Study of electronic states for V thin films deposited on 6H-SiC substrates by soft X-ray emission spectroscopy

Silicon carbide (SiC) is a candidate material for electronic devices to operate upon crucial environment. Electronic states of silicides and/or carbide/graphite formed in metal/SiC contact system is fundamentally important from the view point of device performance.We study interface electronic structure of vanadium (V) thin-film deposited on 6H-SiC(0 0 0 1) Si-face by using a soft X-ray emission spectroscopy (SXES). For specimens of V(38 nm)/6H-SiC (substrate) contact systems annealed at 850 °C, the Si L_2,3 emission spectra indicate different shapes and peak energies from the substrate. The product of materials such as silicides and/or ternary materials is suggested. Similarly, the C Kα emission spectra show the shape and peak energy characteristic of vanadium carbide including substrate 6H-SiC signal.     

Quantum size effect of electron density in ultra-thin metal films and its influence on the interlayer relaxation

Further minimization of electronic devices and microelectromechanical systems (MEMS) requires the feature sizes of relevant materials to be shrunk significantly. In such a case, boundary effects, such as interfaces and surfaces, become remarkable, especially in nanometer scale, which must affect their microstructures and properties. In this work, we have analyzed the distribution of electron charge density in Cu and Al ultra-thin films using free electron model. The results show that an electrostatic field may come into being due to quantum size effect, and the interlayer separations must relax to decrease the Coulomb energy, the thinner the films, the larger the relaxation. More interestingly, two opposite deviating directions of the center of negative charges result in two absolutely distinct interlayer relaxations: inwards for Cu and outwards for Al.     

Comparative photoemission study of the electronic properties of L-CVD SnO2 thin films

In this work we present the results of comparative XPS and PYS studies of electronic properties of the space charge layer of the L-CVD SnO₂ thin films after air exposure and subsequent UHV annealing at 400 °C, with a special emphasis on the interface Fermi level position.From the centre of gravity of binding energy of the main XPS Sn 3d_5/2 line the interface Fermi level position E_F − E_v in the band gap has been determined. It was in a good correlation with the value estimated from the offset of valence band region of the XPS spectrum, as well as from the photoemission yield spectroscopy (PYS) measurements. Moreover, from the valence band region of the XPS spectrum and PYS spectrum two different types of filled electronic band gap states of the L-CVD SnO₂ thin films have been derived, located at 6 and 3 eV with respect to the Fermi level.     

Post deposition purification of PTCDA thin films

The decomposition of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecules during evaporation of unpurified raw material in ultra high vacuum was studied. The fragments were identified by mass spectrometry and the influence of these fragments and further contaminations of the raw material on the electronic structure of PTCDA thin films was measured by photoemission spectroscopy. Annealing of contaminated PTCDA films was tested as cheap and easy to perform method for (partial) post deposition purification of the contaminated films.     

First-principles calculations of magnetic and electronic structures around surfaces and interfaces of magnetic multi-layered structure

Electronic and magnetic structures of ferromagnetic (FM)/non-magnetic (NM) and FM/antiferromagnetic (AF) bi-layer systems are calculated by the first principles approach. For the FM/NM system, we focus on the Co/Cu multi-layered structure whose interfacial layer is assumed to have a mixed composition of Co and Cu atoms, and show a possibility that Co atoms at the interface play a significant role as the spin-dependent scattering potentials. In the FM/AF system, we consider Fe or Co monolayer as FM layer and MnNi as AF layers. It is predicted that the Mn moments adjacent to FM layer are forced to align the FM moments, and those of under layer go gradually to anti-parallel alignment as in the bulk MnNi.     

Loss of adhesion strength of PVD Cu films on carbon substrates after heat treatment and correlated effects on the thermal interface properties

The study of coating-substrate systems consisting of a thin copper film on a flat carbon substrate is of great interest in order to get information on the interfacial behaviour of such systems. This work is focused on the mechanical adhesion strength and the correlated interfacial thermal contact resistance which are influenced by heat treatment. Using plasma-assisted pre-treatment of the carbon substrate prior to the deposition of copper coatings via physical vapor deposition (PVD), the adhesion strength between copper coatings and substrate has increased significantly, while the thermal contact resistance decreased in the as deposited state. After heat treatment at 800 °C for 1 h, considerably decreased adhesion strengths have been observed, accompanied by increased values of the thermal contact resistance.     

Inverse photoemission spectroscopy of the unoccupied electronic structure of Na/Cu(110)

The unoccupied electronic states of Na thin films on a Cu(110) substrate have been measured by inverse photoemission spectroscopy (IPES). The IPES spectrum provides the intensity of the unoccupied states, which decreases with increasing Na coverage at off-normal incidence of the electron beam. The IPES spectra at 17 and 19 eV incident electron energies show a shift towards the Fermi level with increasing Na coverage for the peak at ∼7.8 eV.     

