Application of Density Functional Theory and Photoelectron Spectra to

ARTICLE pubs.acs.org/JPCC

Application of Density Functional Theory and Photoelectron Spectra to the Adsorption and Reaction of H2S on Si (100) Tsung-Fan Teng,†,‡ Chun-Yi Chou,† Wei-Hsiu Hung,*,‡ and Jyh-Chiang Jiang*,† † ‡

Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan Department of Chemistry, National Taiwan Normal University, Taipei 116, Taiwan

bS Supporting Information ABSTRACT: The adsorption and reaction of H2S on a Si(100) surface were investigated using density functional theory (DFT) and X-ray photoelectron spectra (XPS). The decomposition of H2S results in the formation of S adatoms via a HS species. We propose four reaction paths for the decomposition of adsorbed H2S; the corresponding structural conformations of H2S, HS, and S species are presented. The density of states and electron density difference were utilized to illustrate the interaction between S-containing species and surface Si atoms. The interaction of the surface Si atom and the H atom of H2S facilitates the decomposition of adsorbed H2S. The assignments of XPS data are correlated with the proposed intermediates during the thermal decomposition of H2S.

’ INTRODUCTION The investigation of sulfur deposited on silicon surfaces has fundamental importance in chemistry and physics, in particular, in contexts of doping, passivation, and heteroepitaxial growth of group II sulfides.1
5 A S adlayer becomes deposited on a semiconductor surface through its immersion in a solution of (NH4)2S6,7 or on its exposure to gaseous elemental sulfur or H2S.8,9 As a gaseous source of sulfur, H2S undergoes thermal decomposition on a Si surface to produce surface Si hydrides and sulfides.10 The resulting S layer formed from adsorption of H2S shows properties of passivation superior to those from adsorption of elemental S.11 The bonding conformation and the composition upon adsorption of H2S on Si greatly affect the subsequent chemical reaction of a film growth and fabrication and influence properties of thin films and the resulting interfaces. An improved understanding of the adsorption and decomposition of H2S on a Si surface can thus provide insight into the limitations and merits of H2S as a source of sulfur for film growth and surface passivation. The Si(100) surface undergoes reconstruction, whereby surface Si atoms pair to form dimers. The Si
Si bonding of a dimer is described in terms of a strong σ bond and a weak π bond. The Si
Si dimer exhibits a tilted configuration that produces a nonuniform distribution of charge in the dimer Si atoms; the buckled-up Si atom of the dimer becomes an electron-rich and nucleophilic site, whereas the buckled-down Si atom is an electron-poor and electrophilic site.12 The special electronic configuration of Si(100) with dimers has attracted much attention in the adsorption and reaction of organic compounds adsorbed on the Si(100) surface.13
15 The tilted dimer might r 2011 American Chemical Society

exhibit two surface reconstructions with either c(4  2) or p(2  1) symmetry.16 On the basis of high-resolution X-ray photoelectron spectra (XPS), Landemark et al. concluded that similar Si 2p spectra from p(2  1) near 300 K and the c(4  2) structure near 120 K imply the same local structure of the two reconstructions.17 According to ultraviolet photoelectron spectra (UPS), the initial adsorption of H2S is dissociative to form surface SH and H species on Si(100) at 150 K.18 Observations of D2S on Si(100) with a scanning tunneling microscope (STM) indicated that the dissociative DS moiety bridges two Si atoms between two dimer rows and that the D atom bound to a Si atom of the neighboring dimer.19 A saturation of available adsorption sites was accordingly attained at a coverage of 1/3 monolayer DS and D. Other work with an STM showed that adsorption of H2S on a Si surface was site-selective and temperature-dependent because adsorption was affected by surface reconstruction and thermally activated dissociation.20 Measuring Auger electron spectra and temperature-programmed desorption (TPD), Han et al. proposed reaction intermediates and their adsorption geometries following the adsorption and thermal decomposition of H2S on Si(100).21 H2 and SiS were the desorption products with signal maxima at 780 and 820 K during thermal decomposition of H2S, respectively.22 Romero et al. investigated the adsorption energetics and geometries of H2S on the Si(100) surface, using quantum Received: May 17, 2011 Revised: August 20, 2011 Published: August 22, 2011 19203

