Role of Ti/Al relative thickness in the formation mechanism of Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures

In this work, Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures with different Ti/Al relative thicknesses were fabricated and characterized. A contact with Ti/Al relative thickness of 20/180 nm presented non-linear current–voltage response and bumpy surface morphology, while a contact with Ti/Al relative thickness of 20/120 nm demonstrated low contact resistance and flat surface morphology. We verify the existence of two electron transport mechanisms, namely carrier tunnelling and direct link through a TiN conductive pathway, by scanning transmission electron microscopy and corresponding electron dispersive x-ray spectra. Ti/Al relative thickness exerts a decisive influence on the alloy reaction processing. Superfluous Al consumes Ti, thus impeding the formation of low-resistance TiN and N vacancies. As a result, electron transport is restricted by the low-efficiency tunnelling mechanism. At the same time, the consumption of Ni causes damage to its blocking effect and arouses the free diffusion of metal alloys, which should be responsible for the rough surface.