Correlation of reaction sites during the chlorine extraction by hydrogen atom from Cl
Si ( 100 ) 2 × 1
Ming-Feng Hsieh, Jen-Yang Chung, Deng-Sung Lin, and Shiow-Fon Tsay
Citation: The Journal of Chemical Physics 127, 034708 (2007); doi: 10.1063/1.2752502
View online: http://dx.doi.org/10.1063/1.2752502
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/127/3?ver=pdfcov Published by the AIP Publishing
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Correlation of reaction sites during the chlorine extraction by hydrogen
atom from Cl/ Si
„100…-2Ã1
Ming-Feng Hsieh, Jen-Yang Chung, and Deng-Sung Lina兲
Institute of Physics, National Chiao-Tung University, 1001 Ta-Hsueh Road, Hsinchu 300, Taiwan
Shiow-Fon Tsay
Department of Physics, National Sun Yat Sen University, Kaohsiung 804, Taiwan
共Received 20 October 2006; accepted 31 May 2007; published online 20 July 2007兲
The Cl abstraction by gas-phase H atoms from a Cl-terminated Si共100兲 surface was investigated by scanning tunneling microscopy共STM兲, high-resolution core level photoemission spectroscopy, and computer simulation. The core level measurements indicate that some additional reactions occur besides the removal of Cl. The STM images show that the Cl-extracted sites disperse randomly in the initial phase of the reaction, but form small clusters as more Cl is removed, indicating a correlation between Cl-extracted sites. These results suggest that the hot-atom process may occur during the atom-adatom collision. © 2007 American Institute of Physics.
关DOI:10.1063/1.2752502兴 I. INTRODUCTION
The extraction of adsorbates on both metal and semicon-ductor surfaces by impinging hydrogen atoms has attracted attention as a model system for understanding the fundamen-tal dynamics of gas-surface reactions.1–6 One of the many model systems among these studies is the production of HCl gas species from a Cl-terminated Si共100兲 surface 关Cl/Si共100兲兴. In this system, an incident H-atom flux reacts with Cl atoms adsorbed on the Si共100兲 surface and produces gaseous HCl molecules: H共g兲+ Cl共ad兲/ Si共100兲→HCl共g兲 + Si共100兲. This gas-surface reaction has practical applica-tions for Cl reduction in Si atomic layer epitaxy at low temperature7 and for the dry etching process in very large scale integration.
One of the main scientific issues behind these studies is to examine the role of three disparate surface reaction mechanisms at the gas/solid interfaces. In the idealized Langmuir-Hinshelwood共LH兲 mechanism, two reagents react after they have been chemisorbed and are in thermal equilib-rium with the surface. Most surface reactions are believed to occur by this method. In the idealized Eley-Rideal 共ER兲 mechanism, a direct, single gas-surface collision is respon-sible for the reaction between an incident gas-phase species and another adsorbed reagent. The occurrence of this path-way has been clearly demonstrated by Lykke and Kay8 and by Rettner.5In the hot-atom共HA兲 mechanism, a trapped in-cident gas-phase species bounces a few times or diffuses for a short distance before reacting with another adsorbed re-agent. This pathway falls between the two idealized path-ways and has been shown to be the dominant reaction mechanism for the production of both H2 and HCl in the
reaction of H atoms with H- and Cl-covered metal surfaces.2,9
Halogen and hydrogen atoms form strong bonds on a
semiconductor surface and barely diffuse at near room temperature.10 Therefore, surface species are likely to retain their position after an extraction of halogen by an incident H atom.11 Utilizing Auger electron spectroscopy and temperature-programmed desorption 共TPD兲 mass spectros-copy, Cheng et al. found that the halogen removal rate by H共g兲 is first order in both the Cl/ Br surface coverage 共Cl,Br兲 and in the H flux 共FH兲.12 They also reported an
activation energy of 91 meV per Cl removed and concluded that the H-extraction process follows an Eley-Rideal reaction mechanism, where the surface reaction is mainly driven by the high internal energy of incident atomic hydrogen. Using time-of-flight scattering and recoiling spectroscopy to mea-sure the real-time surface H and Br coverage, Koleske and Gates verified that the removal rate of Br on the Si共100兲 surfaces with H atom has a linear dependence on both Br
and FHbelow 500 ° C.6In addition to the linear dependence
onBrand FH, the same reaction on the Si共111兲 surface also
has a linear dependence on the hydrogen coverageH,
indi-cating a more complex kinetics. The linear dependence of the reaction rate onBris consistent with an ER pathway.
