Improved low resistance contacts of Ni/Au and Pd/Au to p-type GaN using a cryogenic treatment

A low resistance Ohmic contact to p-type GaN is essential for reliable operation of electronic and optoelectronic devices. Such contacts have been made using Ni/Au and Pd/Au contacts to p-type Mg-doped GaN (1.41×10¹⁷ cm^-3) grown by metalorganic chemical vapor deposition (MOCVD) on (0001) sapphire substrates. Thermal evaporation was used for the deposition of those metals followed by annealing at temperatures of 400 approx. 700°C in an oxygen and nitrogen mixed gas ambient, then subsequently cooled in liquid nitrogen which reduced the specific contact resistance from the range of 9.46 approx. 2.80×10^-2 Ωcm² to 9.84 approx. 2.65×10^-4 Ωcm² for Ni/Au and from the range of 8.35 approx. 5.01×10^-4 Ωcm² to 3.34 approx. 1.80×10^-4 Ωcm² for Pd/Au. The electrical characteristics for the contacts were examined by the current versus voltage curves and the specific contact resistance was determined by use of the circular transmission line method (c-TLM). The effects of the cryogenic process on improving Ohmic behavior (I-V linearity) and reducing the specific contact resistance will be discussed from a microstructural analysis which reveals the metallurgy of Ohmic contact formation.