Helicon-wave-excited plasma sputtering epitaxy of Nb-doped TiO₂ films on GaN

Rutile- and anatase-phase Nb-doped TiO₂ (TiO₂:Nb) epilayers were selectively grown on a GaN template by the helicon-wave-excited-plasma sputtering epitaxy (HWPSE) method. The rutile (100) TiO₂:Nb films grew on the etched clean (0001) GaN surface with the in-plane epitaxial relationship [010] TiO₂ // [10-10] GaN, regardless of growth temperature. While, the anatase (001) TiO₂:Nb films grew on appropriately oxidized (0001) GaN with the in-plane relations [110] TiO₂ // [10-10] GaN (and [110] TiO₂ // [11-20] GaN) under high temperature and low oxygen partial pressure conditions. The lowest resistivity of the heterostructure was 3×10^-4 Ω·cm. The selective growth of anatase films is assigned as being due to the formation of a sheet of Ga-O layer, possibly (001) or (100) orientation β-Ga₂O₃, on the GaN surface.