We study microcrystalline germanium (mu c-Ge) film as narrow gap semiconductor materials for infrared absorbers by reactive sputtering with inert gas/mixtures. H(2) mixed with Ne, Ar and Xe was used as sputtering gases, in order to research effects of the ion damage. A higher deposition rate is obtained by using inert gases with a larger mass. But the crystallinity becomes lower by the damage due to larger mass ions. In the Ar/H(2) mixtures, the structure changes from crystalline to amorphous with increase in the Ar/H(2) flow rate ratio. The damage of Xe ion is too large to crystallize the films, but the influence of Ne on the crystallinity is not significant. The photo-sensitivity is obtained in the mixed structures between crystalline and amorphous given by proper ion damages. The amorphous parts probably contribute suppression of the grain-boundary defects. The observation of photo-sensitivity indicates the possibility of mu c-Ge as a narrow gap material for PV cells. (c) 2010 Elsevier B.V. All rights reserved.