This report describes an experimental study of the conduction and absorption mechanisms of Germanium in the temperature range 4.2 - 1.5 K. The results of these studies were mainly devoted to the developments of very far infrared detectors. Germanium (Ge) is a well-known semiconductor element used widely, when doped with small concentration of impurities, for detection of far infrared wavelengths up to 100~m. For doping concentrations less 16 3 than 1.0 x 10 atoms/em, the absorption of radiation in the range 100- 1000~m is very weak Because of the lack of the proper absorption mechanisms, except for some photo-hopping absorption in compensated samples around 1000llm.16 -3 In the range of doping between 1-8 x 10 cm ,there exists additional thermal activation energy not present in the lower concentrations. It was thought that this activation energy results from impurity interactions in this doping range, and hence a delocalized energy band is thus formed above the ground state level. However, the electrical conduction, the width of this band and its position, and the relevance of this band to the marked bolometric effect for 10o-lOOOWU wavelength detections are not yet clear. This thesis presents further study on this band together with its relation to the conduction and absorption mechanisms. Comparative studies were usually made for two samples of Ge differing in doping configuration, one of which does not have this additional activation energy (low concentration) . The firs two chapters give a review of the absorption and conduction mechanisms in Ge at low temperatures, and the performance relations and measurements for different types of infrared detectors. In this report, the conduction mechanism is studied for the two samples, and includes galvanometric properties, thermal properties and energy scattering processes for the carriers in the delocalized band. The absorption characteristics, 1n lOO-lOOO~ru range of the two samples were investigated. Germanium elements with absorbing surfaces are also studied using two different techniques, namely, surface ion implantation and metal film deposition. The mutual effects of the implanted surface and the bulk material are discussed and suggestions for the future of this technique are given. Finally, the design and performance of the constructed high sensitivity far infrared Ge detectors using the higher concentration sample are given. Theoretical noise limitations were reached in these detectors. Heasurements and practicaI. astronomical applications are also given.

Mercury cadmium telluride is important in the detection of electromagnetic radiation in the eight to twelve micron atmospheric window for infrared imaging systems. High resolution infrared imaging systems use either large (256x256 element to 1024x1024 element) staring arrays or much smaller (1-6 element) scanned arrays in which the image is optically scanned across the detectors. In scanned arrays, high resolution and sensitivity may result in the scan direction not being parallel to the detector bias current. The response of an infrared detector to uniform illumination is investigated. It is found that variations in the detector thickness result in significant changes in output voltage. Scanned detectors are modeled in five different orientations; scan parallel to bias, scan opposite to bias, scan perpendicular to bias, and two orientations of the scan diagonal to the bias. The response is analyzed for two cases: 1) the size of the scanned radiation equal to the size of the detector and 2) when the pixel width is half of the width of the detector, but of equal length. Results of the simulation show that the fastest response occurs when the scan and bias are parallel. The largest response occurs when the scan direction is diagonal to the bias, but the response time is much slower than when the bias is parallel to the scan. Therefore, a tradeoff must be made between maximum signal and speed of response. Test detectors are being fabricated and will be tested at FLIR Systems Inc., Portland, Oregon, to confirm the model predictions. / Graduation date: 1992

