Crystal Phase-Selective Epitaxy of Rutile and Anatase Nb-Doped TiO2 Films on a GaN Template by the Helicon-Wave-Excited-Plasma Sputtering Epitaxy Method

Crystal phase-selective epitaxy of Nb-doped TiO 2 (TiO 2 :Nb) films on GaN is demonstrated using the helicon-wave-excited-plasma sputtering epitaxy method. On the clean (0001) GaN, rutile (100) TiO 2 :Nb was grown regardless of growth temperature ($T_{\text{g}}$). On the (0001) GaN covered with a monolayer-thick (100) or (001) $\beta$-Ga 2 O 3 , anatase (001) TiO 2 :Nb was grown with the in-plane relation $\langle 110\rangle$TiO 2 $\parallel$ $\langle 11\bar{2}0\rangle$GaN under high $T_{\text{g}}$ and low O 2 partial pressure conditions. The transmittance between 450 and 900 nm for the 200-nm-thick films was approximately 80%. Typical resistivity of the anatase TiO 2 :Nb/GaN heterostructure was as low as $3.6\times 10^{-4}$ $\Omega$$\cdot$cm, due to the interfacial conduction.