Abstract
Medical X-ray image detective device was fabricated by hydrogenated amorphous silicon (a-Si:H), because of a-Si:H have advantage of low cost and large area. A novel X-ray image detectors which was designed by S. S. Fann of our laboratory is composed of PIN photodiode with a dielectric layer inserted between the n layer and aluminum electrode. X-ray was used as the source of sensor characteristics measurement to analyse the sensor properties. The experimental data of S. S. Fann showed the number of charges extracted by read-out circuit was proportional to the total incident light dose, and it proved that the structure of pin photodiode in combination with the silicon nitride layer as a X-ray detector is a valid concept. PIN photodiodes and silicon nitride layers were fabricated in the same camber of a plasma enhanced chemical vapor deposition system (PECVD). In this study, we try to combine barium strontium titanate (BST) with PIN process to obtain a sensor which can be operated at higher X-ray dose range. BST was deposited with sol-gel method to fulfill requirements for large-area and low-cost. The dielectric thin films are needed to combine with the a-Si:H device and they are deposited on glass substrate. The process' temperature must be reduced to avoid a-Si:H device fail and glass melt. When dielectric layer need to be deposited after the a-Si:H devices process, the deposition temperature can't surpass 250℃. When the dielectric layer is needed to deposited on glass, 600℃ is a bounds to avoid the glass melt. Due to limitations temperature, O2 plasma treatment and RTA with oxygen ambiance were used to reduce the leakage current and to increase the capacitance density. BST thin films treated with O2 plasma for proper time can obtain lower surface roughness and get lower leakage current. From the data of our experiments at lower process temperature, it is hard to improve crystallization of the BST thin films. High temperature process might be indispensable to improve the dielectric properties. Although the I-t measurement of BST deposited on the a-Si:H diode with 250℃ show the charging and discharging properties, but leakage is higher than electrical circuit's limit. Decreasing the leakage current and increasing the capacitance of a-Si:H diodes with BST will be an important research topic in future.