SiO Desorption Kinetics of Si ( 111 ) Surface Oxidation Studied by Real-Time Photoelectron Spectroscopy

The kinetics of the initial oxide growth on the Si(111) surface have been investigated using real-time photoelectron spectroscopy and density functional theory (DFT) calculations. Including SiO desorption into the description of the transition from Langmuir-type adsorption to two-dimensional (2D) oxide island growth reveals that oxidation at high temperature T and low oxygen pressure PO2 is not governed by 2D oxide island growth despite sigmoidal oxygen uptake curves. Because SiO desorption during the initial oxide growth depends strongly on temperature and oxide coverage θoxide in the transition region, an initial oxidation model for the transition region is proposed. According to PO2 -dependent experimental results and theoretical calculations, the frequent occurrence of SiO desorption is due to the formation of the transition state tri-ins×2 species, SiO desorption during the initial oxidation is suppressed by the most thermally stable oxygen adsorption species tri-ins×3 formed on Si(111)7×7. [DOI: 10.1380/ejssnt.2013.116]


I. INTRODUCTION
In order to understand the oxidation of Si surfaces owing to the technological importance of silicon oxide thin films in microelectronic devices, a great deal of studies for the reaction kinetics of both the passive oxidation and active oxidation of Si surfaces have been carried out [1][2][3].In particular, the mechanisms of Langmuir-type adsorption without oxide decomposition, two-dimensional (2D) oxide island growth with SiO desorption, and the transition from Langmuir-type adsorption to 2D oxide island growth in the passive oxidation region have been widely studied [4,5].The autocatalytic reaction (ACR) model, which is generally adopted to describe passive oxidation, predicts a slightly temperature-dependent oxide growth because of the thermal activation of desorption within the region between low T -high P O2 Langmuir-type adsorption and high T -low P O2 2D oxide island growth, namely the mode transition region [6].However, oxide growth has also been suggested to proceed via a competition between thermal desorption and conversion of the immediate adsorbates to more stable oxide species [7].The transition from growth to decomposition of the surface oxide was also found kinetically for critical values of the initial ox- * Electronic address: ruffle@mail.tagen.tohoku.ac.jp ide coverage [8].On the other hand, oxide decomposition only occurred at temperatures above 650 • C by the desorption of small oxide nuclei at the beginning of 2D oxide island growth, whereas oxide growth at the periphery of larger nuclei could surmount decomposition [9].Additionally, Auger electron spectroscopy combined with reflection high energy electron diffraction (AES-RHEED) measurements showed that the formation of the desorption precursor SiO * is crucial for separating Langmuirtype adsorption from 2D oxide island growth [5].The oxygen adsorption states changed drastically from the energy barrier-less adsorption to desorption precursor SiO * in pairs with single dimer vacancies in the phase transition region [5].Despite many studies on both the passive and active oxidations processes, the detailed kinetics of SiO desorption and the competition between the oxide growth and oxide decomposition during the initial oxide growth in the transition region on the Si(111)7×7 surface have not been fully elucidated.
In the present study, the kinetics of SiO desorption during the initial oxide growth in the transition has been investigated by a combination of the real-time photoelectron spectroscopy and density functional theory (DFT) calculations.The oxidation at high T and low P O2 is found not to occur via 2D oxide island growth despite sigmoidal oxygen uptake curves.In addition, an oxidation model in the transition region on Si(111)7×7 has been proposed based on the temperature dependent ex-perimental results.Further, according to the oxygen pressure dependent experimental results and theoretical calculations, the oxygen adsorption species tri-ins×2 and tri-ins×3 formed at different initial oxidation stages influence the kinetics of SiO desorption during the initial oxide growth in the transition region on Si(111)7×7.

