Germanium carbon (Ge1 - xCx) thin films were deposited by co-sputtering of small C wafers pasted on a pure Ge target in a H-2/Ar discharge. Spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used to characterize the films deposited at different substrate temperatures. Results show that the optical band gap and the XPS intensity IGe-C and IC-Ge of the Ge1 - xCx films decrease, while the Ge and C contents in the films vary slightly (both within 1.5%) with increasing substrate temperature. XRD patterns indicate that elevated substrate temperatures can cause segregation of Ge and C and crystallization of Ge, while lower substrate temperatures facilitate the formation of Ge-C bonds and germanium carbide grains in the films. Comparison of optical band gap of the co-sputtered Ge1 - xCx films with that of the reactive-sputtered ones implies that co-sputtering can provide higher energy precursor C and Ge atoms than reactive-sputtering, which causes the larger optical band gap of the co-sputtered films. (C) 2013 Elsevier B.V. All rights reserved.