Amorphous selenium (a-Se) avalanche photodetector for applications in Positron Emission Tomography (PET)

Although avalanche amorphous selenium (a-Se) is a very promising photoconductor for a variety of imaging applications, it is currently restricted to applications with electron beam readout. This is performed with a vacuum pick-up tube called a High-gain Avalanche Rushing Photoconductor (HARP) Vidicon. The electron beam readout is compatible with high definition television (HDTV) applications, but for use in medical imaging devices it should be replaced by an solid-state electronic readout with a twodimensional array of metal pixel electrodes. However, due to the high electric field required for avalanche multiplication, it is a technological challenge to avoid dielectric breakdown at the edges, where electric field experiences local enhancement. We show that this problem can be overcome by the use of a Resistive Interface Layer (RIL) deposited between the uniform a-Se layer and the array of metal electrodes. By detailed analysis of charge transport properties we demonstrate that RIL enables stable avalanche operation in a-Se photoconductor without degradation in charge transport or avalanche multiplication gain. Our results showed that a stable gain of 200 is reached at 104 V/μm for a 15-μm thick a-Se layer, which is the maximum theoretical gain for this thickness. We conclude that RIL is an enabling technology for practical implementation of solidstate avalanche a-Se image sensors.