Chemical analysis of semiconducting and metallic SmS thin films by X-ray photoelectron spectroscopy

We studied the chemical state of semiconducting and metallic SmS thin films by X-ray photoelectron spectroscopy (XPS), which were fabricated using dual-target magnetron sputtering by controlling the power applied to both metal and chalcogenide targets. On the basis of the valence band spectra obtained, it was suggested that semiconducting SmS has the final state corresponding to the Sm²⁺(4f⁶) configuration below the Fermi level, and metallic SmS has mainly the Sm³⁺(4f⁵) final state and a virtual band state in the Sm 5d band, contributing to the delocalization of 4f electrons and the emergence of metallic conductivity (4f⁶d⁰-4f⁵d¹). Thus, the spectra of our fabricated SmS thin films correspond to the band structure obtained from the dielectric property. This is the first work performed on the intrinsically prepared metallic SmS while the former works done for the sample transformed from semiconductor to metal phase by hard polishing.     

Electronic band structure of graphite studied by highly angle-resolved ultraviolet photoelectron spectroscopy

The electronic valence band structure of graphite has been experimentally determined with considerable accuracy for the first time by the highly angle-resolved ultraviolet photoelectron spectroscopy. The photoemission measurements were performed on a synthesized single-crystalline graphite (kish graphite) with the He II resonance line as an exciting source. Some high symmetry points in the Brillouin zone have been successfully identified in the experimentally determined band structure. The present experimental result has been compared with early photoemission experiments and some band calculations presented so far.     

Spin- and angle-resolved photoelectron spectroscopy from solid surfaces with circularly polarized light

Spin-polarized photoemission with circularly polarized light represents a relatively new technique in surface science. It became feasible with the increased availability of circularly polarized light in the vacuum ultraviolet and soft X-ray regime, generated in dedicated synchrotron radiation facilities. Another important ingredient was the development of efficient electron spin polarization detectors. Supported by group theoretical considerations, this technique was first employed to study the process of optical spin orientation in single crystalline non-magnetic materials. In these samples, spin-dependent effects arise solely from spin-orbit coupling. The experiments revealed the strong influence of spin-orbit coupling on the details of the electronic band structure, such as the symmetry character of the electronic wave functions, hybridization phenomena, and the behavior of degeneracies. The results opened the way to a detailed understanding of the electronic structure and permit a rigorous test of both state-of-the-art bulk band theories and fully relativistic photoemission calculations. Since 1990, spin-polarized photoemission with circularly polarized light is also used to investigate ferromagnetic systems. These experiments led to the discovery of the counterpart of optical spin orientation in ferromagnets, namely, the magnetic dichroisms. Magnetic dichroism means that the spin-dependent photoexcitation from a ferromagnetically ordered system manifests itself already in the photoelectron intensity distributions. Because of this particular property, techniques based on magnetic dichroisms are currently receiving wide interest in the spectroscopic investigation of magnetic systems and phenomena.

The following article reviews the current status of the field of spin- and angle-resolved photoemission from solids using circularly polarized radiation. We survey the development of this technique over the last 10 years, covering its applications to both non-magnetic and ferromagnetic systems.     

The reaction of oxygen and water with iron films studied by X-Ray photoelectron spectroscopy

Adsorption of oxygen on iron at ambient temperature and low pressure is shown by XPS to give a chemisorbed species and the oxide, Fe₂O₃. At low temperatures a further adsorbed species is detected, similar to the nickel-oxygen system. Correction of the intensity of the oxygen signal for depth results in an oxidation curve in agreement with reported work using other techniques, i.e. oxidation is fast until about four layers of oxide are formed, at an exposure of ca. 10² L, and then proceeds slowly to about ten layers. Adsorption of water vapour produces an overlayer less than one layer in depth at an exposure of 10⁵ L. Comparison of the overlayer depths calculated from the decrease in unoxidised iron signal intensity and from the increase in oxygen intensity gives good agreement for the thick oxide film produced by oxygen adsorption, but not for the thin overlayer formed by exposure to water vapour. This suggests a difference in packing of the ions in the thin overlayer compared to the arrangement in the bulk oxide.     

The reactions of nitric oxide with nickel films as studied by x-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been employed to study the reactions between nitric oxide and freshly deposited nickel films. When the nickel surface is treated with NO at pressures of less than 1 · 10^?4 torr for 60 s (6000L), the NO is dissociatively adsorbed with no oxidation of the nickel surface. When treated with 0.40 torr of NO that has been exposed to varying degrees of oxidation, the nickel surface is oxidized and species such as NO₃^?, NO₂^?, NO and N₂ may be found on the surface. The species found are determined by the extent of oxidation of NO.     

Sputtered metal-silicon thin films studied by auger electron spectroscopy

The silicon KLL Auger spectrum from a range of stainless steel-silicon thin films was studied to provide information on changes in the silicon-silicon to silicon-metal bond ratio. This work complements earlier Auger spectroscopic studies of stainless steel-carbon films. Unlike the carbon KLL spectrum, the silicon KLL spectrum contains little chemical information. Changes in the principal plasmon loss peak associated with the silicon KLL spectrum may however prove useful in identifying the silicon phases.     

