A high degree of crystallinity is obtained in nc-Si:H films deposited by r.f. PECVD, produced from SiF4-H-2-He mixtures. The amorphous-to-nanocrystalline transition is favored because of the presence of F atoms, which preferentially etch the amorphous phase. The addition of He to the SiF4-H-2 gas mixture gives an increase of F and H atoms in the plasma, thus inducing higher crystallinity. A further improvement in the nc-Si:H film structure and properties is obtained by adjusting the r.f. power and the deposition temperature. Under optimized plasma conditions, substrate temperatures as low as 120 degrees C can be reached for the deposition of nc-Si:H having 100% of crystallinity.