Chalcogenide glasses are a class of covalent amorphous semiconductors with interesting properties. The presence of short-range order and the pinned Fermi level are the two important properties that make them suitable for many applications. With flash memory technology reaching the scaling limit as per Moore’s law, alternate materials and techniques are being researched at for realizing next generation non-volatile memories. Two such possibilities that are being looked at are Phase Change Memory (PCM) and Programmable Metallization Cell (PMC) both of which make use of chalcogenide materials. This thesis starts with a survey of the work done so far in realizing PCMs in reality. For chalcogenides to be used as a main memory or as a replacement to FLASH technology, the electrical switching parameters like switching voltage, programming current, ON state and OFF state resistances, switching time and optical parameters like band gap are to be considered. A survey on the work done in this regard has revealed that various parameters such as chemical composition of the PC material, nature of additives used to enhance the performance of PCM, topological thresholds (Rigidity Percolation Threshold and Chemical Threshold), device geometry, thickness of the active volume, etc., influence the electrical switching parameters. This has motivated to further investigate the material and experimental parameters that affect switching and also to explore the possibility of multi level switching. In this thesis work, the feasibility of using two chalcogenide systems namely Si15Te85-xGex and Ge15Te85-xAgx in the form of amorphous thin films for PCM application is explored. In the process, electrical switching experiments have been carried out on thin films belonging to these systems and the results obtained are found to exhibit some interesting anomalies. Further experiments and analysis have been carried out to understand these anomalies. Finally, the dynamics of electrical switching has been investigated and presented for amorphous Si15Te85-xGex thin films. From these studies, it is also seen that multi state switching/multiple resistance levels of the material can be achieved by current controlled switching, the mechanisms of which have been further probed using XRD analysis and AFM studies. In addition, investigations have been carried out on the electrical switching behavior of amorphous Ge15Te85-xAgx thin film devices and optical band gap studies on amorphous Ge15Te85-xAgx thin films. Chapter one of the thesis, gives a brief introduction to the limitations in existing memory technology and the alternative memory technologies that are being researched, based on which it can be inferred that PCM is a promising candidate for the next generation non volatile memory. This chapter also discusses the principle of using PCM to store data, realization of PCM using chalcogenides, the material properties to be considered in designing PCM, the trade offs in the process of design and the current trends in PCM technology. Chapter two provides a brief review of the electrical switching phenomenon observed in various bulk chalcogenide glasses, as electrical switching is the underlying principle behind the working of a PCM. In the process of designing a memory, many parameters like read/write operation speed, data retentivity and life, etc., have to be optimized for which a thorough understanding on the dependence of electrical switching mechanism on various material parameters is essential. In this chapter, the dependence of electrical switching on parameters like network topological thresholds and electrical and thermal properties of the material is discussed. Doping is an efficient way of controlling the electrical parameters of chalcogenides. The nature of dopant also influences switching parameters and this also is briefly discussed. Chapter three provides a brief introduction to the different experimental techniques used for the thesis work such as bulk chalcogenide glass preparation, preparation of thin amorphous films, measurement of film thickness, confirmation of amorphous nature of the films using X-Ray Diffraction (XRD), electrical switching experiments using a custom made setup, crystallization study using XRD and Atomic Force Microscopy (AFM) and optical band gap studies using UV-Vis spectrometer. Vt is an important parameter in the design of a PCM. Chapter four discusses the dependence of Switching voltage, Vt, on input energy. It is already established that the Vt is influenced by the composition of the base glass, nature of dopants, thickness of films and by the ambient temperature. Based on the results of electrical switching experiments in Si15Te74Ge11 amorphous thin films a comprehensive analysis has been done to understand the kinetics of electrical switching. Chapter five discusses a current controlled crystallization technique that can be used to realize multi-bit storage with a single layer of chalcogenide material. In case of PCM, data is stored as structural information; the memory cell in the amorphous state is read as data ‘0’ and the memory cell in crystalline state is read as data ‘1’. This is accomplished through the process of electrical switching. In order to increase the memory density or storage density, multi-bit storage is being probed at by having multiple layers of chalcogenide material. However, with this technique, the problems of inter-diffusion between different layers cannot be ruled out. In this thesis work, a current controlled crystallization technique has been used to achieve multiple stable resistance states in Si15Te75Ge10 thin films. Chapter six discusses the mechanism behind multi state switching exhibited by certain compositions of Si15Te85-xGex thin films. Crystallization studies on certain Si15Te85-xGex films have been carried out using XRD and AFM to understand the phenomenon of multiple states. The results of these experiments and analysis are presented in this chapter. Chapter seven discusses the results of electrical switching experiments and optical band gap studies on amorphous Ge15Te85-xAgx thin films. Chapter eight gives the conclusion and scope for future work.

