Thesis (M.S. in Chemistry)--University of California, Berkeley, Sept. 1957. / Includes bibliographical references (leaf 23).

This paper presents an integrated column-type micro thermoelectric cooler (£g-TEC) constructed with serial connected p-type antimony-tellurium (Sb2Te3) and n-type bismuth-tellurium (Bi2Te3) micro pillars deposited by electrochemical deposited technology. To optimize the power factor, density and uniformity of the TE films and to enhance the reproducibility of £g-TEC device, a cathode with tunable rotary speed and with accurate current controller has been designed in the electroplating system of this research. The electroplating deposited Bi2Te3 and Sb2Te3 with an average thickness of 8 £gm, are connected using Cr/Au layers at the hot junctions and cold junctions. The measured Seebeck coefficient and electrical resistivity are -86 £gV/K and 16 £g£[-m for Bi2Te3 films after annealed at 250¢XC, and are 68 £gV/K and 30 £g£[-m for Sb2Te3 films after annealed at 200¢XC. The optimized power factors of the n-type (2.64¡Ñ10-4 W/K2m) and p-type (2.64¡Ñ10-4 W/K2m) telluride compounds have been demonstrated in this paper. Under 5 volts driven, the integrated £g-TEC device shows average cooling achieved is about 1.3 ¢XC.

Thesis advisor: Zhifeng Ren / Solid-state cooling and power generation based on thermoelectric effects are attractive for a wide range of applications in power generation, waste heat recovery, air-conditioning, and refrigeration. There have been persistent efforts on improving the figure of merit (ZT) since the 1950's; only incremental gains were achieved in increasing ZT, with the (Bi1-xSbx)2(Se1-yTey)3 alloy family remaining the best commercial material with ZT ~ 1. To improve ZT to a higher value, we have been pursuing an approach based on random nanostructures and the idea that the thermal conductivity reduction that is responsible for ZT enhancement in superlattices structures can be realized in such nanostructures. The synthesis and characterization of various nanopowders prepared by wet chemical as well as high energy ball milling methods will be discussed in this dissertation. The solid dense samples from nanopowders were prepared by direct current induced hot press (DC hot press) technique. The thermoelectric properties of the hot pressed samples have been studied in detail. By ball milling ingots of bulk alloy crystals and hot pressing the nanopowders, we had demonstrated a high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this dissertation, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 can be achieved. Our material also exhibits a ZT of 0.7 at 250 °C, close to the value reached when ingot was used. This process is more economical and environmentally friendly than starting from bulk alloy crystals. The ZT improvement is caused mostly by the low thermal conductivity, similar to the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the high density grain boundaries and defects. The performance of thermoelectric materials is determined by its dimensionless figure-of-merit (ZT) which needs to be optimized within a specific temperature range for a desired device performance. Hence, we show that by varying the Bi/Sb ratio, the peak ZT can be shifted to a higher or lower temperature for power generation applications or a cooling mode operation. A peak ZT of about 1.3 is achieved from a Bi0.4Sb1.6Te3 composition which is highest among the different compositions. These nanostructured bulk samples have a significantly low lattice thermal conductivity compared to the bulk samples due to the increased phonon scattering in the grain boundaries and defects. This study shows that Bi0.5Sb1.5Te3 may potentially perform better for cooling devices, while Bi0.3Sb1.7Te3 should be able to show better power generation efficiency. Several issues related to accurate measurement of thermoelectric properties were identified and many of them were solved during my studies and these are discussed in this thesis. With the data we obtained, it is clear that nanopowder-based thermoelectric materials hold significant promise. Therefore, a review of synthesis of nanostructured materials by solution-based methods, including a hydrothermal process for the Bi2Te3, Bi2Se3, and Bi2Te2.25Se0.75 nanoparticles, a solvothermal route for Sb2Te3 nanostructures, and a polyol process for the preparation of Bi nanostructures is presented in this dissertation. These new nanostructures may find applications in enhancing the thermoelectric performance. Although small sized and well dispersed nanopowders of various thermoelectric materials could be prepared by a solution method in large scale, contamination and partial oxidation are always big challenges in a chemical approach. Hence, a high energy ball milling technique to prepare thermoelectric nanopowders in large scale and without major contamination is still found to be more efficient and preferred. / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

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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.