Akitaka Yoshigoe

Commissioning of surface chemistry end-station in BL23SU of SPring-8

An experimental apparatus for surface reaction research is now under commissioning as an end-station of a soft X-ray beamline, BL23SU, in the SPring-8. A supersonic molecular beam (SSMB) technique is applied to control the translational-energy of incident molecules. Owing to keeping low pressure in vacuum chambers during molecular beam operation by differential-pumping systems, the suppression of gas diffusion into the beamline was sufficiently achieved. The translational-energy control and photoemission spectroscopic performance were checked by preliminary experiments concerning the silicon oxidation using a conventional X-ray source. The photoemission intensity from chemisorbed oxygen atoms on an Si(1 0 0) surface was enhanced with increasing the translational-energy of incident oxygen molecules up to 3 eV.

Actively controlled oxidation of Cu{100} with hyperthermal O2 molecular beam

The oxidation of Cu{100} with a hyperthermal O2 molecular beam (HOMB) was investigated using x-ray photoemission spectroscopy in conjunction with a synchrotron light source. The efficiency of oxidation with HOMB is higher than that with ambient thermal O2. Further oxidation under oxygen coverage (Θ)⩾0.5u2009ML occurs rather inefficiently even for the 2.3-eV-HOMB irradiation. We found that such slow oxidation of Cu corresponding to the initial stage of the Cu2O formation can be interpreted in terms of a collision-induced-absorption mechanism. The kinetics of the dissociative adsorption under Θ⩽0.5u2009ML is well described using the first-order kinetics in a simple Langmuir-type adsorption model.

Real-time monitoring of oxidation on the Ti(0 0 0 1) surface by synchrotron radiation photoelectron spectroscopy and RHEED–AES

The oxidation kinetics on the Ti(0 0 0 1) single crystal surface was investigated by real-time Auger electron spectroscopy combined with reflection high-energy electron diffraction (RHEED-AES) and high-resolution photoelectron spectroscopy using synchrotron radiation. The RHEED specular reflection spot intensity showed an oscillatory behavior, at the maxima and minima of which the oxygen uptake curve obtained by O KLL Auger electron intensity shows breaks. This agreement indicates that growth of oxides progresses with changing the surface morphology and the oxygen uptake rate. The O 2 dose dependence of Ti 2p photoelectron spectrum measured at photon energies of 574.6 eV (surface sensitive) and 1549 eV (bulk sensitive) revealed that (1) oxidation of metallic Ti atoms with growing Ti 2 O and TiO on the surface makes the surface morphology roughened up to ∼17 L; (2) the roughened morphology is recovered by oxidation of Ti 2 O and TiO into Ti 2 O 3 , Ti 3 O 5 and TiO 2 up to ∼45 L; (3) after almost saturation of TiO 2 growth in the surface layer the surface morphology is roughened again by further oxidation at the interface up to ∼140 L; and (4) then the oxygen uptake rate is considerably suppressed due to the passivation effect of grown oxides.

Si(001) Surface Layer-by-Layer Oxidation Studied by Real-Time Photoelectron Spectroscopy using Synchrotron Radiation

High-resolution O 1s and Si 2p photoelectron spectroscopy using synchrotron radiation was employed to clarify a layer-by-layer oxidation reaction mechanism on a Si(001) surface from the viewpoint of point defect generation due to an oxidation-induced strain at a SiO2/Si interface. The Siβ and Siα components in Si 2p3/2 spectra, which are assigned to the first and second strained Si layers, respectively, below the transition layer composed of suboxides, Si1+, Si2+, and Si3+, significantly decrease during the step-by-step temperature increase-enhanced growth of the second oxide layer. Because of the corresponding band bending changes measured using the O 1s peak position, which are caused by defect-related band gap states, the observed decreases in Siβ and Siα components, indicating a decrease in interfacial strain, are induced not only by the structural relaxation of a SiO2 network due to a thermal annealing effect, but also due to the generation of point defects at the SiO2/Si interface. Continuous band bending changes with the growth of the third oxide layer also suggest that the point defects are generated during oxide growth, whereas the Siβ and Siα components are maintained almost constant. On the basis of the observed interfacial strain and point defect generation changes, the layer-by-layer growth kinetics of the first, second and third oxide layers is discussed using a unified Si oxidation reaction model mediated by point defect generation at the SiO2/Si interface [S. Ogawa and Y. Takakuwa: Jpn. J. Appl. Phys. 45 (2006) 7063].

