Transmission electron microscopy study of the interfacial reactions in gold-based contacts to gallium arsenide

The interfacial structures resulting from the metallurgical interactions between a multilayer gold-based thin film (i.e. Au, Au/Ge/Au, Ni/Au/Ge/Au) and (100) GaAs substrate have been investigated by transmission electron microscopy in surface-normal and cross-section views. A variety of intermetallic compound phases were identified at the contact-GaAs interface. In particular, the metallurgical reaction of Au/Ge/Au with GaAs at 400°C resulted in the previously unknown hexagonal Au₃Ga phase; after annealing above 400°C, GaAs crystallites were observed to grow epitaxially on the underlying GaAs substrate from the melt and the contact became Ohmic. For Ni/Au/Ge/Au on GaAs, the dominant phase that touched the GaAs substrate after annealing at below 350°C was Au₇Ga₂, which existed as a layer between the polycrystalline NiAs layer and the GaAs substrate; after annealing at 400°C, the NiAs grains grew and touched the GaAs substrate, and the contact became Ohmic. The NiAs phase was polycrystalline after annealing up to 450°C but was highly oriented after annealing at 500°C.