Takuhiro Kakiuchi

Recent progress in coincidence studies on ion desorption induced by core excitation

This paper reports on recent studies of photostimulated ion desorption (PSID) using electron ion coincidence (EICO) spectroscopy combined with synchrotron radiation. H+ desorption from H2O dissociatively adsorbed on Si(111) and SiO2/Si(111) surfaces (H2O/Si(111) and H2O/SiO2/Si(111)) was studied for Si L-edge excitation. The Si 2p–H+ photoelectron photoion coincidence (PEPICO) and Si 2p photoelectron spectra of H2O/Si(111) and H2O/SiO2/Si(111) show that H+ desorption probability increases as the number of positive charges at the Si site increases. The H+ desorption probability per Si 2p ionization for the Si4+ site was estimated and found to be 5–7 × 10−5. We proposed a mechanism that H+ desorption is induced by Si 2p photoionization accompanied by a Si LVVV double-Auger transition. This article also reviews recent EICO work on site-specific ion desorption of 1,1,1-trifluoro-2-propanol-d1 (CF3CD(OH)CH3) adsorbed on Si(100) surfaces, and on the mechanisms of PSID of poly(tetrafluoroethylene) (PTFE) and TiO2(110). Clear site-specific ion desorption was observed for the C 1s core excitation of a CF3CD(OH)CH3 sub-monolayer on Si(100). A spectator-Auger-stimulated ion-desorption mechanism was proposed for F+ desorption induced by a transition from F 1s to σ(C–F)* of PTFE. O+ desorption induced by O 1s excitation of TiO2(110) was attributed mainly to three-hole final states resulting from multi-electron excitation/decay. For O+ desorption induced by Ti core excitation of TiO2(110), on the other hand, charge transfer from an O 2p orbital to a Ti 3d orbital, instead of the interatomic Auger transition, was proposed to be responsible for the desorption. These investigations demonstrate that EICO spectroscopy combined with synchrotron radiation is a useful tool for studying PSID.

Surface-Site-Selective Study of Valence Electronic Structures of Clean Si(100)-2×1 Using Si-L23VV Auger Electron–Si-2p Photoelectron Coincidence Spectroscopy

Valence electronic structures of a clean Si(100)-2×1 surface are investigated in a surface-site-selective way using Si- L 23 VV Auger electron–Si-2 p photoelectron coincidence spectroscopy. The Si- L 23 VV Auger electron spectra measured in coincidence with Si-2 p photoelectrons emitted from the Si up-atoms or Si 2nd-layer of Si(100)-2×1 suggest that the position where the highest density of valence electronic states located in the vicinity of the Si up-atoms is shifted by 0.8 eV towards lower binding energy relative to that in the vicinity of the Si 2nd-layer. Furthermore, the valence band maximum in the vicinity of the Si up-atoms is indicated to be shifted by 0.1 eV towards lower binding energy relative to that in the vicinity of the Si 2nd-layer. These results are direct evidence of the transfer of negative charge from the Si 2nd-layer to the Si up-atoms.

Ion Desorption from Single-Walled Carbon Nanotubes Induced by Soft X-ray Illumination

We have investigated ion desorption from single-walled carbon nanotubes (SWNTs) that is induced by soft X-ray illumination in the C 1s core-excitation energy range in order to have insight to the mechanism of defect formation caused by the similar excitation. The mass of desorbed ions was analyzed by a time-of-flight (TOF) spectrometer and the photon energy dependence of the ion yield or the desorption efficiency spectra were measured as a function of the monochromatic photon energy used for illumination. Experimental results exclude the simple detachment of carbon atoms constituting the nanotubes from the cause of the defect formation. Also the photo-induced etching of carbon atoms associated with C–H bond rupture is ruled out from the defect formation mechanism. Auger mechanisms to explain the photo-induced desorption are discussed.

Local Valence Electronic States of SiO2 Ultrathin Films Grown on Si(100) Studied Using Auger Photoelectron Coincidence Spectroscopy: Observation of Upward Shift of Valence-Band Maximum as a Function of SiO2 Thickness

The local valence electronic states of the surface, interface, and substrate for SiO 2 ultrathin films thermally grown on a Si(100)-2×1 have been investigated using Si- L 23 V V Auger-electron Si n + -2 p photoelectron coincidence spectroscopy ( n represents the number of oxygen atoms bonded to the Si). A series of Si- L 23 V V Auger electron spectra (AES) measured in coincidence with Si n + -2 p photoelectron indicate that the valence electronic states in the vicinity of the Si n + sites shift to the deeper binding-energy side as n increases. Furthermore, the Si 4+ - L 23 V V AES measured as a function of the thickness of the SiO 2 , show that the valence-band maximum of SiO 2 shifts ∼1.6 eV toward the Fermi level when the thickness of the SiO 2 film is decreased to 1.7–1.5 A. This shift is attributed to a decrease in the number of Si 4+ and an increase in the number of Si 3+ , Si 2+ , Si 1+ , and Si 0 in the vicinity of the topmost SiO 2 layer.

