No description available.

Chalcogenide glass based Phase Change Memories (PCMs) are being considered recently as promising alternatives to conventional non-volatile Random Access Memories (NVRAMs). PCMs offer high performance & low power consumption, in addition to other advantages, such as high scalability, high endurance and compatibility with complementary metal oxide semiconductors (CMOS) technologies. Basically PCM is a resistance variable non-volatile memory in which the memory bit state is defined by the resistance of the material. In this case, the initial ‘OFF’ state (logic zero) corresponds to the high resistance amorphous state and the logic 1 or ‘ON’ state corresponds to low resistance crystalline state. The present thesis work deals with electrical, thermal, mechanical and optical characterization of certain tellurium based chalcogenide glasses in bulk and thin film form for phase change memory applications. A comparative study has been done on the electrical switching behavior of Ge-Te-Se & Ge-Te-Si amorphous thin film samples with their bulk counterparts. Further, electrical switching and thermal studies have been undertaken on bulk Ge-Te-Bi and Ge-Te-Sn series of samples. The composition dependence of switching voltages of bulk and thin film samples studied has been explained on the basis of different factors responsible for electrical switching. The thesis contains ten chapters: Chapter 1 deals with a brief introduction on chalcogenides and their applicability in phase change memories. The glass transition phenomenon, synthesis of chalcogenide alloys, different structural models of amorphous semiconductors and electrical switching behavior are also discussed in detail in this chapter. Further, a brief description of optical and mechanical properties along with the principles of few characterization techniques used is discussed. Also, a brief overview on PCM application of chalcogenides is presented. The second chapter provides the details of various experimental techniques used to measure electrical, thermal, optical and mechanical properties of few tellurium based chalcogenide glassy systems. In the third chapter, the electrical switching behavior of amorphous Al23Te77 thin film devices, deposited in co-planar geometry, has been discussed. It is found that these samples exhibit memory type electrical switching. Scanning Electron Microscopic studies show the formation of a crystalline filament in the electrode region which is responsible for switching of the device from high resistance OFF state to low resistance ON state. The switching behavior of thin film Al-Te samples is found to be similar to that of bulk samples, with the threshold fields of bulk samples being higher. This has been understood on the basis of higher thermal conductance in bulk, which reduces the Joule heating and temperature rise in the electrode region. Electrical switching and thermal behavior of bulk; melt quenched Ge18Te82-xBix glasses (1 ≤ x ≤ 4) are presented in chapter 4. Ge-Te-Bi glasses have been found to exhibit memory type electrical switching behavior, which is in agreement with the lower thermal diffusivity values of these samples. A linear variation in switching voltages (also known as threshold voltages) (Vt) has been found with increase in thickness. The switching voltages have been found to decrease with an increase in temperature which is due to the decrease in the activation energy for crystallization at higher temperatures. Further, Vt of Ge18Te82-xBix glasses have been found to decrease with the increase in Bi content, indicating that in the Ge-Te-Bi system, the resistivity of the additive has a stronger role to play in the composition dependence of Vt, in comparison with the network connectivity and rigidity factors. In addition, the composition dependence of crystallization activation energy has been found to show a decrease with an increase in Bi content. X-ray diffraction studies on thermally crystallized samples reveal the presence of hexagonal Te, GeTe and Bi2Te3 phases. The fifth chapter deals with the electrical switching studies and optical band gap measurements on GexSe35-xTe65 (17 ≤ x ≤ 23) amorphous thin film samples. These thin film samples coated with sandwich geometry are found to switch with very low voltages as compared to bulk samples of the same chalcogenide glasses. The switching voltages and optical band gap are found to increase with the addition of Ge at the expense of Se. High structural cross linking with progressive addition of 4-fold coordinated Ge atoms could be the one of the reasons of increasing switching voltage and stronger Ge-Se bond strength could be the reason of