SOLID-STATE PHASE-EQUILIBRIA IN THE FE-GA-AS SYSTEM

Solid state phase equilibria in the ternary Fe-Ga-As diagram were dete rmined at 600 degrees C using powder X-ray diffraction as experimental techniques. Very limited solid solubilities were measured in the bina ry constituent Fe-Ga and Fe-As compounds. In the Fe-rich part of the d iagram, a ternary phase was evidenced which corresponds in fact to a s olid solution into which Ga and As substitute one another on the same sublattice. This phase, which can be expressed by the general formula Fe3Ga2-xAsx (0.20 less than or equal to x less than or equal to 1.125) , crystallizes in hexagonal symmetry; it is structurally derived from the NiAs type structure (B8(1)) and can be considered, owing to its ci a ratio (1.23 less than or equal to c/a less than or equal to 1.28) cl ose to root 3/root 2, as hexagonal-pseudocubic. The original feature o f this experimental diagram is the occurrence of a tie-line between th is ternary phase and the semiconductor GaAs which differs significantl y from the theoretical diagram, estimated from simplified calculations proposed by Schmid-Fetzer. In particular, it is the first time that s uch a ternary phase M-x(Ga, As)(y) (M = transition metal) is found to be in thermodynamic equilibrium with GaAs. This fact suggests that the solid state interdiffusions which occur during the annealing of a Fe/ GaAs heterostructure could lead to a ferromagnetic, epitaxial and stab le Fe3GaAs/GaAs contact.