Effects of Te doping on the optical and electronic properties of a-Se photodetectors

Amorphous selenium (a-Se) is a high resistivity semiconductor used in UV and X-ray detectors due to its high gain, low dark current, and ability to achieve avalanche multiplication at relatively low fields (60-70 V/um). The 2.2 eV band gap of a-Se limits the low energy detection range to UV-blue/green wavelengths; incorporation of Te has been shown to lower the gap and decrease resistivity. However, it has also been determined that the inclusion of Te generates deep and shallow trap states within the a-Se material, lowering both hole and electron mobilities. Still, studies have demonstrated that Te inclusion at 15 wt % can lower the threshold for impact ionization, providing a renewed interest in investigations of Te as a dopant for a-Se detectors targeting lower energy wavelength detection.

In this work, we will review the optoelectronic properties of a-Se_1-xTe_x (x < 0.3) devices and neat films fabricated by thermal evaporation. Characterization will include a study of the absorption coefficient, including the optical bandgap and defect states; hole and electron mobilities as a function of applied field; and conversion efficiency and gain, including a detailed analysis of the onset field for avalanche multiplication.