STRUCTURE OF GLOW-DISCHARGE AMORPHOUS SILICON

The structure of hydrogenated amorphous silicon films, 100 Å thick, prepared from plasma glow discharge of silane, is investigated by electron microscopy and scanning electron diffraction. Films are prepared at plasma pressures of 0.05 and 0.3 Torr onto NaCl and KBr substrates held at room temperature and at 250 °C. The H content of the films ranges from 20 to 40 at%. The radial distribution function of these films shows a near neighbor correlation distance of (2.375 ± ± 0.05) Å (for evaporated films (2.350 ± 0.05) Å), and a second neighbor distance of (3.69 ± ± 0.05) Å. The Si—Si near neighbor coordination numbers decrease monotonically with increasing H content from 3.55 to 3.25 ± 0.1 (for evaporated Si films it is 4.0 ± 0.1). Atoms‐per‐unit volume density ratios which include H (ϱa‐Si/ϱx‐tal Si) of 0.7866, 0.7863, and 0.707 are determined for films containing 20, 25, and 40 at% H. Films prepared at room temperature with 0.3 Torr pressure have an additional small peak at 4.29 Å, not found for films at high substrate temperature and low pressure, from which it is concluded that the former films are partially polymeric. Films containing up to 25 at% H show a weak structural modulation at 4.55 Å which decreases with decreasing H content, and is not seen for evaporated amorphous Si films and films containing 40 at% H. This peak is associated with a larger number of staggered configurations in the random network. The observed radial correlations with the exception of the aforementioned differences are in general agreement with a tetrahedral random network. Bright field electron microscopy of the high pressure films shows an unhomogeneous microstructure consisting of clusters 1000 Å in diameter, with details of the morphology and size depending on the substrate temperature and type. Low pressure films have a homogeneous microstructure. Copyright © 1979 WILEY‐VCH Verlag GmbH & Co. KGaA