Thesis (M.App.Sc.) - Carleton University, 2007. / Includes bibliographical references (p. 135-137). Also available in electronic format on the Internet.

Thesis (M.S.)--Rochester Institute of Technology, 2008. / Typescript. Includes bibliographical references (p. 239).

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.

No description available.

The objective of this dissertation is the development of the necessary processing steps needed to manufacture infrared photodiodes on InAs1-xSbx material. Preliminary surface preparation steps were performed on both InAs and InSb material, thus covering both possible extremes of the antimony mole fraction. The first experiments endeavoured to characterise the effect of several possible etchants with regards to etch rate, repeatability, limitations for photolithographic patterning and the resultant surface roughness. The etchants investigated include a lactic acid based etchant, a sulphuric acid based etchant, an acetic acid based etchant, an ammonium based etchant, a hydrochloric acid based etchant as well as an organic rinse procedure. These cleaning and etching steps were evaluated at several temperatures. Measurements were performed on an Alpha Step stylus profiler as well as an atomic force microscope. Metal-insulator-semiconductor capacitor devices were manufactured, on both InAs and InSb material, in order to investigate the effects of the above-mentioned etchants combined with surface passivation techniques in terms of surface state densities. Capacitance-versus-bias voltage measurements were done to determine the resultant surface state densities and to compare these to the surface state density of an untreated reference sample. The surface passivation techniques included KOH, Na2S as well as (NH4)2S anodisation. Auger electron spectroscopy measurements were done on InAs and InSb material in order to examine possible surface contamination due to the etchants as well as combinations of these etching and anodisation procedures. The extent of surface coverage by contaminants as well as by the intrinsic elements was measured. The results of the cleaning and etching as well as the surface passivation studies were used to manufacture photovoltaic infrared diodes on an MOCVD (metal oxide chemical vapour deposition) grown p-InAs0.91Sb0.09/i- InAs0.91Sb0.09/n-GaSb structure. Current-versus-voltage and electro-optical measurements were performed on the these diodes in order to evaluate the effect of sulphuric acid based etching combined with KOH, Na2S or (NH4)2S anodisation on the detector performance. The results of surface passivated structures were compared to those of an unpassivated reference detector.

All-optical time-resolved measurement techniques provide a powerful tool for investigating critical parameters that determine the performance of infrared photodetector and emitter semiconductor materials. Narrow-bandgap InAs/GaSb type-II superlattices (T2SLs) have shown great promise as next generation materials, due to superior intrinsic properties and versatility. Unfortunately, InAs/GaSb T2SLs are plagued by parasitic Shockley-Read-Hall recombination centers that shorten the carrier lifetime and limit device performance. Ultrafast pump-probe techniques and time-resolved differential-transmission measurements are used here to demonstrate that "Ga-free" InAs/InAs₁₋xSbx T2SLs and InAsSb alloys do not have this same limitation and thus have significantly longer carrier lifetimes. Measurements of unintentionally doped MWIR and LWIR InAs/InAs₁₋xSbx T2SLs demonstrate minority carrier (MC) lifetimes of 18.4 µs and 4.5 µs at 77 K, respectively. This represents a more than two order of magnitude increase compared to the 90 ns MC lifetime measured in a comparable MWIR and LWIR InAs/GaSb T2SL. Through temperature-dependent differential-transmission measurements, the various carrier recombination processes are differentiated and the dominant recombination mechanisms identified for InAs/InAs₁₋xSbx T2SLs. These results demonstrate that these Ga-free materials are viable options over InAs/GaSb T2SLs and potentially bulk Hg₁₋xCdxTe photodetectors. In addition to carrier lifetimes, the drift and diusion of excited charge carriers through the superlattice layers (i.e. in-plane transport) directly aects the performance of photo-detectors and emitters. All-optical ultrafast techniques were successfully used for a direct measure of in-plane diffusion coeffcients in MWIR InAs/InAsSb T2SLs using a photo-generated transient grating technique at various temperatures. Ambipolar diffusion coefficients of approximately 60 cm²/s were reported for MWIR InAs/InAs₁₋xSbxT2SLs at 293 K.

Mid-infrared (mid-IR) detection and imaging over atmospheric transparent 3-5 μm and 8-12 μm bands are increasingly becoming important for various space, defense and civilian applications. Various kinds of microbolometers offer uncooled detection of IR radiation. However, broadband absorption of microbolometers makes them less sensitive to spectrally resolved detection of infrared radiation and the fabrication is also very tedious involving multiple complex lithography steps. In this study, we designed an optical nano-antenna array based detector with narrow frequency band of operation. The structure consists of a two-element antenna array comprised of a perforated metallic hole array coupled with an underneath disk array which trap incident radiation as dipole currents. The energy is dissipated as electron plasma loss on the hole-disk system inducing close to ~100% absorption of the incident radiation. This near perfect absorption originates from simultaneous zero crossing of real component of permittivity and permeability due to the geometrical arrangement of the two antenna elements which nullifies overall charge and current distributions, prohibiting existence of any propagating electromagnetic modes at resonance. Moreover, the continuous perforated film allows probing of the induced "micro-current" plasma loss on each nano hole-disk pair via a weak bias current. Such optical antenna design enables flexible scaling of detector response over the entire mid-infrared regime by change in the antenna dimensions. Furthermore, the development of simple nanoimprint lithography based large area optical antenna array fabrication technique facilitates formation of low cost frequency selective infrared detectors.

