E. Sleeckx

Structural and optical properties of amorphous selenium prepared by plasma-enhanced CVD

The preparation of layers of amorphous Se by plasma-enhanced CVD using the hydride H 2 Se 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.

Thermally and photo-induced irreversible changes in the optical properties of amorphous GexSe100-x films

Abstract Films of amorphous GexSe100-x were prepared by plasma-enhanced chemical vapour deposition (PECVD) using the hydrides GeH4 and H2Se as precursor gases and by standard thermal evaporation. The optical bandgap obtained from a Tauc plot showed a maximum (Eg = 2.27 eV) at the stoichiometric composition (x = 33) for the PECVD films. The bandgap values of the virgin evaporated films were much lower in the range x = 40 to 75, the difference being due to the incorporation of hydrogen in the PECVD films, detected in IR and Raman spectra. Thermal annealing and illumination by bandgap light of the evaporated films induced irreversible bleaching. The PECVD films showed a more complex behaviour of the optical shift upon annealing due to the loss of hydrogen. An IR and Raman investigation demonstrated that the changes in the optical transparency are accompanied by an increase of ordering in the local structure.

Photodarkening induced at low temperatures in amorphous GexSe100−x films

Films of amorphous GexSe100−x with 0⪯x⪯40 were prepared by thermal evaporation and plasma-enhanced chemical vapour deposition. Measurements of photodarkening at three different temperatures (300, 77 and 13 K) showed that the magnitude of optical shift induced by illumination depends on sample temperature. At 13 and 77 K, the optical shift was less for compositions close to stoichiometry and increased for larger Se contents. No effect was observed in samples with an excess of Ge. At 300 K, the photodarkening was much smaller and showed a maximum at x=25. We suppose that the process of photodarkening is the same at 300, 77 and 13 K, but that at higher temperature during illumination part of the metastable states already converts back to the ground state. An important feature of the Tauc plots, used to calculate the bandgaps, was the decrease in their slope upon illumination. These changes give evidence for an increase in disorder due to stronger lone–pair interactions in the illuminated state.

Optical properties of amorphous Se films prepared by PECVD

Abstract The optical transmission spectrum of amorphous Se films prepared by plasma-enhanced chemical vapour deposition was measured over the 500 to 2000 nm spectral region. The absorption coefficient values plotted vs photon energy according to Taucs law yielded an optical gap, Eg, of 1.94 eV. The temperature dependence of Eg was measured between 13 K and 300 K and was found to be well described within Fans one-phonon approximation. Photo-induced darkening of the optical gap was studied by illumination at 13 K and 77 K.

Temperature dependence of the optical gap in thin amorphous films of As2S3, As2Se3 and other basic non-crystalline chalcogenides

Abstract Optical transmittivity in the UV–VIS region of thermally evaporated amorphous As 2 S 3 , and As 2 Se 3 thin films was measured in the temperature region 15 K K . The temperature dependence of the optical gap ( E g ( T )) was found to be in agreement with Fans one-phonon approximation or with Codys one-oscillator model. The parameters necessary to describe the E g ( T ) dependences for non-crystalline Se, As 2 S 3 , As 2 Se 3 , GeS 2 and GeSe 2 , according to a simple linear relation, Varshnis relation, and Fans or Codys model are summarized.

Optical gap and Urbach edge slope in a-Se

Good correspondence between the temperature shift of the optical gap and the Urbach edge slope has been found for a-Se. Such coherence, however, does not exist for photoinduced changes of the gap and Urbach edge slope measured at 77 K. The kinetics of photoinduced changes of the gap takes the form of first-order process, while the kinetics of photoinduced changes of Urbach edge obeys stretched exponential form. It is supposed that overheating due to illumination is most probably not responsible for photoinduced changes. Rather an increase in density of localized states at band edges, together with formation of self-trapped exciton-like states could explain the photoinduced changes observed in the gap and Urbach edge slope.

Plasma-enhanced chemical vapour deposition of amorphous GexSe1−x films

Abstract We describe plasma-enhanced chemical vapour deposition for decomposing a mixture of two gaseous hydrides, GeH 4 and H 2 Se, to yield layers of amorphous Ge x Se 1−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 Ge 0.66 Se 0.34 ) and Se-rich deposits (maximum Ge 0.23 Se 0.77 ). 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.

Comparison between electrical properties and electronic structure of variously-prepared germanium selenide films

Abstract Electronic structure and properties have been compared for amorphous germanium selenide films with compositions near GeSe 2 to GeSe 3 , and prepared by either plasma-enhanced chemical vapour deposition (PECVD) of GeH 4 and H 2 Se mixtures, or by thermal evaporation of bulk germanium selenide compounds. X-ray photoelectron spectroscopy (XPS) is used in conjunction with photo- and dark conductivity measurements. XPS spectra from Ge 3d and Se 3d core levels, and of the valence band, reveal no significant difference in electronic structure, but the electrical measurements point to a different position of the Fermi level in the two types of material. It is suggested that PECVD produces a distribution of localized gap states which pushes the Fermi level closer to the transport band.