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 oper ation of electronic and optoelectronic devices. Such contacts have been mad e using Ni/Au and Pdl Au contacts to p-type Mg-doped GaN(1.41x10(17) cm(-3) ) grown by metalorganic chemical vapor deposition (MOCVD) on (0001) sapphir e substrates. Thermal evaporation was used for the deposition of those meta ls followed by annealing at temperatures of 400 similar to 700 degreesC in an oxygen and nitrogen mixed gas ambient, then subsequently cooled in liqui d nitrogen which reduced the specific contact resistance ham the range of 9 .46 similar to2.80x10(-2) Ohm cm(2) to 9.84 similar to2.65x10(-4) Ohm cm(2) for Ni/Au and from the range of 8.35 similar to5.01x10(-4) Ohm cm(2) to 3. 34 similar to1.80x10(-4) Ohm cm(2) for Pd/Au. The electrical characteristic s for the contacts were examined by the current versus voltage curves and t he specific contact resistance was determined by use of the circular transm ission line method (c-TLM). The effects of the cryogenic process on improvi ng Ohmic behavior (I-V linearity) and reducing the specific contact resista nce will be discussed from a microstructural analysis which reveals the met allurgy of Ohmic contact formation.