Pulsed laser deposition of Bismuth Telluride compounds for human body energy scavengers

The world wide research interest in Bismuth Telluride thin films is due to the fact that they are the most commonly efficient thermoelectric materials at temperatures as low as room temperature, which is typically suitable for implementing such thin films through the fabrication of miniaturized thermoelectric generators and human body energy scavengers. This work aims to characterize various Bismuth Telluride -based thin films deposited by Pulsed Laser Deposition technique in order to optimize their thermoelectric performance represented in their thermoelectric figures of merit. This has been achieved by investigating the electrical and thermoelectric properties of the deposited thin films as well as studying the structural properties of such thin films that is necessary for future micromachining and fabrication of energy scavengers; the results of this effort are really promising. The first chapter is an introductory overview concerning thermoelectric effects and thermoelectric generators. The second chapter deals with the different deposition techniques and the reasoning behind the employment of PLD to deposit Bismuth Telluride thin films. The third chapter includes some of Bismuth Telluride chemical and physical properties in addition to a literature survey of what other groups have already achieved concerning this material. The fourth chapter covers all the experiments and includes the results of this work. Finally, the fifth chapter includes the summary, conclusion and recommendation for future progress in this topic

Nanostructure and thermal power of highly-textured and single-crystal-like Bi2Te3 thin films

Bi2Te3-based alloys are known to have outstanding thermoelectric properties. Although structure-property relations have been studied, still, detailed analysis of the atomic and nano-scale structure of Bi2Te3 thin film in relation to their thermoelectric properties remains poorly explored. Herein, highly-textured (HT) and single-crystal-like (SCL) Bi2Te3 films have been grown using pulsed laser deposition (PLD) on Si wafer covered with (native or thermal) SiOx and mica substrates. All films are highly textured with c-axis out-of-plane, but the in-plane orientation is random for the films grown on oxide and single-crystal-like for the ones grown on mica. The power factor of the film on thermal oxide is about four times higher (56.8 mu W.cm(-1).K-2) than that of the film on mica (12.8 mu W.cm(-1).K-2), which is comparable to the one of the polycrystalline ingot at room temperature (RT). Reduced electron scattering in the textured thin films results in high electrical conductivity, where the SCL film shows the highest conductivity. However, its Seebeck coefficient shows a low value. The measured properties are correlated with the atomic structure details unveiled by scanning transmission electron microscopy. For instance, the high concentration of stacking defects observed in the HT film is considered responsible for the increase of Seebeck coefficient compared to the SCL film. This study demonstrates the influence of nanoscale structural effects on thermoelectric properties, which sheds light on tailoring thermoelectric thin films towards high performance