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chemical calculations of the total energy,23 which focused mainly on the initial adsorption of molecular H2S. Although the adsorption and decomposition of H2S on the Si surface have been characterized experimentally, a detailed mechanism of the decomposition has not been reported. We studied the adsorption and thermal decomposition of H2S on Ge(100) with TPD, XPS, and density functional calculations.24 Here we report the results of the density of states (DOS) for the adsorption energetics and conformations of H2S, HS, and S on Si(100) and correlate the XPS data with these results. We thereby elucidate possible mechanisms of thermal decomposition of H2S on Si(100).

’ EXPERIMENTS AND CALCULATIONS The experiments were conducted in an ultrahigh vacuum (UHV) chamber with base pressure 2  10
10 Torr. The UHV system was equipped with a quadrupole mass filter (EPIC, Hiden), a low-energy electron-diffraction analyzer, and an electron-energy analyzer (HA100, VSW). The thickness of our Si(100) sample (n-type, 1
10 Ω cm) was 0.3 mm. To prevent possible organic residues, the Si surface was cleaned with acetone, methanol, and hot HNO3 (1 M) in a sequence, followed by rinsing with distilled water. A Ta strip (thickness 0.025 mm) was uniformly pressed between two Si samples with Ta foils at both ends, which were in turn mounted on a copper block. The sample could be cooled to 115 K with liquid nitrogen via conduction through the copper block and heated by resistive heating of the Ta strip and the Si sample. The sample temperature was monitored with a thermocouple (K-type) spot-welded onto a thin Ta foil inserted between two Si samples. The Si surface was initially cleaned by means of resistive heating in situ slowly to 1200 K; according to LEED, the surface then exhibited sharp 2  1 and dim c(4  2) patterns revealing two domains.22 The cleanliness of the Si surface was verified with LEED and XPS measurements. H2S (99.5%, Matheson) was introduced to the Si surface without purification. XPS were recorded at the HSGM beamlines of National Synchrotron Radiation Research Center in Taiwan. The incident angle of the photon beam was 55
from the surface normal; the photoelectrons were collected with the electron-energy analyzer normal to the sample surface. All XPS data presented here resulted from Shirley background subtraction with a third-order polynomial to each side of the line and were fitted numerically with Voigt functions. The onset of photoemission from an Au foil attached to the sample holder served as the Fermi level. All calculations were performed using DFT, as implemented in the Vienna ab initio simulation package (VASP).25
27 The Vanderbilt ultrasoft pseudopotential was used in determination of surface structures with energy truncated at 300 eV.28 To sample the Brillouin zone, we performed the calculation on 4  2  1 k points of a Monkhorst-Pack mesh. The validities of all optimized structures and determined TSs were checked through normal-mode frequency analysis. For identification of a real minimum on a potential energy surface, all frequencies had to be positive; a TS had to have one imaginary frequency corresponding to the reaction coordinate. The clean Si(100) surface was then modeled in the form of a slab, the c(4  2) unit cell with dimensions a = 15.36 Å, b = 7.68 Å, and c = 30.86 Å, having nine atomic layers separated by a 20 Å vacuum to ensure that there were no interactions between the surface adsorbates and the preceding slab. The optimized geometry of the model Si surface is depicted in Figure 1.

Figure 1. Optimized Si(100)-c(4  2) surface model: (a) front view and (b) side view.

