DSpace at VNU: A computational study on the adsorption configurations and reactions of SiHx(x=1-4) on clean and H-covered Si(100) surfaces

Accepted Manuscript Title: A Computational Study on the Adsorption Configurations and Reactions of SiHx (x = 1-4) on Clean and H-covered Si(100) Surfaces Author: Thong N-M Le P Raghunath L.K Huynh M.C Lin PII: DOI: Reference: S0169-4332(16)31328-9 http://dx.doi.org/doi:10.1016/j.apsusc.2016.06.099 APSUSC 33469 To appear in: APSUSC Received date: Revised date: Accepted date: 22-3-2016 25-5-2016 17-6-2016 Please cite this article as: Thong N-M Le, P.Raghunath, L.K.Huynh, M.C.Lin, A Computational Study on the Adsorption Configurations and Reactions of SiHx(x = 1-4) on Clean and H-covered Si(100) Surfaces, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.06.099 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain A Computational Study on the Adsorption Configurations and Reactions of SiHx(x=1-4) on Clean and H-covered Si(100) Surfaces Thong N-M Le1, P Raghunath2, L K Huynh3* and M C Lin2* Molecular Science and Nano-Materials Laboratory, Institute for Computational Science and Technology, Quang Trung Software Park, Dist 12, Ho Chi Minh City, Vietnam Center for Interdisciplinary Molecular Science, Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan International University, VNU-HCMC, Quarter 6, Linh Trung, Thu Duc District, Ho Chi Minh City, Vietnam Graphical Abstract Highlight Systematically provides the optimized adsorption configurations of all adsorbates on Si(100) surface The mechanisms leading to the formation of silicon adatoms on the surface are proposed The barriers for hydrogen abstractions from the surface are negligible comparing to the barriers for the hydrogen migrations The barriers for hydrogen abstractions from the adsorbed speices are negligible comparing to the barriers for the decompositions Abstract Possible adsorption configurations of H and SiHx (x=1-4) on clean and H-covered Si(100) surfaces are determined by using spin-polarized DFT calculations The results show that, on the clean surface, the gasphase hydrogen atom and SiH3 radicals effectively adsorb on the top sites, while SiH and SiH2 prefer the bridge sites of the first layer Another possibility for SiH is to reside on the hollow sites with a triple-bond configuration For a partially H-coverd Si(100) surface, the mechanism is similar but with higher adsorption energies in most cases This suggests that the surface species become more stable in the presence of surface hydrogens The minimum energy paths for the adsorption/migration and reactions of H/SiHx species on the surfaces are explored using the climbing image-nudged elastic band method The competitive surface processes for Si thin-film formation from SiHx precursors are also predicted The study reveals that the migration of hydrogen adatom is unimportant with respect to leaving open surface sites because of its high barriers (˃ 29.0 kcal/mol) Alternatively, the abstraction of hydrogen adatoms by H/SiHx radicals is more favorable Moreover, the removal of hydrogen atoms from adsorbed SiHx, an essential step for forming Si layers, is dominated by abstraction rather than the decomposition processes Keywords: silane precursor, silicon surfaces, Si(100), hydrogen abstractions, plasma enhanced Introduction Silicon has played an important role in the fabrication of integrated circuits [1] The advances made from traditional silicon wafers to silicon thin films make it feasible for a wider range of applications related to future microelectronic [2] and photovoltaic [3] devices Plasma-enhanced chemical vapor deposition (PECVD) [4] is a commonly used technique to grow amorphous [5-8] or crystalline [2, 8-10] silicon thin films from silane precursors It has been reported that this technique significantly increased the growth rate of silicon against thermal depositions (CVD) [6] under identical conditions, although these processes were controlled by similar surface kinetics [9] In plasma-enhanced depositions, the reactive radicals are amplified by collisions of energetic electrons with the