Titanium and titanium nitride contacts to n-type gallium nitride

Ti (150 nm), TiN (200 nm) and Ti (5 nm)/TiN (200 nm) contacts have been fabricated on n-type GaN (Si doped 7 x 10(17) cm(-3), 44 Omega/square) and characterized by specific contact resistance measurements and by x-ray photoelectron spectroscopy depth profiling. Ti contacts were annealed between 400 and 900 degrees C in Ar and N-2 to observe the effects of annealing environment on ohmic contact formation. Ti contacts annealed in N-2 became ohmic after 1 min at 700 degrees C and reached a minimum specific contact resistance of 4 x 10(-6) Omega cm(2) after 1 min at 800 degrees C. Ti contacts annealed in Ar required 20-25 min at 700 degrees C to become ohmic and also reached a minimum specific contact resistance of 4 x 10(-6) Omega cm(2) after 1 min at 900 degrees C. Depth profiles of Ti contacts showed that the presence of TiN at the metal-GaN interface was a necessary condition for ohmic contact formation. TiN and Ti/TiN contacts were annealed between 400 and 800 degrees C in Ar. TiN contacts became ohmic after 1 min at 400 degrees C but only reached a minimum specific contact resistance of 4 x 10(-5) Omega cm(2) after being annealed at 800 degrees C. Ti/TiN contacts became ohmic after being annealed for 1 min at 700 degrees C, similar to Ti contacts annealed in N-2. Measurements at room temperature and 77 K revealed that annealed TiN ohmic contacts displayed increased specific contact resistance at 77 K (characteristic of thermionic or thermionic-field emission), while Ti/TiN and Ti contacts annealed in Ar showed no temperature dependence (characteristic of field emission). Based on the results of this study, we conclude that having TiN in intimate contact with n-type GaN was necessary for these contacts to be ohmic, while some reaction between Ti and GaN during the contact anneal caused the ohmic contacts to operate by field emission and to reach a lower specific contact resistance.