This paper summarizes a recently developed continuum theory for the elastic-viscoplastic deformation of amorphous solids such as polymeric and metallic glasses. Introducing an internal-state variable that represents the local free-volume associated with certain metastable states, we are able to capture the highly non-linear stress-strain behavior that precedes the yield-peak and gives rise to post-yield strain-softening. Our theory explicitly accounts for the dependence of the Helmholtz free energy on the plastic deformation in a thermodynamically consistent manner. This dependence leads directly to a backstress in the underlying flow rule, and allows us to model the rapid strain-hardening response after the initial yield-drop in monotonic deformations, as well as the Bauschinger-type reverse-yielding phenomena typically observed in amorphous polymeric solids upon unloading after large plastic deformations. We have implemented a special set of constitutive equations resulting from the general theory in a finite-element computer program. Using this finite-element program, we apply the specialized equations to model the large-deformation response of the amorphous polymeric solid polycarbonate, at ambient temperature and pressure. We show numerical results to some representative problems, and compare them against corresponding results from physical experiments. / Singapore-MIT Alliance (SMA)

Il est bien connu que divers solides amorphes ont de nombreuses propriétés universelles. L'une d'entre elles est la variation de la conductivité thermique en fonction de la température. Cependant, le mécanisme microscopique du transfert de chaleur dans le domaine de température supérieure à 20 K est encore mal compris. Simulations numériques récentes du silicium et de la silice amorphes montrent que les modes de vibration dans la gamme de fréquences correspondante (au-dessus de plusieurs THz) sont délocalisés. En même temps ils sont complètement différents des phonons acoustiques de basse fréquence, dits « diffusions ».Dans ce travail, nous présentons un modèle stable de matrice aléatoire d'un solide amorphe. Dans ce modèle, on peut faire varier le degré de désordre allant du cristal parfait jusqu'au milieu mou extrêmement désordonné sans rigidité macroscopique. Nous montrons que les solides amorphes réels sont proches du deuxième cas limite, et que les diffusions occupent la partie dominante du spectre de vibration. La fréquence de transition entre les phonons acoustiques et diffusons est déterminée par le critère Ioffe-Regel. Fait intéressant, cette fréquence de transition coïncide pratiquement avec la position du pic Boson. Nous montrons également que la diffusivité et la densité d'états de vibration de diffusons sont pratiquement constantes en fonction de la fréquence. Par conséquent, la conductivité thermique est une fonction linéaire de la température dans le domaine allant à des températures relativement élevées, puis elle sature. Cette conclusion est en accord avec de nombreuses données expérimentales. En outre, nous considérons un modèle numérique de matériaux de type de silicium amorphe et étudions le rôle du désordre pour les vibrations longitudinales et transverses. Nous montrons aussi que la théorie des matrices aléatoires peut être appliquée avec succès pour estimer la densité d'états vibrationnels des systèmes granulaires bloqués. / It is well known that various amorphous solids have many universal properties. One of them is the temperature dependence of the thermal conductivity. However, the microscopic mechanism of the heat transfer above 20 K is still poorly understood. Recent numerical simulations of amorphous silicon and silica show that vibrational modes in the corresponding frequency range (above several THz) are delocalized, however they are completely different from low-frequency acoustic phonons, called “diffusons”.In this work we present a stable random matrix model of an amorphous solid. In this model one can vary the strength of disorder going from a perfect crystal to extremely disordered soft medium without macroscopic rigidity. We show that real amorphous solids are close to the second limiting case, and that diffusons occupy the dominant part of the vibrational spectrum. The crossover frequency between acoustic phonons and diffusons is determined by the Ioffe-Regel criterion. Interestingly, this crossover frequency practically coincides with the Boson peak position. We also show that, as a function of frequency, the diffusivity and the vibrational density of states of diffusons are practically constant. As a result, the thermal conductivity is a linear function of temperature up to rather high temperatures and then saturates. This conclusion is in agreement with numerous experimental data.Further, we consider a numerical model of amorphous silicon-like materials and investigate the role of disorder for longitudinal and transverse vibrations. We also show that the random matrix theory can be successfully applied to estimate the vibrational density of states of granular jammed systems.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第24397号 / 理博第4896号 / 新制||理||1699(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 早川 尚男, 教授 佐々 真一, 准教授 藤 定義 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

