Local structure and transport properties of chalcogenide glasses

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.