during the DMCd exposure. All molecular arrangements possible using this cell were considered for each coverage, and the energetically favored coverages were included in the study. The same considerations apply for the H2S exposure, but here a few arrangements had to be calculated by using a large 4 × 2 cell. Default optimization convergence thresholds and an extra large integration grid were adopted in the calculations. Gibbs corrected relative reaction energies were calculated for the DMCd step to estimate the influence of thermodynamics on the half reactions, and infrared vibrational spectra were determined for energetically preferred growth surfaces. S-H and S-Cd bond strengths for surface SH and S-Cd-CH3 species were estimated by assuming that calculated reaction energies originate solely from bond energies associated with chemical bonds formed and broken during the reaction. Notably, analogous computational models with those described above have been previously successfully utilized in the simulation of ZnS ALD.26 The details of the utilized computational techniques are summarized in Table 1. Results and Discussion CdS ALD Reaction Pathways. The PBE0 and RI-MP2 calculated reaction pathways for the DMCd/H2S ALD process

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TABLE 1: Summary of the Computational Techniques Utilized in the Work model cluster periodic slab

calculation

program

method

basis set

structural optimization single-point energy structural optimization

Gaussian 09 TURBOMOLE (v6.1) CRYSTAL2006

PBE0 RI-MP2 PBE0

LANL2TZ/6-31G** def2-TZVP modifieda

a

def2-SVP was used for C and H, and a standard 6-31G** basis for S. For Cd a modified triple valence zeta quality 9-7-6-311-d631G basis was utilized. The modified basis set was necessary for the periodic calculations.

TABLE 2: PBE0/6-31G** and RI-MP2/def2-TZVP Calculated Energies for the ALD Half Reactions SH* + Cd(CH3)2 f S-Cd-CH3* + CH4 and S-Cd-CH3* + H2S f S-Cd-SH* + CH4 in kcal/mol with Respect to the Pristine Cd24S22H46 Clustera ∆ fH o DMCd/H2S ALD process pristine cluster

PBE0

RI-MP2

0.0

0.0

DMCd Pulse 1. adsorption state 2. transition state 3. adsorption state 4. chemisorbed DMCd

-3.0 7.3 -42.4 -43.5

-5.7 12.9 -40.2 -36.6

H2S Pulse 5. adsorption state 6. transition state 7. adsorption state 8. chemisorbed H2S overall ALD reaction

-49.9 -26.8 -71.2 -69.5

-44.5 -13.7 -64.0 -60.4

exp.b

-94

a

The labels 1-8 refer to specific points on the reaction pathways as illustrated in Figure 2. b Experimental reaction enthalpy (∆fHo) for the overall reaction H2S(g) + Cd(CH3)2(g) f CdS(s) + 2CH4 (g).28

Figure 2. (a) PBE0/6-31G** and RI-MP2/def2-TZVP calculated reaction pathways from local optimizations for the DMCd/H2S ALD process according to half reactions SH* + Cd(CH3)2 f S-Cd-CH3* + CH4 (1-4) and Cd-CH3* + H2S f Cd-SH* + CH4 (5-8). (b) The optimized structures for points 1-8. The molecular species reacting with the surface are designated by a “+” and the byproducts leaving the surface are marked “-”.

are illustrated in Figure 2, and the data is summarized in Table 2. A Cd24S22H46 cluster was utilized in the calculations to represent an active SH* adsorption site on the S-terminated (111) surface of zincblende CdS (Figure 1). The points from 1 to 4 on the pathway in Figure 2 refer to the first half reaction, which is the chemisorption of the DMCd precursor to the growth surface according to reaction (1) SH* + Cd(CH3)2 f S-Cd-CH3* + CH4. The calculations suggest that the reaction is initiated by the adsorption of DMCd to the growth surface with PBE0 and MP2-calculated stabilization energies of 3.0 and 5.7 kcal/mol, respectively. The barrier for the reaction to proceed is calculated to be moderate, 10.4 kcal/mol with PBE0 and 18.7 kcal/mol with MP2 as compared

