STUDY OF THIN FILM AND BULK SEMICONDUCTING MATERIALS FOR INTERFACE STRUCTURE AND OTHER PROPERTIES

Abstract

Thermoelectric device has ability to modify thermal energy into electrical energy or vice versa, and such devices may propose a fabulous possibility in settling of energy problem from an environmental-sustainable aspect. Climate change issue and worldwide shortage of energy is creating exaggerated interests in other new spectacular contrivance of power generation with translucent energy sources. Whenever, the absence of any liquid media or other moving parts, a temperature gradient imposed between the hot and cold junction. A thermoelectric material plays an important role in primary power generation (i.e., combustion, chemical reactions, and nuclear decay), and energy conservation both.

Several thermoelectric materials are reported. However, among all of these Tin Telluride (SnTe) displays exclusive features like low-toxicity and eco-friendly behaviour etc. The current prevalence illustrates that at temperature 300 K nano-structuring and band engineering may provide a high thermoelectric performance device of SnTe, which offers a substitute for toxic PbTe in similar operational temperature. The huge area competence and innateness of scaling up with comprehensive constraint of material handling provides possible application of sintering in industrial manufacture of coatings and optoelectronic devices. However, metal chalcogenides of the group IV-VI are gained significant attention due to its fascinating physical properties. These narrow bandgap semiconductors are enormously useful for thermoelectric devices, thermo-voltaic, photovoltaic and as well as various optical applications. Whereas, SnTe semiconductor possesses a cubic rock salt structure with a direct band gap 0.18 eV, which is responsible to make it more attractive. Many more applications such as photo detectors, mid-infrared (3-14 μm) detection and thermoelectric devices of SnTe is a proof of its capacity in material research. A Tin Telluride (SnTe) vacuum evaporated thin films has been synthesized at room temperature (RT) on a glass substrate, which has been proven as a significant enhancement in the figure of merit (ZT) value as p-type material. Moreover, a high-resolution X-ray diffraction (HRXRD) outlines indicated towards a polycrystalline nature in bulk solids as well as in thin films both. Surface morphology of composed grains of variable sizes of these films also investigated using scanning electron microscopy (SEM), which has been further supported by atomic force microscopy (AFM) wherein, the surface parameters (like roughness, skewness, and kurtosis) were measured and analyzed to determine topography of the thin film surface. High-resolution transmission electron microscope (HRTEM) also touches to local microstructural features and crystalline structure, which another level investigation has been confirmed using selected area electron diffraction (SAED) pattern analysis. Four probes method has also been used to determine electrical measurements, which confirm that the thin films behave as a semi-metallic nature. We observed that figure of merit of thin films increases with thickness of the film. The maximum ZT value of ∼ 1.02 for the SnTe thin film with thickness 275 nm and ∼ 1.04 for In-doped SnTe thin film of thickness 450 nm has been observed at room temperature measurement.

A detailed analyzes of SnTe is indicated that SnTe is the utmost promising material for thin film photovoltaics, thermoelectric device and IR detector. Whereas, present research work also deals with a comparative study of bulk solids and thin films of SnTe and In-doped SnTe compound semiconducting material. Whereas, it has been observed that optical, electrical and thermoelectric properties of thin films alter with the thickness of thin film.

