Real-time photoemission spectroscopy for the analysis in situ of Si (001) oxidation induced by supersonic O 2 molecular beams The analysis of chemical phenomena occurring at the gas-solid interface is a fundamental step in research to understand and control surface chemical reactions. In particular, it is considerably essential to realize and control surface chemical reactions with respect to the recent development of nanotechnologies which require the atomicscale manipulation in terms of device sizes and chemical compositions. Photoemission spectroscopy combined with soft X-ray synchrotron radiation, which offers the advantages of energy resolution, intensity, beam siz e and wide energy regions, is a powerful experimental technique for observing the chemical composition and chemical bonds on solid surfaces. Furthermore, we can in situ monitor the time-evolution of chemical species on the solid surface in the core-level photoemission spectra during chemical reactions. H ere, we demonstrate the advantages of real-time in situ photoemission spectroscopy combined with a high-performance soft X-ray synchrotron radiation source for the monitoring of chemical reactions on solid surfaces. The real-time monitoring of oxidation on the S i (001) surface induced by translational k inetic energy of O 2 at room temperature is reported. This reaction system is an important sub j ect, not only for the semiconductor industry, but also in terms of the reaction dynamics in surface science [1 - 4] . The SUREAC2000 [5] , especially designed for the analysis of surface chemical reactions at beamline BL23SU , was used for all experiments. A n important advantage of the SUREAC2000 is that in situ photoemission measurements with synchrotron radiation can be carried out under supersonic molecular beam (SSMB) irradiation on solid surfaces. The photon energy resolution was better than 100 me V and the photon flux was on the order of 10 11 photons · s - 1 . Photoelectrons were collected with a 125 mm hemispherical electron energy analyzer (O micron N ano Technology, G mb H ) with five channeltrons. The excitation energies for the O - 1 s and S i- 2 p core levels were 830 e V and 409 e V , respectively. The analysis depth from the surface was roughly estimated to be 0 . 7 nm. The O 2 beam with the k inetic energy of 2 . 2 e V was exposed to the n-type S i (001) substrate at 9 degrees from the surface normal at room temperature. Figure 1 shows r epresentative high -energy- resolution S i- 2 p photoemission spectra obtained by r e a l - t i m e i n s i t u m e a s u r e m e n t s t a k e n d u r i n g irradiation of the O 2 SSMB . The O 2 dose is indicated in L angmuir units (1L = 1 . 3 × 10 4 Pa • s ) . It too k approximately 40 s to obtain an S i- 2 p and an O - 1 s photoemission spectrum. Pea k deconvolution was performed to obtain the components related to S i oxidation by the curve fitting procedure. The spin- orbit splitting between S i 2 p 1/2 and S i 2 p 3/2 was set to be 0 . 6 e V . Prior to the curve fitting, the bac k grounds 6 5 4 3 2 1 0 - 1 - 2 Relative Electron Binding Energy (eV) Normalized Photoemission Intensity (arb. units) 6 5 4 3 2 1 0 - 1 - 2 6 5 4 3 2 1 0 - 1 - 2 6 5 4 3 2 1 0 - 1 - 2 (a) 1376 L (b) 688 L (c) 172 L (d) 34.4 L Si 4+ Si 3+ Si 2+ Si 1+ Si 0 Fig. 1. Representative high-resolution Si-2 p photoemission spectra for the real-time in situ observation of oxidation on a Si(001)- 2 × 1 surface induced by O 2 molecular beams at room temperature. 