MICROPARACRYSTALLINE STRUCTURE OF AMORPHOUS CD-AS FILMS

A structural model of amorphous CdAs films in the composition range from 45 to 82 at.% As, based on the microparacrystalline theory, has been developed. An analysis of the X-ray diffraction data has been carried out by comparison of both experimental intensity ki(k) (k = 4-pi sin-theta/lambda) and radial distribution 4-pi r2 (rho-rho(o) functions with those of calculated using the concept of paracrystallinity. Two structural region with different composition ranges are distinguished. The first structural domain extends from 45 to about 67 at.% As. In this composition range, the distorted tetrahedral structure, intermediate between CdAs and Si III type structure, is adequate to account for the experimental data. The second structural domain extends from about 74 to 82 at.% As. In this composition range, the ki(k) and 4-pi r2 (rho-rho(o) functions look much like those of glassy CdAs2. The tetrahedral structure model based on the tetragonal CdAs2 structure, which is characterized by As spiral chains linked together by Cd atoms, proves to be suitable for arsenic-rich compositions. It has been shown that the "alpha* relation", empirically established by Hosemann et al., is satisfied for all investigated amorphous cadmium arsenide films as well as CdAs2 glass.