The recent development of a framework called non-affine lattice dynamics made it possible to calculate the elastic moduli of disordered systems directly from their microscopic structure and potential energy landscape at zero temperature. In this thesis different types of disordered systems were studied using this framework. By comparing the shear modulus and vibrational properties of nearest neighbour spring networks based on depleted lattices we were able to show that the dominating quantity of the system’s non-affine reorganisation during shear deformation is the affine force field. Furthermore we found that different implementation of disorder lead to the same behaviour at the isostatic point. Later we studied the effect of long range interaction in such depleted lattices with regard to spatial correlation local elasticity. We found that the implementation of long springs with decaying spring constant reproduced the spatial correlation observed in simulations of Lennard-Jones glasses. Finally we looked at simple freely rotating polymer model chains by extending the framework to angular forces and studied the dependence of the shear modulus and the vibrational density of states (VDOS) and length and bending stiffness of the chains. We found that the effect of chain length on the shear modulus and the vibrational density of states diminishes as it depends on the number of backbone bonds in the system. This number increases fast for short chains as many new backbone bonds are introduced but slows down significantly when the chain length reaches 50 monomers per chain. For the dependence on the bending stiffness we found a rich phenomenology that can be understood by looking at specific motions of the monomers relative the the chain geometry. We were able to trace back the different regimes of the VDOS to the simple model of the triatomic molecule. We also explored the limits of non-affine lattice dynamics when describing systems at temperatures T > 0 and gave an approximate solution for the shear modulus in this case.

Par la mise en œuvre de simulations numériques d'un modèle bidimensionnel de verre de Lennard-Jones, nous étudions l'effet de la température sur les mécanismes élémentaires de la déformation dans les matériaux amorphes. Nous présentons un ensemble très complet de données couvrant plusieurs décades de taux de cisaillement à différentes températures en dessous et jusqu'à la transition vitreuse. Les mesures, qui portent sur la diffusion transverse, la contrainte macroscopique ainsi que sur des champs mésoscopiques (déformation, contrainte) et leurs corrélations spatiales, conduisent à proposer que la dynamique des avalanches identifiée précédemment dans les simulations athermiques continue d'être à l'œuvre - en restant presque inchangée - jusqu'à la transition vitreuse. Nous arguons que dans la gamme de paramètres utilisée l'effet des fluctuations thermiques revient à déplacer les seuils auxquels les événements dissipatifs se produisent, ce qui se traduit par une forte baisse du niveau de contrainte macroscopique aux températures les plus basses / Using numerical simulations of a model two-dimensional Lennard-Jones glass, we study the effect of small temperatures on the elementary mechanisms of deformation in amorphous materials. A very extensive data set covering several decades of shear rate at various temperatures below and up to the glass transition was compiled. Measurements, which include transverse diffusion, macroscopic stress, and coarse-grained fields (strain, stress) and their spatial correlations, lead us to propose that the avalanche dynamics previously identified in athermal simulations continues to be at work -- and nearly unchanged -- up to the glass transition. It is then argued that in this range, thermal fluctuation essentially shift the strains at which dissipative events take place, which results in a sharp drop of the macroscopic stress level at the lowest temperatures

Les films de silice dont sont constitués les traitements antireflets des verres de lunettes sont déposés par vaporisation au canon à électrons, à température ambiante. Ils sont le siège de fortes contraintes résiduelles compressives qui diminuent considérablement leur stabilité mécanique. Ces contraintes sont difficiles à contrôler parce que les paramètres process qui les affectent sont très nombreux: propriétés du substrat, du gaz résiduel, caractéristiques de l’enceinte et du canon à électrons, vitesse de croissance,… Ils ne sont par ailleurs pas tous indépendants et agissent souvent sur plusieurs phénomènes physiques à la fois. Dans cette thèse, nous mettons en œuvre des simulations numériques et des expériences pour identifier les mécanismes à l’origine de la mise en compression des films de silice pendant leur croissance. Les expériences nous permettent de distinguer