Microstructure and chemistry of Cu-Ge ohmic contact layers to GaAs

Abstract

We report systematic studies of microstructure and chemistry of Cu-Ge alloyed ohmic contacts to n-GaAs with very low specific contact resistivity ((4-6) x 10-7 Ωcm2 for n∼l x 1017cm-3). Using transmission electron microscopy, x-ray microanalysis, and secondary ions mass spectroscopy, we investigated chemistry of phase formation, 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 ζ-phase (average composition Cu5Ge) reveal the formation of a nonuniform intermediate layer of hexagonal -Cu3As phase which grows epitaxially on Ga111 planes of GaAs. In this case, released Ga diffuses out and dissolves in the alloyed layer stabilizing ζ-phase, which is formed in the structures with average Ge concentration as low as 5 at.%. Unique properties of the contact layers, namely low specific contact resistivity, high thermal stability, interface sharpness, and high contact layer uniformity are related to the formation of an ordered orthorhombic e1 Cu3Ge phase. In the alloyed layer with Ge concentration >25 at.%, no phases due to the chemical reactions with GaAs in the interface region were found demonstrating the chemical inertness of the e1Cu3Ge ordered phase with respect to GaAs. This results in sharp interfaces and uniform chemical composition, the characteristics needed for superior contacts.