Microstructure and contact resistance temperature dependence of Pt/Ti ohmic contact to Zn-doped GaAs

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Abstract
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 TixGa1−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.