Amorphous selenium (a-Se) and its alloys are important photoconductor materials used in direct conversion flat panel digital x-ray detectors. The performance of these detectors is determined, in part, by the electronic transport properties of the a-Se photoconductor layer namely, the charge carrier mobility m and the deep trapping lifetime t. The product of the mobility and the lifetime mt, referred to as the charge carrier range, determines the average distance that photo-generated charge will travel before being removed from the transport band by deep localized states in the mobility gap of the semiconductor. The loss of carriers to these deep states reduces the amount of charge collected per unit of x-ray exposure, and, hence, limits the x-ray sensitivity of the detector. Two experimental techniques that may be used to measure the transport properties of holes and electrons in high resistivity semiconductors are described in this thesis. The Time-of-Flight (TOF) transient photoconductivity technique is used to evaluate the charge carrier mobility by measuring the time required for the charge carriers to transit a fixed distance under the influence of an applied electric field. The Interrupted-Field Time-of-Flight (IFTOF) technique is used to determine the charge carrier deep trapping time; the drift of the injected carriers is temporarily interrupted at a position in the sample by removing the applied field. When the field is reapplied the number of charge carriers has decreased due to trapping events. The carrier lifetime is determined from the dependence of the fraction of recovered charge carriers before and after the interruption with the interruption time. <p> TOF and IFTOF measurements were carried out on a number of samples of vacuum deposited selenium alloy x-ray photoconductors. Device quality photoconductor films are fabricated by evaporating a-Se source material that has been alloyed with a small quantitiy of As (~0.3 at. %) and doped with a halogen (typically Cl) in the p.p.m. range. The dependence of the carrier range on the composition of the photoreceptor film was accurately measured using both TOF and IFTOF measurements. It was found that the transport properties of the film could be controlled by suitably adjusting the composition of the alloy. Combined IFTOF and TOF measurements were also performed on several samples to examine the effects of trapped electrons on the hole transport properties in a-Se films. It was found that drifting holes recombine with the trapped electrons, and that this process could be described by a Langevin recombination process. This finding is important for the correct modeling of amorphous selenium digital x-ray detector designs. Finally, the effects of x-ray exposure on a-Se films were examined. A temporary reduction in the effective hole lifetime was observed due to an increase in the number of hole capture centers following an x-ray exposure. The capture coefficient between free holes and the x-ray induced hole capture centers was measured using combined TOF and IFTOF measurements. It was shown that this capture process was governed by the Langevin recombination mechanism. From these observations it was concluded that trapped electrons from a previous x-ray exposure act as recombination centers for subsequently generated holes, thereby reducing the effective hole lifetime in the sample.

Amorphous selenium (a-Se) and its alloys are important photoconductor materials used in direct conversion flat panel digital x-ray detectors. The performance of these detectors is determined, in part, by the electronic transport properties of the a-Se photoconductor layer namely, the charge carrier mobility m and the deep trapping lifetime t. The product of the mobility and the lifetime mt, referred to as the charge carrier range, determines the average distance that photo-generated charge will travel before being removed from the transport band by deep localized states in the mobility gap of the semiconductor. The loss of carriers to these deep states reduces the amount of charge collected per unit of x-ray exposure, and, hence, limits the x-ray sensitivity of the detector. Two experimental techniques that may be used to measure the transport properties of holes and electrons in high resistivity semiconductors are described in this thesis. The Time-of-Flight (TOF) transient photoconductivity technique is used to evaluate the charge carrier mobility by measuring the time required for the charge carriers to transit a fixed distance under the influence of an applied electric field. The Interrupted-Field Time-of-Flight (IFTOF) technique is used to determine the charge carrier deep trapping time; the drift of the injected carriers is temporarily interrupted at a position in the sample by removing the applied field. When the field is reapplied the number of charge carriers has decreased due to trapping events. The carrier lifetime is determined from the dependence of the fraction of recovered charge carriers before and after the interruption with the interruption time. <p> TOF and IFTOF measurements were carried out on a number of samples of vacuum deposited selenium alloy x-ray photoconductors. Device quality photoconductor films are fabricated by evaporating a-Se source material that has been alloyed with a small quantitiy of As (~0.3 at. %) and doped with a halogen (typically Cl) in the p.p.m. range. The dependence of the carrier range on the composition of the photoreceptor film was accurately measured using both TOF and IFTOF measurements. It was found that the transport properties of the film could be controlled by suitably adjusting the composition of the alloy. Combined IFTOF and TOF measurements were also performed on several samples to examine the effects of trapped electrons on the hole transport properties in a-Se films. It was found that drifting holes recombine with the trapped electrons, and that this process could be described by a Langevin recombination process. This finding is important for the correct modeling of amorphous selenium digital x-ray detector designs. Finally, the effects of x-ray exposure on a-Se films were examined. A temporary reduction in the effective hole lifetime was observed due to an increase in the number of hole capture centers following an x-ray exposure. The capture coefficient between free holes and the x-ray induced hole capture centers was measured using combined TOF and IFTOF measurements. It was shown that this capture process was governed by the Langevin recombination mechanism. From these observations it was concluded that trapped electrons from a previous x-ray exposure act as recombination centers for subsequently generated holes, thereby reducing the effective hole lifetime in the sample.

