X-ray detection using amorphous silicon technology

Abstract

This thesis describes the design and fabrication of a novel direct-conversion X-ray detector based on Mo/a-Si:H Schottky diodes. The choice of Mo as a Schottky contact follows from its low stress, temperature stability, and relatively large X-ray absorption. Furthermore, it is compatible with the standard hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) technology. Here, the TFTs are intended for use as switching elements in large area imaging applications.

Investigations of device stability at low bias suggest that the time dependent variations of reverse current can be attributed to the release of trapped charge in the amorphous material. Here, the instabilities in reverse current during X-ray detection is limited to less than 2.5%. Various experimental studies of noise characteristics in Mo/a-Si:h Schottky diodes show that flicker noise dominates at low frequencies, where the detectors are expected to be operated. Here, the noise current power spectral density has a quadratic dependence on reverse current.

X-ray sensitivity experiments, performed in a medical environment for a wide range of X-ray energies (40 to 100 kVp), show a linear response of the detector with respect to the number of absorbed X-ray photons. Analysis shows that detector sensitivity reaches its maximum for a Mo layer thickness of around 500 nm at 60kVp. The impact of other geometric and bias conditions on the detector performance has also been studied.