Dependence of the electrical properties of stabilized a-Se on the preparation conditions and the development of a double layer X-ray detector structure

Dark current in multilayer stabilized amorphous selenium X‐ray photoconductors

“…The release of trapped holes may Figure 4 Comparison of dark current density 300 s after the application of an electric field of -10 V/µm. Data for cold deposited i-n sample is taken from [6]. play a role in this "hump" because, considering a typical phonon frequency of 10 12 s -1 , a hole release time of 1000 s would correspond to a deep trap energy of 0.86 eV above the valence band edge which agrees well with accepted models for the density of states for a-Se [9].…”

“…Article duces the dark current by over three orders of magnitude and the current decreases very little after 300 s. Figure 4 shows a comparison of the dark current 300 s after the application of voltage for all three samples and an i-n structure in which a 20 µm n-layer was cold deposited (25 °C substrate) on a 130 µm i-layer with a top electrode of Pt [6]. It is clear that only the addition of an n-like layer is capable of reducing the dark current to below the acceptable level of 100 pA/cm 2 .…”

“…This dark current limits the minimum amount of radiation that can be detected and also reduces the dynamic range of the system. An acceptable level for dark current in a direct conversion commercial detector has been estimated to be ~100 pA/cm 2 [5] but at applied electric fields of 10 V/µm this cannot be achieved with a single uniform layer of a-Se [6]. In practical devices, thin blocking layers are deposited between the electrodes and the intrinsic layer (i-layer) of aSe [7].…”

“…An n-like layer is used to block the injection of holes from the positive electrode and a p-like layer is used to block the injection of electrons from the negative electrode. The definition of the n and p-layers does not necessarily imply the location of the Fermi level in these layers as in traditional semiconductor physics, but only the relative charge carrier transport properties that they possess [6]. The n-like layer is required to trap holes in deep localized states in the bandgap while allowing electrons to drift without trapping.…”

“…In practical devices, the p-like layer is commonly made from a-As 2 Se 3 [8], known for its electron trapping properties [9]. The n-like layer is often a-Se doped with an alkali metal, but recent work has shown that a cold deposited a-Se layer can have the same hole-trapping effect [6].…”