increasing band gap for these chalcogenide glasses. In chapter 6, electrical switching studies on amorphous Ge15Te85-xSix (1 ≤ x ≤ 6) thin film samples have been described and the results are compared with their bulk counterparts. Similar trend has been found for both bulk and film samples when the threshold field is varied with composition. Optical band gap has been measured as a function of composition for these films, which also shows a behavior similar to that of switching voltages. The increasing trend in the variation with composition of electrical switching voltages and optical band gap are due to the increase in network connectivity and rigidity as Si atoms are incorporated into the Ge-Te system. Chapter 7 summarizes the electrical switching and glass forming ability of the Ge-Te-Sn glasses of two different composition tie-lines, namely Ge15Te85-xSnx and Ge17Te83-xSnx. Glasses belonging to both the series have been found to exhibit memory type of electrical switching behavior. The thickness dependence of threshold voltages is also found to support the memory switching behavior of the system. Further, ADSC studies are undertaken to explore the thermal behavior of these glasses which indicates that the crystallization tendency increases as Sn concentration is increased in the Ge-Te network. XRD studies done on two samples from both the series, reveal the fact that Sn atoms do not take part actively to enhance the network connectivity and rigidity. The composition dependence of crystallization temperature, metallicity factor and results of XRD studies are put together to explain the variation with composition of threshold voltages for both the series of samples. In chapter 8, investigations on the electrical switching behavior of Ge15Te85-xSnx (1 ≤ x ≤ 5) and Ge17Te83-xSnx (1 ≤ x ≤ 4) amorphous thin films have been discussed. Both the series of samples have been found to exhibit memory type of electrical switching behavior. The composition dependence of threshold voltage shows a decreasing trend, which has been explained on the basis of the Chemically Ordered Network (CON) model, bond strength and the metallicity factor. The optical band gap variation of both the series also exhibits a similar decreasing trend with composition. The observed behavior has been understood on the basis of higher atomic radius of Sn atom than Ge atom, which makes the energy difference between bonding and anti bonding state less at band edge. Chapter 9 deals with the nano-indentation studies on Ge15Te85-xSix (0 ≤ x ≤ 9) bulk glasses. The composition dependence of young’s modulus and hardness is studied systematically in this glassy system. The density of the samples of different compositions has also been measured, which strongly supports the variation of Young’s Modulus and hardness with composition. The composition dependence of mechanical properties of Ge-Te-Si samples has been understood on the basis of the presence of an intermediate phase and a thermally reversing window in this glassy system. A summary of the significant results obtained in the present thesis work is presented in the last chapter along with the scope for future work.

Silver containing chalcogenide glasses have been extensively studied during the last few decades; the main interest in these materials being their electrical conductivity which changes by several orders of magnitude upon silver doping. Glassy chalcogenides doped with silver have applications in optical elements, gratings, micro-lenses, waveguides, bio & chemical sensors, solid electrolytes, batteries, etc. Chalcohalide glasses have become important in the recent times, from both scientific & technological points of view, due to the interesting properties exhibited by these glasses such as the transparency in the infrared region, the stability against devitrification, solubility of rare earth elements, etc. In this thesis work, the thermal properties and electrical switching behavior of certain silver and iodine doped chalcogenide glasses have been investigated The thesis contains five chapters: Chapter 1: This chapter is an introduction to the fundamental aspects of amorphous semiconductors with a particular reference to chalcogenide glasses. The advantages and applications of chalcogenide glasses are also described. Chapter 2: The methods of preparation and characterization of the glasses investigated are described in this chapter. Also, the details of the experiments undertaken, namely temperature modulated Alternating Differential Scanning Calorimetry (ADSC), electrical switching analysis, Photo-thermal Deflection Spectroscopy (PDS), etc, are outlined. Chapter 3: In this chapter, the