trois régimes de croissance, en fonction de la pression de gaz résiduel. Sous vide très poussé, où le gaz a un rôle négligeable, les films croissent en compression. Ensuite, à mesure que la pression augmente, l’incorporation d’espèces issues du gaz dans les films les comprime légèrement. Enfin, lorsque la pression augmente encore, le ralentissement des particules vaporisées par le gaz diminue fortement le niveau de compression et masque l’effet d’incorporation. Les dépôts de silice par dynamique moléculaire nous permettent d’explorer la limite de vide idéal. Grâce à une étude paramétrique systématique, nous trouvons que la mise en compression des films est exclusivement contrôlée par l’énergie cinétique moyenne des particules incidentes. En outre, les valeurs expérimentales ne peuvent être retrouvées qu’avec une énergie de quelques eV, au moins dix fois plus grande que toutes les prédictions formulées dans la littérature sur le dépôt. Ce résultat inattendu nous conduit à réfuter l’idée que la vaporisation au canon à électrons procéderait par simple échauffement thermique. Nous le confirmons en déposant expérimentalement des films à partir de monoxyde de silicium, évaporé thermiquement ou vaporisé au canon à électrons: les premiers croissent en tension, les seconds en compression. Finalement, pour expliquer les quelques eV prédits, nous proposons que sous irradiation électronique, une concentration de charges se forme à la surface de la silice en raison de sa très faible conductivité électrique. Les particules vaporisées qui sont chargées sont alors accélérées par répulsion Coulombienne / Silica thin films are widely used as low index layers in antireflective coatings. In the ophthalmic industry, they are deposited at ambient temperature, by electron beam vaporization. This process generates large compressive stresses which make the coatings susceptible to damage. It is thus crucial to understand how these stresses emerge. However, this problem is highly complex because many process parameters may play a role: substrate and residual gas properties, characteristics of the deposition chamber, of the electron gun, growth rate,… Moreover, these parameters may depend on each other and affect several phenomena at the same time. In this thesis, numerical simulations and experiments are performed in order to identifiy the mechanisms responsible for the generation of compressive stresses during film growth. The experiments reveal three regimes of growth, depending on the residual gas pressure. Near ultra high vacuum, where the effect of residual gas is negligible, films grow under compression. Then, as pressure increases, incorporation of gas species in the films slightly compresses them. Eventually, when pressure is high enough so that vaporized particles are slowed down by collisions with gas particles, the level of compression significantly decreases; this rapidly masks the incorporation effect. Molecular dynamics simulations allow us to explore the ideal vacuum limit. By depositing silica films in a vast ensemble of conditions, we find that their compressive state of stress is solely controlled by the mean kinetic energy of incident particles. Comparison with experiments suggests that this energy is equal to a few eV, which is at least ten times greater than predictions from the literature on deposition. This unexpected result leads us to refute the idea that electron beam vaporization would be equivalent to simple themal heating. We confirm this experimentally, by comparing films deposited from silicon monoxide either thermally evaporated or vaporized using an electron beam: the formers grow under tension while the latters under compression. Finally, we explain the ejection of particles of a few eV as coming from the very low electrical conductivity of silica: under electronic irradiation, charges accumulate at its surface and accelerate the charged vaporized particles by Coulombian repulsions

Diffusion plays an important role in a number of processes like heterogeneous catalysis, corrosion, separation and purification of chemicals of industrial importance, steel hardening, fuel cells, and solid electrolytes for batteries. It also plays a vital role in several biological processes like transport across biomembranes, nerve impulse, flow of blood and permeation of ingested drug. The elementary process of diffusion in solids is quite different from those in liquids. Similarly, the mode of diffusion in porous solid where different regimes such Knudsen regime exists bears little similarity to those in a dense close-packed crystalline solid. Chapter 1 provides a brief introduction to basics of diffusion in different phases of condensed matter. Among the various phases discussed are liquids, close-packed crystalline solids (e.g., body-centered cubic solids), amorphous solids (e.g. glasses) and microporous crystalline solids (e.g., zeolites). Diffusion in these widely differing phases often bears no resemblance to each other; the rate of diffusion in these phases varies over many orders of magnitude and the