Microstructural analysis of Pd-based ohmic contacts to p-type GaAs

As part of the investigation of the use of Pd-based ohmic contacts to p-type GaAs, cross-sectional transmission electron microscopy, Auger electron spectroscopy, and secondary ion mass spectroscopy were used to explore the uniformity at the metal/GaAs interface and its composition profile after ohmic contact formation. Comparisons were made among Au:Be, Au: Be/Pd, and Au/Pd contacts. Regions of p⁺were formed in n-type GaAs by a spin-on source which was rapid diffused at 950 °C for 6s or by ion implantation at a dose of 3 x 10¹⁴atoms/cm²at 150 keV for 15 min. Metallizations were accomplished by evaporation with a base pressure of 3 x 10^–6Torr. Sintering of the metallizations was done in a furnace at 350 °C for 15 min. Cross-sectional transmission electron microscope studies revealed an absence of spiking when Be is present in the metallization scheme but a broad band diffused into GaAs. An improper metal/GaAs adhesion was observed when Pd is absent. Be assists in confining the reaction of Pd with GaAs and acts as a diffusion barrier to the p⁺dopant. Electrical measurements, taken from transmission line and cross bridge Kelvin resistors, were best for the Pd/Au:Be, which yielded a contact resistance of 0.3 μΩ-cm².