thermal behavior and electrical switching of silver doped Ge-Se and As-Se chalcogenide glasses are described. Bulk, melt-quenched Se-rich Ge0.15Se0.85-xAgx glasses have been found to be microscopically phase separated and composed of Ag2Se clusters and GeSe2-Se network. When the silver concentration exceeds 10 atom %, the Ag2Se clusters embedded in the GeSe2-Se network percolate. The signature of this percolation threshold is clearly observed as the sudden appearance of two exothermic crystallization peaks in ADSC runs. Density, molar volume and micro hardness studies also strongly support the view of a percolation transition. The super-ionic conduction observed earlier in these glasses at higher silver proportions, is likely to be connected with the silver phase percolation. It has been found that Ge0.15Se0.85-xAgx glasses of lower silver concentration (x = 0.07 and 0.08) do not exhibit electrical switching at voltages up to 1100 V. A negative resistance behavior and threshold type electrical switching is seen in Ge0.15Se0.85-xAgx samples with x 0.09. Also, fluctuations are observed in the I-V characteristics of these samples, which have been attributed to the difference in thermal conductivities between the Ag2Se inclusions and the Ge-Se base glass. A sharp drop has been observed in the switching voltage with Ag concentration which is due to the more metallic nature of silver and the presence of Ag+ ions. Further, the saturation in the decrease of VT around x = 0.10, is related to silver phase percolation in these glasses. Bulk As20Se80-xAgx glasses (0 x 15) have been found to exhibit two endothermic glass transitions and two exothermic crystallization reactions on heating. Based on which it is suggested that As20Se80-xAgx glasses are also microscopically phase separated, containing Ag2Se phases embedded in an As-Se backbone. The occurrence of microscopic phase separation in As20Se80-xAgx glasses is also confirmed by SEM studies. With increasing silver concentration, the Ag2Se phase percolates in the As-Se matrix, with a well-defined percolation threshold at x = 8. This silver phase percolation is exemplified by sudden jumps in the composition dependence of the second crystallization peak and non-reversible heat-flow, Hnr obtained at the second glass transition reaction of As20Se80-xAgx glasses. The super-ionic conduction observed earlier in these glasses at higher silver proportions, is likely to be associated with the observed silver phase percolation. Like Ge0.15Se0.85-xAgx glasses, As20Se80-xAgx glasses also exhibit threshold type electrical switching with fluctuations in the I-V characteristics; these fluctuations have been attributed to the difference in thermal conductivities between the Ag2Se inclusions and the As-Se base glass. A sharp drop has been observed in the switching voltage with Ag concentration which is due to the more metallic nature of silver and the presence of Ag+ ions. Further, the saturation in the decrease of VT around x = 8, is found to be related to silver phase percolation in these glasses, which has been proposed on the basis of ADSC experiments. Chapter 4: The chapter 4 deals with thermal studies, electrical switching investigations and Photo-thermal Deflection Spectroscopic (PDS) measurements on certain Ge-Te-I and As-Te-I chalcohalide glasses. It has been found that the compositional variation of the glass transition temperature of Ge22Te78-xIx glasses, obtained by Alternating Differential Scanning Calorimetry (ADSC), exhibits a broad hump around 5 atom % of iodine. Further, a sharp minimum is seen in the composition dependence of non-reversing enthalpy (Hnr) of Ge22Te78-xIx glasses at x = 5, which is suggestive of a thermally reversing window at this composition. Electrical switching studies on Ge22Te78-xIx glasses indicate that these glasses exhibit memory type electrical switching. At lower iodine concentrations, a decrease is seen in switching voltages with an increase in iodine content (in comparison with the base Ge22Te78 glass), which is due to the decrease in network connectivity. The increase seen in switching voltages of Ge22Te78-xIx glasses at higher iodine contents, suggests that the influence of the metallicity is stronger at higher iodine proportions. It is also interesting to note that the composition dependence of the threshold voltages shows a slope change at x = 5, the inverse rigidity percolation threshold of the Ge22Te78-xIx system. . Further, it is found that the thermal diffusivities ( D) of Ge22Te78-xIx glasses decrease with the increase in iodine content, which has been understood on the basis of fragmentation of the Ge-Te network with the addition of