Characterization of interface defects related to negative-bias temperature instability in ultrathin plasma-nitrided SiON/Si systems

Abstract Interface defects related to negative-bias temperature instability (NBTI) in an ultrathin plasma-nitrided SiON/Si〈1xa00xa00〉 system were characterized by using conductance–frequency measurements, electron-spin resonance measurements, and synchrotron radiation X-ray photoelectron spectroscopy. It was confirmed that NBTI is reduced by using D 2 -annealing instead of the usual H 2 -annealing. Interfacial Si dangling bonds (P b1 and P b0 centers) were detected in a sample subjected to negative-bias temperature stress (NBTS). Although we suggest that NBTS also generates non-P b defects, it does not seem to generate nitrogen dangling bonds. These results show that NBTI of the plasma-nitrided SiON/Si system is predominantly due to P b depassivation. Plasma nitridation was also found to increase the P b1 /P b0 density ratio, modify the P b1 defect structure, and increase the latent interface trap density by generating Si suboxides at the interface. These changes are likely to be the causes of NBTI in ultrathin plasma-nitrided SiON/Si systems.

Photoemission spectroscopic study on influence of O2 translational kinetic energy for Si(001) initial oxidation

Abstract The influence of translational kinetic energy of incident O 2 molecules for the passive oxidation of clean Si(0xa00xa01) surfaces at room temperature has been studied by photoemission spectroscopy using high-resolution synchrotron radiation. The O 2 incident energy was controlled up to about 3 eV via supersonic seed molecular beam techniques. Predicted two potential energy barriers for the O 2 dissociative chemisorption were indistinct. Photoemission spectra of Si-2p and O-1s as a function of incident energy revealed the incident energy induced oxidation, in which the oxide layers thickness was controlled to the extent of 0.6 nm.

Time resolved photoemission spectroscopy on Si(001)-2 × 1 surface during oxidation controlled by translational kinetic energy of O2 at room temperature

Abstract We have experimentally studied the dissociative adsorption of O 2 on a clean Si(0xa00xa01)-2xa0×xa01 surface at room temperature from the viewpoint of time resolved photoemission spectroscopy for surface chemical reactions induced by the O 2 incident energy. Combining the high-resolution and intense synchrotron radiation with the supersonic molecular beams technique, we have succeeded in the analysis of the time evolution of Si oxidation states (Si 1+ , Si 2+ , Si 3+ and Si 4+ ) induced by the translational kinetic energy of O 2 ( Et O 2 =0.04, 0.6, 2.0 and 3.0 eV). Furthermore, we proposed that the oxidation on a clean Si(0xa00xa01)-2xa0×xa01 surface progressed in the mechanism of almost consecutive Si oxidation in the low translational kinetic energy region. It was found that the translational kinetic energy of O 2 induced the adsorption of oxygen atom at the bridge site between the Si dimer and at the backbonds of the Si dimers and the subsurface.

Real-Time Observation of Initial Thermal Oxidation on Si(110)-16×2 Surfaces by O 1s Photoemission Spectroscopy Using Synchrotron Radiation

The initial oxidation on a Si(110)-16×2 surface at room temperature and 540 °C has been investigated by real-time X-ray photoemission spectroscopy (O 1s) using 687 eV photons. At both temperatures, the initial oxidation of Si(110) is characterized by its unique rapid oxidation regime immediately after the introduction of oxygen molecules. O 1s spectra are shown to consist of at least four oxidation states. It is likely that oxidation at or around the adatoms of pentagon pairs, reportedly present on the Si(110)-16×2 reconstructed surface, is the predominant process in the very early stage of oxidation.

Oxidation of Si(001) with a hyperthermal O-atom beam at room temperature: Suboxide distribution and residual order structure

Synchrotron radiation photoelectron spectroscopy (SR-PES) and crystal truncation rod (CTR) scattering profiles were used to investigate an ultrathin SiO2 overlayer on a Si(001) surface formed by a 5eV O-atom beam at room temperature. The SR-PES spectra indicated that the suboxides in the O-atom-beam oxidized film were concentrated on the SiO2 surface rather than at the Si∕SiO2 interface. The CTR scattering data of the O-atom-beam oxidation film had a lower intensity near (11L) (0.3<L<0.8), suggesting a lower content of the SiO2 ordered structure in the oxide film. An inverse diffusion of the interstitial Si atoms in the oxidation kinetics can explain the data.

Time-Resolved Photoelectron Spectroscopy of Oxidation on the Ti(0001) Surface

High-resolution photoelectron spectroscopy using synchrotron radiation was applied for monitoring in real time the oxidation kinetics on the Ti(0001) surface at 405 C with dry O{sub 2} gas. The time evolution of O 1s photoelectron intensity showed a linear uptake curve up to {approx}90 L followed by a sudden saturation up to {approx}160 L and then a restart of the linear increase, indicating that O{sub 2} adsorption obeys a zero-order reaction scheme before and after the saturation. Corresponding to the first linear uptake and saturation, the surface core level shift (SCLS) component of Ti 2p decreased predominantly and disappeared completely, and appeared again after the saturation and remained persistently during TiO{sub 2} growth. Thus the zero-order reaction of O{sub 2} adsorption on the Ti(0001) surface at 405 C is concerned with the metallic Ti layer on the outermost surface.