Dark current–voltage characteristics of vacuum deposited multilayer amorphous selenium-alloy detectors and the effect of x-ray irradiation

Abstract

Doped and stabilized amorphous selenium (a-Se) alloys in a multilayer form are currently used as a photoconductor in direct conversion flat panel x-ray imagers in mammography and tomosynthesis. While much progress has been made on the physics of such detectors, there are still unresolved questions on such issues as the principles of operation of the so-called p-i-n detector structure in extinguishing the dark current. The present paper examines dark current transients after the application of a voltage in seven types of a-Se alloy based devices: i-layer, i-n, n-i, i-p, p-i, p-i-n, and n-i-p structures. The substrate was ITO coated glass, the top (radiation receiving) electrode was chromium, and the films were fabricated by vacuum deposition. The nominal device thickness was 200 μm, similar to commercial mammographic detectors. It is shown that n-i, i-n, n-i-p, and p-i-n devices have dark currents less than 1 pA mm−2 at an applied field of 10 V/μm. The dark current in the p-i-n device is the lowest at approximately 0.01 pA mm−2 at an applied field of 10 V μm−1. Experiments have been carried out by subjecting the detector to a staircase voltage-time profile during the voltage application (turn-on) and a staircase voltage ramp-down during the turning-off of the bias voltage. Step-voltage ramp-up and step-voltage ramp-down current transient are typical of expected dark current transient behavior in a semiconductor with traps in which carriers are captured and released from various trap centers in the bandgap. The dark current transients are qualitatively similar to those expected from a capacitor in parallel with a large resistor and both in series with a much smaller resistor. Current transients during charging and discharging experiments were integrated to find how much of the injected charge is released during discharge experiments. It is shown that the majority of this trapped charge is stored in the n-type and p-type blocking layers, near the contacts. The trapped carriers in the i-layer represent a very small portion. The evolution of the dark current upon single and repeated x-ray exposure has also been examined in n-i and p-i-n devices. Right after the cessation of irradiation, there is an excess or residual dark current whose magnitude is roughly ∼20 times higher in p-i-n and about ∼2 times higher in the case of n-i under an exposure of 3.36 R and a mean photon energy of 34.2 keV. The absorbed dose is 1.53 Gy. The excess dark current has a fast decay component with a time constant ∼10 s and a slow component with a time constant ∼100 s. It is shown that the decay in the irradiation induced excess dark current is very similar to the initial dark current. There is no permanent change in the dark current, and within a few hundred seconds, the dark current reaches the same level as that in the unexposed detector. The experimental results in this work highlight the distinct advantages of p-i-n and n-i-type a-Se multilayer structures in x-ray detection applications.

Frequency- and time-resolved photocurrents in vacuum-deposited stabilised a-Se films: the role of valence alternation defects

“…We note that commercial a-Se detectors are based on a three-layer p-i-n structure in which the p-layer and n-layer are thin (a few microns) and the i-layer is approximately 200 μm. The p-layer (next to the substrate) is As 2 Se 3 and the n-layer (between the i-layer and the positive electrode) is an alkaline-(Cs) doped Se 1−x As x (x ≈ 0.02-0.05) alloy which is n-type [21,22]. The thin layers act as "blocking layers" and reduce the dark current, but X-ray absorption and transport occurring in the i-layer dictate device performance; the work reported here is most relevant to that region.…”

Modeling Dark Current Conduction Mechanisms and Mitigation Techniques in Vertically Stacked Amorphous Selenium-Based Photodetectors

“…Configuration 1 is an a -Se-based nonavalanche photodiode, while configurations 2 and 3 are a -Se-based avalanche photodiodes (Table ). ,− The introduction of blocking layers in these three configurations leads to an insufficient number of injection charges for space-charge effect, resulting in a suppression of the nonuniformity in electric field in the a -Se layer in Figure (a). Additionally, the hole Schottky barrier in the nonavalanche photodiode, as shown in Figure (b), is higher than in the MiM structure, as shown in Figure (a) (0.9 as compared to 0.68 eV).…”

Engineering of a Blocking Layer Structure for Low-Lag Operation of the a-PbO-Based X-Ray Detector

“…However, the dark current increases with the electric field: 0.1 pA/mm 2 at 5 V/μm and 0.7 pA/mm 2 at 20 V/μm after biasing for 2 h. This behavior is similar to what has been observed in a-Se-based n-i , ip, and n-ip blocking structures (where the i -layer is stabilized a-Se), and in PI/a-Se, the dark current rapidly decreases with time by 1-3 orders of magnitude (depending on the layer configuration) over several hours. The lowest attainable steady-state dark current was achieved in n-ip and PI/a-Se detectors: 0.01-0.05 pA/mm 2 at 10 V/μm [36], [37]. This minimal dark current was attributed to the bulk thermal generation in the PI/a-Se detector, but in the n-ip detector, it was assigned to the electron injection [38] or balancing of the carrier trapping and detrapping rates due to Schottky emission over the barrier in the n-layer [39].…”