elementary step and mechanism in the diffusion process are very different. Brief introduction to theories for diffusion in these phases is provided. Various experimental techniques to measure diffusivities are discussed. Different microscopic models to explain the Quasi Elastic Neutron Scattering (QENS) spectra of these phases yield an insight into the elementary step of the diffusion process. Notwithstanding the fact that completely different models are invoked to explain diffusion in different phases, there are certain underlying generic behaviour across these widely differing phases as the recent work on size dependence of diffusion in these phases demonstrate. Diffusion of a molecule or species (in the context of diffusion within condensed phases) without loss of generality may be said to occur in a medium. A universal behaviour observed is that self diffusivity exhibits a maximum as a function of the size of the diffusant when the diffusant is confined to a medium, as a result of what is known as the Levitation Effect. Such a maximum in self diffusivity has been seen in widely differing medium: microporous solids, dense liquids, ions in polar solvents, etc. The aim of the thesis is to investigate and further explore such universal behaviour and demonstrate for the first time the existence of common trends across different condensed phases in spite of difference in the detail at the microscopic level. In Chapter 2, we report a molecular dynamics study of diffusion of diatomic species AB within zeolite Y. The bond length of A-B as well as the interaction of A and B with the host zeolite atoms are varied. The results demonstrate that for the symmetric case (when A=B or AA), there exists a preferred bond length (determined by the bottleneck or window diameter) when the diffusivity is maximum. This is in agreement with previous results on monatomic species which also exhibit a similar diffusivity maximum. More importantly, no such maximum is seen when the interaction asymmetric is introduced in AB. Slight asymmetry in the interaction gives rise to a weak maximum while large asymmetry in interaction obliterates the diffusivity maximum. These results suggest that the importance of interaction between the diffusant and the medium in Levitation Effect or size-dependent diffusivity maximum. Further, it also demonstrates for the first time the close association between an inversion centre (in a statistical sense and not in the crystallographic sense) and the Levitation Effect. In Chapter 3, a study of size dependence of solutes in a Lennard-Jones liquid is reported. Einstein and others derived the reciprocal dependence of the self-diffusivity D on the solute radius ru for large solutes based on kinetic theory. We examine here (a) the range of ru over which Stokes-Einstein (SE) dependence is valid and (b) the precise dependence for small solutes outside of the SE regime. We show through molecular dynamics simulations that there are two distinct regimes for smaller solutes: (i) the interaction or Levitation Effect (LE) regime for solutes of intermediate sizes and (ii) the D 1/ru2 for still smaller solutes. We show that as the solute-solvent size ratio decreases, the breakdown in the Stokes-Einstein relationship leading to the LE regime has its origin in dispersion interaction between the solute and the solvent. These results explain reports of enhanced solute diffusion in solvents existing in the literature seen for small solutes for which no explanation exists. Several properties have been computed to understand the nature of solute motion in different regimes. We investigate in Chapter 4, the dependence of self diffusivity on the size of the diffusant in a disordered medium with the objective of understanding the experimentally observed correlation between self diffusivity and activation energy seen in a wide variety of glasses. Typically, it is found in many ionic glasses that a higher conductivity is associated with lower activation energy and vice versa. Our understanding of transport in glasses as provided by existing theories does not offer an explanation of this correlation. We have carried out molecular dynamics simulation as a function of the size of the impurity atom or diffusant (both neutral and charged) in a model host amorphous matrix. We find that there is a maximum in self diffusivity as a function of the size of the impurity atom suggesting that there is an appropriate size for which the diffusivity is maximum. The activation energy is found to be the lowest for this size of the impurity. A similar maximum has previously been found in other condensed phases such as confined fluids and dense liquids and has its origin in the Levitation Effect. The implications of this result for understanding ionic conductivity in glasses are discussed. Our result suggests that there is a relation between microscopic structure of the amorphous solid, diffusivity or conductivity and activation energy. The nature of this relationship is discussed in terms of the Levitation parameter showing that diffusivity