Device Architectures for Improved Temporal Response with Amorphous Selenium Radiation Detectors

Abstract

Amorphous selenium (a-Se) is a commercially mature direct conversion photoconductor capable of very high spatial resolution that enables the early detection of small and subtle lesions in breast cancer screening and diagnostics. However, a-Se exhibits poor collection efficiency due to low carrier mobility and charge trapping related to its amorphous nature. Release of trapped electrons gives rise memory artifacts such as photocurrent lag, that can last for several seconds after the cessation of the X-ray pulse, thus making a-Se a challenging material for use in high spatial resolution dynamic imaging applications.

In this research, the intrinsic causes of temporal behavior in a-Se photoconductor are investigated using lag, ghosting and pulse height spectroscopy (PHS) measurements on conventional a-Se detectors. The measured data is compared to the data obtained with alternative a-Se imaging device architectures for improved dynamic imaging performance. The alternative device architectures include: (1) use of a polyimide blocking layer that permits the operation of a-Se devices with higher electric fields, (2) a high field capable solid-state unipolar charge sensing detector that can achieve hole-mostly charge sensing and (3), small pixel geometries to obtain the small pixel effect (SPE).

Theoretical and experimental results show that an image lag of less than 1.5% is achieved using the solid-state unipolar charge sensing detector for dynamic imaging which is in stark contrast to measurements performed on conventional a-Se imaging devices that exhibit a lag of up to 16%. PHS measurements are also presented that demonstrate, for the first time, a measured energy resolution of 8.3 keV at 59.5 keV was for the unipolar charge sensing device in contrast to 22 keV at 59.5 keV for conventional a-Se devices. Also, the photon counting ability of a-Se photoconductor was demonstrated by integrating a SPE capable a-Se detector with a CMOS pixel array having 11 x 11-μm pixels. Measured results on this CMOS array using a mono-energetic radioactive source are presented which indicate for the first time, that amorphous semiconductors can be used for photon-counting X-ray imaging applications.

The research results indicate that mature large-area a-Se photoconductor, when incorporated with a single polarity charge sensing device design such as SPE or the solid-state unipolar charge sensing detector, can meet the requirements of high spatial resolution dynamic medical imaging applications such as spectral mammography or even micro-angiography without resorting to new sensor materials or crystalline semiconductors that are challenging to scale up to larger areas because of cost and yield issues associated with growth and bonding technology.