Ohmic Contacts to p-GaN Using Au/Ni-Zn-O Metallization
We have studied the electrical properties and depth concentration profiles of Au/Ni-Zn-O/p-GaN and Au/Ni-Zn/p-GaN, thus of structures containing zinc as a p-type dopant utilized to increase the concentration of free charge carriers in the subsurface region of GaN and to gain a low-resistance ohmic contact. The layers were deposited on p-GaN by DC reactive magnetron sputtering. The prepared contact structures were annealed in N2, structure Au/Ni-Zn/p-GaN also in O2. The contact structures containing zinc exhibited lower values of contact resistivity in comparison with those without zinc. It was also found that the values of contact resistivity for both Au/Ni-Zn-O/p-GaN and Au/Ni-Zn/p-GaN were the same, thus it was not affected by the presence of a small content of oxygen (< 0.2 at%) in the working atmosphere during the deposition of layers. Similarly, various gaseous ambients (N2 or a mixture of N2 + O2) during subsequent annealing of the contacts had no observable influence upon the magnitude of the contact resistivity. In our opinion the ohmic nature of the Au/Ni-Zn-O/p-GaN and Au/Ni-Zn/p-GaN contacts is related to a reconstruction of the contacts to a sequence metal/p-NiO/p-GaN due to annealing in nitrogen or in a mixture of oxygen and nitrogen and the ohmic properties of the contacts are predetermined by creating a thin NiO oxide layer on the metal/p-GaN interface. We believe that the lower values of the contact resistance in Au/Ni-Zn-O/p-GaN and Au/Ni-Zn/p-GaN contacts than in structure Au/Ni-O/p-GaN are caused by an enhanced hole concentration in the surface region of p-GaN due to zinc diffusion from the zinc-doped contact layer.