Optical and structural properties of co-sputtered Ge1 − xCx thin films as a function of the substrate temperature

• Observed optical band gap evolution with substrate temperature in co-sputtered germanium carbon film
• Obtained germanium carbide grains in the film at low temperature
• Revealed high energy precursor C and Ge atoms are essential to forming GeC bonds.

Germanium carbon (Ge1 − xCx) thin films were deposited by co-sputtering of small C wafers pasted on a pure Ge target in a H2/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 IGeC and ICGe 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 GeC 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.