Electrical, spectroscopic, and morphological investigation of chromium diffusion through gold films

Gold films approximately 1000 Å thick exhibit a characteristic increase in resistance upon adsorption of mercury. Using scanning tunneling microscopy (STM), it has been observed that mercury adsorbs on the gold surface and subsequently migrates to grain boundaries and other defective regions on the surface. However, after heating a gold film having a thin underlying chromium adhesive layer in air to above 500 K, the resistance change of the gold film after mercury adsorption decreases dramatically. This is attributed to the passivation of the gold surface to mercury at grain boundaries by chromium oxide. Both STM and scanning electron microscopy reveal increased grain size upon prolonged annealing at 573 K. An enlargement of the grain boundaries due to chromium diffusion to the surface is also observed. The chromium diffuses to the surface via grain boundaries and forms Cr₂O₃, as confirmed by X-ray photoelectron spectroscopy. The Cr₂O₃ occupies grain boundaries that otherwise would be available for mercury. With increased annealing time at 573 K, there is an increase in surface chromium concentration and a corresponding decrease in surface mercury concentration.