InAlN high electron mobility transistor Ti/Al/Ni/Au Ohmic contact optimisation assisted by in-situ high temperature transmission electron microscopy

This paper correlates the micro-structural and electrical characteristics associated with annealing of metallic multi-layers typically used in the formation of Ohmic contacts to InAlN high electron mobility transistors. The multi-layers comprised Ti/Al/Ni/Au and were annealed via rapid thermal processing at temperatures up to 925°C with electrical current-voltage analysis establishing the onset of Ohmic (linear IV) behaviour at 750-800°C. In-situ temperature dependent transmission electron microscopy established that metallic diffusion and inter-mixing were initiated near a temperature of 500°C. Around 800°C, inter-diffusion of the metal and semiconductor (nitride) was observed, correlating with the onset of Ohmic electrical behaviour. The sheet resistance associated with the InAlN/AlN/GaN interface is highly sensitive to the anneal temperature, with the range depending on the Ti layer thickness. The relationship between contact resistivity and measurement temperature follow that predicted by thermionic field emission for contacts annealed below 850°C, but deviated above this due to excessive metal-semiconductor inter-diffusion.