This work focuses on the growth of vertically aligned CdTe nanowire arrays of controllable diameter and length using cathodic electro deposition in anodized alumina templates. This step was followed by annealing at 250° C in a reducing environment (95% Ar + 5% H2). AAO template over ITO-glass was used as starting template for the device fabrication. The deposited nanowires showed nanocrystalline cubic phase structures with a strong preference in [111] direction. First gold (Au) was deposited into AAO using cathodic electro deposition. This was followed by CdTe deposition into the pore. Gold was deposited first as it aids the growth of CdTe inside AAO and it makes Schottky contact with the deposited n type CdTe. CdTe was determined to be n-type from the fact that back to back diode was obtained with Au-CdTe-Au test structure. Aluminum (Al) was sputtered on the top to make the ohmic contact to the n type CdTe deposited in AAO. Analysis of Schottky diodes yielded a diode ideality factor of 10.03 under dark and 10.08 under light and reverse saturation current density of 34.9μA/cm2 under dark and 39.7μA/cm2 under light.

This thesis reports the fabrication and characterization of long wavelength infrared mercury cadmium telluride (Hg1-xCdxTe) photodiodes and 128x128 focal plane arrays grown on lattice matched cadmium zinc telluride (Cd1-yZnyTe) substrates by metal organic vapor phase epitaxy (MOVPE). The dark current modeling of 33x33 mm2 Hg1-xCdxTe photodiodes has shown the dark current is dominated by trap assisted tunneling under small reverse bias voltages typically used to bias these detectors. The dominant dark current mechanisms under high reverse bias and low forward bias are band&ndash / to&ndash / band tunneling and generation&ndash / recombination, respectively. The photodiodes have yielded a peak 77 K detectivity of 3.2x1010 cm&amp / #8730 / Hz/W with a cut-off wavelength (50%) of 10.92 mm. It has also been found that the 1/f noise current of the detectors at 1 Hz is related to the trap-assisted tunneling current through the empirical relation in=&amp / #945 / TAT(ITAT)&amp / #946 / with &amp / #945 / TAT=7.0 x 10-5 and &amp / #946 / =0.65. In the course of the focal plane array (FPA) fabrication process development work, ohmic contact formation on p-type Hg1-xCdxTe and mesa wet etch were studied in detail. Contacts with chromium, gold, platinum and copper on p-type Hg1-xCdxTe resulted in bad ohmic contacts, which did not seem to improve with annealing. On the other hand a HgTe cap layer on p-type Hg1-xCdxTe resulted in good ohmic contact with acceptably low resistance. Among the etchants studied for mesa etching of the diode structures, Br2/HBr solution yielded the best performance. After developing all of the steps of FPA processing, 128x128 Hg1-xCdxTe FPAs were successfully fabricated and tested in a thermal imager. While thermal imaging was performed with the FPAs, high nonuniformity of the material and low R0A product of the pixels did not allow high sensitivity imaging.

Title: Photoluminescence of CdTe crystals Author: Jan Procházka Department: Institute of Physics of Charles University in Prague Supervisor: Doc. RNDr. Pavel Hlídek, CSc. Abstract: Energy levels connected with defects in nominally undoped crystals CdTe, indium- doped crystals and chlorine-doped crystals were studied using low-temperature photoluminescence. The crystals are intended for X- and gamma- ray detectors operated at room temperature. An effect of annealing in cadmium or tellurium vapor on luminescence spectra was investigated. Some changes were interpreted by filling of vacancies not only by atoms coming from gaseous phase but also by impurities from defects like interstitials, precipitates, inclusions, grain boundaries etc. The luminescence bands assigned to defects important for compensation mechanism were examined, namely A-centers (complexes of vacancy in cadmium sublattice and impurity shallow donor) and complexes of two donors bound to a vacancy. It was shown, that temperature dependence of the luminescence bands results from more complicated processes than a simple thermal escape of bound excitons or thermal excitation of electrons (holes) from defects to bands. We observed expressive "selective pair luminescence" bands (SPL) on partially compensated In-doped samples during sub-gap...

