Microstructural and electrical investigation of Pd/Ge/Ti/Au ohmic contact to pseudomorphic high electron mobility transistor with undoped cap layer

Microstructural reactions of Pd/Ge/Ti/Au contact to AlGaAs/InGaAs pseudomorphic high electron mobility transistor with an undoped GaAs/AlGaAs cap layer have been investigated using cross-sectional transmission electron microscopy, and the results are used to interpret the electrical properties of the ohmic contact. In the as-deposited state, a quaternary phase of PdxAlGaAs containing excess Ge atoms is formed at the interface of Pd/AlGaAs and some microvoids exist at the Pd layer in the vicinity of the interface. When the ohmic metals deposited on the undoped cap layer were annealed, the lowest contact resistivity of 9.1×10−5 Ω cm2 is obtained at 380 °C. AuGa compound is formed at the PdGe/undoped-AlGaAs interface as a result of the reaction between the ohmic metal and the undoped GaAs cap. This is due to the fast in-diffusion of Au toward the undoped AlGaAs through grain boundaries of the PdGe compound. The layer structure is changed to TiO/AuGa/PdGe/AuGa(TiAs+epi-Ge)/undoped-AlGaAs. The AuGa compound en...