Secondary ion mass spectrometry study of Pd-based ohmic contacts to GaAs and AlGaAs/GaAs

Secondary ion mass spectrometry (SIMS) has been used to study nonalloyed Zn/Pd/Au (p-type) and Ge/Pd/Au (n-type) ohmic contacts to GaAs. Both contacts have very low contact resistances and smooth surface morphologies. It appears that Pd, a fast diffuser in GaAs, helps the diffusion of Zn into GaAs in the Zn/Pd/Au contact. On the other hand, it was found that Pd and Ge diffused together during the heat treatment of the Pd/Ge/Au contact. SIMS profiles of a Pd/Ge/Au ohmic contact fabricated upon a high-electron mobility transistor (HEMT) structure provide two possible explanations for its significantly higher contact resistance than a conventional Ni/Ge/Au alloyed contact. The SIMS data indicates that Pd may have caused Al and Ga interdiffusion at the AlGaAs/GaAs interface and that the metallization failed to make contact to the electron gas because of the interdiffusion.