Two-dimensional (2D) materials have properties that are often different from their three-dimensional (3D) bulk form. Many of these materials are stable at ambient conditions, which allows them to be integrated with other 2D- or 3D-materials to form heterostructures. Integration of various dimensional materials attains unique electrical and optical properties that aid in developing novel electronic devices. The interface of the heterogeneous integration of these films can exhibit a weak van der Waals-like bonding. In this thesis, an advanced characterization (from atomic to millimeter resolution) of various dimensional materials with weakly bonded interfaces is developed and employed to understand their behavior at scale. First, a large-area single-crystal cadmium telluride thin film is grown incommensurately and strain-free to a sapphire substrate despite a significant 3.7% lattice mismatch. The film remarkably delaminates as a bulk single crystal film due to an atomically thin tellurium that spontaneously forms at the interface. Aberration-corrected electron microscopy and spectroscopy reveal both the van der Waals-like structure and bonding at the film/substrate interface. Second, a large-area atomically thin gallium is intercalated at the interface of epitaxial graphene. Correlative microscopy workflows are applied to understand the thickness uniformity and area coverage of the 2D–gallium over few millimeters of the sample. Utilizing multiple correlative methods, SEM image contrast is found to be directly related to the presence of the intercalated gallium. The origin of the SEM contrast is investigated as a function of the surface potential. Then, the heterostructure characterization is scaled up over a few square millimeter areas by segmenting SEM images, each acquired with nanometer-scale resolution. Additionally, transmission electron microscopy is applied to investigate the interface of gallium–SiC, the gallium air–stability, and the role of the substrate on the heteroepitaxial growth of 2D–gallium, which charts a path for further development of these materials. / Thesis / Doctor of Philosophy (PhD)

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<p>The growth of CdTe thin films on Al₂O₃(0001) substrates by pulsed laser deposition from undoped pressed powder targets was studied. Thin film crystal structure was investigated by x-ray texture analysis as a function of plume flux, growth temperature and film thickness. Crystal texture increased for a decrease in plume flux. Single crystal CdTe (111) films were obtained by optimizing the plume flux. Increasing the growth temperature demonstrated a reduction in twin density. An optimum temperature of 300°C minimized the twin density without adverse desorption effects. The twin density decreased as an inverse squared function of film thickness. Single crystal CdTe films with comparable structural quality to Bridgeman single crystal wafers were grown under optimal conditions.</p> <p>The optoelectronic properties of CdTe films were investigated by photoluminescence and photoreflectance spectroscopy. The room temperature bandgap energy of 1.51 eV was consistent between spectroscopic measurements. Broadening parameters for spectra were consistent with reference high quality material. Low temperature photoluminescence spectra had a dominant emission consistent with bound excitons found in bulk CdTe. Emissions consistent with self-compensation or doping were not found. Hall effect and conductivity measurements at 300 K demonstrated high resistivity for undoped material and electron mobilities comparable to bulk CdTe for lightly doped films. Spectroscopic and electrical measurements of high structural quality CdTe films were consistent with high optoelectronic quality.</p> <p>An as-grown ability of the films to detach from their substrate was discovered. X-ray texture analysis and photoluminescence spectroscopy of films released onto rigid secondary carriers demonstrated that they maintained their structural and optoelectronic quality proceeding lift-off. Substrates having films released from them were found to be suitable for repeated growth. The technological relevance of this discovery is likely to drive further study into the lift-off phenomena and controlled doping of CdTe thin films.</p> / Master of Applied Science (MASc)

This thesis on multicarrier effects in the magnetotransport and optical properties of Mercury Cadmium Telluride (MCT or HgCdTe) covers mainly: design, construction and calibration of a 12T 4K and 19T 77K pulsed high magnetic field systems; temperature dependent magnetotransport measurements upto 15T performed on the home-built pulsed magnet systems; computational techniques developed to extract densities and mobilities of various carriers, especially low mobility heavy holes, participating in conduction; theoretical analysis of heavy hole mobility based on Boltzmann transport equation; temperature dependent optical absorption experiments in the Mid and Far-IR on bulk and thin film samples; and theoretical modelling of optical absorption below bandgap. The work essentially probes the low and high frequency conductivity of the semiconductor