Thermoelectric (TE) materials directly convert heat energy into electrical energy. The conversion efficiency of the TE devices depends on the performance of the materials. The conversion efficiency of available thermoelectric materials and devices is low. Therefore, the development of new materials for improving thermoelectric device performance is a highly essential. As the performance of the TE materials depends on TE figure of merit [zT=S2P T ] which consist of three material properties such as Seebeck coefficient (S), electrical resistivity ( ) and thermal conductivity ( ). Thermoelectric figure of merit can be improved by either increase of power factor or decreasing of thermal conductivity or by both. In the present thesis, Cu based chalcogenide compounds are chosen for the study of thermoelectric properties because of their complex crystal structure, which leads to lower values of thermal conductivity. Also, the power factor of these materials can be tuned by the partial substitution doping. In the present thesis, Cu based chalcogenide compounds quaternary chalcogenide compound (Cu2ZnSnSe4), ternary compounds (Cu2SnSe3 and Cu2GeSe3) and tetrahedrite materials (Cu12Sb4S13) have been prepared by solid state synthesis. The prepared compounds are characterized by XRD for the phase identification, Raman Spectroscopy used as complementary technique for XRD, SEM for surface morphology and EPMA for the phase purity and elemental composition analysis respectively. For the evaluation of zT, thermoelectric properties of all the samples have been studied by measuring Seebeck coefficient, resistivity and thermal diffusivity. In the chapter 1, a brief introduction about thermoelectricity and its effects is discussed. Thermoelectric materials parameters such as electrical resistivity, Seebeck coefficient and thermal conductivity for different class of materials are mentioned. The selection of thermoelectric materials and the motivation for choosing the Cu based chalcogenide compounds for thermoelectric applications are discussed. In chapter 2, the details of the experiments carried out for Cu based chalcogenide compounds are presented. In chapter 3, the effect on thermoelectric properties by the cation substitution on quaternary chalcogenide compound Cu2+xZnSn1 xSe4 (0, 0.025, 0.05, 0.075, 0.1, 0.125, and 0.15) is studied. The electrical resistivity of all the samples decreases with an increase in Cu content except for Cu21ZnSn09Se4, most likely due to a higher content of the ZnSe. All the samples showed positive Seebeck coefficients indicating that holes are the majority charge carriers. The thermal conductivity of doped samples was higher as compared to Cu2ZnSnSe4 and this may be due to the larger electronic contribution and the presence of the ZnSe phase in the doped samples. The maximum zT = 0.23 at 673 K is obtained for Cu205ZnSn095Se4. In chapter 4, the effect of multi{substitution of Cu21ZnSn1 xInxSe4 (0, 0.05, 0.075, and 0.1) on transport properties were studied. The Rietveld powder X-ray diffraction data accompanied by electron probe microanalysis (EPMA) and Raman spectra of all the samples con firmed the formation of a tetragonal kesterite structure. The electrical resistivity of all the samples exhibits metallic-like behavior. The positive values of the Seebeck coefficient and the Hall coefficient reveal that holes are the majority charge carriers. The co-doping of copper and indium leads to a significant increase of the electrical resistivity and the Seebeck coefficient as a function of temperature above 650 K. The thermal conductivity of all the samples decreases with increasing temperature. Lattice thermal conductivity is not significantly modified as the doping content may infer negligible mass fluctuation scattering for copper zinc and indium tin substitution. Even though, the power factors (S2 ) of indium-doped samples Cu21ZnSn1 xInxSe4 (x=0.05, 0.075) are almost the same, the maximum zT=0.45 at 773 K was obtained for Cu21Zn09Sn0925In0075Se4 due to its smaller value of thermal conductivity. In chapter 5, thermoelectric properties of Zn doped ternary compounds Cu2ZnxSn1 xSe3 (x = 0, 0.025, 0.05, 0.075) were studied. The undoped com\pound showed a monoclinic crystal structure as a major phase, while the doped compounds showed a cubic crystal structure confirmed by powder XRD (X-Ray Diffraction). The electrical resistivity decreased up to the samples with Zn content x=0.05 in Cu2ZnxSn1 xSe3, and slightly increased in the sample Cu2Zn0075Sn0925Se3 . This behavior is consistent with the changes in the carrier concentration confirmed by room temperature Hall coefficient data. Temperature dependent electrical resistivity of all samples showed heavily doped semiconductor behavior. All the samples exhibit positive Seebeck coefficient (S) and Hall coefficient indicating that the majority of the carriers are holes. A linear increase in Seebeck coefficient with increase in temperature indicates the degenerate semiconductor behavior. The total thermal conductivity of the doped samples increased with a higher amount of doping, due to the increase in the carrier contribution. The total and lattice thermal conductivity of all samples decreased with increasing of temperature, which points toward the dominance of phonon scattering at high temperatures. The maximum zT = 0.34 at 723 K is obtained for the sample Cu2SnSe3 due to a low thermal conductivity compared to the doped samples. In chapter 6, thermoelectric properties of Cu2Ge1 xInxSe3 (x = 0, 0.05, 0.1, 0.15) compounds is studied. The powder X-ray diffraction pattern of the undoped sample revealed an orthorhombic phase. The increase in doping content led to the appearance of additional peaks related to cubic and tetragonal phases along with the orthorhombic phase. This may be due to the substitutional disorder created by indium doping. The electrical resistivity ( ) systematically decreased with an increase in doping content, but increased with the temperature indicating a heavily doped semiconductor behavior. A positive Seebeck coefficient (S) of all samples in the entire temperature range reveal holes as predominant charge carriers. Positive Hall coefficient data for the compounds Cu2Ge1 xInxSe3 (x= 0, 0.1) at room temperature (RT) con rm the sign of Seebeck coefficient. The trend of as a function of doping content for the samples Cu2Ge1 xInxSe3 with x = 0 and 0.1 agrees with the measured charge carrier density calculated from Hall data. The total thermal conductivity increased with rising doping content, attributed to an increase in carrier thermal conductivity. The thermal conductivity decreases with increasing temperature, which indicates the dominance of Umklapp phonon scattering at elevated temperatures. The maximum thermoelectric figure of merit (zT) = 0.23 at 723 K was obtained for Cu2In01Ge09Se3. In chapter 7, thermoelectric properties of Cu12 xMn1 xSb4S13 (x = 0, 0.5, 1.0, 1.5, 2.0) samples were studied. The Rietveld powder XRD pattern and Electron Probe Micro Analysis revealed that all the Mn substituted samples showed a single tetrahedrite phase. The electrical resistivity increased with increasing Mn due to substitution of Mn2+ on the Cu1+ site. The positive Seebeck coefficient for all samples indicates that the dominant carriers are holes. Even though the thermal conductivity decreased as a function of increasing Mn, the thermoelectric figure of merit (zT) decreased, because the decrease of the power factor is stronger than the decrease of the thermal conductivity. The maximum zT = 0.76 at 623 K is obtained for Cu12Sb4S13. In chapter 8, the summary and conclusion of the present work is presented.

