Low‐Resistivity Ti/Al/TiN/Au Ohmic Contacts to Ga‐ and N‐Face n‐GaN for Vertical Power Devices

Abstract

Low‐resistivity Ohmic contacts to Gallium‐face (G‐aface) and/or Nitrogen‐face (‐Nface) n‐GaN are undoubtedly needed for high‐quality vertical power devices. Contrary to Ga‐face n‐GaN, for which the Ohmic contact formation is a well‐established process, the formation of low‐resistivity Ohmic contacts to N‐face n‐GaN is much more difficult due to the different surface properties. Despite many efforts, obtaining low‐resistivity ohmic contacts, with ρc < 1 × 10−5 Ω cm2 to N face n‐GaN, still remains difficult to achieve. Herein, results of fabrication and characterization of Ti/Al/TiN/Au ohmic contacts to Ga‐ and N‐face n‐GaN for vertical power devices are presented. The optimum annealing temperature is examined and it is noticed that Ohmic contacts to Ga‐face n‐GaN (n ≈ 5 × 1018 cm−3 , 200 nm on bulk GaN substrate) are formed after 550 °C annealing and continuously lower their ρc up to temperature of 750 °C for which ρc is ≈1.86 × 10−6 Ω cm2. Ohmic contact to N‐face n‐GaN (bulk ammonothermally grown substrate, ρ ≈ 10−3–10−2 Ω cm2) becomes quasilinear after annealing at 650 °C and becomes linear after annealing at 700 °C, reaching a minimum value of ρc 6 × 10−6 Ω cm2. Further annealing at higher temperaturesincreases the contact resistivity to the value of ≈5.8 × 10−5 cm2 for annealing at 800 °C. The electrical measurements results were accompanied with the results of structural characterization for determination of Ohmic contact formation mechanism.