<p>Direct conversion flat panel x-ray image sensors based on using a photoconductor with an active matrix array provide excellent images. These image sensors are suitable for replacing the present day x-ray film/screen cassette to capture an x-ray image electronically, and hence enable a clinical transition to digital radiography. The performance of these sensors depends critically on the selection and design of the photoconductor. This work quantitatively studies the combined effects of the detector geometry (pixel size and detector thickness), operating conditions (x-ray energy and applied electric field) and charge transport properties (e.g., carrier trapping and recombination) of the photoconductor on the detector performance by developing appropriate detector models. In this thesis, the models for calculating the x-ray sensitivity, resolution in terms of the modulation transfer function (MTF), detective quantum efficiency (DQE), and ghosting of x-ray image detectors have been developed. The modeling works are based on the physics of the individual phenomena and the systematic solution of the fundamental physical equations in the photoconductor layer: (1) semiconductor continuity equation (2) Poissons equation (3) trapping rate equations. The general approach of this work is to develop models in normalized coordinates to describe the results of different photoconductive x-ray image detectors. These models are applied to a-Se, polycrystalline HgI_2 and polycrystalline CdZnTe photoconductive detectors for diagnostic medical x-ray imaging applications (e,g., mammography, chest radiography and fluoroscopy). The models show a very good agreement with the experimental results.</p><p>The research presented in this thesis shows that the imaging performances (e.g., sensitivity, MTF, DQE and ghosting) can be improved by insuring that the carrier with higher mobility-lifetime product is drifted towards the pixel electrodes. The carrier schubwegs have to be several times greater, and the absorption depth has to be at least two times smaller than the photoconductor thickness for achieving sufficient sensitivity. Having smaller pixels is advantageous in terms of higher sensitivity by ensuring that the carrier with the higher mobility-lifetime product is drifted towards the pixel electrodes. </p><p>A model for calculating zero spatial frequency detective quantum efficiency, DQE (0), has been developed by including incomplete charge collection and x-ray interaction depth dependent conversion gain. The DQE(0) analyses of a-Se detectors for fluoroscopic applications show that there is an optimum photoconductor thickness, which maximizes the DQE(0) under a constant voltage operation. The application of DQE(0) model to different potential photoconductive detectors for fluoroscopic applications show that, in addition to high quantum efficiency, both high conversion gain and high charge collection efficiency are required to improve the DQE performance of an x-ray image detector.</p><p>An analytical expression of MTF due to distributed carrier trapping in the bulk of the photoconductor has been derived using the trapped charge distribution across the photoconductor. Trapping of the carriers that move towards the pixel electrodes degrades the MTF performance, whereas trapping of the other type of carriers improves the sharpness of the x-ray image.</p><p>The large signal model calculations in this thesis show an upper limit of small signal models of x-ray image detectors. The bimolecular recombination between drifting carriers plays practically no role on charge collection in a-Se detectors up to the total carrier generation rate q0 of 10^18 EHPs/m^2-s. The bimolecular recombination has practically no effect on charge collection in a-Se detectors for diagnostic medical x-ray imaging applications. </p><p>A model for examining the sensitivity fluctuation mechanisms in a-Se detectors has been developed. The comparison of the model with the experimental data reveals that the recombination between trapped and the oppositely charged drifting carriers, electric field dependent charge carrier generation and x-ray induced new deep trap centers are mainly responsible for the sensitivity fluctuation in biased a-Se x-ray detectors. </p><p>The modeling works in this thesis identify the important factors that limit the detector performance, which can ultimately lead to the reduction of patient exposure/dose consistent with better diagnosis for different diagnostic medical x-ray imaging modalities. The quantitative analyses presented in this thesis show that the detector structure is just as important to the overall performance of the detector as the material properties of the photoconductor itself.</p>

