STUDY OF TUNGSTEN AND WSI REFRACTORY OHMIC CONTACTS TO GRADED-GAP INGAAS/GAAS/ALGAAS HETEROSTRUCTURES

Non-alloyed tungsten and WSi ohmic contacts to NxGa1-xAs/GaAs/AlxGa1-xAs graded-gap heterostructures were investigated. The contacts showed a low specific contact resistance, below 1×10-5 Ω cm2, and an excellent surface morphology following rapid thermal annealing (RTA) up to 800°C for 10 s. The minimum values for the specific contact resistance, 1.3 × 10-6 and 6.7 × 10-6 Ω cm2 for tungsten and WSi contacts, respectively, were obtained following RTA at 600 °C for 10 s. The increase in the contact resistivity at higher temperatures was attributed to the increase in the heterostructure sheet resistance which resulted from the intermixing of layers and indium and gallium outdiffusion. Transmission electron microscopy examination revealed grain growth and polygonization of tungsten while the WSi films transformed from being amorphous to a crystalline state. The amorphous WSi was shown to perform better than tungsten as a barrier to indium and gallium outdiffusion. In contrast, the significantly lower resistivity of tungsten films, 18 μΩ cm compared with 185 μΩ cm for WSi, made them more attractive for fabrication of non-alloyed refractory ohmic contacts. © 1990.