Ultrathin oxide films treatments

On the other hand, the progress of wet-processes as preparative techniques of metal oxide films has been remarkable. The so-called soft solution process that provides oxide layers by means of electrochemical oxidation of a metal surface is expanding as a synthetic method of various mixed metal oxides with controlled thickness [2], The two-dimensional (2D) sol-gel process based on the hydrolysis of metal alkoxides at the air/water interface has been reported as a preparative technique of ultrathin oxide films (Fig. 6.1a) [3]. It is also known that LB films of metal complexes of long-chain alkyl carboxylic acid can be converted to metal oxide films after removal of organic component by oxygen plasma [4] and UV-ozone treatments (Fig. 6.1b) [5]. Preparation of metal oxide... 

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. 

The electrochemical oxidation of silicon is of interest for the formation of ultrathin dielectric layers due to its low temperatures and easy controllability of the process [3]. In this case, the efficiency of the electrochemical treatment of the materials when oxide layers of low thicknesses are synthesized on their surfaces depends on the uniformity of the charge distribution at the solid phase/electrolyte interface. A great impact on this factor is made by adsorption processes, particularly, chemisorption ones, but their role in the formation of nano-sized films is, not yet understood. 

This approach, developed by Fauchet, Striemer, and coworkers, creates mesoporosity via thermal anneal treatments of ultrathin (10-50 nm) silicon films (Fang et al. 2010). A typical process flow is shown schematically in Fig. 2. For silicon Aims sandwiched between silica layers, porosities up to 15 % were achieved. The potential in vitro and in vivo biomedical uses of such membranes are reviewed in the handbook chapter Porous Silicon in Immunoisolation and Biofiltration. Very recently, higher levels of porosity have been achieved using silicon nitride rather than oxide as the barrier layers (Qi et al. 2014). 

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