In the structural optimizations, the positions of the last three silicon layers and an H-passivated layer on the bottom were fixed upon adsorption of H2S, and the remaining substrate atoms were allowed to relax with the adsorbates. For economy of computing time, only the upper two atomic layers of the surface and adsorbates were relaxed in the calculations of vibrational frequencies. The adsorption energies were obtained according to an equation Eads ¼ Etotal
ðEgas þ Esurf Þ in which Etotal, Egas, and Esurf are the calculated electronic energies of the adsorbed species on the surface, a gaseous molecule, and a clean surface, respectively. A negative value of Eads indicates an exothermic adsorption. The relative energies (Erel) were defined according to the following equation Erel ¼ EðLM or FSÞ
EH2 S ðadÞ in which EH2S(ad)and E(LM or FS) are the calculated adsorption energies of H2S and local minima or final products, respectively. The nudged elastic-band (NEB) method was applied to locate the transition structures that were located on interpolating a series of images of the system between the initial and final states on the potential energy surface.29,30 A spring force between the adjacent images was employed to maintain constant the spacing between the images, and a true force was applied to impel the images toward the path of minimum energy (MEP), mimicking an elastic band. Each image was optimized with the NEB algorithm based on a constrained algorithm of molecular dynamics. The highest point on the MEP corresponded to a transition structure on the proposed reaction path, and its energy, relative to that of the initial state, became the activation barrier of the reaction.

’ RESULTS AND DISCUSSION To ensure the reliability of the computational method, we calculated the optimized lattice parameters for bulk silicon with various pseudopotentials. An ultrasoft pseudopotential with the generalized gradient approximation (US-GGA)31 exhibited the least discrepancy between the calculated and experimental lattice parameters among the tested pseudopotentials (Table S1 in the Supporting Information). Accordingly, the US-GGA method 19204

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Table 1. Geometrical Parameters of H2S Molecule in Gas Phase H
S bond length (Å) this work H2S

1.347

exptl

calcd

1.328a 1.349,d 1.346,c

H
S
H angle (deg) this work 91.7

exptl

calcd

91.6a 91.6,d 91.7,c

b

90.5b

1.345 a

b

c

d

Ref 34. Ref 35. Ref 36. Ref 37.

Figure 3. DOS of a H2S molecule before (dotted line) and after (solid line) adsorption on Si(100) and DOS of a dimeric Si atom (dashed line) bound to H2S.

Figure 2. Optimized structures of H2S adsorbed on Si(100): (a) H2Sad, (b) H2Sad-a, and (c) H2Sad-b.

Table 2. Adsorption Energies (Eads) and Structural Parameters for Hydrogen Sulphide (H2S) Adsorption on Si(100) Si
Si tilting d(H
S) d(Si
S) d(Si
Si)

H
S
H

angle

Eads

(Å)

(Å)

(Å)

(deg)

(deg)

(eV)

H2Sad

1.36/1.45

2.37

2.43

92.0

16


0.74

H2Sad-a

1.36/1.42

2.38

2.42

91.7

16


0.70

H2Sad-b

1.36/1.42

2.50

2.43

91.7

16


0.65

was utilized to elucidate the evolution of H2S during the thermal reaction on a Si surface. On a clean c(4  2) Si, the calculated bond length and tilt angle of the Si
Si dimer are 2.35 Å and 19
, respectively, consistent with previous reports.32,33 Besides c(4  2), we also used c(4  4) unit cell to model Si(100) surface. The calculated bond length and tilt angle of the Si
Si dimer are the same as those of c(4  2) unit cell (Table S2 in the Supporting Information). The calculated H
S bond length is 1.347 Å, and the — HSH bond angle is 91.7
for a free H2S molecule (Table 1), in agreement with experimental34 and computational35
37 data. A. Molecular Adsorption of H2S. Upon adsorption of H2S on Si(100), the S atom of H2S favors offering its lone-pair electrons to the electron-poor buckled-down Si atom of a dimer. Our calculation shows three stable adsorption conformations with orientations of the S
H bond, as depicted in Figure 2, denoted H2Sad, H2Sad-a, and H2Sad-b. The calculated adsorption energies and structural parameters for each conformation are summarized in Table 2. The difference of adsorption energies is less than only 0.09 eV. Among these three conformations, H2Sad is the most stable with adsorption energy
0.74 eV. In H2Sad, the bond of the Si-dimer bound to a H2S molecule is stretched to 2.43 Å from 2.35 Å on a clean Si surface. The tilt angle of a Si dimer is decreased to 16
from 19
for a clean surface, indicating the decreased zwitterionic and π-bond character of the Si dimer

