Recently developed direct conversion flat-panel X-ray image detectors provide not only potentially superior images but also enable a simple and convenient means of achieving digital radiography. The present flat-panel sensors use stabilized a-Se as an X-ray photoconductor to convert absorbed X-ray photons to collectable charge carriers. The present paper outlines the basics of the direct conversion flat-panel X-ray detector, formulates and reviews the required X-ray photoconductor properties and examines to what extent stabilized amorphous selenium (a-Se) fulfills these requirements and how it compares with other potential X-ray photoconductors that have been proposed. Charge transport and the electron–hole pair creation energy in stabilized a-Se are reviewed within the context of our present knowledge. Current materials problems and future potential developments are also critically discussed.

Review X-ray photoconductors and stabilized a-Se for direct conversion digital flat-panel X-ray image-detectors

We report on electron and hole drift mobility, lifetime and range measurements on vacuum coated stabilized a-Se (a-Se alloyed with As and doped with Cl) X-ray photoconductor type layers as a function of the substrate temperature T substrate during deposition; T substrate has been varied from 6 to 66 °C covering the glass transition temperature of a-Se (about 40 °C). We find the electron mobility, lifetime and range show essentially no dependence on T substrate , whereas for holes, although the drift mobility is not significantly affected, the lifetime deteriorates rapidly with decreasing T substrate . We also re-examine and discuss the charge conversion efficiency of a-Se X-ray photoconductors by reanalyzing the electron–hole pair creation energy ( W ± ) vs. field data, and evaluating the saturated (minimum) W ± from the W ± vs. reciprocal field plot.

Amorphous selenium as an X-ray photoconductor

The suitability of stabilized amorphous selenium (a-Se:0.2-0.5% As, 10-20 ppm Cl) as an x-ray imaging photoconductor is largely determined by its charge-generation, transport and trapping properties. The product of the charge-carrier drift mobility, , deep-trapping lifetime, , and applied electrical field, F, known as the Schubweg, represents the average distance that a charge carrier travels in the transport band before being trapped in deep localized states within the mobility gap. Time-of-flight and interrupted-field time-of-flight measurements on stabilized a-Se layers suitable for use in x-ray image detectors show that the hole and electron lifetimes are about 500 s and about 750 s respectively which are much longer than typical transit times in these photoconductors. The observed field dependence of the x-ray sensitivity is therefore not due to any Schubweg limitations. The energy required to create a free pair of electrons and holes, , was measured by integrating the x-ray-induced photocurrent to find the number of free charge carriers and dividing that by the energy absorbed in the selenium layer. evinced a strong field dependence which was extrapolated at the highest fields to obtain the intrinsic electron-hole-pair creation energy which was found to be about 6 eV. was shown to be temperature independent over the range 263-300 K. This result is in accord with the columnar recombination theory for the origin of the field dependence of for a-Se proposed by Hirsch and Jahankhani.

https://iopscience.iop.org/article/10.1088/0022-3727/32/3/004/pdf

Charge transport and electron-hole-pair creation energy in stabilized a-Se x-ray photoconductors

The present paper examines why a-Se has become the key photoconductor material in flat panel direct conversion X-ray image detectors. We critically examine the X-ray sensitivity of stabilized a-Se layers as a function of electric field and mean photon energy. The radiation energy W± absorbed per free electron–hole pair liberated for photoconduction in both a-Se and a-Si:H seem to follow the Que–Rowlands rule of W ± ≈2.2 E g +E phonon for amorphous semiconductors.

Properties of a-Se for use in flat panel X-ray image detectors

We present amorphous silicon p‐i‐n diodes able to sustain a reverse bias corresponding to 106 V/cm with a reasonably low leakage current. The influence of the p‐layer thickness on the reverse bias current and the breakdown voltage is investigated. The high‐voltage reverse current at room temperature is attributed to two different mechanisms: field enhanced thermal generation in the p‐i interface region and, at the highest bias, electron injection through the p layer. Variable range hopping is also contributing to the low‐temperature reverse current. Charge collection measurements after pulsed photogeneration were also performed up to the maximum voltage. No evidence for signal amplification is found, which sets a lower limit of 106 V/cm for impact ionization and avalanche phenomena.

High electric‐field amorphous silicon p‐i‐n diodes: Effect of the p‐layer thickness

Prototypes of thin film p-i-n amorphous silicon detectors, having intrinsic layer thicknesses ranging from 3 to 18 μm, were designed and fabricated, and their response to protons of 1.3 to 11.6 MeV, and to α particles of 1.6 to 14.6 MeV was studied. The collection efficiency of the charges generated by a particle is found to decrease sharply with increasing dE dx , for dE dx values greater than 10 keV μm . However, there exists a domain of oblique angles of incidence, relatively high applied biases and low dE dx values over which the signal amplitude varies linearly with energy loss. Pulse shape studies show the signal to consist of a steep leading edge, attributed to prompt electron collection, and a slow component showing evidence of multiple-trapping transport of holes, which is a possible cause for incomplete collection of the generated charge at the maximum attainable field of 60 V μm . Finally, all detectors give signal amplitudes compatible with a mean electron-hole pair creation energy of 3.4 to 4.4 eV in a-Si: H.

Response of amorphous silicon p-i-n detectors to ionizing particles

New types of a‐Si:H p‐i‐n diodes, capable of sustaining high reverse electric fields up to 6×105 V/cm with very low leakage currents, have been designed specifically for charged particle detection. Because of the low hole mobility and lifetime in this type of material, very high applied fields are required to collect a large fraction of the charge carriers generated by an ionizing particle. Since the leakage current is shown to be essentially due to electron tunneling through the p‐doped layer, the new design includes a thicker p layer (300 nm) at the expense of an increased surface leakage current. An appropriate etching of the top doped layer around the electrode is then required to eliminate such currents. A model, taking into account the doped layer conductivities, is presented to roughly compare the reverse current‐voltage characteristics of the samples before and after etching.

High reverse voltage amorphous silicon p-i-n diodes

We measured the response of reverse-biased a-Si:H diodes, in the p-i-n geometry, to charged particles as a function of ionization, detector bias (V) and angle of incidence, in the ionization range of 10–300 keV/µm. In the two cases where the amount of signal was weak, namely at low biases or at low ionizations, the signal was extracted from the noise using a coincidence technique. The response was found to be roughly proportional to, at all values of dE/dx, in the bias range of 7 V to 50 V. The dependence of the charge collection efficiency on the ionization density indicates a strong plasma effect in the detector. This collection efficiency increases steeply when the ionization is decreased but no plateau has yet been reached, indicating that the plasma effect is still important at 10 keV/µm.

Response of a-Si:H Detectors to Protons and Alphas

The signal produced by protons in a-Si:H detectors has been measured. The charge collected from a 5.8 μm p-i-n diode is studied as a function of the reverse bias and the energy deposited in the detector. The charge collection process is discussed. Limits on the pair-creation energy, Ep, are obtained (3.4 eV<Ep< 4.5 eV).

Charge collection in a-Si:H particle detectors

Thin film radiation detectors consisting of p-i-n layers of a-Si:H designed to sustain high biases were exposed to protons of 7 to 12 MeV. The charge induced by the ionizing particle was studied as a function of applied voltage. A model taking into account the dispersive hole transport is used to fit the data.

Modeling of the signal induced in a-Si:H particle detectors by protons

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