Uncooled microbolometers have attracted significant interest due to their small size, low cost and low power consumption. As the application range of microbolometers broadens, increasing the dynamic range becomes one of the main objectives of microbolometer research. Targeting this objective, tunable thermal conductance microbolometers have been proposed recently, in which the thermal conductance is tuned by electrostatic actuation. Being a new concept in the field, the current tunable thermal conductance microbolometers have significant potential for improvement in design and performance. In this thesis, an extensive analysis of tunable thermal conductance microbolometers is made, an analytical model is constructed for this purpose, and solutions are proposed to some potential problems such as in-use stiction and variation in spectral response. The current thermal conductance tuning mechanisms use the substrate for electrostatic actuation, which does not support pixel-by-pixel actuation. In this thesis, a new thermal conductance tuning mechanism is demonstrated, that enables pixel-by-pixel actuation by using the micromirror as an actuation terminal instead of the substrate. In addition, a stopper mechanism is used to decrease the risk of in-use stiction. With this new mechanism, the thermal conductance can be tuned by a factor of three at relatively low voltages, making it a promising thermal conductance tuning mechanism for adaptive infrared detectors. Effective estimation of the performance parameters of a tunable thermal conductance microbolometer in the design state requires an analytical model that combines the physics of infrared radiation detection and the thermal conductance tuning mechanisms. As a part of this research, an extensive analytical model is presented, which includes the electrostatic-structural modeling of the thermal conductance tuning mechanism, and electromagnetic and thermal modeling of the microbolometer. The accuracy of the thermal model is of significant importance as the operation of the tuning mechanism within the desired range should be verified in the design stage. A thermal model based on the solution of the microbolometer heat conduction equation is established, which is easily applicable to conventional and tunable thermal conductance microbolometers of various shapes. The constructed microbolometer model is validated by experiments and finite element model simulations. Furthermore, the effect of thermal conductance tuning on spectral response is analyzed. The present thermal conductance tuning mechanisms result in variations in spectral response, which is an undesired effect in many applications. As a solution, a new microbolometer architecture is proposed, in which the spectral response is not affected by thermal conductance. The microbolometer is designed using an analytical model and its performance is characterized by finite element model simulations. To realize the proposed design, a fabrication process flow is offered. It is shown that the proposed microbolometer exhibits high performance, tunable thermal conductance and constant spectral response.

Uncooled microbolometers have attracted significant interest due to their small size, low cost and low power consumption. As the application range of microbolometers broadens, increasing the dynamic range becomes one of the main objectives of microbolometer research. Targeting this objective, tunable thermal conductance microbolometers have been proposed recently, in which the thermal conductance is tuned by electrostatic actuation. Being a new concept in the field, the current tunable thermal conductance microbolometers have significant potential for improvement in design and performance. In this thesis, an extensive analysis of tunable thermal conductance microbolometers is made, an analytical model is constructed for this purpose, and solutions are proposed to some potential problems such as in-use stiction and variation in spectral response. The current thermal conductance tuning mechanisms use the substrate for electrostatic actuation, which does not support pixel-by-pixel actuation. In this thesis, a new thermal conductance tuning mechanism is demonstrated, that enables pixel-by-pixel actuation by using the micromirror as an actuation terminal instead of the substrate. In addition, a stopper mechanism is used to decrease the risk of in-use stiction. With this new mechanism, the thermal conductance can be tuned by a factor of three at relatively low voltages, making it a promising thermal conductance tuning mechanism for adaptive infrared detectors. Effective estimation of the performance parameters of a tunable thermal conductance microbolometer in the design state requires an analytical model that combines the physics of infrared radiation detection and the thermal conductance tuning mechanisms. As a part of this research, an extensive analytical model is presented, which includes the electrostatic-structural modeling of the thermal conductance tuning mechanism, and electromagnetic and thermal modeling of the microbolometer. The accuracy of the thermal model is of significant importance as the operation of the tuning mechanism within the desired range should be verified in the design stage. A thermal model based on the solution of the microbolometer heat conduction equation is established, which is easily applicable to conventional and tunable thermal conductance microbolometers of various shapes. The constructed microbolometer model is validated by experiments and finite element model simulations. Furthermore, the effect of thermal conductance tuning on spectral response is analyzed. The present thermal conductance tuning mechanisms result in variations in spectral response, which is an undesired effect in many applications. As a solution, a new microbolometer architecture is proposed, in which the spectral response is not affected by thermal conductance. The microbolometer is designed using an analytical model and its performance is characterized by finite element model simulations. To realize the proposed design, a fabrication process flow is offered. It is shown that the proposed microbolometer exhibits high performance, tunable thermal conductance and constant spectral response.

