Stability of InAs contact layers on GaAs/AlGaAs heterojunction bipolar transistors during implant isolation annealing

The thermal stability of thin (300-500 AA) InAs contact layers on GaAs/AlGaAs heterojunction bipolar transistors (HBTS) during implant isolation anneals (450-540 degrees C, 60 s) was examined by looking at the contact characteristics of AuGe/Au or TiPtAu metallization subsequently deposited on the InAs, and also by transmission electron microscopy. Thicker (500 AA) InAs layers annealed with dielectric (Si₃N₄) protection, followed by deposition and alloying (400 degrees C, 30 s) of AuGe-based metallization, produce the least degradation of ohmic contacts for high-speed HBT operation. Higher annealing temperatures (<or=570 degrees C) produce the lowest isolation currents between devices, but these temperatures induce severe degradation of the InAs quality and compromise the ohmic contact resistance. The best solution is to perform the isolation anneal under an overpressure of arsenic vapour-in this condition, no degradation of the InAs was observed.