Sunday, June 16, 2013

Identification of Chemical Phases

A typical task in thin-film analysis is the identification of chemical phases. In many thin-film projects a materials system is to be developed and the prepared layers have to be checked as to whether an intended chemical phase has successfully been prepared. Information on phase composition is also needed for the deposition process in order to optimize the process conditions. X-ray(cheap medical equipment) diffraction yields different diffraction patterns for different chemical phases and this chemical phase selectivity makes it well suited for such an analytical task. It might be conjectured that the majority of x-ray diffraction investigations of thin solid films are devoted to this type of analysis. 
This chapter outlines how a symmetric B/2B scan has to be analyzed to identify the chemical phases present in a sample. The x-ray attenuation coefficient is con- sidered in more detail because of its close relation to the chemical nature of the thin film. In addition, the attenuation coefficient and the absorption coefficient that derive from it can be used to quantify the layer thickness when a substrate reflection or harmonic reflections occur in the pattern. Also the question of phase mixtures and amorphous phases is addressed. The composition of alloy compounds or stoichiometry deviations may be tackled by Vegard's rule, and, therefore, the obstacles to determining accurate lattice parameters are considered. Since these investigations are all based on a symmetric 9/20 scan, further information(lead glass window) on beam condi-tioning in the B/2B diffractometer is given in Instrumental Box 3. 
The illustrative examples of this chapter are related to polycrystalline and amor-phous semiconductors composed from either group IV elements or II-VI com-pounds.Structure Boxes 2 and 3 are thus devoted to the diamond structure and the sphalerite and wurtzite structure.