alloy Hg1?xCdxTe by performing microscopic calculations of scattering related phenomena of its free carriers at higher temperatures (200 K–300 K) and comparing with experimental data. Special attention is given to properties of heavy holes as the effects due to these carriers appear only at higher magnetic fields. It is demonstrated that in this temperature range and at high magnetic fields, taking both measured resistivity and derived conductivity in the multicarrier analysis gives better results which are then applied to explain both heavy hole mobility as well as free carrier absorption without further fitting parameters and using a minimal set of necessary intrinsic properties. The agreement thus obtained with experimental data is shown to be excellent. The bulk and epilayer samples used in this thesis were grown by the MCT group headed by R. K. Sharma (SSPL, Delhi). The organization of the thesis is as follows: Chapter 1 The importance of Mercury Cadmium Telluride as a narrow gap semiconductor for infrared detection is introduced. The relevant physical and material properties of HgCdTe are reviewed. Chapter 2 A low cost 12T pulsed magnet system has been integrated with a closed-cycle Helium refrigerator (CCR) for performing magnetotransport measurements. Minimal delay between pulses and AC current excitation with software lock-in to reduce noise enable quick but accurate measurements to be performed at temperatures 4K-300K upto 12T. An additional pulsed magnet operating with a liquid nitrogen cryostat extends the range upto 19T. The instrument has been calibrated against a commercial superconducting magnet by comparing quantum Hall effect data in a p-channel SiGe/Si heterostructure and common issues arising out of pulsed magnet usage have been addressed. The versatility of the system is demonstrated through magnetotransport measurements in a variety of samples such as heterostructures, narrow gap semiconductors and those exhibiting giant magnetoresistance. Chapter 3 The necessity of employing multicarrier methods in magnetotransport of narrow gap semiconductors is brought out. In these materials, mixed conduction is seen to exist at nearly all temperatures of interest. Methods of extracting two of the most important transport parameters of device interest, density and mobility, from the variable magnetic field Hall and magnetoresistance measurements are elaborated. Improvements have been made to the conventional non-linear least squares fitting procedure and are demonstrated. Chapter 4 Magnetotransport measurements in pulsed fields upto 15 Tesla have been performed on Mercury Cadmium Telluride (Hg1?xCdxTe, x?0.2) bulk as well as liquid phase epitaxially grown samples to obtain the resistivity and conductivity tensors in the temperature range 220K to 300 K. Mobilities and densities of various carriers participating in conduction have been extracted using both conventional multicarrier fitting (MCF) and Mobility Spectrum Analysis(MSA). The fits to experimental data, particularly at the highest magnetic fields, were substantially improved when MCF is applied to minimize errors simultaneously on both resistivity and conductivity tensors. The semiclassical Boltzmann Transport Equation (BTE) has been solved without using adjustable parameters by incorporating the following scattering mechanisms to fit the mobility: ionized impurity, polar and nonpolar optical phonon, acoustic deformation potential and alloy disorder. Compared to previous estimates based on the relaxation time approximation with out-scattering only, polar optical scattering and ionized impurity scattering limited mobilities are shown to be larger due to the correct incorporation of the in-scattering term taking into account the overlap integrals in the valence band. Chapter 5 Optical absorption measurements have been performed on bulk Mercury Cadmium Telluride (Hg1?xCdxTe, x?0.2) samples between 4K and 300 K. After fitting the Urbach part of the spectrum in the mid-infrared, below bandgap absorption is modeled using only basic processes and mechanisms, i.e. intervalence transitions and free carrier absorption (FCA). The additive FCA coefficients for individual carriers have been calculated using known quantum mechanically derived expressions for scattering due to polar and nonpolar optical phonons, ionized impurities and acoustic deformation potential mechanisms found to be relevant for electrical transport in this temperature range. The densities of carriers used in the calculations are derived from a modified multicarrier fitting (MCF) procedure on both resistivity and conductivity tensors from magnetotransport measurements in pulsed fields upto 15 Tesla from 220K to 300 K, thus making hole density more reliable. It is found that such a treatment is sufficient to model the absorption spectra below bandgap quite accurately without introducing any additional mechanical or compositional defect related phenomena. Chapter 6 A summary of the work carried out in this thesis is presented. Some future directions including preliminary work to measure carrier mobilities at high electric fields and effect of hydrogen passivation in MCT are briefly discussed.

