Gregorios, Mathew; Dr. Jacob, Philip(Cochin University of Science and Technology, December , 1998)
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Abstract:
In this thesis, we present the results of our investigations on the photoconducting
and electrical switching properties of selected chalcogenide glass systems. We have used
XRD and X-ray photoelectron spectroscopy (XPS) analysis for confinuing the amorphous
nature of these materials and for confirming their constituents respectively.Photoconductivity is the enhancement in electrical conductivity of materials
brought about by the motion of charge carriers excited by absorbed radiation. The
phenomenon involves absorption, photogeneration, recombination and transport processes
and it gives good insight into the density of states in the energy gap of solids due to the
presence of impurities and lattice defects. Photoconductivity measurements lead to the
determination of such important parameters as quantum efficiency, photosensiti\'ity,
spectral sensitivity and carrier lifetime. Extensive research work on photoconducting
properties of amorphous semiconductors has resulted in the development of a variety of
very sensitive photodetectors. Photoconductors are finding newer and newer uses eyery
day. CdS, CdSe. Sb2S3, Se, ZnO etc, are typical photoconducting materials which are used
in devices like vidicons, light amplifiers, xerography equipment etc.Electrical switching is another interesting and important property possessed by
several Te based chalcogenides. Switching is the rapid and reversible transition between a
highly resistive OFF state, driven by an external electric field and characterized by a
threshold voltage, and a low resistivity ON state, Switching can be either threshold type
or memory type. The phenomenon of switching could find applications in areas like
infonnation storage, electrical power control etc. Investigations on electrical switching in
chalcogenide glasses help in understanding the mechanism of switching which is
necessary to select and modify materials for specific switching applications.Analysis of XRD pattern gives no further infonuation about amorphous materials
than revealing their disordered structure whereas x-ray photoelectron spectroscopy,XPS)
provides information about the different constituents present in the material. Also it gives binding energies (b.e.) of an element in different compounds and hence b.e. shift from the
elemental form.Our investigations have been concentrated on the bulk glasses, Ge-In-Se, Ge-Bi-Se
and As-Sb-Se for photoconductivity measurements and In-Te for electrical switching. The
photoconducting properties of Ge-Sb-Se thin films prepared by sputtering technique have
also been studied. The bulk glasses for the present investigations are prepared by the melt
quenching technique and are annealed for half an hour at temperatures just below their
respective glass transition temperatures. The dependence of photoconducting propenies on
composition and temperature are investigated in each system. The electrical switching
characteristics of In-Te system are also studied with different compositions and by varying
the temperature.
Description:
Department of Instrumentation,
Cochin University of Science and Technology