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.

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.

In this work, we have designed, simulated, fabricated and characterized homojunction Hg1-xCdxTe detector for high operating temperature in the MWIR region. The IR photon detectors need cryogenic cooling to suppress thermal generation. The temperature of operation in narrow gap semiconductor devices is limited by the noise due to statistical nature of thermal generation-recombination in narrow gap semiconductors. To make IR systems affordable they have to be operated without cooling or with minimal cooling compatible with low cost, low power and long life. Several fundamental and technological limitations to uncooled operation of photon detectors have been discussed in Chapter-1 of this thesis. Way and means adopted to increase the operating temperature, such as non-equilibrium operation, use of multilayer stacked hetero¬structures, optical immersion etc. have also been discussed. Key to improving the detector performance at any temperature is reduction of dark currents to level below the photocurrent and ultimately to the level where detector noise is determined by the fluctuations in photon flux from the scene (BLIP limit). In addition, design of present generation uncooled Hg1-xCdxTe infrared photon detectors relies on complex hetero-structures with a basic unit cell of type n+/π/p+. Theoretical modeling and numerical simulations on TLHJ device consisting of backside illuminated n+/π/p+ photodiodes have been performed. A numerical model for solving carrier transport equations for Hg1-xCdxTe infrared photodiodes was developed in MATLAB. Finite difference discretization of carrier transport equations and successive over relaxation method have been adopted. Numerical models are more appropriate than analytical models when analyzing multi-layer hetero-structures because we can account for realistic doping profiles, compositional grading and hetero-structures using this model. The model can be suitably modified to accommodate different device architectures, designs, material properties and operating temperature. Such a generalized model is useful to a device designer to customize the detector performance as per the availability of the material to suit the application specific requirements. The present work therefore proposes a more flexible, accurate and generalized methodology to accommodate the user needs by simulating the position dependence of carrier concentration, electrostatic potential and g-r rates and their effect on detector performance vis-à¬vis contact doping, absorber doping and absorber width on device performance. We detail aspects of our simulation model by developing a library of Hg1-xCdxTe properties using analytical and empirical expressions for material parameters (energy band gap, electron affinity, intrinsic carrier concentration, carrier effective mass, carrier mobility, dielectric constant and absorption coefficient). The PDEs were solved using the FDM coupled with SOR method. Behavior of Hg1-xCdxTe diodes (homo/hetero-junction) under different biasing, illumination and non equilibrium situations were modeled. Model has been validated for experimental measured data on n on p Hg1-xCdxTe photodiodes. The numerical computations are next applied to simulation/modeling of MWIR (λc=4.5 μm) n+/π/p+ TLHJ device for operation at T=250K. Several recombination processes occur in Hg1¬-xCdxTe depending on material quality, operating temperature, device design and processing conditions. Detailed mathematical models of radiative, Auger, Shockley Read Hall (SRH), surface recombination and optical g-r are analyzed and their effect on carrier lifetime have been evaluated. Analytical models for dark currents affecting the performance of Hg1-xCdxTe diodes at different temperatures are discussed. The mechanisms contributing to dark current are: (i) the thermal diffusion of minority carriers from the neutral regions (IDiff); (ii) generation-recombination from the space charge region of diode (IG-R) (iii) trap assisted tunneling currents, wherein the traps in the depletion region or the traps in the quasi neutral p region close to the depletion edge participate in the tunneling process(ITAT); (iii) band-to-band tunneling currents (IBTB) and (iv) surface leakage currents due to shunt resistance. Total current of a photodiode is ITOT=IDiff+IG-R+ITAT+IBTB+ISH-IP, where IP is the photocurrent. We evaluate the variation of electrostatic potential, carrier concentration, and electric field and g-r profiles as a function of position. The effect of variation in absorber width, doping and contact doping on D* is also analyzed. The mathematical models of different g-r processes (Auger, SRH, radiative, surface recombination and optical generation) affecting the device performance analyzed and their affect on carrier lifetimes are investigated. Responsivity ~3.25Amp-Watt-1, noise current~2pA/Hz1/2 and D* ~8x109 cmHz1/2watt-1 at 0.1V reverse bias have been calculated using optimized values of doping concentration, absorber width and carrier lifetime. The suitability of the method has been illustrated by demonstrating the feasibility of achieving the optimum device performance by carefully