Ga-doped ZnO films were deposited by pulsed laser deposition (PLD) at 200 degrees C and 10 mTorr in either pure argon (Ar films) or in oxygen (O-2 films). The bulk resistivity of the Ar films is <2 x 10(-4) Omega cm at 300 K, two orders of magnitude lower than that of the O-2 films. In the Ar films, the donor concentration N-D as determined by a detailed Hall-effect analysis is close to 100% of the total Ga concentration [Gal measured by secondary ion mass spectrometry (SIMS), while in the O-2 films ND is less than 50% of [Gal. Furthermore, the compensation ratio K=N-A/N-D is >90% for the O-2 films and <60% for the Ar films. Yet, when the oxygen pressure is reduced to 0.2 mTorr, the O-2 films have resistivities of about 5 x 10(-4) Omega cm, approaching those of the Ar films. These results suggest that oxygen-rich environments produce Ga/O complexes that reduce the dopant activation efficiency and thus decrease ND and increase K. Some of these complexes may also contribute to the increase in deep centers observed in photoluminescence. Published by Elsevier B.V.