Amorphous semiconductors, being intrinsically metastable in nature, exhibit a wide variety of changes in their physical properties, particularly when photoinduced using bandgap illumination. This article reviews the photoinduced phenomena exhibited by amorphous semiconductors such as amorphous hydrogenated silicon (and other tetrahedrally coordinated materials) and chalcogenide glasses. Features exhibited in common by all types of amorphous semiconductors, whether in the experimentally observed photoinduced metastability or the theoretical models used to account for such behaviour, are stressed.

Photoinduced effects and metastability in amorphous semiconductors and insulators

Virtually all electronic and optoelectronic devices necessitate a challenging assembly of conducting, semiconducting and insulating materials into specific geometries with low-scattering interfaces and microscopic feature dimensions. A variety of wafer-based processing approaches have been developed to address these requirements, which although successful are at the same time inherently restricted by the wafer size, its planar geometry and the complexity associated with sequential high-precision processing steps. In contrast, optical-fibre drawing from a macroscopic preformed rod is simpler and yields extended lengths of uniform fibres. Recently, a new family of fibres composed of conductors, semiconductors and insulators has emerged. These fibres share the basic device attributes of their traditional electronic and optoelectronic counterparts, yet are fabricated using conventional preform-based fibre-processing methods, yielding kilometres of functional fibre devices. Two complementary approaches towards realizing sophisticated functions are explored: on the single-fibre level, the integration of a multiplicity of functional components into one fibre, and on the multiple-fibre level, the assembly of large-scale two- and three-dimensional geometric constructs made of many fibres. When applied together these two approaches pave the way to multifunctional fabric systems.

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Towards multimaterial multifunctional fibres that see, hear, sense and communicate

The author provides an update of: glass preparation in bulk, fibre and film form; optical and thermal properties, and potential applications of chalcogenide glasses.

Chalcogenide glasses: a review of their preparation, properties and applications

It is proposed that the first sharp diffraction peak (FSDP) in the structure factor of network glasses and liquids is a pre-peak in the concentration-concentration structure factor due to the chemical ordering of interstitial voids around cation-centred clusters in the structure. This model can predict quantitatively the positions of the FSDP for a wide range of oxide and chalcogenide glasses and liquids, and can also successfully rationalize the anomalous temperature and pressure behaviour of the FSDP intensity, as well as the effect of the incorporation of network modifiers.

https://iopscience.iop.org/article/10.1088/0953-8984/4/38/003/pdf

The origin of the first sharp diffraction peak in the structure factor of covalent glasses and liquids

Illumination with bandgap light induces changes in physical properties of many chalcogenide semiconductors. Fundamental aspects of strike reversible photodarkening (the light-induced red-shift in the optical absorption edge) in arsenic-chalcogen glasses are critically reviewed. For understanding photodarkening at the microscopic level, details of the changes in the atomic structure that accompany the shift in the absorption edge are of particular importance. Study of the structural changes by a variety of techniques has revealed a phenomenon rich in basic physics but has not led a coherent picture of the underlying microscopic mechanism. Application of advanced experimental probes providing more detailed structural information has clarified some of the fundamental changes in the atomic structure and their relation to changes in the electronic and mechanical properties. Modifications in short-range and intermediate-range order accompany photodarkening. The changes in short-range order in the form of increased AsAs bonding are very small and probably do not play a predominant role in the changes in the electronic structure. Evidence suggests that the primary effect of the light-induced changes is the modification of intermediate-range correlations.

Reversible photodarkening of amorphous arsenic chalcogens

The pressure dependence of reversible photodarkening was determined for glassy As 2 S 3 in the range from 1 to 60 kBar. Annealed samples were pressurized in a diamond anvil cell for successive light soaking and thermal annealing cycles at constant pressure. Similar to PD at atmospheric pressure, the absorption edge, determined from in-situ transmission measurements, exhibits a parallel shift to lower energy under illumination with band gap light. Hydrostatic pressure causes the magnitude of the reversible edge shift to increase in the low pressure region and then to decrease rapidly at higher pressures, leading to a vanishing of the effect at 60 kBar. The results are interpreted in terms of two IRO structural states: Application of low pressures further promotes a structural configuration responsible for PD at atmospheric pressure whereas at higher pressures, a second structural configuration that exhibits no PD becomes increasingly dominant.

Identification of IRO structures in photodarkening: A high pressure study of As2S3

In laser-induced darkening of a-As2S3, the polarity of the optical anisotropy (the anisotropic shift in optical absorption edge) is critically dependent upon the intensity of darkening band-gap irradiation. Structural entities associated with these optical anisotropic effects are investigated through time-resolved x-ray absorption spectroscopy. Experimental results suggest a model of the darkening mechanism involving specific intermediate range order structural modifications.

Photon intensity-dependent darkening kinetics in optical and structural anisotropy in a-As2S3: A study of X-ray absorption spectroscopy

Models of the atomic structure and the photostructural changes of amorphous arsenic sulfide, a-As 2 S 3 , are discussed and compared with experiment It is shown that structural units involving two coupled AsS 3 pyramids surrounding a shared S atom provide elements of intermediate range order, IRO, sufficient to describe available data These elements are shown to be configured such that the dihedral angle relationships defined by the two bonds of the shared S-atom make helical and bridging S-atom sites identifiable Reversible photostructural changes are modeled in terms of these units

Modeling the structure and photostructural changes in amorphous arsenic sulfide

Stick and ball models of As 2 S 3 were computer-relaxed in an interatomic potential consisting of bond bending, bond stretching, lone-pair and core repulsion terms. The RDF and pair distribution functions calculated from the relaxed models show good agreement with experimental data. The relaxed models exhibit helical ordering without indications of planar structures.

Gerd Pfeiffer

Papers

Reversible photodarkening of amorphous arsenic chalcogens

Identification of IRO structures in photodarkening: A high pressure study of As2S3

Photon intensity-dependent darkening kinetics in optical and structural anisotropy in a-As2S3: A study of X-ray absorption spectroscopy

Modeling the structure and photostructural changes in amorphous arsenic sulfide

Modelling the structure of amorphous As2S3: Helical structures and a lack of planarity