precursor molecule such as SiH4 Co-existing in the gas phase, besides SiH4 (silane), SiH2 (silylene) and silylidyne (SiH), hydrogen (H) and SiH3 (silyl) are known as the most abundant radicals in this regime [5, 7] Fast growth rates are continuously achieved by the adsorption and reactions of the generated radicals with the silicon substrate surface The deposition techniques have evolved in practical applications with large-scale production, but the key process of silicon thin-film growth has not been fully understood at the molecular level, although the reactions of SiHx on silicon surfaces have been extensively studied both experimentally [8, 10-12] and computationally [13-21] Based on experimental studies, Gate and co-workers [11, 12] proposed the growth mechanism of silicon which became the preliminary source for a large number of later works In these studies, a steady-state surface kinetic model was also developed to estimate the silicon growth rate The model provided a rather good description on both mechanism and kinetics for CVD processes Gate’s mechanism not only contributed to the understanding of CVD but also PECVD because of their similarities in surface kinetic occurrences, although PECVD is more complex than CVD in the aspect of the plasma chemistry producing reactive species for surface reactions to occur However, the drawback of this model was to assume the overall growth rate to be equivalent to the dissociative adsorption of SiH4 [12] Srinivasan [10] showed that hydrogen abstractions and hydrogen etching via Eley-Rideal (ER) [22] mechanism were important pathways for crystalline silicon formation at temperatures between 25 and 300 o C In another experiment, Srinivasan [8] found that there was a transition from amorphous silicon to crystalline silicon by hydrogen abstractions at temperatures lower than 250 oC Obviously, these experiments concentrated on abstraction processes for activating adsorption sites on the surface, which was absolutely necessary in the plasma-enhanced deposition On the other hand, computational studies can critically cover various approaches from classical to ab initio calculations conducted on both cluster and slab models These calculations, moreover, can extend over a wide range of surface processes occurring throughout realistic deposition conditions involving abstractions of surface hydrogens, adsorptions of gas-phase radicals, diffusions of surface radicals, decompositions of adsorbed species, and abstractions of hydrogens from adsorbed radical species Ramalingam et al [13] showed that there was no energetic difference for hydrogen abstraction by SiH3 from the crystalline or amorphous Si(100)-(2x1) surface However, the mobility of the radicals was higher for the amorphous than the crystalline surface In another work, Ramalingam and co-workers [14] mentioned that the “valley-filling mechanism” accounts for the surface roughness, i.e., the mobile precursors such as SiH3 diffuse and react with dangling bonds on the valleys Cereda [15] estimated the probability of 60% for the barrierless abstraction of surface hydrogens by silyl radicals via the ER mechanism Bakos and coworkers [16] proposed three pathways for H abstraction by SiH3, i.e., ER, Langmuir-Hinshelwood (LH) [22] and precursor-mediated (PM) [22], where ER was barrierless, while the barriers for LH and PM models were 17.5-18.0 and 9.0 kcal/mol, respectively The calculation results from Kang and Musgrave [17] showed that the barrier for surface hydrogen abstractions was less than 1.0 kcal/mol As can be seen from the above summary, SiH3 radicals can diffuse on the surface and then abstract the surface hydrogens via Eley-Rideal mechanism without a significant barrier When the dangling bonds are available, further steps are the adsorptions and reactions of radicals on the surface The dissociative adsorption of silane was also calculated by Kang and his coworker [17] with a favorable barrier of 7.4 kcal/mol This barrier was also found to be 12-14 kcal/mol by Brown and coworkers [18, 19] Smardon [20] confirmed that silanes adsorbed dissociatively on