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It is well established that by bonding with the dangling bonds of silicon, hydrogen reduces the density of states of amorphous silicon and renders this material suitable to electronic applications. For so-called “standard” a-Si : H films deposited by the RF glow discharge decomposition of silane at low deposition rates (≈1 Å/s) and over a large range of deposition temperatures, we observed the usual correlation between the hydrogen bonding and the defect density in the as-deposited material only. It clearly appears that the widely accepted correlation between the hydrogen bonding, the microstructure and the defect density only applies to a limited set of deposition parameters. Moreover we found that the deposition of a-Si : H from mixtures of ...............

The optical and structural properties of hydrogenated amorphous silicongermanium (a-Si 1−x Ge x :H) films, deposited at high growth rate by radiofrequency (RF) glow discharge on the powered electrode in a conventional parallel plate reactor, were investigated by infrared transmission measurements, standard optical transmission measurements and the photothermal deflection spectroscopy (PDS) method. By varying the electrode gap spacing, D, from 3.2 to 0.8 cm, and keeping all others parameters of the plasma constant, a gradual change in the hydrogen-bonding configurations was observed. The effects of this change on the structure of the material, on the optical gap, and on the midgap density are investigated and discussed at high fixed composition (x = 0.8) and at constant total bonded hydrogen content (C H (at.%) = 8.5). The optical gaps (1.16 eV E T 1.22 eV) of the films are less than those of conventional films, reported in the literature. The level of absorption at energies bellow the Urbach tail decreases with D and reaches an optimum at D = 0.8 cm. The disorder parameter (E 0v ≈ 46 ± 1 meV) and the level of absorption in optimized samples are of the same order as those of device-quality a-Si:H films. It is suggested that the remarkable improvement of the electronic properties of the alloys is mainly due to the incorporation of the hydrogen in the bulk of the material as Ge-H and Si-H bonds, with a preferential attachment to Ge rather than to both Ge and Si.