OHMIC CONTACTS TO N-GAAS WITH A PT/GE/AU CONTACTING LAYER AND A TA-SI-N BARRIER - ELECTRICAL AND METALLURGICAL CHARACTERISTICS

Pt/Ge/Au trilayers of various Pt:Ge compositions, overlaid with a Ta-Si-N barrier layer and an Au metallization layer, are investigated as ohmic contacts to n-type GaAs. After annealing in flowing argon at 450-degrees-C for 15 min, a Contact resistivity of 3.7 x 10(-6) OMEGA CM2 is obtained for the sample of atomic ratio Pt/Ge = 1. The contact resistivity of this sample degrades only slightly to 5.0 X 10(-6) OMEGA cm2 upon aging at 450-degrees-C for 60 h, while the surface stays smooth. Contact resistivities of samples with other Pt/Ge atomic ratios are in the range of 10(-5)-10(-4) OMEGA CM2. TO understand this electrical behavior, the contacts are characterized by backscattering spectrometry, x-ray diffraction, and transmission electron microscopy in conjunction with energy-dispersive analysis of x rays. The reaction products vary with the Pt:Ge compositions due to the difference of the chemical reactivity between Pt, Ge, and GaAs. The formation and distribution of a ternary PtGe:As phase are the determining factors for the contact resistivity. The outstanding thermal stability of the contact is due to the Ta-Si-N barrier layer which closes the GaAs-trilayer system and protects their chemical equilibria from being disrupted by an inflow of Au from the metallization layer. Without the barrier layer, the morphology of the contact degrades badly at 450-degrees-C after 20 h or less.