Silicon nanowires (SiNWs) and the copper chalcogenides, namely copper sulfide (Cu2S) and selenide Cu2Se, have diverse applications in renewable energy technology. For example, SiNWs which have direct band gaps unlike bulk Si, have the potential to radically reduce the cost of Si based photovoltaic cells. However, they degrade quickly under ambient conditions. Various surface passivations have therefore been investigated for enhancing their stability but it is not yet well understood how they affect the electronic structure of SiNWs at a fundamental level. Here, we will explore, from first-principles simulation, how fluorine, methyl and hydrogen surface passivations alter the electronic structures of [111] and [110] SiNWs via strain and quantum confinement. We also show how electronic charge states in [111] and [110] SiNWs can be effectively modelled by simple quantum wells. In addition, we address the issue of why [111] SiNWs are less influenced by their surface passivation than [110] SiNWs. Like SiNWs, Cu2S and Cu2Se also make excellent photovoltaic cells. However, they are most well known for their exceptional thermoelectric performance. This is by virtue of their even more unique solid-liquid hybrid nature which combines the low thermal conductivity and good electrical characteristics required for a high thermoelectric efficiency. We use first-principles molecular dynamics simulations to show that Cu diffusion rates in Cu2S and Cu2Se can be as high as 10-5cm2s-1. We also relate their phonon power spectra to their low thermal conductivities. Furthermore, we evaluate the thermoelectric properties of Cu2S and Cu2Se using a combination of Boltzmann transport theory and first-principles electronic structure calculations. Our results show that both Cu2S and Cu2Se are capable of maintaining high Seebeck coefficients in excess of 200μVK-1 for hole concentrations as high as 3x1020cm-3.

Les matériaux à changement de phase montrent la combinaison exceptionnelle d’un contraste énorme dans leurs propriétés physiques entre la phase amorphe et cristalline allié à une cinétique de changement de phase extrêmement rapide. La grande différence en résistivité permet leur application dans les mémoires numériques. De plus, cette classe de matériaux montre dans leur état vitreux des phénomènes de transport électronique caractéristiques. Le seuil de commutation dénote la chute de la résistivité dans l’état amorphe au delà d’un champ électrique critique. Le phénomène de seuil de commutation permet la transition de phase en appliquant des tensions relativement faibles. Au-dessous de cette valeur critique l’état désordonné montre une conductivité d’obscurité activée en température ainsi qu’une résistance - dans les cellules mémoires et les couches minces également – qui augmente avec le temps. Cette évolution de la résistivité amorphe entrave le stockage à plusieurs niveaux, qui offrirait la possibilité d'accroître la capacité ou la densité de stockage considérablement. Comprendre les origines physiques de ces deux phénomènes est crucial pour développer de meilleures mémoires à changement de phase. Bien que ces deux phénomènes soient généralement attribués aux défauts localisés, la connaissance de la distribution de défauts dans les matériaux amorphes à changement de phase est assez limitée. Cette thèse se concentre sur la densité des défauts mesurée dans différents verres chalcogénures présentant l’effet de seuil de commutation. Sur la base d’expériences de photo courant modulé (MPC) et de spectroscopie par déviation photothermique, un modèle sophistiqué des défauts a été développé pour GeTe amorphe (a-GeTe) mettant en évidence les états de la bande de valence et plusieurs défauts. Cette étude sur a-GeTe montre que l’analyse des données MPC peut être grandement améliorée en prenant en compte la variation de la bande de l’énergie interdite avec la température. Afin de mieux appréhender l’évolution de la résistivité amorphe, la présente étude porte sur l’évolution avec les recuits et le vieillissement de la résistivité, de l’énergie d’activation du courant d’obscurité, de la densité des défauts, du stress mécanique, de l'environnement atomique et de l’énergie de la bande interdite mesurée par des méthodes optiques sur les couches minces de a-GeTe. Le recuit d’un échantillon entraîne un élargissement de la bande interdite et de l’énergie d’activation du courant d’obscurité. De plus, la technique MPC a révélé une diminution des défauts profonds dans les couches minces de a-GeTe vieillies. Ces résultats illustrent l’impact de l’annihilation des défauts et de l’élargissement de la bande interdite sur l’évolution de la résistivité des matériaux à changement de phase amorphe. Cette thèse présente également une étude sur les alliages à changement de phase GeSnTe. En augmentant la concentration d’étain, on observe une décroissance systématique de la résistivité amorphe, de l’énergie d’activation du courant d’obscurité, de la largeur de bande interdite et de la densité des défauts, qui conduisent à une résistivité amorphe plus stables dans les compositions riches en étain comme a-Ge2Sn2Te4. L’étude sur les alliages GeSnTe montre que les matériaux à changement de phase ayant une résistivité amorphe plus stable présentent une faible énergie d’activation du courant d’obscurité. À l’exemple du Ge2Sn2Te4 et GeTe la présente étude montre un lien étroit entre l’évolution de la résistivité et la relaxation du stress mécanique. L’étude sur les verres chalcogénures montrent que les matériaux ayant un grand champ d’électrique de seuil, bien connu d’après la littérature, présentent aussi une grande densité de défauts. Ce résultat implique que l’origine du phénomène de seuil de commutation se trouve dans un mécanisme de génération à travers la bande interdite et de recombinaison dans les défauts profonds comme proposé par D. Adler. / Phase change materials combine a pronounced contrast in resistivity and reflectivity between their disordered amorphous and ordered crystalline state with very fast crystallization kinetics. Due to this exceptional combination of properties phase-change materials find broad application in non-volatile optical memories such as CD, DVD or Bluray Disc. Furthermore, this class of materials demonstrates remarkable electrical transport phenomena in their disordered state, which have shown to be crucial