Valence-band offsets for Nb-doped (100) rutile (R-TiO 2) epilayer on (0001) GaN and (001) anatase (A-TiO 2) epilayer mixed with R-TiO 2 on (0001) GaN were determined using x-ray photoelectron spectroscopy to be 0.2 eV and 0.6 eV, respectively. Accordingly, they form type-I and type-II heterojunctions, respectively. The electron mobility as high as 260 cm 2 V -1s -1 was measured for the A (+R)-TiO 2: Nb epilayer on undoped GaN, which is quantitatively explained in terms of electron accumulation at the interfacial region of GaN. The intrinsic mobility of approximately 30 cm 2 V -1s -1 at 300 K was obtained for the A (+R)-TiO 2: Nb epilayer grown on a p-type GaN.
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