DFT study of F atom adsorption on Si(001) surface
Lemya. Bouamama, Amel. Lounis, Abdelhamid. Ziane Laboratoire de Physique et Chimie Quantique(LPCQ)
Faculté des Sciences, UMMTO, Algérie E-mail: lemya.bouamama@yahoo.fr
Arezki. Mokrani
Institut des Matériaux Jean Rouxel, BP 32229 2 rue de la Houssinière, F-44322
Nantes Cedex, France
Abstract— We have investigated the initial adsorption of an fluorine (F) atom on Si(001) surface by means of first-principles calculations using pseudopotential method implemented in SIESTA code. Three high-symmetric adsorption sites of F on Si(100) were examined : top, bridge and hollow sites. For an F atom adsorbed on perfect Si(001) surface, we found that F atom prefer the bridge adsorption site with a high adsorption energy of 6.47eV. However, in the relaxed case, the adsorption of an F atom leads to a Si(001)-2×1 surface reconstruction and the most stable adsorption site corresponds to the dangling bond site of a Si(100) surface top layer. The Si-F bond is calculated to be 1.67 Å and the adsorption energy of the F atom is evaluated to be 6.51eV.
Keywords— DFT, SIESTA, Silicon, fluorine, adsorption
I. INTRODUCTION
The interaction of F with Si surfaces is of great importance from both scientific and technological viewpoints. Since F is the most reactive atom it is of interest how it reacts with the Si surfaces. The chemical etching of silicon by fluorine is one of the most important materials tailoring techniques in the manufacture of semiconductor devices [1]. There are many theoretical as well as experimental studies but still the mechanism of fluorine etching process is not comprehensively understood [2-8]. It is found that during the initial exposure the F adsorbs rapidly on the Si(001)-2×1 surface up to about 1.5 ML (monolayer)[2]. As the F exposure increases a fluorosily adlayer is formed, which consists of the reaction intermediates SiF, SiF2, and SiF3[3,4]. The steady state exposure of F then leads to spontaneous etching at room temperature with the major gas phase product being SiF4[5].
In the present study we carried out density functional calculations related to initial fluorination of Si(100) surface.
The aim of this work is thus to attempt to answer some of the basic questions concerning the F-Si reaction and to determine the various geometries adopted by the F/Si(001) system.
II. COMPUTATIONAL METHOD
Our calculations were based on the density functional theory, as implemented in the SIESTA code, with the Local density approximations (LDA) for exchange correlation potential as parametrized by D. M. Ceperley and B. J.
Alder[6], This method is based on norm-conserving pseudopotentials and linear combinations of atomic orbitals as basis sets. We have used the supercell technique consisting of a periodical repetition of a seven layers slab of Si, with a
p(2×2) surface unit mesh and a coverage of 0.25 ML for F atoms. The images of the Si slabs are separated by a vacuum space of 19 Å for Si(001) slab. This vacuum space is sufficient to cancel any interaction between two consecutive slabs. The energy cut-off used in all calculations is 300 eV; the structural relaxation, carried out with the conjugate gradient method, stops when the forces on each relaxed atom are smaller than 0.001 eV/Å. The minimization of the total energy procedure yields a Si bulk lattice constant of 5.47 Å which is somewhat higher than the experimental value of 5.43 Å[7], but in good agreement with other theoretical estimations using similar methods [8, 9]. In the calculation of the fluorine adsorption on the Si(001) non reconstructed surface, the slab atoms are fixed in a bulk-like structure and the F atom is full relaxed in three directions. In the relaxed case, the fluorine atom and the three uppermost layers of Si atoms are allowed to relax in three dimensions and others atoms are fixed in a bulk-like structure.
III. RESULTS AND DISCUSSION A. F adsorption on unreconstructed surface
We begin with determining stable adsorption sites for an F atom on the Si(001) perfect surface. Three high symmetry pathways and consequent adsorption sites were tested: (i) adsorption of fluorine atom directly on a silicon dangling bond (Top site) to form a mono coordinated species (ii) adsorption of fluorine atom directly between two atoms of surface (bridge site) (iii) adsorption of a fluorine atom at the center of a square of first-layer atoms (hollow).These sites are shown in Fig.1.