to the adsorption state. The activation energy originates from the breaking of the Cd-C and S-H* bonds with bond strengths of 6427 and 37 kcal/mol (estimated value, see Computational Details section), respectively, and from the concomitant formation of C-H and S-Cd bonds with bond strengths of 10427 and 25 kcal/mol (estimated value). The transition vector is dominated by the motion of surface SH-hydrogen toward the carbon of the CH3 ligand, with the S-H distance in the transition state elongated by 0.24 Å with respect to the S-H distance in the pristine cluster. The overall reaction for the DMCd chemisorption is exoenergetic with respect to the pristine cluster by 43.5 and 36.6 kcal/mol at the PBE0 and MP2 levels of theory, respectively (point 4 on the pathway). The exoenergicity originates primarily from the formation of CH4, which leaves the surface via an adsorption state that is not stabilized with respect to the final product with PBE0, and slightly stabilized with MP2, as illustrated by points 3 and 4 on the pathway. The subsequent half reaction is the dissociative chemisorption of H2S, i.e., the reaction S-Cd-CH3* + H2S f S-Cd-SH* + CH4 (see points 5-8 in Figure 2), where the active surface site is the S-Cd-CH3* species formed during the previous DMCd pulse (vide supra). The reaction is initiated by coordination of H2S to the surface Cd through S with PBE0 and MP2 calculated stabilization energies of 6.4 and 7.9 kcal/ mol, respectively. The activation energy with respect to the precursor state for the reaction to proceed is high, 23.0 kcal/ mol with PBE0 and 30.7 kcal/mol with MP2, suggesting a slow reaction and a requirement for long pulse times at room temperature. However, the reaction is likely to occur faster at typical experimental deposition temperatures above 100 °C.6 The barrier, which is about twice the barrier of the DMCd half reaction, originates primarily from the dissociation of a strong

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TABLE 3: PBE0-Calculated Reaction Energies (∆Er) for the ALD Half Reactions (1) SH* + Cd(CH3)2 f S-Cd-CH3* + CH4 and (2) S-Cd-CH3* + H2S f S-Cd-SH* + CH4, Together with the Corresponding Gibbs-Corrected Energies (∆G298K , 1 atm) in kcal per mol r DMCd for Reaction 1 half reaction

precursor coverage

Growth Surface (SH)∞ (1) 0.25 (1) 0.50 (1) 0.75 (1) 1.00

ML ML ML ML

DMCd DMCd DMCd DMCd

∆Er -36.3 -43.9 -43.7 -41.8

0.50 ML Growth Surface Formed during DMCd Pulse (2) 0.25 ML H2S -32.9 (2) 0.50 ML H2S -37.0 (2) 0.75 ML H2S -40.2 (2) 1.00 ML H2S -41.6

