In order to better understand the effects of hydrogen incorporation and departure on the defects and the disorder in undoped hydrogenated amorphous silicon (a-Si:H͒, we performed a comparative study on samples deposited under different plasma conditions. We used a combination of IR absorption spectroscopy, electronspin resonance, photothermal deflection spectroscopy, constant photocurrent method, and elastic recoil detection analysis measurements to determine the changes in the defect density and in the disorder, as well as in the hydrogen concentration and bonding modes, after isochronal annealing cycles at temperatures up to 500-600°C. The results, which show a better stability of the bonded hydrogen in the films deposited at high rates, are interpreted as a whole in terms of specific local hydrogen bonding environments, related to different growth mechanisms.

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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.

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