How-ever, the structure dependence of the reaction leads to the suggestion that the H atom may be partially accommodated at the surface in a mobile “hot precursor” state before the reaction with the adsorbed Br. From the theoretical aspect, Kim et al. studied the H共g兲+ Cl共ad兲/ Si共100兲 system using the classical trajectory approach and concluded that all reactive events occur through a localized ER mechanism.13
As mentioned earlier, previous experimental studies em-ployed various spectroscopic techniques to measure the ki-netics and dynamics of the gas-surface reaction. Hattori et al. first investigated the fact that atomic hydrogen extracts chlo-rine from Si共111兲-7⫻7 using a scanning tunneling micro-scope 共STM兲.14 The authors showed that Cl atoms are ex-tracted from the Cl-covered Si共111兲 surface by atomic H, and that the surface Si atoms, after H bombardment, are termi-nated with H atoms. The clean Si共100兲 surface after Cl
ter-a兲Electronic mail: dslin@mail.nctu.edu.tw
0021-9606/2007/127共3兲/034708/6/$23.00 127, 034708-1 © 2007 American Institute of Physics
mination at room temperature has a relatively simple struc-ture: The silicon dimers retain their bonding and the surface layer consists of rows of Cl–Si–Si–Cl species.15,16The sur-face species exhibit the same dimerized structure, namely, –Si–Si–Cl, –Si–Si–, H–Si–Si–, and H–Si–Si–H after imme-diate Cl extraction and further H adsorption.16,17 Taking ad-vantage of these facts, we utilized both the synchrotron ra-diation photoemission spectroscope and the STM to observe the Cl/ Si共100兲 surface in atomic resolution after H-atom ex-posure. By comparing the results from the measurement with those from the computer simulation, it is evident that the reaction does not occur simply as the result of a single col-lision with unitary reaction probability between the gas atom and the adatom.
II. EXPERIMENT
The Si共100兲 samples were sliced from Boron-doped wa-fers with a dopant concentration of approximately 1.5 ⫻1015cm−3. After outgassing at ⬃900 K for ⬃12 h, a
dimerized clean Si共100兲 surface was obtained by dc Joule heating to⬃1450 K for a few seconds. After direct heating, chlorine molecules were introduced through a leak valve and a stainless-steel tube to the sample surface at room tempera-ture to form the Cl-terminated Si共100兲-2⫻1 structure. A hot tungsten-spiral filament was used to produce atomic hydro-gen. The filament was ⬃5 cm away from the Si共100兲 sub-strate and heated to⬃1800 K when the chamber was back-filled for a period of time T with H2to a pressure P of about 2⫻10−7 torr without sensitivity correction. Maxwell’s
distri-bution expects the kinetic energy of the dissociated H atoms from the hot filament surface to be 0 – 230 meV. From the geometry of the filament and the samples, it was estimated that the incident angles of H atoms was less than⬃25° from normal. The apparent H2exposure, i.e., P⫻T, is presumably
proportional to the actual dosage of hydrogen atoms on the surfaces. The atomic hydrogen flux was not measured di-rectly in the present study. Instead, the apparent exposure in Langmuir共1 L=10−6torr s兲 is used as the relative measure-ment of H dosage on the Cl–Si共100兲 surface.
