Photon Factory Activity Report 2005 #23Part B (2006)
Surface and Interface

11D/2004G026

Photostimulated ion desorption from H2O dissociatively adsorbed on Si(111)
studied using photoelectron photoion coincidence spectroscopy [1]Eiichi KOBAYASHI1,2)*, Akira NAMBU1), Junya Seo3), Kazuhiko MASE1, 4)
1)
Institute of Materials Structure Science, 1-1 Oho, Tsukuba 305-0801, Japan
2)
Inoue Foundation for Science, 15-15 Nanpeidaicho, Shibuya-ku, Tokyo 150-0036, Japan
3)
Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
4)
PRESTO, JST, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
Introduction
When a core-level of a surface molecule is ionized, anion desorption may be stimulated by an Auger process
leaving multiple valence holes (Auger stimulated ion
desorption (ASID)). Since core-level photoelectron
spectroscopy can distinguish two atomic sites whose
chemical shifts are different, the coincidence
measurement of photoelectrons and photoions(photoelectron photoion coincidence, PEPICO) is an ideal
technique to study ion desorption at a specific site [2].
Furthermore, site-specific ion desorption is a potential
tool for analyzing geometry and properties at specificsurface sites. We selected water (H2O) adsorbed on
Si(111) (H2O/Si(111)) as a target, because it is importantfor fundamental science, and for understanding themechanisms of wet etching in semiconductor device
processing. Tanaka et al. have investigated ASID+
mechanism of H desorbed from H2O/Si(100) in the O Kedge region using PEPICO and Auger electron photoion
coincidence (AEPICO) spectroscopy, and concluded that+n-valence-hole states (n > 2) are responsible for Hdesorption [3]. In the present paper we report a study of+
H desorption from H2O/Si(111) in the Si L-edge region
using an improved electron ion coincidence (EICO)
analyzer.

where CPEPICO and CPE denote Si 2p – H+ PEPICO, and thephotoelectron counts measured with the EICO analyzer
simultaneously at a specific photoelectron kinetic energy.
The latter is the same as the number of the trigger for theMCS during the coincidence measurement. RSi2p is theratio of the Si 2p photoelectron counts to the electron
counts at a specific electron kinetic energy. EPI is thephotoion detection efficiency of the TOF-MS, given by
the following equation:

E PI = (TMesh ) 3 × E MCP ,where TMesh is the transmittance of one mesh in the TOFMS (TMesh = 0.77) and EMCP is the photoion detection
efficiency of the MCPs (EMCP = 0.6). The values of PD
were estimated to be <4 × 10-6, 5-7 × 10-6 and 2-4 × 10-5
for the Si0, Si+ and Si2+ sites, respectively. We assigned the+2++Si and Si sites responsible to the H desorption to theSi-OH and Si(O)-OH sites, respectively.
+We also carried out Si 2p – H PEPICO measurementsof Si(111) exposed to H2O of about 5 L, on which Si-OH+and Si-H sites exist. The H PEPICO signal, however,was below the detection limit of our apparatus (PD < 4 ×-6+10 ). The results show that H desorption probability isquite small at the Si-OH and Si-H sites.

Experimental-6
Clean Si(111) was exposed to 1000 L (1 L = 1 ×10Torr s) of water at room temperature. H2O is known to bedissociatively adsorbed on Si(111) forming Si-OH and SiH surface species. Synchrotron radiation at hν = 130 eV
was used for photoelectron and PEPICO spectroscopy.
Results and Discussion+Figure 1 shows the Si 2p - H PEPICO data and Si 2p
photoelectron spectrum of a H2O/Si(111) surface at hν =130 eV. The accumulation time was 3900 sec for each+
PEPICO data. The H PEPICO peak was observed at a
relative binding energy of +1.5 eV, corresponding to the+2++Si and Si sites. We estimated the H desorption
probability per Si 2p ionization (PD) by the followingequation:

PD =C PEPICO,C PE × RSi 2 p × E PI

Fig. 1. Si 2p - H+ PEPICO (filled circles) and Si 2p
photoelectron (solid line) spectra of H2O/Si(111) at hν =130 eV. Both spectra are normalized by the ion counts
simultaneously measured with the EICO analyzer
References[1] E. Kobayashi et al., J. Phys.: Condens. Matter in press.
[2] E. Kobayashi et al., Surf. Sci. 528 (2003) 255.
[3] S. Tanaka et al., J. Chem. Phys. 117 (2002) 4479.
*eiichi.kobayashi@aist.go.jp

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