Chemical Vapor Deposition

Hydrogen plays an important role in the CVD growth of the wide bandgap GaN and AlGaN materials system and also in the subsequent processing of these semiconductors. It diffuses rapidly in these materials even at quite low temperatures and can profoundly affect the electrical properties. A review is given of the present understanding of hydrogen in the wide bandgap nitrides.

The production mechanisms and the gas-phase diagnostic techniques for H atoms, which play important roles in catalytic CVD (also called hot-wire CVD) and plasma-enhanced CVD processes, were reviewed. Experimentally determined absolute H-atom densities in typical CVD processes are compiled in a table. Procedures for producing and detecting vibrationally excited H₂ molecules, which can be another active species in CVD processes, are also discussed.

The state-of-the-art performance, obtained through the reduction of precursor gas residence time in hematite photoanodes produced by APCVD, is explained in this report by an enhanced preferential orientation of the highly conductive crystal planes perpendicular to the electron-collecting substrate.

Spinel-type Co₃O₄ nanopyramids grown by CVD starting from Co(hfa)₂•TMEDA display attractive features for hydrogen production by photo-reforming of methanol solutions, especially in the presence of oxygen in the reaction medium. To the best of our knowledge, this is the first example of photo-activated H₂ generation by means of supported cobalt oxide nanosystems.

We report the observation of hydrogen production from photodiodes, when irradiated with UV light. These photodiodes were composed of a stainless steel substrate, with a TiO₂ photocatalyst deposited suing APCVD on one side and Pt deposited using sputter coating on the other. In contrast similar devices made using a titanium substrate show no measurable H₂ production.

Monometallic Ru and bimetallic Ru-Sn heterogeneous catalysts were prepared by CVD of Ru(TMHD)₃ and Sn(n-Bu)₄ precursors on Mg(Al)O mixed oxide support. Ru/Mg(Al)O system was very efficient and durable in the hydrogen production by glycerol steam reforming. The effect of tin addiction was investigated and at low loadings it selectively decorates Ru faces sites leaving exposed the most active Ru sites.

Dense and porous La_0.8Sr_0.2CrO₃ films for use in Solid Oxide Fuel Cells are fabricated on stainless steel using a flame-assisted CVD route, starting from a solution of the corresponding metal nitrates. The interrelations between the processing parameters and the film morphology and microstructure are investigated by SEM and XRD. The functional properties of the obtained systems in H₂ production by methane steam reforming are also presented and discussed.

A systematic study of CVD-based synthesis strategies for the fabrication of zeolite-templated carbons, using commercially available zeolite Y as hard template, has generated predominantly microporous carbons with surface area of 850−2160m²/g and pore volume of 0.87−1.26cm³/g. The templated carbons exhibit variable hydrogen uptake capacity in the range 1.9–4.9wt% (at −196°C and 20 bar), with the uptake dependent on the textural properties. The study clarifies on the effect of the nature of carbon precursor and the influence of using two CVD steps or a combination of liquid impregnation and CVD.

The adsorption and desorption kinetics of hydrogen and hydrogen chloride on a silicon surface were computationally investigated using density functional and coupled cluster theories. The investigated mechanisms were the pairwise intradimer and the 4H-2H reaction pathways. The calculations reveal that, while the H₂ surface desorption dynamics is dominated by the 4H-2H mechanisms, the fastest HCl desorption mechanism proceeds through the intradimer pathway.

Chemical vapor deposition of copper oxide films using a hot-wall tubular reactor and a cuprous iodide and oxygen precursor system is studied experimentally and by developing a sequence of empirical response surface models and physically based transport and deposition chemistry models. In particular, the source of an unusually sharp transition between distinct cuprous and cupric oxide phases is attributed to the O2 gradient resulting from the back-diffusion of this reactant and surface reactions producing the copper oxide films.