is maximum when the size of the neck or doorway radius is comparable with the size of the diffusant. Our computational results here are in excellent agreement with independent experimental results which show that structural features of the glass are important in determining the ionic conductivity. In Chapter 5, we report results of molecular dynamics investigations into neutral impurity diffusing within an amorphous solid as a function of the size of the diffusant and density of the host amorphous matrix. We find that self diffusivity exhibits an anomalous maximum as a function of the size of the impurity species. An analysis of properties of the impurity atom with maximum diffusivity shows that it is associated with lower mean square force, reduced backscattering of velocity autocorrelation function, near-exponential decay of the intermediate scattering function (as compared to stretched-exponential decay for other sizes of the impurity species) and lower activation energy. These results demonstrate the existence of well known size-dependent diffusivity maximum in disordered solids. Further, we show that the diffusivity maximum is observed at lower impurity diameters with increase in density. This is explained in terms of the levitation parameter and the void structure of the amorphous solid. We demonstrate that these results imply contrasting dependence of self diffusivity (D) on the density of the amorphous matrix, . D increases with  for small sizes of the impurity but shows an increase followed by a decrease for intermediate sizes of the impurity atom. For large sizes of the impurity atom, D decreases with increase in . These contrasting dependence arises naturally from the existence of Levitation Effect. In Chapter 6, we discuss size dependence of impurity diffusion in an ordered system. We report molecular dynamics simulation studies to understand the role of impurity size and impurity-host interaction on impurity diffusivity in a body centered cubic solid. The simulation studies have been performed for a set of impurity-host interaction parameter ih (i=impurity, h=host atom) for a range of impurity sizes in rigid and flexible bcc solids. A double maximum is seen corresponding to two different sizes of the impurity species. Anomalous maximum is seen for a larger size of the impurity species in the case of the rigid host as compared to flexible host. The second anomalous diffusivity disappears with decrease in ih in flexible bcc solid. For one of the ih where double diffusivity maxima are observed, various properties are analysed to understand the anomalous diffusion behaviour. The impurity with anomalous diffusion has lower activation energy as compared to other impurities. Among the two anomalous impurities, the impurity with higher diffusivity has lower activation energy. The anomalous regime impurities as associated with velocity autocorrelation function with little or no backscattering, minimum average mean square force due to host atoms, lower activation energy. The self intermediate scattering function shows faster decay and a single relaxation time for anomalous regime impurity and two relaxation times for other impurity sizes. The wavenumber dependence of diffusivity of impurities shows oscillatory behaviour except for the anomalous regime impurities which show monotonic dependence on wavenumber. Chapter 7 discusses the influence of temperature induced solid-liquid phase transition on the size-dependent diffusivity. We report results for two distinct cases: (a) when the phase change is associated with corresponding changes in density and (b) when the phase change occurs at constant density. The latter is carried out so as to obtain the influence of disorder on the size-dependent diffusion or Levitation Effect. Studies with variable density are useful to understand the effect of disorder as well as change in density on size-dependent diffusivity. Two diffusivity maxima in the solid face-centred cubic phase is seen when the impurity-medium interaction is sufficiently large. One of these diffusivity maximum disappears with decrease in h. The impurities near the diffusivity maximum show velocity autocorrelation function with little backscattering, minimum in the average mean square force, lower activation energy, faster decay of self intermediate scattering function with a single relaxation time and a monotonic decay in wavevector dependence of diffusivity. Chapter 8 reports molecular dynamics simulations of a model guest tetrahedral molecule AX4 with differing bond lengths lAX have been carried out in a sphere with different surface roughness. The rotational-diffusion coefficient Dr shows a maximum for a particular value of lAX. This corresponds to the distance at which the interaction of the guest with the atoms of the host is most favourable. Although, the intensity of the maximum decreases with increase in the roughness of the confining surface, it is seen that the maximum exists even for a reasonably high degree of roughness. The observed maximum arises from the minimum in