Influence of annealing temperature on the properties of ZnO thin films deposited by thermal evaporation

ZnO thin films were deposited by thermal evaporation of a ZnO powder. The as-deposited films are dark brown, rich zinc and present a low transmittance. Then, these films were annealed in air atmosphere at different temperatures between 100 and 400 °C. Their microstructure and composition were studied using XRD and RBS measurements respectively. By increasing the temperature, it was found that film oxidation starts at 250 °C. XRD peaks related to ZnO appear and peaks related to Zn decrease. At 300 °C, zinc was totally oxidised and the films became totally transparent. The electrical conductivity measurement that were carried out in function of the annealing temperature showed the transition from highly conductive Zn thin film to a lower conductive ZnO thin film. The optical gap (E_g) was deduced from the UV-vis transmittance, and its variation was linked to the formation of ZnO.     

Onset of ferromagnetism in ultrathin Fe films on semiconductors

Very thin Fe films have been grown by molecular beam epitaxy on Ge(001), GaAs(001) and ZnSe(001) substrates, under identical preparation conditions. The electronic and magnetic properties of such interfaces have been studied, as a function of the Fe thickness, by means of spin resolved inverse photoemission. From the spin dependence of Fe empty states, we observe the onset of room temperature ferromagnetism to occur at a Fe thickness as low as three monolayers (ML) for Fe/Ge, while 5 and 8 ML have been found for Fe/GaAs and Fe/ZnSe, respectively.     

Composition of the sputter deposited W-Ti thin films

Tungsten-titanium (W-Ti) thin film was deposited by dc Ar⁺ sputtering of W(70 at.%)-Ti(30 at.%) target. The thin film composition, determined by X-ray photoelectron spectroscopy (XPS) depth profiling, is W_(0.77±0.07)Ti_(0.08±0.03)O_(0.15±0.03). The presence of oxygen in the deposit is due to the rather poor vacuum conditions during the deposition, while significant deficiency of Ti, as compared to the sputtering target composition cannot be explained straightforwardly. Monte Carlo simulations of both, transport of sputtered particles from target to the substrate through the background gas (SRIM 2003 program) and thin film sputtering during the XPS depth profiling (program TRIDYN_FZR) are presented. The simulations show that the particle transport through the background gas is mainly responsible for the Ti depletion: the estimated composition of the thin film is W_0.61Ti_0.16O_0.23. Additional apparent Ti depletion occurs due to the preferential sputtering during the thin film composition analysis. The simulation of the sputtering process show that the surface concentration measured by XPS should be about W_0.74Ti_0.11O_0.15. The discrepancy between the estimated surface composition and the actual experimental result is in the range of the experimental error.     

Structural study of thin films prepared from tungstate glass matrix by Raman and X-ray absorption spectroscopy

Thin films were prepared using glass precursors obtained in the ternary system NaPO₃BaF₂WO₃ and the binary system NaPO₃WO₃ with high concentrations of WO₃ (above 40% molar). Vitreous samples have been used as a target to prepare thin films. Such films were deposited using the electron beam evaporation method onto soda-lime glass substrates. Several structural characterizations were performed by Raman spectroscopy and X-ray Absorption Near Edge Spectroscopy (XANES) at the tungsten L_I and L_III absorption edges. XANES investigations showed that tungsten atoms are only sixfold coordinated (octahedral WO₆) and that these films are free of tungstate tetrahedral units (WO₄). In addition, Raman spectroscopy allowed identifying a break in the linear phosphate chains as the amount of WO₃ increases and the formation of POW bonds in the films network indicating the intermediary behavior of WO₆ octahedra in the film network. Based on XANES data, we suggested a new attribution of several Raman absorption bands which allowed identifying the presence of WO⁻ and WO terminal bonds and a progressive apparition of WOW bridging bonds for the most WO₃ concentrated samples (above 40% molar) attributed to the formation of WO₆ clusters.     

Comparison of electronic structures of orthorhombic and hexagonal manganites studied by X-ray absorption spectroscopy

Electronic structures of orthorhombic perovskite-type manganites (OM) and hexagonal manganites (HM) are compared by using X-ray absorption near edge structure spectroscopy. This study reveals that the environments around Mn and O ions in both the systems are different. These are reflected primarily in the lifting of degeneracy of the Mn 3d orbitals. Interestingly, the unoccupied Mn 3dx²−y² orbital in HM is strongly hybridized with O 2p states and appears to be responsible for both long range orderings: ferroelectric ordering at high temperatures and ferromagnetic ordering at low temperatures.     

Oxidation studies of niobium thin films at room temperature by X-ray reflectivity

We report the results of growth kinetics of oxidation process on niobium thin film surfaces exposed to air at room temperature by using a surface sensitive non-destructive X-ray reflectivity technique. The oxidation process follows a modified Cabrera-Mott model of thin films. We have shown that the oxide growth is limited by the internal field due to the contact potential which develops during the initial stage of oxidation. The calculated contact potential for 100 and 230 Å thick films is 0.81 ± 0.14 and 1.20 ± 0.11 V respectively. We report that 40% increase in the contact potential increases the growth rate for the first few mono layers of Nb₂O₅ from ∼2.18 to ∼2790 Å/s. The growth rates of oxidation on these samples become similar after the oxide thicknesses of ∼25 Å are reached. We report on the basis of our studies that a protective layer should be grown in situ to avoid oxidation of Nb thin film surface of Nb/Cu cavities.     