Figure 4. EDD contours of H2Sad on a plane containing (a) a S
Si bond and a dimeric Si
Si bond and (b) a S
Si bond and a S
H bond. Each contour line represents a change of 0.002 eV. The solid and dashed lines of contours represent the increase and the decrease in electron density, respectively, compared with a free H2S molecule and Si(100).

upon absorption of H2S. The bond length of S
Si is 2.37 Å, and two H
S bonds of adsorbed H2S are lengthened to 1.36 and 1.45 Å, respectively. The structural parameters of H2Sad-a and H2Sad-b are similar to those of H2Sad, consistent with their comparable adsorption energies. In addition, the adsorption energy of H2Sad-b was calculated on Si(100)-c(4  4) surface. The value,
0.63 eV, was very close to that on the Si(100)-c(4  2) surface. Therefore, it is acceptable to examine the interaction between H2S and Si surface with Si(100)-c(4  2) surface model. The electron density difference (EDD) is presented to illustrate the variation of the charge distribution upon adsorption of H2S on a Si surface. Figures 3 and 4 show the DOS and EDD of a H2S molecule before and after adsorption on the Si(100) surface, respectively. The H2S molecule possesses C2v symmetry; four valence states of H2S are labeled 4a1, 2b2, 5a1, and 2b1 according to their orbital symmetries, as shown in Figure 3. The partial DOS (PDOS) indicates that states 4a1 and 2b1 are contributed mainly from atomic orbitals 3s and 3pz of the S atom, respectively, whereas 2b2 and 5a1 states arise from the hybridization of orbitals 3px and 3py of a S atom (Figure S1 in Supporting Information). Figure 3 shows that all S 3p atomic orbitals (2b2, 5a1, and 2b1 states), especially 3px and 3py, of adsorbed H2S have significant overlaps with the d orbitals of the buckled-down Si 19205

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Figure 5. Potential energy surfaces for adsorption and decomposition of H2S on Si(100). All potential energies of intermediates, transition structures, and final products are referred to the H2S molecule adsorbed on the surface.

that is bound to the H2S molecule. The electron densities of these states became broadened because of interaction with the surface, except the 3s orbital (4a1 state). Figure 4a,b shows the EDD contours on the planes containing the S
Si bond with the dimeric Si
Si bond and the S
H bond, respectively. The electron density increases between the S atom and the Si atom bound to H2S, whereas the electron density decreases between the S atom and H atoms, indicating a weakened S
H bond of adsorbed H2S. An increased electron density is observed also between the H atom and the Si atom of a neighboring Si dimer along direction [010], which interacts with each other through a hydrogen bond; this interaction induces a weakened S
H bond that is elongated to 1.45 Å and readily undergoes scission. B. Reaction Mechanism of H2S on Si(100). Previous authors discussing H2S on Si(100) indicated that an initial adsorption of H2S is dissociative to form SH and H species adsorbed on Si(100) at 150 K. The HS species undergoes further dehydrogenation to form S adatom at 550 K.18,39 As stated above, three stable structures of H2S adsorbed on Si(100) surface are possible. We calculated the structural conformations of surface SH and S and utilized the NEB method to map the reaction PES for decomposition of H2S, considering only the most stable conformation, H2Sad. Figure 5 shows the PES for the adsorption and dissociation of H2S on the Si(100) surface; the potential energy is referred to the energy of the molecularly adsorbed H2S (H2Sad). The relative energies and structural parameters of intermediates and final

products are listed in Table 3; the reaction energies, reaction barriers, and imaginary frequencies of transition structures are summarized in Table 4. Their corresponding structures of intermediates are illustrated in Figure 6; top views of their corresponding transition structures are shown in Figure S2 available in the Supporting Information. According to the calculations, the possible reaction paths are summarized in the following scheme:

The H2Sad species can undergo a first dehydrogenation through three paths. The first path produces intermediate LM1I (Figure 6a) through transition structure TS1I, with a barrier 0.67 eV; one H atom of H2S dissociates and migrates to the Si atom of an adjacent dimer along direction [001]. The second path forms intermediate LM1II (Figure 6b) through TS1II with a barrier 0.72 eV; in this intermediate, the dissociated 19206

