| dc.description.abstract | The study o f the structure and properties o f disordered materials continues to be a
topic o f interest in condensed matter physics. Glassy semiconductors, in particular,
chalcogenide glasses have been important historically because o f their technological
usefulness in various fields. In the group o f chalcogenides, glasses based on tellurium are
o f considerable interest because o f their applications in the fabrication o f electrical and
optical memory devices, optical fibers etc.
The main aim o f the present thesis is to investigate the structure and transport
properties o f certain chalcogenide and metal doped chalcogenide glasses. Experimental
investigations on the structure and electrical properties o f Al-Te & Al-As-Te glasses and
carrier type reversal in Pb modified chalcogenide glasses, have been carried out in this
work.
The first chapter o f the thesis gives a brief introduction to the structure and
transport properties o f amorphous semiconductors, in particular chalcogenide glasses.
The phenomenon o f glass transition, glass formation, factors which determine the glass
formation and classifications o f amorphous semiconductors are discussed. A more
detailed account o f the atomic structure o f amorphous materials, different experimental
used in the study o f local structure in amorphous semiconductors, such as MAS NMR
have been described. Further, the electronic band structure o f amorphous semiconductors
and the role o f defect states are discussed with reference to electrical transport properties
o f chalcogenide glasses. A brief overview o f some o f the properties o f glassy
chalcogenides such as DC conductivity, thermopower, Hall effect, electrical switching,
carrier type reversal, optical absorption and photo-structural changes are discussed.
The second chapter describes the instrumentation used in the present
investigations namely, electrical switching study, photoconductivity and thermopower
measurements.
The third chapter deals with the local structure and electrical properties o f Al-Te
samples. The present MAS NMR investigations (27A1) clearly indicate that there are two
different Al environments in Al-Te glasses and non-stoichiometric crystalline Al-Te
samples, corresponding to four fold and six fold coordinated Al atoms. In stoichiometric
crystalline compound, only an octahedral coordination o f Al is found. The variation with
composition, o f the ratio between six fold and four fold coordinated Al atoms, provides
an explanation for the composition dependence o f glass forming ability o f Al-Te glasses.
The composition dependence o f electrical activation energy o f Al-Te glasses has been
understood on the basis o f the introduction o f additional charged defect states, due to the
tetrahedral and octahedral coordination o f Al atoms.
The fourth chapter presents the results o f the thermal, structural and electrical
transport studies on Al-As-Te glasses. DSC studies have been undertaken to obtain
various thermal parameters and also to estimate the thermal stability o f Al-As-Te glasses.
27A1 Solid State NMR measurements, undertaken on Al-As-Te glasses, reveal that in
these glasses also, Al resides in two different structural environments corresponding to
four fold and six fold coordinations respectively. The fraction o f four fold coordinated Al
atoms in these glasses are found to decrease with the decrease in Te content. Further, Al-
As-Te glasses are found to exhibit composition dependent change in electrical switching
behaviour (memory to threshold type). The four fold coordinated Al atoms provide
structural flexibility, and promote memory switching at low As and Al content. At higher
As and Al content, the growth in the fraction o f six fold coordinated Al atoms leads to an
increased network connectivity, which promotes threshold switching. The compositional
dependence o f thermal stability o f Al-As-Te glasses is also found to contribute to the
memory-threshold change in the switching behaviour. Investigations on temperature &
thickness dependence o f switching and structural studies on switched Al-As-Te samples
suggest that the thermal and electronic mechanisms o f switching are operative in memory
and threshold samples respectively.
The local structure in Al-As-Te glasses are found to be characterized by three
different structural species namely, As-Te, [Al-Te(As)]6 and [Al-Te(As)]4 structural units.
The compositional dependence o f these structural species is found to clearly dictate the
medium range structural ordering in these glasses. The time dependent photocurrent
measurements on AhoAsxTeso-x glasses at low temperatures indicate no appreciable
decrease in photocurrent, which is consistent with the behaviour o f other small band-gap
chalcogenide glasses. Further, the photosensitivity is found to be maximum for the glass
with maximized medium range ordering.
In chapter five, the carrier type reversal (p to n) in a Pb modified chalcogenide
glassy system is described. Carrier type reversal has been observed in Pb-Ge-Se-Te
glasses, at 8 atom % o f lead. The composition dependence o f DC electrical resistivity (p)
and activation energy for electrical conduction (AE), are found to show notable changes
at the p-n transition threshold, which are associated with the change in electron
concentration at p-n transition. The observed p-n transition has been understood, in the
light o f the Kolobov model, on the basis o f modification o f charged defect states by the
introduction o f lead. Also, the influence o f other factors such as band structure,
polarizability o f the dopant, etc., on the carrier type reversal, has been discussed.
The chapter six summarizes the essential features o f the present work and also
highlights a few possible directions along which further work can be carried out.
The references are numbered in the series and they occur in the text and are listed
at the end o f the each chapter. | |