The photoemission spectra were observed at the Taiwan Light Source laboratory in Hsinchu, Taiwan. Synchrotron ra-diation from a 1.5 GeV storage ring was dispersed by a wide-range spherical grating monochromator. The photocur-rent from a gold mesh positioned in the synchrotron beam path was monitored to calibrate the incident photon flux. Photoelectrons were collected 15° from the surface normal and analyzed by a 125 mm hemispherical analyzer in a
-metal shielded UHV system. The overall energy resolution was less than 120 meV. The STM measurement was per-formed in a separated UHV chamber.
III. RESULTS
A. Photoemission results
High-resolution core level photoemission spectroscopy can be used to distinguish atoms at nonequivalent sites and in different chemical bonding configurations, according to shifts in their binding energy.18 Figures1共a兲and1共b兲 show the respective surface-sensitive Cl 2p and Si 2p core level
spectra 共circles兲, and their decomposition into constituent components from the Cl– Si共100兲-2⫻1 surface before and after H bombardment at 325 K for various dosages. All fit-ting was least-squares fitfit-ting.19Each component that consists of a pair of spin-orbit split doublets is assumed to have the same Voigt line shape.
The Cl 2p spectra in Fig. 1共a兲 can be analyzed with a component that consists of a pair of split doublets separated by 1.60 eV. The binding energy of these Cl 2p spectra rela-tive to that of the corresponding Si 2p remains at 99.60 eV, suggesting that the Cl atoms form similar Si–Cl bonds. Fig-ure 2 plots the integrated intensities of the Cl 2p spectra 共ICl兲, which are proportional to the surface Cl coverage. The
integrated intensity of the bottom spectrum is normalized to be 1.0 because the chlorine coverage is nominally 1 ML for the Cl-saturated Si共100兲 surface prior to H-atom bombard-ment. ICl decreases linearly with the dosage of H atoms in
the early stage, indicating that Cl atoms were removed by impinging H atoms. This result is consistent with a previous study.12
The bottom spectrum in Fig.1共b兲 shows the Si 2p core level spectra for the Cl– Si共100兲-2⫻1 surface. This Si 2p spectrum consists of two components, B and Si+, separated
by about 0.9 eV. The B component is responsible for emis-FIG. 1.共Color online兲 The 共a兲 Cl 2p and 共b兲 Si 2p core level photoemission spectra共circles兲 for the Cl–Si共100兲-2⫻1 surface and the same surface after various apparent H-atom dosages as labeled. The solid curves are fits to the spectra. The curves labeled B共long dashed curves兲, Si+共dashed dot兲, and
Si2+ 共short dashed curves兲 are the results of decomposition of the Si 2p
spectra into contributions from the bulk, Si–Cl, and Cl–Si–Cl species, re-spectively. The energy zero in共b兲 refers to the 2p3/2bulk position for the
Cl– Si共100兲-2⫻1 surface. To eliminate the band bending effect, the relative binding energy for the Cl 2p refers to the corresponding Si 2p3/2line of the
B component in共b兲.
034708-2 Hsieh et al. J. Chem. Phys. 127, 034708共2007兲
sion from the bulk and the Si+component from the surface Si–Cl species.20 As the exposure of atomic hydrogen in-creases, both the intensities of the Si+component and the Cl 2p spectra drop off. This occurrence suggests that H atoms reduce the surface Cl coverage, similar to the findings of a previous report.12After⬎1000 L of apparent exposure, the line shape of Si 2p is similar to that 关top spectrum in Fig.