67 Fig. 2. Variation of (a) oxide thickness, (b) the O-1 s photoemission intensity and (c) - (f) each Si-2 p oxide component area intensity as a function of O 2 irradiation dose. Intensities for the Si-2 p oxide components were normalized by the oxide thickness. References [1] A. Yoshigoe and Y. Teraoka: Appl. Surf. Sci. 190 (2002) 60. [2] A. Yoshigoe and Y. Teraoka: Surf. Sci. 532-535 (2003) 690. [3] A. Yoshigoe and Y. Teraoka: Surf. Interface Anal. 34 (2002) 432. [4] A. Yoshigoe, K. Moritani and Y. Teraoka: Jpn. J. App. Phys. 42 (2003) 4676. [5] Y. Teraoka et al. : Jpn. J. Appl. Phys. 38 Suppl. 3 8- 1 (1999) 642 ; Appl. Surf. Sci. 169-170 (2001) 73 8 . in the photoe m ission spectru m w ere nu m erically e l i m i n a t e d u s i n g t h e T o u g a a r d m e t h o d . T h e photoe m ission intensity ratio (Si2 p 1 / 2 / Si2 p 3 / 2 ) w as assu m ed to b e 0.5 according to their statistical w eights. C ore le v el shifts related to the Si o x idation w ere esti m ated to b e 1.00 (Si 1 + ), 1. 8 2 (Si 2 + ), 2.62 (S 3 + ) and 3.67 (Si 4 + ) e V , w ith respect to the b inding energy of the b ulk Si - 2 p 3 / 2 peak position. F urther m ore, α (0.23e V ) and β ( - 0.26e V ) w ere due to the slight distortion in the b ond angles at the interface Si ato m s. W ith increasing m olecular b ea m e x posure, the o x ide co m ponents gradually increased w ithout shifts of their peak positions. F igure 2 sho w s the ti m e e v olution of (a) the o x ide thickness, ( b ) the o x ygen a m ount and (c) - (f) the intensities of each o x ide co m ponent ( Si 4 + , Si 3 + , Si 2 + and Si 1 + ) nor m ali z ed b y the o x ide thickness. As sho w n in F igs. 2 (a) and ( b ), the o x ide thickness and the O- 1 s photoe m ission intensity drastically increased w ith irradiation doses less than 50 L and then gradually increased. The saturated thickness of the o x ide layers w as esti m ated to b e appro x i m ately 0.57 n m fro m the b ack b ond o x idation of the su b surface Si layers. Ti m e e v olutions of the su b o x ide co m ponents are illustrated as a function of the O 2 dose in F igs. 2 (c) to (f). Since the Si 4 + species is not o b ser v ed at the early stage of m olecular b ea m irradiation, up to 34.4 L , the top m ost Si di m er ato m s are hardly surrounded b y four o x ygen ato m s in the initial o x idation stage. As sho w n in F ig. 2 (d) and (f), the Si 1 + and the Si 3 + co m ponents gradually decrease w ith increasing the o x ide thickness. This result indicates that the nu mb er of Si 1 + and Si 3 + ato m s per unit o x ide thickness is nearly constant , regardless of the o x ide gro w th. The Si 1 + species re m ain at the interface b et w een the o x ide layers and Si(001) su b strate, w hereas the Si 3 + species are distri b uted o v er all o x ide layers. As sho w n in F ig. 2(e), the a m ount of Si 2 + species decreased w ith increasing a m ounts of Si 4 + , w hereas the a m ounts of Si 1 + and Si 3 + w ere v irtually unchanged. This result indicates that the Si 4 + species w as directly con v erted fro m the Si 2 + species. U sing real - ti m e photoe m ission spectroscopy, w e clarified the reaction m echanis m s of o x idation on a Si(001) surface induced b y the incident energy of O 2 at roo m te m perature. In conclusion, w e succeeded in sho w ing that real - ti m e photoe m ission spectroscopy using synchrotron radiation is a po w erful m ethod for in v estigating the surface che m ical reactions fro m the v ie w points of kinetics and dyna m ics. Akitaka Yoshigoe and Yuden Teraoka SPring -8 / JA ER I E-m ail: yoshigoe @ spring 8 .or. j p O 2 dose (Langmuir) (a) Thickness of oxidized layer (b) O - 1 s (c) Si 4+ (d) Si 3+ (e) Si 2+ (f) Si 1+ 0.8 0.6 0.4 0.2 0.0 2000 1000 0 0.4 0.2 0.0 0.4 0.2 0.0 0.4 0.2 0.0 0.4 0.2 0.0 0.57 nm 0.3 nm 0 200 400 600 800 1000 1200 1400 Normalized Intensity by Oxide Thickness (arb. units) Photoemission Intensity (a. u.) Oxide Thickness (nm) 68
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