Low‐Resistivity Ti/Al/TiN/Au Ohmic Contacts to Ga‐ and N‐Face n‐GaN for Vertical Power Devices
Oskar Sadowski, Maciej Kamiński, Andrzej Taube, Jarosław Tarenko, M. Guziewicz, M. Wzorek, Justyna Maleszyk, I. Jóźwik, Anna Szerling, P. Prystawko, Michał Boćkowski, I. Grzegory
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 ρ c < 1 × 10 −5 Ω cm 2 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 × 10 18 cm −3 , 200 nm on bulk GaN substrate) are formed after 550 °C annealing and continuously lower their ρ c up to temperature of 750 °C for which ρ c is ≈1.86 × 10 −6 Ω cm 2 . Ohmic contact to N‐face n‐GaN (bulk ammonothermally grown substrate, ρ ≈ 10 −3 –10 −2 Ω cm 2 ) becomes quasilinear after annealing at 650 °C and becomes linear after annealing at 700 °C, reaching a minimum value of ρ c 6 × 10 −6 Ω cm 2 . Further annealing at higher temperatures increases the contact resistivity to the value of ≈5.8 × 10 −5 cm 2 for annealing at 800 °C. The electrical measurements results were accompanied with the results of structural characterization for determination of Ohmic contact formation mechanism.