Formation and deterioration mechanisms of low-resistance TaTi ohmic contacts for p-GaN

We demonstrated in our previous paper [Appl. Phys. Lett. 74, 275 (1999)] that TaTi contacts annealed at temperature of 800 °C provided a specific contact resistance ${\mathrm{(\rho }}_c)$ of around ${\text{3×10}}^{\text{−5}}$ Ω cm² for p-GaN epilayers with hole concentration of ${\text{7.0×10}}^{17}\hspace{0.5em}{\mathrm{cm}}^{\mathrm{-3}}.$ The reduction of the contact resistances was believed to be due to reactivation of Mg atoms doped in the GaN epilayer by removing H. In the present article, diffusion behavior of hydrogen atoms in the p-GaN was extensively studied by secondary-ion mass spectroscopy and correlated with the electrical properties of the TaTi contacts. It was found that formation of the low-resistance TaTi ohmic contacts was well explained by diffusion behavior of hydrogen in the p-GaN. Although the deterioration rates of the TaTi contacts during low temperature storage after ohmic contact formation were coincident with the diffusion rates of hydrogen atoms in the GaN, the deterioration mechanism could not be explained simply by hydrogen release from the TaTi contacts into the GaN. We believe that selection of a contact metal which would remove the hydrogen without inducing crystalline defects in the GaN after the metal and GaN reaction is a key to prepare highly reliable, low-resistance ohmic contacts for p-GaN.