A Comparative Study on the Formation of Cr and Ti Ohmic Contacts to (001) β-Ga2O3

A comparative study of Cr/Au and Ti/Au ohmic contacts on (001) β-Ga2O3 was conducted. The electrical behavior from current-voltage measurements and the interfacial composition and microstructure as determined from high-resolution transmission electron microscopy with energy dispersive x-ray analysis were compared for the different contacts at selected points in an annealing series (300 - 700{degree sign}C, 1 min anneals in N2). Cr/Au contacts became ohmic at temperatures (300 - 350{degree sign}C) approximately 50 - 100{degree sign}C lower than Ti/Au contacts (400 - 450{degree sign}C). Cr/Au and Ti/Au contacts demonstrated optimal ohmic behavior (lowest resistance) when annealed to 450 - 500{degree sign}C and 500 - 600{degree sign}C, respectively, with Ti/Au contacts yielding a lower total resistance than Cr/Au. Cross-sectional TEM images of Cr/Au contacts annealed at 450{degree sign}C revealed the presence of Au nanoclusters at the Ga2O3 interface and CrOx layers at both the top of the contact and the Ga2O3 interface. Whereas TiOx also formed at the top and bottom interfaces in 450{degree sign}C-annealed Ti/Au contacts, the TiOx surface layer appeared to be discontinuous, unlike the CrOx surface layer. Au nanoclusters were not detected at the interface in the Ti/Au contacts. The interdiffusion and oxidation observed in both contact metallizations point to the need for diffusion barriers that may allow these contacts to be used in future Ga2O3-based devices that operate at elevated temperatures.