The Science behind amorphous Chalcogenide materials opened up new technologies in the arena of Phase Change Memories. The Ovonic universal phase change memory is called universal because it can replace flash memory, DRAM and SRAM. These are not only basic computer memory devices but also are becoming the driving force for the ongoing revolutionary growth of cell phones and other mobile devices, which are in desperate need of memory providing higher density, faster speed and lower power consumption. In this thesis, compositional dependence of various properties of different chalcogenide glasses are investigated, to explore the possibility of their application in Phase Change Memories. Efforts are also made to understand the effect of rigidity and extended rigidity transition on the composition dependence of properties investigated. This thesis comprises of 9 chapters; a brief summary is given below. Chapter 1 deals with fundamental aspects of amorphous semiconductors with a particular reference to chalcogenide glasses. The advantages and applications of chalcogenide glasses are also described. Chapter 2 outlines preparation and characterization of the glasses investigated. The sample preparation and various experimental setup used in the present thesis work like Raman Scattering, Nanoindentation, Alternating Differential Scanning Calorimetry (ADSC), Photo-thermal Deflection Spectroscopy (PDS), Electrical Switching are summarized here. Chapter 3 deals with Micro-Raman studies in Ge15Te85-x Inx Glasses. Micro-Raman studies reveal that as-quenched Ge15Te85-xInx samples exhibit two prominent peaks, at 123 and 155 cm-1. In thermally annealed samples, the peaks at 120 cm-1 and 140 cm-1, which are due to crystalline Te, emerge as the strongest peaks. The Raman spectra of polished samples are similar to those of annealed samples, with strong peaks at 123 cm-1 and 141 cm-1. The spectra of lightly polished samples outside the thermally reversing window resemble those of thermally annealed samples; however, the spectra of glasses with compositions in the thermally reversing window resemble those of as-quenched samples. This observation confirms the earlier idea that compositions in the thermally reversing window are non-ageing and are more stable. Chapter 4 explains nanoindentation studies undertaken on Ge15Te85-xInx glasse (1 ≤ x ≤ 11). Nanoindentation studies on Ge15Te85-xInx glasses indicate that the hardness and elastic modulus of these glasses increase with indium concentration. While a pronounced plateau is seen in the elastic modulus in the composition range 3 ≤ x ≤ 7, the hardness exhibits a change in slope at compositions x = 3 and x = 7. Also, the density exhibits a broad maximum in this composition range. The observed changes in the mechanical properties and density are clearly associated with the thermally reversing window in Ge15Te85-xInx glasses in the composition range 3 ≤ x ≤ 7. In addition, a local minimum is seen in density and hardness around x = 9, the chemical threshold of the system. Chapter 5 deals with crystallization kinetics of Ge15Te85-xInx glasses. The crystallization kinetics of Ge15Te85Inx glasses have been studied by non-isothermal method. The composition dependence of Tg and Tc at different heating rates, is investigated. The activation energy of crystallization is calculated using the Kissinger’s plot. It is found that the composition dependence of the glass transition temperature, Tg and the crystallization temperature, Tc, the activation energy of crystallization, Ec, and the stability factor, (ΔT= Tc-Tg) exhibit specific signatures of intermediate phase in the composition rang 3 ≤ x ≤ 7 and Chemical Threshold at x = 9. Chapter 6 explains Alternating Differential Scanning Calorimetric and XRD studies on silver doped Ge15Te80In5 glasses. X-ray diffraction studies on quaternary Ge15Te80-xIn5Agx glasses (2 ≤ x ≤ 24) reveal the presence of Te, GeTe, Ag8GeTe6, AgTe, In2Te3 and In4Te3. Thermal studies on quaternary Ge15Te80-xIn5Agx glasses exhibit signatures of Intermediate Phase (IP) in the variation of Tg, ∆HNR and ∆Cp with Ag addition in the composition range 8 ≤ x ≤ 16. The composition x = 16 has been identified to be the Chemical Threshold (CT) based on the saturation of flexible Ag-Te bonds. Micro-Raman, molar volume, thermal diffusivity studies on Ge15Te80-xIn5Agx glasses reveal a clear evidence of intermediate phase in the composition range 8 ≤ x ≤ 16 as depicted in the ADSC studies. Chapter 7 deals with Micro-Raman studies on as-quenched Ge15Te80-xIn5Agx glasses reveal the presence of tetrahedral structural units. Further, the Raman peak positions are found to shift with silver addition. In addition, specific signatures of the Intermediate Phase (IP) are observed in the composition dependence of Raman frequencies and corresponding intensities of different modes in the composition range, 8 ≤ x ≤ 16. In thermally annealed samples, the observed Raman peaks can be attributed to crystalline tellurium and silver lattice vibrational modes; significant increase in intensity is observed at 93 and 141cm-1 with silver addition in annealed samples, suggesting an increase in silver lattice vibrational modes. Also, the compositional dependence of density, molar volume and thermal diffusivity confirms the presence of the intermediate phase. Chapter 8 contains the current-voltage characteristics and electrical switching behavior of Ge15Te80-xIn5Agx glasses. The glasses are found to exhibit memory type switching for 3mA current in the voltage range 70 -120 V, for a sample thickness 0.3 mm. But when the current is lowered to 1mA the samples exhibit threshold switching. The compositional studies indicate the presence of an intermediate phase in the composition range 8 ≤ x ≤ 16. SET-RESET studies have been carried out using a triangular pulse of 6 mA amplitude for SET and 21 mA amplitude for RESET for a sample thickness 0.3 mm. Raman studies on SET and RESET indicates SET state resemble annealed samples and RESET state resemble as-quenched samples. It is interesting to note that the samples in the intermediate phase, especially compositions at x =10, 12, 14 withstand more set-reset cycles. This indicates compositions in the intermediate phase are suitable for PCM devices. Chapter 9 summarizes the significant results obtained and explains the scope of this thesis.

