Crystal-polarity dependence of Ti/Al contacts to freestanding n-GaN substrate

The effect of crystal polarity on the electrical properties of Ti/Al contacts to $\mathrm{n-}\mathrm{GaN}$ substrate has been investigated. The Ti/Al contacts prepared on Ga-face $\mathrm{n-}\mathrm{GaN}$ substrate became ohmic with a contact resistivity of ${\text{2×10}}^{\text{−5}}\mathrm{ \Omega }{\mathrm{cm}}^{\mathrm{2}}$ after annealing at temperatures higher than 600 °C for 30 s. On the contrary, the contacts on N-face $\mathrm{n-}\mathrm{GaN}$ substrate exhibited nonlinear current–voltage curve and high Schottky barrier heights over 1 eV were measured at the same annealing conditions. These results could be explained by opposite piezoelectric-field at GaN/AlN heterostructure resulted from different polarity of the GaN substrate.