the torque on the tetrahedral molecule from its interaction with the confining medium due to mutual cancellation of forces. Activation energy for rotation is seen to be also a minimum for the bond length for which Dr is a maximum. These results suggest that there is a maximum in the rotational-diffusion coefficient when the rotating molecule is confined to a sphere of comparable size similar to the maximum in translational diffusion coefficient seen in porous solids and known as the Levitation Effect. On increase in the roughness of the sphere surface, the value of lAX at which the maximum in Dr is seen decreases. This is similar to the shift seen in the size of the diffusant corresponding to maximum diffusivity in the case of translational diffusivity. In Chapter 9 possible extensions to the work reported in the previous chapters and the directions to take are discussed. Symmetry plays an important role in size dependent diffusivity maximum in microporous crystalline solids; it would be interesting to investigate if similar role of symmetry exists in case of liquids and other disordered solids. Previous work from this laboratory on ions in water has shown the importance of electrostatic interactions. In the light of this, it would be interesting to see the influence of long-range interactions in breakdown of Stokes-Einstein relationship in liquids. Effect of density of the medium on impurity diffusion can be studied over a wide range of densities in case of supercritical fluids such as ions in water (where electrostatic interactions are present) and these can provide greater insight into dependence of diffusion on density. The origin of two diffusivity maxima in case of body-centered and face-centred cubic solids needs a detailed investigation to understand its origin. Quantification of disorder and its effect on size dependence of diffusion would be of interest. A detailed comparison with experimental data of matrix isolated molecules to understand and verify the dependence of rotational diffusivity on the size of the molecule as well as the cavity radius would be instructive.

GeTe and Ge2Sb2Te5 alloys are potential candidates for non-volatile phase change random access memories (PCRAM). For electrical data storage applications the materials should have stable amorphous and crystalline phases, fast crystallization time, low power to switch, and high crystallization activation energy (to be stable at normal operating temperatures). Phase change memories can be tuned through compositional variations to achieve sufficient phase change contrast and thermal stability for data retention. Selenium is one of the attractive choices to use as an additive material owing to its flexible amorphous structure and a variety of possible applications in optoelectronics and solar cells. GeSb2Te3Se alloy, in which 25 at.% of Se substituted for Te, show a higher room temperature resistance with respect to parent GeSb2Te4 alloy, but the transition temperature is lowered which will affect the thermal stability. The RESET current observed for Sb65Se35 alloys were reduced and the crystallization speed increased 25 % faster with respect to Ge2Sb2Te5. Alloys of Ga-Sb-Se possess advantages such as higher crystallization temperatures, better data retention, higher switching speed, lower thermal conductivity and lower melting point than the GST, but the resistance ratio is limited to about two orders of magnitude. This affects the resistance contrast and data readability. It is with this background a study has been carried out in GeTe and GeSbTe system with Se doping. Studies on structural, thermal and optical properties of these materials all through the phase transition temperatures would be helpful to explore the feasibility of phase change memory uses. Thin films along with their bulk counterparts such as (GeTe)1-x Sex ( 0 < x ≤ 0.50) and (GST)1-xSex (0 < x ≤ 0.50), including GeTe and GST alloys, have been prepared. The results are presented in four chapters apart from the Introduction and Experimental techniques chapters. The final chapter summarizes the results. Chapter 1 provides an introduction to chalcogenide glasses, phase change memory materials and their applications. The fundamental properties of amorphous solids, basic phase change properties of Ge2Sb2Te5 and GeTe alloys and their applications are presented in detail. Various doping studies on GeTe and Ge2Sb2Te5 reported in literatures are reviewed. The limitations, challenges, future and scope of the present work are presented. In chapter 2, the experimental techniques used for thin film preparation, electrical characterizations, optical characterization and surface characterizations etc. are explained. Chapter 3 deals entirely on Ge2Sb2Te5 films studied throughout the phase transition, by annealing at different temperatures. Changes in sheet resistance, optical transmission, morphology and surface bonding characteristics are analyzed. The crystallization leads to an increase of roughness and the resistance changes to three orders of magnitude at 125 oC. Optical