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.

The Phase Change Memories (PCM) based on chalcogenide glasses are being considered recently as a possible replacement for conventional Non Volatile Random Access Memories (NVRAM). The main advantages of chalcogenide phase change memories are their direct write/overwrite capability, lower voltages of operation, large write/erase cycles, easiness to integrate with logic, etc. The phase change random access memories work on the principle of memory switching exhibited by chalcogenide glasses during which a local structural change (between amorphous and crystalline states) occurs due to an applied electric field. The development of newer phase change materials for NVRAM applications is based on synthesizing newer glass compositions and investigating their electrical switching characteristics by applying current/voltage pulses of different waveforms. The thermal studies on chalcogenide glasses which provide information about thermal stability, glass forming ability, etc., are also important while selecting a chalcogenide glass for PCM applications. The present thesis work deals with electrical switching and thermal studies on certain silicon based ternary telluride glasses (As-Te-Si, Ge-Te-Si and Al-Te-Si). The effect of network topological thresholds on the composition dependence of switching voltages and thermal parameters such as glass transition temperature, specific heat capacity, non-reversing enthalpy, etc., of these glasses has been investigated. The first chapter of the thesis provides an introduction to various properties of chalcogenide glasses, including their applications in phase change memories. The fundamental aspects of amorphous solids such as glass formation, glass transition, etc., are presented. Further, the concepts of rigidity percolation and self organization in glassy networks and the influence of local structural effects on the properties of glassy chalcogenides are discussed. Also, a brief history of evolution of phase change memories is presented. The second chapter deals with the experimental techniques employed in this thesis work; for sample preparation and for electrical switching studies, Alternating Differential Scanning Calorimetry (ADSC), Raman spectroscopy, NMR spectroscopy, etc. The third chapter discusses the electrical switching and thermal studies on As30Te70-xSix (2 ≤ x ≤ 22) and As40Te60-xSix (2 ≤ x ≤ 17) glasses. The composition dependence of electrical switching voltage (VT) and thermal parameters such as glass transition temperature (Tg), crystallization temperature (Tc), thermal stability (Tc-Tg), etc., reveals the occurrence of extended rigidity percolation and chemical thresholds in As30Te70-xSix and As40Te60-xSix glasses. Chapter 4 presents the electrical switching and thermal studies on Ge15Te85-xSix glasses (2 ≤ x ≤ 12). These glasses have been found to exhibit memory type electrical switching. While Ge15Te85-xSix glasses with x ≤ 5 exhibit a normal electrical switching, an unstable behavior is seen in the I-V characteristics of Ge15Te85-xSix glasses with x > 5 during the transition to ON state. Further, the switching voltage (VT) and initial resistance (R) are found to increase with addition of Si, exhibiting a change in slope at the rigidity percolation threshold of the Ge15Te85-xSix system. The ADSC studies on these glasses indicate the presence of an extended stiffness transition and a thermally reversing window in Ge15Te85-xSix in the composition range of 2 ≤ x ≤ 6. The fifth chapter deals with electrical switching investigations, thermal and structural studies on Al15Te85-xSix glasses (2 ≤ x ≤ 12). These glasses have been found to exhibit two crystallization reactions (Tc1 and Tc2) for compositions with x < 8 and a single stage crystallization is seen for compositions above x = 8. Also, a trough is seen in the composition dependence of non-reversing enthalpy (ΔHNR), based on which it is proposed that there is a thermally reversing window in Al15Te85-xSix glasses in the composition range 4 ≤ x ≤ 8. Further, Al15Te85-xSix glasses are found to exhibit a threshold type electrical switching at ON state currents less than 2 mA. The start and the end of the thermally reversing window seen in the thermal studies are exemplified by a kink and saturation in the composition dependence of switching voltages respectively. 27Al Solid State NMR measurements reveal that in Al15Te85-xSix glasses, Al atoms reside in 4-fold as well as 6-fold coordinated environments. Unlike in Al-As-Te glasses, there is no correlation seen between the composition dependence of the fraction of 4-fold and 6-fold coordinated aluminum atoms and the switching behavior of Al-Te-Si samples. Chapter 6 provides a comparison of the properties of the three glassy systems studied (As-Te-Si, Ge-Te-Si and Al-Te-Si), made to identify the system better suited for phase change memory applications. It is found that the Ge-Te-Si glassy system has better electrical/thermal properties for phase change memory applications. The seventh chapter describes easily reversible SET-RESET processes in Ge15Te83Si2 glass which is a promising candidate for phase change memory applications. This sample exhibits memory switching at a comparatively low threshold electric field (Eth) of 7.3 kV/cm. The SET and RESET processes have been achieved with 1 mA triangular current pulse for the SET process and 1 mA rectangle pulse (of 10 msec width) for RESET operation respectively. Further, a self-resetting effect is seen in this material upon excitation with a saw-tooth/square pulse. About 6.5x104 SET-RESET cycles have been achieved without any damage to the device. In chapter 8, results of in-situ Raman scattering studies on the structural changes occurring during the SET and RESET processes in Ge15Te83Si2 sample, are presented. It is found that the degree of disorder in the glass is reduced from OFF to SET state. The local structure of the sample under RESET condition is similar to that in the OFF state. The Raman results are found to be consistent with the switching results which indicate that the Ge15Te83Si2 glass can be SET and RESET easily. Further, Electron Microscopic studies on switched samples indicate the formation of nanometer sized particles of cSiTe2. A summary of the results obtained and the scope for future work are included in the chapter 9 of the thesis.