Amorphous Ga2O3 thin films have been grown on SiO2/Si substrates by atomic layer deposition (ALD) using tris (2,2,6,6-tetramethyl-3,5-heptanedionato) gallium(III) [Ga(TMHD)(3)] as a gallium source and O-2 plasma as reactant. A constant growth rate of 0.1 angstrom per cycle was obtained in a broad temperature range starting from 100 to 400 degrees C. X-ray photoelectron spectroscopy (XPS) analysis revealed stoichiometric Ga2O3 thin films with no detectable carbon contamination. A double beam - double monochromator spectrophotometer was used to measure the transmittance of Ga2O3 thin films deposited on a quartz substrate and analysis of the adsorption edge yielded a band gap energy of 4.95 eV. The refractive index of the Ga2O3 films was determined from spectroscopic ellipsometry measurements and found to be 1.84 at a wavelength of 632.8 nm. Atomic force microscopic (AFM) analysis showed surface roughness values of 0.15 and 0.51 nm for films deposited at 200 and 400 degrees C, respectively. Finally, all the films could be crystallized into a monoclinic beta-Ga2O3 crystal structure by a post deposition annealing in He as indicated by X-ray diffraction (XRD) measurements.