studies show distinct changes in transmittance during phase transitions and the optical parameters are calculated. Band gap contrast and disorder variation with annealing temperatures are explained. The surface bonding characteristics studied by XPS show Ge-Te, Sb-Te bonds are present in both amorphous and crystalline phases. The temperature dependent modifications of the band structure of amorphous GST films at low temperatures have been little explored. The band gap increment of around 0.2 eV is observed at low temperature (4.2 K) compared to room temperature 300 K. Other optical parameters like Urbach energy and B1/2 are studied at different temperatures and are evaluated. The observed changes in optical band gap (Eopt) are fitted to Fan’s one phonon approximation, from which a phonon energy (ћω) corresponding to a frequency of 3.59 THz resulted. The frequency of 3.66 THz optical phonons has already been reported by coherent phonon spectroscopy experiment in amorphous GST. This opens up an indirect method of calculating the phonon frequency of the amorphous phase change materials. Chapter 4 constitutes comparison of optical, electrical and structural investigation of GST and (GST)1-xSex films. It is well known that GST alloys have vacancy in their structure, which leads to the possibility of switching between the amorphous and crystalline states with minimum damage. Added Se may occupy the vacancy or change the bonding characteristics which intern may manifest in the possibility of change in optical and electrical parameters. The structural studies show a direct amorphous to hexagonal transition in (GST)1-xSex, where x ≥ 0.10 at.%. Raman spectra of the as deposited and annealed (GST)1-xSex films show structural modifications. The infrared transmission spectra indicate a shift in absorption edges from low to high photon energy when Se concentration increases in GST. Band gap values calculated from Tauc plot show the band gap increment with Se doping. It is noted that a small amount of Se doping increases the resistance of the amorphous and crystalline phases and maintains the same orders of resistance contrast. This will be beneficial as it improves the thermal stability and reduces the write current in a device. Switching studies show an increasing threshold voltage as the Se doping concentration increases. Chapter 5 comprises compositional dependent investigations of the bulk GeTe chalcogenides alloys added with different selenium concentrations. The XRD investigations on bulk (GeTe)1-xSex (x = 0.0, 0.02, 0.10, 0.20 and 0.50 at.%) alloys show that the crystalline structure of GeTe alloys does not affect ≤ 0.20 at.% of Se concentration. With increasing amount of Se concentration the alloys gets modified in to a homogeneous amorphous structure. This result has been verified from the XRD, Raman, XPS, SEM and DSC measurements. The possibility that Se occupying the Ge vacancy sites in GeTe structure is explained. Since Se is an easy glass former, the amorphousness increases in the alloys due to new amorphous phases formed by the Se with other elements. It is shown from Raman and XPS analysis that the Ge-Te bonds exists up to Se 0.20 at.% alloys. Ge-Se and GeTe2 bonds are increasing with increasing Se at.%. Melting temperature has found decreases and the reduction in melting point may reduces the RESET current. Further studies on switching behavior may bring out its usefulness. Chapter 6 deals with studies on (GeTe)1-xSex films for phase change memory applications based on the insight received from their bulk study. Even at low at.% addition of Se makes the as prepared (GeTe)1-xSex film amorphous. At 200 oC, GeTe crystalline structure is evolved and the intensity of the peaks reduces in the alloys with increase of Se content. At 300 oC, more evolved GeTe crystalline structure is seen compared to 200 oC annealed films whereas 0.20 at.% Se alloy remain amorphous. Resistance and thermal studies shows increase in crystallization temperature. It is expected that Se sits in the vacancies of the GeTe crystalline structural formation. This may also account for the increased threshold voltages with increasing Se doping. The band gap increase with increase of Se at.% signifying the possibility of band gap tuning in the material. Possible explanation for the increased order in GeTe due to Se doping is presented. The modifications in the alloy with Se addition can be explained with the help of chemical bond energy approach. Those bonds having higher energy leads to increased average bond energy of the system and hence the band gap. The XPS core level spectra and Raman spectra investigation clearly shows the GeTe bonds are replaced by Ge-Se bonds and GeTe2 bonds. The 0.10 at.% Se alloy is found to have a higher thermal stability in the amorphous state and maintains a gigantic resistance contrast compared to other Se concentration alloys. This alloy can be considered as an ideal candidate for multilevel PCM applications. Chapter 7 summarizes the major findings from this work and the scope for future work.