Electrical and microstructural investigation of Au/Pd/Ti ohmic contacts for AlGaAs/GaAs heterojunction bipolar transistors

The microstructure and electrical properties of as deposited and annealed Au (400 nm)/Pd (75 nm)/Ti (10 nm) contact structures to p type GaAs, C doped with a concentration of 5 x 10(18) and 5 x 10(19) cm(-3), have been investigated using transmission electron microscopy, and current-voltage measurements as a function of temperature in the range 198-348 K. The specific contact resistivities have also been measured using the transmission line method. It was found that increasing the epilayer doping level by an order of magnitude, from 5 x 10(18) to 5 x 10(19) cm(-3), caused the dominant current transport mechanism to change from thermionic field emission to field emission. For the lower level doped epilayers generation-recombination within the depletion region was found to be the dominant current transport mechanism for temperatures below 298 K. The contacts to the more highly doped epilayers (C doped, 5 x 10(19) cm(-3)) had specific contact resistivities of 0.08 +/- 0.03 Ohm mm and 0.05 +/- 0.06 Ohm mm, respectively. These values, together with a minimal metal penetration in the semiconductor of <15 nm, indicate that these contacts are suitable for heterojunction bipolar device applications.