| Summary: | 碩士 === 國立交通大學 === 工學院半導體材料與製程設備學程 === 101 === This research fabricated high-gain avalanche rushing amorphous photoconductors (HARP) for medical x-ray detector applications, using amorphous selenium (a-Se) as the photoconductive material because of its high photoconversion efficiency. Under a strong electric field, photogenerated holes travel in the photoconductive layer with a high velocity and collide with atoms on the drift path, resulting in more photogenerated carriers. The successive impact ionization process induces the so-called “avalanche multiplication” of the electrical signal. For medical x-ray imagers of low irradiation exposure, HARP devices should have a high photoconversion gain with a very low noise so that a high image contrast can be obtained. The injection of holes and electrons into the a-Se layer via the top and bottom contact electrodes is the primary noise source. We used thermal evaporation to deposit a-Se thin films at temperatures below 40 °C. The CeO2 &; ZnO hole blocking layers, which sandwiched between the ITO substrate and the a-Se layer, were prepared by sputter deposition. In order to observe the correlation between the dark current and traps inside the CeO2 &; ZnO thin film, we controlled the flow rate of oxygen when sputtering CeO2 &; ZnO. In addition, we also deposited distributed resistive layer on the a-Se layer to improve the electrical breakdown voltage for the HARP structure.
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