∆Gr298K -44.1 -49.4 -43.9 -44.7

-34.1

0.75 ML Growth Surface Formed during DMCd Pulse (2) 0.33 ML H2S -36.9 (2) 0.66 ML H2S -40.4 (2) 1.00 ML H2S -44.7

S-H bond in H2S with a bond strength of 91 kcal/mol. For comparison, the bond strength of S-H bond in surface SH* species, being broken during the DMCd half reaction, is 37 kcal/ mol. The transition state complex for the H2S treatment is similar to the one determined for the DMCd reaction, and the transition vector is dominated by the motion of H atom in H2S toward the carbon of the chemisorbed S-Cd-CH3* species (see point 6 on the pathway). The reaction proceeds via an exoenergetic step with the formation of a surface S-Cd-SH* group and a CH4 molecule, which is bound to the surface by interaction energies of 1.7 and 3.6 kcal/mol at the PBE0 and MP2 levels of theory, respectively. The desorption of the adsorbed CH4 gives rise to overall reaction energies of 26.0 kcal/mol with PBE0 and 23.8 kcal/mol with MP2 for the H2S half reaction with respect to the system formed after the previous DMCd pulse. Overall, PBE0 and MP2 provided similar energetic trends for the half reactions suggesting that the simulation of ALD growth is possible at the DFT level of theory. The differences in the energetics originate from the tendency of MP2 to overbind physisorbed systems and to overestimate barrier heights, whereas DFT methods are known to underestimate activation energies and often fail to describe physisorbed systems accurately due to insufficient description of dispersion interactions.29,30 As an example, the adsorption state 1 is stabilized by about 2 kcal/mol more with MP2 as compared to PBE0, whereas the barrier for the DMCd half reaction (point 2) is calculated to be about 8 kcal/mol higher at MP2 level. The calculated pathways suggest that the DMCd/H2S process is highly exoenergetic, agreeing with the experimental enthalpy of formation for CdS from DMCd and H2S (see Table 2). The metal half reaction has a moderate activation barrier, whereas the barrier for the H2S step is calculated to be noticeably higher, especially at MP2 level. Notably, similar findings have been reported for ZnS ALD from analogous DMZn and H2S precursors.26 Influence of Precursor Surface Coverage on CdS ALD Growth. The active sites on the (SH*)∞ slab for the DMCd chemisorption according to reaction 1 are SH* surface species (see Figure 1), and a 2 × 2 slab model with four active sites was utilized to model the fractional precursor surface coverages of 0.25, 0.50, 0.75, and 1.0. The PBE0-calculated relative reaction energies for the half reactions (1) and (2) are sum-

Figure 3. (a) PBE0-calculated relative energies from local optimizations for DMCd dissociative chemisorption on hydrogenated Sterminated zincblende CdS (111) surface according to ALD half reaction SH* + Cd(CH3)2 f S-Cd-CH3* + CH4 as a function of precursor surface coverage. (b) Top and (c) side views of the growth surfaces with deposited Cd to surface S* ratios of 0.25, 0.50, 0.75, and 1.00.

Figure 4. (a) PBE0-calculated relative energies from local optimizations for H2S dissociative chemisorption on a partially (0.50 ML) methyl-covered zincblende CdS (111) surface according to reaction S-Cd-CH3* + H2S f S-Cd-SH* + CH4 as a function of precursor surface coverage. (b) Top and (c) side views of the growth surfaces with deposited S to surface Cd* ratios of 0.25, 0.50, 0.75, and 1.00.

marized in Table 3 and illustrated in Figures 3 and 4. The relative energies have been obtained by partial optimization calculations, and the energies are given per chemisorbed precursor molecule to enable comparison between different precursor fractional coverages, and Gibbs corrected relative energies are reported for the DMCd step to investigate the influence of thermodynamics on the relative energies. In agreement with the cluster calculations, DMCd deposition to the (SH*)∞ surface is exoenergetic by about 36-44 kcal per mol DMCd, and the reaction energies are influenced by the precursor coverage on the surface as illustrated in Figure 3. The relative energies demonstrate that the chemisorption of DMCd on the growth surface is increasingly favorable up to the surface with a deposited Cd to surface S* ratio of 0.50, which is a half

