The work described herein deals with the synthesis and characterization of lead chalcogenide thin films and nanocrystals. The first part of thesis describes the properties of semiconductors followed by an analysis on the chemical vapour deposition and nanoparticulate formation. In the next part of thesis, single-source precursors of type thioselenophosphinato, selenoureato, dithiocarbamato and dithiocarbanato complexes of lead have been synthesised and characterised. As-synthesised compounds have been utilised for the fabrication of lead sulfide and lead selenide thin films by aerosol-assisted chemical vapour deposition as well as nanocrystals by colloidal injection method. Lead sulfide thin films were also deposited by liquid-liquid interface from lead dithiocarbanato at room temperature. The as grown thin films of lead sulfide and lead selenide have been characterised by XRD, SEM and energy dispersive x-ray (EDX) analysis. In the second part of the thesis, preparation of lead sulfide and lead selenide nanocrystals in olive oil at low growth temperatures (50-60°C) is described and have shown that by controlling experimental conditions, well-defined particles with tunable emission in mid and far-infrared region can be synthesised. Furthermore, compositionally-tuned PbSxSe1-x nanocrystals has also been prepared by adding controlled amount of sulur and selenium ingredients into lead oxide. Homogenous distribution of sulfur and selenium within alloyed nanocrystals is confirmed by transmission electron microscope studies. Moreover, attempts have been made to prepare quaternary (PbTe/Se/S) nanocrystals of lead chalcogenides and depth (1.9-5.8 nm) profile analysis by x-ray photoelectron spectroscopy confirmed the formation of core/shell/shell type structure i.e. PbTe/S/Se.

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.

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.