Zinc Diffusion in GaAs-AIGaAs Heterojunction Bipolar Transistor Structures

ABSTRACTWe have examined the diffusion of Zn from the base of GaAs-AIGaAs heterojunction bipolar transistor (HBT) structures during growth by organometallic vapor phase epitaxy. The role of Si doping in the emitter-contact, emitter, and collector/subcollector in enhancing the Zn diffusion has been determined by separately doping each layer. For a growth temperature of 675°C Zn shows no observable redistribution up to concentrations of 3x1019 cm−3 without Si doping. The addition of Si to the adjacent AIGaAs emitter and GaAs collector/subcollector layers causes significant diffusion from the base, while Si doping of the GaAs emitter-contact results in even greater Zn redistribution. Silicon counter-doping in the base region retards the Zn diffusion. These results are consistent with a recent model which shows that the n-type surface layer enhances the formation of gallium interstitials which diffuse into the structure and displace the Zn in the base via a kick-out mechanism.