Analysis of an a-Si:H P-I-N Diode for the flat panel detector

Abstract

This dissertation is primarily concerned with the characterization of the pixel diodes for a flat-panel detector. Photodiode signal with no incident radiation, which is known as leakage current, is an important parameter. Leakage current will reduce the signal capacity available for incident x-ray and could result in additional noise. It is therefore desirable to minimize the magnitude of the leakage current. In addition to the leakage current, the degree to which radiation affects current must necessarily be investigated. In this dissertation leakage current and radiation damage of a-Si:H P-I-N diodes are measured and analyzed. The new P-I-N diodes, base on the ion shower doping process instead of the conventional PECVD method, are fabricated and characterized. Comparison between ion shower and PECVD diodes is discussed. This method is chosen for two reasons. First, diodes made by conventional PECVD showed poor uniformity of leakage current in a large-area substrate. Among the components of the leakage current, the injection current seems to be the major cause of the poor uniformity. Second, when we fabricate a TFT array only for an Active Matrix Liquid Crystal Display (AMLCD) application, no boron doping is required. However, diode fabrication requires boron doping, which may contaminate the PECVD chamber and affects the TFT deposition process. Since the ion shower doping is performed in another process chamber, it doesn`t result in boron contamination of the PECVD chamber and it may provide high reproducibility of the photodiode leakage current. Also, a reduction of bad pixels is expected. The experiments are performed to investigate the radiation damage to the leakage current of a-Si:H diode made by PECVD and ion shower doping methods, and to study the effect of a copper plate used in portal imaging. Basically the damage effects seem to be same for two diodes. Leakage current as a function of absorbed dose shows that there exists a threshold dose. However t...