Epitaxial GaN films deposited on sapphire substrates prepared by the sol-gel method

Thin films of GaN have been successfully deposited on Al₂O₃ (0001) substrates by the sol-gel technique. The method, in addition to being is simple and cost-effective, results in epitaxial films. The films have been characterized by photoluminescence spectroscopy.     

Ion-induced secondary electron emission from ZnS thin films deposited by closed-spaced sublimation

ZnS thin films were deposited on soda lime glass and aluminum substrates by close-spaced sublimation technique. The change in composition, structural and optical properties of the films was investigated as a function of the substrate temperature. The deposited films were stoichiometric and crystalline in nature having cubic structure oriented only along (1 1 1) plane. The energy band gap of the films deposited at the substrate temperature of 150, 250 and 350 °C was 3.52, 3.58 and 3.63 eV respectively. These films were then bombarded with 2-10 keV energy pulsed Ar⁺ beam and their electron yield was determined from impinging ion and emitted electron currents. The electron yield of ZnS films was much high as compared to the metals. The electron yield of ZnS films increased with energy of the incident ion and got saturated at about 8 keV. The most important result of this study was that the electron yield of ZnS films having same composition was different. Monte Carlo simulations performed to interpret these experimental findings showed that the dissimilar electron yields of ZnS films is due to the combined effect of energy band gap, surface barrier potential and density of the films.     

Surface electronic structure of the (3 × 2) reconstruction induced by Yb on a Si(1 1 1) surface

We have investigated the electronic structure of the Yb/Si(1 1 1)-(3 × 2) surface using angle-resolved photoelectron spectroscopy. Five surface states have been identified in the gap of the bulk band projection. Among these five surface state, the dispersions of three of them agree well with those of the surface states of monovalent atom adsorbed Si(1 1 1)-(3 × 1) surfaces. The dispersions of the two other surface states agree well with those observed on the Ca/Si(1 1 1)-(3 × 2) surface, whose basic structure is the same as that of monovalent atom adsorbed Si(1 1 1)-(3 × 1) surfaces. Taking these results into account, we conclude that the five surface states observed in the band gap originate from the orbitals of Si atoms that form a honeycomb-chain-channel structure.     

Transparent and conducting Zn-Sn-O thin films prepared by combinatorial approach

Zn-Sn-O (ZTO) films with continuous compositional gradient of Sn 16-89 at.% were prepared by co-sputtering of two targets of ZnO and SnO₂ in a combinatorial method. The resistivities of the ZTO films were severely dependent on oxygen content in sputtering gas and Zn/Sn ratio. Except for the films with Sn 16 at.%, all the as-prepared films were amorphous and maintaining the stable amorphous states up to the annealing temperature of 450 °C. Annealing at 650 °C resulted in crystallization for all the composition, in which ZnO, Zn₂SnO₄, ZnSnO₃, and SnO₂ peaks were appeared successively with increasing Sn content. Above Sn 54 at.%, the ZTO films were deduced to have a local structure mixed with ZnSnO₃ and SnO₂ phases which were more conductive and stable in thermal oxidation than ZnO and Zn₂SnO₄ phases. The lowest resistivity of 1.9 × 10⁻³ Ω cm was obtained for the films with Sn 89 at.% when annealed at 450 °C in a vacuum. The carrier concentrations of the amorphous ZTO films that contained Sn contents higher than 36 at.% and annealed at 450 °C in a vacuum were proportional to the Sn contents, while the Hall mobilities were insensitive to Sn contents and leveling in the range of 23-26 cm²/V s.     

The effects of substrate temperature on the structure, morphology and photoluminescence properties of pulsed laser deposited SrAl2O4:Eu,Dy thin films

In this study, SrAl₂O₄:Eu²⁺,Dy³⁺ thin film phosphors were deposited on Si (1 0 0) substrates using the pulsed laser deposition (PLD) technique. The films were deposited at different substrate temperatures in the range of 40-700 °C. The structure, morphology and topography of the films were determined by using X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). Photoluminescence (PL) data was collected in air at room temperature using a 325 nm He-Cd laser as an excitation source. The PL spectra of all the films were characterized by green phosphorescent photoluminescence at ∼530 nm. This emission was attributed to 4f⁶5d¹→4f⁷ transition of Eu²⁺. The highest PL intensity was observed from the films deposited at a substrate temperature of 400 °C. The effects of varying substrate temperature on the PL intensity were discussed.     

Effect of interface traps on Debye thickness semiconductor films

The presence of the boundary interface trapping states and their role in determining the properties of Debye thickness thin semiconductor films, are demonstrated experimentally, using PbTe films deposited on mica. These charged states could not be observed earlier and be studied directly, because of the screening by the relatively high carrier density of the ordinary PbTe. Thin, Debye length thickness, PbTe films with a high concentration of interface trapping centers, possess an extraordinary high resistance. In this case the thermostimulated capacitor discharge method can be successfully applied to determine the energy of these levels, their carrier capture cross-sections and their donor- or acceptor-like character. The experimental results and theoretical calculations are discussed.     

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.