iodine. Also, a cusp is seen in the composition dependence of thermal diffusivity at the composition x = 5 (average coordination number, r = 2.39), which has been identified to be the inverse rigidity percolation threshold of the system at which the network connectivity is lost. ADSC studies on As45Te55-xIx chalcohalide glasses (3 x 10) reveal that there is not much variation in the glass transition temperature of As45Te55-xIx glasses, even though there is a wide variation in r . Based on this observation we suggest that the variation in glass transition temperature of network glasses is dictated by the variation in average bond energy rather than the average coordination number. Further, the non-reversing enthalpy Hnr of As45Te55-xIx glasses is found to exhibit a sharp minimum at the composition x = 6. A broad hump is also seen in glass transition and crystallization temperatures in the composition range 5 x 7. These results indicate a narrow thermally reversing window in As45Te55-xIx glasses around the composition x = 6. As45Te55-xIx glasses have been found to exhibit a memory to threshold type change in switching behavior with iodine content (x 6), which has been understood on the basis of the sharp increase in thermal diffusivity above x = 6. It is also observed that the switching voltages do not change appreciably with composition/average coordination number. Though no pronounced signature of a stiffness transition is seen in the variation with composition of VT, fluctuations are seen in the switching voltages around x = 6, the composition corresponding to the sharp thermally revering window. PDS studies indicate that the thermal diffusivities () of As45Te55-xIx chalcohalide exhibit a sharp minimum at the composition x = 6. This result reasserts the presence of a sharp thermally reversing window in As45Te55-xIx glasses around the composition x = 6. Chapter 5: The significant results obtained in the present thesis work have been summarized in this chapter. Further, the scope for future work is also presented.

Optical amplifiers are highly sought-after in optical communications to power boost light signals carrying information. Rare Earth doped glasses have been the medium of choice for optical amplification. It is, therefore, essential to understand the interaction of light with potential host glasses for rare-earths before they could be proposed as suitable candidates. In this research, we have optically characterized three different rare earth doped bulk glasses. The glass samples investigated were Neodymium doped Gallium Lanthanum Sulfide (GLS:Nd), Erbium doped Germanium Gallium Sulfide (GeGaS:Er) and Erbium doped Fluorochlorozirconate (FCZ:Er). The transmission spectra, T(λ), was used in identifying the absorption transitions of rare earth ions from the ground level to the various excited levels and in obtaining the optical absorption coefficient, α(λ). This in turn was used in determining the Judd-Ofelt parameters, which were then used in obtaining radiative lifetimes of the energy levels of interest. Photoluminescence emission bands were also identified and their shapes were investigated. Finally, a comparison of the Judd-Ofelt lifetime with the experimental decay time was also done. From which, the major decay mechanism of the rare earth ions from the energy level under investigation was concluded.

Optical amplifiers are highly sought-after in optical communications to power boost light signals carrying information. Rare Earth doped glasses have been the medium of choice for optical amplification. It is, therefore, essential to understand the interaction of light with potential host glasses for rare-earths before they could be proposed as suitable candidates. In this research, we have optically characterized three different rare earth doped bulk glasses. The glass samples investigated were Neodymium doped Gallium Lanthanum Sulfide (GLS:Nd), Erbium doped Germanium Gallium Sulfide (GeGaS:Er) and Erbium doped Fluorochlorozirconate (FCZ:Er). The transmission spectra, T(λ), was used in identifying the absorption transitions of rare earth ions from the ground level to the various excited levels and in obtaining the optical absorption coefficient, α(λ). This in turn was used in determining the Judd-Ofelt parameters, which were then used in obtaining radiative lifetimes of the energy levels of interest. Photoluminescence emission bands were also identified and their shapes were investigated. Finally, a comparison of the Judd-Ofelt lifetime with the experimental decay time was also done. From which, the major decay mechanism of the rare earth ions from the energy level under investigation was concluded.