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monolayer (ML), and the relative energy at 0.75 ML coverage is nearly equal to that at 0.50 ML. The full ML is unfavored by about 2 kcal/mol per DMCd with respect to the 0.50 ML system, and the preference for the 0.50 ML coverage was suggested also by Gibbs corrected relative energies. The reported energetic trends can be understood by both electronic and steric factors which arise from attractive interactions between the chemisorbed Cd atoms and the neighboring surface S atoms, and from repulsion between the CH3 ligands of the chemisorbed Cd species. The beneficial electronic interaction between deposited Cd and neighboring S can be separated into orbital and electrostatic interactions, the latter originating from the negative partial charge of surface S and from the positive partial charge of Cd. At 0.25 ML coverage the beneficial electronic effect is disrupted by partially positive H atoms of the SH* surface groups surrounding the chemisorbed DMCd species (see Figure 3b), whereas the steric repulsion between CH3 groups is insignificant. At 0.50 ML coverage chemisorbed DMCd molecules occupy adjacent S sites on the surface in a row-like fashion which enables the electronic attractions between deposited Cd and neighboring surface S atoms that are not bound to H atoms. This applies also for the higher coverages, and the electronic interactions lead to strong tilting of the chemisorbed precursors at 0.50 ML and higher fractional coverages (see Figure 3b,c). The steric ligand repulsion increases with coverage and as a consequence the 1.0 ML coverage is unfavored in relative energy with respect to the 0.50 and 0.75 ML systems. The preference for the row arrangement at 0.50 ML coverage was verified by performing an optimization using a 4 × 2 slab unit cell on the corresponding zigzag arrangement, which was found to be unfavorable by 2.5 kcal per mol DMCd compared to the row system due to increased steric repulsion between the ligands. Overall, the reported energetic trends for the DMCd pulse suggest that steric effects become significant at coverages above approximately 0.75 ML, and the access of the DMCd molecules on the surface to form the full ML is expected to be prevented by a significant energy barrier. While our calculations provide evidence for the submonolayer coverage, future work in the form of periodic reaction pathway calculations is required to distinguish the most likely coverage to be formed during the DMCd exposure. The half ML coverage system, which is the most favorable in terms of relative energy of the considered coverages (see Figure 3), was chosen as the growth surface for the subsequent H2S treatment according to reaction (2). The precursor surface coverage for this step is defined as the deposited S to previously deposited Cd* ratio, and the half reaction process was investigated using 2 × 2 and 4 × 2 slab models. The active sites on the growth surface are S-Cd-CH3* species that occupy half of the surface sites in a row-like fashion (see Figure 3), while the remaining SH* sites of the initial substrate are inactive toward the H2S exposure. For comparison, H2S treatment of the 0.75 ML coverage system, which is slightly unfavored in relative energy with respect to the 0.50 ML system, of the previous DMCd exposure was investigated by the same computational approach using 2 × 2 slab model and by considering 0.33, 0.66, and 1.0 ML surface coverages. As suggested by cluster calculations, the H2S chemisorption is exoenergetic and the relative energies, which range from about 33 to 42 kcal per mol H2S, are influenced by the deposited S to surface Cd* ratio as illustrated in Figure 4. The dissociative chemisorption of H2S on the 0.50 ML growth surface results in the removal of the tilting of the surface Cd atoms, which adapt

Tanskanen et al.

Figure 5. Experimental DMCd/H2S ALD growth rates in ML/cycle as a function of deposition temperature (data from ref 6).

to orientations close to the ones expected for the cubic [111] direction. The relative energies converge toward the full replacement of the surface CH3 species by SH groups, suggesting the preference for the full removal of the surface alkyl species during the H2S treatment. This finding originates from the formation of [-Cd-SH-]n “chains” on the growth surface with the energetically favored bulk-like ring structure (see Figure 4b,c). Also, the full removal of the surface CH3 species during the H2S pulse is energetically favored for the 0.75 ML growth surface investigated for comparison (see Table 3). The H2S chemisorption on the 0.75 ML DMCd surface is calculated to be slightly more exoenergetic than on the 0.50 ML surface due to sulfur’s ability to coordinate to more than two Cd atoms on this surface. Notably, similar ALD growth characteristics have been identified for ZnS by quantum chemical calculations, and this can be understood by the similar structural and electronic characteristics of the ALD grown CdS and ZnS films. Namely, both films are group II-VI semiconductors with a preferred cubic [111] growth direction at low deposition temperatures, wurtzite phase becoming dominant at higher temperatures.6,31 It should be noted that the reported growth behavior is described using the geometries from local optimizations and no reaction path has been determined due to computational limitations (see the Computational Details section for further explanation). Overall, our periodic calculations provide evidence for 0.50-0.75 ML growth during CdS ALD and hence suggest submonolayer growth during DMCd pulse, whereas the subsequent H2S treatment results in full replacement of the previously deposited alkyl groups from the growth surface. However, based on the experimental observation that crystalline films are deposited by this ALD process, we expect that subsequent ALD cycles will result in the deposition of Cd atoms at the initially unreacted SH* sites. The submonolayer growth per ALD cycle is also experimentally observed, and the experimental growth rates are illustrated in Figure 5 for comparison. Notably, 1 ML/ cycle growth rates have also been reported for CdS ALD in UHV environment and on ZnSe substrates, but those rates reflect ALD growth directly at the substrate, i.e. during the first few ALD cycles.12 Spectral Features. In situ IR experiments provide a possible way for the investigation of ALD growth mechanisms, but such studies have not yet been carried out. To provide a point of comparison for future experiments, we determined vibrational features within the harmonic oscillator approach for the initial substrate, i.e., the (SH*)∞ slab, for the 0.50 ML surface formed due to the DMCd exposure, and for the alkyl-free surface formed due to the H2S treatment. The main features of the calculated spectra, comprising SH and CH stretching features, are summarized in Table 4.

