Interface dynamics in ohmic contact optimization on AlGaN/GaN heterostructure by the formation of TiN

We illustrate a systematic approach to reduce the ohmic contact resistance on AlGaN/GaN heterostructure. We have optimized the ohmic contact resistance by varying the thickness of AlGaN barrier layer, Ti layer, Al/Ti ratio, and temperature. We have observed nonlinear dependency on various process conditions. Careful observations on performed experiments bring out the interface dynamics of the metal and semiconductor layers. We have demonstrated a very low typical contact resistance of 0.06 Omega.mm for AlGaN barrier thickness of 13 nm, Ti/Al/Ni/Au of 12/150/40/50 nm, and annealed at 775 degrees C. The minimum contact resistance is found to be 0.05 Omega mm when measured across devices. This R-C value is appropriate for high power RF applications of AlGaN/GaN based high electron mobility transistors. This value of the contact resistance is the lowest among the values reported on AlGaN/GaN heterostructure using alloyed contacts. A lower thickness AlGaN barrier can help to reduce the thermal budget for achieving low ohmic contact resistance. This is achieved at lower critical thickness for Ti which is dependent on the AlGaN barrier thickness. It is also observed that Ti:Al ratio should be increasingly larger for decreasing thickness of Ti. This trend is observed till 7 nm of Ti.