Investigation of thermopower waves based energy sources

Miniaturisation of energy sources is critical for the development of the next generation electronic devices. However, reduction in dimensions of none of the commonly used energy generation technologies including batteries, fuel cells, heat engines and supercapacitors have resulted in efficient and reliable energy sources with high specific powers (power-to-mass ratio). Recently, the new concept of energy generation based on thermopower waves has shown promise for miniaturization. In such sources, exothermic chemical reactions of a reactive fuel are coupled to charge carriers of a thermoelectric (TE) material in its affinity, resulting in an intense thermal wave that self-propagates along the surface of the TE materials. This wave simultaneously entrains charge carriers, resulting in a large current. If the TE material also has a high Seebeck coefficient, a large output voltage and subsequently large specific power output are obtained. As the thermal wave results in a power output, it is called a thermopower wave. In the first stage of the PhD research, the author demonstrated thermopower wave systems based on thin films of Bi 2 Te 3 . Bi 2 Te 3 was implemented due to its high S (~ –200 μV/K) and σ (10 5 S/m). As Bi 2 Te 3 exhibits a low κ , the author devised a novel strategy by placing it on thermally conductive alumina (Al 2 O 3 ) substrate to compensate for this deficiency. The Bi 2 Te 3 based thermopower wave sources generated voltages and oscillations higher (at least 150 %) than the previously reported multi-walled carbon nanotube (MWNT) based thermopower wave sources, while maintaining a high specific power in the order of 1 kW/kg. In the second stage, the author implemented a novel combination of p-type Sb 2 Te 3 and n-type Bi 2 Te 3 as the core TE materials with complimentary semiconducting properties, to show the generation of voltage signals with alternating polarities. In the third stage, the author implemented zinc oxide (ZnO), which is a TE transition metal oxide (TMO), for the first time as the core material in thermopower wave sources. It was shown that both S (~ –500 μV/K at 300 °C) and σ (~ 4×10 3 S/m at 300 °C) of ZnO increased at elevated temperatures. By incorporating ZnO as the core TE material, the PhD candidate obtained voltages and oscillation amplitudes at least 200 % higher than any previously demonstrated thermopower wave systems (in the order of > 500mV), while maintaining a high specific power (~ 0.5 kW/kg). In the final stage, in order to exceed voltages larger than 1 V, the PhD candidate identified that manganese dioxide (MnO 2 ), which is another TE TMO, can exhibit exceptionally large S and moderate σ at elevated temperatures. As a result, the author implemented MnO 2 as the core TE material. It was shown that the S of MnO 2 increased dramatically with temperature, exhibiting a peak value of approximately –1900 μV/K at 350 °C. Consequently, voltages large enough (~1.8 V) to drive small electronic circuits were obtained, while maintaining high specific powers in the order of 1 kW/kg

Magnetic sensors based on topological insulators

The ever-increasing demands for higher computing capabilities and low energy consumption has necessitated the developing of micro or nano electronics and sensors. This results in increasing demand for faster, higher performance, more compact and low energy consumption devices and sensors which pushes microelectronics to its physical limit. Driven by size, cost, sensitivity, reliability and power consumption, the electronic and magnetic related devices are entering a completely new age where innovations on new materials and physics are being explored. Among the most promising materials, magnetoelectric multiferroic (MEMF) and topological insulators (TI) have attracted a great deal of interest, since they are promising for their unique properties and innovative applications. The coupling of electric and magnetic properties of MEMF and the ultrahigh surface carrier mobility of TI enlighten the design of devices with extremely low thermal losses and energy cost. However, most of the device implementations of these material systems are still in status of ideas and laboratory prototypes. The prospects of practical realization of devices based on MEMF and TI encounter several critical challenges: the low ME coupling coefficient and current leakage in magnetoelectric(ME) sensor; fabrication large scale, low roughness and large terrace width of TI thin films for industry utilization; the high bulk conductivity and low sensitivity of TI based magnetic sensors. The present thesis will address some problems and challenges based on the above questions. In this work, several aspects regarding to achieve a high performance and low energy consuming devices were investigated including: systemically studied and manipulated the energy band structure of TI for nanosized electronics and sensors application; developed Hall effect sensor and anomalous Hall effect sensor based on magnetically doped topological insulator; explored a method to increase the ME coupling coefficient of ME sensors; There are nine chapters in this dissertation. Chapter 1 gives general background to readers on magnetic sensors which used widely in daily life. Basic physics of two kinds of important materials: topological insulators and MEMF composites will also be introduced. Besides that, chapter 1 will also introduce a proposed switching device which integrates both two kinds of materials. The last part of chapter 1 will be the motivation and objectives of work in this dissertation. Chapter 2 will review the experiments, techniques and equipment used for research in this dissertation including sample fabrication methods and testing methods. Starting from chapter 3, topological insulators material fabrication and sensor application will be introduced based on different kind of TIs. Study on MEMF sensors will be introduced in chapter 8. Chapter 9 is a summary of all the work and gives some general conclusions of this dissertation