Quantum well infrared photodetectors (QWIPs) have recently emerged as a potential alternative to the conventional detectors utilizing low bandgap semiconductors for infrared applications. There has been a considerable amount of experimental and theoretical work towards a better understanding of QWIP operation, whereas there is a lack of knowledge on the underlying physics. This work provides a better understanding of QWIP operation and underlying physics through particle simulations using the ensemble Monte Carlo method. The simulator incorporates Gamma, L, and X valleys of conduction band as well as the size quantization in the quantum wells. In the course of this work, the dependence of QWIP performance on different device parameters is investigated for the optimization of the QWIP structure. The simulations on AlGaAs/GaAs QWIPs with the typical Al mole fraction of 0.3 have shown that the L valley of the conduction band plays an important role in the electron capture. A detailed investigation of the important scattering mechanisms indicates that the capture of the electrons through the L valley quantum well (L-QW) affects the device performance significantly when Gamma and L valley separation is small. The characteristics of electron capture have been further investigated by repeating the simulations on QWIPs for quantum well widths of 36 and 44 &Aring / . The results suggest that the gain in the shorter well width device is considerably higher, which is attributed to the much longer lifetime of the photoexcited electrons as a result of lower capture probability (pc) in the device. The effects of the L-QW height on the QWIP characteristics have also been studied by artificially increasing this height from 63 to 95 meV in Al0.3Ga0.7As/GaAs QWIPs. The increase in the L valley (L-QW) height resulted in higher pc and lower gain due to high rate of capturing of these electrons when Gamma and L valley separation is small.

Thesis (M.S. in Systems Engineering (Electronic Warfare))--Naval Postgraduate School, September 1990. / Thesis Advisor(s): Milne, Edmund Alexander. Second Reader: Hughes, Wayne Philo. "September 1990." Description based on title screen as viewed on December 29, 2009. DTIC Identifier(s): Infrared detectors, electrooptical detectors, theses. Author(s) subject terms: Electronic warfare, EO/IR system, ground EW system, tactical EW system, Korea. Includes bibliographical references (p. 72-76). Also available in print.

Remote sensing of the Earth's atmosphere, typically performed in the infrared region of the spectrum, plays an important role in scientific research. In the past the instruments used to perform these observations have been large, massive devices and correspondingly have only been able to be placed on large satellites. There is currently a trend toward smaller Earth observing platforms, so-called micro-satellites, and there is therefore a need for smaller, less massive instruments. Typically these instruments utilise a semiconductor device that responds to incoming infrared radiation in a known way. Such devices are subject to a number of noise sources that reduce their performance. By cooling them to temperatures around 80K it is possible to significantly reduce the amplitude of this noise compared to the incoming radiation of interest, thus increasing the signal-to-noise ratio (SNR). Typically this cooling is performed by a mechanical cooler, but currently many of them are too massive and require too much power to be suitable for use on a small remote sensing satellite. By considering a typical application, a performance target for a miniature cooler was determined to be a heat lift of 200mW at a cold tip temperature of 80K. Hardware has been created to investigate the feasibility of achieving this aim with a hybrid cooler/radiator. The cooler is a miniature integral Stirling machine and uses flat spiral flexures with a newly designed linear motor to drive the compressor piston; the displacer is driven pneumatically. The prototype initially underwent characterisation without first being pre-cooled by the radiator. Although significant cooling was observed (to below 170K), initial characterisation highlighted a low thermal resistance between the warm end of the cooler and the cold tip. With pre-cooling the cold tip was able to reach a minimum no-load cold tip temperature of 92K, and with 200mW applied to the cold tip a temperature of 122.4 K was sustainable. Attempts were made to increase the thermal resistance between the warm end and cold tip by introducing thermal breaks into the regenerator, and whilst these did increase the thermal resistance, the overall performance of the cooler decreased. The concept of a hybrid miniature cooler/radiator has been shown to be feasible. To achieve the target performance of a heat lift of 200mW at 80K further work needs to be performed to characterise loss processes within the cooler and increase the thermal resistance between the warm end and cold tip.

Thesis (M.S.)--Naval Postgraduate School, Sept. 2004. / Title from title screen (viewed June 24, 2005). "September 2004." Includes bibliographical references (p. 73-74). Also issued in paper format.

Thesis (M.S. in Systems Engineering)--Naval Postgraduate School, Sept. 2004. / Thesis advisor(s): Phillip E. Pace, Murali Tummala. Includes bibliographical references (p. 73-74). Also available online.

Thesis (M.S. in Combat Systems Technology)--Naval Postgraduate School, Dec. 2004. / Thesis Advisor(s): Monique P. Fargues, Alfred W. Cooper. Includes bibliographical references (p. 73-75). Also available online.