En 2010, une nouvelle méthode de production de verres de chalcogénure transparents dans l'infrarouge a été brevetée, alliant la mécanosynthèse au frittage Spark Plasma Sintering. Ce travail de thèse intègre le transfert de technologie vers la société Diafir dans le cadre d'un projet RAPID, financé par la DGA. Il a pour but un travail d'optimisation des procédés de synthèse et de mise en forme a été mené afin de permettre d'adapter cette technologie innovante à un volume industriel. Les verres de composition Ge28Sb12Se60, 80GeSe2-20Ga2Se3, Te20As30Se50, As2Se3 et As2S3 ont été synthétisés par fusion-trempe, broyés finement puis frittés par SPS. Des disques avec des transmissions comparables au verre obtenu par voie classique ont été obtenus. Des traitements antireflets ont été déposés avec succès à leur surface. Le développement de moules spécifiques au SPS a conduit à la production de lentilles sphériques adaptables à des dispositifs optiques. Ces compositions vitreuses ont ensuite été synthétisées par mécanosynthèse puis frittées par SPS. Les disques obtenus présentant des phénomènes de diffusion récurrents. Les principales causes de diffusion ont été analysées afin d'optimiser le procédé de synthèse par voie mécanique. / In 2010, a new method of production of chalcogenide glasses, transparent in the infrared, has been patented. It combines mechanosynthesis to Spark Plasma Sintering. In this thesis work, there is the technology transfer to the Diafir society included in the RAPID project, founded by the DGA. The aim is to optimize the processes of synthesis and forming in order to adapt this innovating technology to an industrial scale. The glasses of composition Ge28Sb12Se60, 80GeSe2-20Ga2Se3, Te20As30Se50, As2Se3 and As2S3 have been synthetized by melt-quench method then crushedfinely before to be sintered by SPS.

No description available.

Une nouvelle famille de vitrocéramiques, avec une microstructure inédite, a été fabriquée par une cristallisation contrôlée des verres dans le système GeSe2-Sb2Se3-CuI. L'influence de la composition et du processus de cristallisation des verres de base, sur la microstructure et sur l’intensité du photo-courant des vitrocéramiques a été étudiée. Une composition optimisée, le 40GeSe2-40Sb2Se3-20CuI, a été particulièrement étudiée avec des résultats suivants: (1) Après une étude systématique , il a été constaté que cette composition donne la plus forte intensité de photo-courant parmi tous les verres étudiés dans ce système pseudo-ternaire GeSe2-Sb2Se3-Cul. Il a été également démontré que le photo-courant généré par différentes vitrocéramiques est non seulement déterminé par la composition, mais aussi par la microstructure composite de la vitrocéramique, qui est déterminée par le processus de céramisation. Ce processus de céramisation a ensuite été optimisé. Par rapport au procédé de traitement thermique en deux étapes, le procédé en une seule étape à basse température est une stratégie plus appropriée pour obtenir une microstructure efficace, favorisant la séparation des charges, construisant des canaux conducteurs et donnant une intensité de photo-courant élevée dans la vitrocéramique. (2) La microstructure composite inédite, discutée ci-dessus est composée de micro-domaines conducteurs interconnectés, formées par des cristaux Sb2Se3 faiblement conducteur en forme de tiges, couverts par des nano-cristaux de Cu2GeSe3 beaucoup plus conducteurs. Le procédé le plus probable de la photo-génération efficace des charges est le suivant: les photons sont efficacement et essentiellement absorbés par Sb2Se3 ainsi que par Cu2GeSe3. Les hétérojonctions formées par les Sb2Se3 du type n et les Cu2GeSe3 du type p, favorisent la séparation de charges, tandis que les Cu2GeSe3 interconnectées et conductrices fournissent des