Si(100)-(2x2); the fragments could be either on the same dimer or on adjacent dimers with the energy difference of 4.2 kcal/mol Kang and his coworker [17] determined that the barriers for hydrogen removals from adsorbed SiH3, along with transforming to SiH2 bridging structure, were 5.7 and 32.9 kcal/mol with and without H(g), respectively The full decomposition of SiH3 radicals to Si and H adatoms was also mentioned in Ceriotti’s calculations [21], which are in good agreement with the experimental data from Gate and coworkers [11] Overall, the deposition process extensively incorporates competitive surface processes including diffusions, abstractions and decompositions These surface processes should be taken into consideration for constructing a full thin-film growth mechanism It has been known that such a growth mechanism remains unclear Moreover, the kinetics for most of these processes is not fully characterized experimentally or computationally The purpose of the study is to investigate the mechanisms of the adsorption and decomposition mechanisms of SiHx(x=1-4) radicals on the Si(100) surface leading to the thin-film growth using the density functional theory (DFT) The scope of this study includes: (1) hydrogen migrations on the clean surface, (2) hydrogen abstractions from the surface by either hydrogen atoms or silyl radicals, (3) decompositions of surface silicon-hydride species, and (4) hydrogen abstractions from the surface siliconhydride species These surface reactions involve both ER and LH mechanisms that contribute to the thinfilm growth by producing of surface dangling bonds and dehydrogenation of adsorbed species The microscopic understanding on the growth mechanism is essential to establish the kinetics as well as to develop a realistic simulation model which may help predict the evolution of each surface species under practical conditions When validated, the mechanism may provide a greater opportunity to effectively optimize the production of silicon thin films in industrial scales Computational methods All calculations have been carried out using the Vienna Ab initio Simulation Package (VASP) [23-26] based on periodic density functional theory (DFT) The frozen ionic cores are described by the projector augmented wave (PAW) method [27], and the Kohn-Sham valence states are expanded in the plane wave basis sets up to 380 eV The exchange-correlation energy is described by the generalized gradient approximation with the Perdew-Burke-Ernzerhof (PBE) functional [28-30] The p(2×2) cell of the Si(100) surface was modeled as periodically repeated slabs with six atomic layers and four Si atoms on each layer For the surface calculations, slabs were separated by a vacuum spacing greater than 17 Å in the direction perpendicular to the surface, which guarantees no interaction between the slabs The three top layers were allowed to relax for all geometry optimizations, while the three bottom layers were fixed at bulk positions with the experimental bulk lattice constant (5.43 Å) [31] The surface Brillouin zone was sampled with the Monkhorst-Pack scheme [32] using a (6×6×1) k-point mesh converging with respect to the electronic energy The ionic relaxation was stopped until the forces on all free atoms were less than 0.02 eV/Å The Gaussian smearing method [26, 33] with a smearing parameter of 0.01 eV was applied The total energy of all gas-phase species were calculated in a box with dimensions of 15Å on each side, large enough to ensure negligible interactions between neighboring cells Spin-polarized calculations were perforface hydrogens plays a critical role in promoting the growth rate of thin films by providing reactive surface dangling bonds [8, 10, 15] The reaction may occur through Eley-Rideal mechanism by which the reactive gas-phase radicals abstract adsorbed hydrogens from the surface and create adsorption sites with dangling bonds In our present study, H and SiH3 were found to successfully abstract the surface hydrogens to release H2 and SiH4 gases, respectively, without well-defined in transition barriers.The SiH radical was found to form an