for their application in electronic storage devices. The threshold switching phenomenon denotes the sudden decrease in resistivity beyond a critical electrical threshold field. The threshold switching phenomenon facilitates the phase transitions at practical small voltages. Below this threshold the amorphous state resistivity is thermally activated and is observed to increase with time. This effect known as resistance drift seriously hampers the development of multi-level storage devices. Hence, understanding the physical origins of threshold switching and resistance drift phenomena is crucial to improve non-volatile phase-change memories. Even though both phenomena are often attributed to localized defect states in the band gap, the defect state density in amorphous phase-change materials has remained poorly studied. This thesis presents defects state densities measured on different amorphous phase-change materials and chalcogenides showing electrical threshold switching. On the basis of Modulated Photo Current (MPC) Experiments and Photothermal Spectroscopy a sophisticated band model for a-GeTe has been developed, which is shown to consist of defect bands and band tail states. This study on a-GeTe has shown that the data analysis within MPC experiments can be drastically improved by taking the temperature dependence of the optical band gap into account. To get a better understanding of resistance drift phenomena this study focuses on the evolution of resistivity on heating and ageing, activation energy of electronic conduction, optical band gap, defect state density, mechanical stress and nearest neighbour ordering in a-GeTe thin films. After heating the samples one hour at 140°C the activation energy for electric conduction increases by 30 meV, while the optical band gap increases by 60 meV. Additionally, MPC experiments revealed a decreasing concentration of mid gap states in aged a-GeTe thin films. These findings demonstrate the impact of band gap opening and defect annihilation on resistance drift. Furthermore, the stoichiometric dependence of resistance drift phenomena in a-GeSnTe phase-change alloys is studied in this thesis. A systematic decrease in the amorphous state resistivity, activation energy for electric conduction, optical band gap and defect density is observed with increasing tin content resulting in a low resistance drift for tin rich compositions such as a-Ge2Sn2Te4. This study on GeSnTe systems demonstrates, that phase change alloys showing a more stable amorphous state resistivity are characterized by a low activation energy of electronic conduction. This finding found in GeSnTe alloys holds also true for GeSbTe and AgInSbTe systems. On the example of a-Ge2Sn2Te4 and a-GeTe exhibiting a strong resistance drift, the evolution of the amorphous state resistivity is shown to be closely linked to the relaxation of internal mechanical stresses resulting in an improving structural ordering of the amorphous phase. For the investigated alloys showing electrical switching, the measured density of midgap states is observed to decreases with decreasing threshold field known from literature. This result favours a generation-recombination model behind electrical switching in amorphous chalcogenides as originally proposed by Adler.

Les verres du système binaire Ag₂S-As₂S₃ sont connus pour être de très bons conducteurs ioniques et l’ajout de HgI₂ permet d’envisager une application des verres du pseudo-ternaire HgI₂-Ag₂S-As₂S₃ en tant que membrane ionique spécifique dédiée au dosage du mercure en solution aqueuse. Les limites de son domaine vitreux ont été vérifiées à l’aide de la diffraction des rayons X. Les évolutions des propriétés macroscopiques des verres, incluant les densités et les températures caractéristiques (Tg, Tc et Tf) ont été analysées de façon systématique. Les propriétés de conduction des verres HgI₂-Ag₂S-As₂S₃ ont été évaluées à l’aide de la spectroscopie d’impédance complexe et de la diffusion du traceur radioactif 108mAg. Un des résultats les plus marquants dans ces verres conducteurs ioniques est l’augmentation de la conductivité lorsque Ag₂S est substitué par HgI₂. Afin de comprendre les mécanismes de conduction mis en jeu, des études structurales ont été menées par spectroscopie Raman, diffusion de neutrons et diffraction des rayons X haute énergie. Pour appréhender la structure de ces verres complexes, des études préalables sur les deux systèmes pseudo-binaires Ag₂S-As₂S₃ et HgI₂-As₂S₃ ont aussi été menées. Les différentes techniques utilisées ont notamment permis de montrer que des réactions d’échanges se produisaient lors de la synthèse. Enfin, la dernière partie de cette thèse est entièrement consacrée à la caractérisation de nouveaux capteurs chimiques pour la détection des ions Hg²+ en solution. Différentes compositions sont testées afin de définir la sensibilité, la limite de détection et les coefficients de sélectivité en présence d’ions interférents. / Glasses of the pseudo-binary system Ag₂S-As₂S₃ are well known to be good ionic conductors and the addition of HgI₂ allows considering the glasses of the pseudo-ternary system HgI₂-Ag₂S-As₂S₃ as ion-membrane dedicated to mercury sensing in aqueous solution. The limits of its vitreous domain were verified by X-ray diffraction. Changes in macroscopic properties of glasses, including density and characteristic temperatures (Tg, Tc et Tm) were systematically investigated. Conduction properties of HgI₂-Ag₂S-As₂S₃ glasses were evaluated using the complex impedance spectroscopy and 108mAg tracer diffusion measurements. One of the most interesting results concerns the conductivity increase if Ag₂S is substituted by HgI₂. To understand the conduction mechanisms involved, structural studies were carried out by Raman spectroscopy, neutron scattering and high-energy X-ray diffraction. To understand the structure of these complex glasses, preliminary studies on the two pseudo-binary systems Ag₂S-As₂S₃ and HgI₂-As₂S₃ were also undertaken. These techniques have underlined exchange reactions that occur during the synthesis. The last part of this research work is entirely devoted to the characterization of new chemical sensors for the detection of Hg²+ ions in solution. Different compositions were tested to determine the sensitivity, detection limit and selectivity coefficients in the presence of interfering ions.

