S A Shivashankar
Articles written in Bulletin of Materials Science
Volume 25 Issue 2 April 2002 pp 163-168 Thin Films
Spectroscopic ellipsometry was used to characterize carbonaceous, crystalline aluminium oxide films grown on Si(100) by low-pressure metal organic chemical vapour deposition, using aluminium acetylacetonate as the precursor. The presence of carbon in the films, attribured to the use of a metalorganic precursor for the deposition of films, was identified and analysed by secondary ion mass spectroscopy and X-ray photoelectron sectroscopy, for the elemental distribution and the chemical nature of the carbon in the films, respectively. Ellipsometry measurments over the photon energy range 1.5-5 eV were used to derive the pseudo-dielectric function of the aluminium oxide-containing films. Multi-layer modelling using linear regression techniques and the effective medium approximation were carried out to extract the structural details of the specimens. The excellent fit between the simulated and experimental optical data validates the empirical model for alumina-containing coatings grown by MOCVD.
Volume 25 Issue 5 October 2002 pp 391-398 Thin Films
Equilibrium concentrations of various condensed and gaseous phases have been thermodynamically calculated, using the free energy minimization criterion, for the metalorganic chemical vapour deposition (MOCVD) of copper films using
Volume 28 Issue 1 February 2005 pp 49-54 Thin Films
Thin films of Sm2O3 have been grown on Si(100) and fused quartz by low-pressure chemical vapour deposition using an adducted 𝛽-diketonate precursor. The films on quartz are cubic, with no preferred orientation at lower growth temperatures (∼ 550°C), while they grow with a strong (111) orientation as the temperature is raised (to 625°C). On Si(100), highly oriented films of cubic Sm2O3 at 625°C, and a mixture of monoclinic and cubic polymorphs of Sm2O3 at higher temperatures, are formed. Films grown on either substrate are very smooth and fine-grained. Infrared spectroscopic study reveals that films grown above 600°C are free of carbon.
Volume 28 Issue 3 June 2005 pp 243-248 Sensors
Thin films of a hydrated phase of tungsten oxide, viz. hydrotungstite, have been prepared on glass substrates by dip-coating method using ammonium tungstate precursor solution. X-ray diffraction shows the films to have a strong 𝑏-axis orientation. The resistance of the films is observed to be sensitive to the humidity content of the ambient, indicating possible applications of these films for humidity sensing. A homemade apparatus designed to measure the d.c. electrical resistance in response to exposure to controlled pulses of a sensing gas has been employed to evaluate the sensitivity of the hydrotungstite films towards humidity.
Volume 31 Issue 1 February 2008 pp 67-72 Thin Films
The equilibrium concentrations of various condensed and gaseous phases were calculated from thermodynamic modeling of MOCVD of ZrO2 films using a 𝛽-ketoesterate complex of zirconium as precursor. This leads to the construction of the `CVD phase stability diagram’ for the formation of solid phases. In the reactive ambient of oxygen, the calculations predict carbon-free ZrO2 film over a wide range of process conditions. The thermodynamic yields are in reasonable agreement with experimental observations, though the removal of carbon from the MOCVD grown films is not as complete as the thermodynamic calculations predict.
Volume 31 Issue 5 October 2008 pp 723-728 Thin Films
Thermodynamic calculations, using the criterion of minimization of total Gibbs free energy of the system, have been carried out for the metalorganic chemical vapour deposition (MOCVD) process involving the 𝛽-ketoesterate complex of iron [tris(𝑡-butyl-3-oxo-butanoato)iron(III) or Fe(tbob)3] and molecular oxygen. The calculations predict, under different CVD conditions such as temperature and pressure, the deposition of carbon-free pure Fe3O4, mixtures of different proportions of Fe3O4 and Fe2O3, and pure Fe2O3. The regimes of these thermodynamic CVD parameters required for the deposition of these pure and mixed phases have been depicted in a `CVD phase stability diagram’. In attempts at verification of the thermodynamic calculations, it has been found by XRD and SEM analysis that, under different conditions, MOCVD results in the deposition of films comprising pure and mixed phases of iron oxide, with no carbonaceous impurities. This is consistent with the calculations.