Thermoelectric Transport Phenomena in Semiconducting Nanostructures

The efficiencies of state-of-the-art thermoelectric devices made from bulk materials remain too low for widespread application. Early predictions by Hicks and Dresselhaus indicated that one potential route for improving the thermoelectric properties of materials was through nanostructuring. This predicted improvement was due to two effects: an increase in the thermoelectric power factor and a decrease in the lattice thermal conductivity. In this thesis, new models are developed for calculation of the thermoelectric transport properties of nanostructures. The results of these models are in line with what has been seen experimentally in the field of nanostructured thermoelectrics: the power factor of nanostructures falls below the bulk value for sizes accessible by current experimental techniques. While this is demonstrated first for a particular system (cylindrical InSb nanowires), this result is shown to hold true regardless of the dimensionality of the system, the material of interest or the temperature. Using the analytical forms of the transport properties of nanostructured systems, we derive universal scaling relations for the power factor which further point to the fundamental and general nature of this result. Calculations done for nanostructured systems in which the scattering time is a function of carrier energy indicate that the introduction of nanoscale grain boundaries can lead to improvements in the power factor. We present experimental methods for the fabrication and characterization of porous bismuth-antimony-telluride (Bi2-xSbxTe3) thin films using a templated deposition technique. Preliminary results from this experimental work indicate that the nanostructured morphology of the templates used for the deposition of porous films limits diffusion during grain growth, and thus the crystal structure of these porous films differs from that of films deposited on dense substrates. For fundamental investigation of the effects of porosity on thermoelectric transport, future studies should therefore focus on Bi2-xSbxTe3 thin films made by top-down patterning techniques

Epitaxial chalcogenide Ge-Sb-Te thin films and superlattices by pulsed laser deposition

This thesis deals with the deposition of epitaxial chalcogenide (Ge2Sb2Te5 (GST225), GeTe and Sb2Te3) thin films and superlattice (SL) arrangement based on GeTe-Sb2Te3 using pulsed laser deposition (PLD) technique on (111)-oriented Si substrates. The thin films are characterized using in-situ RHEED, XRD, SEM, AFM and TEM. The epitaxial trigonal GST225 films with out-of-plane c-plane orientation were grown in 2D growth mode. For the first group of the films (substrate-target distance (dts) of ~7.5 cm), the epitaxial window was observed from 200 °C to 300 °C. By varying laser frequency, deposition rate as high as 42 nm/ min can be achieved. The deposition with a slight reduction of dts to ~6 cm (second group) at moderate Ts of 220 °C results in the epitaxial films with heterogeneous vacancy structures (coexisting metastable phases. i.e. with random and ordered vacancies, and stable trigonal phase). Thermal annealing (at 220 °C) leads to a phase transformation towards a pure trigonal phase. The epitaxial Sb2Te3 films with out-of-plane (0001) oriented trigonal structure were grown at Ts from 140 to 280 °C in 2D growth mode. The optimum Ts in terms of deposition rate and film quality was determined to be 240 °C. The epitaxial growth of Sb2Te3 thin films is initiated by the self-organized formation of a Sb/Te single-atomic passivation layer on the Si surface. The growth of GeTe was initialized by the formation of an ultra-thin amorphous layer. The films were predominantly grown in the mix of 2D and 3D growth modes. The deposited films possesses trigonal structure out-of-plane (0001)-orientated on Si(111). By employing a 2D-bonded Sb2Te3 as a seeding layer on Si(111), the epitaxial window of GeTe can be extended especially towards the lower temperature regime, up to 145 °C. Additionally, the surface topography can be significantly improved, indicating that the films are grown in 2D growth mode on the buffered substrate. The epitaxial SLs can be grown starting at Ts = 140 °C. Each layer of the SLs, i.e. Sb2Te3 and GeTe layer, was grown in 2D growth mode. An intermixing of GeTe and Sb2Te3 layers occurred at a higher temperature deposition. Studies on local structure of 140 °C-deposited SL showed that the SL consists of Ge-rich Ge(x+y)Sb(2–y)Tez and Sb2Te3 units intercalated by Van der Waals gaps with the inhomogeneity of layer thickness across the SL. The obtained results demonstrate the feasibility of PLD for deposition of good quality epitaxial chalcogenide thin films and SL structure on Si(111)