Effect of hydrogen on the microstructural, optical, and electronic properties of a-Si:H thin films deposited by direct current magnetron reactive sputtering

Device quality hydrogenated amorphous silicon (a-Si:H) films have been deposited under a wide range of deposition conditions using dc magnetron reactive sputtering. The total hydrogen content (CH) has been varied from 0 to ~40 at. % by changing the substrate temperature (Ts) or hydrogen partial pressure (PH) independent of other deposition parameters. The films that contain CH between 10 and 28 at. % have the highest quality. The optical band gap (Eg) varies linearly with CH for all deposition conditions studied. With increasing CH the dark conductivity at 300 K decreases from ~ 1× 10-4 to ~1× 10-12 (Ω cm)-1; however, the photoconductivity under AM-1 illumination, for the highest quality films, remains in the 0.8-3.5 × 10-5 (Ω cm)-1 range. The dark conductivity activation energy (Ea) was measured to determine the Fermi-level (Ef) position with respect to the conduction band (Ec) and a linear correlation between Eg and Ea is found for high-quality films. The films having low hydrogen content (10 = CH = 17 at. %) are slightly n type and become intrinsic as CH is increased. The subgap density of states diminishes with the increase in Ec — Ef(taken to be Ea) as the films become intrinsic. In short, the microstructural, optical, and electronic properties of these films vary monotonically with the total hydrogen content. Therefore we propose that CH is the appropriate structural parameter to correlate changes in the properties of a-Si:H.