Atomic ordering in intermetallic CoAl alloys epitaxially grown on GaAs (001)

Epitaxially grown intermetallic CoAl alloys on GaAs (001) are investigated by transmission electron microscopy (TEM). Nanobeam diffraction proves the coexistence of the ordered B2 (CsCl) structure and its disordered BCC version, which is also verified by high-resolution TEM. From dark-field imaging, it is observed that the relative volume fractions of these two phases depend on the growth condition: the lower the growth temperature, the larger the proportion of B2 phase. The mechanism is discussed with respect to the diffusivity of vacancies and Co atoms.