Enhanced Dark Current Suppression of Amorphous Selenium Detector With Use of IGZO Hole Blocking Layer
We examined the potential application of indium gallium zinc oxide (IGZO) as a hole-blocking layer for an amorphous selenium (a-Se)-based detector to reduce the dark current and improve the sensitivity of the detector. By employing a thin layer of IGZO (375 nm), the dark current of an a-Se detector remains below 1 pA/mm2 up to electric fields as high as 60 V/μm. The measured dark current at different electric fields is comparable to the thermal generation currents in a-Se, thus demonstrating the good hole-blocking properties of IGZO. The detector's photo response was characterized using a blue light-emitting diode at different electric fields. A factor of three improvement in external quantum efficiency was observed by increasing the electric field of the detector from 10 to 50 V/μm.
Modeling of Transient Photocurrent and Lag Signal in X-ray Imaging Detectors: Application to amorphous selenium
Flat-panel detectors (FPDs) are digital detectors that are widely utilised in medical applications such as general radiography and mammography. They are exposed to electromagnetic excitation to produce digital images of internal body organs. The electromagnetic radiation (optical or X-rays) creates electrons and holes in the photoconductor layer. These photogenerated electrons and holes drift under the influence of the applied electric field and constitute a photocurrent. Apart from the photocurrent, there is an undesirable current known as the lag signal detected in the devices after the removal of the excitation. The lag signal causes image artifacts in the digital image output that can lead to inaccurate or misleading medical diagnosis. Generally, the photocurrent and lag signals in FPDs are analyzed through experimental means. To the best of our knowledge, a complete mathematical model does not exist in literature to represent the entire current profile, which includes the photocurrent and the lag signal, for exponential carrier generation in FPDs. 
This thesis is concerned with developing a mathematical model for transient photocurrent and lag signal in FPDs for X-ray and optical excitation by considering charge carrier trapping and detrapping in the energy distributed defect states under exponentially distributed carrier generation across the photoconductor. The model for the transient and steady-state carrier distributions and hence the photocurrent has been developed by solving the carrier continuity equation for both holes and electrons. The lag signal is modeled by solving the trapping rate equations considering the thermal release and trap filling effects. The model is applied to amorphous selenium (a-Se) detectors for both chest radiography and mammography. The dependence of the lag signal on various factors such as X-ray exposure, applied electric field, and temperature is analyzed. The lag signal is found to be more prominent in chest radiographic detector than in mammographic detectors. Moreover, the transient rise and decay of the photocurrent profile as a function of time is studied. The quick rise and decay parts, and then the slow rise and decay parts of the photocurrent profile are due to the hole and electron transports, respectively. The model calculations are compared with the published experimental data and show a good agreement within the limits of experimental error. The satisfactory fittings of the experimental data with the model reveals the origin of the residual signal in the detector, which could be helpful for predicting the lag signal and designing the readout circuit and finding the ways to reduce lag.
EFFECT OF SAMPLE PREPARATION ON WATER TREE RESISTANCE OF XLPE/SEBS BLENDS
The effects of an aromatic polymer, such as poly[styrene-b-(ethylene-co-butylene)b-styrene] (SEBS), and its preparation processes on water tree resistance, morphology, crystallinity, dielectric constant and dielectric loss tangent of crosslinked polyethylene (XLPE)/SEBS and XLPE/SEBS/ethylene vinyl acetate (EVA) blends were investigated. The XLPE/SEBS samples were characterized by means of differential scanning calorimetry, scanning electron microscopy, dielectric spectral analysis and the accelerated water treeing experiment. The results clearly showed that although SEBS was partially compatible with LDPE and it was dispersed evenly in XLPE matrix, the blends demonstrated excellent dielectric performances, such as dielectric constant and dissipation factor. The crystallinity of XLPE/SEBS blends was decreased with increase in SEBS content and it was further decreased with the addition of EVA. SEBS could effectively improve the water-tree resistance of XLPE and both SEBS and EVA were found to improve the water tree resistance of XLPE synergistically as expected. The differences in sample preparation processes could change the phase morphology and dielectric performance of the blends, and could also influence the dispersion of SEBS in XLPE matrix as well as the crystallinity of the blends. Different preparation processes led to different water tree resistance behaviours of the samples. The improvement of SEBS dispersion in XLPE matrix helped to improve the water tree resistance of XLPE.