Availability of amorphous semiconductors in the form of high quality multilayers provide potential applications in the field of micro- and optoelectronics. Although the misfit problems in amorphous multilayers (AML) are considerably reduced compared to crystalline superlattices, there are still some physical processes (e.g. quantum confinement, diffusion) that are not well understood. Recently chalcogenide glass multilayers were prepared with high quality nanomodulation, which demonstrated their potential for tailoring the optical properties. Moreover studies on amorphous nanolayered chalcogenide structures (ANC) are still at the infant stage. These ANCs are similar to the crystalline superlattices, yet distinct from ideal crystalline superlattices produced by molecular beam epitaxy. The ANCs can be considered as well-correlated layers with good periodicity and smooth interface. They are attractive because of the prominent photoinduced effects, similar to those exhibited by uniform thin films. For example, photoinduced diffusion in short period multilayer systems is important because of its potential applications in holographic recording and fabrication of phase gratings. In spite of its practical usefulness, the mechanism of photoinduced interdiffusion is not properly understood. Since most structural changes are related to atomic diffusion, understanding of the structural transformation must be based on the diffusion process. Moreover, in AML, the process of interdiffusion is not well understood. The main aim of this thesis is to study the photoinduced interdiffusion in Se/As2S3 and Bi/As2S3 nanolayered films. In literature, there are reports about the photoinduced interdiffusion in Se/As2S3 and Bi/As2S3 nanolayered films, but the mechanisms of photoinduced interdiffusion of these elements are not properly understood. Raman scattering and infrared spectroscopy techniques were used to study the photoinduced interdiffusion in Se/As2S3 and Bi/As2S3 nanolayered films by Kikineshi et al, but the results were not satisfactory. The characteristic spectra of components in the multilayer and those of the mixed layer were rather similar. In the present thesis, photoinduced interdiffusion in Se/As2S3 and Bi/As2S3 nanolayered samples are studied by optical absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and Photoluminescence (PL). The detailed information about the distribution of electronic states in the absorption edge, localized states and the new bonds formed between the components due to photoinduced interdiffusion elucidated from the above studies will give more insight into the mechanism and kinetics of photoinduced interdiffusion. The thesis consists of six chapters. References are given at the end of each chapter. Various general and unique physical properties of amorphous chalcogenides are discussed in Chapter 1. This chapter summarizes the fundamental aspects of amorphous state, such as the structure and its models, electronic band structure, defects as well as the physical properties like d.c conductivity, a.c conductivity, optical absorption, photoconductivity and PL. A more detailed account of the various photoinduced effects are also discussed. Apart from this, similar photoinduced effects observed in other systems like a-Si:H, oxide glasses, Polymers etc are described in brief. Finally, the scope of present investigations is furnished. Chapter 2 has been devoted to photoinduced interdiffusion and related changes in optical properties of nanolayered Se/As2S3 films. It begins with a brief introduction followed by a survey of the earlier work done on these multilayered films. The theory of optical absorption and experimental procedures are discussed. Photoinduced interdiffusion was observed with above band gap light in nanolayered Se/As2S3 films. It is discussed in terms of the optical parameters such as bandgap, Urbach edge (Ee) and Tauc’s parameter (B1/2). From the analysis of the optical absorption spectra, it was concluded that the optical bandgap, Ee and B1/2 change with photoinduced interdiffusion. These changes in properties are ascribed to the solid solution formation due to the intermixing of adjacent layers. The photoinduced intermixing of the adjacent layers are obviously related to the photoinduced viscous flow and it depends on the number of excited chalcogen bridge atoms, which determine the local deformations due to the bond switching and displacements. Experimental data of B1/2 and Ee for as prepared samples do not show a clear correlation implied by the Mott-Davis model. It is also observed that the optical parameters can be changed with a change in the Se sublayer thickness. Variations of these optical parameters as a function of modulation period and photoinduced interdiffusion were discussed in terms of the quantum confinement effect and changes in the valence and conduction bands. Chapter 3 deals with the PL studies on as prepared and irradiated samples of Se/As2S3 nanolayered films. The theory of PL, experimental procedures and data analysis are discussed in detail. PL studies were carried out on as prepared and irradiated nanolayered samples of Se/As2S3 films. None of the samples showed PL at 77 K, which clearly indicate that there exists a competitive non-radiative mechanism. We observed a broad PL in the range of 0.8–1.2 eV for as prepared and irradiated samples at 4.2 K. The observed stoke shift in PL is discussed in terms of the strong electron-phonon coupling at the recombination centers. We found that the PL intensity can be increased by several orders of magnitude by irradiating the samples with appropriate wavelengths in the range of the absorption edge. The broadening of luminescence bands takes place either with a decrease in Se layer thickness or with irradiation. The former is due to the change in interface roughness while the latter is due to photoinduced interdiffusion. Deconvolution showed that the PL spectrum consists of five transitions. The deconvoluted peak PL intensity, PL quantum efficiency and full width at half maximum are varying according to the function of