1共b兲兴 obtained by direct, high-dosage hydrogen exposure on the clean Si共100兲-2⫻1 surface at room temperature.21
This observation indicates that hydrogen atoms terminate nearly all surface dangling bonds and form a mixture of dihydride and monohydride surface when most Cl atoms are extracted. It should be noted that a small component labeled Si2+
emerges in Fig.1共b兲after H impingement. The chemical shift of Si2+, around 1.78 eV on the higher bonding energy side of
B, is consistent with a charged state of +2 for Si atoms and is responsible for SiCl2 species.15 Presumably, the SiCl2 spe-cies were formed as a consequence of the highly exothermic uptake of halogens during the extraction. Although more study is needed, the emersion of the dichloride species im-plies that impinging H atoms induces other surface reactions besides extracting upon collision with a surface adatom.
B. STM results
The clean Si共100兲 surface consists of rows of dimers, where the two dangling bonds from the two atoms in a dimer form a weak bond.22 Cl adsorption on a clean Si共100兲 surface saturates the dimer dangling bonds while preserving the basic共2⫻1兲 dimer structure without buckling, as shown in Fig.3共a兲.23,24 In Fig.3共a兲, a handful of dark sites can be discerned, each occupying one side of a Cl–Si–Si–Cl spe-cies. As Figs. 3共b兲 and3共c兲show, the density of these dark sites increase with the H exposure. The dangling bonds gen-erated during the Cl removal exhibit a higher apparent height due to enhanced tunneling near the Fermi level, and they are highly reactive to further H adsorption.25 The dark sites in Fig.3 are H-terminated sites. The initial H coverage on the Cl/ Si共100兲 surface is less than 0.02 ML. The presence of some initial surface H is likely due to the residue in the cleaning process and/or the adsorption of impurity by the HCl molecules in the Cl2gas source. The remaining Cl
cov-erage after H exposure can be obtained by directly counting its density in the STM images. The results are plotted in Fig.
2. Since the STM and photoemission measurements were performed in different chambers, the actual H dosages for the two measurements are different but proportional, as shown in Fig.2.
When the substrate temperature is held at RT during H-atom exposure, a reaction site, where a Cl atom is re-moved by an H atom and an H atom is subsequently ad-sorbed, presumably undergoes no diffusion.11,26The brightest humps in the images are likely weakly bonded terrace SiCl2 moieties, as evident from the photoemission spectra and as discussed in the previous section. In addition, the remaining Cl-terminated sites and bright humps, and most of the re-acted sites in Figs.3共b兲and3共c兲, appear to be H terminated. At first glance, the H-terminated sites, or the Cl-extracted sites, appear to be randomly dispersed. However, as will be analyzed and discussed in the following section, the density and the sizes of the clusters grouped together in neighboring Cl-extracted sites are larger than those created by random FIG. 2.共Color online兲 Cl coverage calculated from the integrated intensities
of the Cl 2p core level spectra in Fig.1共a兲共solid squares兲 and from those
counting from the STM images共open circles兲. The initial coverage is nomi-nally 1.0 ML based on the STM result.
FIG. 3.共Color online兲 STM images of the Cl/Si共100兲-2⫻1 surface after 共a兲 0,共b兲 36, and 共c兲 90 L apparent dosages of H atoms. The sample bias used was +2 V. In共a兲 the green rectangle box, running from the upper right to the lower left, encloses a row of five Cl–Si–Si–Cl共monochloride兲 species. A surface Cl atom appears as a bright protrusion and forms a narrow ellipse with another in the neighboring monochloride row in the image. Notably each of two nearest Cl adatoms in the empty state images are not from a single Cl–Si–Si–Cl species, but are a part of two adjacent Cl–Si–Si–Cl species. The green and blue arrows point to a missing dimer defect site and a H-termination site, respectively. The inset in共c兲 shows a 2⫻1 area of nearly complete H termination after Cl extraction. The size of a 1⫻1 unit cell in the image is 3.84⫻3.84 Å2.
extraction. At higher H-atom exposure, even two-dimensional islands with a H / Si共100兲-2⫻1 structure, as shown in Fig. 3共c兲, can be easily found. Figure4 shows an STM image for the H共g兲+ Cl共ad兲/ Si共100兲 reaction at a sub-strate temperature of⬃600 K. Similar isolated dark sites oc-cupying one side of a dimer can be easily identified, since they are H-terminated sites after Cl extraction. The density of the Cl-extracted sites increases as the H-atom dosage and the clustering of reaction sites become evident at higher H-atom dosage. The results are similar to those obtained at near room temperature.
