A study was made of Pd contacts, to both n-type and p-type material, by using positron annihilation spectroscopy. The results were used to explain the effect of Ga vacancies upon band bending below the contacts. The concentration of Ga vacancies in Si-doped GaN was higher than that in Mg-doped material. In the case of Si-doped GaN, implanted positrons were annihilated in the nearer-surface region, and the Pd/n-GaN interface was detected by positrons being clearly shifted towards the Pd surface. This suggested that Ga vacancies could act as an interface state; pinning the Fermi level at the Pd/GaN interface and leading to the production of a negative electric field below the interface.

Positron Annihilation Study of Pd Contacts on Impurity-Doped GaN. J.L.Lee, J.K.Kim, M.H.Weber, K.G.Lynn: Applied Physics Letters, 2001, 78[26], 4142-4