unstable complex on the surface, while the SiH2 was found to directly insert into a surface Si-H bond to form adsorbed SiH3 The H and SiH3 abstraction 18 reactions are described as (1) H(g) + H(a) → H2(g) + db, and (2) SiH3(g) + H(a) → SiH4(g) + db, respectively, where db means a dangling bond The plots of instrinsic reaction coordinates along the reaction path of these abstractions are showed in Figure and Figure As can be seen from the reaction path figures, there are zero barriers for the hydrogen abstractions by H and SiH3 regardless of the coverages of surface hydrogens This findings is in good agreement with the conclusions from Cereda et al [15], Bakos and coworkers [16], while Kang’s results [17] showed that the barrier was less than 1.0 kcal/mol for silane formation 19 (a) (b) Figure 13 Energy path comparision between decomposition and H abstraction from surface SiH involving the two-most stable configurations, (a) intra-dimer bridging and (b) inter-dimer bridging The solid lines with +H(g) symbol represents the abstraction channels, one H atom from the gas phase eliminates one H from the surface SiH to produce H2(g) The dash lines with TS are the decomposition channels For both mechanisms in (a) and (b), the zero-energy levels are chosen to be the initial states on each reaction channel For (a), they are HSi_b1(a) and HSi_b1(a) + H(g) for the decomposition and abstraction channel, respectively It is similar in (b) Conclusions In summary, the DFT method has been employed to investigate the surface processes for the silicon thin-film growth The most stable adsorption configurations are provided from which surface reactions may favorably occur The barrier for the dissociative chemisorption of silane is estimated to be 5.4 kcal/mol, which is comparable to the value of 7.4 kcal/mol from Kang and Musgrave [17] The SiH3 radicals prefer the top adsorption sites, from there sequentially decomposition reactions losing its hydrogens first giving rise to bridged SiH2 and then SiH, following two distinctive reaction paths with the energy barrier difference by at least kcal/mol Although the gas-phase SiH2 radicals favorably adsorb on bridge sites with the same fashion as the SiH2 decomposing from SiH3, further decomposition reactions of adsorbed SiH2 are energetically and kinetically similar for both channels Hydrogen migrations were found to be insignificant factors in providing surface dangling bonds because of their rather high barriers, ranging from 29.0 to 45.0 kcal/mol In contrast, abstractions of surface hydrogens were found to occur without energy barriers; they are more favorable in providing adsorption sites, in accordance with the previous conclusions [15-17] Additionally, the barrierless abstractions from adspecies by H atoms and 29 SiHx radicals, comparing to the barrier energies ranging from to 31 kcal/mol for their decomposition reactions, suggest that it is kinetically more favorable for hydrogen abstractions from adsorbed species for the growth of Si-Si bonds under the PECVD conditions Acknowledgements This research was funded by the Department of Science and Technology - Ho Chi Minh City, Vietnam to the Institute for Computational Science and Technology (ICST) at Ho Chi Minh City (contract no 172/2015/HĐ-SKHCN) and the Ministry of Science and Technology to the Center for Interdisciplinary Molecular Science, Department of Applied Chemistry, National Chiao Tung University P.R gratefully acknowledges financial support for this study from the Ministry of Science and Technology, Taiwan under contract MOST 103-2113-M-009-011-MY2 The authors are grateful to the National Center for High-performance Computing, Hsinchu Science Park, Hsinchu City, Taiwan for computing resources This work has also been benefited in the computer time and facility from the ICST and International University, VNU-HCMC 30 References [1] G.E Moore, The role of Fairchild in silicon technology in the early days of "Silicon Valley", Proceedings of the IEEE, 86 (1998) 53-62 [2] N Yamauchi, R Reif, Polycrystalline silicon thin films processed with silicon ion implantation and