Growth evolution and characterization of ultra-thin CoGe2 films synthesized via a catalytic solid–vapour reaction technique

Abstract

A low temperature process, relying on the catalytic chemical vapor reaction of GeH₄ with solid Co layers, is developed to synthesize ultra-thin CoGe₂ layers (10–20 nm) on 300 mm Si wafers. The selective reaction of Co with GeH₄ results in the direct formation of CoGe₂ films crystallizing in the orthorhombic structure. Detailed studies under germanidation reaction conditions show that the optimal temperature (325 °C vs. 400 °C) to obtain low resistive, defect free and continuous CoGe₂ films depends on the initial Co thickness. Investigation on various stages of cobalt germanide growth evolution demonstrates that the complete conversion of Co, leading to closed and stoichiometric CoGe₂ films, occurs after 60 s of GeH₄ exposure as evidenced by atomic force microscopy (AFM), Rutherford back scattering (RBS) and sheet resistance (R_s) analysis. The CoGe₂ formation is found to exhibit self-limiting growth. The layers demonstrate constant film properties (phase purity, resistivity, composition homogeneity, uniformity, and smooth morphology with insignificant rms roughness increase) within a large process window, negligibly influenced by excessive GeH₄ exposure conditions. Independent of the post-deposition annealing employed (RTA or furnace annealing), results showed the CoGe₂ layers to be thermally stable up to 500 °C without phase or morphological degradation or film roughening. Resistivity values in the range of 70–50 μΩ cm were observed for 10–20 nm CoGe₂ films.