Changes in the gait characteristics, such as walking speed and stride length, of a person living at home can be used to presage cognitive decline, predict fall potential, monitor long-term changes in cognitive impairment, test drug regimens, and more. This thesis presents a novel approach to gait analysis in a smart-home environment by leveraging new advances in inexpensive sensors and embedded systems to create novel solutions for in-home gait analysis. Using a simple, non-invasive hardware system consisting entirely of wall-mounted infrared and radio frequency sensor arrays, data is collected on the gait of subjects as they pass by. This data is then analyzed and sent to a clinician for further study. The system is non-invasive in that it does not use cameras and could be built into the molding of a home so that it would be nearly invisible. In a finished prototype version, the system presented in this thesis could be used to analyze the gait characteristics of one or more subjects living in a home environment while ignoring the data of visitors and other non-subject cohabitants. The ability to constantly collect data from a home environment could provide thousands of observations per year for clinical analysis. Providing such a robust data set may allow people with gait impairment to live at home longer and more safely before transitioning to a care facility, have a reduced fall risk due to better prediction, and live a healthier life in old age.

Capacitance-voltage-frequency measurements on n+-GaN/AlxGa1−xN UV/IR dual-band detectors are reported. The presence of shallow Si-donor, deep Si-donor, and C-donor/N-vacancy defect states were found to significantly alter the electrical characteristics of the detectors. The barrier Al fraction was found to change the position of the interface defect states relative to the Fermi level. The sample with Al fraction of 0.1 shows a distinct capacitance-step and hysteresis, which is attributed to C-donor/N-vacancy electron trap states located above the Fermi level (200 meV) at the heterointerface; whereas, the sample with Al fraction of 0.026 shows negative capacitance and dispersion, indicating C-donor/N-vacancy and deep Si-donor defect states located below the Fermi level (88 meV). When an i-GaN buffer layer was added to the structure, an anomalous high-frequency capacitance peak was observed and attributed to resonance scattering due to hybridization of localized Si-donor states in the band gap with conduction band states at the i-GaN/n+-GaN interface.

This thesis presents the design, fabrication and characterization of high performance and high fill factor surface micromachined uncooled infrared resistive microbolometer detectors which can be used in large format focal plane arrays (FPAs). The detector pixels, which have a pixel pitch of 25 &mu / m, are designed and fabricated as two-level structures using the enhanced sandwich type resistor while the active material is selected as Yttrium Barium Copper Oxide (YBCO). First level of the pixel structure is allocated for the formation of the support arms in order to obtain longer support arms hence lower thermal conductance values to get the desired high performance levels. The pixel body is built in the second level such that the fill factor and absorption of the detector is maximized. Structural and sacrificial layer thicknesses are also optimized in order to increase the absorption coefficient of the pixel in the 8-12 &mu / m wavelength range. The thermal simulations are conducted using finite element method (FEM) by CoventorWare software. The designed pixel has a fill factor of 92 % together with the thermal conductance and thermal time constant values calculated as 16.8 nW/K and 19.3 ms in the simulations, respectively. The pixels are fabricated at METU MEMS facilities after the design of a CMOS compatible process flow. All process steps are optimized individually to obtain the expected high performance. Characterization step of the pixels includes the measurements of temperature coefficient of resistance (TCR), noise and thermal conductance value together with the thermal time constant. Effective TCR of the pixel is measured as -2.81 %/K for a pixel with a support arm resistance of 8 k&Omega / and total resistance of 55 k&Omega / . The corner frequency of 1/f noise in the pixel is 9.5 kHz and 1.4 kHz under 20 &mu / A and 10 &mu / A current bias, respectively. The total rms noise is 192 pA within 8.4 kHz bandwidth for a current bias of 20 &mu / A. Thermal conductance, Gth, of the pixel is measured as 17.4 nW/K with a time constant of 17.5 ms. The measurement results indicate that the single pixels designed and fabricated in the scope of this thesis are applicable to large format FPAs in order to obtain a high performance imager. The expected NETD values are 33 mK and 36 mK for 384x288 and 640x480 format FPAs, respectively.

