M.H. Farías

Optical and surface analysis of DC-reactive sputtered AlN films

A set of aluminium nitride (AlN), thin films were prepared by DC-reactive magnetron sputtering.The deposition parameters, such as substrate temperature, sputtering gas composition and plasma current were varied.Spectroscopic ellipsometry, XPS, RBS, XRD, SEM, AFM and FTIR techniques were utilized to study the relationship between film properties and preparation conditions. We observed that the optical and surface properties have a strong dependence on the deposition rate.All prepared films present a composition close to AlN stoichiometry, even for nitrogen to argon gas concentrations below 1:2.The near surface of AlN films exposed to atmosphere was primarily composed of Al O while the bulk was AlN with some minor contamination of 23 oxygen and carbon.The thickness of the oxide layer was reduced when higher plasma current and lower nitrogen concentration were used.Deposits prepared at 400 8C presented the best refractive index and deposition rate, for both utilized plasma currents. 2003 Elsevier Science B.V. All rights reserved.

Tungsten nitride films grown via pulsed laser deposition studied in situ by electron spectroscopies

Abstract Tungsten nitride (WNx) films were grown on silicon and glass slide substrates by laser ablating a tungsten target in molecular nitrogen ambient. By in situ Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), the films density, elemental composition and chemical state were determined. Ex situ, the films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Also, the transmittance and resistivity of the film on glass substrates were determined. The results show that the reaction of tungsten and nitrogen is effective; the nitrogen is integrated in the tungsten matrix changing gradually the electronic configuration, chemical states and film properties. Since with this preparation method the obtained films are of high quality, low resistivity and dense, this makes attractive to growth tungsten nitride films for technological applications.

XPS and HRTEM characterization of cobalt-nickel silicide thin films

Abstract We studied by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) films of Co–Ni/p-Si deposited by PLD on Si(100) substrates. They were thermally treated in vacuum to promote silicide formation. By means of XPS in-depth profiles, it was observed that the deposited metal film contains more Co than Ni. The Co and Ni 2p transitions present shifts characteristic of silicide at respective ranges of 778.3–778.6 and 853.2–853.6 eV, while the Si2p transition appears at 99.2–99.5 eV, as determined by XPS. By means of HRTEM, nanocrystalline regions belonging to CoSi2, Ni2Si and NiSi2 structures were identified. Some grains of CoSi2 are large in size, more than 20 nm in diameter, while Ni2Si and NiSi2 nanocrystals are of the order of 10 nm. There are several regions where no crystalline ordering seems to be apparent. The SiO2 layer acted as an effective diffusion barrier suppressing mobility of metal into the Si(100) substrate. The observed tendencies of the Co and Ni concentrations as a function of depth agree with a model of CoSi and NiSi structure separation and subsequent formation of CoSi2 and NiSi2.

Lack of chemical interaction of hydrogenated amorphous silicon with indium-doped zinc oxide transparent conductive films

Abstract Indium-doped zinc oxide films have been prepared by the spray pyrolysis technique using air as a carrier gas at atmospheric pressure. These films show the excellent optical and electrical characteristics of a transparent conductive coating, with a sheet resistance of 15 Ω/square and an average optical transmission of about 88%. All the studied films show the wurtzite hexagonal crystalline structure. The results of Auger electron spectroscopy show that there was no reduction of the oxide film when an a-Si : H film was deposited by the plasma decomposition of silane at substrate temperatures in the range 150–300°C. A steep interface between the transparent conductive film and the hydrogenated amorphous silicon film was observed. Also, it was found that the depth distribution of the constituent atoms zinc, oxygen and indium in the ZnO : In film was homogeneous.

Electronic structure of β-Be3N2

Abstract Ab initio all electron Hartree-Fock periodical calculations were performed on the hexagonal phase of Be 3 N 2 . The electronic structure was studied and the structural parameters were determined. By performing total energy calculations, equilibrium lattice parameters, bulk modulus and its derivative, and cohesion energy were determined. The lattice parameters obtained, a = 2.847 A and c = 9.714 A , differ by less than 1% from the corresponding experimental values. The calculated bulk modulus, 2.61 Mbar, locates this compound among the hard materials. Based on the band structure and density of states obtained, the hexagonal Be 3 N 2 is a wide band gap semiconductor with an indirect gap of 12.50 eV. The charge density difference along the BeN bonds indicates that the character of the bond is more covalent than ionic.