selecting the device design and other parameters. The numerical models provided insight about the operation and performance of Hg1-xCdxTe Auger-suppressed infrared photodiodes. Hetero-junction configuration increases the dynamic resistance, while the heavily doped contacts reduce the contact resistance. Wide gap/heavily doped contacts present a barrier to injection of minority carries into the absorber layer. At the same time they allow collection of minority carriers generated in the absorber region at the contacts. Hg1-xCdxTe hetero-diodes are grown by MOCVD and MBE with precise doping and compositional gradient control to reduce g-r contributions from defects and dislocations to the dark current in order to reap advantages of Auger suppression. Measured dark currents in hetero-junction photodiodes continue to be larger than expected in spite of the advancements in MBE technique. Delineation of an array on hetero-structures involves mesa separation of the diodes thus creating additional surface requiring passivation. Overall, the whole effort of fabricating a hetero Hg1-xCdxTe detector array is disproportionate to the overall gain in the performance. Therefore, we employ a much simpler fabrication process of homo-junction Hg1-xCdxTe detectors. It involves a planar device fabrication approach thus minimizing the surface passivation problem. We have deliberated upon the specific growth, characterization techniques and processing steps employed in our study. We discuss some of the experimental issues. We also presented results on the novel processing techniques developed that are potentially applicable to HOT technology and Hg1-xCdxTe technology in general. Hg1-xCdxTe (x=0.27-0.31) layer of ~ 15×15mm2 area and 15-20µm thickness is grown on CdZnTe substrate by Liquid Phase Epitaxy (LPE) in-house. As grown wafer is vacancy doped p-type with a carrier concentration of ~5×1015-1x1016 cm-3 and hole mobility of ~400cm2V-1s-1@80K. Planar n+/ν/p junction ~2-3µm deep is formed by B+ ion implantation and subsequent annealing; details are outlined in Chapter-4. Hall measurements and differential Hall measurements were used to find the carrier concentration, carrier mobility, resistivity of the wafer. The diodes are formed in the form of a 2D array along with various PEV’s for process characterization. Composition of Hg1-xCdxTe wafers used for the work is in the range of 0.27¬ 0.31 as determined by FTIR, corresponding to cutoff wavelength of 4.5-6.5µm. Junction depth and doping profile of the diodes after ion implantation was characterized by differential Hall technique. Transient minority carrier lifetime in fabricated MWIR n+/ν/p Hg1-xCdxTe (x=0.27) diodes were characterized using diode reverse-recovery technique. We prefer this method because it is a direct indicator of device as well as material quality post processing. By this time the device has undergone all the chemical/mechanical treatments and the measured lifetime is the cumulative of g-r mechanisms operative in bulk, space charge region and surface of diode. The value of lifetime extracted from the measured data lies in the range of 80-160ns. Variable temperature lifetime data was also extracted to determine the prevalent g-r process operative in the device. Diode dark I-V and junction C-V measurements were also made to correlate the observed behavior of the measured lifetime with g-r processes. Evidence of Auger suppression at room temperature is seen in the dark I-V characteristics via observation of negative differential resistance in the homo-junction Hg1-xCdxTe diodes. The experimental data is fitted using the numerical and analytical models developed. Based on this fitting, the current mechanisms limiting the dark current in these photodiodes are extracted. An improved analytical I-V model is reported by incorporating TAT and electric field enhanced Shockley-Read-Hall generation recombination process due to dislocations. Tunneling currents are fitted before and after the Auger suppression of carriers with energy level of trap (Et), trap density (Nt) and the doping concentrations of n+ and νregions as fitting parameters. Values of Et and Nt were determined as 0.78-0.80Eg and ~7-9×1014 cm-3 respectively in all cases. Doping concentration of νregion was found to exhibit non-equilibrium depletion from a value of 2×1016 to 4×1015 cm-3. Quantum efficiency of the diodes was found to ~25-30%. Note, that these are wafer level measurements on unpackaged device without backside AR coating. In addition to junction diodes, we present results on several PEV's such as VADA, MIS/MIM capacitors and TLM structures both at room and low temperature. Variable temperature measurements for a VADA tile and subsequent analysis provide evidence of g-r processes originating from defects, dislocations and dislocation loops, which are non-uniformly distributed across the Hg1-xCdxTe wafer and contributes to TAT current at high temperatures. MIS analysis yielded surface charge density lying between 3×1010-1×1011 cm-2 for ZnS/CdTe surface corresponding to a near flat band condition. Results of low and variable temperature measurements on the devices have also been shown to correlate it with the possibility of operating the device at mid temperatures such as 180-250K.