subsequent solid‐phase crystallization: Theory, experiments, and thin-film transistor applications, Journal of Applied Physics, 75 (1994) 3235-3257 [3] A Shah, P Torres, R Tscharner, N Wyrsch, H Keppner, Photovoltaic Technology: The Case for Thin-Film Solar Cells, Science, 285 (1999) 692-698 [4] T.J Donahue, R Reif, Silicon epitaxy at 650-800  °C using low‐pressure chemical vapor deposition both with and without plasma enhancement, Journal of Applied Physics, 57 (1985) 2757-2765 [5] M Moravej, S.E Babayan, GRNowling, X Yang, R.F Hicks, Plasma enhanced chemical vapour deposition of hydrogenated amorphous silicon at atmospheric pressure, Plasma Sources Sci Technol., 13 (2004) 8-14 [6] H Wiesmann, A.K Ghosh, T McMahon, M Strongin, a‐Si  :  H produced by high‐temperature thermal decomposition of silane, Journal of Applied Physics, 50 (1979) 3752-3754 [7] A Gallagher, Neutral radical deposition from silane discharges, Journal of Applied Physics, 63 (1988) 2406-2413 [8] E Srinivasan, G.N Parsons, Hydrogen elimination and phase transitions in pulsed-gas plasma deposition of amorphous and microcrystalline silicon, Journal of Applied Physics, 81 (1997) 28472855 [9] J.H Comfort, R Reif, Plasma‐enhanced deposition of high‐quality epitaxial silicon at low temperatures, Applied Physics Letters, 51 (1987) 2016-2018 [10] E Srinivasan, G.N Parsons, Hydrogen abstraction kinetics and crystallization in low temperature plasma deposition of silicon, Applied Physics Letters, 72 (1998) 456-458 [11] S.M Gates, C.M Greenlief, D.B Beach, Decomposition mechanisms of SiHx species on Si(100)‐ (2×1) for x=2, 3, and 4, The Journal of Chemical Physics, 93 (1990) 7493-7503 [12] S.M Gates, S.K Kulkarni, Kinetics of surface reactions in very low‐pressure chemical vapor deposition of Si from SiH4, Applied Physics Letters, 58 (1991) 2963-2965 [13] S Ramalingam, D Maroudas, E.S Aydil, Atomistic simulation study of the interactions of SiH3 radicals with silicon surfaces, Journal of Applied Physics, 86 (1999) 2872-2888 [14] S Ramalingam, S Sriraman, E.S Aydil, D Maroudas, Evolution of structure, morphology, and reactivity of hydrogenated amorphous silicon film surfaces grown by molecular-dynamics simulation, Applied Physics Letters, 78 (2001) 2685-2687 [15] S Cereda, M Ceriotti, F Montalenti, M Bernasconi, L Miglio, Quantitative estimate of H abstraction by thermal SiH3 on hydrogenated Si(001)(2×1), Physical Review B, 75 (2007) 235311-1 – 235311-8 [16] T Bakos, M.S Valipa, D Maroudas, Thermally activated mechanisms of hydrogen abstraction by growth precursors during plasma deposition of silicon thin films, The Journal of Chemical Physics, 122 (2005) 054703-1 – 054703-9 [17] J Ku Kang, C.B Musgrave, Theoretical study of the chemical vapor deposition of (100) silicon from silane, Physical Review B, 64 (2001) 245330-1 – 245330-11 [18] A.R Brown, D.J Doren, Dissociative adsorption of silane on the Si(100)-(2×1) surface, The Journal of Chemical Physics, 110 (1999) 2643-2651 [19] A Robinson Brown, D.J Doren, Energetics of silicon hydrides on the Si(100)-(2×1) surface, The Journal of Chemical Physics, 109 (1998) 2442-2449 [20] R.D Smardon, G.P Srivastava, Ab initio surface reaction energetics of SiH4 and Si2H6 on Si(001)(2×2), The Journal of Chemical Physics, 123 (2005) 174703-1 – 104703-8 31 [21] M Ceriotti, S Cereda, F Montalenti, L Miglio, M Bernasconi, Ab initiostudy of the diffusion and decomposition pathways of SiHx species on Si(100), Physical Review B, 79 (2009) 165437-1 – 165437-13 [22] R.I Masel, Introduction to surface reactions, in: Principles of adsorption and reaction on solid surfaces, Wiley, New York, 1996, pp 445 [23] G Kresse, J Hafner, Ab initio molecular dynamics for liquid metals, Physical Review B, 47 (1993) 558-561 [24] G Kresse, J Hafner, Ab initio molecular-dynamics simulation of the liquid-metal–amorphoussemiconductor transition in germanium, Physical Review B, 49 (1994) 14251-14269 [25] G Kresse, J Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Physical Review B, 54 (1996) 11169-11186 [26] G Kresse, J Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Computational