J.X. Wu

The interaction of oxygen with niobium studied by XPS and UPS

Abstract The adsorption and desorption of O2 on Nb (110) surface are studied by XPS and UPS. The changes of structure in the electron energy distribution curves for different oxygen exposures and desorption temperatures are correlated with formation of different oxides. The result indicates a reversible conversion of Nb, NbO and Nb2O5 in the absorption and desorption process.

The effect of Rb coverage on the oxidation of RbInSb(111) interfaces

Abstract Core-level and valence-band photoemission and work-function measurements have been used to investigate the effect of Rb coverage on the oxidation of Rb InSb (111) interfaces as a function of oxygen exposure at room temperature. The results show that a rapid oxygen uptake at low oxygen exposures is basically independent of the Rb coverage. For low Rb coverages, the oxygen adsorption is mainly related to the chemisorbed atomic oxygen and/or the oxidation of Sb on the surface, which is associated with an increase in the work function. In the case of high Rb coverages, the oxidation of the Sb atoms takes place at the interface between the peroxide (Rb 2 O 2 ) and the substrate at low oxygen exposures, causing an initial decrease in the work function. The O 2− 2 ion appears to have four features in the valence band. The O − 2 species of the superoxide (RbO 2 ) located in the topmost layer is stabilized only after a certain amount of O 2 has been dosed to the surface. At high oxygen exposure the intensity ratio of O − 2 to O 2− 2 increases with Rb coverage.

X-ray photoemission studies of praseodymium thin films on SiO2/Si(100)

Pr overlayers, with thicknesses of about 1.8 and 0.6 nm, were deposited on 1.2 nm SiO2/Si(100) at room temperature and x-ray photoelectron spectroscopy (XPS) was used to investigate the reactions at the Pr/SiO2/Si interfaces as a function of annealing temperature. The results show that the Pr overlayers reduce SiO2 at room temperature, forming Pr2O3 and a Pr silicate (Pr–O–Si). For the 1.8 nm Pr/SiO2/Si system, the 1.2 nm SiO2 layer is mostly reduced at room temperature and a Pr silicide was also observed. The Pr–O–Si silicate increases in intensity while the Pr2O3 intensity decreases with annealing temperature. Angle-dependent XPS, taken after annealing, indicates that the silicate is located at the top surface. In the case of the 0.6 nm Pr/SiO2/Si system, the Pr overlayer is almost oxidized after annealing, and some SiO2 remains even after annealing at 1090 K, which can serve as a low-defect-density buffer layer between the high-κ film (Pr2O3 and Pr–O–Si) and the silicon.

Photoemission study of the desorption and reaction of C60 and K4C60 films on Si(111) surfaces

Abstract X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) have been used to investigate the desorption and reaction of C60 and K4C60 films on Si(111) surfaces. For C60/Si system, C60 multilayers desorb from the surface at 450 K, except the first monolayer. In the case of K4C60/Si system, the desorption of both potassium and C60 begins at 820 K. The potassium atoms are completely removed from the surface at 1000 K, causing the disruption of C60 cage structure and the formations of both SiC and C–Si alloy. Although the C 1s peak due to SiC becomes detectable at 950 K in both cases, the SiC intensity converted by C–Si alloy for K4C60/Si system grows faster at temperatures above 1000 K.

Thermal effect on K-promoted oxidation of β-SiC

Abstract Thermal effect on potassium-promoted oxidation of β-SiC has been investigated by means of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and the work function measurement from the shift of the slow secondary electron cut-off in the ultraviolet photoemission for different potassium coverage and substrate temperature. It is found that the substrate temperature influences the adsorption kinetics of oxygen on the alkali metal atoms and Si in the substrate; an elevated substrate temperature can promote the oxygen transferring from the catalyst to Si and penetrating into the substrate; the K-bonded oxygen is in the state of O 2 2− at room temperature, and in the state of O 2− at 500 K; a SiO 2 layer only forms from silicon suboxide after the sample was annealed at 700 K. The experimental result shows that a proper selection of temperature is even more important to the promoted oxidation than the amount of the promoter does in the present case.