canaux conducteurs et jouent ainsi le rôle de collecteur efficace de charges. Il en résulte ainsi une très longue durée de vie des porteurs de charge et un fort photo-courant. (3) La formation de nano-hétérojonctions entre les cristaux Sb2Se3 et Cu2GeSe3 dans un seul micro-domaine peut conduire à une séparation efficace des électrons et des trous photo-générés. Par conséquent, pour application photo-catalytique, il n’est pas nécessaire de former des canaux conducteurs (conducteurs interconnectés des micro-domaines) dans l'ensemble de la vitrocéramique. De plus, la formation de ces canaux conducteurs, nécessiterait une augmentation de la durée ou/et la température de recuit, pouvant conduire à une diminution de l'activité photo-catalytique à cause de la taille relativement grande des grains cristallins. Les vitrocéramiques optimisées montrent une bonne capacité de désamination oxydative et une forte activité photo-catalytique en général, démontrant ainsi son potentiel en tant que photo-catalyseur efficace. / A totally new family of glass ceramics with a unique microstructure was fabricated by controlling the crystallization of the GeSe2-Sb2Se3-CuI glass system. The influences of the material composition and the crystallizing process of the precursor glasses on the microstructure and photocurrent of the prepared glass ceramics were investigated. An optimized composition, 40GeSe2-40Sb2Se3-20CuI, was particularly studied with the following significant results: (1) After a systematic study, it was found that this particular composition shows the highest photocurrent density among all studied glasses in the pseudo-ternary GeSe2-Sb2Se3-CuI system. It is also demonstrated that the photocurrent generated by different glass ceramics is not only determined by the composition, but also by the composite microstructure of the glass ceramic, which is determined by the ceramisation process. This process was then carefully studied. Compared with the two-step heat treatment process, the single-step process at a low temperature is a more efficient strategy to build up an efficient composite microstructure, which promotes charge carrier separation and provides a conductive channel, leading to a high photocurrent intensity in the glass ceramic. (2) The above-mentioned unique composite microstructure is composed of interconnected conductive microdomains, formed by low conductive rod-like Sb2Se3 crystals, covered by relatively high conductive Cu2GeSe3 nanocrystals. The most likely process for efficient photogeneration of charges is proposed as follows: photons are efficiently and essentially absorbed by Sb2Se3 as well as by Cu2GeSe3, and then the heterojunction formed by n-type Sb2Se3 and p-type Cu2GeSe3 promotes the charge separation, whereas the oriented and relatively conductive Cu2GeSe3 aggregate provides a conductive channel and plays the role of efficient charge collector. This structure results in exceptionally long lifetime of charge carriers (around 16 µs) and high photocurrent (at least 100 times higher than any of Sb2Se3 and Cu2GeSe3 individually). (3) The formation of nano-heterojunctions between Sb2Se3 and Cu2GeSe3 crystals within a single conductive microdomain can fully lead to an efficient separation of photo-generated electrons and holes. Therefore, for the photocatalytic application, it is unnecessary to form conductive channels (interconnected conductive microdomains) in the whole glass ceramic. Moreover, in order to form conductive channels, the necessary increase of annealing time or/and temperature may decrease the photocatalytic activity due to its relatively large crystal grain size. The optimized glass ceramic exhibits a good oxidative deamination ability and high photocatalytic activity, demonstrating its potential as an efficient photocatalyst.