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Table 3. Structural Parameters and Relative Energies for Dissociatively Adsorbed H2S on Si(100) local minima and final products

d(Si
Si) (Å)a

d(Si
Si) (Å)b

d(Si
S) (Å)

d(Si
H) (Å)

tilting angle

tilting angle

(Si
Si) (deg)a

(Si
Si) (deg)b

Erel (eV)

LM1I

2.43

2.40

2.17

1.50

8

8


0.95

LM1II

2.41 (2.39c)

2.41 (2.39c)

2.16 (2.15c)

1.50 (1.53c)

2 (2c)

2 (2c)


1.43

LM1III

2.45 (2.42c)

2.39 (2.37c)

2.18 (2.16c)

1.50 (1.53c)

8 (7c)

14 (11c)


1.31

LM2I

2.44

2.40

2.03

1.50

0

8


1.05

LM2II

2.44

2.38

2.02

1.50

0

2


1.33

LM3I

2.45

2.38

2.04

1.50

0

5


1.13

LM3II LM3III

2.44 2.43

2.41 2.39

2.01 2.02

1.50 1.50

16 0

0 0


1.45
1.45

LM3IV

3.30

2.45

2.18/2.14

1.50

0

2


1.46

LM3V

3.44

2.40

2.18/2.16

1.50

0

0


1.59

FSI

2.67

2.67

2.30

1.50

FSII

2.32 (2.30c)

2.40 (2.38c)

2.18 (2.18c)

1.50 (1.53c)

0 (0c)

0 (0c)


2.63

FSIII

2.41 (2.40c)

2.41 (2.40c)

0

0

FSIV

3.45 (3.50c,3.48d)


1.38

2.27 (2.26c)

1.50 (1.53c)

2.15/2.18 (2.16c,2.22d)

1.50 (1.53c, 1.53d)

0 (0c, 0d)


2.12
1.99

a

Bond length of a Si(I)
Si(II) dimer bounded to HS, or S. b Bond length of a Si(III)
Si(IV) or Si(V)
Si(VI) dimer bounded to dissociative H. c Ref 23. d Ref 38.

Table 4. Calculated Reaction Barriers (‡E, eV), Reaction Energies (ΔE, eV), and Imaginary Frequencies (IMF, cm
1) for the Transition States of H2S Reaction on Si(100) Surface E (eV)

ΔE (eV)

IMF (cm
1)

H2Sad f TS1I f LM1I

0.67


0.95

664i

H2Sad f TS1II f LM1II

0.72


1.43

893i

H2Sad f TS1III f LM1III

0.01


1.31

538i

LM1I f TS2I-a f LM2I

0.97


0.10

337i

LM1II f TS2I-b f LM2I

1.24

0.38

625i

LM1III f TS2II-a f LM2II LM1II f TS2II-b f LM2II

0.87 0.73


0.02 0.10

928i 966i

LM2I f TS3I f LM3I

1.19


0.08

603i

LM2II f TS3II f LM3II

1.63


0.12

430i

LM2II f TS3III f LM3III

1.54


0.12

750i

LM2I f TS3IV f LM3IV

0.42


0.41

131i

LM2II f TS3V f LM3V

0.50


0.26

163i

LM3I f TS4I f FSI

0.15


0.25

151i

LM3II f TS4II f FSII LM3III f TS4III f FSIII

0.06 0.19


1.18
0.67

137i 141i

LM3IV f TS4IV f FSIV

1.22


0.53

143i

LM3V f TS4V f FSIV

1.54


0.40

1019i

reaction path

‡

H and HS adsorb separately on each Si atom of a dimer. The third path yields intermediate LM1III (Figure 6c) via TS1III with the smallest barrier, 0.01 eV; the resulting H and HS adsorb on Si atoms of adjacent dimers along direction [010]. As listed in Table 4, the reaction energies of the first dehydrogenation for these three paths are
0.95,
1.43, and
1.31 eV, respectively. Compared with H2Sad, the S
Si bonds of the HS species
LM1I, LM1II, and LM1III
are shortened by ∼0.5 Å, whereas the S
H bonds become longer, and the Si
Si bonds of the Si dimer slightly decrease. The tilt angles of the dimeric Si
Si bond also decrease after the dehydrogenation of H2Sad to form HS. Both LM1I and LM1II can undergo further dehydrogenation to produce LM2I (Figure 6d) via transition structures TS2I-a and TS2I-b with energy barriers 0.97 and 1.24 eV, respectively. LM2II