The results for the adsorption energies and the bond length Si- F of fluorine atom on Si(001) are summarized in Table 1.
It can be seen from the calculated energies (Table 1) that the interaction between F and Si surfaces is important and the bridge site is found to be the lowest in energy for the perfect surface. The Si-F bond length is calculated to be 1.96 Å . The adsorption energy of the F atom is evaluated to be 6.47eV, which is in reasonable agreement with the experimental bond dissociation energy for removal of a F atom from SiF4 (6.95 eV) [10]. Top site is only 0.27 eV higher in energy than the bridge site, the bond length Si-F is found to be 1.64 Å in good agreement with the only theoretical study of 1.463 Å [11]. The hollow site her is 2.08 eV higher than the stable state (bridge).
The population of this site should be negligible.
Fig. 1. Representation of the high-symmetry sites for the adsorption of F on the Si(001) perfect surface: bridge, top and hollow. Top panel: top view;
bottom panel: side view.
TABLE I. ADSORPTION ENERGY (EADS) AND THE SI-F BOND LENGTH OF
F ON PERFECT SURFACE SI(001) FOR HIGH SYMMETRY SITES:BRIDGE,TOP AND HOLLOW
Adsorption sites Eads(eV) dF-Si(Å)
Bridge (This work) 6.47 1.96
Theo / /
Top (This work) 6.20 1.64
Theo[11] 6.341 1.643
Hollow (This work) 4.39 2.75
Theo / /
We also calculated the trajectory for F attacking Si(001) surface along the dangling bond direction (top) for the unreconstructed surface. The potential curves calculated at the DFT levels are plotted in Fig. 2. It is observed that the minimum energy lies at Si–F distance of 1.64 Å and then it slowly increases and reaches a plateau at a distance of 2.5 Å onwards. It shows that the reaction does not have an activation barrier, this is consistent with empirical [12] and ab-initio [13]
derived molecular dynamic simulations, which recommends a near unity sticking probability.
B. F adsorption on the reconstructed surface
At room temperature the unreconstructed surface is instable [14] and the Si(001) surface have several reconstruction. For this reason in this second case of calculation, we examined the same three site of adsorption.
The fluorine atom and the three uppermost layers of Si atoms are allowed to relax in three dimensions and others atoms are fixed in a bulk-like. The relaxation of the three uppermost layers of substrate leads to a reconstruction in the surface. It is found that there are one stable adsorption and two metastable sites. The most stable adsorption site corresponds to the dangling bond (DB) site of a top layer Si atom as shown in Fig. 1. The first metastable stat (MS1) is only 0.31 eV higher in energy than the DB site and the F atom bonds to a second layer Si atom. The other metastable adsorption site (MS2) is the center of the dimer Si atoms. Since the MS2 site is 1.34 higher than the DB site. The results for the adsorption energies and the bond length Si-F of fluorine atom on Si(001) surface
are summarized in Table 2, wish are in good agreement with the early theoretical studies [1,2].
Fig. 2. Potential curve for F atom approaching a Si perfect surface along the dangling bond direction (top) as a function of Si–F distance.
Fig. 3. Representation of the bonding structures of F atom on the Si(001) reconstructed surface. Top panel: top view; bottom panel: side view.
TABLE II. ADSORPTION ENERGY (EADS) AND THE SI-FBOND LENGTH OF
F ON THE SI (001)RECONSTRUCTED SURFACE
Adsorption sites Eads(eV) dF-Si(Å)
DB (This work) 6.51 1.662
Theo 7.21 6.222 1.601 1.6462
MS1 (This work) 6.28 1.69
Theo 6.61 1.611
MS2(This work) 5.17 1.96
Theo 5.61 1.851
IV. CONCLUSION
We have presented a first-principles studies examining the reaction of atomic fluorine with the Si(100) surface both reconstructed and unreconstructed. The favorable surface binding sites were determined. It was found that the bridge Site wish is the most stable in the unreconstructed surface become non preferred in the reconstructed surface and the fluorine atom occupied the dangling bonds.
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