Comment on “Phase equilibria of the Ga–Ni–As ternary system” [J. Appl. Phys. 80, 543 (1996)]

The phase equilibria of the Ga–Ni–As ternary system, especially in its upper part, has given rise to much controversy. An attempt to clarify this situation is given in this comment. We show that the re-examined diagram proposed by Ingerly et al. [J. Appl. Phys. 80, 543 (1996)] is not right owing to a much too limited number of synthesized samples, and that this diagram cannot rationalize the complex solid-state interdiffusion which occurs in the Ni/GaAs contacts. We confirm the occurrence of ordered ternary phases which crystallize in hexagonal superlattices a√3,3c and 2a,4c derivative from the NiAs-type structure. Finally, we point out that these superlattices index successfully all the extra lines of the x-ray diffraction pattern that Ingerly et al. were unable to explain.