Degradation of scanned-electron-beam-alloyed Ohmic contacts on n-GaAs
Degradation of scanned-electron-beam- (SEB) and furnace-alloyed AuGe-Ni Ohmic contacts to n-type GaAs with time, above room temperature, has been studied with and without current flow through the contacts. The degradation is characterised in terms of contact resistivity, source-to-drain resistance Rsd, and surface and subsurface morphology. The results show that SEB-alloyed contacts are more stable than furnace-alloyed contacts when aging is performed at temperatures below 300 degrees C without current flow, and the smooth surface morphology of the SEB-alloyed contacts is lost when they are aged above 300 degrees C. SEB-alloyed contacts show an overall reducing contact resistivity with aging time. In the case of gradual degradation of contacts due to current flow above room temperature, for typical GaAs field-effect-transistor contact structures, the SEB-alloyed contacts do not show any performance advantage over furnace-alloyed contacts. The threshold voltage for metal migration from the drain (anode) to the source (cathode) resulting in catastrophic breakdown of the device is found to be the same for furnace- and SEB-alloyed contacts.