MICROSTRUCTURE AND CONTACT RESISTANCE TEMPERATURE-DEPENDENCE OF PT/TI OHMIC CONTACT TO ZN-DOPED GAAS

Ohmic contacts to p-GaAs, Zn doped at 5×1018 and 1×1019 cm-3, with the best specific contact resistance of 1.2×10-5 and 2.4×10-6 Ω cm2, respectively, have been formed with deposited layers of Ti and Pt. These layers, which were sequentially evaporated and then rapid thermally processed at 450 °C for 30 s, contained an interfacial layer constituted mainly of the TiAs phase adjacent to the substrate and the TixGa 1-x solid solution in between it and the Ti layer. In addition, a significant amount of the Pt3Ti intermetallic was found at the Ti/Pt interface. The same metallization scheme, applied to 1×1018 cm-3 Zn-doped GaAs, produced a Schottky contact for the as-deposited and heat-treated samples. The temperature dependence characteristic of the specific contact resistance of the as-deposited Pt/Ti contacts to 5×1018 and 1×1019 cm-3 Zn-doped GaAs revealed a thermionic emission dominated carrier transport mechanism with an apparent barrier height of about 0.118 and 0.115 eV, respectively. This mechanism remained as the dominated one for the heat-treated contacts to the lower doped substrate. The contact resistance of the heat-treated contacts to the more heavily doped substrate, however, revealed a weaker temperature dependence. This indicates a conversion to a mixture of thermionic and field-emission carrier transport mechanisms across the interfacial barrier, and a reduced barrier height to a minimum value of 0.068 eV measured after sintering at 450 °C.