Surface ScienceThe effect of Rb coverage on the oxidation of RbInSb(111) interfaces

Abstract Core-level and valence-band photoemission and work-function measurements have been used to investigate the effect of Rb coverage on the oxidation of Rb InSb (111) interfaces as a function of oxygen exposure at room temperature. The results show that a rapid oxygen uptake at low oxygen exposures is basically independent of the Rb coverage. For low Rb coverages, the oxygen adsorption is mainly related to the chemisorbed atomic oxygen and/or the oxidation of Sb on the surface, which is associated with an increase in the work function. In the case of high Rb coverages, the oxidation of the Sb atoms takes place at the interface between the peroxide (Rb 2 O 2 ) and the substrate at low oxygen exposures, causing an initial decrease in the work function. The O 2− 2 ion appears to have four features in the valence band. The O − 2 species of the superoxide (RbO 2 ) located in the topmost layer is stabilized only after a certain amount of O 2 has been dosed to the surface. At high oxygen exposure the intensity ratio of O − 2 to O 2− 2 increases with Rb coverage.

X-RAY AND ULTRAVIOLET PHOTOEMISSION SPECTRA STUDY OF TL0.3MOO3

X-ray and ultraviolet photoemission spectra of thallium blue bronze Tl0.3MoO3 are presented. The results show that the conduction band extends to 3.2eV below the Fermi level, the valence band lies between 3.2 and 10 eV and the gap between conduction and valence band is insignificant. The O 1s and Tl 4f core lines exhibit the distinctive feature of weak shoulders, understood as excitation of small energy plasmon (homega approximately 2eV). Asymmetric line shape of O ls is due to contribution from inequivalent oxygen sites. The Mo 3d core-electron spectrum is dominated by two final-state effect, representing alternate screening channels.

Photoemission study of oxygen adsorption on Rb6C60 film surface

Abstract X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) have been used to investigate oxygen adsorption on the surface of Rb 6 C 60 film at room temperature. The results show that the Rb 6 C 60 surface is sensitive to oxygen adsorption. The absorbed oxygen changes the crystal structure of the film by pulling intercalated Rb atoms to the surface, forming Rb oxides. An initial rapid oxygen uptake is mainly related to the formation of rubidium peroxide (Rb 2 O 2 ). There exists a strong reaction between C 60 and Rb 2 O 2 , leading to the formation of rubidium carbonate (Rb 2 CO 3 ). At high oxygen exposures, a slow further adsorption of oxygen is associated with the intensity increases of both Rb 2 CO 3 and rubidium superoxide (RbO 2 ) species.

Interaction of oxygen with a potassium-covered GaSb(110) surface at 500 K

The interaction of oxygen with a GaSb(110) surface precovered with 0.9 monolayers of potassium at 500 K has been studied by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and change of work function measurements. It is found that potassium can diffuse into the substrate and react with antimony atoms in the GaSb surface. At low exposures oxygen penetrates below the topmost layer, inducing a disruption of the KSb bonds and reacting with potassium, antimony and/or gallium to form their corresponding oxides (O2−). At the same time, potassium atoms are gradually drown back to the surface with oxygen exposure. At higher exposure, potassium peroxide (O2−2) on the surface is observed follow the formation of the monoxide K2O. The elevated temperature promotes oxygen diffusion into the substrate and delays the appearance of potassium peroxide, resulting in an increase of the saturated adsorption of oxygen, and this is beneficial for further oxidation of the GaSb surface.

Interaction of Ce overlayers with the GaSb(111) surface and oxidation of the Ce/GaSb interface

Abstract In this paper we use XPS to study the interaction of Ce overlayers with the semiconductor GaSb(111) surface and Ce-promoted oxidation of the substrate. The results show that when cerium deposits on GaSb(111), it forms weak bonds of Ce–Ga and Ce–Sb with the substrate and makes Ga atoms diffuse out. A Ce–Ga intermetallic phase is formed on the surface. When the exposure of O 2 reaches 50 L, the oxide of cerium, Ce 2 O 3 , begins to change into unstable CeO 2 . The dissociation of CeO 2 results in obvious oxidation of the substrate. The main products are Ga 2 O 3 , Sb 2 O 3 and then Sb 2 O 5 . After annealing at 250 and 400°C, respectively, some of the oxygen atoms transfer from cerium dioxide toward Ga and Sb in the substrate, by which it strongly promotes the oxidation of the substrate.