A. J. Hartmann

Influence of interface and Al structure on layer exchange during aluminum-induced crystallization of amorphous silicon

Aluminum-induced crystallization of amorphous silicon (a-Si) is studied using various microscopy techniques and x-ray photoelectron spectroscopy. During the isothermal annealing of subsequently deposited aluminum and a-Si films on glass, a layer exchange process is induced, while a continuous polycrystalline silicon film (poly-Si) on glass is formed within the initial metal layer and therefore displaces it. This crystallization process is conducted at temperatures ranging from 350 °C to 500 °C, significantly below the eutectic temperature of the Si–Al binary system of 577 °C. The results presented focus on the influences of the polycrystalline structure of the evaporated Al, the Si–Al layer sequence, and the interface layer between the Al and Si films on the overall crystallization process. They reveal that the larger the Al grain size of the initial polycrystalline Al layer, the larger the grain size of the final poly-Si film and the slower the entire layer exchange process. It is further shown that the ...

Electronic characteristics of the SrBi2Ta2O9–Pt junction

The voltage and film composition dependence of leakage currents of ferroelectric SrBi2Ta2O9 thin films, sandwiched between Pt has been studied. Schottky emission dominated the leakage current at voltages above the ohmic conduction regime, while space charge limited currents (SCLC), for which the observed temperature dependence is correctly predicted in Rose’s theory, appeared to dominate the leakage current in high conductivity SrBi2Ta2O9 thin films including bismuth-excess samples. A consequence of the latter was the observation of negative differential resistivity in high conductivity SrBi2Ta2O9 thin films. X-ray photoemission spectroscopy depth profiling indicated that Bi has diffused into the ferroelectric–metal interface and also influenced the electronic conduction mechanism of the ferroelectric capacitors. Confirmation of this was found through the current–voltage dependence of Pt/SrBi2Ta2O9/Bi, which is compatible with space charge limited currents. The theory of Rose was successfully applied to t...

Valence Band and Bandgap States of Ferroelectric SrBi2Ta2O9 Thin Films.

Ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and surface photovoltage spectroscopy (SPV) were used to determine the electronic structure near the bandgap of strontium bismuth tantalate (SBT) thin films. The UPS results for nearly stoichiometric SBT have been compared with tight-binding calculations. The spectra for bismuth-excess SBT indicate additional density of states (DOS) in the wide bandgap of the material. SPV studies indicate that the surface bandgap of bismuth-excess SBT is approximately 2 eV, which also confirms that there are additional surface states in the bandgap. These electronic structural data are used to explain the observed dependency of the electrical properties of the SBT/electrode junction on the bismuth concentration.

The bandgap of SrBi2Ta2O9

Abstract The electronic structure of SrBi2Ta2O9 thin films, which are of interest for ferroelectric random access memories, has been investigated. The valence band has been analysed using surface sensitive X-ray photoemission spectroscopy (XPS), while the partial density of unoccupied electronic states associated with the conduction band has been investigated using near edge X-ray absorption spectroscopy (NEXAS). The shapes of the NEXAFS spectra are roughly in agreement with calculations presented previously, however, the metal states (especially bismuth) appear to be broader. The NEXAFS measurements were performed for two different information depths (∼100A and ∼2000A) which indicated no significant electronic structural differences between the bulk and the near-surface region. The bandgap has been measured using bulk sensitive UV-visible absorption and surface sensitive electron energy loss spectroscopy. The data indicated that the bandgap of the thin film material is ca. 4 eV and lower than suggested p...

Influence of Copper Dopants on the Resistivity of ZnO Films

Copper 2p 3/2 x-ray photoemission spectroscopy (XPS) measurements and near-edge x-ray absorption spectroscopy (NEXAFS) studies of copper-doped ZnO thin films were carried out in order to understand the role of the dopants in increasing the film resistivity. c-Axis-oriented ZnO films (thickness 120 nm) were deposited by r.f. sputtering on Cr-coated SiO 2 /Si substrates and subsequently implanted with metallic copper (0.5 at.%). After implantation, XPS results indicated that the dopant consists of a mixture of Cu + and Cu 0 . The c-axis film resistivity was found to decrease to 7 x 10 7 Ω. cm (prior to implantation : 5 x10 10 Ω.cm). Oxygen annealing resulted in complete oxidation of the dopants in the film surface and bulk (Cu 2+ ) as indicated by XPS and bulk-sensitive fluorescence NEXAFS. The film resistivity increased to 2 x 10 11 Ω.cm, which can be explained in terms of electron trapping by Cu 3d hole states of Cu 2+ . The NEXAFS results suggested that the copper dopants after oxygen annealing have preferred orientations in the ZnO matrix. Additional vacuum annealing reduced Cu 2+ ions to Cu + , which demonstrated that the observed decrease in film resistivity from 2 x 10 11 -10 7 Ω. cm is due to the Cu + oxidation state.

