Les matériaux à changement de phase (PCM) sont utilisés pour la réalisation de mémoire non volatile. Ces matériaux possèdent la particularité de passer d’un état cristallin à un état amorphe à l’aide d’une impulsion de chaleur, créant ainsi un processus propre au stockage de l’information. Les PCMs sont généralement basés sur des composés ternaires de type Ge-Sb-Te (GST) avec une température de transition de l’ordre de 125°C, rendent ces matériaux inutilisable dans le domaine de l’automobile et pour des applications militaires. Pour contourner cette limitation, le GST est remplacé par le composé In-Sb-Te (IST) possèdent une température de transition plus élevé et un temps de transition beaucoup plus rapide (nanoseconde). Les propriétés thermiques de l’IST et de ses interfaces au sein de la cellule PCM peuvent influencer la température de transition. C’est pourquoi la mesure de la conductivité thermique nous donnera une estimation de la valeur de cette transition.Différentes techniques ont été misent en oeuvre pour mesurer la conductivité thermique des couches minces d’IST en fonction de la concentration en Te, à savoir ; la radiométrie photo-thermique modulée (MPTR) et la méthode 3ω dans une gamme de température allant de l’ambiant jusqu'à 550°C.Les résultats obtenus par les deux techniques de caractérisation thermiques démontrent que la conductivité thermique de l'IST diminue lorsque l'on augmente la teneur en Te. L'augmentation de la teneur en Te pourrait donc conduire à un alliage thermiquement plus résistif, qui est censé apporter l'avantage d'un flux de chaleur plus confiné et limiter la cross-talk thermique dans le dispositif de mémoire à changement de phase. / Phase change memories (PCM) are typically based on compounds of the Ge-Sb-Te (GST) ternary system. Nevertheless, a major drawback of PCM devices is the failure to fulfill automotive-level or military-grade requirements (125°C continuous operation), due to the low crystallization temperature of GST. To overcome this limitation, alloys belonging to the In-Sb-Te (IST) system have been proposed, which have demonstrated high crystallization temperature, and fast switching. Thermal properties of the chalcogenide alloy and of its interfaces within the PCM cell can influence the programming current, reliability and optimized scaling of PCM devices. The two methods, namely: 3ω and Modulated Photothermal Radiometry (MPTR) technique was implemented to measure the thermal conductivity of IST thin films as well as the thermal boundary resistance at the interface with other surrounding materials (a metal and a dielectric). The experiment was carried outin situ from room temperature up to 550oC in order to investigate the intrinsic thermal properties at different temperatures and the significant structural rearrangement upon the phase transition.The results obtained from the two thermal characterization techniques demonstrate that the thermal conductivity of IST decreases when increasing the Te content. Increasing the Te content could thus lead to a more thermally resistive alloy, which is expected to bring the advantage of a more confined heat flow and limiting the thermal cross-talk in the phase change memory device.

