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The preparation of layers of amorphous Se by plasma-enhanced CVD using the hydride H2Se as precursor gas is described. Using a mixture of 15 vol.% H2Se in H2, partly crystallized films were obtained. Information concerning the structure of the films was obtained from Raman spectroscopy. The spectra of amorphous Se indicated that the dominant molecular structure is the eight-membered ring and/or a chain with Se8molecular fragments. The optical transmission spectrum was recorded at different temperatures in the range 77-300 K. The optical bandgap ETwas calculated from the optical absorption coefficients α using Tauc law : αν=C(hν-Er)2, where hν is the photon energy. The temperature dependence of ETcan be approximated by a linear relation : [MATH].

We describe plasma-enhanced chemical vapour deposition for decomposing a mixture of two gaseous hydrides, GeH4and H2S or H2Se, to yield layers of amorphous GexS1-xand GexSe1-x. We discuss the influence of the gas ratios and the deposition conditions (pressure, rf power input) on the chemical composition and homogeneity of the films. For Ge-Se samples, the composition varied between Ge-rich (typically Ge0.66Se0.34) and Se-rich deposits (maximum Ge0.23Se0.77). The incorporation of sulfur was less effective : maximum 57 at.% S in Ge-S layers (GeH4/H2S = 1/96 and p = 0.1 mbar) i.e. less than the stoichiometric composition GeS2. Information concerning the structure of as-deposited Ge-S and Ge-Se layers was obtained from infrared and Raman spectroscopy. Ge-rich films of both Ge-S and Ge-Se were unstable in air and showed in their IR spectra broad absorption bands of Ge-O vibrations, increasing in intensity when keeping them in air for a long time.

Optical transmittivity in the UV–VIS region of thermally evaporated amorphous As2S3, and As2Se3thin films was measured in the temperature region 15K

We describe plasma-enhanced chemical vapour deposition for decomposing a mixture of two gaseous hydrides, GeH4and H2Se, to yield layers of amorphous GexSe1−x. We discuss the influence of the gas ratios and the deposition conditions (pressure, rf power input) on the chemical composition and homogeneity of the films. The composition of the GeSe samples varied between Ge-rich (typically Ge0.66Se0.34) and Se-rich deposits (maximum Ge0.23Se0.77). Information concerning the structure of as-deposited GeSe layers was obtained from infrared and Raman spectroscopy. Ge-rich films were unstable in air and showed in their IR spectra broad absorption bands of GeO vibrations, increasing in intensity when keeping them in air for a long time.

The photoinduced shift in the optical gap (Eg) was studied in amorphous GeSe2 thin films at temperatures T = 77 K, 300 ? and 400 K. It was found that at T = 77 K, both virgin and well-annealed films are darkened by band-gap illumination. By annealing the film up to room temperature, the darkening is reduced and the film slowly relaxes to a state close to the initial one. Room-temperature illumination results in a blue shift of the gap. The process can be described by first-order kinetics. Annealed films, when illuminated at room temperature, are 'quasireversibly' darkened, meaning that after annealing of the previously darkened film, a small increase of the gap was observed in comparison with the preceding annealed state. The light-saturated state at ? = 400 ? has been found to be more ordered, and a blue shift of the gap was observed, in contrast with the well-annealed state. It is speculated that the photoinduced changes have their origin in self-trapped exciton-like defects and random-pair formation. An equilibrium between these defects is most probably determined by both the temperature and light intensity. At higher annealing temperatures (TA), these defects (induced by illumination) probably exist as short life-time intermediate states during the transition of the initial state of the film to a more ordered state, which otherwise can probably be reached by annealing at higher TA, i.e. at TA close to the glass-transition temperature. Thus, as proposed by Fritzsche (1993, Phil. Mag. B, 68, 561), recombination bond-induced rearrangements could be responsible for the observed light-induced changes of the gap.

The preparation of layers of amorphous Se by plasma-enhanced CVD using the hydride H2Se as precursor gas is described. Information concerning the structure of the films was obtained from Raman spectroscopy. The spectra of amorphous Se indicated that the dominant molecular structure is the eight-membered ring and/or a chain with Se8 molecular fragments. This material exhibited reversible photodarkening when illuminated at 77 K. In order to explain this phenomenon, we propose a mechanism which takes into account the role of the lone-pair electron orbitals of Se in their contribution to structural ordering. Illumination can cause a distortion in the normal bonding direction between nearest-neighbour Se atoms and induce in this way intrinsic defect states located at the band edges. In the photo-darkened state, optical transition will occur between these defect states.

Irreversible photo-induced and thermally induced changes in the optical properties of amorphous Ge31.4S68.6 thin films were studied using visible and far-infrared transmission and using Raman spectroscopy. It was found that the blue shift of the optical gap induced by illumination (+ 0.07 eV) and by annealing (up to + 0.19 eV) is accompanied by an increase in the density of heteropolar Ge-Se bonds. Simultaneously the medium-range order increases as inferred from an increase in the relative intensity of the 'companion' line (at about 220 cm-1) in the Raman spectra. Hence a simultaneous increase in the short- and medium-range order accompanies the blue shift of the gap. It is, however, supposed that a key role in the blue shift of the gap is played by changes in the short-range order.