IV. DISCUSSION
In the ER mechanism, a Cl-extraction reaction occurs via a collision-induced reaction. The calculated cross section is smaller than a unit cell within a small proximity around the spot where an H atom strikes.13The gas-phase H atoms impinge on the surface in a random fashion. In this scenario, a new Cl-extracted site is generated no matter what neigh-boring chemical environment surrounds the site where an H atom strikes. In other words, the extraction probability upon collision with an H atom is not changed when a Cl–Si sur-face species is neighboring one or more dangling bond sites or monohydride sites. If this scenario is valid, then the dis-tribution of the Cl-extracted sites by the random and sequen-tial impingement of gas-phase H atoms will be completely random in the STM images.
Figures5共a兲–5共c兲show the results of the impingement-site distribution from the computer simulation based on this assumption. In the simulation, a reactive site is randomly generated since the impact parameter found in the classical trajectory approach is small.13 In Fig. 5, the Cl-extracted sites are classified into eight categories, and are marked in different colored intensity scales共labeled 0–8兲, according to the degree of reaction-site clustering. A site in categories numbered k = 0, 1, 2, and 3 is a Cl-extracted site with 0, 1, 2, and 3 of its four nearest neighboring Cl-extracted sites 共la-beled s = 1 – 4 in Fig. 6兲, respectively. If a Cl-extracted site
with its four nearest neighboring Cl extracted is called a “surrounded site,” a site in category 4, 5, 6, 7, and 8 is a surrounded site and has 0, 1, 2, 3, and 4 nearest neighboring surrounded sites, respectively. In the classification scheme, the category number k of a Cl-extracted site indicates the number of other extraction reactions occurring in its
imme-diate vicinity. Therefore, the larger the cluster formed by the Cl-extracted sites, the darker the cluster appears in the im-age.
Figure 6 displays the “unnormalized” pair distribution function共g
⬘
兲 of Cl-extracted sites,g
⬘
共s兲 = g共s兲= 1 N兺
i=1N
ni共s兲
m共s兲, 共1兲
where ni共s兲 is the number of sth-nearest-neighbor
Cl-extracted sites around the ith Cl-Cl-extracted site共labeled 0 in Fig.6兲, denotes the coverage of the Cl-extracted sites, and m共s兲 denotes the number of sth-neighbor sites.27 As ex-pected, the pair distribution g
⬘
obtained from the simulated images is roughly equal to , independent of site index s. Figure 6 shows that g⬘
calculated from simulation images such as Figs. 5共d兲–5共f兲is in agreement with those expected for a completely random distribution. In contrast, g⬘
for near-est neighboring sites s = 1 – 4 obtained from STM images is boosted by about 20%. g⬘
for next nearest neighboring sites s = 5 – 10 is also boosted at higher coverage. The deviation of g⬘
from the mean coverage suggests the existence of cor-relation and interaction between Cl-extracted sites and, there-fore, rules out the pure ER process with unitary reaction probability.28To further examine whether or not the cluster formation of Cl-extracted sites results from random H impingement, STM images taken after the reaction were digitized and are shown in Figs.5共d兲–5共f兲in a similar fashion to those in Figs. FIG. 4. 共Color online兲 STM images of the Cl/Si共100兲-2⫻1 surface after
12 L apparent dosages of H atoms at a sample temperature of 600 K. The sample bias used was +2 V.
FIG. 5.共Color online兲 Distribution of Cl-extracted sites obtained from 关共a兲– 共c兲兴 simulation and 关共e兲–共f兲兴 STM. The coverage of Cl-extracted sites in monolayer is labeled. The simulation starts on an area which initially con-sists of 50⫻50 Cl adatom sites. A Cl-extracted site is classified into eight categories and represented by a 1⫻1 cell of different colors as indicated. The “digitalized” STM images were obtained from parts of STM images typically about 30⫻30 nm2in size.