諸多的研究顯示，和塊材相比，低維度的材料其物理性質會有所不同，為了探究熱電材料在低維度下對其熱電性質所造成的效應，我們合成了BixSb2-xTe3-y奈米線並量測其熱電性質。本實驗藉由熱處理薄膜製備奈米線的方法合成單晶BixSb2-x Te3-y奈米線。我們先利用脈衝雷射沉積系統將BixSb2-x Te3鍍在矽基板上形成薄膜，再將薄膜以350 ℃至490 ℃熱處理5到21天，奈米線即為了平衡因薄膜與矽基板彼此熱膨脹係數不同所造成的應力而自薄膜上長出，其直徑為幾十奈米至幾百奈米不等，長度則為幾微米至幾十微米。為了瞭解奈米線之構成與量測其熱電性質，我們結合微影製程及操縱技術，將單根奈米線架空於附有電極、加熱元件及溫度感測元件之量測平台上，由於奈米線已被架空，我們便能透過選區繞射分析奈米線其結晶性，並使用能量散射分析儀得知奈米線之成分，利用四點量測可得知奈米線的電阻率ρ，以加熱元件在奈米線兩端產生溫差，並量測因西貝克效應 (Seebeck effect) 所造成之電壓差即能得到西貝克係數 S (Seebeck coefficient)，三倍頻技術要求所量測的樣品必須要架空於基板上，運用三倍頻技術 (3ω method) 可量測奈米線之熱導率κ及比熱。結合微影製程、操縱技術以及量測系統，我們成功得到單根奈米線的三個熱電係數ρ、S以及κ，並了解低維度對熱電性質所造成的影響。 / Compare with the bulk materials, many researches had revealed that physical properties were different in low dimensional materials. To study the low-dimensional effects on thermoelectric properties of thermoelectric materials, BixSb2-xTe3-y nanowires were synthesized and studied for their thermoelectric properties. Single-crystallized BixSb2-xTe3-y nanowires were synthesized by on-film formation of nanowires. First, BixSb2-xTe3 thin films were deposited on SiO2/Si substrates by using the pulsed laser deposition system. BixSb2-xTe3-y nanowires grew from the films by annealing the films at 350~490 ℃ for 5~21 days through the stress release of the thermal expansion mismatch between the film and the substrate. A series of BixSb2-xTe3-y nanowires were prepared with the diameter from few tens of nanometers to few hundreds of nanometers and the length from few micrometers to few tens of micrometers. In order to analyze the components and measure the thermoelectric properties of the nanowires, the technique of combining microfabrication and manipulation for suspending a single BixSb2-xTe3-y nanowire on a measurement platform with electrodes, heater and thermometers was developed. As long as the wire is suspended, the crystallization of the nanowire is able to be analyzed by the selected area electron diffraction (SAED). The composition of the nanowire can be analyzed by the STEM-EDX. Resistivity ρ is measured by the four-point probe method. In order to get the Seebeck coefficient S, temperature difference were generated by the heater and thermoelectric voltage generated by Seebeck effect were measured. The 3ω method which demands that the wire should be suspended was applied to measure the thermal conductivity κ and specific heat c. By using the developed technique and the measurement system, three thermoelectric parameter ρ, S, κ of a single nanowire were successfully measured and the low-dimensional effect on thermoelectric properties were examined.

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Climate change is a subject of deep distress in today’s world. Over dependence on hydrocarbon has resulted in serious environmental problems. Rising sea level, global warming and ozone layer depletion are the mainstream of any discuss world over. The collective goal of cutting carbon emission by the year 2020has prompted the search for clean, alternative energy sources. This effort are already yielding good reward as other forms of energy such as solar, wind, nuclear and hydro have received huge investment and renew interest over the past decade. Thermoelectric materials over the past decades have been tipped to replace conventional means of power generations as these materials have the ability to convert heat to electrical energy and vice versa. They are simple, have no moving parts and use no greenhouse gases. But the major drawback of these materials is their low conversion efficiency. Hence there is a need to enhance the efficiency of thermoelectric material to fulfill their undeniable potentials. A parameter called the thermoelectric figure of merit, ZT defines the efficiency of a thermoelectric material. ZT relates three non-mutually exclusive transport properties namely Seebeck coefficient, electrical conductivity and thermal conductivity. Efficient thermoelectric material should possess high Seebeck coefficient, high electrical conductivity and low thermal conductivity. Hence, one of the interesting ideas in the area of thermoelectric research is the concept of designing a bulk material with high density of phonon scattering centers so has to reduce the lattice contribution to thermal conductivity but at the same time have minimum impact oncharge carriers. This is usually achieved by utilizing interphase and grain boundaries which are localized defects to scatter phonons. The volume fraction of the grain/interphase boundaries can be control through phase modification and microstructure design. This thesis is centered on Bi-Sb-Te systems which are the present room temperature state of the earth thermoelectric material. The investigation revolves around developing a new kind of microstructure in the well-studied Bi-Sb-Te system that shows tremendous potential as a means to reduce lattice contribution to thermal conductivity. The idea of having both p and n-type thermoelectric material preferably from the same material