Microstructural characterisation of TiAlTiAu and TiAlPdAu ohmic contacts on AlGaN/GaN
Analysis of the TiAlTiAu sample annealed at 950°C concludes that the inclusions are TiN, which is a semi-metal with a low work function, and would therefore satisfy the requirement for ohmic contacting to n-type GaN. However, despite the significant change in interfacial morphology observed between TiAlTiAu samples annealed at 850°C and 950°C, the electrical behaviour showed only a slight improvement. This indicates that it is the formation of the interfacial TiN phase that is the key to ohmic behaviour in this contact. The TiAlPdAu contact annealed at 950°C showed a much lower density of inclusions than the TiAlTiAu sample. It is believed that the Al/Au diffusion front is essential to the formation of the inclusions, indicating that the Pd layer is more effective than the second Ti layer at inhibiting the diffusion of Au to the interfacial region. However, the lack of ohmic behaviour in the TiAlPdAu contact suggests that this sample contained inadequate activities of Ti at the interfacing to form adequate levels of interfacial TiN, which is necessary for the formation of an ohmic contact in these schemes.