Multiple Ti/Al stacks induced thermal stability enhancement in Ti/Al/Ni/Au Ohmic contact on AlGaN/GaN heterostructure

Compared with Ti/Al/Ni/Au Ohmic contacts, Ti/Al/Ti/Al/Ti/Al/Ti/Al/Ti/Al/Ni/Au with multiple Ti/Al stacks improved the thermal stability. Multiple-stacked Ohmic contacts showed lower degradation during long-time thermal aging at 600°C. The samples after degradation were tested with Transmission Electron Microscopy (TEM) to research the structural reasons. TEM results show that multiple stacks can avoid the producing of voids, which usually appear in the single Ti/Al stacked Ohmic metals. Al-Au alloy is believed to be the origin of the voids. While multiple Ti/Al stacks can reduce the size and quantity of Al-Au alloy and thus avoid the appearance of voids. The thermal stability enhancement of Ti/Al/..../Ti/Al/Ni/Au Ohmic contacts rendered it a great application in high temperature AlGaN/GaN HEMTs.