Characterisation of thin films of bismuth oxide by X-ray photoelectron spectroscopy

The XPS (or ESCA) technique has been used for the characterisation of vacuum-deposited thin films of bismuth oxide. The spectra of Bi metal and Bi₂O₃ powder are used for comparison. The characterisation is carried out by consideration of the positions of the Bi 4f_{$\text{7}\text{2}$} and 4f_{$\text{5}\text{2}$} peaks and by using peak-fitting routines. A lower suboxide of bismuth, and metallic bismuth are observed in bismuth oxide films as evaporated. Oxidation of these films by heating in air results in bismuth(III) oxide. A linear relation is found between the binding energies and oxidation state. The corresponding O 1s spectra for the two types of film are also discussed.     

X-ray photoelectron spectroscopy applied to insular films

For a given experimental arrangement, X-ray photoelectron spectra measured at different points on a sample surface may show fluctuations which cannot be attributed to counting statistics. These fluctuations are caused by overlayers having an insular structure. The XPS information obtained can be evaluated by a statistical method to yield the dimensions of the islands.     

Surface electronic state on stepped Cu(755) studied by angle-resolved ultraviolet photoelectron spectroscopy using synchrotron radiation

The surface electronic state on the stepped surface of Cu(755) has been investigated by means of angle-resolved ultraviolet photoelectron spectroscopy using synchrotron radiation(SR-ARUPS). We have observed a free-electron-like surface state below the Fermi level. In spite of the anisotropy of the atomic arrangement due to steps, the surface state is shown to be isotropic since the dispersion profile and the peak shape are almost identical in the directions parallel and perpendicular to the steps. This result makes a clear contrast with the previous SR-ARUPS results on Ni(755) surfaces which have the surface structure similar to Cu(755). Those experimental evidences are discussed based upon the electron configurations of both metal substrates.     

X-ray photoelectron spectroscopy study and thermoelectric properties of Al-doped ZnO thin films

In this paper, high quality Al-doped ZnO (AZO) thin films were prepared by direct current (DC) reactive magnetron sputtering using a Zn target (99.99%) containing Al of 1.5 wt.%. The films obtained were characterized by X-ray photoelectron spectroscopy (XPS) and thermoelectric measurements. The XPS results reveal that Zn and Al exist only in oxidized state, while there are dominant crystal lattice and rare adsorbed oxygen for O in the annealed AZO thin films. The studies of thermoelectric property show a striking thermoelectric effect in the AZO thin films. On the one hand, the thermoelectromotive and magnetothermoelectromotive forces increase linearly with increasing temperature difference (ΔT). On the other hand, the thermoelectric power (TEP) decreases with the electrical resistance of the sample. But the TEP increases with the increase of temperature below 300 K, and it nearly does not change around room temperature. The experimental results also demonstrate that the annealing treatment increases TEP, while the external magnetic field degrades TEP.     

Thickness determination of molecularly thin lubricant films by angle-dependent X-ray photoelectron spectroscopy

An angle-dependent X-ray photoelectron spectroscopy (XPS) method used to measure the thickness of molecularly thin lubricants was developed. The method was built based on an island model of patched overlayer on a flat substrate by using the photoemission signal solely from the lubricant film. Typical molecularly thin Zdol films on the CHx overcoat of unused commercial magnetic disks were measured to verify the metrology. The lubricant thickness determined by the metrology was equal to the recent result by thermostatic high vacuum atomic force microscopy. The measured deduction in the thickness of the molecularly thin lubricant films, successively irradiated by the monochromatic source operated at 14 kV/250 W, was as low as 1 ? during the first irradiation hour. XPS spectra showed that no hydrocarbons, water or oxygen were adsorbed over the Zdol outer surfaces in the tested XPS conditions. The inelastic mean free path (IMFP) of C 1s in Zdol or in CHx was found to be independent of take off angle (TOA) when TOA < 40°. The IMFP of C 1s in Zdol was ∼63.5 ? and the lubricant island thickness was ∼35 ?.     

Sb掺杂SrTio3透明导电薄膜的光电子能谱研究

用X射线光电子能谱和同步辐射光电子能谱研究了Sb掺杂的钙钛矿型氧化物SrTi1-xSbxO3(x=0.05,0.10,0.15,0.20)薄膜的电子结构.薄膜由紫外脉冲激光淀积在SrTiO3(001)单晶衬底上.该薄膜系列在可见光波段透明,透过率均超过90%.其导电性与掺杂浓度有关,当Sb掺杂浓度x=0.05时,薄膜显示金属型导电性.X射线光电子能谱和同步辐射光电子能谱研究结果表明,Sb掺杂在母化合物SrTiO3的禁带内引入了浅杂质能级和深杂质能级.浅杂质能级上的退局域化电子离化到导带中会产生一定的传导电 关键词： 光电子能谱 光学透过率 脉冲激光沉积薄膜     

Silicon diffusion through thin tungsten films on silicon,studied with Auger spectroscopy

Silicon out-diffusion through ? 3000 Å tungsten films deposited on silicon by r.f. sputtering was studied using Auger spectroscopy. Silicon first diffuses to the tungsten film surface by grain boundary diffusion and surface migration. The out-diffusion kinetics were most strongly dependent on the thickness of the silicon dioxide layer between tungsten and silicon, and this (native) oxide thickness varied with substrate doping. The out-diffusion rate was independent of tungsten film thickness at 540 Å and 2400 Å. For substrates from which the native oxide was removed by backsputtering just prior to tungsten deposition, no Si out-diffusion to the W film surface was observed until almost the entire film had converted to WSi².     

