MICROSTRUCTURAL ANALYSIS OF A AU PT/PD/ZN OHMIC CONTACT TO AN ALGAAS/GAAS HETEROJUNCTION BIPOLAR-TRANSISTOR/

Gold-based ohmic contacts, incorporating Pt, Pd, and Zn layers, to AlG aAs/GaAs heterojunction bipolar transistors (HBTs) have been character ized using transmission electron microscopy (TEM). The metallization w as deposited onto a 30 nm graded emitter layer of n-type AlxGa1-xAs, w hich was on a 30 nm emitter layer of n-type Al0.3Ga0.7As, with the aim of contacting the underlying 80 nm thick graded base layer of p-type AlxGa1-xAs. Metal layers were deposited sequentially using electron be am evaporation and the resultant metallizations were annealed at tempe ratures ranging from 250-500 degrees C for up to several minutes. A mi nimum contact resistance of approximate to 8.5 x 10(-7) Ohm-cm(2) was achieved, which corresponded to the decomposition of ternary phases at the metallization/semiconductor interface, to binary phases, i.e., Pd Ga and PtAs2. Long term stability tests were done on the optimum conta cts. Anneals at 270 degrees C for up to four weeks in duration produce d virtually no change in microstructure, with the exception of some ou tward diffusion of Ga and As.