The present thesis has been discussed into seven chapters, which brief discussion has been indicated in the following paragraphs. Chapter 1 introduces to the fundamental aspects of thermoelectric materials. Various kinds of thermoelectric materials such as Metal chalcogenides, Superionic conductors, Metal oxides, Silicon-based materials, PGEC thermoelectric materials categorized as Skutterudites, clathrates, half- Heusler alloys, and SnTe has been discussed in this segment (introduction) of the thesis. Literature review of SnTe compound is discussed thoroughly including the detailed explanation about crystalline structures, phase diagrams and related applications. The effect of doping in pure SnTe has also been discussed in detail with an optimal literature survey. Overall, the semiconducting compound materials discussed in this part. This chapter ends with the motivation for the present work. Finally, the objectives of the thesis based on the review of the literature have been incorporated. Chapter 2 describes a brief description of experimental and characterization techniques used in the present investigation for the synthesis of bare SnTe and Indium doped SnTe bulk and thin films. Whereas, a primarily, vertical directional solidification (VDS) and subsequently thermal evaporation technique have been used for the synthesis of the bulk SnTe, In-doped SnTe and their thin films. This chapter includes the details of sophisticated analytical experimental tools like, XRD/ HRXRD, VP-SEM, EDS, TEM/HRTEM, AFM, FTIR, micro-Raman, PL, UV-NIR, TOF-SIMS, EPR, Four probe method (-T) and Herman method (ZT) measurements for different properties. Bulk SnTe and In-doped SnTe in the form of ingot was prepared by the physical route via VDS technique in a vertical muffle furnace. The thin film deposition has been performed on glass, silicon, and NaCl substrate using the thermal vapor deposition instrument and discussed in detail in this chapter. Chapter 3 provides the detailed investigation of the bulk SnTe ingot. This chapter emphasized that SnTe material was prepared by vertical directional solidification (VDS) technique at high temperature via the physical route. Bulk SnTe pallets were used for the structural characterization (XRD). XRD pattern of bulk SnTe confirms the formation of polycrystalline SnTe as well as its atomic-scale range of structural periodicity. From the XRD analysis it is established that both cubic and orthorhombic phases co-exist in bulk SnTe compound. Rietveld refinement of four-time repeated XRD data indicates all best-fitting parameters for analysis of bulk SnTe. SEM and EDX analyses show the existence of cleavage planes on the morphological surface of the bulk SnTe compound. The results obtained from the EDX revealed that the stoichiometry of Sn and Te is maintained perfectly. The microstructural investigations were achieved by employing HRTEM and SAED, which confirm that inter-planar spacing values correlate with XRD data, and various sizes of grains are present in the material. Some grain boundaries have been occurred, establishing about the formations of imperfections in the material. HRTEM micrographs show the disorder in the grain boundaries of different grains, and hence here in most cases, lattice fringes of different grains were merged with each other. FTIR and Micro - Raman spectra revealed that the SnTe is a suitable compound for IR applications. EPR results revealed that holes are present in an abundant concentration, and voids presence makes the material highly paramagnetic. As SIMS spectra revealed the presence of unreacted Tin, Tellurium, SnTe compound, and impurities in the ingot up to ppm level, therefore this may be an appropriate reason for paramagnetic behavior. The confirmation of the p-type nature of SnTe indicates the presence of holes or vacancies in the material, which is also responsible for paramagnetic resonance. The reaction of very – very few oxygen atoms with the material may also be responsible for free electrons in the material, which seems a strongly correlated with Micro-Raman and FTIR results. Electrical properties confirmed that P-type SnTe is semi-metallic, and resistivity is temperature-dependent. These studies explore the feasibility of employing the material in the industrial production line of infrared detectors. Chapter 4 reports the detailed study of SnTe thin films of different thickness deposited on various substrates. By using thermal evaporation equipment, a series of Tin Telluride (SnTe) thin films of varied thicknesses are deposited onto different substrates at 300 K. The morphology, microstructure, topology, optical, elemental mass isotope spectrum, electrical, and thermoelectric properties of SnTe thin films having thicknesses 33 nm to 275 nm have reported here. High-resolution x-ray diffraction (HRXRD) patterns of SnTe thin films revealed the polycrystalline nature with [200], which orientation possessed a cubic structure. Rietveld refinement of XRD data of these thin films indicates all best-fitting parameters for the analysis of crystalline features. The microstructural and morphological structures of all thin films were examined using HRTEM and SEM-EDS, respectively. The distribution of isotopes of various elements in the thin film along with facet and longitudinal channels was expolred by using depth profile determination through the TOF – SIMS technique. Fourier transform infrared spectroscopy spectra reveal the molecular vibrations, narrow bandgap property of material, and suitability of materials in infrared applications. Longitudinal – optical phonon scattering due to the [222] plane orientation is also observed in the micro-Raman spectra at room temperature, which corresponds to a peak in the range 120–130 Raman shift/cm−1. Hence, the change in optical and microstructural properties at the nano-regime resulted in a shift towards the near-infrared region with an enhancment in the thickness of the thin films. Electrical properties enhance with the decrease of thin-film thickness. Whereas, figure of merit (ZT) equal to 1.02 is the highest value for a thin film of thickness 275 nm among all four thin films. Chapter 5 reports the detailed study of doping of the Indium (In) element in SnTe bulk compound and In-doped SnTe thin films with thickness of 50 nm, 245 nm and 450 nm. Rietveld refinement of XRD data of bulk In0.1Sn0.9Te compound and thin films indicates the formation of polycrystalline bulk and thin films. Rietveld refinement of XRD data indicates all best-fitting parameters for analysis of In-doped SnTe thin films. Morphological, microstructural, topological, optical, and thermoelectric properties have also described in this chapter. SEM, TEM and AFM micrographs revealed that nanoparticles (NPs) of different size from 50 nm to 500 nm have been spread along the whole of surface of thin film. These different size NPs can affect the optical properties of these thin films, because due to absorption of light in visible region the variable size of NPs can emit diverse colors radiations. However, the metallic NPs show dissimilar physical and chemical properties in comparison of bulk metals. It is clear that NPs have large surface area to volume ratio hence in the case of NPs huge interactive interface exist between the adjacent particle and thier local surroundings. As per use of any compound rather in the form of bulk material or in the form of nanomaterials the properties of similar chemical compositional material changed significantly. Thermoelectric properties (figure of merit) revealed that ZT = 1.04 is highest for the film of thickness 450 nm. Chapter 6 describes the detailed study of Ultra-fast spectroscopy for SnTe and In-doped SnTe thin films. This chapter describes relationship about the study of optical properties with dynamics of thin film. Ultrafast laser spectroscopy is a sophisticated technique in which ultrashort pulse lasers generally used to study the dynamics of reaction mechanism up to tremendously small-time scales. Various techniques are practiced for the study of the dynamics of holes and electrons, atoms or molecules. The time domain part of frequency-resolved spectroscopy is the main component of ultrafast molecular spectroscopy. In the case of ultrafast spectroscopy, coherent quantum levels lead to time-dependent dynamics which is belongs to the traditional mechanical movement. In ultrafast spectroscopy, a 70-fs pulse is applied to pump the specimen, which is generated as a result of a mode-locked laser beam, amplifier, and optical parametric amplifier (OPA). The mode-locked laser is of MICRA, which generates a 35-fs pulse of 800 nm with a 320mW average power. The pulsed laser is then amplified using a COHERENT amplifier in which the Ti: Sapphire crystal is used to amplifies the pulse laser to 4 W. This laser pulse is then split into 70:30 using a beam splitter in which 30% of the laser beam is passed through the delay stage to provide a 6 ns long delay. The delayed beam is then passed through the Ti: Sapphire crystal to generate a continuum in the NIR range of 800-1600 nm. Simultaneously, the remaining 70% of the laser beam is passed through TOPAs, which is an OPA. The HELIOS spectrometer is used to detect the differential reflectance in which the InGaAs detector is used. The system is calibrated through the in-house built BND in CSIR-NPL. Chapter 7 provides a summary of the research work done in this thesis. It also suggests and outlines open issues and the future course of research in the area of Chalcogenide materials and compound semiconductors.