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TABLE 4: PBE0-Calculated Infrared C-H and S-H Stretching Vibrations of the (SH*)∞ Growth Surface, the 0.50 ML Surface Formed Due to the DMCd Pulse, and the Surface Formed Due to the H2S Exposure growth surface wavenumber (cm-1) (SH*)∞ DMCd pulsea H2S pulseb

2660 2620 3080 - 3180 2510 2670

assignment SH of surface SH* SH of unreacted surface SH* CH of surface Cd-CH3* SH of unreacted SH* SH of chain SH*

a

Calculated for energetically favored 0.50 ML coverage surface (see Figure 3 and Table 3). b Calculated for energetically favored 1.0 ML coverage surface (see Figure 4 and Table 3).

The spectrum of the initial growth surface shows a strong S-H stretching feature around 2660 cm-1, the corresponding bending modes and CdS-related modes appearing at wavenumbers below about 500 cm-1. The DMCd exposure and the concomitant formation of surface Cd-CH3 groups introduce strong C-H stretching modes above 3080 cm-1 and weaker C-H bending modes to 1100-1500 cm-1 region. In addition, the stretching SH features of the unreacted SH* species are shifted slightly toward lower wavenumbers. The formation of the previously described CdS chain surface structure due to the H2S pulse results in two strong SH stretching modes around 2500-2700 and cm-1, the features at lower and higher wavenumbers corresponding to the initial SH* adsorption sites and the chain SH sites, respectively (see Figure 4). Overall, the experimental observation of both SH and CH stretching modes after the DMCd pulse, and of more than one SH mode after the H2S treatment would provide support for the submonolayer growth during one DMCd/H2S cycle. Conclusions The ALD growth characteristics of CdS films deposited from DMCd and H2S precursors have been studied by hybrid DFT and ab initio MP2 calculations with a focus on simulating material deposition during the steady-state ALD growth according to the half reactions (1) and (2). Cluster models were utilized to determine reaction pathways for the film growth, whereas periodic optimization calculations were performed to investigate coverage effects on the half reactions and to understand the growth mechanisms of the films. Infrared vibrational features were determined at the PBE0 level of theory for relevant growth surfaces to enable comparison of the proposed mechanistic details with experiments. PBE0 method and MP2 methods provided similar trends for the half reactions, and the energetics were in agreement with experimental reaction enthalpies for CdS growth. The reaction barrier for the DMCd reaction was moderate, whereas significant activation energy of over 20 kcal/mol is required for the H2S chemisorption to the growth surface according to the second half reaction. However, at typical experimental deposition temperatures above 100 °C the H2S reactant molecules are likely to have sufficient energy to overcome this barrier. Periodic calculations provided evidence for submonolayer growth during a DMCd/H2S cycle by suggesting that not all SH adsorption sites are active during the DMCd pulse, whereas all Cd-CH3 surface sites are likely to react during the subsequent H2S exposure. Analysis of the infrared features calculated for the relevant growth surfaces demonstrated that the mechanistic details of the proposed growth characteristics could be confirmed by in situ IR experiments.

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