A SIMS and TEM investigation of Au/Ti/Pd solid state Ohmic contacts on p-GaAs

The effects of annealing on the distribution of elements in a Au(400 nm)/Ti(75 nm)/Pd(75 nm) Ohmic contact structure on zinc-doped p-GaAs epilayers, have been investigated using secondary ion mass spectrometry and cross-sectional transmission electron microscopy. The structure remained layered upon heat treatment up to 380 degrees C in spite of considerable elemental mixing and the formation of new phases. The metallisation/semiconductor interfacial region was found to be very reactive. At room temperature, interaction between the contact and the GaAs resulted in the formation of a 20 nm thick Pd-Ga-As ternary layer (phase I) adjacent to the substrate, Annealing the structure at temperatures of 200 and 260 degrees C led to further interaction at the contact/GaAs boundary and to the creation of protrusions, composed of a second Pd-Ga-As ternary compound (phase II), extending 90 nm into the semiconductor substrate, Heat treatments at 320 and 380 degrees C resulted in a uniform multi-phase layer without protrusions, of total thickness 170 nm, next to the GaAs substrate.