Materials Science, (1996) 15-50 [27] P.E Blöchl, Projector augmented-wave method, Physical Review B, 50 (1994) 17953-17979 [28] J.P Perdew, J.A Chevary, S.H Vosko, K.A Jackson, M.R Pederson, D.J Singh, C Fiolhais, Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation, Physical Review B, 46 (1992) 6671-6687 [29] J.P Perdew, K Burke, M Ernzerhof, Generalized Gradient Approximation Made Simple, Physical Review Letters, 77 (1996) 3865-3868 [30] J.P Perdew, K Burke, M Ernzerhof, Perdew, Burke, and Ernzerhof Reply, Physical Review Letters, 80 (1998) 891-891 [31] Y Okada, Y Tokumaru, Precise determination of lattice parameter and thermal expansion coefficient of silicon between 300 and 1500 K, Journal of Applied Physics, 56 (1984) 314-320 [32] H.J Monkhorst, J.D Pack, Special points for Brillouin-zone integrations, Physical Review B, 13 (1976) 5188-5192 [33] C.L Fu, K.M Ho, First-principles calculation of the equilibrium ground-state properties of transition metals: Applications to Nb and Mo, Physical Review B, 28 (1983) 5480-5486 [34] G Henkelman, B.P Uberuaga, H Jónsson, A climbing image nudged elastic band method for finding saddle points and minimum energy paths, The Journal of Chemical Physics, 113 (2000) 9901-9904 [35] K.P Huber, G Herzberg, Molecular spectra and molecular structure IV Constants of diatomic molecules, Van Nostrand Reinhold Co, 1979, 1-716 [36] K.K Irikura, Experimental vibrational zero-point energies: diatomic molecules, Journal of Physical and Chemical Reference Data, 36 (2007) 389 [37] G.R Gupte, R Prasad, Ground state geometries and vibrational spectra of small hydrogenated silicon clusters using nonorthogonal tight-binding molecular dynamics, International Journal of Modern Physics B, 12 (1998) 1607-1622 [38] M.E Jacox, Vibrational and electronic energy levels of polyatomic transient molecules, J Phys Chem.Ref Data, Monograph (1994), 1-461 [39] C Yamada, E Hirota, Detection of the silyl radical SiH3 by infrared diode-laser spectroscopy, Phys Rev Lett, 56 (1986) 923-925 [40] D.R.J Boyd, Infrared spectrum of trideuterosilane and the structure of the silane molecule, The Journal of Chemical Physics, 23 (1955) 922 [41] T Shimanouchi, Tables of Molecular vibrational frequencies consolidated volume I, National Bureau of Standards (1972) 1-160 [42] P Raghunath, Y.-M Lee, S.-Y Wu, J.-S Wu, M.-C Lin, Ab initio chemical kinetics for reactions of H atoms with SiHx (x=1-3) radicals and related unimolecular decomposition processes, International Journal of Quantum Chemistry, 113 (2013) 1735-1746 [43] N Roberts, R.J Needs, Total energy calculations of dimer reconstructions on the silicon (001) surface, Surface Science, 236 (1990) 112-121 32 [44] S Tang, A.J Freeman, B Delley, Structure of the Si(100)2×1 surface: Total-energy and force analysis of the dimer models, Physical Review B, 45 (1992) 1776-1783 [45] R.M Tromp, R.G Smeenk, F.W Saris, D.J Chadi, Ion beam crystallography of silicon surfaces II Si(100)-(2×1), Surface Science, 133 (1983) 137-158 [46] H.-T Chen, C.-C Hwang, H.-J Chiang, J.-G Chang, Adsorption and reaction of Si2H5 on clean and H-Covered Si(100)-(2×1) surfaces: A computational study, The Journal of Physical Chemistry C, 115 (2011) 15369-15374 [47] F.C.H Lim, E.S Tok, H.C Kang, Decomposition of SiH3 to SiH2 on Si(100)-(2×1), Physical Review B, 74 (2006) 205333-1 – 205333-7 [48] E Srinivasan, H Yang, G.N Parsons, Abinitio calculation of hydrogen abstraction energetics from silicon hydrides, The Journal of Chemical Physics, 105 (1996) 5467-5471 33 .. .A Computational Study on the Adsorption Configurations and Reactions of SiHx(x=1-4) on Clean and H-covered Si(100) Surfaces Thong N-M Le1, P Raghunath2, L K Huynh3* and M C Lin2* Molecular... steps are the adsorptions and reactions of radicals on the surface The dissociative adsorption of silane was also calculated by Kang and his coworker [17] with a favorable barrier of 7.4 kcal/mol... Graphical Abstract Highlight Systematically provides the optimized adsorption configurations of all adsorbates on Si(100) surface The mechanisms leading to the formation of silicon adatoms on the