Single crystalline gallium-doped ZnO films have been hydrothermally grown at 90 C using a ZnO seed layer on MgAl2O4(111) substrates. High resolution X-ray diffraction showed an epitaxial relationship between the ZnO single crystalline film and the spinel substrate with an out-of-plane orientation of Ga:ZnO < 001 >parallel to MgAl2O4 < 111 > and an in-plane orientation of Ga:ZnO[110]parallel to MgAl2O4[(1) over bar(1) over bar2] and Ga:ZnO[(1) over bar 10]parallel to MgAl2O4[(1) over bar 10]. The effects of the doping concentration as well as the postannealing treatments on the electrical and optical properties were examined. After thermal treatment, the carrier concentration in the 2.48% Ga-doped ZnO films was 2 orders of magnitude higher at 3.1 x 10(20) cm(-3) with a carrier mobility of 28 cm(2)/V s. The blueshift of the optical bandgap was also observed from the photoluminescence and optical absorption measurements as the doping concentration increased. This can be explained in the framework of the induced stress and Burstein-Moss effects at high carrier concentration. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3430099] All rights reserved.