Effect of hole blocking dielectric layer on microstructure and photoconducting properties of polycrystalline Se thin films

We fabricated polycrystalline selenium (c-Se) based photodetectors using three different dielectrics (HfO₂, Al₂O₃ and SiO₂) as the hole blocking layer (HBL), and studied the influence of the HBLs on the photoconducting performance of the photodetectors. The microstructure of the c-Se layer is greatly influenced by the nucleation behavior of the tellurium (Te) adhesion layer deposited between the c-Se layer and the HBLs. The photoconducting performance of the photodetectors is basically dependent on the barrier height at the junctions of the HBL with the tin-doped indium oxide (ITO) anode and with the c-Se layer. A higher barrier height at the HBL/ITO junction leads to a lower dark current density (I_D) of the photodetectors. However, the photodetector with the SiO₂ HBL exhibits the largest I_D as the bias exceeds 2 V although it has the highest junction barrier height. We attribute the abnormity to the rugged morphology of the c-Se layer, which is a result of a less dense Te nucleation on the SiO₂ HBL. The photocurrent density (I_ph) is inversely related to the junction barrier height at the HBL/c-Se contact. The c-Se photodetector with the HfO₂ HBL has the largest I_Ph and the one with the SiO₂ HBL has the smallest. The photodetector with the HfO₂ HBL exhibits a quantum efficiency of ~ 89% at 6 V.