Formation energies of metal impurities in GaN

We present ab initio local density pseudopotential calculations on neutral nickel, titanium and gold impurities in GaN. Formation energies are calculated for substitution on the gallium site MGa, the nitrogen site MN and for incorporation in the octahedral interstitial site Mi. These elements are commonly employed for metal contacts in GaN-based LED, LD and FET devices and may be present in significant concentrations within the GaN lattice due to the use of high anneal temperatures. The majority of defects studied were found to have a high formation energy in excess of 4 eV. TiGa is found to have a particularly low formation energy of 1.2 eV and can be expected to incorporate readily into GaN. Also, deep gap states are found to be introduced by Aui,Tii,Nii,TiN,AuGa and by NiGa. TiGa and NiGa are found to act as single donors whereas NiN and AuN act as double donors.