Fluorinated amorphous silicon-germanium alloys deposited from disilane-germane mixture

We have grown amorphous silicon-germanium alloys by rf glow discharge decomposition of disilane, germane, silicon tetrafluoride and hydrogen. Films with an optical band gap in the range of 1.72 to 1.2e V have been made. The optimized films show the following properties: 1) incorporation of germanium is uniform across the length of the sample; 2) hydrogen is bonded to silicon and germanium uniformly without any preference; 3) the slope of the valence band tail remains constant as the optical gap is lowered down to 1.25eV and 4) use of silicon tetrafluoride in the gas mixture improves the quality of the material. Photothermal deflection spectroscopy and space charge limited conduction were used to obtain information about the deep states. Even for the lowest band gap material, the density of states at the Fermi level is about 10 17 cm −3 eV −1 which demonstrates the high quality of the material.