sublayer thickness and interdiffusion. All these results indicate the high impact of interdiffusion on the luminescence intensity in the given AML is due to changes of defect states, which in turn are not directly connected to the band structure, i.e., confinement effects are not essential for this type of luminescence. The whole picture is complex due to more complicated carrier relaxation and recombination process, possibly with several interconnected effects, which are not properly understood, but the possibility for tuning the optical parameters of the Se/As2S3 nanolayered films, including the low temperature luminescence, is established. Chapter 4 is on kinetics and chemical analysis of photoinduced interdiffusion in nanolayered Se/As2S3 films. The basic formalism of X-ray photoelectron spectroscopy and in situ optical absorption spectroscopy together with a brief description of the theory and data analysis adopted in the present studies are given. We have studied the kinetics of photoinduced interdiffusion in nanolayered Se/As2S3 film by in situ optical absorption measurements. All previous measurements were performed ex situ, i.e., a film exposed under light irradiation during the measurement was never studied. In situ changes in the transmission spectra were measured, but at a fixed wavelength. Since the measurements were done on a single wavelength, the kinetics of the variation of optical bandgap and Tauc parameter were missing. In short, information has been missing about the metastable changes in the multilayer structure during photoinduced interdiffusion. In situ changes in transmission spectra were recorded over the wavelength range λ=400-1000 nm, and also at fixed wavelengths to understand the changes in absorption coefficient, optical bandgap and Tauc parameter during photoinduced interdiffusion. The in situ optical absorption measurements reveal that the photo darkening in amorphous nanolayered Se/As2S3 film is followed by photoinduced interdiffusion. An increase in disorder during photodarkening and its subsequent decrease during photoinduced interdiffusion was also observed. The observation of photodarkening of Se at room temperature when confined between As2S3 layers suggests that the glass transition temperature of Se shifts to higher temperature. The analysis shows that the atoms, which take part in photodarkening, play a vital role in photoinduced interdiffusion. We used XPS to analyze the new bonds formed between the components due to photoinduced interdiffusion. The XPS results showed that there is a considerable decrease in the As-S, As-As and S-S bonds after photoinduced interdiffusion; As-O and some of the S-S homopolar bonds are retained. There was a considerable decrease in As-S bond followed by an increase in As-Se and S-Se bonds. XPS analysis also shows that during photodiffusion, heteropolar bonds replace homopolar bonds, i.e., the irradiated samples are chemically ordered than the corresponding as prepared samples. Chapter 5 is concerned with the photoinduced interdiffusion in Bi/As2S3 nanolayered films. A brief description about the photoinduced interdiffusion of metals such as Ag, Zn, etc is given in the introduction. The experimental procedures and data analysis are also given. Two sets of samples with different ratios of sublayer thickness (d), d-Bi/d-As2S3 = 1/12 and 1/6 prepared by cyclic thermal evaporation are employed for the present study. A pump probe optical absorption technique was used to study the photoinduced interdiffusion in Bi/As2S3 nanolayered samples. Photoinduced interdiffusion of Bi into As2S3 was observed in both the films. The XPS analysis shows that the as prepared samples contain a large number of wrong As–As bonds and some of the As-As bonds are converted to As-S bonds during irradiation. The XPS analysis also reveals that the Bi is forming only bond with S during photoinduced interdiffusion. Chapter 6 summarizes the essential features of the present work and also points a few possible directions along which further work can be carried out.

Systematic synthetic investigations of the quaternary systems Ag - Bi - Q - X (Q = S, Se; X = Cl, Br) led to a variety of quaternary phases that exhibit considerable structural diversity with increasing complexity. These include Ag1.2Bi17.6S23Cl8, AgBi4Se5Br3 and numerous members of the homologous double series Agx(N+1)Bi2+(1-x)(N+1)Q2+(2-2x)(N+1)X2+(2x-1)(N+1) denoted (N, x)P. N represents the order number of a given homologue and x is the degree of substitution of Bi by Ag with 1/2 <= x <= 1. Their structures are built up from two alternating types of modules denoted A and B that are stacked parallel to (001). In module A, rows of edge-sharing [MZ6] octahedra (Z = X and/or Q); M = Ag and/or Bi) running parallel to [010] alternate along [100] with parallel chains of paired monocapped trigonal prisms around Bi atoms. The module type denoted B represents NaCl-type fragments of varying thickness. It is defined by the number N of octahedra within the chain of edge-sharing octahedra running diagonally across it in the (010) plane of the structure. The thickness of module B for current members of the series extends from N = 0 to N = 7. All structures exhibit Ag/Bi disorder in octahedrally coordinated metal positions and Q/X (Q = S, Se; X = Cl, Br) mixed occupation of some anion positions. Some of these compounds are narrow gap semiconductors.

