Conventional transmission electron microscopy and high‐resolution electron microscopy were used to study the cross sections of Au/Zn/Au ohmic contacts to a [001]‐orientedp‐type GaAs substrate. The metal contacts fabricated by sequential vapor deposition were capped with an Al2O3thin film to form a closed system when subjected to a 420 °C anneal for 3 min. For an as‐deposited specimen, the Zn layer was found to react with the two adjacent Au layers and the reaction products were three room‐temperature phases of Au3Zn. One of these phases is identified for the first time and its structure is characterized. Upon annealing, these phases remained stable and no interfacial reaction between the metal contact and GaAs was observed, as opposed to an uncapped contact. The interface between the annealed contact and GaAs exhibited the same degree of uniformity and planarity as that of the as‐deposited specimen. This study demonstrates that the formation of metallic protrusions into GaAs, as commonly found in an uncapped contact, is not necessary for the transition from rectifying to ohmic behavior of the contact.