Transmission electron microscopy assessment of the Si enhancement of Ti∕Al∕Ni∕Au Ohmic contacts to undoped AlGaN∕GaN heterostructures

The microstructure of Si∕Ti∕Al∕Ni∕Au was investigated using transmission electron microscopy and energy dispersive x-ray spectroscopy. The dependence of the contact resistance on the silicon layer thickness and the temperature was correlated to the microstructure of the alloyed contacts. The enhancement of the contact resistance by inserting a 30Å thick Si layer under the Ti∕Al∕Ni∕Au metallization was attributed to diffusion of the contact into the AlGaN layer. Increasing the Si thickness and or the temperature resulted in the formation of Gold (Au)-based silicides, which prevent the formation of low interfacial TiN or AlN layers.