This work describes multiband photon detection techniques based on novel semiconductor device concepts and detector designs with simultaneous detection of dierent wavelength radiation such as UV and IR. One aim of this investigation is to examine UV and IR detection concepts with a view to resolve some of the issues of existing IR detectors such as high dark current, non uniformity, and low operating temperature and to avoid having additional optical components such as filters in multiband detection. Structures were fabricated to demonstrate the UV and IR detection concepts and determine detector parameters: (i) UV/IR detection based on GaN/AlGaN heterostructures, (ii) Optical characterization of p-type InP thin films were carried out with the idea of developing InP based detectors, (iii) Intervalence band transitions in InGaAsP/InP heterojunction interfacial workfunction internal photoemission (HEIWIP) detectors. Device concepts, detector structures, and experimental results are discussed. In order to reduce reflection, TiO2 and SiO2 nanostructured thin film characterization and application of these as anti-reflection coatings on above mentioned detectors is also discussed.

Pedestrian movement is unconstrained. For this reason it is not amenable to mathematical modelling in the same way as road trac. Individual pedestrians are notoriously difficult to monitor at a microscopic level. This has led to a lack of primary data that can be used to develop reliable models. Although video surveillance is cheap to install and operate, video data is extremely expensive to process for this purpose. An alternative approach is to use passive infrared detectors that are able to track individuals unobtrusively. This thesis describesthe use of a low cost infrared sensor for use in tracking pedestrians. The sensor itself, manufactured by a British company, is designed to count people crossing an arbitrary datum line. However, with the development of additional software, the functionality of these sensors can be extended beyond their original design specication. This allows the trajectories of individual pedestrians to be tracked. Although the field of view of each sensor is relatively small (44 m), five were deployed in a busy indoor corridor, covering most of its length. In this research, the technical challenges involved in using the sensors in this way are addressed. Statistics derived from the data collected are then compared to other studies at this scale.