During the last few years the scientific interest in chalcogenides glasses has been provoked on account of their properties and new application possibilities. These materials exhibit electrical and optical properties, which make them useful for several potential applications. Specifically the threshold and memory switching behavior and the infrared transmission of many of these glasses make the materials to be well suitable for use in memory devices and in fiber optics. Multicomponent glasses have been found to be more useful for many of these applications since the properties could be tailored for the specific uses. On account of this there has been great deal of interest in recent years in understanding the composition dependent variations of physical properties in these glasses. Models based on network topology and chemical ordering have been proposed to explain the composition dependence of physical properties. The Chemically Ordered Covalent Network (COCN) model is one of the best efforts put forth in this subject. This model predicts distinctive physical properties of these glasses for compositions at which there is a maximum number of heteropolar bonds. A physical model based on changes in network topology with composition has been proposed recently. This model predicts the rigidity to percolate in the network at the mean coordination number <r> = 2.40. This critical value of <r> at which the rigidity percolates is called the mechanical threshold or the rigidity percolation threshold. One more argument based on medium range interactions, existing in these glassy networks, suggests that the mechanical threshold should occur at <r> = 2.67. A general lack of consensus in the existing experimental reports on the mechanical threshold in some chalcogenides glasses prevents one from identifying the correct threshold value of <r>. A systematic study of the composition dependence of glasses with a large glass-forming region is necessary to resolve this controversy. The correct threshold value of <r> and the reason for the departure from this value in the other cases is the first step towards verifying the applicability of this model to chalcogenide glasses. Glasses belonging to IV — V — VI groups are natural candidates for this study because of their large glass forming region. It also seems possible to isolate the chemical threshold from interfering with the mechanical threshold in some of these glasses. In device applications of any semiconductor the optical and the electrical band gaps need to be varied and this is commonly done by doping. The large density of valence alteration pairs and intrinsic disorder of amorphous semiconductors counter-balances the effects of external additives. As a result, it is hard to electrically dope these materials. Non-equilibrium experimental techniques have been used to some extent, but one of the limitations is that they are confined to the thin film state. The finding that p to n type conduction sign changes can be induced by Bi and Pb in bulk Ge-M (M= S, Se and Te) glasses has therefore created special interest. This thesis deals with Ge-Se glass matrix doped with Te, Bi and Pb. The optical, thermal and electrical properties have been studied. The present thesis work is arranged in several chapters. The basic introduction of chalcogenide glasses is given in chapter one. This includes an introduction to chalcogenide glasses followed by a brief discussion on the important structural models, the possible defects in chalcogenide glasses and the electrical, optical and thermal properties of chalcogenide glasses. The second chapter discusses the experimental techniques used in the present investigations. The basic principles and theory behind the experiments, the experimental setup and the experimental procedure leading to the determination of the physical properties are given here. These include information about Differential Scanning Calorimetry (DSC), Photo acoustic (PA) spectroscopy and Photoluminescence studies. In the third chapter the experimental investigations on Ge-Se-Te glasses are presented. The chapter starts with the preparation and characterization of these glasses. It then gives an account of the earlier studies on Ge-Se-Te glasses that are relevant to the present work. The results of the DSC and PA studies are discussed in the following two sections. In the systems with Gex Se80-x Te20 and Gex Se75.x Te25, glasses with less than 20 at. % of Ge do not show any crystallization peak due to Se rich content. But Te and Ge-rich glasses show strong crystallization tendency. The composition dependence of Tg of this glassy system gives an evidence for the occurrence of the topological threshold or mechanical threshold at <r> = 2.40 and chemical threshold at <r> = 2.67. These can be explained on the basis of COCN model. The optical band gap and thermal diffusivity studies also show anomalous behavior at <r> = 2.40 and <r> = 2.67. The experimental results on Ge-Se-Te glasses are summarized in the last section of this chapter. The investigations on Bi doped Ge-Se and Ge-Se-Te glasses are given in the fourth chapter. The chapter starts with a brief introduction of preparation, characterization