The motivation of this thesis is to understand the physical properties of semiconductor quantum dots (QDs) and to get insight on the basic physics of charge separation in composites made from QDs with graphene oxide (GO)/organic semiconductors. The flexion phonon interactions is one of fundamental issues in solid state physics, which has a significant effect on both electrical and optical properties of solid state materials. This thesis investigates the physical properties of aqueous grown QDs through exciton-phonon coupling and non-radiative relaxation of excited carriers which have been carried out by temperature dependent photoluminescence spectroscopy. Several e orts have been made in order to understand the basic physics of photo induced charge separation in the hybrid systems made from QDs with graphene oxide and organic semiconductors. Investigations on the photoconductivity of the devices made from these hybrid composites have been carried out keeping the motive of its application in nanotechnology. This thesis work is presented in six chapters inclusive of summary and directions for future work. Chapter 1 discusses the background knowledge and information of the general properties of semiconductor nanostructures, QDs and their hybrid nanocomposites. Chapter 2 deals with the sample preparation and experimental techniques used in this thesis. Chapter 3 elaborates the exciton-phonon scattering and nonradiative relaxations of excited carriers in visible emitting cadmium telluride QDs with help of temperature and size dependent photoluminescence. Chapter 4 presents the investigations on time resolved photoluminescence dynamics and temperature dependent photoluminescence properties of near infrared (NIR) emitting mercury cadmium telluride (CdHgTe). Chapter 5 discusses the importance of NIR emitting silver sulphide (Ag2S) QDs and gives insight of nonradiative recombinations through defect/trap states. Chapter 6 investigates the excited state interactions between CdHgTe QDs and GO. Chapter 7 focuses on the understanding of basic physics of charge separation/transfer between poly (3hexylthiophene) and Ag2S QDs. Chapter 1: Semiconductor nanostructures have attracted significant scientific attention due to their fundamental physical properties and technological interests. Quasi zero dimensional nanocrystals or quantum dots (QDs) have shown unique optical and electrical properties compared to its bulk counterpart. These QDs show discrete energy levels due to the quantum confinement effect hence known as arti cial atoms. Large surface to volume ratio in these QDs is expected to play a crucial role in determing the photo-physical properties. Temperature dependent photoluminescence is a powerful tool for understanding the role of the large surface area on exciton recombination process in QDs. Inorganic QDs combined with different materials like graphene oxide or organic semiconductors forms an exciting class of synthetic materials which integrates the properties of organic and inorganic semiconductors. It is quite important to understand the basic physics of electronic interactions in these composites for its future application in many elds. Chapter 2: Synthesis of the inorganic QDs, graphene oxide, composites and fabrication of devices is an important and integral part of this thesis. Hydrothermal and three necked ask technique is adopted to get highly dispersible colloidal quantum dots in solvents. Synthesis of graphene oxide from graphite through oxidation and ultrasonication has been carried out to obtain homogenous dispersed graphene oxide in water. Structural properties have been studied by techniques like X ray diffraction, Raman spectroscopy, X ray photoelectron spectroscopy and high resolution transmission electron microscopy. Morphological properties are studied by atomic force microscopy and transmission electron microscopy. Optical properties are investigated by absorption spectroscopy, steady state and time resolved photoluminescence spectroscopy. Photoconductivity characteristics are analyzed to understand the basics of enhanced current in the various devices made from QDs composites. Chapter 3:Investigations on exciton phonon coupling and nonradiative relaxations in various sizes of visible light emitting cadmium telluride (CdTe) QDs size have been presented. Due to the