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 pseu domorphic high electron mobility transistor with an undoped GaAs/AlGaA s cap layer have been investigated using cross-sectional transmission electron microscopy, and the results are used to interpret the electri cal properties of the ohmic contact. In the as-deposited state, a quat ernary phase of PdxAlGaAs containing excess Ge atoms is formed at the interface of Pd/AlGaAs and some microvoids exist at the Pd layer in th e vicinity of the interface. When the ohmic metals deposited on the un doped cap layer were annealed, the lowest contact resistivity of 9.1 x 10(-5) Ohm cm(2) is obtained at 380 degrees C. AuGa compound is forme d at the PdGe/undoped-AlGaAs interface as a result of the reaction bet ween 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 enhances the cr eation of group III vacancies, and the in-diffused Ge atoms occupy the vacancies. Thus, a number of electrons are produced below the contact , which plays a role in reducing the contact resistivity. For the ohmi c metals deposited on n-AlGaAs by removing the undoped cap layer, the annealing temperature at which the contact resistivity has the minimum value of 2.3 x 10(-6) Ohm cm2 increases to 460 degrees C. The Au2Al i s additionally observed at the PdGe/n-AlGaAs interface. Consequently, the layer structure is changed to TiO/AuGa/PdGe/AuGa + Au2Al(TiAs + ep i-Ge)/n- type AlGaAs. The formation of Au2Al at the PdGe/n-AlGaAs inte rface creates more group III vacancies. Thus, the contact resistivity is further reduced by the incorporation of the in-diffused Ge into the group III vacancies. The InGaAs channel layer is observed to be inter mixed in the annealed sample. This evidences the production of a large number of the group III vacancies via electrons below the contact. (C ) 1998 American Institute of Physics. [S0021-8979(98)04914-7].