II. EXPERIMENTAL AND THEORETICAL METHODS
The sample for oxidation is a highly boron-doped ptype Si(111) wafer cut to a suitable size.The Si wafer was cleaned by the RCA washing [10] before its introduction into the experimental apparatuses.Real-time ultraviolet photoelectron spectroscopy (UPS) was carried out using the integrated surface analysis apparatus of the RHEED-AES-UPS facility at Tohoku University, Japan.The photon energy of the UV light was 21.22 eV with a take-off angle of 90 • (surface normal) and a surface sensitivity of 1 nm.The X-ray photoelectron spectroscopy (XPS) was performed at SUREAC2000 at BL23SU at Spring-8 [11,12], Japan.The photon energy and take-off angle values were 710 eV and 20 • , respectively.The surface sensitivities of O 1s and Si 2p spectra were 0.4 nm and 0.8 nm, respectively.Before beginning the oxidation experiments, the sample was annealed at 1000 • C for 10 min to remove the oxide from the Si surface.For thermal oxidation, oxygen gas with 99.999% purity was introduced into the reaction chamber with a base pressure at ∼1.0×10 −8 Pa.The oxidation temperature of the Si wafer ranged from 590 • C to 710 • C.
The DFT calculations were performed using the hybrid functional B3LYP [13,14] and 6-31G(d) basis set [15,16] using Gaussian 09.In the present work, the adsorption energies of various oxygen adsorption species on Si(111)7×7 were determined by means of a Si 27 H 24 cluster model, in which hydrogen atoms terminated the 27-Si atom cluster except for the adatom and the rest atom [17,18].

III. RESULTS AND DISCUSSION
In Figs.1(a) and (b), the oxygen uptake curves obtained from the time evolution of the O 2p photoelectron intensity, display a transition from a Langmuir-Hinshelwood (LH) curve to a more slowly increasing sigmoidal curve with increasing T and decreasing P O2 , respectively.This transition in the uptake curve indicates a change in the mode of oxide growth from Langmuir-type adsorption to 2D oxide island growth.The difference between the two modes was analyzed by fits to the oxygen uptake curves based on the ACR model [7], as given in Eq. (1).
where κ = (θ 0 + 1)/θ 0 , τ θ 0 is a measure of the incubation time for the nucleation of oxide clusters, and θ 0 defines the oxide coverage at which the 2D island growth commences.In this case, Eq. ( 1) shows an LH form for θ ≫ 1, while The saturation levels of the oxygen uptake curves in Fig. 1(a) increase slightly with increasing T in the LH form region but begin to decrease in the sigmoidal region.Hence, less oxygen remained on the surface during the initial oxide growth in the sigmoidal mode, which is responsible for the occurrence of SiO desorption, this growth mode is generally called 2D oxide island growth.However, as shown in Fig. 1(b), the saturation levels of the oxygen uptake curves rose slightly from the sigmoidal region to the LH region by increasing P O2 , which suggests a low oxide growth rate in the sigmoidal mode but without SiO desorption at 600 • C. Hence, at high T and low P O2 , no 2D oxide island growth is observed despite the sigmoidal form of the oxygen uptake curve, in discord with the results on Si(001) in the previous work [6,7].To describe the time evolutions of SiO desorption in the Langmuir-type adsorption region, the oxygen uptake curves for oxidation with interrupted O 2 supply were fitted to a sigmoid curve as follows.
The fits based on Eq. ( 2) (pink solid lines in Figs.2(b)-(d)) show excellent agreement with the experiment so that the oxidation kinetics model in the Langmuir-type adsorption region is extended to include both the oxide growth and decomposition according to The first term in Eq. ( 3) is the conventional normalized Langmuir-type adsorption formulation, and the latter term is the normalized analytical solution of Eq. ( 2).The terms 1/τ g and 1/τ d denote the growth and decomposition rate, respectively.The competition coefficient k c measures the competition between the oxide growth and decomposition during the initial oxidation, which is considered to be temperature-dependent.
As depicted in Fig. 3, the oxygen uptake curve falls more drastically after interrupting O 2 supply at θ oxide of approximately 50% compares to when θ oxide was approximately 90%.Because the thermal stability of the oxygen adsorption species is expected to affect the SiO desorption during the initial oxidation, the oxygen adsorption species formed at θ oxide of approximately 50% are probably less stable than those formed at θ oxide of approximately 90%, which accelerates the oxide decomposition.
Next, the dependence of the SiO desorption kinetics on P O2 during the initial oxide growth in the transition region is investigated by comparing the oxygen uptake curves as a function of O 2 supply mode, as shown in Fig. 4. Because the oxygen uptake curves for the interrupted O 2 supply at 600 • C cannot be fitted well to Eq. ( 2), the mechanism of SiO desorption at 600 • C is likely different from that at ≥610 • C, as proposed above.To elucidate the competition between the oxide growth and decomposition, the oxygen uptake curves with interrupted O 2 supply were fitted to Eq. (4).
A 0 reflects the competition between the two processes for the initial oxidation at 600 • C. As already known, oxide growth and decomposition dominate for A 0 < 0 and A 0 > 0, respectively.According to the analyzed A 0 in Fig. 4 and the comparison of the oxygen uptake curves shown in Fig. 5 is convex upward for θ oxide > 50% and convex downward for θ oxide < 50%.This result indicates that A 0 changed from positive to negative at the critical θ oxide value of 50%.Hence, the competition between the oxide growth and decomposition exists at the critical θ oxide of approximately 50% at 600 • C. The P O2 dependence of the amount of oxygen desorbed from the surface until 1000 s after interrupting the O 2 supply is shown in Fig. 5(b).With increasing P O2 , the oxide growth mode changes from sigmoid type to LH type, but oxygen desorbed more drastically below 3.5×10 −4 Pa.This result agrees well with the suggestion given above that oxidation at high T and low P O2 does not correspond to 2D oxide island growth.The change of the O 2p intensity after interrupting the O 2 supply ∆I O2p exhibits its maximum at P O2 = 1.5×10 −4 Pa and 3.5×10 −4 Pa, thus indicating that the oxygen adsorption species formed at pressure between 1.5×10 −4 Pa and 3.5×10 −4 Pa are more stable than those formed outside this pressure range at θ oxide ∼ 50% and 600 • C.
Table I displays the calculated adsorption energies of various absorbed oxygen adsorption species, thereby revealing their energetic stability at different stages during the initial oxide growth on the Si(111)7×7 surface.The adsorption energy of tri-ins×2 (13.37 eV) is lower than those of ad-ins×2 (13.85 eV) and ins×3 (14.37 eV), and reveals that the tri-ins×2 species is the transition state for transitions from ad-ins×2 or ins×3 to tri-ins×3.In addition, the tri-ins×3 adsorption species, whose adsorption energy is 18.79 eV, is thermally most stable during the initial oxide growth in the transition region on Si( 111