X-ray photoelectron spectroscopy study of CdTe oxide films grown by rf sputtering with an Ar-NH3 plasma

Abstract CdTe oxide films were prepared on glass substrates using the rf-sputtering technique and a controlled plasma of Ar–NH 3 . Films were studied by X-ray photoelectron spectroscopy and X-ray diffraction. The changes in chemical composition as a function of NH 3 partial pressure during deposition indicate that oxygen incorporates in the films up to approximately 62 at.%, while the Cd and Te contents decrease from ∼43 to 19 at.%. At NH 3 partial pressure of 1.3×10 −2 Pa, the Te–Cd bonds are strongly reduced and the Te in the films is mainly bonded to oxygen. The Cd MNN X-ray Auger feature shows a shift in energy of approximately 0.8 eV as a function of NH 3 partial pressure. This shift appears to be related to the change in Cd bonding from Cd–Te to Cd–O. Films prepared at NH 3 partial pressure of 4×10 −4 Pa present crystallinity with a [111] cubic CdTe orientation, while those prepared at higher NH 3 partial pressure show an amorphous structure. The amorphous material formed at NH 3 partial pressure saturation appears to be mainly amorphous CdTeO 3 .

Structural and catalytic properties of Pd/Al2O3–La2O3 catalysts

Abstract A new Pd/Al2O3–La2O3 catalyst has been synthesized for the reduction of NO with hydrogen. This catalyst is more active than coprecipitated Pd/Al2O3 catalysts. The revealed effect of the improvement of the catalytic activity at medium temperature and the increase of NH3 formation at high temperatures for Pd catalyst supported on alumina–lanthana prepared by the sol–gel method are ascribed to a new lanthanum-containing phase observed by X-ray powder diffraction and high resolution electron microscopy.

Hydrodesulfurization catalysts prepared by two methods analyzed by transmission electron microscopy

Abstract Samples of molybdenum sulfide, cobalt sulfide and mixtures in atomic ratios r = Co/(Co + Mo) of 0.0, 0.3, 0.5, 0.7, and 1.0 were prepared by two different methods, homogeneous sulfide precipitation (HSP) and impregnated thiosalt decomposition (ITD). Samples were observed by high-resolution electron microscopy using imaging and diffraction modes. Both preparation methods present the “rag structure” typical of MoS 2 2H with some structural differences between them. The average number of layers (n) in molybdenum disulfide crystals is about the same in both preparation methods, while the average length (L) of the molybdenum disulfide crystallites obtained by HSP is larger than that of those obtained by ITD. The particle size is smaller for ITD samples. The presence of cobalt does not greatly modify the number of layers of the MoS 2 2H stacks in mixed samples. An increase in the intracrystallite disorder is observed.

Nuclear reactions as a probe of fluorine content in SnO2:F thin films

Abstract The resonant nuclear reaction technique has been applied to the determination of fluorine content in SnO 2 thin films prepared by spray pyrolysis. γ rays from the nuclear reaction 19 F(p, αγ) 16 O, which presents a narrow resonance at a 340 keV proton bombarding energy, were observed. The excitation curves obtained indicate the fluorine concentration and distribution in the sample. The interface between the film and the substrate is also clearly marked. Auger electron spectroscopy was used as an alternative technique to observe the fluorine, but it was very difficult to detect. The reason for this difficulty is assumed to be electron-induced desorption of fluorine atoms.

Plasma enhanced chemical vapor deposition of SiO 2 films at low temperatures using SiCl 4 and O 2

Silicon dioxide films have been deposited by Plasma-Enhanced Chemical Vapor Deposition (PECVD) technique using SiCl4 and O2 as reactive materials. Infra-red transmittance, Auger electron spectroscopy analysis, ellipsometry, electrical, and chemical etch measurements have been used to characterize these films. It is possible to obtain good quality oxides at a substrate temperature of 200° C using a low flow of reactant gases. High flow of reactant gases results in highly non-homogeneous porous films. The best oxide films obtained show destructive breakdown at electrical fields above 4 MV/cm and a fixed charge density of the order of 2.6 × 1011 charges/cm2.