Abstract
AbstractThis thesis presents an innovative hydrogenated amorphous silicon (a-Si:H) p-i-n photodiode based x-ray detector for the medical imaging applications, as well as a novel method for the amorphous material defect study. The thesis consists of several parts; and the major ones include the innovative photo detectors: n-i-p-SiNx and p-i-n-SiNx; and the interesting phenomenon arisen from trapped electrons in the a-SiC:H p-layer of the novel detectors, in which the trapped electrons have lower than 10-2 Hz attempt to escape frequencies at room temperature. The operating principles of the detectors are described in details in this thesis with significantly improved performances, and use of this new detector structure is proposed in the a-Si:H p-i-n photodiode based active matrix, flat panel, x-ray medical imaging array. The trap energy level is measured, and the energy barrier to release the trapped electrons is also calculated based on a three-configuration-state model, which is modified from the Adler’s atomic relaxation model for amorphous materials. Some fabricated detectors for this thesis have a much longer data retention time, which can be attributed to the low attempt to escape frequencies of the trapped electrons.For the medical applications, the image quality and the least exposure of patient to x-ray are most seriously concerned. Therefore, the diode must have low leakage current, low noise, and high sensitivity to light. The light converted signal is kept in the diode till it is read-out. As a result, data retention becomes the most essential when the imager size is extensively increased for the high spatial resolution and full field applications, in which, the data and gate sizes each may be up to 2000 lines. The requirements of a-Si:H p-i-n photodiode for medical imaging applications are very critical. In this thesis, two modified p-i-n photodiodes, n-i-p-SiNx and p-i-n-SiNx, with enlarged charge storage capacity and more flexibility in the trade-off between the photo sensitivity and charge storage capability, are proposed in the medical imaging applications. The p-i-n photodiode based active matrix array is very promising in the digital x-ray applications. To date, it is also the leader among the commercially available systems. However, the novel p-i-n based photon detectors have a larger charge storage capacity than the conventional p-i-n photodiode and have a very simple structure. As a consequence, the performances of the detectors are significantly improved. The scheme is more effective than any other modifications of the photodiode structure for the active matrix medical imaging array, such as to build an amplifier in each pixel, which should preserve the unique feature of simplicity of this novel detector structure. The operating principles and performance of this novel detector are described in this thesis, and the fundamentals of the detector are also discussed. The experimental results proved that use of this novel structure is valid, and can be applied to construct effectively a two-dimensional detector array, offering considerable advantages in the x-ray medical image applications. Furthermore, the possible applications of these novel detectors, such as the input device for the digital information systems, are also proposed. These detectors are described in details in this thesis, beginning with a survey of the state-of-the-art of the digital x-ray imagers. The proposed detectors are made from a-Si:H. The low-cost and capability to fabricate devices or circuitries over a large area is the primary advantage of a-Si:H material, but its leakage current is also much larger than the crystalline silicon. The intrinsic properties of the a-Si:H p-i-n photodiode which negatively affect its applications in the medical imaging array are also described. The performance improvements due to the enlarged charge storage capacity of the novel detectors are analyzed based on their operating principles. And the largely enhanced data retention capability of the detectors is attributed to the low attempt escaped frequencies of trapped electrons in the p-layer or in the interface to SiNx, which is explained by the modified atomic relaxation model.