A comparison of Schottky-limited and space-charge-limited currents in SrBi2Ta2O9 thin films

Abstract Stoichiometric SrBi2Ta2O9 (SBT)-Pt contact exhibits a Schottky contact with a barrier height of 0.9 eV, but small amounts of excess Bi produce ohmic contacts at low voltages, followed by perfectly quadratic space-charge-limited current-voltage dependences (SCLC). Attempts to fit Schottky models to the temperature dependence of leakage currents in Bi-rich SBT films yield unphysical results, including a negative barrier height. We have generalized the SCLC theory of Rose to incorporate the temperature dependence of the dielectric constant. This theory has a single parameter T∗ which is a measure of trap energy distribution. A single T∗ value fits our data at all temperatures and all voltages. These macroscopic leakage current data are correlated with X-ray photoelectron spectroscopy (XPS) depth profiles that show how excess Bi has diffused into the electrode-ferroelectric interface region.

Thermopower of ultra-high purity YBa2Cu3O7−δ single crystals

Abstract We have measured the thermopower of Y 2 O 3 crucible grown, ultra-high quality twinned YBa 2 Cu 3 O 7−δ single crystal in both the a - b (in-plane) and c axis (out-of-plane) orientations. We report negative values for the in-plane thermopower and positive values for the out-of-plane thermopower of overdoped YBCO single crystals. The in-plane measurements correlate with results obtained from sintered polycrystalline YBCO [1] where the a - b plane dominates the electronic conduction process. We discuss the implications of these intrinsic results in terms of the high temperature metallic thermopower relation.

X-ray photoemission spectroscopy of thin films

Abstract Developments in XPS (X-ray photoemission spectroscopy) over the past few years have seen, above all other surface analytical techniques, a successful bridging of the gap between ideal (single crystal) research and practical materials analysis. There has been a number of improvements in spectroscopic resolution through advances in both light sources (e.g. synchrotron) and analyser/detector design together with orders of magnitude refinement in the spatial resolution of the new breed of imaging spectrometers. As a result, in combination with in situ film growth techniques, a large range of film features can be examined with relative simplicity. These include interface chemistry and structure, electronic characterisation and quantifiable chemical imaging. All of which can be achieved through state-of-the-art commercially available instrumentation. A number of general improvements have been made in XPS over the past few years and in practical applications, in the area of thin film and coatings.

Electronic Structure of Nd0.5Y0.5Ba2Cu3O7‐δ Single Crystals

The new bulk, melt-textured grown composite Nd 0.5 Y 0.5 Ba 2 Cu 3 O 7-δ , which also forms large single crystals, has the standard high-T c 123 orthorhomhic structure when oxygenated and a very high critical current density in a magnetic field of 0.5 T. Both of these results were not anticipated, as the Nd and Y ions are significantly different in size, while the absence of Y211 and/or Nd422 precipitates within the 123 grains indicates that a new pinning mechanism is responsible for the high J c . A possible mechanism has been attributed in part to the partial substitution of Nd 3+ in the Ba 2+ site, but to further understand this material and to further optimize its processing route for application purposes, we have carried out an XPS study of its electronic structure using cleaved surfaces of single crystals The data obtained are analysed and discussed in terms of that found for high-quality Re 123 single crystals where Re is either Nd or Y.

Investigation of the Interfacial Region Formed During ZnO Growth on Si(100) Substrate Using Single‐source CVD

The growth process of ZnO films on Si(100) by single-source chemical vapour deposition (CVD) was investigated. During the initial stages of growth (film thickness <30A), oxidation of the Si substrate was observed which resulted in an interfacial region consisting of ZnO and Si oxides. For film thicknesses in excess of 40 A the composition of the film approaches that of a continuous ZnO film. It is suggested that the mixed interfacial region strongly influences the adhesion of the film on the substrate by providing Zn-O-Si-type bonds. The oxidation of the substrate during the initial film growth may have direct implications on the type of contact layers that can be used in ZnO thin-film devices using single-source CVD techniques.