Structure and lateral diffusion of ohmic contacts in AlGaAs/GaAs high electron mobility transistors and GaAs devices

Abstract

A conventional Au–Ge–Ni based metallization scheme is used to obtain low resistivity ohmic contacts (≤0.2 Ω mm) to GaAs–AlGaAs based high electron mobility transistor devices and GaAs devices. Thermal stressing reveals that contact resistance remains stable to about 250 °C. At 300 °C an accelerated resistance degradation occurs and instability sets in after 40 h. Cross-sectional electron transmission and energy dispersive x-ray analysis reveals the extent of alloying and the composition of contact region. In the unstressed samples alloying and singular protrusions extending to 0.2 μm below the crystal surface, are observed. At the edge of the contact considerable lateral (0.12–0.15 μm) and vertical protrusions are regularly observed. The extent of the vertical penetration at the contact edge implies that current flow between the alloyed region and the two-dimensional electron gas region occurs in the lateral direction. The lateral contact penetration decreases the nominal source–drain separation and has to be taken into account for accurate device modeling.