MECHANISMS FOR THE FORMATION OF LOW-TEMPERATURE, NONALLOYED AU-GE OHMIC CONTACTS TO NORMAL-GAAS

Low temperature, non-alloyed Au-Ge contact formation to n-GaAs is a multi-step process. During the first 5 min of annealing at 320-degrees-C the Au and Ge segregate into regions a few microns in size and extend over the entire thickness of the metal layer and significant in-diffusion of the Au and Ge and out-diffusion of the Ga and As occurs. This intermixing reduces the barrier height from 0.75 to 0.40 eV. The contact does not show ohmic behavior until it has been annealed for 3 hr. During this time Ge continues to in-diffuse but at a slower rate than it did initially. The rate of Ge in-diffusion is enhanced by the presence of Au since samples containing less Au require longer anneals to show ohmic behavior and have higher specific contact resistances. The presence of excess As, which is prevented from evaporating by a Si3N4 cap has the opposite effect since capped layers have higher specific contact resistances. Au-Ge phases appear after approximately 3 hr of annealing, therefore, Au-Ge phases cannot be responsible for the reduction in barrier height. The interface morphology is smooth, differing from that of pure Au and alloyed contacts that often contain spiking of the metals into the semiconductor. The orientation relationship for the Au grains differs from that pure Au.