Comparative study on radiation damage of a-Si:H p-i-n diodes made by PECVD and ion shower doping

Flat panel detectors using amorphous silicon p-i-n diodes have been developed for chest and portal imaging. In radiation therapy, an imaging device is exposed to a high radiation dose. So the radiation damage affects the leakage current of diode and may decrease the whole system performance. In this research we have investigated the radiation damage to the leakage current of a-Si:H diode made by PECVD and ion-shower doping methods, and studied the effect of a copper plate used in portal imaging. Leakage current as a function of absorbed dose shows that the damage effects seem to be same for two types of diodes when a copper plate is used. They have the threshold value below which leakage current does not change. Both of two diodes show different threshold behavior when a copper plated is not used. Threshold value shifts to a higher dose in the absence of a copper plate. In addition an ion-shower diode shows a higher leakage current at high absorbed dose compared to a PECVD diode. Since the p-n junction property of an ion-shower diode is not as good as a PECVD diode, the leakage current increase induced by radiation can be higher at high absorbed dose. Monte Carlo simulation tells that the copper plate used in portal imaging generates more secondary electrons, which transfer energy to the diode as they travel through.