Effect of surface treatment by (NH4)2Sx solution on the reduction of ohmic contact resistivity of p-type GaN

Surface treatment using aqua regia and (NH4)2Sx solution in sequence prior to Pd metal deposition was effective in reducing the contact resistivity of Pd to p-type GaN. The contact resistivity was drastically decreased from 3.6×10−1 to 2.9×10−4 Ω cm2 by the treatment. The surface oxides formed on p-type GaN during epitaxial growth were effectively removed using aqua regia, and the following (NH4)2Sx treatment protected the surface from the formation of oxides during air exposure. The reduction of the contact resistivity is due to the direct contact of Pd to the clean surface of p-type GaN, via shift of the Fermi level to an energy level near the valence band, resulting in the reduction of the barrier height for holes at the interface of Pd/p-type GaN.