Wide Bandgap SSPMs

Many homeland security applications require the construction of large radiation detectors, or imagers, to provide adequate sensitivity. Existing scintillation materials, such as plastic and sodium iodide, provide a relatively low-cost solution. However, emerging scintillation materials such as CYLC emit UV to blue light which promises improved sensitivity and capabilities.

In constructing these detectors, large-area PMTs, with their associated high-voltage electronics, often represents the most costly component. To combat this issue, RMFD is looking at silicon-based solid-state photomultiplier (SSPMs). While SSPMs provide ideal readout for small scintillation detectors, the dark current associated with ~1eV Bandgap of silicon limit the effective detector area when operating above room temperature.

Recent advances in the fabrication of semiconductor devices, such as avalanche photodiodes (APDs), in SiC and AlGaAs provide the necessary foundation for constructing SSPM devices. RMD has fabricated APD devices in these materials and the quality of the starting wafers has demonstrated significant improvements.

The solid-state photomultipler (SSPM) is fabricated using an array of avalanche photodiodes operated above breakdown, termed Geiger photodiodes (GPD). The inherent segmentation of the SSPM configuration and the isolation provided by the integrated quenching resistor mitigates the effect of defects associated with the starting wafer.

RMD is investigating large area wide-band-gap SSPM’s using wide-band-gap semiconductor materials. These materials provide low dark noise, are radiation hard, and have a high sensitivity in the blue to UV spectrum. These photodetectors will also be able to provide large gains (~105-106), an important characteristic for detecting low signals from scintillation materials. The application of SSPMs to wide Bandgap materials is transformational because it surmounts many potential limitations in the use of non-silicon semiconductor materials to provide improved optical detector for UV scintillation materials with high detection efficiency and low noise. In addition, the increased popularity of light emitting diodes provides the basis for accelerating the matriculation of many wide band gap materials.