Ce travail de thèse présente les caractéristiques optiques et électriques de dopants dans des couches de CdHgTe, CdZnTe et CdS. Ces 3 matériaux II-VI ont pour point commun d'être utilisés dans des dispositifs de détection, que ce soit la détection de lumière infrarouge pour les couches de CdHgTe et CdZnTe ou la détection visible comme c'est le cas pour le CdS. La caractérisation optique de ces couches de matériaux II-VI a été réalisée par la technique de photoluminescence et corrélée à des mesures électriques effectuées par effet Hall en température. Dans un premier temps, une étude du dopage intrinsèque par les lacunes de mercure et du dopage extrinsèque par incorporation d'arsenic de l'alliage CdHgTe, couche active des détecteurs IR a été réalisée. Pour cela, des mesures optiques par photoluminescence (sur un banc mis en place au laboratoire pendant la 1ere année de thèse permettant de travailler depuis les basses températures jusqu'à l'ambiante entre 1µm et 12 µm dans l'IR) sur des couches de CdHgTe réalisées par épitaxie en phase liquide (EPL) de différentes compositions en Cd ont été effectuées. La corrélation de ces mesures optiques avec des mesures électriques par effet Hall en température a permis d'identifier les énergies d'activation des 2 niveaux de la lacune de mercure ainsi que de démontrer le phénomène de U-négativité de la lacune de mercure dans le CdHgTe. De plus, la comparaison de spectres de PL d'échantillons dopés arsenic pendant la croissance par épitaxie par jets moléculaires (EJM) avec des mesures disponibles réalisées par absorption de rayons X (EXAFS) a permis d'observer des transitions optiques associées aux différents complexes arsenic formés avant et pendant le recuit d'activation. Par ailleurs, un travail de modélisation du phénomène de désordre d'alliage dans le CdHgTe a été réalisé. Plus précisément, un modèle basé sur une statistique gaussienne associée aux fluctuations d'alliage autour d'un gap moyen et une statistique de Boltzman a été développé pour ajuster dans un premier temps des spectres d'absorption puis pour ajuster les spectres de photoluminescence. Ce modèle nous a permis d'ajuster étroitement les spectres de photoluminescence et d'absorption, tout en prenant en compte intrinsèquement le désordre d'alliage du matériau. Nous avons ainsi constaté que l'ajustement des spectres par des fonctions gaussiennes comme il est réalisé communément dans la littérature permet de trouver les bons écarts entre les pics d'émission et donc les bonnes énergies d'ionisation. Dans un deuxième temps, toujours dans le cas de la détection infrarouge, le travail a porté sur l'étude du substrat CdZnTe utilisé pour l'épitaxie du CdHgTe. Des comparaisons des spectres de PL avec les paramètres de croissance ont été effectuées. Plus particulièrement, une étude sur une zone spécifique de certains échantillons présentant une absorption du rayonnement IR a été réalisée afin d'en comprendre l'origine. Enfin, nous nous sommes intéressés à la couche de CdS, matériau II-VI dopé intrinsèquement (type n) utilisé comme fenêtre transparente et formant la jonction p-n avec le CdTe dans les cellules solaires, détecteurs de lumière visible. Dans cette partie, nous avons cherché à étudier l'influence des différentes méthodes de dépôt, sublimation ou bain chimique de la couche de CdS sur un substrat de verre, en comparant les spectres d'émission de photoluminescence obtenus ainsi que les types de traitements thermiques effectués après dépôts. Ces mesures ont été corrélées avec le rendement des cellules solaires finales. / This thesis presents a study of the optical and electrical characteristics of dopants in HgCdTe, CdZnTe and CdS, three materials that play a major role in industry. HgCdTe and CdZnTe are important for infrared light detection and CdS for the fabrication of solar cells. Layers of those II VI materials are characterized by photoluminescence (PL) and temperature-dependent Hall effect. The PL apparatus, built in house, is equipped with a liquid helium cryostat and allows one to scan the entire range of interest between 1 and 12 µm. We address one of the major problems in current HgCdTe technology: p-type doping by both Hg vacancies and arsenic. Low temperature PL and temperature-dependent Hall measurements are first carried out on HgCdTe layers grown by liquid phase epitaxy (LPE). They yield the activation energies of the 2 the Hg vacancy acceptor levels and reveal the U-negative nature of the Hg vacancy in HgCdTe. HgCdTe layers doped with As during molecular beam epitaxial (MBE) growth are also investigated by PL and the results correlated with existing X-ray absorption fine structure (EXAFS) studies. This work allows us to assign the observed optical transitions to the different arsenic complexes formed before and during activation annealing. Furthermore, a model for alloy disorder is developed to correctly fit the data. More precisely, a Gaussian model for the statistical fluctuation of the gap energy due to alloying is formulated and Boltzmann statistics is then introduced in order to fit both the transmission and the PL spectra. The introduction of such model allows one to find the correct emission peak energies, hence the correct ionization energies. Complementary to the study of HgCdTe layer growth, we examine the optical properties of CdZnTe substrates used for HgCdTe epitaxy. In particular, we correlate the PL spectra with the growth parameters, to understand the anomalous infrared absorption of certain CdZnTe ingots. Last, we became interested in CdS, an intrinsically doped (n-type) material used as transparent junction partner in CdTe in solar cells and visible light detectors. We study the impact of different deposition methods, namely sublimation or chemical bath on a glass substrate, by obtaining the PL spectra of as-grown layers and layers that underwent thermal treatment after deposition. These measurements are finally correlated with the final solar cell efficiency.

No description available.

Approved for public release; distribution in unlimited. / Silicon Blocked Impurity Band (BIB) detectors are highly efficient, radiation-hardened photodetectors that operate in the range of 5-40 æm. To further extend BIB coverage to 40-350 æm, Ge and GaAs BIB detectors are under development; however, these new detectors face fabrication issues that have delayed their introduction. This thesis will describe the use of a numerical model to examine alternate operating modes for GaAs BIB detectors in order to bypass current fabrication issues. The numerical simulations provide an understanding of the fundamental physics that governs detector transport. The proposed alternatives to standard operation are created by reversing the detector's bias and varying the blocking layer thickness. Modeling indicates that reversing the bias on these detectors provides a larger signal current than standard configurations, while preserving the principal benefits gained from a multilayered device. At the same time, the alternate bias configuration allows for the use of thicker blocking layers, while preserving overall detector responsivity and reducing shot noise. This proposed new model of operation should allow for the relaxation of fabrication constraints without sacrificing the inherent benefits associated with BIB detectors. These devices are of potential interest for missile defense and terahertz surveillance applications. / Lieutenant Commander, United States Navy