and a short review of earlier work. In PA studies the anomalous behavior is observed in thermal diffusivity and thermal diffusion length plot at 8-9 at. % of Bi doping of the Ge-Se and Ge-Se-Te glasses where the conduction changes from p to n type. These results are explained on the basis of percolation model and the formation of Bi2Se3 microcrystalline phase. Finally these results are summarized at the end of the chapter. The fifth chapter is devoted to the investigations on Pb doped Ge-Se glasses. It is arranged in five sections; preparation and characterization, earlier work, Photo acoustic and Photoluminescence studies. In PA studies the composition dependence of thermal diffusivity show anomalous behavior at x =F 9 at % of Pb in Pbx Ge42-x Sesg glasses and y = 21 at. % of Ge in Pb2o Gey Seso-y glasses where the conduction changes from p to n type. After that it reaches the maximum. After the conduction sign changes the conductivity increases with addition of respective Pb and Ge concentration in both series of glasses, which is reflected in thermal diffusivity value also. The results have been explained on the basis of COCN model. From PL studies, the PL intensity is high in un-doped Ge42 Scss glasses. With the addition of Pb into Ge-Se system the PL intensity goes down drastically up to 9 at. % of Pb, beyond 9 at. % the PL intensity is approximately the same up to 15 at. %. In the last section the results are summarized. Chapter six summarizes the essential features of the work reported in the thesis. These conclusions are drawn from the present and the earlier reported studies on Ge-Se-Te glasses, Bi doped Ge-Se and Ge-Se-Te glasses and Pb doped Ge-Se glasses. Finally based on the present experimental results, some future work has been suggested which could throw some light on a better understanding of/? to n transition and defects state of these glasses. It is worth extending the microscopic phase separation studies in these glasses. Highly sensitive experimental techniques are needed in this regard. Also some simulation work like Monte-Carlo simulation and Molecular dynamics simulation needs to be undertaken for understanding the microscopic phase separation and the role of defects in carrier type reversal in these glassy materials. All the references cited in the thesis are collected and listed at the end of the thesis.

Owing to their excellent infrared transmittance and good rheological properties, selenium based chalcogenide glasses have been materials of choice for a number of technological applications. However, chalcogenide glasses can undergo substantial structural relaxation even at room temperature due to their low glass transition temperatures. The origins of these dynamic processes and their correlation to the glass structure is therefore of fundamental and practical interest. In particular, a deep understanding of the dynamic response near the glass transition region could help elucidate the mechanism of these structural relaxation processes. The correlation between structure and dynamic properties of selenium based glass systems were therefore investigated. NMR and Raman spectroscopy measurements reveal that the structure of AsₓSe₁₋ₓ glass follow the chain crossing model in selenium-rich glasses but contain increasing amounts of cage molecules in arsenic-rich compositions. This structural pattern leads to systematic extrema in physical properties at the stoichiometric composition As₄₀Se₆₀.The dynamic response of AsₓSe₁₋ₓ glasses investigated by heat capacity spectroscopy shows two minima in melt fragility as a function of composition which correlate well with the dimensionality of the glassy network. The structure evolves from 2D to 3D during crosslinking of selenium chains by arsenic but reduces into a 2D layer-like structure at the stoichiometric composition. Upon precipitation of arsenic-rich cages the network first reverts back to 3D and eventually becomes a mix of 2D and 0D structural units. The presence of molecular clusters in the network is evidenced by a strong bimodal dynamic response at high arsenic contents. NMR and Raman spectroscopy measurements of GeₓSe₁₋ₓ glasses suggest a structure composed of aggregated tetrahedral units and long selenium chains with little or no connectivity. Distinct dynamic responses of these two separated structural motifs are revealed by heat capacity spectroscopy. A non-Gaussian distribution of the imaginary heat capacity peak provided further evidence for the structural heterogeneity. This behavior is consistent with high temperature NMR measurements which show that the dynamic response of floppy selenium chains is distinct from that of rigid tetrahedral units. Finally, heat capacity spectroscopy applied to pure selenium provides strong evidence for the microscopic origin of the non-exponential structural relaxation, a universal feature of fragile glasses. The evolution of the imaginary heat capacity peak shape during annealing shows a non-monotonic trend which remarkably matches model predictions based on the enthalpy landscape. These results indicate that the non-exponential character of the relaxation process is linked to density fluctuations in the glass.