Oxide films stoichiometry

Thin oxide films may be prepared by substrate oxidation or by vapour deposition onto a suitable substrate. An example of the fomrer method is the preparation of silicon oxide thin-films by oxidation of a silicon wafer. In general, however, the thickness and stoichiometry of a film prepared by this method are difficult to control. 

The stoichiometry of the redox reactions of conducting polymers (n and m in reactions 1 and 2) is quite variable. Under the most widely used conditions, polypyrroles and polythiophenes can be reversibly oxidized to a level of one hole per ca. 3 monomer units (i.e., a degree of oxidation, n, of ca. 0.3).7 However, this limit is dictated by the stability of the oxidized film under the conditions employed (Section V). With particularly dry and unreactive solvents, degrees of oxidation of 0.5 can be reversibly attained,37 and for poly-(4,4 -dimethoxybithiophene), a value of n = 1 has been reported.38 Although much fewer data are available for n-doping, it appears to involve similar stoichiometries [i.e., m in Eq. (2) is typically ca. 0.3].34,39"41 Polyanilines can in principle be reversibly p-doped to one... 

In addition to the stoichiometry of the anodic oxide the knowledge about electronic and band structure properties is of importance for the understanding of electrochemical reactions and in situ optical data. As has been described above, valence band spectroscopy, preferably performed using UPS, provides information about the distribution of the density of electronic states close to the Fermi level and about the position of the valence band with respect to the Fermi level in the case of semiconductors. The UPS data for an anodic oxide film on a gold electrode in Fig. 17 clearly proves the semiconducting properties of the oxide with a band gap of roughly 1.6 eV (assuming n-type behaviour). 

The XPS valence band spectra for the dioxides of the transuranium elements (from Np to Bk) have been presented in an extensive and pioneering work that also includes core level spectra and has been for a long time the only photoemission study on highly radioactive compounds. High resolution XPS spectra (AE = 0.55 eV) were recorded on oxidized thin metal films (30 A) deposited on platinum substrates with an isotope separator. (The oxide films for Pu and the heavier actinides may contain some oxides with lower stoichiometry, since starting with Pu, the sesquioxides of the heavier actinides begin to form in high vacuum conditions.)... 

Electrical conductivity measurements of the as-deposited ln(OH)s showed an expectedly high resistivity of ca. 10 ff-cm. That of the annealed oxide film decreased to 33 O-cm (carrier concentration = 1.85 X 10 cm mobility = 10 cm V sec ). The resistivity is high compared to many other ln20s films (which are often used as transparent conductors), mainly due to the low carrier concentration, implying a high degree of stoichiometry. 

The only other plasma-enhanced CVD film that has seen wide use in integrated circuit manufacture is the plasma oxide film. We say "so-called" because it is not truly Si02, but rather SiOxNyHz. In fact, it is just this ability to modify the film stoichiometry that makes these films so valuable. Many of the film characteristics change depending on this stoichiometry, so it allows a freedom to alter film characteristics that is not possible with thermally-grown films. 

UV Raman studies of ferroelectricity in strain-free non-stoichiometric and nominally stoichiometric SrTiOs films, in combination with dielectric, ferroelectric, nonlinear optical and nanoscale piezoelectric property measurements highlighted the sensitive role of stoichiometry when exploring strain and epitaxy-induced electronic phenomena in oxide films, heterostmctures, and interfaces. 

Aerosol-assisted CVD introduces rapid evaporation of the precursor and short delivery time of vapor precursor to the reaction zone. The small diffusion distance between the reactant and intermediates leads to higher deposition rates at relatively low temperatures. Single precursors are more inclined to be used in AACVD therefore, due to good molecular mixing of precursors, the stoichiometry in the synthesis of multicomponent materials can be well controlled. In addition, AACVD can be preformed in an open atmosphere to produce thin or thick oxide films, hence its cost is low compared to sophisticated vacuum systems. CVD methods have also been modified and developed to deposit solid phase from gaseous precursors on highly porous substrates or inside porous media. The two most used deposition methods are known as electrochemical vapor deposition (EVD) and chemical vapor infiltration (CVI). 

In contrast to the processes described above, the electrooxidation of metals and alloys still cannot be considered as an accepted electrosynthetic method as yet only its principal possibilities have been demonstrated. At the same time, the anodic oxidation of transition metals, which forms the basis for a number of semiconductor technologies, is extremely effective and convenient for varying and controlling the thickness, morphology, and stoichiometry of oxide films [233]. It therefore cannot be mled out that, as the concepts concerning the anodic behavior of metal components of HTSCs in various media are developed, new approaches will be found. The development of combined methods that include anodic oxidation can also be expected, by analogy with hydrothermal-electrochemical methods used for obtaining perovskites based on titanium [234,235], even at room temperature [236]. 

In situ STM studies of the oxidation of a Pd film in the SMSI state at elevated temperature show a thickening of the encapsulating film (Fig. 8.7a-c). The film prior to oxidation had a hexagonal pin-wheel structure on the raised triangular island on the Pd film. After oxidation (Fig. 8.7c), the island was decorated heavily with a thickened, rough layer of Ti implying the formation of an oxide film of higher stoichiometry (possibly TP+), or mass transport of Ti to the Pd surface from the Ti surface. 

For STM and many other surface-science techniques, conductive samples are needed. However, many oxides with perfect stoichiometry are insulators. This problem has been addressed by studying thin oxide films grown on metal singlecrystal substrates. Epitaxial, thin oxide films can be grown for the right choice of metal substrates, which exhibit surfaces with structures similar to those for bulk oxide samples. 

The most commonly used methods for the preparation of ultrathin oxide films are (1) direct oxidation of the parent metal surface, (2) preferential oxidation of one metal of choice from a suitable binary alloy, and (3) simultaneous deposition and oxidation of a metal on a refractory metal substrate. The detailed procedures for (1) and (2) are discussed elsewhere [7,56,57] procedure (3) is discussed here in detail. Preparation of a model thin-film oxide on a refractory metal substrate (such as Mo, Re, or Ta) is usually carried out by vapor-depositing the parent metal in an oxygen environment. These substrate refractory metals are typically cleaned by repeated cycles of Ar sputtering followed by high-temperature annealing and oxygen treatment. The choice of substrate is critical because film stoichiometry and crystallinity depend on lattice mismatch and other interfacial properties. Thin films of several oxides have been prepared in our laboratories and are discussed below. 

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