Micro-structural evaluation of Ti/Al/Ni/Au ohmic contacts with different Ti/Al thicknesses in AlGaN/GaN HEMTs

• Ohmic contacts with four different Ti/Al metal thickness ratios were fabricated on undoped Al.28Ga.72N/GaN HEMT under various annealing cycles using Rapid thermal processor.
• Ohmic contact with Ti/Al metal thickness ratio, 1/5 showed superior quality in terms of Rc and surface morphology.
• Ni–Al alloy segregation on the surface as analyzed by EDAX seems to be related to bumpy surface morphology.
• Ohmic contact with Ti/Al metal ratio 1/5 showed the presence of TiN by X-ray diffraction analysis. This was further confirmed by SIMS analysis.
• The lower level of elemental Ti and Al at the surface of the contact with Ti/Al metal thickness ratio 1/5 compared to the other contacts indicated lower oxidation possibility and therefore superior quality ohmic contacts with the metal ratio 1/5.

In this work, ohmic contacts were formed by varying the Ti/Al thickness ratio in the metal stack of Ti/Al/Ni/Au on Al.28Ga.72N/GaN HEMT epistructure followed by annealing in the temperature range 740–860 °C by rapid thermal processor (RTP). The contacts were electrically characterized for contact resistance (Rc) and the sheet resistance (Rs) of AlGaN/GaN epistructure. The ohmic contacts formed by Ti/Al metal thickness ratio of 1/5 exhibited lowest Rc values and better surface morphology compared to the contacts formed by other Ti/Al metal thickness ratios. The difference observed in the electrical characterization of these contacts was correlated with their X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS) analyses. The surface morphology of the ohmic metal post annealing showed two distinct regions in scanning electron microscope (SEM) images. The energy dispersive X-ray analysis (EDAX) identified these regions as Ni–Al and Au–Al rich. Ni–Al rich region is believed to be responsible for rough morphology. Further, the contact formed with Ti/Al metal thickness ratio 1/5 showed less number of elemental Al and Ti atoms and therefore was correlated with lower oxidation probability of the contact compared to ohmic contact formed by other metal thickness ratios.