<p>Direct conversion flat panel x-ray image sensors based on using a photoconductor with an active matrix array provide excellent images. These image sensors are suitable for replacing the present day x-ray film/screen cassette to capture an x-ray image electronically, and hence enable a clinical transition to digital radiography. The performance of these sensors depends critically on the selection and design of the photoconductor. This work quantitatively studies the combined effects of the detector geometry (pixel size and detector thickness), operating conditions (x-ray energy and applied electric field) and charge transport properties (e.g., carrier trapping and recombination) of the photoconductor on the detector performance by developing appropriate detector models. In this thesis, the models for calculating the x-ray sensitivity, resolution in terms of the modulation transfer function (MTF), detective quantum efficiency (DQE), and ghosting of x-ray image detectors have been developed. The modeling works are based on the physics of the individual phenomena and the systematic solution of the fundamental physical equations in the photoconductor layer: (1) semiconductor continuity equation (2) Poissons equation (3) trapping rate equations. The general approach of this work is to develop models in normalized coordinates to describe the results of different photoconductive x-ray image detectors. These models are applied to a-Se, polycrystalline HgI_2 and polycrystalline CdZnTe photoconductive detectors for diagnostic medical x-ray imaging applications (e,g., mammography, chest radiography and fluoroscopy). The models show a very good agreement with the experimental results.</p><p>The research presented in this thesis shows that the imaging performances (e.g., sensitivity, MTF, DQE and ghosting) can be improved by insuring that the carrier with higher mobility-lifetime product is drifted towards the pixel electrodes. The carrier schubwegs have to be several times greater, and the absorption depth has to be at least two times smaller than the photoconductor thickness for achieving sufficient sensitivity. Having smaller pixels is advantageous in terms of higher sensitivity by ensuring that the carrier with the higher mobility-lifetime product is drifted towards the pixel electrodes. </p><p>A model for calculating zero spatial frequency detective quantum efficiency, DQE (0), has been developed by including incomplete charge collection and x-ray interaction depth dependent conversion gain. The DQE(0) analyses of a-Se detectors for fluoroscopic applications show that there is an optimum photoconductor thickness, which maximizes the DQE(0) under a constant voltage operation. The application of DQE(0) model to different potential photoconductive detectors for fluoroscopic applications show that, in addition to high quantum efficiency, both high conversion gain and high charge collection efficiency are required to improve the DQE performance of an x-ray image detector.</p><p>An analytical expression of MTF due to distributed carrier trapping in the bulk of the photoconductor has been derived using the trapped charge distribution across the photoconductor. Trapping of the carriers that move towards the pixel electrodes degrades the MTF performance, whereas trapping of the other type of carriers improves the sharpness of the x-ray image.</p><p>The large signal model calculations in this thesis show an upper limit of small signal models of x-ray image detectors. The bimolecular recombination between drifting carriers plays practically no role on charge collection in a-Se detectors up to the total carrier generation rate q0 of 10^18 EHPs/m^2-s. The bimolecular recombination has practically no effect on charge collection in a-Se detectors for diagnostic medical x-ray imaging applications. </p><p>A model for examining the sensitivity fluctuation mechanisms in a-Se detectors has been developed. The comparison of the model with the experimental data reveals that the recombination between trapped and the oppositely charged drifting carriers, electric field dependent charge carrier generation and x-ray induced new deep trap centers are mainly responsible for the sensitivity fluctuation in biased a-Se x-ray detectors. </p><p>The modeling works in this thesis identify the important factors that limit the detector performance, which can ultimately lead to the reduction of patient exposure/dose consistent with better diagnosis for different diagnostic medical x-ray imaging modalities. The quantitative analyses presented in this thesis show that the detector structure is just as important to the overall performance of the detector as the material properties of the photoconductor itself.</p>