Effects of surface treatments on the electrical and the microstructural changes of Pd contact on p-type GaN

We investigated the effects of surface treatments by aqua regia and (NH₄)₂S_x on the electrical and the microstructural changes of Pd contact on p-type GaN during annealing. The formation of a surface oxide was suppressed by the (NH₄)₂S_x treatment, and S-Ga and S-N bonds with binding energy of 162.1 eV and 163.6 eV were formed, degrading the structural ordering of Pd. After 300°C annealing, the contact resistivity in the aqua regia-treated sample increased significantly. This could be attributed to the outdiffusion of N atoms leaving N vacancies below the contact, as confirmed by the increase of the Pd (111) plane spacing probably due to the dissolution of N atoms in Pd interstitial sites. Meanwhile, the contact resistivity in the (NH₄)₂S_x-treated sample was not degraded and no change was observed in the Pd (111) plane spacing. These results suggest that S-Ga and S-N bonds formed on (NH₄)₂S_x-treated GaN could act as a diffusion barrier for the outdiffusion of N atoms. The contact resistivity for the aqua regia-treated sample decreased again, probably due to the outdiffusion of Ga as well as N atoms at 500°C.