In situ synchrotron radiation studies of functional materials prepared through CVD techniques

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Chemical vapour deposition is an enormously important technique for the formation of thin films. It is the most widely used coating technique in industry for coatings especially for the flat glass industry where it is used to make energy efficient and self cleaning coatings through to the glass bottle industry where it is used to make low friction coatings ( total value > 10B pa). It is also one of the key techniques used in microelectronics for the fabrication of integrated circuits. Despite its immense value the study of the formation of CVD films by in situ analysis has been surprisingly sparse. Typically films are studied after deposition and cool-down and not as they are grown. Some in situ measurements have been made especially by elipsometry, Raman, and reflectometry techniques. Direct measurements of the growth process by XRD or by EXAFS has been difficult to achieve because the samples are in thin-film form typically contain few atomic layers (1-100 nm thick) and construction of reactors cells that accommodate the CVD experiment and the source have proved exacting. The process is only possible with a brilliant light source such as that provided by a synchrotron. The ability to study what is happening during the initial growth phases in a CVD process will bring immense benefit to understanding the process and will enable better design and control of a CVD experiment. For example to determine the position of a dopant atom in a structure and relate this to functionality and to understand how preferred orientation and growth changes during a CVD process. This has societal benefits- for example low-energy window coatings (such as K-glass) prepared by CVD have three times better performance if grown with the optimum growth direction. The performance of ZnO thin films for use as a transparent oxide conductor is magnified if grown with a (1 0 0) orientation. The position of the nitrogen dopant atom within titanium dioxide in N-doped titania is greatly effected by CVD conditions and both substitutional and interstitial doping is seen. However only the interstitial doping is important in extending the band gap and making a visible light photocatalyst. Such a material could find widespread usage as an antimicrobial coating for use in hospitals (to reduce MRSA transmission) and also as a visible light photocatalysts that can be used for water splitting. To realize that potential to improve on existing materials the growth process needs to be studied in detail and the conditions developed and understood that lead only to interstitial N- doping. Such complex problems can now be tackled with the next generation of synchrotron sources such as ESRF and DIAMOND.