Decay Processes of Si 2s Core Holes in Si(111)-7 × 7 Revealed by Si Auger Electron Si 2s Photoelectron Coincidence Measurements

Decay processes of Si 2s core holes in a clean Si(111)-7 × 7 surface are investigated using coincidence measurements of Si Auger electrons and Si 2s photoelectrons at a photon energy of 180 eV. We show that Si 2s core holes exhibit two nonradiative decay processes: the first being a Si L1L23V Coster–Kronig transition followed by delocalization of the valence hole and Si L23VV Auger decay, and the second being Si L1VV Auger decay. The branching ratio of the Si L1L23V Coster–Kronig transition to the Si L1VV Auger decay is estimated to be 96.7% ± 0.4% to 3.2% ± 0.4%.

Study of Local Valence Electronic States of SiO2 Ultrathin Films Grown on Si(111) by Using Auger Photoelectron Coincidence Spectroscopy: Upward Shift of Valence-Band Maximum Depending on the Interface Structure

To clarify the factors governing the local valence electronic states of SiO 2 ultrathin films, we have measured the Si- L 23 V V –Si n + -2 p Auger-electron photoelectron coincidence spectra ( n = 0, 1, 2, 3, 4, where n represents the number of oxygen atoms bonded to Si) of SiO 2 thermally grown on a Si(111)-7×7 surface [SiO 2 /Si(111)]. The results indicate that the valence electronic states in the vicinity of the Si n + sites shift to deeper binding energies as n increases. Furthermore, Si 4+ - L 23 V V Auger electron spectra, measured as a function of SiO 2 thickness, taken in coincidence with Si 4+ -2 p photoelectron emission show that the valence-band maximum (VBM) of the SiO 2 layer shifts by 2.7 ±1.0 eV toward the Fermi level when SiO 2 thickness is decreased to ≈1 monolayer (ML). This upward shift is much larger than that for a SiO 2 layer with a thickness of ≈1 ML thermally grown on Si(100)-2×1 (about 1.6 eV). We attribute the large shift in the VBM of SiO 2 /Si(111) with a 1-ML-thick SiO 2 layer...

Site-Specific Electron-Relaxation Caused by Si:2p Core-Level Photoionization: Comparison between F3SiCH2CH2Si(CH3)3 and Cl3SiCH2CH2Si(CH3)3 Vapors by Means of Photoelectron Auger Electron Coincidence Spectroscopy

Site-specific electron relaxations caused by Si:2p core-level photoionizations in F3SiCH2CH2Si(CH3)3 and Cl3SiCH2CH2Si(CH3)3 vapors have been studied by means of the photoelectron Auger electron coincidence spectroscopy. F3SiCH2CH2Si(CH3)3 shows almost 100% site-specificity in fragmentation caused by the Si:2p ionization. However, substitution of Cl for F of F3SiCH2CH2Si(CH3)3 considerably reduces the site-specificity at the Si atom bonded to three halogen atoms, with the site-specificity at the Si site bonded to three methyl groups remaining largely unchanged. The site-specificity reduction in Cl3SiCH2CH2Si(CH3)3 is considered to take place during the transient period between Si:L23VV Auger electron emission and the subsequent fragmentation. The reason for the reduction can be explained in terms of some differences between these two molecules in the L23VV Auger decay at the Si site bonded to the three halogen atoms.

Local Valence Electronic States and Valence-Band Maximum of Ultrathin Silicon Nitride Films on Si(111) Studied by Auger Photoelectron Coincidence Spectroscopy: Thickness and Interface Structure Dependence

The local valence electronic states of Si3N4 films grown on Si(111)-7 × 7 [Si3N4/Si(111)] have been investigated by coincidence spectroscopy. The Si L23VV Auger electron spectra (AES) measured in coincidence with the Sin+ 2p photoelectrons of β-Si3N4(0001)/Si(111)-8 × 8 indicate that the binding energy of the local valence electronic state at Sin+ increases as n increases. Si4+ L23VV AES measured as a function of the β-Si3N4(0001) thickness show that the binding energy at the valence-band maximum (BEVBM) of β-Si3N4(0001) decreases by 1.6 ± 0.5 eV as the Si3N4 thickness decreases from 3.6 to 0.7 A. The large decrease is attributed to the hybridization of the valence electronic state of Si3N4 with those of neighboring subnitrides and to the formation of β-Si3N4(0001) islands. The BEVBM value of the 3.6-A-Si3N4/Si(111)-quadruplet decreases by 0.7 ± 0.6 eV from that of 3.6-A-β-Si3N4(0001)/Si(111)-8 × 8. The decrease in BEVBM is attributed to the different interface structures.

Auger electron spectra of hydrogenated Si(111)-1×1 surface obtained from Si-L23VV Auger electron Si-2p photoelectron coincidence measurements

The Si-L23VV Auger electron Si-2p photoelectron coincidence spectra (Si-L23VV-Si-2p APECS) of a hydrogenated Si(111)-1×1 surface are measured with an analyzer that is used for electron-electron and electron-ion coincidence measurements of surfaces. The Si-L23VV-Si-2p APECS show a sharp peak, a small shoulder, and a broad large peak at electron kinetic energies of about 85, about 80.5, and about 70 eV, respectively. The clarity of the last two peaks is lower in the Si-L23VV-Si-2p APECS of a clean Si(111)-7×7 surface. We attribute the three peaks to the Si-L23V3pV3p, Si-L23V3pV3s, and Si-L23V3sV3s Auger lines, respectively. The three-peak structure indicates that the local density of states of the 3s + 3p band is reduced in the case of the hydrogenated Si(111)-1×1 surface. This finding is consistent with that of a previous study on valence electronic states of hydrogenated Si(111) using ultraviolet photoemission spectroscopy.