Atomic-scale studies on the growth of palladium and titanium on GaN(0 0 0 1)

We have used elevated-temperature scanning tunnelling microscopy (STM) to investigate the initial stages of growth of Pd and Ti on GaN(0 0 0 1). Deposition of Pd onto a Ga-rich GaN(0 0 0 1) surface followed by annealing at ∼600 °C leads to the formation of an ordered Pd/Ga alloy layer, followed by the nucleation and growth of hexagonal Pd nanocrystals. When Pd is deposited on a less Ga-rich surface, the Pd initially wets the surface. With increased Pd coverage the wetted 2D islands coalesce into 3D islands without a wetting layer. The 3D islands have hexagonal shapes with a flat (1 1 1) top surface, and STM images of the nanocrystal tops show that they are reconstructed. A separate series of experiments was performed investigating Ti deposition on GaN(0 0 0 1), where the Ti reacts with GaN to form TiN. Deposition of Ti onto GaN(0 0 0 1) followed by annealing in UHV results in the formation of 2D irregular TiN islands, which can be transformed into more regular triangular TiN nanocrystals upon annealing in an ammonia atmosphere. This study shows overall that the growth modes of Pd and Ti are strongly influenced by the stoichiometry and cleanliness of the GaN substrate. Therefore, to create reliable electrical contacts between metals and GaN, the atomic level order and stoichiometry of the GaN substrates needs to be controlled.