Effect of TiN barrier layer in Cu-based ohmic contact of AlGaN/GaN high electron mobility transistor

Ti/TiN/Cu is established to be an enabling alternative to the better-known Au-based ohmic contact metals such as Ti/Al/Ni/Au. The Cu-based option delivers lower contact resistance and smoother surface morphology and is proven to be compatible with AlGaN/GaN high-electron-mobility transistors (HEMTs) device processing. The TiN layer serves as an effective Cu-diffusion barrier as no detectable Cu-diffusion was observed when subjected to thermal treatment up to 600 °C. There is a tendency of N-diffusion across the Ti/GaN interface near which N-deficiency in the GaN epitaxial layer and formation of a nano-sheath of TiN were found. This ultrathin layer of TiN works to further improve the ohmic performance of the electric contact, as reflected in lowered contact resistivity ρ _C . It is possible to manufacture the TiN thin films with low sheet resistance at a high deposition rate by adjusting the ratios between argon and nitrogen gas flows during sputtering deposition. Contact resistivity ρ _C , tested for the AlGaN/GaN HEMT devices fabricated on Si substrate according to the transmission line method standard was found to be as low as 3.65 × 10⁻⁶ Ω cm² (R _C = 0.54 Ω mm). The outcomes benchmark favorably against many reported metal-stacking structures for ohmic contacts. The robustness of surface morphology and interface sharpness against thermal treatments make the established ohmic stack structures suitable for scalable device fabrications.