(Figure 6e) can be formed from dehydrogenation of LM1III and LM1II via TS2II-a and TS2II-b with barriers 0.87 and 0.73 eV, respectively. Except for the Si
S bond length and tilt angle, the structural parameters of LM2I and LM2II are similar and also near those of HS intermediates LM1I, LM1II, and LM1III shown in Table 3. The Si
S bond of the LM2 species is much shorter than that of species LM1, indicating a stronger Si
S bond in species LM2. Table 4 shows that all reaction energies of further dehydrogenations, LM1 f LM2, are near zero, and the barriers are higher than those of H2Sad f LM1. Species LM2I and LM2II can transform to more stable intermediates of two types, LM3. The first type includes LM3I (Figure 6f), LM3II (Figure 6g), and LM3III (Figure 6h), which are formed through the migration of a H atom of LM2I or LM2II. The S adatom remains bound to a Si atom and has a dangling bond in LM3I∼III. The formation barriers of LM3I, LM3II, and LM3III through transition structures TS3I, TS3II, and TS3III are 1.19, 1.63, and 1.54 eV, respectively. LM3I, LM3II, and LM3III can further transform with small barriers to their final stable states, FSI, FSII, and FSIII. In the final product, the S adatom bridges two Si atoms of a dimer (FSII; Figure 6l) or of two neighboring dimers along [001] (FSI; Figure 6k) or along [010] (FSIII; Figure 6m), whereas H adatoms remain at their original adsorption sites. In the second type LM3IV and LM3V, the S adatom bridges two Si atoms of a dimer, as shown in Figure 6i,j. The formation barriers of LM3IV and LM3V are 0.42 (TS3IV) and 0.50 eV (TS3 V), respectively, much smaller than those of the first type. Through migration of one H adatom with large barriers, LM3IV and LM3V further reconstruct to form the same final product, FSIV (Figure 6n). The dimeric Si
Si bond is preserved, and its length remains nearly constant at 2.43 ( 0.02 Å in the intermediates or final products, except LM3IV, LM3V, FSI, and FSIV. In the structure of FSI, the bond of dimeric Si becomes longer because of the ring stress from the Si
S
Si bonds. Upon formation of LM3IV, LM3V, or FSIV, the dimeric Si
Si bond, which is bound to S adatom, is cleaved, and the resulting distance between the Si atoms is >3 Å. 19207

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Figure 6. Optimized structures of intermediates and final products of the H2S reaction on the Si(100) surface: (a) LM1I, (b) LM1II, (c) LM1III, (d) LM2I, (e) LM2II, (f) LM3I, (g) LM3II, (h) LM3III, (i) LM3IV, (j) LM3V, (k) FSI, (l) FSII, (m) FSIII, and (n) FSIV.

Figure 7. XPS spectra of S 2p for a Si(100) surface exposed to 20 L H2S at 115 K and subsequently heated to indicated temperatures.

On the basis of these calculated results, we propose four reaction paths for the dissociation of H2S on Si(100): H2Sad f LM1I f LM2I f LM3I f FSI, H2Sad f LM1II/LM1III f LM2II f LM3II f FSII, H2Sad f LM1II/LM1III f LM2II f LM3III f FSIII, and H2Sad f LM1I/LM1II f LM2I/LM2II f LM3IV/ LM3V f FSIV. Their barriers at the rate-determining step are 1.19, 1.63, 1.54, and 1.22 eV, respectively. Accordingly, FSI and FSIV are expected to be the major products, but FSII and FSIII are more thermodynamically stable than FSI and FSIV and are therefore likely formed at a high reaction temperature from H2S. C. Analysis of XPS Spectra for Decomposition of Adsorbed H2S. The thermal evolution of XPS spectra served to characterize the variation of the surface composition during the thermal decomposition of H2S on Si(100). We correlate our calculated results with the XPS experimental data. Figure 7 shows S 2p spectra for a Si(100) surface at 115 K exposed to 20 L H2S and