In this work, homojunction interfacial workfunction internal photoemission (HIWIP) detectors based on GaAs, and heterojunction interfacial workfunction internal photoemission (HEIWIP) detectors based mainly on the Gallium Arsenide/Aluminum Gallium Arsenide material system are presented. Design principles of HIWIP and HEIWIP detectors, such as free carrier absorption, photocarrier generation, photoemission, and responsivity, are discussed in detail. Results of p-type HIWIPs based on GaAs material are presented. Homojunction detectors based on p-type GaAs were found to limit their operating wavelength range. This is mainly due to band depletion arising through carrier transitions from the heavy/light hole bands to the split off band. Designing n-type GaAs HIWIP detectors is difficult as it is strenuous to control their workfunction. Heterojunction detectors based on Gallium Arsenide/Aluminum Gallium Arsenide material system will allow tuning their threshold wavelength by adjusting the alloy composition of the Aluminum Gallium Arsenide/Gallium Arsenide barrier, while keeping a fixed doping density in the emitter. The detectors covered in this work operate from 1 to 128 micron (300 to 2.3 THz). Enhancement of detector response using resonance cavity architecture is demonstrated. Threshold wavelength extension of HEIWIPs by varying the Al composition of the barrier was investigated. The threshold limit of approximately 3.3 THz (92 micron), due to a practical Al fraction limit of approximately 0.005, can be overcome by replacing GaAs emitters in Gallium Arsenide/Aluminum Gallium Arsenide HEIWIPs with Aluminum Gallium Arsenide/Gallium Arsenide emitters. As the initial step, terahertz absorption for 1 micron-thick Be-doped Aluminum Gallium Arsenide epilayers (with different Al fraction and doping density) grown on GaAs substrates was measured. The absorption probability of the epilayers was derived from these absorption measurements. Based on the terahertz absorption results, an Aluminum Gallium Arsenide/Gallium Arsenide HEIWIP detector was designed and the extension of threshold frequency (f0) to 2.3 THz was successfully demonstrated. In a different study, switching in Gallium Arsenide/Aluminum Gallium Arsenide heterostructures from a tunneling dominated low conductance branch to a thermal emission dominated high conductance branch was investigated. This bistability leads to neuron-like voltage pulses observed in some heterostructure devices. The bias field that initiates the switching was determined from an iterative method that uses feedback information, such as carrier drift velocity and electron temperature, from hot carrier transport. The bias voltage needed to switch the device was found to decrease with the increasing device temperature.

This work describes infrared (IR) photon detector techniques based on novel semiconductor device concepts and detector designs. The aim of the investigation was to examine alternative IR detection concepts with a view to resolve some of the issues of existing IR detectors such as operating temperature and response range. Systems were fabricated to demonstrate the following IR detection concepts and determine detector parameters: (i) Near-infrared (NIR) detection based on dye-sensitization of nanostructured semiconductors, (ii) Displacement currents in semiconductor quantum dots (QDs) embedded dielectric media, (iii) Split-off band transitions in GaAs/AlGaAs heterojunction interfacial workfunction internal photoemission (HEIWIP) detectors. A far-infrared detector based on GaSb homojunction interfacial workfunction internal photoemission (HIWIP) structure is also discussed. Device concepts, detector structures, and experimental results discussed in the text are summarized below. Dye-sensitized (DS) detector structures consisting of n-TiO2/Dye/p-CuSCN heterostructures with several IR-sensitive dyes showed response peaks at 808, 812, 858, 866, 876, and 1056 nm at room temperature. The peak specific detectivity (D*) was 9.5E+10 Jones at 812 nm at room temperature. Radiation induced carrier generation alters the electronic polarizability of QDs provided the quenching of excitation is suppressed by separation of the QDs. A device constructed to illustrate this concept by embedding PbS QDs in paraffin wax showed a peak D* of 3E+8 Jones at ~540 nm at ambient temperature. A typical HEIWIP/HIWIP detector structures consist of single (or multiple) period(s) of doped emitter(s) and undoped barrier(s) which are sandwiched between two highly doped contact layers. A p-GaAs/AlGaAs HEIWIP structure showed enhanced absorption in NIR range due to heavy/light-hole band to split-off band transitions and leading to the development of GaAs based uncooled sensors for IR detection in the 2 5 μm wavelength range with a peak D* of 6.8E+5 Jones. A HIWIP detector based on p-GaSb/GaSb showed a free carrier response threshold wavelength at 97 µm (~3 THz)with a peak D* of 5.7E+11 Jones at 36 μm and 4.9 K. In this detector, a bolometric type response in the 97 - 200 µm (3-1.5 THz) range was also observed.