IV. CONCLUSIONS
The reaction mechanism of the initial oxidation for transition from Langmuir-type adsorption to 2D oxide island growth has been investigated using real-time photoelectron spectroscopy and DFT calculations.At high T and low P O2 , oxidation did not proceed via 2D oxide island growth despite sigmoidal oxygen uptake curves.The SiO desorption is found to depend on temperature and θ oxide during the oxidation in the transition region.Fur-ther, a model is proposed to explain the competition between the oxide growth and decomposition in the transition region on Si(111)7×7.Based on the P O2 dependent experimental results and adsorption energies of oxygen adsorption states calculated by DFT, the oxygen adsorption species tri-ins×2 is suggested as the transition state on Si(111)7×7, which facilitates the SiO desorption.In contrast, the oxygen adsorption species tri-ins×3 formed during the initial oxide growth, is considered to be the most thermally stable species, and prevents SiO desorption in the transition region.

Figure 2
compares the oxygen uptake curves during the continuous oxidation and oxidation with O 2 supply interrupted at the same oxide coverage with θ oxide = 50% at temperatures ranging from 590 to 650 • C. According to Fig. 1(c), the uptake curves between 590 • C and 650 • C belong to the LH-type curve, indicating that the oxide growth mode shows the Langmuir-type adsorption.As seen from Fig. 2, the amount of oxygen adsorbed on the surface after interrupting the O 2 supply rapidly fell at temperatures above 610 • C. In particular, no oxygen remained on the surface at 8000 s, 4000 s and 2000 s after interrupting the O 2 supply at 610 • C, 630 • C and 650 • C, respectively.These findings suggest that SiO desorption becomes energetically favorable with increasing temperature during the initial oxide growth in the transition region.This suggestion questions the validity of the Langmuir-type adsorption model for the growth mode in the temperature range 610-650 • C.
FIG. 2: Comparison of oxygen uptake curves between continuous oxidation and oxidation with O2 supply interrupted at an oxide coverage θ oxide of approximately 50% and at of (a) 590 • C, (b) 610 • C, (c) 630 • C, and (d) 650 • C in the Langmuirtype adsorption region.Curves of oxygen uptake with interrupted O2 supply were fitted with Eq. (2) (pink solid lines).
FIG. 5: (a) Comparison of oxygen uptake curves for oxidation with O2 supply interrupted at various PO 2 at 600 • C. (b) Change of O 2p intensity after stopping O2 supply ∆IO2p as a function of PO 2 .

FIG. 6 :
FIG. 6: (a) Curve-fitting results of Si 2p spectrum at an oxidation time of 1218 s at 600 • C and 3.5×10 −4 Pa.Open circles denote experimental results, and solid lines show the fits.(b) Intensities of oxidation states Si 1+ , Si 2+ , Si 3+ , Si 4+ from Si 2p spectra as a function of oxide coverage.