034708-4 Hsieh et al. J. Chem. Phys. 127, 034708共2007兲
5共a兲–5共c兲. A direct visual comparison between Figs.5共b兲and
5共e兲, and Figs.5共c兲and5共f兲suggests that the site population for categories with a greater k obtained from the STM mea-surement 关PSTM共k兲兴 is greater than that 关PSim共k兲兴 from the corresponding simulated images. Their ratios PSTM共k兲/ PSim共k兲, plotted in Fig.7, deviate significantly from 1.0, especially for k⬎4. This finding also indicates that the simulation based on the assumption of a pure ER process deviates from the experimental results. The cluster formation of Cl-extracted sites can only be realized if an impinging H atom “senses” the chemical environment in a small共HA兲 or
large共LH兲 range beyond the collision spot. The sizes of the clusters in Figs. 5共d兲–5共f兲 are not large; the larger-than-expected pair correlation found in the small coverage of the extraction reaction sites is limited to the nearest neighboring sites 共s=1–4兲. These facts suggest that the reaction of Cl-extraction likely follows that of the HA process.
V. CONCLUSION
Distinguishing a detailed surface reaction mechanism has been an important but difficult issue. The H共g兲 + Cl共ad兲/ Si共100兲 is an important prototypical system for the study of the ER, HA, and LH mechanisms. In our work, a combination of atomic resolved STM images, spectroscopic measurements of core level photoemission, and computer simulations provide a detailed picture of the atomic pro-cesses involved in this seemingly simple gas-surface reac-tion. The core level measurement and STM images observed the formation of SiCl2 surface species, indicating that
“some” additional reactions occur beside the removal of Cl upon impingement of H atoms. Analysis of the STM and simulated images shows that the Cl-extracted sites are corre-lated to the neighboring Cl-extracted sites. These experimen-tal results cannot be explained by the pure Eley-Rideal pro-cess with unitary reaction probability. We recognize that other mechanisms, for example, an Eley-Rideal abstraction processes with a reaction probability which depends on the local surface coverage of Cl and maybe H, might possibly lead to our results. However, our findings and consideration lead us to believe that the HA process likely occurs during the atom-adatom collision. Further study is needed to better understand the nature of the gas-solid reactions.
ACKNOWLEDGMENTS
This work is supported by the National Science Council of Taiwan under Contract No. NSC 95-2112-M009-0039-MY4 共D.S.L.兲 and NSC 95-2112-M-110-016-MY3 共S.F.T.兲 and by the National Synchrotron Radiation Research Center and National Center for High-Performance Computing. The authors thank Professor Jyh-Yang Wu for discussion of the site distribution analysis.
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unnor-malized pair distribution function of Cl-extracted sites vs the neighboring site s obtained from a set of the STM images共circles兲 and the simulation 共squares兲, and the completely random distribution calculation 共dashed curves兲. The coverage of Cl-extracted sites in monolayer is labeled. The inset shows the Cl/ Si共100兲-2⫻1 lattice; each circle corresponds to an initial Cl adatom site. Numbers mark the respective sites around a Cl-extracted site at the center position共labeled 0兲. The distribution functions obtained from several STM measurements and simulations vary slightly for each s, but their overall trends are the same.
FIG. 7. 共Color online兲 The ratio of the population density obtained from a set of STM images PSTMto that from simulated images PSimvs site
cat-egory. The coverage of Cl-extracted sites in monolayer for each curve is labeled. The ratios obtained from several STM measurements and simula-tions vary slightly for each category, but their overall trends are the same.
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034708-6 Hsieh et al. J. Chem. Phys. 127, 034708共2007兲
