Microstructural investigation of oxidized Ni/Au ohmic contact to p-type GaN

The microstructure of oxidized Ni/Au films on p-GaN was examined to elucida te the formation of a low resistance ohmic contact to p-GaN with a field-em ission gun transmission electron microscope in conjunction with composition analyses. The p-GaN/Ni/Au samples were heat treated at 500 degrees C in ai r mainly composed of a mixture of crystalline NiO, Au, and amorphous Ni-Ga- O phases. Small voids adjacent to the p-GaN film were also observed. The as -deposited Au film converted into discontinuous islands containing small am ounts of Ni that connect with p-GaN. NiO formed a continuous film at the su rface that covers the Au islands and the amorphous Ni-Ga-O phases. Moreover , NiO partially contacts p-GaN as well as Au islands and the amorphous Ni-G a-O phase. The orientation relationship of the crystalline NiO, Au-rich isl ands, and p-GaN film was identified as NiO(111)//Au(11 (1) over bar)//GaN(0 002) and NiO[1 (1) over bar 0]//Au[1 (1) over bar 0]//GaN[11 (2) over bar 0 ]. The results suggested that Ni atoms diffuse through the Au layer onto th e surface and react with oxygen to form NiO, whereas Au atoms diffuse towar ds the inside to form a Au-Ni alloy. The microstructural examination indica ted that the crystalline NiO and/or the amorphous Ni-Ga-O phases may signif icantly affect the low resistance ohmic contact to p-GaN. (C) 1999 American Institute of Physics. [S0021-8979(99)05219-6].