Adsorption and diffusion of 3d transition metal atoms on the GaN(0001) surface

We carried out first-principles spin-polarized calculations in order to study the adsorption and diffusion of 3d transition metal (TM=Ti, V, Cr, Mn, Fe, Co, and Ni) atoms on a GaN(0001)- 2 × 2 surface using density functional theory within a plane-wave ultrasoft pseudopotential scheme. The results show that, for Ti, V, Cr, and Mn atoms, the most stable adsorption sites are all at the T4 site (the top site of the N-surface atom), whereas Fe, Co, and Ni slightly prefer the H3 hollow site. The adsorption energies can vary significantly with different TM atoms. A comparative study suggests that the TM-N bond formation is energetically more favorable for Ti, V, and Cr atoms, while the formation of a TM-Ga surface alloy is more favorable for elements such as Fe, Co, and Ni, as experimental results have shown. We found that the 3d TM adatom diffusion energy barrier between the H3 and T4 sites is around 0.40 eV, which is an indication of a significant TM adatom diffusion on the GaN(0001) surface. Furthermore, the total magnetic moment increases for Ti, V, Cr, and Mn adsorbates successively and then decreases for Fe, Co, and Ni adsorbates. The density of states indicates that the adsorption of Ti, V, and Cr atoms results in semiconductor behavior, while the adsorption of Mn, Fe, and Co atoms presents a half-metallic character. These properties make the TM/GaN systems promising for yielding high-efficiency metal-semiconductor spin injection devices.