Τα άμορφα υλικά είναι μια ευρεία κατηγορία υλικών με σημαντικές ιδιότητες πολλές από τις οποίες δεν απαντώνται όταν αυτά βρίσκονται στην αντίστοιχη κρυσταλλική τους φάση. Στην παρούσα εργασία μελετώνται επιλεγμένα υμένια (πάχους ~ 1μm) από μια ειδική κατηγορία άμορφων υλικών, τις χαλκογονούχες ενώσεις (chalcogenides). Ως “χαλκογενή” (chlacogens) αναφέρονται τα στοιχεία της ομάδας VIA του περιοδικού πίνακα, δηλαδή το Θείο (S), το Σελήνιο (Se) και το Τελλούριο (Te) και συνεπώς οι ενώσεις που περιέχουν ένα ή περισσότερα από αυτά τα στοιχεία μαζί με στοιχεία όπως τα As, Ge, P, Bi, Si, Sb, Ga, Ag, κλπ. σχηματίζουν τις χαλκογονούχες ενώσεις. Το γεγονός ότι το ενεργειακό χάσμα των ενώσεων αυτό εμπίπτει στην φασματική περιοχή του ορατού φωτός και του κοντινού υπερύθρου έχει ως αποτέλεσμα την εμφάνιση πλήθους φωτο-επαγόμενων (μη-θερμικών) φαινομένων όταν τα υλικά αυτά ακτινοβοληθούν με φως κατάλληλου μήκους κύματος και πυκνότητας ισχύος. Τα φωτο-επαγόμενα φαινόμενα περιλαμβάνουν αλλαγές σε δομικές, μηχανικές, χημικές, οπτικές, ηλεκτρικές κ.α. ιδιότητες. Πιο συγκεκριμένα, μέσω της μελέτης των φωτο-επαγόμενων φαινομένων παρέχεται η δυνατότητα για ελεγχόμενη μεταβολή δομικών (μικροσκοπικών) αλλά και μακροσκοπικών ιδιοτήτων του υλικού. Επομένως τα υλικά αυτά έχουν έντονο τεχνολογικό ενδιαφέρον, σε εφαρμογές όπως στην οπτική, στην μικροηλεκτρονική και στην ανάπτυξη στοιχείων αποθήκευσης πληροφορίας (οπτικές μνήμες). Στόχος της παρούσας εργασίας είναι η μελέτη και η κατανόηση σε βασικό επίπεδο των μικρο-δομικών χαρακτηριστικών, υμενίων επιλεγμένων άμορφων χαλκογονούχων ενώσεων υπό την επίδραση διαφόρων εξωτερικών ερεθισμάτων καθώς και η επίτευξη συσχετισμού μεταξύ μικροσκοπικών χαρακτηριστικών και χρήσιμων για εφαρμογές μακροσκοπικών ιδιοτήτων. Πιο συγκεκριμένα, μελετήθηκε συστηματικά η επιφανειακή ηλεκτρονιακή δομή υμενίων του συστήματος AsxSe100-x, παρασκευασμένων με θερμική εναπόθεση (thermal evaporation, TE) και εναπόθεση με παλμικό laser (pulsed laser deposition, PLD) με επιφανειακά ευαίσθητες τεχνικές όπως Φασματοσκοπία Φωτοηλεκτρονίων από Ακτίνες-x (XPS) και από Υπεριώδες (UPS). H Φασματοσκοπία Φωτοηλεκτρονίων από Ακτίνες-x (XPS) χρησιμοποιείται για τον καθορισμό της χημικής σύστασης της επιφάνειας του στερεού. Η πολλαπλότητα των χημικών καταστάσεων για ένα συγκεκριμένο είδος ατόμου υποδηλώνει την ύπαρξη μιας ποικιλίας τοπικών ατομικών διατάξεων στην επιφάνεια του υμενίου. Επομένως οι αλλαγές των ηλεκτρονιακών ιδιοτήτων στην επιφάνεια μπορούν να συσχετιστούν άμεσα με αλλαγές που αφορούν στην επιφανειακή δομή, οι οποίες προκαλούνται είτε μεταβάλλοντας διάφορες παραμέτρους όπως η σύσταση του υλικού είτε με την επιβολή κάποιου εξωτερικού ερεθίσματος όπως η θέρμανση και η ακτινοβόληση, είτε με τη φωτο-διάλυση ατόμων μετάλλου (Ag) στο εσωτερικό τους. Μεταβάλλοντας την σύσταση σε PLD υμένια AsxSe100-x και υποβάλλοντας τα σε θέρμανση, σε θερμοκρασία 150ºC (δηλαδή λίγο πιο κάτω από το Τg) οι πιο έντονες αλλαγές παρατηρήθηκαν στο ηλεκτρονικό περιβάλλον των ατόμων αρσενικού στα υμένια με ενδιάμεσες συστάσεις (As50Se50, As60Se40). Στην συνέχεια, η συμμετρική σύσταση As50Se50 μελετήθηκε διεξοδικότερα λόγω της μεγάλης ποικιλομορφίας και ετερογένειας σε νανο-κλίμακα. Τα αποτελέσματα έδειξαν ότι η ακτινοβόληση και η θέρμανση οδηγούν την δομή σε δύο διαφορετικές άμορφες καταστάσεις με διαφορετικό ποσοστό δομικών μονάδων. Το φαινόμενο είναι αντιστρεπτό και επαναλήψιμο σε διαδοχικούς κύκλους θέρμανσης και ακτινοβόλησης για τα PLD υμένια ενώ δεν ισχύει το ίδιο για τα ΤΕ υμένια. Ο προσδιορισμός του δείκτη διάθλασης με την χρήση φασματοσκοπικής ελλειψομετρίας σε PLD και ΤΕ As50Se50 υμένια, σε διαδοχικές διεγέρσεις ακτινοβόλησης και θέρμανσης, αποκάλυψε την συσχέτιση των αλλαγών στη μικροδομή των υμενίων με τις μεταβολές σε αυτή την μακροσκοπική ιδιότητα του υμενίου. Επιπλέον, εκπονήθηκε μελέτη του φωτο-επαγόμενου φαινομένου της διάχυσης και διάλυσης ατόμων μετάλλου όπως ο Ag στην δομή των υμενίων PLD και ΤΕ As50Se50 με ακτινοβόληση ακτίνων- x και ορατού φωτός (laser ενέργειας συγκρίσιμης με το ενεργειακό χάσμα του ημιαγωγού). Σκοπός ήταν η μελέτη της εξέλιξης των σχηματιζόμενων χημικών ειδών κατά τα διάφορα στάδια του φαινομένου σε αντίθεση με την έως τώρα υπάρχουσα πρακτική που εστιάζει κυρίως στον μηχανισμό της κινητικής του φαινομένου. Μετρήσεις ανάλυσης σε βάθος με XPS και SIMS έλαβαν χώρα με σκοπό την διερεύνηση του προφίλ της συγκέντρωσης του μετάλλου στο εσωτερικό του υμενίου, πριν και μετά την επαγωγή του φαινομένου. / Amorphous, are a wide category of materials with significant properties that do not occur in their respective crystalline phase. In this work, a special category of selected amorphous chalcogenide compounds (chalcogenides) in the form of thin (1μm) films, is studied experimentally. Chalcogens are the elements from Group VIA, namely Sulfur (S), selenium (Se) and tellurium (Te) and therefore compounds containing one or more of these elements together with elements such as As, Ge, P, Bi, Si, Sb, Ga, Ag, etc. form chalcogenide compounds. The fact that their energy gap is within the range of visible light and near infrared has given rise to numerous of photo-induced (non-thermal) phenomena when these materials are irradiated with light of appropriate wavelength and power density. The photo-induced effects include changes in structural, mechanical, chemical, optical, electrical, etc. properties. More specifically, through the study of photo-induced effects it is possible to control micro-structural changes and macroscopic properties of the material. Therefore these materials have a strong technological interest for applications in optics, in microelectronics and as elements in circuits for optical data storage (optical memories). The aim of this work is to study and understand at a basic level the micro-structural characteristics of chalcogenide films of selected compounds under the influence of various external stimuli as well as to achieve a correlation between microscopic characteristics and useful for applications macroscopic properties. In the present work the electronic surface structure of AsxSe100-x films prepared by thermal evaporation (TE) and by pulsed laser deposition (PLD) was studied systematically with surface sensitive techniques such as X-ray and Ultraviolet Photoelectron Spectroscopies ( XPS, UPS). X-ray photoelectron spectroscopy is used to determine the chemical composition of the surface of a solid. The multiplicity of chemical states for a specific type of atom suggests the existence of a variety of local individual arrangements on the surface of the film. Therefore, the changes of electronic properties on the surface can be directly correlated with changes on the surface structure, which are caused either by altering various parameters such as the composition of the material or by imposing