large surface area, QDs are prone to have defect/trap states which can affect the exciton relaxation. Hence, understanding the role of such defect/trap states on photoluminescence is very essential for achieving the optimum optical properties. Temperature dependent (15 300 K) photoluminescence has been used to understand nonradiative relaxation of excited carriers. Thermally activated processes and multiple phonons scattering is thoroughly investigated to understand the quenching of photoluminescence with temperature. The strength of exciton-phonon coupling is investigated which determines the variation in energy bandgap of QDs with temperature. Role of exciton phonon scattering is also discussed to understand the basic physics of photoluminescence line width broadening in QDs. Chapter 4 and 5: This part of thesis focuses on the size and temperature pho-toluminescence properties of near infra red emitting ternary alloyed CdHgTe and Ag2S QDs. Near infrared emitting semiconductor quantum dots (QDs) have attracted significant scientific and technological interests due to their potential applications in the fields of photosensor, solar energy harvesting cells, telecommunication and biological tissue imaging etc. Structural and photophysical properties of CdHgTe QDs have been analyzed by high resolution transmission electron microscopy, X rayphotoelectron microscopy, photoluminescence decay kinetics and low temperature photoluminescence. Investigations on the nonradiative recombinations through trap/defects states and exciton phonon coupling are carried out in colloidal Ag 2S QDs which emits in the range of 1065 1260 nm. Particularly, the photoluminescence quenching mechanism with increasing temperature is analyzed in the presence of multiple nonradiative relaxation channels, where the excited carriers are thermally stimulated to the surface defect/trap states of QDs. Chapter 6 and 7: The aim of these chapters is to understand the basic physics of photo induced charge separation in the hybrid systems made from the inorganic QDs with graphene oxide and organic semiconductors. In chapter 6, CdHgTe QDs are decorated on graphene oxide sheets through physisorption. The excited state electronic interactions have been studied by optical and electrical characterizations in these CdHgTe QDs GO hybrid systems. In chapter 7, investigations are carried out for understanding the basic physics of charge separation in the composites of Ag2S QDs and poly (3hexylthiophene 2,5 diyl)(P3HT). These composites of inorganic organic materials are made by simple mixing with help of ultrasonication technique. Steady state and time resolved photoluminescence measurements are used as powerful technique to gain insight of energy/charge transfer process between P3HT and Ag2S QDs. Furthermore, investigations have been carried out on the photoconductivity of the devices made from these hybrid composites keeping the motive of its application in nanotechnology. Chapter 8: The conclusions of the work presented in this thesis are coherently summarized in this chapter. Thoughts and prospective for future directions are also summed up.

Numerical simulations of current-voltage characteristics of nanowire CdS/CdTe solar cells are performed as a function of temperature using SCAPS-1D. This research compares the experimental current-voltage (I-V) characteristics with the numerical (I-V) simulations obtained from SCAPS-1D at various temperatures. Various device parameters were studied which can affect the efficiency of the nanowire-CdS/CdTe solar cell. It was observed that the present simulated model explains the important effects of these solar cell devices, such as the crossover and the rollover effect. It was shown that the removal of defect in i-SnO2 is responsible for producing the crossover effect. In the past, the rollover effect has been explained by using back to back diode model in the literature. In this work, simulations were performed in order to validate this theory. At the back electrode, the majority carrier barrier height was varied from 0.4 to 0.5 eV, the curve corresponding to the 0.5 eV barrier showed a strong rollover effect, while this effect disappeared when the barrier was reduced to 0.4 eV. Thus, it was shown that the change of barrier height at the contact is a critical parameter in the rollover effect.