We investigated the effects of the annealing ambient during solid phase crystallization (SPC) on the crystallization temperature of amorphous Ge and the electrical properties of a back-gated poly-crystalline germanium thin film transistor (poly-Ge TFT). Then, it was suggested that a slight amount of oxygen in Ge film and residual oxygen in the annealing ambient play important roles in Ge SPC. We found that the leakage current of poly-Ge TFT was well suppressed by performing SPC in N-2 or Ar which contains a slight but appreciable amount of residual oxygen, without decreasing the field effect mobility. In this paper, we discuss effects of the residual oxygen from the view point of defect passivation of Ge. (C) 2013 Published by Elsevier B.V.