Microstructural characterization of AlGaAs-GaAs modulation-doped field-effect transistor ohmic contacts formed by transient annealing

Transient annealing of a sequentially deposited metallization scheme, Au/Ni/Au/Ge/Ni, was used to obtain low resistivity ohmic contacts to GaAs-AlGaAs based modulation-doped field-effect transistors. Cross-sectional transmission electron microscopy and energy dispersive x-ray analysis techniques were employed to determine the type and distribution of various phases formed. Three different phases, namely, Au rich, Ni-Ge, and Ni-As(Ge), were observed. The Ni-As(Ge) phase was found to be in contact with GaAs. The Ni-Ge phase was present in localized regions in the Au-rich phase. Also, the Ni-Ge phase was found to be in direct contact with either Ni-As(Ge) or with GaAs. The Au-rich region manifested itself as a layered structure and was observed above the Ni-As(Ge) phase. As a consequence of the anneal cycle, movement of contact material both parallel and perpendicular to the device surface occurred at the metallization edges.