Ge redistribution in solid-phase Ge/Pd/GaAs ohmic contact formation

A backside secondary ion mass spectrometry technique is employed to examine elemental redistribution in the Ge/Pd/GaAs ohmic contact as a function of annealing conditions. Dilute Al containing marker layers (Ga1−x Alx As) in the GaAs permit precise calibration and alignment of the elemental depth profiles. Double etch-stop thinning yields high depth resolution. The onset of ohmic behavior is found to occur when Ge is detected at the GaAs surface. Good ohmic behavior is observed when an interfacial layer of reacted Pd4GaAs is dispersed and complete coverage of Ge occurs. The Ge/GaAs interface is abrupt with the Ge concentration dropping by over three orders of magnitude within 100 Å. About 40 Å of GaAs is found to be consumed during the ohmic contact formation. Degradation of the ohmic contacts, as a result of further heat treatment, was found to correlate with Ge in-diffusion into the GaAs. The results place strict limitations on doping and heterointerface models of ohmic behavior for this contact.