subsequently annealed to various temperatures. Only a S 2p3/2 binding energy is observed at 163.48 eV. According to previous UPS and STM measurements, H2S dissociates to form H and HS species at 150 K.18 In our work, there is no significant difference between the binding energies of S 2p for a Si surface exposed to H2S at 115 and 150 K. This S 2p3/2 feature is thus attributed to surface HS, indicating that adsorbed H2S undergoes hydrogenation at the adsorption temperature, 115 K. According to our calculation, adsorbed H2S can undergo the first dehydrogenation to form an HS intermediate with a barrier as small as 0.01 eV, consistent with the XPS data that show dehydrogenation of H2S on Si(100) at a temperature as low as 115 K. Upon annealing the sample above 420 K, the intensity of the S 2p line at 163.48 eV due to HS gradually attenuated. A line appeared at 162.68 eV and is attributed to surface S adatoms produced from further dehydrogenation of the HS species. Our XPS spectra show that the binding energy of S 2p3/2 remains nearly constant before the HS species begins to dissociate at 420 K, but our calculation indicates that the possible HS intermediates, LM2 and LM3, have similar bonding configurations of the S atom. These HS intermediates thus exhibit similar binding energies of S 2p; the XPS measurement is inadequate to resolve their differences. The intensity of S 2p assigned to S adatoms decreases upon annealing the sample and disappears at a temperature up to 850 K, consistent with the desorption of SiS in this temperature range.22 The signal of S 2p3/2 corresponding to S adatoms gradually shifts from 162.86 eV at 570 K to 162.94 eV at 770 K. This change of the 2p3/2 binding energy is likely due to the structural transformation of S adatoms among the proposed LM2, LM3, and FS. The calculated results and reaction paths are hence consistent with the XPS measurement.

’ CONCLUSIONS We employed DFT calculations and XPS measurements to examine the adsorption configurations and the possible reaction paths of H2S on Si(100). The calculation of DOS elucidates that except for the 4a1 state all valence states of the H2S molecule involve the interaction of H2S with a Si surface, resulting in a large 19208

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The Journal of Physical Chemistry C adsorption energy. The EDD contour reveals that the dissociation of S
H is catalyzed by the Si surface via a concerted mechanism of formation and breaking of bonds that involve surface Si atoms. We propose a two-step mechanism involving intermediates (LM1) during the decomposition of adsorbed H2S to form S adatoms. In the first step, adsorbed H2S partially dissociates to form HS and H. This HS species undergoes further dehydrogenation to form a S adatom in the second step. The resulting S adatoms can transform to form four final products, FSI
IV, with more stable conformations via LM2 and LM3 intermediates. FSI and FSIV are kinetically favored products, whereas FSII and FSIII are thermodynamically favored products. According to the DFT calculation, we propose four possible paths for the decomposition of H2S, which have activation barriers 1.19, 1.63, 1.54, and 1.22 eV, respectively. Two S 2p features of XPS spectra observed during the thermal decomposition of H2S are attributed to surface HS and S species, respectively.

’ ASSOCIATED CONTENT

bS

Supporting Information. Lattice parameters of Si obtained with various pseudopotentials, partial DOS of H2S adsorbed on the Si(100) surface, and top view of all transition structures on the reaction paths of adsorbed H2S. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*(J.-C.J.) E-mail: [email protected]. Phone: +886-227376653. Fax: +886-2-27376644. (W.-H.H.) E-mail: whung@ntnu. edu.tw. Phone: +886-2-77346125. Fax: +886-2-29324249.

’ ACKNOWLEDGMENT National Science Council of Taiwan (NSC 96-2120-M-011001 and NSC-97-2113-M-011-001) supported this research. We are also grateful to National Center of High-Performance Computing, Institute of Nuclear Energy Research, and Atomic Energy Council in Taiwan for their support.

ARTICLE

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