MICROSTRUCTURE AND CHEMISTRY OF CU-GE OHMIC CONTACT LAYERS TO GAAS

We report systematic studies of microstructure and chemistry of Cu-Ge alloyed ohmic contacts to n-GaAs with very low specific contact resist ivity ((4-6) x 10(-7) Omega . cm(2) for n similar to 1 x 10(17) cm(-3) ). Using transmission electron microscopy, x-ray microanalysis, and se condary ions mass spectroscopy, we investigated chemistry of phase for mation, crystal structure, and mechanism of ohmic contact formation in Cu-Ge alloyed layers with Ge concentration in the range of 0-40 at.%. Layers with Ge deficiency to form zeta-phase (average composition Cu, Ge) reveal the formation of a nonuniform intermediate layer of hexagon al beta-Cu3As phase which grows epitaxially on Ga{111} planes of GaAs. In this case, released Ga diffuses out and dissolves in the alloyed l ayer stabilizing zeta-phase, which is formed in the structures with av erage Ge concentration as low as 5 at.%. Unique properties of the cont act layers, namely low specific contact resistivity, high thermal stab ility, interface sharpness, and high contact layer uniformity are rela ted to the formation of an ordered orthorhombic epsilon(1)-Cu3Ge phase . In the alloyed layer with Ge concentration >25 at.%, no phases due t o the chemical reactions with GaAs in the interface region were found demonstrating the chemical inertness of the epsilon(1)-Cu3Ge ordered p hase with respect to GaAs. This results in sharp interfaces and unifor m chemical composition, the characteristics needed for superior contac ts.