Volume 31 Issue 6 November 2008 pp 835-839 Thin Films and Nanomatter
Thin films of the hydrated phase of tungsten oxide, hydrotungstite (H2WO4.H2O), have been grown on glass substrates using a dip-coating technique. The 𝑏-axis oriented films have been characterized by X-ray diffraction and scanning electron microscopy. The electrical conductivity of the films is observed to vary with humidity and selectively show high sensitivity to moisture at room temperature. In order to understand the mechanism of sensing, the films were examined by X-ray diffraction at elevated temperatures and in controlled atmospheres. Based on these observations and on conductivity measurements, a novel sensing mechanism based on protonic conduction within the surface layers adsorbed onto the hydrotungstite film is proposed.
Volume 33 Issue 3 June 2010 pp 251-255 Polymers
Polycrystalline diamond coatings are grown on Si (100) substrate by hot filament CVD technique. We investigate here the effect of substrate roughening on the substrate temperature and methane concentration required to maintain high quality, high growth rate and faceted morphology of the diamond coatings. It has been shown that as we increase the substrate roughness from 0.05 𝜇m to 0.91 𝜇m (centre line average or CLA) there is enhancement in deposited film quality (Raman peak intensity ratio of 𝑠𝑝3 to non-𝑠𝑝3 content increases from 1.65 to 7.13) and the substrate temperature can be brought down to 640°C without any additional substrate heating. The coatings grown at adverse conditions for 𝑠𝑝3 deposition has cauliflower morphology with nanocrystalline grains and coatings grown under favourable 𝑠𝑝3 condition gives clear faceted grains.
Volume 34 Issue 1 February 2011 pp 11-18
Equilibrium thermodynamic analysis has been applied to the low-pressure MOCVD process using manganese acetylacetonate as the precursor. ``CVD phase stability diagrams” have been constructed separately for the processes carried out in argon and oxygen ambient, depicting the compositions of the resulting films as functions of CVD parameters. For the process conduced in argon ambient, the analysis predicts the simultaneous deposition of MnO and elemental carbon in 1:3 molar proportion, over a range of temperatures. The analysis predicts also that, if CVD is carried out in oxygen ambient, even a very low flow of oxygen leads to the complete absence of carbon in the film deposited oxygen, with greater oxygen flow resulting in the simultaneous deposition of two different manganese oxides under certain conditions. The results of thermodynamic modeling have been verified quantitatively for lowpressure CVD conducted in argon ambient. Indeed, the large excess of carbon in the deposit is found to constitute a MnO/C nanocomposite, the associated cauliflower-like morphology making it a promising candidate for electrode material in supercapacitors. CVD carried out in oxygen flow, under specific conditions, leads to the deposition of more than one manganese oxide, as expected from thermodynamic analysis (and forming an oxide–oxide nanocomposite). These results together demonstrate that thermodynamic analysis of the MOCVD process can be employed to synthesize thin films in a predictive manner, thus avoiding the inefficient trial-and-error method usually associated with MOCVD process development. The prospect of developing thin films of novel compositions and characteristics in a predictive manner, through the appropriate choice of CVD precursors and process conditions, emerges from the present work.
Volume 34 Issue 6 October 2011 pp 1163-1171
Nanopowders of TiO2 has been prepared using a microwave irradiation-assisted route, starting from a metalorganic precursor,
Volume 34 Issue 7 December 2011 pp 1633-1637
A method for the estimation of vapour pressure and partial pressure of subliming compounds under reduced pressure, using rising temperature thermogravimetry, is described in this paper. The method is based on our recently developed procedure to estimate the vapour pressure from ambient pressure thermogravimetric data using Langmuir equation. Using benzoic acid as the calibration standard, vapour pressure–temperature curves are calculated at 80, 160 and 1000 mbar for salicylic acid and vanadyl
Volume 42 | Issue 3