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 ε₁ 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 ε₁Cu₃Ge ordered phase with respect to GaAs. This results in sharp interfaces and uniform chemical composition, the characteristics needed for superior contacts.