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

Microstructural changes in Ni/Au contacts to p-type GaN as a function of an nealing temperature were investigated using X-ray diffraction, field emissi on scanning electron microscopy, and cross-sectional transmission electron microscopy combined with energy dispersive spectroscopy. The results obtain ed were used to interpret the electrical properties of Ni/Au ohmic contacts to p-type GaN. The contact resistivity decreased from 1.4 X 10(-2) to 6.1 X 10(-4) Omega cm(2) after annealing at 600 degreesC. The reduction in cont act resistivity resulted from the dissolution of Ga atoms into the Au-Ni so lid solution produced during annealing. Au atoms diffused to the: GaN subst rate through grain boundaries and reacted with Ni atoms at the grain bounda ries of Ni, producing an Au-Ni solid solution. This resulted in the evoluti on of microstructure with island-shaped Ni grains surrounded by the Au-Ni s olid solution. At this stage, Ga atoms outdiffused from the GaN substrate a nd dissolved into the Au-Ni solid solution, leading to the generation of Ga vacancies below the contact. Thus, the net concentration of holes below th e contact increased, and so the contact resistivity was reduced. When the s ample was annealed at 800 degreesC, the Au layer completely reacted with th e Ni layer, uniformly producing an Au-Ni solid solution containing Au-Ga an d Ni-Ga compounds. Simultaneously, N atoms reacted with Ni, and produced cu bic Ni4N in the vicinity of the metal/GaN interface. Consequently, N vacanc ies, acting as donors in GaN, were generated below the contact, leading to an increase of contact resistivity to 4.0 X 10(-2) Omega cm(2). (C) 2000 Th e Electrochemical Society. S0013-4651(00)02-111-X. All rights reserved.