Characterization of low-resistance ohmic contacts to n- and p-type InGaAs

Multilayer ohmic contacts with differing first metal layers (M = Mo, Pd, Pt) beneath a Ti/Pt diffusion barrier and Au cap were fabricated on n+ and p+-InGaAs, and the relationship between their specific contact resistance and interfacial chemistry was examined. Palladium-based contacts offered the lowest specific contact resistances of ρc=3.2×10−8 and 1.9×10−8 Ω-cm2 to n+- and p+-InGaAs, respectively. The low resistances of the Pd-based contact were correlated with the formation of a uniform PdxInGaAs phase in direct contact with InGaAs, as observed using transmission electron microscopy and energy dispersive spectroscopy. On the other hand, the Mo-based contact to n+ and p+-InGaAs had much higher specific contact resistances, even though its specific contact resistance on lightly doped n-InGaAs was nearly the same as that of the Pd-based contact. The cause of this discrepancy was identified to be the native oxide layer that remained between the contact and semiconductor in the Mo-based contacts, as revealed using transmission electron microscopy, energy dispersive spectroscopy, and electron energy loss spectroscopy. The effect of the native oxide in series with the Schottky barrier is significant only when the contact is doped heavily enough that tunneling of carriers through the Schottky barrier introduces a very low resistance.