Surface plasmon resonance from metallic columnar thin films

Surface plasmon (SP) waves on the interface of a dielectric (such as water) and a metallic columnar thin film (CTF) of porosity as high as 0.55 were experimentally and theoretically investigated. The CTFs were made of Al, Au, Ag, or Cr. As the porosity increases, the SP resonance (SPR) dip was found to widen, shift to higher wave numbers, and become asymmetric due to increasing scattering losses. With further increase of porosity, the SPR dip was found to disappear, leaving behind only a peak near the onset to the total internal reflection regime. The shape of the nanoislands constituting the CTF is better described as ellipsoidal than as spherical or spheroidal, indicating thereby the existence of orientational biaxial anisotropy even for CTFs thinner than 60 nm. For a best fit between the theoretical calculations and the experimental data, the CTF was divided into two layers having different porosity and nanoisland shape, particularly for the Ag- and Au-CTFs. The sensitivity of the CTF-based SPR signal to refractive index variations of an analyte infiltrating the nanopores of and in the region adjoining the metallic CTF was found to be doubly enhanced compared to that for the SPR signal from a nonporous metallic film.     

Characterization of thin films on the nanometer scale by Auger electron spectroscopy and X-ray photoelectron spectroscopy

We describe two NIST databases that can be used to characterize thin films from Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) measurements. First, the NIST Electron Effective-Attenuation-Length Database provides values of effective attenuation lengths (EALs) for user-specified materials and measurement conditions. The EALs differ from the corresponding inelastic mean free paths on account of elastic-scattering of the signal electrons. The database supplies “practical” EALs that can be used to determine overlayer-film thicknesses. Practical EALs are plotted as a function of film thickness, and an average value is shown for a user-selected thickness. The average practical EAL can be utilized as the “lambda parameter” to obtain film thicknesses from simple equations in which the effects of elastic-scattering are neglected. A single average practical EAL can generally be employed for a useful range of film thicknesses and for electron emission angles of up to about 60°. For larger emission angles, the practical EAL should be found for the particular conditions. Second, we describe a new NIST database for the Simulation of Electron Spectra for Surface Analysis (SESSA) to be released in 2004. This database provides data for many parameters needed in quantitative AES and XPS (e.g., excitation cross-sections, electron-scattering cross-sections, lineshapes, fluorescence yields, and backscattering factors). Relevant data for a user-specified experiment are automatically retrieved by a small expert system. In addition, Auger electron and photoelectron spectra can be simulated for layered samples. The simulated spectra, for layer compositions and thicknesses specified by the user, can be compared with measured spectra. The layer compositions and thicknesses can then be adjusted to find maximum consistency between simulated and measured spectra, and thus, provide more detailed characterizations of multilayer thin-film materials. SESSA can also provide practical EALs, and we compare values provided by the NIST EAL database and SESSA for hafnium dioxide. Differences of up to 10% were found for film thicknesses less than 20 Å due to the use of different physical models in each database.     

A time- and angle-resolved X-ray photoelectron spectroscopy study of polystyrene exposed to a nitrogen plasma

Angle-resolved X-ray photoelectron spectroscopy (ARXPS) measurements were made on a polystyrene sample that had been exposed to a nitrogen plasma. It was observed that both oxygen and nitrogen were present in the sample surface, and that both were lost over a period of 5 h under X-ray irradiation in the vacuum of the spectrometer. The ARXPS data sets were corrected for the time displacement between consecutive measurements at different photoemission angles, and fitted with boxcar models in order to extract simple nitrogen and oxygen concentration depth profiles, consistent with the data, as a function of time. Both depth profiles were found to evolve in a consistent manner, indicating a loss of both the average concentration and thickness in the oxygen profile, and a stable concentration but similar loss of thickness in the nitrogen profile.     

Alkyl chain effects in thin films of substituted phthalocyanines studied using infrared spectroscopy

Thin films (2-50 nm) of unsubstituted and 1,4-octa-alkyl substituted zinc phthalocyanines were investigated using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, whereas the alkyl chains are C₄H₇, C₇H₁₃, C₁₀H₁₉. The absorption bands in the whole spectral range are discussed. We observe distinct differences in the spectra between the alkyl substituted Phthalocyanine (Pc) compounds. In contrast to PcZn and (but)₈PcZn, the spectra of (hep)₈PcZn and (dec)₈PcZn show two additional features in the spectral range between 3700 and 3000 cm⁻¹, which are discussed in detail.