an external stimulus such as annealing and irradiation, or by photo-dissolution of silver atoms (Ag) in their structure. Changing the composition of PLD AsxSe100-x films and submitting them to annealing below the Tg, the most pronounced changes occurred in the electronic environment of atoms in films with intermediate compositions (As50Se50, As60Se40). The symmetrical composition As50Se50 was chosen and studied thoroughly because of the great diversity and heterogeneity of its micro-structural units in nano-scale. The results showed that irradiation and annealing lead the film to two different amorphous states, with different percentage of structural units. The phenomenon is reversible and repeatable in successive cycles of annealing and irradiation for the PLD films while this is not true for the TE films. The determination of the refractive index using spectroscopic ellipsometry in PLD and TE As50Se50 films, in successive irradiation and annealing stimuli, revealed the correlation of the changes in the microstructure of films with the changes in this macroscopic property. Furthermore, the photo-induced diffusion and dissolution of silver (Αg) atoms in the structure of PLD and TE As50Se50 films induced by x-rays and visible light (laser energy comparable to the energy gap of semiconductor) was studied. The purpose of these experiments was to follow the chemical species formed during the various stages of the diffusion procedure with XPS in contrast to most studies so far focusing mainly on the mechanism of kinetics of the diffusion reaction. Depth profile analysis by XPS and SIMS took place in order to investigate the concentration profile of the metal atoms in depth of the films, before and after the induction of the effect.

A novel photonic frequency discriminator has been developed. The discriminator utilizes a Mach Zehnder interferometer-assisted ring resonator to achieve enhanced linearity. A numerical frequency-domain two-tone test is performed to evaluate the unique design of the discriminator, particularly for suppression of the third order intermodulation distortion. The discriminator is switchable between linear-intensity and linear-field regimes by adjusting a phase delay on one arm of the Mach Zehnder interferometer. Through the simulation, the linear<intensity discriminator is shown to be advantageous. The discriminator is an optical ring resonator-Mach Zehnder interferometer synthesized passive filter. The ring resonator is made of Arsenic trisulfide (As2S3) and the bus waveguide is a Titanium<diffused Lithium niobate (LiNbO3) waveguide. This As2S3 ring-on-Ti:LiNbO3 hybrid structure offers electro-optic tunability of the device owing to a strong electro-optic effect of the substrate material. A large optical confinement factor achieved by vertical integration of the As2S3 strip waveguide on a LiNbO3 substrate enables a low loss ring resonator. The Mach Zehnder interferometer is formed by the optical path length difference of the birefringent LiNbO3 substrate instead of a physical Y-branch structure, which makes the fabrication tolerances relaxed. In order for this highly birefringent device to be characterized, each polarization mode must be measured separately. A novel algorithm which can measure the wavelength-swept Jones matrix including its phase response is devised. The efficacy of the algorithm is demonstrated by characterizing a ring resonator. Finally, the fabricated discriminator is fully characterized using the algorithm.

The content of this thesis encompasses the fundamentals, modelling, chip design, nanofabrication process, measurement setup, and experimental results of devices exploiting the optical properties of phase-change chalcogenide materials. Special attention is paid to integrated Si₃N₄ nanophotonic circuits for optical switching and memory applications, as well as to multilayer stacks for colour modulation. Herein, the implementation of the first robust, non-volatile, phase-change photonic memory is presented. By utilising optical near-field effects for Read, Write and Erase operations, bit storage of up to eight transmission levels is demonstrated in a single device employing Ge₂Sb₂Te₅ as the active material. These on-chip memory cells feature single-shot read-out of the transmission state and switching energies as low as 13.4pJ at speeds approaching 1GHz. The capability to readily switch between intermediate states is also demonstrated, a feature that requires complex iteration-based algorithms in electronic phase-change memories. This photonic memory is not only the first truly non-volatile memory---a long-term elusive goal in integrated photonics---but could also potentially represent the first multi-level memory, including electronic counterparts, that requires no computational post-processing or drift correction. These findings provide a pathway towards solving the throughput limitations of current computer architectures by eliminating the so-called von-Neumann bottleneck and portend a new paradigm in all-photonic memory, non-conventional computing, and tunable photonic devices. Finally, novel capabilities in electro-optic colour modulation using phase-change materials are demonstrated. In particular, this thesis offers the first implementation of Ag₃In₄Sb₇₆Te₁₇-based optical cavities for colour modulation on low-dimensional multilayer stacks. Moreover, "gray-scale" image writing is demonstrated by establishing intermediate levels of crystallisation via voltage modulation. This finding, in turn, corresponds to the first demonstration of nonvolatile colour-depth modulation in the emerging phase-change materials nanodisplay technology, featuring resolutions down to 50nm. Furthermore, a comprehensive comparison is carried out for two types of materials: growth- (Ag₃In₄Sb₇₆Te₁₇) and nucleation-dominated (Ge₂Sb₂Te₅) alloys in terms of colour, energy efficiency, and resolution. These results provide new tools for the new generation of bistable and ultra-high-resolution displays and smart glasses while allowing for other potential applications in photonics and optoelectronics.