Infrared (IR) dipole antenna-coupled metal-oxide-metal (MOM) tunnel diodes provide a unique detection mechanism that allows for determination of the polarization and wavelength of an optical field. By integrating the MOM diode into a phased-array antenna, the angle of arrival and degree of coherence of received IR radiation can be determined. The angular response characteristics of IR dipole antennas are determined by boundary conditions imposed by the surrounding dielectric or conductive environment on the radiated fields. To explore the influence of the substrate configuration, single dipole antennas are fabricated on both planar and hemispherical lens substrates. Measurements demonstrate that the angular response can be tailored by the thickness of the electrical isolation stand-off layer on which the detector is fabricated and/or the inclusion of a ground plane. Directional detection of IR radiation is achieved with a pair of dipole antennas coupled to a MOM diode through a coplanar strip transmission line. The direction of maximum angular response is altered by varying the position of the diode along the transmission line connecting the antenna elements. By fabricating the devices on a quarter wave layer above a ground plane, narrow beam widths of 35° full width at half maximum and reception angles of ± 50° are achievable with minimal side-lobe contributions. Phased-array antennas can also be used to assess the degree of coherence of a partially coherent field. For a two-element array, the degree of coherence is a measure of the correlation of electric fields received by the antennas as a function of the element separation.

[Truncated abstract] Infrared (IR) detectors have many applications, from homeland security and defense, to medical imaging, to environmental monitoring, to astronomy, etc. Increasingly, the wave- length dependence of the IR radiation is becoming important in many applications, not just the total intensity of infrared radiation. There are many types of infrared detectors that can be broadly categorized as either photon detectors (narrow band-gap materials or quantum structures that provide the necessary energy transitions to generate free car- riers) or thermal detectors. Photon detectors generally provide the highest sensitivity, however the small transition energy of the detector also means cooling is required to limit the noise due to intrinsic thermal generation. This thesis is concerned with the tech- nique of resonant-cavity-enhancement of detectors, which is the process of placing the detector within an optically resonant cavity. Resonant-cavity-enhanced detectors have many favourable properties including a reduced detector volume, which allows improved operating temperature, or an improved signal to noise ratio (or some balance between the two), along with a narrow spectral bandwidth. ... Responsivity of another sample annealed for 20 hours at 250C in a Hg atmosphere (ex-situ) also shows resonant performance, but indicates significant shunting due the mirror layers. There is good agreement with model data, and the peak responsivity due to the absorber layer is 9.5×103 V/W for a 100 'm ×100 'm photoconductor at 80K. An effective lifetime of 50.4 ns is extracted for this responsivity measurement. The responsivity was measured as a function of varying field, and sweepout was observed for bias fields greater than 50 V/cm. The effective lifetime extracted from this measurement was 224 ns, but is an over estimate. Photodiodes were also fabricated by annealing p-type Hg(1x)Cd(x)Te for 10 hours at 250C in vacuum and type converting in a CH4/H2 reactive ion etch plasma process to form the n-p junction. There is some degradation to the mirror structure due to the anneal in vacuum, but a clear region of high reflection is observed. Measurements of current-voltage characteristics at various temperatures show diode-like characteristics with a peak R0 of 10 G measured at 80K (corresponding to an R0A of approximately 104 cm2. There was significant signal from the mirror layers, however only negligible signal from the absorber layer, and no conclusive resonant peaks.

Approved for public release; distribution in unlimited. / Different sensors exploit different regions of the electromagnetic spectrum; therefore, a multi-sensor image fusion system can take full advantage of the complementary capabilities of individual sensors in the suit; to produce information that cannot be obtained by viewing the images separately. In this thesis, a framework for the multiresolution fusion of the night vision devices and thermal infrared imagery is presented. It encompasses a wavelet-based approach that supports both pixel-level and region-based fusion, and aims to maximize scene content by incorporating spectral information from both the source images. In pixel-level fusion, source images are decomposed into different scales, and salient directional features are extracted and selectively fused together by comparing the corresponding wavelet coefficients. To increase the degree of subject relevance in the fusion process, a region-based approach which uses a multiresolution segmentation algorithm to partition the image domain at different scales is proposed. The region's characteristics are then determined and used to guide the fusion process. The experimental results obtained demonstrate the feasibility of the approach. Potential applications of this development include improvements in night piloting (navigation and target discrimination), law enforcement etc. / Civilian, Republic of Singapore