[Truncated abstract] HgCdTe is the leading semiconductor material for the fabrication of high performance infrared photon detectors, in particular, for detection of radiation beyond the near infrared. State-of-the-art infrared detection and imaging systems are currently based around high density focal plane arrays consisting of HgCdTe photodiodes as detector elements. Despite the high performance of HgCdTe infrared detectors, and the many benefits they can offer to industry and society, their utilisation remains limited due to the high cost of production. The chemical composition and narrow bandgap of the HgCdTe material used for infrared detection means that the material is inherently very susceptible to defect formation caused by the processing procedures required for device fabrication. Consequently, fabrication of HgCdTe photodiode arrays have traditionally been characterised by low yields and high costs for arrays that meet required operability specifications. In this thesis a new photodiode fabrication technology with the potential to improve device yields over traditional fabrication technologies is presented. This new fabrication technology is distinguished from others by the use of plasma-induced p-to-n type conversion of HgCdTe for junction formation. This allows great simplification of the fabrication process and avoids high temperature processing during and after junction formation, and keeps the junction protected from the atmosphere at all stages of fabrication. The development of the photodiode fabrication technology using plasma-induced junction formation has involved characterising the electrical transport properties of the type-converted layers, fabrication and characterisation of photodiodes, and photodiode dark current modelling

Thesis (M.S. in Applied Physics)--Naval Postgraduate School, June 2005. / Thesis Advisor(s): Richard M. Harkins, Gamani Karunasiri. Includes bibliographical references (p. 77-78). Also available online.

No description available.

It is desirable to modify the spectral signature of a surface, particularly in the infrared (IR) region of the electromagnetic spectrum. To alter the surface signature in the IR, two methods are investigated: thin film application and antenna array application. The former approach is a common and straightforward incorporation of optically-thin film coatings on the surface designated for signature modification. The latter technique requires the complex design of a periodic array of passive microantenna elements to cover the surface in order to modify its signature. This technology is known as frequency selective surface (FSS) technology and is established in the millimeter-wave spectral regime, but is a challenging technology to scale for IR application. Incorporation of thin films and FSS antenna elements on a surface permits the signature of a surface to be changed in a deterministic manner. In the seminal application of this work, both technologies are integrated to comprise a circuit-analog absorbing IR FSS. The design and modeling of surface treatments are accomplished using commercially-available electromagnetic simulation software. Fabrication of microstructured antenna arrays is accomplished via microlithographic technology, particularly using an industrial direct-write electron-beam lithography system. Comprehensive measurement methods are utilized to study the patterned surfaces, including infrared spectral radiometry and Fourier-transform infrared spectrometry. These systems allow for direct and complementary spectral signature measurements--the radiometer measures the absorption or emission of the surface, and the spectrometer measures its transmission and reflection. For the circuit-analog absorbing square-loop IR FSS, the spectral modulation in emission is measured to be greater than 85% at resonance. Other desirable modifications of surface signature are also explored; these include the ability to filter radiation based on its polarization orientation and the ability to dynamically tune the surface signature. An array of spiral FSS elements allows for circular polarization conditioning. Three techniques for tuning the IR FSS signature via voltage application are explored, including the incorporation of a pn junction substrate, a piezoelectric substrate and a liquid crystal superstrate. These studies will ignite future explorations of IR FSS technology, enabling various unique applications.

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.

The IRMA III infrared radiometer is a passive atmospheric water vapor detector designed for use with interferometric submillimeter arrays as a method of phase correction. The IRMA III instrument employs a distributed, multi-tasking software control system permitting precise fine-grained control at remote locations over a low-bandwidth network connection. IRMA's software is divided among three processors tasked with performing three primary functions: command interpretation, data collection and motor control of IRMA's Alt-Az mount. IRMA's hardware control and communication functionality is based on compact, low cost, energy efficient Rabbit 2000 microcontroller modules, selected to meet IRMA's limited space and power requirements. IRMA accepts scripts defined in a custom, high level control language as its method of control, which the operator can write or dynamically generated by a separate GUI front-end program. / xi, 193 leaves : ill. ; 28 cm.

Our group has developed a 20 μm passive atmospheric water vapour monitor. The Infrared Radiometer for Millimetre Astronomy (IRMA) has been commissioned and deployed for site testing for the Thirty Meter Telescope (TMT) and the Giant Magellan Telescope (GMT). Measuring precipitable water vapour (PWV) requires both a sophisticated atmospheric model (BTRAM) and an instrument (IRMA). Atmospheric models depend on atmospheric profiles. Most profiles are generic in nature, representing only a latitude in some cases. Site-specific atmospheric profiles are required to accurately simulate the atmosphere above any location on Earth. These profiles can be created from publicly available archives of radiosonde data, that offer nearly global coverage. Having created a site-specific profile and model, it is necessary to determine the PWV sensitivity to the input parameter uncertainties used in the model. The instrument must also be properly calibrated. In this thesis, I describe the radiometric calibration of the IRMA instrument, and the creation and analysis of site-specific atmospheric models for use with the IRMA instrument in its capacity as an atmospheric water vapour monitor for site testing. / xii, 135 leaves : ill. ; 28 cm. --

The Infrared Radiometer for Millimetre Astronomy (IRMA) is a passive atmospheric water vapour monitor developed at the University of Lethbridge. It is a compact, robust, and autonomous instrument, which is capable of being operated remotely. The latest model is based on a PC/104 running an AMD 133 MHz SC520 processor, which allows for more flexible control of the unit. The modifications and upgrades to the software required for the transition to this new platform are discussed in this thesis. In addition to software optimisation, a new calibration method has been developed as the unit has become better understood. This method has been verified through test campaigns carried out in Lethbridge and Chile. The results of these tests are included in this thesis. / xii, 141 leaves : ill. (some col.) ; 28 cm. --

No description available.

Call number: LD2668 .T4 1948 N49 / Master of Science