was also a motivation in our investigation. The thesis isdivided into six chapters. The first chapter introduces the concept of thermoelectricity i.e. the direct conversion of thermal energy into electricity. The physics involved and contribution of individual to the science of thermoelectricity were enumerated. Efficiency, optimization and material selection for better thermoelectric performance were briefly enumerated. Prospective materials that are currently been investigated for better thermoelectric properties were also mentioned. The structure of the Bi-Sb-Te system which is the focus of this thesis is present in this chapter including doping effect on the thermoelectric performance of the system as well as the various methods present been employed to improve the thermoelectric properties of the system. Finally the chapter enumerates the scope and object of the present thesis. The different experimental procedures adopted in the present thesis arediscussed in chapter 2. The details of different processing routes followed to synthesize flame-melted ingots, flame-melted + low temperature milled (cryo milling) + spark plasma sintering (SPS) alloy and flame-melted + melt spinning + spark plasma sintering (SPS) alloy, are discussed followed by the various structural and functional characterization techniques. The unique advantage of the spark plasma sintering techniques over the conventional sintering method was talked out in detail. The structural characterizations performed on the synthesized alloys include XRD, SEM and whilethe functional characterizations comprised of Hall measurement, Seebeck coefficient, electrical resistivity and thermal conductivity measurements. Thermoelectric properties of selected composition of Bi-Sb-Te synthesized via flame-melting are presented in chapter 3.Detail study of four analyzed compositions namelyBi24Sb20Te56, Bi20Sb12Te69, Bi16Sb5Te79 and Bi29Sb11Te60resulted in four unique microstructure and different volume fraction of primary and secondary phases. The resultant morphologies of the microstructure were observed to have influence the thermoelectric behavior corresponding to each composition. The sole influence of anti-structural defects on the conductivity type and the role of microstructure morphologies and length scale were understood in this chapter. Samples with segregated Te and a solid solution BiSbTe3(eutectic morphology) form an n-type thermoelectric material while samples with only solid solution BiSbTe3 forms a p-type thermoelectric material. Pair of n-type and p-type material was obtained without the introduction of external dopant.The pair shows good compatibility factorsuitable for thermoelectric device. In chapter 4, the thermoelectric properties of four selected composition of Bi-Sb-Te synthesized via low temperature milling plus spark plasma sintering is addressed. The analyzed compositions are as follows Bi24Sb20Te56, Bi18Sb11Te71, Bi17Sb6Te77, and Bi28Sb15Te57 respectively. The effect of low temperature milling combine with the prospect of minimum grain growth of spark plasma sintering on the thermoelectric properties of the selected compositions were determined. Samples with eutectic morphology which would otherwise scatter charge carriers were observed to have the highest carrier mobility as a result of high volume fraction of Te phase which serves as a donor injecting excess electrons into the system. The impact of small grain size was observed on the transport properties of the sample Bi28Sb15Te57 with the highest electrical resistivity, the best Seebeck coefficient and the lowest thermal conductivity. Pair of n-type and p-type material was obtained without the introduction of external doping elements. The pairshows good compatibility factor suitable for segmented thermoelectric device. Chapter 5 narrates the thermoelectric properties of four compositions namely Bi30Sb13Te58, Bi23Sb13Te65, Bi18Sb5Te77 and Bi23Sb20Te58subjected to melt spinning plus spark plasma sintering.High cooling rate obtained during melt spinning process was observed in this chapter to cause a shift of composition which resulted in a microstructure morphology with eutectic colonies that is predominantly Te rich. These Te rich colonies in the sample Bi30Sb13Te58 was observed to change the conductivity type of the sample from an otherwise p-type to n-type while also aiding bipolar conduction which was detrimental to the overall thermoelectric performance of the alloy. Segregated Te in the form of eutectic morphology helps to inject excess electron into the bulk of the sample Bi23Sb13Te65 and Bi18Sb5Te77hereby increases the observed electrical conductivity which by virtue of the microstructure morphology is expected to be low. As a result of the processing routes, all four compositions in this chapter shown-type conductivity. Chapter 6 presents the summary of the important conclusions drawn from this work.

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.